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@settitle GNAT Reference Manual
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GNAT Reference Manual , May 19, 2026

AdaCore

Copyright @copyright{} 2008-2026, Free Software Foundation
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@title GNAT Reference Manual
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@contents

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`GNAT, The GNU Ada Development Environment'


@include gcc-common.texi
GCC version @value{version-GCC}@*
AdaCore

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being “GNAT Reference
Manual”, and with no Back-Cover Texts.  A copy of the license is
included in the section entitled @ref{1,,GNU Free Documentation License}.

@menu
* About This Guide:: 
* Implementation Defined Pragmas:: 
* Implementation Defined Aspects:: 
* Implementation Defined Attributes:: 
* Standard and Implementation Defined Restrictions:: 
* Implementation Advice:: 
* GNAT Implementation Mode (the -gnatg switch): GNAT Implementation Mode the -gnatg switch. 
* Implementation Defined Characteristics:: 
* Intrinsic Subprograms:: 
* Representation Clauses and Pragmas:: 
* Standard Library Routines:: 
* The Implementation of Standard I/O:: 
* The GNAT Library:: 
* Interfacing to Other Languages:: 
* Specialized Needs Annexes:: 
* Implementation of Specific Ada Features:: 
* Implementation of Ada 2022 Features:: 
* GNAT language extensions:: 
* Security Hardening Features:: 
* Obsolescent Features:: 
* Compatibility and Porting Guide:: 
* GNU Free Documentation License:: 
* Index:: 

@detailmenu
 --- The Detailed Node Listing ---

About This Guide

* What This Reference Manual Contains:: 
* Conventions:: 
* Related Information:: 

Implementation Defined Pragmas

* Pragma Abort_Defer:: 
* Pragma Abstract_State:: 
* Pragma Ada_83:: 
* Pragma Ada_95:: 
* Pragma Ada_05:: 
* Pragma Ada_2005:: 
* Pragma Ada_12:: 
* Pragma Ada_2012:: 
* Pragma Ada_2022:: 
* Pragma Aggregate_Individually_Assign:: 
* Pragma Allow_Integer_Address:: 
* Pragma Always_Terminates:: 
* Pragma Annotate:: 
* Pragma Assert:: 
* Pragma Assert_And_Cut:: 
* Pragma Assertion_Level:: 
* Pragma Assertion_Policy:: 
* Pragma Assume:: 
* Pragma Assume_No_Invalid_Values:: 
* Pragma Async_Readers:: 
* Pragma Async_Writers:: 
* Pragma Attribute_Definition:: 
* Pragma C_Pass_By_Copy:: 
* Pragma Check:: 
* Pragma Check_Float_Overflow:: 
* Pragma Check_Name:: 
* Pragma Check_Policy:: 
* Pragma Comment:: 
* Pragma Common_Object:: 
* Pragma Compile_Time_Error:: 
* Pragma Compile_Time_Warning:: 
* Pragma Complete_Representation:: 
* Pragma Complex_Representation:: 
* Pragma Component_Alignment:: 
* Pragma Constant_After_Elaboration:: 
* Pragma Contract_Cases:: 
* Pragma Convention_Identifier:: 
* Pragma CPP_Class:: 
* Pragma CPP_Constructor:: 
* Pragma CPP_Virtual:: 
* Pragma CPP_Vtable:: 
* Pragma CPU:: 
* Pragma Deadline_Floor:: 
* Pragma Debug:: 
* Pragma Debug_Policy:: 
* Pragma Default_Initial_Condition:: 
* Pragma Default_Scalar_Storage_Order:: 
* Pragma Default_Storage_Pool:: 
* Pragma Depends:: 
* Pragma Detect_Blocking:: 
* Pragma Disable_Atomic_Synchronization:: 
* Pragma Dispatching_Domain:: 
* Pragma Effective_Reads:: 
* Pragma Effective_Writes:: 
* Pragma Elaboration_Checks:: 
* Pragma Eliminate:: 
* Pragma Enable_Atomic_Synchronization:: 
* Pragma Exceptional_Cases:: 
* Pragma Exit_Cases:: 
* Pragma Export_Function:: 
* Pragma Export_Object:: 
* Pragma Export_Procedure:: 
* Pragma Export_Valued_Procedure:: 
* Pragma Extend_System:: 
* Pragma Extensions_Allowed:: 
* Pragma Extensions_Visible:: 
* Pragma External:: 
* Pragma External_Name_Casing:: 
* Pragma Fast_Math:: 
* Pragma Favor_Top_Level:: 
* Pragma Finalize_Storage_Only:: 
* Pragma Float_Representation:: 
* Pragma Ghost:: 
* Pragma Global:: 
* Pragma Ident:: 
* Pragma Ignore_Pragma:: 
* Pragma Implementation_Defined:: 
* Pragma Implemented:: 
* Pragma Implicit_Packing:: 
* Pragma Import_Function:: 
* Pragma Import_Object:: 
* Pragma Import_Procedure:: 
* Pragma Import_Valued_Procedure:: 
* Pragma Independent:: 
* Pragma Independent_Components:: 
* Pragma Initial_Condition:: 
* Pragma Initialize_Scalars:: 
* Pragma Initializes:: 
* Pragma Inline_Always:: 
* Pragma Inline_Generic:: 
* Pragma Interface:: 
* Pragma Interface_Name:: 
* Pragma Interrupt_Handler:: 
* Pragma Interrupt_State:: 
* Pragma Interrupts_System_By_Default:: 
* Pragma Invariant:: 
* Pragma Keep_Names:: 
* Pragma License:: 
* Pragma Link_With:: 
* Pragma Linker_Alias:: 
* Pragma Linker_Constructor:: 
* Pragma Linker_Destructor:: 
* Pragma Linker_Section:: 
* Pragma Lock_Free:: 
* Pragma Loop_Invariant:: 
* Pragma Loop_Optimize:: 
* Pragma Loop_Variant:: 
* Pragma Machine_Attribute:: 
* Pragma Main:: 
* Pragma Main_Storage:: 
* Pragma Max_Queue_Length:: 
* Pragma No_Body:: 
* Pragma No_Caching:: 
* Pragma No_Component_Reordering:: 
* Pragma No_Elaboration_Code_All:: 
* Pragma No_Heap_Finalization:: 
* Pragma No_Inline:: 
* Pragma No_Raise:: 
* Pragma No_Return:: 
* Pragma No_Strict_Aliasing:: 
* Pragma No_Tagged_Streams:: 
* Pragma Normalize_Scalars:: 
* Pragma Obsolescent:: 
* Pragma Optimize_Alignment:: 
* Pragma Ordered:: 
* Pragma Overflow_Mode:: 
* Pragma Overriding_Renamings:: 
* Pragma Part_Of:: 
* Pragma Partition_Elaboration_Policy:: 
* Pragma Passive:: 
* Pragma Persistent_BSS:: 
* Pragma Post:: 
* Pragma Postcondition:: 
* Pragma Post_Class:: 
* Pragma Pre:: 
* Pragma Precondition:: 
* Pragma Predicate:: 
* Pragma Predicate_Failure:: 
* Pragma Preelaborable_Initialization:: 
* Pragma Prefix_Exception_Messages:: 
* Pragma Pre_Class:: 
* Pragma Priority_Specific_Dispatching:: 
* Pragma Profile:: 
* Pragma Profile_Warnings:: 
* Pragma Program_Exit:: 
* Pragma Propagate_Exceptions:: 
* Pragma Provide_Shift_Operators:: 
* Pragma Psect_Object:: 
* Pragma Pure_Function:: 
* Pragma Rational:: 
* Pragma Ravenscar:: 
* Pragma Refined_Depends:: 
* Pragma Refined_Global:: 
* Pragma Refined_Post:: 
* Pragma Refined_State:: 
* Pragma Relative_Deadline:: 
* Pragma Remote_Access_Type:: 
* Pragma Rename_Pragma:: 
* Pragma Restricted_Run_Time:: 
* Pragma Restriction_Warnings:: 
* Pragma Reviewable:: 
* Pragma Secondary_Stack_Size:: 
* Pragma Share_Generic:: 
* Pragma Shared:: 
* Pragma Short_Circuit_And_Or:: 
* Pragma Short_Descriptors:: 
* Pragma Side_Effects:: 
* Pragma Simple_Storage_Pool_Type:: 
* Pragma Source_File_Name:: 
* Pragma Source_File_Name_Project:: 
* Pragma Source_Reference:: 
* Pragma SPARK_Mode:: 
* Pragma Static_Elaboration_Desired:: 
* Pragma Stream_Convert:: 
* Pragma Style_Checks:: 
* Pragma Subprogram_Variant:: 
* Pragma Subtitle:: 
* Pragma Suppress:: 
* Pragma Suppress_All:: 
* Pragma Suppress_Debug_Info:: 
* Pragma Suppress_Exception_Locations:: 
* Pragma Suppress_Initialization:: 
* Pragma Task_Name:: 
* Pragma Task_Storage:: 
* Pragma Test_Case:: 
* Pragma Thread_Local_Storage:: 
* Pragma Time_Slice:: 
* Pragma Title:: 
* Pragma Type_Invariant:: 
* Pragma Type_Invariant_Class:: 
* Pragma Unchecked_Union:: 
* Pragma Unevaluated_Use_Of_Old:: 
* Pragma User_Aspect_Definition:: 
* Pragma Unimplemented_Unit:: 
* Pragma Universal_Aliasing:: 
* Pragma Unmodified:: 
* Pragma Unreferenced:: 
* Pragma Unreferenced_Objects:: 
* Pragma Unreserve_All_Interrupts:: 
* Pragma Unsuppress:: 
* Pragma Unused:: 
* Pragma Use_VADS_Size:: 
* Pragma Validity_Checks:: 
* Pragma Volatile:: 
* Pragma Volatile_Full_Access:: 
* Pragma Volatile_Function:: 
* Pragma Warning_As_Error:: 
* Pragma Warnings:: 
* Pragma Weak_External:: 
* Pragma Wide_Character_Encoding:: 

Implementation Defined Aspects

* Aspect Abstract_State:: 
* Aspect Always_Terminates:: 
* Aspect Annotate:: 
* Aspect Async_Readers:: 
* Aspect Async_Writers:: 
* Aspect Constant_After_Elaboration:: 
* Aspect Contract_Cases:: 
* Aspect Depends:: 
* Aspect Default_Initial_Condition:: 
* Aspect Dimension:: 
* Aspect Dimension_System:: 
* Aspect Disable_Controlled:: 
* Aspect Effective_Reads:: 
* Aspect Effective_Writes:: 
* Aspect Exceptional_Cases:: 
* Aspect Exit_Cases:: 
* Aspect Extended_Access:: 
* Aspect Extensions_Visible:: 
* Aspect Favor_Top_Level:: 
* Aspect Ghost:: 
* Aspect Ghost_Predicate:: 
* Aspect Global:: 
* Aspect Initial_Condition:: 
* Aspect Initializes:: 
* Aspect Inline_Always:: 
* Aspect Invariant:: 
* Aspect Invariant’Class:: 
* Aspect Iterable:: 
* Aspect Linker_Section:: 
* Aspect Local_Restrictions:: 
* Aspect Lock_Free:: 
* Aspect Max_Queue_Length:: 
* Aspect No_Caching:: 
* Aspect No_Elaboration_Code_All:: 
* Aspect No_Inline:: 
* Aspect No_Raise:: 
* Aspect No_Tagged_Streams:: 
* Aspect No_Task_Parts:: 
* Aspect Object_Size:: 
* Aspect Obsolescent:: 
* Aspect Part_Of:: 
* Aspect Persistent_BSS:: 
* Aspect Potentially_Invalid:: 
* Aspect Predicate:: 
* Aspect Program_Exit:: 
* Aspect Pure_Function:: 
* Aspect Refined_Depends:: 
* Aspect Refined_Global:: 
* Aspect Refined_Post:: 
* Aspect Refined_State:: 
* Aspect Relaxed_Initialization:: 
* Aspect Remote_Access_Type:: 
* Aspect Scalar_Storage_Order:: 
* Aspect Secondary_Stack_Size:: 
* Aspect Shared:: 
* Aspect Side_Effects:: 
* Aspect Simple_Storage_Pool:: 
* Aspect Simple_Storage_Pool_Type:: 
* Aspect SPARK_Mode:: 
* Aspect Subprogram_Variant:: 
* Aspect Suppress_Debug_Info:: 
* Aspect Suppress_Initialization:: 
* Aspect Test_Case:: 
* Aspect Thread_Local_Storage:: 
* Aspect Universal_Aliasing:: 
* Aspect Unmodified:: 
* Aspect Unreferenced:: 
* Aspect Unreferenced_Objects:: 
* Aspect User_Aspect:: 
* Aspect Value_Size:: 
* Aspect Volatile_Full_Access:: 
* Aspect Volatile_Function:: 
* Aspect Warnings:: 

Implementation Defined Attributes

* Attribute Abort_Signal:: 
* Attribute Address_Size:: 
* Attribute Asm_Input:: 
* Attribute Asm_Output:: 
* Attribute Atomic_Always_Lock_Free:: 
* Attribute Bit:: 
* Attribute Bit_Position:: 
* Attribute Code_Address:: 
* Attribute Compiler_Version:: 
* Attribute Constrained:: 
* Attribute Default_Bit_Order:: 
* Attribute Default_Scalar_Storage_Order:: 
* Attribute Deref:: 
* Attribute Descriptor_Size:: 
* Attribute Elaborated:: 
* Attribute Elab_Body:: 
* Attribute Elab_Spec:: 
* Attribute Elab_Subp_Body:: 
* Attribute Emax:: 
* Attribute Enabled:: 
* Attribute Enum_Rep:: 
* Attribute Enum_Val:: 
* Attribute Epsilon:: 
* Attribute Fast_Math:: 
* Attribute Finalization_Size:: 
* Attribute Fixed_Value:: 
* Attribute From_Address:: 
* Attribute From_Any:: 
* Attribute Has_Access_Values:: 
* Attribute Has_Discriminants:: 
* Attribute Has_Tagged_Values:: 
* Attribute Img:: 
* Attribute Initialized:: 
* Attribute Integer_Value:: 
* Attribute Invalid_Value:: 
* Attribute Large:: 
* Attribute Library_Level:: 
* Attribute Loop_Entry:: 
* Attribute Machine_Size:: 
* Attribute Mantissa:: 
* Attribute Maximum_Alignment:: 
* Attribute Max_Integer_Size:: 
* Attribute Mechanism_Code:: 
* Attribute Null_Parameter:: 
* Attribute Object_Size:: 
* Attribute Old:: 
* Attribute Passed_By_Reference:: 
* Attribute Pool_Address:: 
* Attribute Range_Length:: 
* Attribute Restriction_Set:: 
* Attribute Result:: 
* Attribute Round:: 
* Attribute Safe_Emax:: 
* Attribute Safe_Large:: 
* Attribute Safe_Small:: 
* Attribute Scalar_Storage_Order:: 
* Attribute Simple_Storage_Pool:: 
* Attribute Small:: 
* Attribute Small_Denominator:: 
* Attribute Small_Numerator:: 
* Attribute Storage_Unit:: 
* Attribute Stub_Type:: 
* Attribute System_Allocator_Alignment:: 
* Attribute Target_Name:: 
* Attribute To_Address:: 
* Attribute To_Any:: 
* Attribute Type_Class:: 
* Attribute Type_Key:: 
* Attribute TypeCode:: 
* Attribute Unconstrained_Array:: 
* Attribute Universal_Literal_String:: 
* Attribute Unrestricted_Access:: 
* Attribute Update:: 
* Attribute Valid_Value:: 
* Attribute Valid_Scalars:: 
* Attribute VADS_Size:: 
* Attribute Value_Size:: 
* Attribute Wchar_T_Size:: 
* Attribute Word_Size:: 

Standard and Implementation Defined Restrictions

* Partition-Wide Restrictions:: 
* Program Unit Level Restrictions:: 

Partition-Wide Restrictions

* Immediate_Reclamation:: 
* Max_Asynchronous_Select_Nesting:: 
* Max_Entry_Queue_Length:: 
* Max_Protected_Entries:: 
* Max_Select_Alternatives:: 
* Max_Storage_At_Blocking:: 
* Max_Task_Entries:: 
* Max_Tasks:: 
* No_Abort_Statements:: 
* No_Access_Parameter_Allocators:: 
* No_Access_Subprograms:: 
* No_Allocators:: 
* No_Anonymous_Allocators:: 
* No_Asynchronous_Control:: 
* No_Calendar:: 
* No_Coextensions:: 
* No_Default_Initialization:: 
* No_Delay:: 
* No_Dependence:: 
* No_Direct_Boolean_Operators:: 
* No_Dispatch:: 
* No_Dispatching_Calls:: 
* No_Dynamic_Attachment:: 
* No_Dynamic_Priorities:: 
* No_Entry_Calls_In_Elaboration_Code:: 
* No_Enumeration_Maps:: 
* No_Exception_Handlers:: 
* No_Exception_Propagation:: 
* No_Exception_Registration:: 
* No_Exceptions:: 
* No_Finalization:: 
* No_Fixed_Point:: 
* No_Floating_Point:: 
* No_Implicit_Conditionals:: 
* No_Implicit_Dynamic_Code:: 
* No_Implicit_Heap_Allocations:: 
* No_Implicit_Protected_Object_Allocations:: 
* No_Implicit_Task_Allocations:: 
* No_Initialize_Scalars:: 
* No_IO:: 
* No_Local_Allocators:: 
* No_Local_Protected_Objects:: 
* No_Local_Tagged_Types:: 
* No_Local_Timing_Events:: 
* No_Long_Long_Integers:: 
* No_Multiple_Elaboration:: 
* No_Nested_Finalization:: 
* No_Protected_Type_Allocators:: 
* No_Protected_Types:: 
* No_Recursion:: 
* No_Reentrancy:: 
* No_Relative_Delay:: 
* No_Requeue_Statements:: 
* No_Secondary_Stack:: 
* No_Select_Statements:: 
* No_Specific_Termination_Handlers:: 
* No_Specification_of_Aspect:: 
* No_Standard_Allocators_After_Elaboration:: 
* No_Standard_Storage_Pools:: 
* No_Stream_Optimizations:: 
* No_Streams:: 
* No_Tagged_Type_Registration:: 
* No_Task_Allocators:: 
* No_Task_At_Interrupt_Priority:: 
* No_Task_Attributes_Package:: 
* No_Task_Hierarchy:: 
* No_Task_Termination:: 
* No_Tasking:: 
* No_Terminate_Alternatives:: 
* No_Unchecked_Access:: 
* No_Unchecked_Conversion:: 
* No_Unchecked_Deallocation:: 
* No_Use_Of_Attribute:: 
* No_Use_Of_Entity:: 
* No_Use_Of_Pragma:: 
* Pure_Barriers:: 
* Simple_Barriers:: 
* Static_Priorities:: 
* Static_Storage_Size:: 

Program Unit Level Restrictions

* No_Elaboration_Code:: 
* No_Dynamic_Accessibility_Checks:: 
* No_Dynamic_Sized_Objects:: 
* No_Entry_Queue:: 
* No_Implementation_Aspect_Specifications:: 
* No_Implementation_Attributes:: 
* No_Implementation_Identifiers:: 
* No_Implementation_Pragmas:: 
* No_Implementation_Restrictions:: 
* No_Implementation_Units:: 
* No_Implicit_Aliasing:: 
* No_Implicit_Loops:: 
* No_Obsolescent_Features:: 
* No_Wide_Characters:: 
* Static_Dispatch_Tables:: 
* SPARK_05:: 

Implementation Advice

* RM 1.1.3(20); Error Detection: RM 1 1 3 20 Error Detection. 
* RM 1.1.3(31); Child Units: RM 1 1 3 31 Child Units. 
* RM 1.1.5(12); Bounded Errors: RM 1 1 5 12 Bounded Errors. 
* RM 2.8(16); Pragmas: RM 2 8 16 Pragmas. 
* RM 2.8(17-19); Pragmas: RM 2 8 17-19 Pragmas. 
* RM 3.5.2(5); Alternative Character Sets: RM 3 5 2 5 Alternative Character Sets. 
* RM 3.5.4(28); Integer Types: RM 3 5 4 28 Integer Types. 
* RM 3.5.4(29); Integer Types: RM 3 5 4 29 Integer Types. 
* RM 3.5.5(8); Enumeration Values: RM 3 5 5 8 Enumeration Values. 
* RM 3.5.7(17); Float Types: RM 3 5 7 17 Float Types. 
* RM 3.6.2(11); Multidimensional Arrays: RM 3 6 2 11 Multidimensional Arrays. 
* RM 9.6(30-31); Duration’Small: RM 9 6 30-31 Duration’Small. 
* RM 10.2.1(12); Consistent Representation: RM 10 2 1 12 Consistent Representation. 
* RM 11.4.1(19); Exception Information: RM 11 4 1 19 Exception Information. 
* RM 11.5(28); Suppression of Checks: RM 11 5 28 Suppression of Checks. 
* RM 13.1 (21-24); Representation Clauses: RM 13 1 21-24 Representation Clauses. 
* RM 13.2(6-8); Packed Types: RM 13 2 6-8 Packed Types. 
* RM 13.3(14-19); Address Clauses: RM 13 3 14-19 Address Clauses. 
* RM 13.3(29-35); Alignment Clauses: RM 13 3 29-35 Alignment Clauses. 
* RM 13.3(42-43); Size Clauses: RM 13 3 42-43 Size Clauses. 
* RM 13.3(50-56); Size Clauses: RM 13 3 50-56 Size Clauses. 
* RM 13.3(71-73); Component Size Clauses: RM 13 3 71-73 Component Size Clauses. 
* RM 13.4(9-10); Enumeration Representation Clauses: RM 13 4 9-10 Enumeration Representation Clauses. 
* RM 13.5.1(17-22); Record Representation Clauses: RM 13 5 1 17-22 Record Representation Clauses. 
* RM 13.5.2(5); Storage Place Attributes: RM 13 5 2 5 Storage Place Attributes. 
* RM 13.5.3(7-8); Bit Ordering: RM 13 5 3 7-8 Bit Ordering. 
* RM 13.7(37); Address as Private: RM 13 7 37 Address as Private. 
* RM 13.7.1(16); Address Operations: RM 13 7 1 16 Address Operations. 
* RM 13.9(14-17); Unchecked Conversion: RM 13 9 14-17 Unchecked Conversion. 
* RM 13.11(23-25); Implicit Heap Usage: RM 13 11 23-25 Implicit Heap Usage. 
* RM 13.11.2(17); Unchecked Deallocation: RM 13 11 2 17 Unchecked Deallocation. 
* RM 13.13.2(1.6); Stream Oriented Attributes: RM 13 13 2 1 6 Stream Oriented Attributes. 
* RM A.1(52); Names of Predefined Numeric Types: RM A 1 52 Names of Predefined Numeric Types. 
* RM A.3.2(49); Ada.Characters.Handling: RM A 3 2 49 Ada Characters Handling. 
* RM A.4.4(106); Bounded-Length String Handling: RM A 4 4 106 Bounded-Length String Handling. 
* RM A.5.2(46-47); Random Number Generation: RM A 5 2 46-47 Random Number Generation. 
* RM A.10.7(23); Get_Immediate: RM A 10 7 23 Get_Immediate. 
* RM A.18; Containers: RM A 18 Containers. 
* RM B.1(39-41); Pragma Export: RM B 1 39-41 Pragma Export. 
* RM B.2(12-13); Package Interfaces: RM B 2 12-13 Package Interfaces. 
* RM B.3(63-71); Interfacing with C: RM B 3 63-71 Interfacing with C. 
* RM B.4(95-98); Interfacing with COBOL: RM B 4 95-98 Interfacing with COBOL. 
* RM B.5(22-26); Interfacing with Fortran: RM B 5 22-26 Interfacing with Fortran. 
* RM C.1(3-5); Access to Machine Operations: RM C 1 3-5 Access to Machine Operations. 
* RM C.1(10-16); Access to Machine Operations: RM C 1 10-16 Access to Machine Operations. 
* RM C.3(28); Interrupt Support: RM C 3 28 Interrupt Support. 
* RM C.3.1(20-21); Protected Procedure Handlers: RM C 3 1 20-21 Protected Procedure Handlers. 
* RM C.3.2(25); Package Interrupts: RM C 3 2 25 Package Interrupts. 
* RM C.4(14); Pre-elaboration Requirements: RM C 4 14 Pre-elaboration Requirements. 
* RM C.5(8); Pragma Discard_Names: RM C 5 8 Pragma Discard_Names. 
* RM C.7.2(30); The Package Task_Attributes: RM C 7 2 30 The Package Task_Attributes. 
* RM D.3(17); Locking Policies: RM D 3 17 Locking Policies. 
* RM D.4(16); Entry Queuing Policies: RM D 4 16 Entry Queuing Policies. 
* RM D.6(9-10); Preemptive Abort: RM D 6 9-10 Preemptive Abort. 
* RM D.7(21); Tasking Restrictions: RM D 7 21 Tasking Restrictions. 
* RM D.8(47-49); Monotonic Time: RM D 8 47-49 Monotonic Time. 
* RM E.5(28-29); Partition Communication Subsystem: RM E 5 28-29 Partition Communication Subsystem. 
* RM F(7); COBOL Support: RM F 7 COBOL Support. 
* RM F.1(2); Decimal Radix Support: RM F 1 2 Decimal Radix Support. 
* RM G; Numerics: RM G Numerics. 
* RM G.1.1(56-58); Complex Types: RM G 1 1 56-58 Complex Types. 
* RM G.1.2(49); Complex Elementary Functions: RM G 1 2 49 Complex Elementary Functions. 
* RM G.2.4(19); Accuracy Requirements: RM G 2 4 19 Accuracy Requirements. 
* RM G.2.6(15); Complex Arithmetic Accuracy: RM G 2 6 15 Complex Arithmetic Accuracy. 
* RM H.6(15/2); Pragma Partition_Elaboration_Policy: RM H 6 15/2 Pragma Partition_Elaboration_Policy. 

GNAT Implementation Mode (the -gnatg switch)

* Language & Semantic Changes:: 

Language & Semantic Changes

* Ada Version and Identifier Rules:: 
* Limited Types:: 
* Preelaboration and Categorization:: 
* Overflow:: 
* Miscellaneous:: 
* ALI Files and Recompilation:: 

Intrinsic Subprograms

* Intrinsic Operators:: 
* Compilation_ISO_Date:: 
* Compilation_Date:: 
* Compilation_Time:: 
* Enclosing_Entity:: 
* Exception_Information:: 
* Exception_Message:: 
* Exception_Name:: 
* File:: 
* Line:: 
* Shifts and Rotates:: 
* Source_Location:: 

Representation Clauses and Pragmas

* Alignment Clauses:: 
* Size Clauses:: 
* Storage_Size Clauses:: 
* Size of Variant Record Objects:: 
* Biased Representation:: 
* Value_Size and Object_Size Clauses:: 
* Component_Size Clauses:: 
* Bit_Order Clauses:: 
* Effect of Bit_Order on Byte Ordering:: 
* Pragma Pack for Arrays:: 
* Pragma Pack for Records:: 
* Record Representation Clauses:: 
* Handling of Records with Holes:: 
* Enumeration Clauses:: 
* Address Clauses:: 
* Use of Address Clauses for Memory-Mapped I/O:: 
* Effect of Convention on Representation:: 
* Conventions and Anonymous Access Types:: 
* Determining the Representations chosen by GNAT:: 

The Implementation of Standard I/O

* Standard I/O Packages:: 
* FORM Strings:: 
* Direct_IO:: 
* Sequential_IO:: 
* Text_IO:: 
* Wide_Text_IO:: 
* Wide_Wide_Text_IO:: 
* Stream_IO:: 
* Text Translation:: 
* Shared Files:: 
* Filenames encoding:: 
* File content encoding:: 
* Open Modes:: 
* Operations on C Streams:: 
* Interfacing to C Streams:: 

Text_IO

* Stream Pointer Positioning:: 
* Reading and Writing Non-Regular Files:: 
* Get_Immediate:: 
* Treating Text_IO Files as Streams:: 
* Text_IO Extensions:: 
* Text_IO Facilities for Unbounded Strings:: 

Wide_Text_IO

* Stream Pointer Positioning: Stream Pointer Positioning<2>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<2>. 

Wide_Wide_Text_IO

* Stream Pointer Positioning: Stream Pointer Positioning<3>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<3>. 

The GNAT Library

* Ada.Characters.Latin_9 (a-chlat9.ads): Ada Characters Latin_9 a-chlat9 ads. 
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads): Ada Characters Wide_Latin_1 a-cwila1 ads. 
* Ada.Characters.Wide_Latin_9 (a-cwila9.ads): Ada Characters Wide_Latin_9 a-cwila9 ads. 
* Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads): Ada Characters Wide_Wide_Latin_1 a-chzla1 ads. 
* Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads): Ada Characters Wide_Wide_Latin_9 a-chzla9 ads. 
* Ada.Containers.Bounded_Holders (a-coboho.ads): Ada Containers Bounded_Holders a-coboho ads. 
* Ada.Command_Line.Environment (a-colien.ads): Ada Command_Line Environment a-colien ads. 
* Ada.Command_Line.Remove (a-colire.ads): Ada Command_Line Remove a-colire ads. 
* Ada.Command_Line.Response_File (a-clrefi.ads): Ada Command_Line Response_File a-clrefi ads. 
* Ada.Direct_IO.C_Streams (a-diocst.ads): Ada Direct_IO C_Streams a-diocst ads. 
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads): Ada Exceptions Is_Null_Occurrence a-einuoc ads. 
* Ada.Exceptions.Last_Chance_Handler (a-elchha.ads): Ada Exceptions Last_Chance_Handler a-elchha ads. 
* Ada.Exceptions.Traceback (a-exctra.ads): Ada Exceptions Traceback a-exctra ads. 
* Ada.Sequential_IO.C_Streams (a-siocst.ads): Ada Sequential_IO C_Streams a-siocst ads. 
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads): Ada Streams Stream_IO C_Streams a-ssicst ads. 
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads): Ada Strings Unbounded Text_IO a-suteio ads. 
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads): Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads. 
* Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads): Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads. 
* Ada.Task_Initialization (a-tasini.ads): Ada Task_Initialization a-tasini ads. 
* Ada.Text_IO.C_Streams (a-tiocst.ads): Ada Text_IO C_Streams a-tiocst ads. 
* Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads): Ada Text_IO Reset_Standard_Files a-tirsfi ads. 
* Ada.Wide_Characters.Unicode (a-wichun.ads): Ada Wide_Characters Unicode a-wichun ads. 
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads): Ada Wide_Text_IO C_Streams a-wtcstr ads. 
* Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads): Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads. 
* Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads): Ada Wide_Wide_Characters Unicode a-zchuni ads. 
* Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads): Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads. 
* Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads): Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads. 
* GNAT.Altivec (g-altive.ads): GNAT Altivec g-altive ads. 
* GNAT.Altivec.Conversions (g-altcon.ads): GNAT Altivec Conversions g-altcon ads. 
* GNAT.Altivec.Vector_Operations (g-alveop.ads): GNAT Altivec Vector_Operations g-alveop ads. 
* GNAT.Altivec.Vector_Types (g-alvety.ads): GNAT Altivec Vector_Types g-alvety ads. 
* GNAT.Altivec.Vector_Views (g-alvevi.ads): GNAT Altivec Vector_Views g-alvevi ads. 
* GNAT.Array_Split (g-arrspl.ads): GNAT Array_Split g-arrspl ads. 
* GNAT.AWK (g-awk.ads): GNAT AWK g-awk ads. 
* GNAT.Binary_Search (g-binsea.ads): GNAT Binary_Search g-binsea ads. 
* GNAT.Bind_Environment (g-binenv.ads): GNAT Bind_Environment g-binenv ads. 
* GNAT.Branch_Prediction (g-brapre.ads): GNAT Branch_Prediction g-brapre ads. 
* GNAT.Bounded_Buffers (g-boubuf.ads): GNAT Bounded_Buffers g-boubuf ads. 
* GNAT.Bounded_Mailboxes (g-boumai.ads): GNAT Bounded_Mailboxes g-boumai ads. 
* GNAT.Bubble_Sort (g-bubsor.ads): GNAT Bubble_Sort g-bubsor ads. 
* GNAT.Bubble_Sort_A (g-busora.ads): GNAT Bubble_Sort_A g-busora ads. 
* GNAT.Bubble_Sort_G (g-busorg.ads): GNAT Bubble_Sort_G g-busorg ads. 
* GNAT.Byte_Order_Mark (g-byorma.ads): GNAT Byte_Order_Mark g-byorma ads. 
* GNAT.Byte_Swapping (g-bytswa.ads): GNAT Byte_Swapping g-bytswa ads. 
* GNAT.C_Time (g-c_time.ads): GNAT C_Time g-c_time ads. 
* GNAT.Calendar (g-calend.ads): GNAT Calendar g-calend ads. 
* GNAT.Calendar.Time_IO (g-catiio.ads): GNAT Calendar Time_IO g-catiio ads. 
* GNAT.CRC32 (g-crc32.ads): GNAT CRC32 g-crc32 ads. 
* GNAT.Case_Util (g-casuti.ads): GNAT Case_Util g-casuti ads. 
* GNAT.CGI (g-cgi.ads): GNAT CGI g-cgi ads. 
* GNAT.CGI.Cookie (g-cgicoo.ads): GNAT CGI Cookie g-cgicoo ads. 
* GNAT.CGI.Debug (g-cgideb.ads): GNAT CGI Debug g-cgideb ads. 
* GNAT.Command_Line (g-comlin.ads): GNAT Command_Line g-comlin ads. 
* GNAT.Compiler_Version (g-comver.ads): GNAT Compiler_Version g-comver ads. 
* GNAT.Ctrl_C (g-ctrl_c.ads): GNAT Ctrl_C g-ctrl_c ads. 
* GNAT.Current_Exception (g-curexc.ads): GNAT Current_Exception g-curexc ads. 
* GNAT.Debug_Pools (g-debpoo.ads): GNAT Debug_Pools g-debpoo ads. 
* GNAT.Debug_Utilities (g-debuti.ads): GNAT Debug_Utilities g-debuti ads. 
* GNAT.Decode_String (g-decstr.ads): GNAT Decode_String g-decstr ads. 
* GNAT.Decode_UTF8_String (g-deutst.ads): GNAT Decode_UTF8_String g-deutst ads. 
* GNAT.Directory_Operations (g-dirope.ads): GNAT Directory_Operations g-dirope ads. 
* GNAT.Directory_Operations.Iteration (g-diopit.ads): GNAT Directory_Operations Iteration g-diopit ads. 
* GNAT.Dynamic_HTables (g-dynhta.ads): GNAT Dynamic_HTables g-dynhta ads. 
* GNAT.Dynamic_Tables (g-dyntab.ads): GNAT Dynamic_Tables g-dyntab ads. 
* GNAT.Encode_String (g-encstr.ads): GNAT Encode_String g-encstr ads. 
* GNAT.Encode_UTF8_String (g-enutst.ads): GNAT Encode_UTF8_String g-enutst ads. 
* GNAT.Exception_Actions (g-excact.ads): GNAT Exception_Actions g-excact ads. 
* GNAT.Exception_Traces (g-exctra.ads): GNAT Exception_Traces g-exctra ads. 
* GNAT.Exceptions (g-except.ads): GNAT Exceptions g-except ads. 
* GNAT.Expect (g-expect.ads): GNAT Expect g-expect ads. 
* GNAT.Expect.TTY (g-exptty.ads): GNAT Expect TTY g-exptty ads. 
* GNAT.Float_Control (g-flocon.ads): GNAT Float_Control g-flocon ads. 
* GNAT.Formatted_String (g-forstr.ads): GNAT Formatted_String g-forstr ads. 
* GNAT.Generic_Fast_Math_Functions (g-gfmafu.ads): GNAT Generic_Fast_Math_Functions g-gfmafu ads. 
* GNAT.Heap_Sort (g-heasor.ads): GNAT Heap_Sort g-heasor ads. 
* GNAT.Heap_Sort_A (g-hesora.ads): GNAT Heap_Sort_A g-hesora ads. 
* GNAT.Heap_Sort_G (g-hesorg.ads): GNAT Heap_Sort_G g-hesorg ads. 
* GNAT.HTable (g-htable.ads): GNAT HTable g-htable ads. 
* GNAT.IO (g-io.ads): GNAT IO g-io ads. 
* GNAT.IO_Aux (g-io_aux.ads): GNAT IO_Aux g-io_aux ads. 
* GNAT.Lock_Files (g-locfil.ads): GNAT Lock_Files g-locfil ads. 
* GNAT.MBBS_Discrete_Random (g-mbdira.ads): GNAT MBBS_Discrete_Random g-mbdira ads. 
* GNAT.MBBS_Float_Random (g-mbflra.ads): GNAT MBBS_Float_Random g-mbflra ads. 
* GNAT.MD5 (g-md5.ads): GNAT MD5 g-md5 ads. 
* GNAT.Memory_Dump (g-memdum.ads): GNAT Memory_Dump g-memdum ads. 
* GNAT.Most_Recent_Exception (g-moreex.ads): GNAT Most_Recent_Exception g-moreex ads. 
* GNAT.OS_Lib (g-os_lib.ads): GNAT OS_Lib g-os_lib ads. 
* GNAT.Perfect_Hash_Generators (g-pehage.ads): GNAT Perfect_Hash_Generators g-pehage ads. 
* GNAT.Random_Numbers (g-rannum.ads): GNAT Random_Numbers g-rannum ads. 
* GNAT.Regexp (g-regexp.ads): GNAT Regexp g-regexp ads. 
* GNAT.Registry (g-regist.ads): GNAT Registry g-regist ads. 
* GNAT.Regpat (g-regpat.ads): GNAT Regpat g-regpat ads. 
* GNAT.Rewrite_Data (g-rewdat.ads): GNAT Rewrite_Data g-rewdat ads. 
* GNAT.Secondary_Stack_Info (g-sestin.ads): GNAT Secondary_Stack_Info g-sestin ads. 
* GNAT.Semaphores (g-semaph.ads): GNAT Semaphores g-semaph ads. 
* GNAT.Serial_Communications (g-sercom.ads): GNAT Serial_Communications g-sercom ads. 
* GNAT.SHA1 (g-sha1.ads): GNAT SHA1 g-sha1 ads. 
* GNAT.SHA224 (g-sha224.ads): GNAT SHA224 g-sha224 ads. 
* GNAT.SHA256 (g-sha256.ads): GNAT SHA256 g-sha256 ads. 
* GNAT.SHA384 (g-sha384.ads): GNAT SHA384 g-sha384 ads. 
* GNAT.SHA512 (g-sha512.ads): GNAT SHA512 g-sha512 ads. 
* GNAT.Signals (g-signal.ads): GNAT Signals g-signal ads. 
* GNAT.Sockets (g-socket.ads): GNAT Sockets g-socket ads. 
* GNAT.Source_Info (g-souinf.ads): GNAT Source_Info g-souinf ads. 
* GNAT.Spelling_Checker (g-speche.ads): GNAT Spelling_Checker g-speche ads. 
* GNAT.Spelling_Checker_Generic (g-spchge.ads): GNAT Spelling_Checker_Generic g-spchge ads. 
* GNAT.Spitbol.Patterns (g-spipat.ads): GNAT Spitbol Patterns g-spipat ads. 
* GNAT.Spitbol (g-spitbo.ads): GNAT Spitbol g-spitbo ads. 
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads): GNAT Spitbol Table_Boolean g-sptabo ads. 
* GNAT.Spitbol.Table_Integer (g-sptain.ads): GNAT Spitbol Table_Integer g-sptain ads. 
* GNAT.Spitbol.Table_VString (g-sptavs.ads): GNAT Spitbol Table_VString g-sptavs ads. 
* GNAT.SSE (g-sse.ads): GNAT SSE g-sse ads. 
* GNAT.SSE.Vector_Types (g-ssvety.ads): GNAT SSE Vector_Types g-ssvety ads. 
* GNAT.String_Hash (g-strhas.ads): GNAT String_Hash g-strhas ads. 
* GNAT.Strings (g-string.ads): GNAT Strings g-string ads. 
* GNAT.String_Split (g-strspl.ads): GNAT String_Split g-strspl ads. 
* GNAT.Table (g-table.ads): GNAT Table g-table ads. 
* GNAT.Task_Lock (g-tasloc.ads): GNAT Task_Lock g-tasloc ads. 
* GNAT.Time_Stamp (g-timsta.ads): GNAT Time_Stamp g-timsta ads. 
* GNAT.Threads (g-thread.ads): GNAT Threads g-thread ads. 
* GNAT.Traceback (g-traceb.ads): GNAT Traceback g-traceb ads. 
* GNAT.Traceback.Symbolic (g-trasym.ads): GNAT Traceback Symbolic g-trasym ads. 
* GNAT.UTF_32 (g-utf_32.ads): GNAT UTF_32 g-utf_32 ads. 
* GNAT.UTF_32_Spelling_Checker (g-u3spch.ads): GNAT UTF_32_Spelling_Checker g-u3spch ads. 
* GNAT.Wide_Spelling_Checker (g-wispch.ads): GNAT Wide_Spelling_Checker g-wispch ads. 
* GNAT.Wide_String_Split (g-wistsp.ads): GNAT Wide_String_Split g-wistsp ads. 
* GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads): GNAT Wide_Wide_Spelling_Checker g-zspche ads. 
* GNAT.Wide_Wide_String_Split (g-zistsp.ads): GNAT Wide_Wide_String_Split g-zistsp ads. 
* Interfaces.C.Extensions (i-cexten.ads): Interfaces C Extensions i-cexten ads. 
* Interfaces.C.Streams (i-cstrea.ads): Interfaces C Streams i-cstrea ads. 
* Interfaces.Packed_Decimal (i-pacdec.ads): Interfaces Packed_Decimal i-pacdec ads. 
* Interfaces.VxWorks (i-vxwork.ads): Interfaces VxWorks i-vxwork ads. 
* Interfaces.VxWorks.IO (i-vxwoio.ads): Interfaces VxWorks IO i-vxwoio ads. 
* System.Address_Image (s-addima.ads): System Address_Image s-addima ads. 
* System.Assertions (s-assert.ads): System Assertions s-assert ads. 
* System.Atomic_Counters (s-atocou.ads): System Atomic_Counters s-atocou ads. 
* System.Memory (s-memory.ads): System Memory s-memory ads. 
* System.Multiprocessors (s-multip.ads): System Multiprocessors s-multip ads. 
* System.Multiprocessors.Dispatching_Domains (s-mudido.ads): System Multiprocessors Dispatching_Domains s-mudido ads. 
* System.Partition_Interface (s-parint.ads): System Partition_Interface s-parint ads. 
* System.Pool_Global (s-pooglo.ads): System Pool_Global s-pooglo ads. 
* System.Pool_Local (s-pooloc.ads): System Pool_Local s-pooloc ads. 
* System.Restrictions (s-restri.ads): System Restrictions s-restri ads. 
* System.Rident (s-rident.ads): System Rident s-rident ads. 
* System.Strings.Stream_Ops (s-ststop.ads): System Strings Stream_Ops s-ststop ads. 
* System.Unsigned_Types (s-unstyp.ads): System Unsigned_Types s-unstyp ads. 
* System.Wch_Cnv (s-wchcnv.ads): System Wch_Cnv s-wchcnv ads. 
* System.Wch_Con (s-wchcon.ads): System Wch_Con s-wchcon ads. 

Interfacing to Other Languages

* Interfacing to C:: 
* Interfacing to C++:: 
* Interfacing to COBOL:: 
* Interfacing to Fortran:: 
* Interfacing to non-GNAT Ada code:: 

Implementation of Specific Ada Features

* Machine Code Insertions:: 
* GNAT Implementation of Tasking:: 
* GNAT Implementation of Shared Passive Packages:: 
* Code Generation for Array Aggregates:: 
* The Size of Discriminated Records with Default Discriminants:: 
* Image Values For Nonscalar Types:: 
* Strict Conformance to the Ada Reference Manual:: 

GNAT Implementation of Tasking

* Mapping Ada Tasks onto the Underlying Kernel Threads:: 
* Ensuring Compliance with the Real-Time Annex:: 
* Support for Locking Policies:: 

Code Generation for Array Aggregates

* Static constant aggregates with static bounds:: 
* Constant aggregates with unconstrained nominal types:: 
* Aggregates with static bounds:: 
* Aggregates with nonstatic bounds:: 
* Aggregates in assignment statements:: 

GNAT language extensions

* How to activate the extended GNAT Ada superset:: 
* Curated Extensions:: 
* Experimental Language Extensions:: 

Curated Extensions

* Local Declarations Without Block:: 
* Deep delta Aggregates:: 
* Fixed lower bounds for array types and subtypes:: 
* Prefixed-view notation for calls to primitive subprograms of untagged types:: 
* Expression defaults for generic formal functions:: 
* String interpolation:: 
* Constrained attribute for generic objects:: 
* Static aspect on intrinsic functions:: 
* First Controlling Parameter:: 
* Unsigned_Base_Range aspect:: 
* Generalized Finalization:: 

Deep delta Aggregates

* Syntax:: 
* Legality Rules:: 
* Dynamic Semantics:: 
* Examples:: 

Generalized Finalization

* Finalizable tagged types:: 
* Composite types:: 
* Interoperability with controlled types:: 

Experimental Language Extensions

* Conditional when constructs:: 
* Implicit With:: 
* Storage Model:: 
* Attribute Super:: 
* Simpler Accessibility Model:: 
* Case pattern matching:: 
* Mutably Tagged Types with Size’Class Aspect:: 
* No_Raise aspect:: 
* Inference of Dependent Types in Generic Instantiations:: 
* External_Initialization Aspect:: 
* Finally construct:: 
* Continue statement:: 
* Destructors:: 
* Structural Generic Instantiation:: 

Storage Model

* Aspect Storage_Model_Type:: 
* Aspect Designated_Storage_Model:: 
* Legacy Storage Pools:: 

Simpler Accessibility Model

* Stand-alone objects:: 
* Subprogram parameters:: 
* Function results:: 

Finally construct

* Syntax: Syntax<2>. 
* Legality Rules: Legality Rules<2>. 
* Dynamic Semantics: Dynamic Semantics<2>. 

Structural Generic Instantiation

* Syntax: Syntax<3>. 
* Legality Rules: Legality Rules<3>. 
* Static Semantics:: 

Security Hardening Features

* Register Scrubbing:: 
* Stack Scrubbing:: 
* Hardened Conditionals:: 
* Hardened Booleans:: 
* Control Flow Redundancy:: 

Obsolescent Features

* PolyORB:: 
* pragma No_Run_Time:: 
* pragma Ravenscar:: 
* pragma Restricted_Run_Time:: 
* pragma Task_Info:: 
* package System.Task_Info (s-tasinf.ads): package System Task_Info s-tasinf ads. 

Compatibility and Porting Guide

* Writing Portable Fixed-Point Declarations:: 
* Compatibility with Ada 83:: 
* Compatibility between Ada 95 and Ada 2005:: 
* Implementation-dependent characteristics:: 
* Compatibility with Other Ada Systems:: 
* Representation Clauses:: 
* Compatibility with HP Ada 83:: 

Compatibility with Ada 83

* Legal Ada 83 programs that are illegal in Ada 95:: 
* More deterministic semantics:: 
* Changed semantics:: 
* Other language compatibility issues:: 

Implementation-dependent characteristics

* Implementation-defined pragmas:: 
* Implementation-defined attributes:: 
* Libraries:: 
* Elaboration order:: 
* Target-specific aspects:: 

@end detailmenu
@end menu

@node About This Guide,Implementation Defined Pragmas,Top,Top
@anchor{gnat_rm/about_this_guide doc}@anchor{2}@anchor{gnat_rm/about_this_guide about-this-guide}@anchor{3}@anchor{gnat_rm/about_this_guide gnat-reference-manual}@anchor{4}@anchor{gnat_rm/about_this_guide id1}@anchor{5}
@chapter About This Guide



This manual contains useful information in writing programs using the
GNAT compiler.  It includes information on implementation dependent
characteristics of GNAT, including all the information required by
Annex M of the Ada language standard.

GNAT implements Ada 95, Ada 2005, Ada 2012 and Ada 2022, and it may also be
invoked in Ada 83 compatibility mode.
By default, GNAT assumes Ada 2012,
but you can override with a compiler switch
to explicitly specify the language version.
(Please refer to the `GNAT User’s Guide' for details on these switches.)
Throughout this manual, references to ‘Ada’ without a year suffix
apply to all the Ada versions of the language.

Ada is designed to be highly portable.
In general, a program will have the same effect even when compiled by
different compilers on different platforms.
However, since Ada is designed to be used in a
wide variety of applications, it also contains a number of system
dependent features to be used in interfacing to the external world.

@geindex Implementation-dependent features

@geindex Portability

Note: Any program that makes use of implementation-dependent features
may be non-portable.  You should follow good programming practice and
isolate and clearly document any sections of your program that make use
of these features in a non-portable manner.

@menu
* What This Reference Manual Contains:: 
* Conventions:: 
* Related Information:: 

@end menu

@node What This Reference Manual Contains,Conventions,,About This Guide
@anchor{gnat_rm/about_this_guide what-this-reference-manual-contains}@anchor{6}
@section What This Reference Manual Contains


This reference manual contains the following chapters:


@itemize *

@item 
@ref{7,,Implementation Defined Pragmas}, lists GNAT implementation-dependent
pragmas, which can be used to extend and enhance the functionality of the
compiler.

@item 
@ref{8,,Implementation Defined Attributes}, lists GNAT
implementation-dependent attributes, which can be used to extend and
enhance the functionality of the compiler.

@item 
@ref{9,,Standard and Implementation Defined Restrictions}, lists GNAT
implementation-dependent restrictions, which can be used to extend and
enhance the functionality of the compiler.

@item 
@ref{a,,Implementation Advice}, provides information on generally
desirable behavior which are not requirements that all compilers must
follow since it cannot be provided on all systems, or which may be
undesirable on some systems.

@item 
@ref{b,,GNAT Implementation Mode (the -gnatg switch)} details the features and restrictions that
are enabled while building the compiler and its runtime libraries.

@item 
@ref{c,,Implementation Defined Characteristics}, provides a guide to
minimizing implementation dependent features.

@item 
@ref{d,,Intrinsic Subprograms}, describes the intrinsic subprograms
implemented by GNAT, and how they can be imported into user
application programs.

@item 
@ref{e,,Representation Clauses and Pragmas}, describes in detail the
way that GNAT represents data, and in particular the exact set
of representation clauses and pragmas that is accepted.

@item 
@ref{f,,Standard Library Routines}, provides a listing of packages and a
brief description of the functionality that is provided by Ada’s
extensive set of standard library routines as implemented by GNAT.

@item 
@ref{10,,The Implementation of Standard I/O}, details how the GNAT
implementation of the input-output facilities.

@item 
@ref{11,,The GNAT Library}, is a catalog of packages that complement
the Ada predefined library.

@item 
@ref{12,,Interfacing to Other Languages}, describes how programs
written in Ada using GNAT can be interfaced to other programming
languages.

@item 
@ref{13,,Specialized Needs Annexes}, describes the GNAT implementation of all
of the specialized needs annexes.

@item 
@ref{14,,Implementation of Specific Ada Features}, discusses issues related
to GNAT’s implementation of machine code insertions, tasking, and several
other features.

@item 
@ref{15,,Implementation of Ada 2022 Features}, describes the status of the
GNAT implementation of the Ada 2022 language standard.

@item 
@ref{16,,Security Hardening Features} documents GNAT extensions aimed
at security hardening.

@item 
@ref{17,,Obsolescent Features} documents implementation dependent features,
including pragmas and attributes, which are considered obsolescent, since
there are other preferred ways of achieving the same results. These
obsolescent forms are retained for backwards compatibility.

@item 
@ref{18,,Compatibility and Porting Guide} presents some guidelines for
developing portable Ada code, describes the compatibility issues that
may arise between GNAT and other Ada compilation systems (including those
for Ada 83), and shows how GNAT can expedite porting applications
developed in other Ada environments.

@item 
@ref{1,,GNU Free Documentation License} contains the license for this document.
@end itemize

@geindex Ada 95 Language Reference Manual

@geindex Ada 2005 Language Reference Manual

This reference manual assumes a basic familiarity with the Ada 95 language, as
described in the
@cite{International Standard ANSI/ISO/IEC-8652:1995}.
It does not require knowledge of the new features introduced by Ada 2005 or
Ada 2012.
All three reference manuals are included in the GNAT documentation
package.

@node Conventions,Related Information,What This Reference Manual Contains,About This Guide
@anchor{gnat_rm/about_this_guide conventions}@anchor{19}
@section Conventions


@geindex Conventions
@geindex typographical

@geindex Typographical conventions

Following are examples of the typographical and graphic conventions used
in this guide:


@itemize *

@item 
@code{Functions}, @code{utility program names}, @code{standard names},
and @code{classes}.

@item 
@code{Option flags}

@item 
@code{File names}

@item 
@code{Variables}

@item 
`Emphasis'

@item 
[optional information or parameters]

@item 
Examples are described by text

@example
and then shown this way.
@end example

@item 
Commands that are entered by the user are shown as preceded by a prompt string
comprising the @code{$} character followed by a space.
@end itemize

@node Related Information,,Conventions,About This Guide
@anchor{gnat_rm/about_this_guide related-information}@anchor{1a}
@section Related Information


See the following documents for further information on GNAT:


@itemize *

@item 
@cite{GNAT User’s Guide for Native Platforms},
which provides information on how to use the
GNAT development environment.

@item 
@cite{Ada 95 Reference Manual}, the Ada 95 programming language standard.

@item 
@cite{Ada 95 Annotated Reference Manual}, which is an annotated version
of the Ada 95 standard.  The annotations describe
detailed aspects of the design decision, and in particular contain useful
sections on Ada 83 compatibility.

@item 
@cite{Ada 2005 Reference Manual}, the Ada 2005 programming language standard.

@item 
@cite{Ada 2005 Annotated Reference Manual}, which is an annotated version
of the Ada 2005 standard.  The annotations describe
detailed aspects of the design decision.

@item 
@cite{Ada 2012 Reference Manual}, the Ada 2012 programming language standard.

@item 
@cite{DEC Ada@comma{} Technical Overview and Comparison on DIGITAL Platforms},
which contains specific information on compatibility between GNAT and
DEC Ada 83 systems.

@item 
@cite{DEC Ada@comma{} Language Reference Manual}, part number AA-PYZAB-TK, which
describes in detail the pragmas and attributes provided by the DEC Ada 83
compiler system.
@end itemize

@node Implementation Defined Pragmas,Implementation Defined Aspects,About This Guide,Top
@anchor{gnat_rm/implementation_defined_pragmas doc}@anchor{1b}@anchor{gnat_rm/implementation_defined_pragmas id1}@anchor{1c}@anchor{gnat_rm/implementation_defined_pragmas implementation-defined-pragmas}@anchor{7}
@chapter Implementation Defined Pragmas


Ada defines a set of pragmas that can be used to supply additional
information to the compiler.  These language defined pragmas are
implemented in GNAT and work as described in the Ada Reference Manual.

In addition, Ada allows implementations to define additional pragmas
whose meaning is defined by the implementation.  GNAT provides a number
of these implementation-defined pragmas, which can be used to extend
and enhance the functionality of the compiler.  This section of the GNAT
Reference Manual describes these additional pragmas.

Note that any program using these pragmas might not be portable to other
compilers (although GNAT implements this set of pragmas on all
platforms).  Therefore if portability to other compilers is an important
consideration, the use of these pragmas should be minimized.

@menu
* Pragma Abort_Defer:: 
* Pragma Abstract_State:: 
* Pragma Ada_83:: 
* Pragma Ada_95:: 
* Pragma Ada_05:: 
* Pragma Ada_2005:: 
* Pragma Ada_12:: 
* Pragma Ada_2012:: 
* Pragma Ada_2022:: 
* Pragma Aggregate_Individually_Assign:: 
* Pragma Allow_Integer_Address:: 
* Pragma Always_Terminates:: 
* Pragma Annotate:: 
* Pragma Assert:: 
* Pragma Assert_And_Cut:: 
* Pragma Assertion_Level:: 
* Pragma Assertion_Policy:: 
* Pragma Assume:: 
* Pragma Assume_No_Invalid_Values:: 
* Pragma Async_Readers:: 
* Pragma Async_Writers:: 
* Pragma Attribute_Definition:: 
* Pragma C_Pass_By_Copy:: 
* Pragma Check:: 
* Pragma Check_Float_Overflow:: 
* Pragma Check_Name:: 
* Pragma Check_Policy:: 
* Pragma Comment:: 
* Pragma Common_Object:: 
* Pragma Compile_Time_Error:: 
* Pragma Compile_Time_Warning:: 
* Pragma Complete_Representation:: 
* Pragma Complex_Representation:: 
* Pragma Component_Alignment:: 
* Pragma Constant_After_Elaboration:: 
* Pragma Contract_Cases:: 
* Pragma Convention_Identifier:: 
* Pragma CPP_Class:: 
* Pragma CPP_Constructor:: 
* Pragma CPP_Virtual:: 
* Pragma CPP_Vtable:: 
* Pragma CPU:: 
* Pragma Deadline_Floor:: 
* Pragma Debug:: 
* Pragma Debug_Policy:: 
* Pragma Default_Initial_Condition:: 
* Pragma Default_Scalar_Storage_Order:: 
* Pragma Default_Storage_Pool:: 
* Pragma Depends:: 
* Pragma Detect_Blocking:: 
* Pragma Disable_Atomic_Synchronization:: 
* Pragma Dispatching_Domain:: 
* Pragma Effective_Reads:: 
* Pragma Effective_Writes:: 
* Pragma Elaboration_Checks:: 
* Pragma Eliminate:: 
* Pragma Enable_Atomic_Synchronization:: 
* Pragma Exceptional_Cases:: 
* Pragma Exit_Cases:: 
* Pragma Export_Function:: 
* Pragma Export_Object:: 
* Pragma Export_Procedure:: 
* Pragma Export_Valued_Procedure:: 
* Pragma Extend_System:: 
* Pragma Extensions_Allowed:: 
* Pragma Extensions_Visible:: 
* Pragma External:: 
* Pragma External_Name_Casing:: 
* Pragma Fast_Math:: 
* Pragma Favor_Top_Level:: 
* Pragma Finalize_Storage_Only:: 
* Pragma Float_Representation:: 
* Pragma Ghost:: 
* Pragma Global:: 
* Pragma Ident:: 
* Pragma Ignore_Pragma:: 
* Pragma Implementation_Defined:: 
* Pragma Implemented:: 
* Pragma Implicit_Packing:: 
* Pragma Import_Function:: 
* Pragma Import_Object:: 
* Pragma Import_Procedure:: 
* Pragma Import_Valued_Procedure:: 
* Pragma Independent:: 
* Pragma Independent_Components:: 
* Pragma Initial_Condition:: 
* Pragma Initialize_Scalars:: 
* Pragma Initializes:: 
* Pragma Inline_Always:: 
* Pragma Inline_Generic:: 
* Pragma Interface:: 
* Pragma Interface_Name:: 
* Pragma Interrupt_Handler:: 
* Pragma Interrupt_State:: 
* Pragma Interrupts_System_By_Default:: 
* Pragma Invariant:: 
* Pragma Keep_Names:: 
* Pragma License:: 
* Pragma Link_With:: 
* Pragma Linker_Alias:: 
* Pragma Linker_Constructor:: 
* Pragma Linker_Destructor:: 
* Pragma Linker_Section:: 
* Pragma Lock_Free:: 
* Pragma Loop_Invariant:: 
* Pragma Loop_Optimize:: 
* Pragma Loop_Variant:: 
* Pragma Machine_Attribute:: 
* Pragma Main:: 
* Pragma Main_Storage:: 
* Pragma Max_Queue_Length:: 
* Pragma No_Body:: 
* Pragma No_Caching:: 
* Pragma No_Component_Reordering:: 
* Pragma No_Elaboration_Code_All:: 
* Pragma No_Heap_Finalization:: 
* Pragma No_Inline:: 
* Pragma No_Raise:: 
* Pragma No_Return:: 
* Pragma No_Strict_Aliasing:: 
* Pragma No_Tagged_Streams:: 
* Pragma Normalize_Scalars:: 
* Pragma Obsolescent:: 
* Pragma Optimize_Alignment:: 
* Pragma Ordered:: 
* Pragma Overflow_Mode:: 
* Pragma Overriding_Renamings:: 
* Pragma Part_Of:: 
* Pragma Partition_Elaboration_Policy:: 
* Pragma Passive:: 
* Pragma Persistent_BSS:: 
* Pragma Post:: 
* Pragma Postcondition:: 
* Pragma Post_Class:: 
* Pragma Pre:: 
* Pragma Precondition:: 
* Pragma Predicate:: 
* Pragma Predicate_Failure:: 
* Pragma Preelaborable_Initialization:: 
* Pragma Prefix_Exception_Messages:: 
* Pragma Pre_Class:: 
* Pragma Priority_Specific_Dispatching:: 
* Pragma Profile:: 
* Pragma Profile_Warnings:: 
* Pragma Program_Exit:: 
* Pragma Propagate_Exceptions:: 
* Pragma Provide_Shift_Operators:: 
* Pragma Psect_Object:: 
* Pragma Pure_Function:: 
* Pragma Rational:: 
* Pragma Ravenscar:: 
* Pragma Refined_Depends:: 
* Pragma Refined_Global:: 
* Pragma Refined_Post:: 
* Pragma Refined_State:: 
* Pragma Relative_Deadline:: 
* Pragma Remote_Access_Type:: 
* Pragma Rename_Pragma:: 
* Pragma Restricted_Run_Time:: 
* Pragma Restriction_Warnings:: 
* Pragma Reviewable:: 
* Pragma Secondary_Stack_Size:: 
* Pragma Share_Generic:: 
* Pragma Shared:: 
* Pragma Short_Circuit_And_Or:: 
* Pragma Short_Descriptors:: 
* Pragma Side_Effects:: 
* Pragma Simple_Storage_Pool_Type:: 
* Pragma Source_File_Name:: 
* Pragma Source_File_Name_Project:: 
* Pragma Source_Reference:: 
* Pragma SPARK_Mode:: 
* Pragma Static_Elaboration_Desired:: 
* Pragma Stream_Convert:: 
* Pragma Style_Checks:: 
* Pragma Subprogram_Variant:: 
* Pragma Subtitle:: 
* Pragma Suppress:: 
* Pragma Suppress_All:: 
* Pragma Suppress_Debug_Info:: 
* Pragma Suppress_Exception_Locations:: 
* Pragma Suppress_Initialization:: 
* Pragma Task_Name:: 
* Pragma Task_Storage:: 
* Pragma Test_Case:: 
* Pragma Thread_Local_Storage:: 
* Pragma Time_Slice:: 
* Pragma Title:: 
* Pragma Type_Invariant:: 
* Pragma Type_Invariant_Class:: 
* Pragma Unchecked_Union:: 
* Pragma Unevaluated_Use_Of_Old:: 
* Pragma User_Aspect_Definition:: 
* Pragma Unimplemented_Unit:: 
* Pragma Universal_Aliasing:: 
* Pragma Unmodified:: 
* Pragma Unreferenced:: 
* Pragma Unreferenced_Objects:: 
* Pragma Unreserve_All_Interrupts:: 
* Pragma Unsuppress:: 
* Pragma Unused:: 
* Pragma Use_VADS_Size:: 
* Pragma Validity_Checks:: 
* Pragma Volatile:: 
* Pragma Volatile_Full_Access:: 
* Pragma Volatile_Function:: 
* Pragma Warning_As_Error:: 
* Pragma Warnings:: 
* Pragma Weak_External:: 
* Pragma Wide_Character_Encoding:: 

@end menu

@node Pragma Abort_Defer,Pragma Abstract_State,,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-abort-defer}@anchor{1d}
@section Pragma Abort_Defer


@geindex Deferring aborts

Syntax:

@example
pragma Abort_Defer;
@end example

This pragma must appear at the start of the statement sequence of a
handled sequence of statements (right after the @code{begin}).  It has
the effect of deferring aborts for the sequence of statements (but not
for the declarations or handlers, if any, associated with this statement
sequence). This can also be useful for adding a polling point in Ada code,
where asynchronous abort of tasks is checked when leaving the statement
sequence, and is lighter than, for example, using @code{delay 0.0;}, since with
zero-cost exception handling, propagating exceptions (implicitly used to
implement task abort) cannot be done reliably in an asynchronous way.

An example of usage would be:

@example
--  Add a polling point to check for task aborts

begin
   pragma Abort_Defer;
end;
@end example

@node Pragma Abstract_State,Pragma Ada_83,Pragma Abort_Defer,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id2}@anchor{1e}@anchor{gnat_rm/implementation_defined_pragmas pragma-abstract-state}@anchor{1f}
@section Pragma Abstract_State


Syntax:

@example
pragma Abstract_State (ABSTRACT_STATE_LIST);

ABSTRACT_STATE_LIST ::=
     null
  |  STATE_NAME_WITH_OPTIONS
  | (STATE_NAME_WITH_OPTIONS @{, STATE_NAME_WITH_OPTIONS@} )

STATE_NAME_WITH_OPTIONS ::=
     STATE_NAME
  | (STATE_NAME with OPTION_LIST)

OPTION_LIST ::= OPTION @{, OPTION@}

OPTION ::=
    SIMPLE_OPTION
  | NAME_VALUE_OPTION

SIMPLE_OPTION ::= Ghost | Synchronous

NAME_VALUE_OPTION ::=
    Part_Of => ABSTRACT_STATE
  | External [=> EXTERNAL_PROPERTY_LIST]

EXTERNAL_PROPERTY_LIST ::=
     EXTERNAL_PROPERTY
  | (EXTERNAL_PROPERTY @{, EXTERNAL_PROPERTY@} )

EXTERNAL_PROPERTY ::=
    Async_Readers    [=> static_boolean_EXPRESSION]
  | Async_Writers    [=> static_boolean_EXPRESSION]
  | Effective_Reads  [=> static_boolean_EXPRESSION]
  | Effective_Writes [=> static_boolean_EXPRESSION]
    others            => static_boolean_EXPRESSION

STATE_NAME ::= defining_identifier

ABSTRACT_STATE ::= name
@end example

For the semantics of this pragma, see the entry for aspect @code{Abstract_State} in
the SPARK 2014 Reference Manual, section 7.1.4.

@node Pragma Ada_83,Pragma Ada_95,Pragma Abstract_State,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-83}@anchor{20}
@section Pragma Ada_83


Syntax:

@example
pragma Ada_83;
@end example

A configuration pragma that establishes Ada 83 mode for the unit to
which it applies, regardless of the mode set by the command line
switches.  In Ada 83 mode, GNAT attempts to be as compatible with
the syntax and semantics of Ada 83, as defined in the original Ada
83 Reference Manual as possible.  In particular, the keywords added by Ada 95
and Ada 2005 are not recognized, optional package bodies are allowed,
and generics may name types with unknown discriminants without using
the @code{(<>)} notation.  In addition, some but not all of the additional
restrictions of Ada 83 are enforced.

Like all configuration pragmas, if the pragma is placed before a library
level package specification it is not propagated to the corresponding
package body (see RM 10.1.5(8)); it must be added explicitly to the
package body.

Ada 83 mode is intended for two purposes.  Firstly, it allows existing
Ada 83 code to be compiled and adapted to GNAT with less effort.
Secondly, it aids in keeping code backwards compatible with Ada 83.
However, there is no guarantee that code that is processed correctly
by GNAT in Ada 83 mode will in fact compile and execute with an Ada
83 compiler, since GNAT does not enforce all the additional checks
required by Ada 83.

@node Pragma Ada_95,Pragma Ada_05,Pragma Ada_83,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-95}@anchor{21}
@section Pragma Ada_95


Syntax:

@example
pragma Ada_95;
@end example

A configuration pragma that establishes Ada 95 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the @code{Ada} and @code{System}
packages and their children, so you need not specify it in these
contexts.  This pragma is useful when writing a reusable component that
itself uses Ada 95 features, but which is intended to be usable from
either Ada 83 or Ada 95 programs.

Like all configuration pragmas, if the pragma is placed before a library
level package specification it is not propagated to the corresponding
package body (see RM 10.1.5(8)); it must be added explicitly to the
package body.

@node Pragma Ada_05,Pragma Ada_2005,Pragma Ada_95,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-05}@anchor{22}
@section Pragma Ada_05


Syntax:

@example
pragma Ada_05;
pragma Ada_05 (local_NAME);
@end example

A configuration pragma that establishes Ada 2005 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This pragma is useful when writing a reusable component that
itself uses Ada 2005 features, but which is intended to be usable from
either Ada 83 or Ada 95 programs.

Like all configuration pragmas, if the pragma is placed before a library
level package specification it is not propagated to the corresponding
package body (see RM 10.1.5(8)); it must be added explicitly to the
package body.

The one argument form (which is not a configuration pragma)
is used for managing the transition from
Ada 95 to Ada 2005 in the run-time library. If an entity is marked
as Ada_2005 only, then referencing the entity in Ada_83 or Ada_95
mode will generate a warning. In addition, in Ada_83 or Ada_95
mode, a preference rule is established which does not choose
such an entity unless it is unambiguously specified. This avoids
extra subprograms marked this way from generating ambiguities in
otherwise legal pre-Ada_2005 programs. The one argument form is
intended for exclusive use in the GNAT run-time library.

@node Pragma Ada_2005,Pragma Ada_12,Pragma Ada_05,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-2005}@anchor{23}
@section Pragma Ada_2005


Syntax:

@example
pragma Ada_2005;
@end example

This configuration pragma is a synonym for pragma Ada_05 and has the
same syntax and effect.

@node Pragma Ada_12,Pragma Ada_2012,Pragma Ada_2005,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-12}@anchor{24}
@section Pragma Ada_12


Syntax:

@example
pragma Ada_12;
pragma Ada_12 (local_NAME);
@end example

A configuration pragma that establishes Ada 2012 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the @code{Ada} and @code{System}
packages and their children, so you need not specify it in these
contexts.  This pragma is useful when writing a reusable component that
itself uses Ada 2012 features, but which is intended to be usable from
Ada 83, Ada 95, or Ada 2005 programs.

Like all configuration pragmas, if the pragma is placed before a library
level package specification it is not propagated to the corresponding
package body (see RM 10.1.5(8)); it must be added explicitly to the
package body.

The one argument form, which is not a configuration pragma,
is used for managing the transition from Ada
2005 to Ada 2012 in the run-time library. If an entity is marked
as Ada_2012 only, then referencing the entity in any pre-Ada_2012
mode will generate a warning. In addition, in any pre-Ada_2012
mode, a preference rule is established which does not choose
such an entity unless it is unambiguously specified. This avoids
extra subprograms marked this way from generating ambiguities in
otherwise legal pre-Ada_2012 programs. The one argument form is
intended for exclusive use in the GNAT run-time library.

@node Pragma Ada_2012,Pragma Ada_2022,Pragma Ada_12,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-2012}@anchor{25}
@section Pragma Ada_2012


Syntax:

@example
pragma Ada_2012;
@end example

This configuration pragma is a synonym for pragma Ada_12 and has the
same syntax and effect.

@node Pragma Ada_2022,Pragma Aggregate_Individually_Assign,Pragma Ada_2012,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-2022}@anchor{26}
@section Pragma Ada_2022


Syntax:

@example
pragma Ada_2022;
pragma Ada_2022 (local_NAME);
@end example

A configuration pragma that establishes Ada 2022 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the @code{Ada} and @code{System}
packages and their children, so you need not specify it in these
contexts.  This pragma is useful when writing a reusable component that
itself uses Ada 2022 features, but which is intended to be usable from
Ada 83, Ada 95, Ada 2005 or Ada 2012 programs.

Like all configuration pragmas, if the pragma is placed before a library
level package specification it is not propagated to the corresponding
package body (see RM 10.1.5(8)); it must be added explicitly to the
package body.

The one argument form, which is not a configuration pragma,
is used for managing the transition from Ada
2012 to Ada 2022 in the run-time library. If an entity is marked
as Ada_2022 only, then referencing the entity in any pre-Ada_2022
mode will generate a warning. In addition, in any pre-Ada_2012
mode, a preference rule is established which does not choose
such an entity unless it is unambiguously specified. This avoids
extra subprograms marked this way from generating ambiguities in
otherwise legal pre-Ada_2022 programs. The one argument form is
intended for exclusive use in the GNAT run-time library.

@node Pragma Aggregate_Individually_Assign,Pragma Allow_Integer_Address,Pragma Ada_2022,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-aggregate-individually-assign}@anchor{27}
@section Pragma Aggregate_Individually_Assign


Syntax:

@example
pragma Aggregate_Individually_Assign;
@end example

Where possible, GNAT will store the binary representation of a record aggregate
in memory for space and performance reasons. This configuration pragma changes
this behavior so that record aggregates are instead always converted into
individual assignment statements.

@node Pragma Allow_Integer_Address,Pragma Always_Terminates,Pragma Aggregate_Individually_Assign,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-allow-integer-address}@anchor{28}
@section Pragma Allow_Integer_Address


Syntax:

@example
pragma Allow_Integer_Address;
@end example

In almost all versions of GNAT, @code{System.Address} is a private
type in accordance with the implementation advice in the RM. This
means that integer values,
in particular integer literals, are not allowed as address values.
If the configuration pragma
@code{Allow_Integer_Address} is given, then integer expressions may
be used anywhere a value of type @code{System.Address} is required.
The effect is to introduce an implicit unchecked conversion from the
integer value to type @code{System.Address}. The reverse case of using
an address where an integer type is required is handled analogously.
The following example compiles without errors:

@example
pragma Allow_Integer_Address;
with System; use System;
package AddrAsInt is
   X : Integer;
   Y : Integer;
   for X'Address use 16#1240#;
   for Y use at 16#3230#;
   m : Address := 16#4000#;
   n : constant Address := 4000;
   p : constant Address := Address (X + Y);
   v : Integer := y'Address;
   w : constant Integer := Integer (Y'Address);
   type R is new integer;
   RR : R := 1000;
   Z : Integer;
   for Z'Address use RR;
end AddrAsInt;
@end example

Note that pragma @code{Allow_Integer_Address} is ignored if @code{System.Address}
is not a private type. In implementations of @code{GNAT} where
System.Address is a visible integer type,
this pragma serves no purpose but is ignored
rather than rejected to allow common sets of sources to be used
in the two situations.

@node Pragma Always_Terminates,Pragma Annotate,Pragma Allow_Integer_Address,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id3}@anchor{29}@anchor{gnat_rm/implementation_defined_pragmas pragma-always-terminates}@anchor{2a}
@section Pragma Always_Terminates


Syntax:

@example
pragma Always_Terminates [ (boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Always_Terminates}
in the SPARK 2014 Reference Manual, section 6.1.11.

@node Pragma Annotate,Pragma Assert,Pragma Always_Terminates,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id4}@anchor{2b}@anchor{gnat_rm/implementation_defined_pragmas pragma-annotate}@anchor{2c}
@section Pragma Annotate


Syntax:

@example
pragma Annotate (IDENTIFIER [, IDENTIFIER @{, ARG@}] [, entity => local_NAME]);

ARG ::= NAME | EXPRESSION
@end example

This pragma is used to annotate programs.  IDENTIFIER identifies
the type of annotation.  GNAT verifies that it is an identifier, but does
not otherwise analyze it. The second optional identifier is also left
unanalyzed, and by convention is used to control the action of the tool to
which the annotation is addressed.  The remaining ARG arguments
can be either string literals or more generally expressions.
String literals (and concatenations of string literals) are assumed to be
either of type
@code{Standard.String} or else @code{Wide_String} or @code{Wide_Wide_String}
depending on the character literals they contain.
All other kinds of arguments are analyzed as expressions, and must be
unambiguous. The last argument if present must have the identifier
@code{Entity} and GNAT verifies that a local name is given.

The analyzed pragma is retained in the tree, but not otherwise processed
by any part of the GNAT compiler, except to generate corresponding note
lines in the generated ALI file. For the format of these note lines, see
the compiler source file lib-writ.ads. This pragma is intended for use by
external tools. The use of pragma Annotate does not affect the compilation
process in any way. This pragma may be used as a configuration pragma.

@node Pragma Assert,Pragma Assert_And_Cut,Pragma Annotate,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assert}@anchor{2d}
@section Pragma Assert


Syntax:

@example
pragma Assert (
  boolean_EXPRESSION
  [, string_EXPRESSION]);
@end example

The effect of this pragma depends on whether the corresponding command
line switch is set to activate assertions.  The pragma expands into code
equivalent to the following:

@example
if assertions-enabled then
   if not boolean_EXPRESSION then
      System.Assertions.Raise_Assert_Failure
        (string_EXPRESSION);
   end if;
end if;
@end example

The string argument, if given, is the message that will be associated
with the exception occurrence if the exception is raised.  If no second
argument is given, the default message is @code{file}:@code{nnn},
where @code{file} is the name of the source file containing the assert,
and @code{nnn} is the line number of the assert.

Note that, as with the @code{if} statement to which it is equivalent, the
type of the expression is either @code{Standard.Boolean}, or any type derived
from this standard type.

Assert checks can be either checked or ignored. By default they are ignored.
They will be checked if either the command line switch `-gnata' is
used, or if an @code{Assertion_Policy} or @code{Check_Policy} pragma is used
to enable @code{Assert_Checks}.

If assertions are ignored, then there
is no run-time effect (and in particular, any side effects from the
expression will not occur at run time).  (The expression is still
analyzed at compile time, and may cause types to be frozen if they are
mentioned here for the first time).

If assertions are checked, then the given expression is tested, and if
it is @code{False} then @code{System.Assertions.Raise_Assert_Failure} is called
which results in the raising of @code{Assert_Failure} with the given message.

You should generally avoid side effects in the expression arguments of
this pragma, because these side effects will turn on and off with the
setting of the assertions mode, resulting in assertions that have an
effect on the program.  However, the expressions are analyzed for
semantic correctness whether or not assertions are enabled, so turning
assertions on and off cannot affect the legality of a program.

Note that the implementation defined policy @code{DISABLE}, given in a
pragma @code{Assertion_Policy}, can be used to suppress this semantic analysis.

Note: this is a standard language-defined pragma in versions
of Ada from 2005 on. In GNAT, it is implemented in all versions
of Ada, and the DISABLE policy is an implementation-defined
addition.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Assert_And_Cut,Pragma Assertion_Level,Pragma Assert,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assert-and-cut}@anchor{2e}
@section Pragma Assert_And_Cut


Syntax:

@example
pragma Assert_And_Cut (
  boolean_EXPRESSION
  [, string_EXPRESSION]);
@end example

The effect of this pragma is identical to that of pragma @code{Assert},
except that in an @code{Assertion_Policy} pragma, the identifier
@code{Assert_And_Cut} is used to control whether it is ignored or checked
(or disabled).

The intention is that this be used within a subprogram when the
given test expression sums up all the work done so far in the
subprogram, so that the rest of the subprogram can be verified
(informally or formally) using only the entry preconditions,
and the expression in this pragma. This allows dividing up
a subprogram into sections for the purposes of testing or
formal verification. The pragma also serves as useful
documentation.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Assertion_Level,Pragma Assertion_Policy,Pragma Assert_And_Cut,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assertion-level}@anchor{2f}
@section Pragma Assertion_Level


Syntax:

@example
pragma Assertion_Level (LEVEL_IDENTIFIER
                        [, depends => DEPENDENCY_DESCRIPTOR]);

DEPENDENCY_DESCRIPTOR ::= LEVEL_IDENTIFIER | LEVEL_IDENTIFIER_LIST

LEVEL_IDENTIFIER_LIST ::= '[' LEVEL_IDENTIFIER @{, LEVEL_IDENTIFIER@} ']'
@end example

For the semantics of this pragma, see the SPARK 2014 Reference Manual,
section 11.4.3.

@node Pragma Assertion_Policy,Pragma Assume,Pragma Assertion_Level,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assertion-policy}@anchor{30}
@section Pragma Assertion_Policy


Syntax:

@example
pragma Assertion_Policy (CHECK | DISABLE | IGNORE | SUPPRESSIBLE);

pragma Assertion_Policy (
    ASSERTION_KIND => POLICY_IDENTIFIER
 @{, ASSERTION_KIND => POLICY_IDENTIFIER@});

ASSERTION_KIND ::= RM_ASSERTION_KIND | ID_ASSERTION_KIND | ASSERTION_LEVEL

RM_ASSERTION_KIND ::= Assert                    |
                      Static_Predicate          |
                      Dynamic_Predicate         |
                      Pre                       |
                      Pre'Class                 |
                      Post                      |
                      Post'Class                |
                      Type_Invariant            |
                      Type_Invariant'Class      |
                      Default_Initial_Condition

ID_ASSERTION_KIND ::= Assertions           |
                      Assert_And_Cut       |
                      Assume               |
                      Contract_Cases       |
                      Debug                |
                      Ghost                |
                      Initial_Condition    |
                      Invariant            |
                      Invariant'Class      |
                      Loop_Invariant       |
                      Loop_Variant         |
                      Postcondition        |
                      Precondition         |
                      Predicate            |
                      Refined_Post         |
                      Statement_Assertions |
                      Subprogram_Variant

POLICY_IDENTIFIER ::= Check | Disable | Ignore | Suppressible
@end example

This is a standard Ada 2012 pragma that is available as an
implementation-defined pragma in earlier versions of Ada.
The assertion kinds @code{RM_ASSERTION_KIND} are those defined in
the Ada standard. The assertion kinds @code{ID_ASSERTION_KIND}
are implementation defined additions recognized by the GNAT compiler.

Additionally the pragma can apply to an assertion level defined by the
@code{Assertion_Level} pragma. For more details see the SPARK 2014 Reference
Manual, section 11.4.2.

The pragma applies in both cases to pragmas and aspects with matching
names, e.g. @code{Pre} applies to the Pre aspect, and @code{Precondition}
applies to both the @code{Precondition} pragma
and the aspect @code{Precondition}. Note that the identifiers for
pragmas Pre_Class and Post_Class are Pre’Class and Post’Class (not
Pre_Class and Post_Class), since these pragmas are intended to be
identical to the corresponding aspects.

If the policy is @code{CHECK}, then assertions are enabled, i.e.
the corresponding pragma or aspect is activated.
If the policy is @code{IGNORE}, then assertions are ignored, i.e.
the corresponding pragma or aspect is deactivated.
This pragma overrides the effect of the `-gnata' switch on the
command line.
If the policy is @code{SUPPRESSIBLE}, then assertions are enabled by default,
however, if the `-gnatp' switch is specified all assertions are ignored.

The implementation defined policy @code{DISABLE} is like
@code{IGNORE} except that it completely disables semantic
checking of the corresponding pragma or aspect. This is
useful when the pragma or aspect argument references subprograms
in a with’ed package which is replaced by a dummy package
for the final build.

The implementation defined assertion kind @code{Assertions} applies to all
assertion kinds. The form with no assertion kind given implies this
choice, so it applies to all assertion kinds (RM defined, and
implementation defined).

The implementation defined assertion kind @code{Statement_Assertions}
applies to @code{Assert}, @code{Assert_And_Cut},
@code{Assume}, @code{Loop_Invariant}, and @code{Loop_Variant}.

@node Pragma Assume,Pragma Assume_No_Invalid_Values,Pragma Assertion_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assume}@anchor{31}
@section Pragma Assume


Syntax:

@example
pragma Assume (
  boolean_EXPRESSION
  [, string_EXPRESSION]);
@end example

The effect of this pragma is identical to that of pragma @code{Assert},
except that in an @code{Assertion_Policy} pragma, the identifier
@code{Assume} is used to control whether it is ignored or checked
(or disabled).

The intention is that this be used for assumptions about the
external environment. So you cannot expect to verify formally
or informally that the condition is met, this must be
established by examining things outside the program itself.
For example, we may have code that depends on the size of
@code{Long_Long_Integer} being at least 64. So we could write:

@example
pragma Assume (Long_Long_Integer'Size >= 64);
@end example

This assumption cannot be proved from the program itself,
but it acts as a useful run-time check that the assumption
is met, and documents the need to ensure that it is met by
reference to information outside the program.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Assume_No_Invalid_Values,Pragma Async_Readers,Pragma Assume,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assume-no-invalid-values}@anchor{32}
@section Pragma Assume_No_Invalid_Values


@geindex Invalid representations

@geindex Invalid values

Syntax:

@example
pragma Assume_No_Invalid_Values (On | Off);
@end example

This is a configuration pragma that controls the assumptions made by the
compiler about the occurrence of invalid representations (invalid values)
in the code.

The default behavior (corresponding to an Off argument for this pragma), is
to assume that values may in general be invalid unless the compiler can
prove they are valid. Consider the following example:

@example
V1 : Integer range 1 .. 10;
V2 : Integer range 11 .. 20;
...
for J in V2 .. V1 loop
   ...
end loop;
@end example

if V1 and V2 have valid values, then the loop is known at compile
time not to execute since the lower bound must be greater than the
upper bound. However in default mode, no such assumption is made,
and the loop may execute. If @code{Assume_No_Invalid_Values (On)}
is given, the compiler will assume that any occurrence of a variable
other than in an explicit @code{'Valid} test always has a valid
value, and the loop above will be optimized away.

The use of @code{Assume_No_Invalid_Values (On)} is appropriate if
you know your code is free of uninitialized variables and other
possible sources of invalid representations, and may result in
more efficient code. A program that accesses an invalid representation
with this pragma in effect is erroneous, so no guarantees can be made
about its behavior.

It is peculiar though permissible to use this pragma in conjunction
with validity checking (-gnatVa). In such cases, accessing invalid
values will generally give an exception, though formally the program
is erroneous so there are no guarantees that this will always be the
case, and it is recommended that these two options not be used together.

@node Pragma Async_Readers,Pragma Async_Writers,Pragma Assume_No_Invalid_Values,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id5}@anchor{33}@anchor{gnat_rm/implementation_defined_pragmas pragma-async-readers}@anchor{34}
@section Pragma Async_Readers


Syntax:

@example
pragma Async_Readers [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Async_Readers} in
the SPARK 2014 Reference Manual, section 7.1.2.

@node Pragma Async_Writers,Pragma Attribute_Definition,Pragma Async_Readers,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id6}@anchor{35}@anchor{gnat_rm/implementation_defined_pragmas pragma-async-writers}@anchor{36}
@section Pragma Async_Writers


Syntax:

@example
pragma Async_Writers [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Async_Writers} in
the SPARK 2014 Reference Manual, section 7.1.2.

@node Pragma Attribute_Definition,Pragma C_Pass_By_Copy,Pragma Async_Writers,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-attribute-definition}@anchor{37}
@section Pragma Attribute_Definition


Syntax:

@example
pragma Attribute_Definition
  ([Attribute  =>] ATTRIBUTE_DESIGNATOR,
   [Entity     =>] LOCAL_NAME,
   [Expression =>] EXPRESSION | NAME);
@end example

If @code{Attribute} is a known attribute name, this pragma is equivalent to
the attribute definition clause:

@example
for Entity'Attribute use Expression;
@end example

If @code{Attribute} is not a recognized attribute name, the pragma is
ignored, and a warning is emitted. This allows source
code to be written that takes advantage of some new attribute, while remaining
compilable with earlier compilers.

@node Pragma C_Pass_By_Copy,Pragma Check,Pragma Attribute_Definition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-c-pass-by-copy}@anchor{38}
@section Pragma C_Pass_By_Copy


@geindex Passing by copy

Syntax:

@example
pragma C_Pass_By_Copy
  ([Max_Size =>] static_integer_EXPRESSION);
@end example

Normally the default mechanism for passing C convention records to C
convention subprograms is to pass them by reference, as suggested by RM
B.3(69).  Use the configuration pragma @code{C_Pass_By_Copy} to change
this default, by requiring that record formal parameters be passed by
copy if all of the following conditions are met:


@itemize *

@item 
The size of the record type does not exceed the value specified for
@code{Max_Size}.

@item 
The record type has @code{Convention C}.

@item 
The formal parameter has this record type, and the subprogram has a
foreign (non-Ada) convention.
@end itemize

If these conditions are met the argument is passed by copy; i.e., in a
manner consistent with what C expects if the corresponding formal in the
C prototype is a struct (rather than a pointer to a struct).

You can also pass records by copy by specifying the convention
@code{C_Pass_By_Copy} for the record type, or by using the extended
@code{Import} and @code{Export} pragmas, which allow specification of
passing mechanisms on a parameter by parameter basis.

@node Pragma Check,Pragma Check_Float_Overflow,Pragma C_Pass_By_Copy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check}@anchor{39}
@section Pragma Check


@geindex Assertions

@geindex Named assertions

Syntax:

@example
pragma Check (
     [Name    =>] CHECK_KIND,
     [Check   =>] Boolean_EXPRESSION
  [, [Message =>] string_EXPRESSION] );

CHECK_KIND ::= IDENTIFIER           |
               Pre'Class            |
               Post'Class           |
               Type_Invariant'Class |
               Invariant'Class
@end example

This pragma is similar to the predefined pragma @code{Assert} except that an
extra identifier argument is present. In conjunction with pragma
@code{Check_Policy}, this can be used to define groups of assertions that can
be independently controlled. The identifier @code{Assertion} is special, it
refers to the normal set of pragma @code{Assert} statements.

Checks introduced by this pragma are normally deactivated by default. They can
be activated either by the command line option `-gnata', which turns on
all checks, or individually controlled using pragma @code{Check_Policy}.

The identifiers @code{Assertions} and @code{Statement_Assertions} are not
permitted as check kinds, since this would cause confusion with the use
of these identifiers in @code{Assertion_Policy} and @code{Check_Policy}
pragmas, where they are used to refer to sets of assertions.

@node Pragma Check_Float_Overflow,Pragma Check_Name,Pragma Check,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check-float-overflow}@anchor{3a}
@section Pragma Check_Float_Overflow


@geindex Floating-point overflow

Syntax:

@example
pragma Check_Float_Overflow;
@end example

In Ada, the predefined floating-point types (@code{Short_Float},
@code{Float}, @code{Long_Float}, @code{Long_Long_Float}) are
defined to be `unconstrained'. This means that even though each
has a well-defined base range, an operation that delivers a result
outside this base range is not required to raise an exception.
This implementation permission accommodates the notion
of infinities in IEEE floating-point, and corresponds to the
efficient execution mode on most machines. GNAT will not raise
overflow exceptions on these machines; instead it will generate
infinities and NaN’s as defined in the IEEE standard.

Generating infinities, although efficient, is not always desirable.
Often the preferable approach is to check for overflow, even at the
(perhaps considerable) expense of run-time performance.
This can be accomplished by defining your own constrained floating-point subtypes – i.e., by supplying explicit
range constraints – and indeed such a subtype
can have the same base range as its base type. For example:

@example
subtype My_Float is Float range Float'Range;
@end example

Here @code{My_Float} has the same range as
@code{Float} but is constrained, so operations on
@code{My_Float} values will be checked for overflow
against this range.

This style will achieve the desired goal, but
it is often more convenient to be able to simply use
the standard predefined floating-point types as long
as overflow checking could be guaranteed.
The @code{Check_Float_Overflow}
configuration pragma achieves this effect. If a unit is compiled
subject to this configuration pragma, then all operations
on predefined floating-point types including operations on
base types of these floating-point types will be treated as
though those types were constrained, and overflow checks
will be generated. The @code{Constraint_Error}
exception is raised if the result is out of range.

This mode can also be set by use of the compiler
switch `-gnateF'.

@node Pragma Check_Name,Pragma Check_Policy,Pragma Check_Float_Overflow,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check-name}@anchor{3b}
@section Pragma Check_Name


@geindex Defining check names

@geindex Check names
@geindex defining

Syntax:

@example
pragma Check_Name (check_name_IDENTIFIER);
@end example

This is a configuration pragma that defines a new implementation
defined check name (unless IDENTIFIER matches one of the predefined
check names, in which case the pragma has no effect). Check names
are global to a partition, so if two or more configuration pragmas
are present in a partition mentioning the same name, only one new
check name is introduced.

An implementation defined check name introduced with this pragma may
be used in only three contexts: @code{pragma Suppress},
@code{pragma Unsuppress},
and as the prefix of a @code{Check_Name'Enabled} attribute reference. For
any of these three cases, the check name must be visible. A check
name is visible if it is in the configuration pragmas applying to
the current unit, or if it appears at the start of any unit that
is part of the dependency set of the current unit (e.g., units that
are mentioned in @code{with} clauses).

Check names introduced by this pragma are subject to control by compiler
switches (in particular -gnatp) in the usual manner.

@node Pragma Check_Policy,Pragma Comment,Pragma Check_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check-policy}@anchor{3c}
@section Pragma Check_Policy


@geindex Controlling assertions

@geindex Assertions
@geindex control

@geindex Check pragma control

@geindex Named assertions

Syntax:

@example
pragma Check_Policy
 ([Name   =>] CHECK_KIND,
  [Policy =>] POLICY_IDENTIFIER);

pragma Check_Policy (
    CHECK_KIND => POLICY_IDENTIFIER
 @{, CHECK_KIND => POLICY_IDENTIFIER@});

ASSERTION_KIND ::= RM_ASSERTION_KIND | ID_ASSERTION_KIND

CHECK_KIND ::= IDENTIFIER           |
               Pre'Class            |
               Post'Class           |
               Type_Invariant'Class |
               Invariant'Class

The identifiers Name and Policy are not allowed as CHECK_KIND values. This
avoids confusion between the two possible syntax forms for this pragma.

POLICY_IDENTIFIER ::= ON | OFF | CHECK | DISABLE | IGNORE
@end example

This pragma is used to set the checking policy for assertions (specified
by aspects or pragmas), the @code{Debug} pragma, or additional checks
to be checked using the @code{Check} pragma. It may appear either as
a configuration pragma, or within a declarative part of package. In the
latter case, it applies from the point where it appears to the end of
the declarative region (like pragma @code{Suppress}).

The @code{Check_Policy} pragma is similar to the
predefined @code{Assertion_Policy} pragma,
and if the check kind corresponds to one of the assertion kinds that
are allowed by @code{Assertion_Policy}, then the effect is identical.

If the first argument is Debug, then the policy applies to Debug pragmas,
disabling their effect if the policy is @code{OFF}, @code{DISABLE}, or
@code{IGNORE}, and allowing them to execute with normal semantics if
the policy is @code{ON} or @code{CHECK}. In addition if the policy is
@code{DISABLE}, then the procedure call in @code{Debug} pragmas will
be totally ignored and not analyzed semantically.

Finally the first argument may be some other identifier than the above
possibilities, in which case it controls a set of named assertions
that can be checked using pragma @code{Check}. For example, if the pragma:

@example
pragma Check_Policy (Critical_Error, OFF);
@end example

is given, then subsequent @code{Check} pragmas whose first argument is also
@code{Critical_Error} will be disabled.

The check policy is @code{OFF} to turn off corresponding checks, and @code{ON}
to turn on corresponding checks. The default for a set of checks for which no
@code{Check_Policy} is given is @code{OFF} unless the compiler switch
`-gnata' is given, which turns on all checks by default.

The check policy settings @code{CHECK} and @code{IGNORE} are recognized
as synonyms for @code{ON} and @code{OFF}. These synonyms are provided for
compatibility with the standard @code{Assertion_Policy} pragma. The check
policy setting @code{DISABLE} causes the second argument of a corresponding
@code{Check} pragma to be completely ignored and not analyzed.

@node Pragma Comment,Pragma Common_Object,Pragma Check_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-comment}@anchor{3d}
@section Pragma Comment


Syntax:

@example
pragma Comment (static_string_EXPRESSION);
@end example

This is almost identical in effect to pragma @code{Ident}.  It allows the
placement of a comment into the object file and hence into the
executable file if the operating system permits such usage.  The
difference is that @code{Comment}, unlike @code{Ident}, has
no limitations on placement of the pragma (it can be placed
anywhere in the main source unit), and if more than one pragma
is used, all comments are retained.

@node Pragma Common_Object,Pragma Compile_Time_Error,Pragma Comment,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-common-object}@anchor{3e}
@section Pragma Common_Object


Syntax:

@example
pragma Common_Object (
     [Internal =>] LOCAL_NAME
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL] );

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma enables the shared use of variables stored in overlaid
linker areas corresponding to the use of @code{COMMON}
in Fortran.  The single
object @code{LOCAL_NAME} is assigned to the area designated by
the @code{External} argument.
You may define a record to correspond to a series
of fields.  The @code{Size} argument
is syntax checked in GNAT, but otherwise ignored.

@code{Common_Object} is not supported on all platforms.  If no
support is available, then the code generator will issue a message
indicating that the necessary attribute for implementation of this
pragma is not available.

@node Pragma Compile_Time_Error,Pragma Compile_Time_Warning,Pragma Common_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas compile-time-error}@anchor{3f}@anchor{gnat_rm/implementation_defined_pragmas pragma-compile-time-error}@anchor{40}
@section Pragma Compile_Time_Error


Syntax:

@example
pragma Compile_Time_Error
         (boolean_EXPRESSION, static_string_EXPRESSION);
@end example

This pragma can be used to generate additional compile time
error messages. It
is particularly useful in generics, where errors can be issued for
specific problematic instantiations. The first parameter is a boolean
expression. The pragma ensures that the value of an expression
is known at compile time, and has the value False. The set of expressions
whose values are known at compile time includes all static boolean
expressions, and also other values which the compiler can determine
at compile time (e.g., the size of a record type set by an explicit
size representation clause, or the value of a variable which was
initialized to a constant and is known not to have been modified).
If these conditions are not met, an error message is generated using
the value given as the second argument. This string value may contain
embedded ASCII.LF characters to break the message into multiple lines.

@node Pragma Compile_Time_Warning,Pragma Complete_Representation,Pragma Compile_Time_Error,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-compile-time-warning}@anchor{41}
@section Pragma Compile_Time_Warning


Syntax:

@example
pragma Compile_Time_Warning
         (boolean_EXPRESSION, static_string_EXPRESSION);
@end example

Same as pragma Compile_Time_Error, except a warning is issued instead
of an error message. If switch `-gnatw_C' is used, a warning is only issued
if the value of the expression is known to be True at compile time, not when
the value of the expression is not known at compile time.
Note that if this pragma is used in a package that
is with’ed by a client, the client will get the warning even though it
is issued by a with’ed package (normally warnings in with’ed units are
suppressed, but this is a special exception to that rule).

One typical use is within a generic where compile time known characteristics
of formal parameters are tested, and warnings given appropriately. Another use
with a first parameter of True is to warn a client about use of a package,
for example that it is not fully implemented.

In previous versions of the compiler, combining `-gnatwe' with
Compile_Time_Warning resulted in a fatal error. Now the compiler always emits
a warning. You can use @ref{3f,,Pragma Compile_Time_Error} to force the generation of
an error.

@node Pragma Complete_Representation,Pragma Complex_Representation,Pragma Compile_Time_Warning,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-complete-representation}@anchor{42}
@section Pragma Complete_Representation


Syntax:

@example
pragma Complete_Representation;
@end example

This pragma must appear immediately within a record representation
clause. Typical placements are before the first component clause
or after the last component clause. The effect is to give an error
message if any component is missing a component clause. This pragma
may be used to ensure that a record representation clause is
complete, and that this invariant is maintained if fields are
added to the record in the future.

@node Pragma Complex_Representation,Pragma Component_Alignment,Pragma Complete_Representation,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-complex-representation}@anchor{43}
@section Pragma Complex_Representation


Syntax:

@example
pragma Complex_Representation
        ([Entity =>] LOCAL_NAME);
@end example

The @code{Entity} argument must be the name of a record type which has
two fields of the same floating-point type.  The effect of this pragma is
to force gcc to use the special internal complex representation form for
this record, which may be more efficient.  Note that this may result in
the code for this type not conforming to standard ABI (application
binary interface) requirements for the handling of record types.  For
example, in some environments, there is a requirement for passing
records by pointer, and the use of this pragma may result in passing
this type in floating-point registers.

@node Pragma Component_Alignment,Pragma Constant_After_Elaboration,Pragma Complex_Representation,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-component-alignment}@anchor{44}
@section Pragma Component_Alignment


@geindex Alignments of components

@geindex Pragma Component_Alignment

Syntax:

@example
pragma Component_Alignment (
     [Form =>] ALIGNMENT_CHOICE
  [, [Name =>] type_LOCAL_NAME]);

ALIGNMENT_CHOICE ::=
  Component_Size
| Component_Size_4
| Storage_Unit
| Default
@end example

Specifies the alignment of components in array or record types.
The meaning of the @code{Form} argument is as follows:

@quotation

@geindex Component_Size (in pragma Component_Alignment)
@end quotation


@table @asis

@item `Component_Size'

Aligns scalar components and subcomponents of the array or record type
on boundaries appropriate to their inherent size (naturally
aligned).  For example, 1-byte components are aligned on byte boundaries,
2-byte integer components are aligned on 2-byte boundaries, 4-byte
integer components are aligned on 4-byte boundaries and so on.  These
alignment rules correspond to the normal rules for C compilers on all
machines except the VAX.

@geindex Component_Size_4 (in pragma Component_Alignment)

@item `Component_Size_4'

Naturally aligns components with a size of four or fewer
bytes.  Components that are larger than 4 bytes are placed on the next
4-byte boundary.

@geindex Storage_Unit (in pragma Component_Alignment)

@item `Storage_Unit'

Specifies that array or record components are byte aligned, i.e.,
aligned on boundaries determined by the value of the constant
@code{System.Storage_Unit}.

@geindex Default (in pragma Component_Alignment)

@item `Default'

Specifies that array or record components are aligned on default
boundaries, appropriate to the underlying hardware or operating system or
both. The @code{Default} choice is the same as @code{Component_Size} (natural
alignment).
@end table

If the @code{Name} parameter is present, @code{type_LOCAL_NAME} must
refer to a local record or array type, and the specified alignment
choice applies to the specified type.  The use of
@code{Component_Alignment} together with a pragma @code{Pack} causes the
@code{Component_Alignment} pragma to be ignored.  The use of
@code{Component_Alignment} together with a record representation clause
is only effective for fields not specified by the representation clause.

If the @code{Name} parameter is absent, the pragma can be used as either
a configuration pragma, in which case it applies to one or more units in
accordance with the normal rules for configuration pragmas, or it can be
used within a declarative part, in which case it applies to types that
are declared within this declarative part, or within any nested scope
within this declarative part.  In either case it specifies the alignment
to be applied to any record or array type which has otherwise standard
representation.

If the alignment for a record or array type is not specified (using
pragma @code{Pack}, pragma @code{Component_Alignment}, or a record rep
clause), the GNAT uses the default alignment as described previously.

@node Pragma Constant_After_Elaboration,Pragma Contract_Cases,Pragma Component_Alignment,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id7}@anchor{45}@anchor{gnat_rm/implementation_defined_pragmas pragma-constant-after-elaboration}@anchor{46}
@section Pragma Constant_After_Elaboration


Syntax:

@example
pragma Constant_After_Elaboration [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect
@code{Constant_After_Elaboration} in the SPARK 2014 Reference Manual, section 3.3.1.

@node Pragma Contract_Cases,Pragma Convention_Identifier,Pragma Constant_After_Elaboration,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id8}@anchor{47}@anchor{gnat_rm/implementation_defined_pragmas pragma-contract-cases}@anchor{48}
@section Pragma Contract_Cases


@geindex Contract cases

Syntax:

@example
pragma Contract_Cases (CONTRACT_CASE @{, CONTRACT_CASE@});

CONTRACT_CASE ::= CASE_GUARD => CONSEQUENCE

CASE_GUARD ::= boolean_EXPRESSION | others

CONSEQUENCE ::= boolean_EXPRESSION
@end example

The @code{Contract_Cases} pragma allows defining fine-grain specifications
that can complement or replace the contract given by a precondition and a
postcondition. Additionally, the @code{Contract_Cases} pragma can be used
by testing and formal verification tools. The compiler checks its validity and,
depending on the assertion policy at the point of declaration of the pragma,
it may insert a check in the executable. For code generation, the contract
cases

@example
pragma Contract_Cases (
  Cond1 => Pred1,
  Cond2 => Pred2);
@end example

are equivalent to

@example
C1 : constant Boolean := Cond1;  --  evaluated at subprogram entry
C2 : constant Boolean := Cond2;  --  evaluated at subprogram entry
pragma Precondition ((C1 and not C2) or (C2 and not C1));
pragma Postcondition (if C1 then Pred1);
pragma Postcondition (if C2 then Pred2);
@end example

The precondition ensures that one and only one of the case guards is
satisfied on entry to the subprogram.
The postcondition ensures that for the case guard that was True on entry,
the corresponding consequence is True on exit. Other consequence expressions
are not evaluated.

A precondition @code{P} and postcondition @code{Q} can also be
expressed as contract cases:

@example
pragma Contract_Cases (P => Q);
@end example

The placement and visibility rules for @code{Contract_Cases} pragmas are
identical to those described for preconditions and postconditions.

The compiler checks that boolean expressions given in case guards and
consequences are valid, where the rules for case guards are the same as
the rule for an expression in @code{Precondition} and the rules for
consequences are the same as the rule for an expression in
@code{Postcondition}. In particular, attributes @code{'Old} and
@code{'Result} can only be used within consequence expressions.
The case guard for the last contract case may be @code{others}, to denote
any case not captured by the previous cases. The
following is an example of use within a package spec:

@example
package Math_Functions is
   ...
   function Sqrt (Arg : Float) return Float;
   pragma Contract_Cases (((Arg in 0.0 .. 99.0) => Sqrt'Result < 10.0,
                           Arg >= 100.0         => Sqrt'Result >= 10.0,
                           others               => Sqrt'Result = 0.0));
   ...
end Math_Functions;
@end example

The meaning of contract cases is that only one case should apply at each
call, as determined by the corresponding case guard evaluating to True,
and that the consequence for this case should hold when the subprogram
returns.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Convention_Identifier,Pragma CPP_Class,Pragma Contract_Cases,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-convention-identifier}@anchor{49}
@section Pragma Convention_Identifier


@geindex Conventions
@geindex synonyms

Syntax:

@example
pragma Convention_Identifier (
         [Name =>]       IDENTIFIER,
         [Convention =>] convention_IDENTIFIER);
@end example

This pragma provides a mechanism for supplying synonyms for existing
convention identifiers. The @code{Name} identifier can subsequently
be used as a synonym for the given convention in other pragmas (including
for example pragma @code{Import} or another @code{Convention_Identifier}
pragma). As an example of the use of this, suppose you had legacy code
which used Fortran77 as the identifier for Fortran. Then the pragma:

@example
pragma Convention_Identifier (Fortran77, Fortran);
@end example

would allow the use of the convention identifier @code{Fortran77} in
subsequent code, avoiding the need to modify the sources. As another
example, you could use this to parameterize convention requirements
according to systems. Suppose you needed to use @code{Stdcall} on
windows systems, and @code{C} on some other system, then you could
define a convention identifier @code{Library} and use a single
@code{Convention_Identifier} pragma to specify which convention
would be used system-wide.

@node Pragma CPP_Class,Pragma CPP_Constructor,Pragma Convention_Identifier,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-class}@anchor{4a}
@section Pragma CPP_Class


@geindex Interfacing with C++

Syntax:

@example
pragma CPP_Class ([Entity =>] LOCAL_NAME);
@end example

The argument denotes an entity in the current declarative region that is
declared as a record type. It indicates that the type corresponds to an
externally declared C++ class type, and is to be laid out the same way
that C++ would lay out the type. If the C++ class has virtual primitives
then the record must be declared as a tagged record type.

Types for which @code{CPP_Class} is specified do not have assignment or
equality operators defined (such operations can be imported or declared
as subprograms as required). Initialization is allowed only by constructor
functions (see pragma @code{CPP_Constructor}). Such types are implicitly
limited if not explicitly declared as limited or derived from a limited
type, and an error is issued in that case.

See @ref{4b,,Interfacing to C++} for related information.

Note: Pragma @code{CPP_Class} is currently obsolete. It is supported
for backward compatibility but its functionality is available
using pragma @code{Import} with @code{Convention} = @code{CPP}.

@node Pragma CPP_Constructor,Pragma CPP_Virtual,Pragma CPP_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-constructor}@anchor{4c}
@section Pragma CPP_Constructor


@geindex Interfacing with C++

Syntax:

@example
pragma CPP_Constructor ([Entity =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION ]
  [, [Link_Name     =>] static_string_EXPRESSION ]);
@end example

This pragma identifies an imported function (imported in the usual way
with pragma @code{Import}) as corresponding to a C++ constructor. If
@code{External_Name} and @code{Link_Name} are not specified then the
@code{Entity} argument is a name that must have been previously mentioned
in a pragma @code{Import} with @code{Convention} = @code{CPP}. Such name
must be of one of the following forms:


@itemize *

@item 
`function' @code{Fname} `return' T`

@item 
`function' @code{Fname} `return' T’Class

@item 
`function' @code{Fname} (…) `return' T`

@item 
`function' @code{Fname} (…) `return' T’Class
@end itemize

where @code{T} is a limited record type imported from C++ with pragma
@code{Import} and @code{Convention} = @code{CPP}.

The first two forms import the default constructor, used when an object
of type @code{T} is created on the Ada side with no explicit constructor.
The latter two forms cover all the non-default constructors of the type.
See the GNAT User’s Guide for details.

If no constructors are imported, it is impossible to create any objects
on the Ada side and the type is implicitly declared abstract.

Pragma @code{CPP_Constructor} is intended primarily for automatic generation
using an automatic binding generator tool (such as the @code{-fdump-ada-spec}
GCC switch).
See @ref{4b,,Interfacing to C++} for more related information.

Note: The use of functions returning class-wide types for constructors is
currently obsolete. They are supported for backward compatibility. The
use of functions returning the type T leave the Ada sources more clear
because the imported C++ constructors always return an object of type T;
that is, they never return an object whose type is a descendant of type T.

@node Pragma CPP_Virtual,Pragma CPP_Vtable,Pragma CPP_Constructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-virtual}@anchor{4d}
@section Pragma CPP_Virtual


@geindex Interfacing to C++

This pragma is now obsolete and, other than generating a warning if warnings
on obsolescent features are enabled, is completely ignored.
It is retained for compatibility
purposes. It used to be required to ensure compatibility with C++, but
is no longer required for that purpose because GNAT generates
the same object layout as the G++ compiler by default.

See @ref{4b,,Interfacing to C++} for related information.

@node Pragma CPP_Vtable,Pragma CPU,Pragma CPP_Virtual,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-vtable}@anchor{4e}
@section Pragma CPP_Vtable


@geindex Interfacing with C++

This pragma is now obsolete and, other than generating a warning if warnings
on obsolescent features are enabled, is completely ignored.
It used to be required to ensure compatibility with C++, but
is no longer required for that purpose because GNAT generates
the same object layout as the G++ compiler by default.

See @ref{4b,,Interfacing to C++} for related information.

@node Pragma CPU,Pragma Deadline_Floor,Pragma CPP_Vtable,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpu}@anchor{4f}
@section Pragma CPU


Syntax:

@example
pragma CPU (EXPRESSION);
@end example

This pragma is standard in Ada 2012, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Deadline_Floor,Pragma Debug,Pragma CPU,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-deadline-floor}@anchor{50}
@section Pragma Deadline_Floor


Syntax:

@example
pragma Deadline_Floor (time_span_EXPRESSION);
@end example

This pragma applies only to protected types and specifies the floor
deadline inherited by a task when the task enters a protected object.
It is effective only when the EDF scheduling policy is used.

@node Pragma Debug,Pragma Debug_Policy,Pragma Deadline_Floor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-debug}@anchor{51}
@section Pragma Debug


Syntax:

@example
pragma Debug ([CONDITION, ]PROCEDURE_CALL_WITHOUT_SEMICOLON);

PROCEDURE_CALL_WITHOUT_SEMICOLON ::=
  PROCEDURE_NAME
| PROCEDURE_PREFIX ACTUAL_PARAMETER_PART
@end example

The procedure call argument has the syntactic form of an expression, meeting
the syntactic requirements for pragmas.

If debug pragmas are not enabled or if the condition is present and evaluates
to False, this pragma has no effect. If debug pragmas are enabled, the
semantics of the pragma is exactly equivalent to the procedure call statement
corresponding to the argument with a terminating semicolon. Pragmas are
permitted in sequences of declarations, so you can use pragma @code{Debug} to
intersperse calls to debug procedures in the middle of declarations. Debug
pragmas can be enabled either by use of the command line switch `-gnata'
or by use of the pragma @code{Check_Policy} with a first argument of
@code{Debug}.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Debug_Policy,Pragma Default_Initial_Condition,Pragma Debug,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-debug-policy}@anchor{52}
@section Pragma Debug_Policy


Syntax:

@example
pragma Debug_Policy (CHECK | DISABLE | IGNORE | ON | OFF);
@end example

This pragma is equivalent to a corresponding @code{Check_Policy} pragma
with a first argument of @code{Debug}. It is retained for historical
compatibility reasons.

@node Pragma Default_Initial_Condition,Pragma Default_Scalar_Storage_Order,Pragma Debug_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id9}@anchor{53}@anchor{gnat_rm/implementation_defined_pragmas pragma-default-initial-condition}@anchor{54}
@section Pragma Default_Initial_Condition


Syntax:

@example
pragma Default_Initial_Condition [ (null | boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect
@code{Default_Initial_Condition} in the SPARK 2014 Reference Manual, section 7.3.3.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Default_Scalar_Storage_Order,Pragma Default_Storage_Pool,Pragma Default_Initial_Condition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-default-scalar-storage-order}@anchor{55}
@section Pragma Default_Scalar_Storage_Order


@geindex Default_Scalar_Storage_Order

@geindex Scalar_Storage_Order

Syntax:

@example
pragma Default_Scalar_Storage_Order (High_Order_First | Low_Order_First);
@end example

Normally if no explicit @code{Scalar_Storage_Order} is given for a record
type or array type, then the scalar storage order defaults to the ordinary
default for the target. But this default may be overridden using this pragma.
The pragma may appear as a configuration pragma, or locally within a package
spec or declarative part. In the latter case, it applies to all subsequent
types declared within that package spec or declarative part.

The following example shows the use of this pragma:

@example
pragma Default_Scalar_Storage_Order (High_Order_First);
with System; use System;
package DSSO1 is
   type H1 is record
      a : Integer;
   end record;

   type L2 is record
      a : Integer;
   end record;
   for L2'Scalar_Storage_Order use Low_Order_First;

   type L2a is new L2;

   package Inner is
      type H3 is record
         a : Integer;
      end record;

      pragma Default_Scalar_Storage_Order (Low_Order_First);

      type L4 is record
         a : Integer;
      end record;
   end Inner;

   type H4a is new Inner.L4;

   type H5 is record
      a : Integer;
   end record;
end DSSO1;
@end example

In this example record types with names starting with `L' have @cite{Low_Order_First} scalar
storage order, and record types with names starting with `H' have @code{High_Order_First}.
Note that in the case of @code{H4a}, the order is not inherited
from the parent type. Only an explicitly set @code{Scalar_Storage_Order}
gets inherited on type derivation.

If this pragma is used as a configuration pragma which appears within a
configuration pragma file (as opposed to appearing explicitly at the start
of a single unit), then the binder will require that all units in a partition
be compiled in a similar manner, other than run-time units, which are not
affected by this pragma. Note that the use of this form is discouraged because
it may significantly degrade the run-time performance of the software, instead
the default scalar storage order ought to be changed only on a local basis.

@node Pragma Default_Storage_Pool,Pragma Depends,Pragma Default_Scalar_Storage_Order,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-default-storage-pool}@anchor{56}
@section Pragma Default_Storage_Pool


@geindex Default_Storage_Pool

Syntax:

@example
pragma Default_Storage_Pool (storage_pool_NAME | null);
@end example

This pragma is standard in Ada 2012, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Depends,Pragma Detect_Blocking,Pragma Default_Storage_Pool,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id10}@anchor{57}@anchor{gnat_rm/implementation_defined_pragmas pragma-depends}@anchor{58}
@section Pragma Depends


Syntax:

@example
pragma Depends (DEPENDENCY_RELATION);

DEPENDENCY_RELATION ::=
     null
  | (DEPENDENCY_CLAUSE @{, DEPENDENCY_CLAUSE@})

DEPENDENCY_CLAUSE ::=
    OUTPUT_LIST =>[+] INPUT_LIST
  | NULL_DEPENDENCY_CLAUSE

NULL_DEPENDENCY_CLAUSE ::= null => INPUT_LIST

OUTPUT_LIST ::= OUTPUT | (OUTPUT @{, OUTPUT@})

INPUT_LIST ::= null | INPUT | (INPUT @{, INPUT@})

OUTPUT ::= NAME | FUNCTION_RESULT
INPUT  ::= NAME

where FUNCTION_RESULT is a function Result attribute_reference
@end example

For the semantics of this pragma, see the entry for aspect @code{Depends} in the
SPARK 2014 Reference Manual, section 6.1.5.

@node Pragma Detect_Blocking,Pragma Disable_Atomic_Synchronization,Pragma Depends,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-detect-blocking}@anchor{59}
@section Pragma Detect_Blocking


Syntax:

@example
pragma Detect_Blocking;
@end example

This is a standard pragma in Ada 2005, that is available in all earlier
versions of Ada as an implementation-defined pragma.

This is a configuration pragma that forces the detection of potentially
blocking operations within a protected operation, and to raise Program_Error
if that happens.

@node Pragma Disable_Atomic_Synchronization,Pragma Dispatching_Domain,Pragma Detect_Blocking,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-disable-atomic-synchronization}@anchor{5a}
@section Pragma Disable_Atomic_Synchronization


@geindex Atomic Synchronization

Syntax:

@example
pragma Disable_Atomic_Synchronization [(Entity)];

pragma Enable_Atomic_Synchronization [(Entity)];
@end example

Ada requires that accesses (reads or writes) of an atomic variable be
regarded as synchronization points in the case of multiple tasks.
Particularly in the case of multi-processors this may require special
handling, e.g. the generation of memory barriers. This synchronization
is performed by default, but can be turned off using pragma
@code{Disable_Atomic_Synchronization}.
The @code{Enable_Atomic_Synchronization} pragma turns it back on.

The placement and scope rules for these pragmas are the same as those
for @code{pragma Suppress}. In particular they can be used as
configuration pragmas, or in a declaration sequence where they apply
until the end of the scope. If an @code{Entity} argument is present,
the action applies only to that entity.

@node Pragma Dispatching_Domain,Pragma Effective_Reads,Pragma Disable_Atomic_Synchronization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-dispatching-domain}@anchor{5b}
@section Pragma Dispatching_Domain


Syntax:

@example
pragma Dispatching_Domain (EXPRESSION);
@end example

This pragma is standard in Ada 2012, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Effective_Reads,Pragma Effective_Writes,Pragma Dispatching_Domain,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id11}@anchor{5c}@anchor{gnat_rm/implementation_defined_pragmas pragma-effective-reads}@anchor{5d}
@section Pragma Effective_Reads


Syntax:

@example
pragma Effective_Reads [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Effective_Reads} in
the SPARK 2014 Reference Manual, section 7.1.2.

@node Pragma Effective_Writes,Pragma Elaboration_Checks,Pragma Effective_Reads,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id12}@anchor{5e}@anchor{gnat_rm/implementation_defined_pragmas pragma-effective-writes}@anchor{5f}
@section Pragma Effective_Writes


Syntax:

@example
pragma Effective_Writes [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Effective_Writes}
in the SPARK 2014 Reference Manual, section 7.1.2.

@node Pragma Elaboration_Checks,Pragma Eliminate,Pragma Effective_Writes,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-elaboration-checks}@anchor{60}
@section Pragma Elaboration_Checks


@geindex Elaboration control

Syntax:

@example
pragma Elaboration_Checks (Dynamic | Static);
@end example

This is a configuration pragma which specifies the elaboration model to be
used during compilation. For more information on the elaboration models of
GNAT, consult the chapter on elaboration order handling in the `GNAT User’s
Guide'.

The pragma may appear in the following contexts:


@itemize *

@item 
Configuration pragmas file

@item 
Prior to the context clauses of a compilation unit’s initial declaration
@end itemize

Any other placement of the pragma will result in a warning and the effects of
the offending pragma will be ignored.

If the pragma argument is @code{Dynamic}, then the dynamic elaboration model is in
effect. If the pragma argument is @code{Static}, then the static elaboration model
is in effect.

@node Pragma Eliminate,Pragma Enable_Atomic_Synchronization,Pragma Elaboration_Checks,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-eliminate}@anchor{61}
@section Pragma Eliminate


@geindex Elimination of unused subprograms

Syntax:

@example
pragma Eliminate (
            [  Unit_Name       => ] IDENTIFIER | SELECTED_COMPONENT ,
            [  Entity          => ] IDENTIFIER |
                                    SELECTED_COMPONENT |
                                    STRING_LITERAL
            [, Source_Location =>   SOURCE_TRACE ] );

        SOURCE_TRACE    ::= STRING_LITERAL
@end example

This pragma indicates that the given entity is not used in the program to be
compiled and built, thus allowing the compiler to
eliminate the code or data associated with the named entity. Any reference to
an eliminated entity causes a compile-time or link-time error.

The pragma has the following semantics, where @code{U} is the unit specified by
the @code{Unit_Name} argument and @code{E} is the entity specified by the @code{Entity}
argument:


@itemize *

@item 
@code{E} must be a subprogram that is explicitly declared either:


@itemize *

@item 
Within @code{U}, or

@item 
Within a generic package that is instantiated in @code{U}, or

@item 
As an instance of generic subprogram instantiated in @code{U}.
@end itemize

Otherwise the pragma is ignored.

@item 
If @code{E} is overloaded within @code{U} then, in the absence of a
@code{Source_Location} argument, all overloadings are eliminated.

@item 
If @code{E} is overloaded within @code{U} and only some overloadings
are to be eliminated, then each overloading to be eliminated
must be specified in a corresponding pragma @code{Eliminate}
with a @code{Source_Location} argument identifying the line where the
declaration appears, as described below.

@item 
If @code{E} is declared as the result of a generic instantiation, then
a @code{Source_Location} argument is needed, as described below.
@end itemize

Pragma @code{Eliminate} allows a program to be compiled in a system-independent
manner, so that unused entities are eliminated but without
needing to modify the source text. Normally the required set of
@code{Eliminate} pragmas is constructed automatically using the @code{gnatelim} tool.

Any source file change that removes, splits, or
adds lines may make the set of @code{Eliminate} pragmas invalid because their
@code{Source_Location} argument values may get out of date.

Pragma @code{Eliminate} may be used where the referenced entity is a dispatching
operation. In this case all the subprograms to which the given operation can
dispatch are considered to be unused (are never called as a result of a direct
or a dispatching call).

The string literal given for the source location specifies the line number
of the declaration of the entity, using the following syntax for @code{SOURCE_TRACE}:

@example
SOURCE_TRACE     ::= SOURCE_REFERENCE [ LBRACKET SOURCE_TRACE RBRACKET ]

LBRACKET         ::= '['
RBRACKET         ::= ']'

SOURCE_REFERENCE ::= FILE_NAME : LINE_NUMBER

LINE_NUMBER      ::= DIGIT @{DIGIT@}
@end example

Spaces around the colon in a @code{SOURCE_REFERENCE} are optional.

The source trace that is given as the @code{Source_Location} must obey the
following rules (or else the pragma is ignored), where @code{U} is
the unit @code{U} specified by the @code{Unit_Name} argument and @code{E} is the
subprogram specified by the @code{Entity} argument:


@itemize *

@item 
@code{FILE_NAME} is the short name (with no directory
information) of the Ada source file for @code{U}, using the required syntax
for the underlying file system (e.g. case is significant if the underlying
operating system is case sensitive).
If @code{U} is a package and @code{E} is a subprogram declared in the package
specification and its full declaration appears in the package body,
then the  relevant source file is the one for the package specification;
analogously if @code{U} is a generic package.

@item 
If @code{E} is not declared in a generic instantiation (this includes
generic subprogram instances), the source trace includes only one source
line reference. @code{LINE_NUMBER} gives the line number of the occurrence
of the declaration of @code{E} within the source file (as a decimal literal
without an exponent or point).

@item 
If @code{E} is declared by a generic instantiation, its source trace
(from left to right) starts with the source location of the
declaration of @code{E} in the generic unit and ends with the source
location of the instantiation, given in square brackets. This approach is
applied recursively with nested instantiations: the rightmost (nested
most deeply in square brackets) element of the source trace is the location
of the outermost instantiation, and the leftmost element (that is, outside
of any square brackets) is the location of the declaration of @code{E} in
the generic unit.
@end itemize

Examples:

@quotation

@example
pragma Eliminate (Pkg0, Proc);
-- Eliminate (all overloadings of) Proc in Pkg0

pragma Eliminate (Pkg1, Proc,
                  Source_Location => "pkg1.ads:8");
-- Eliminate overloading of Proc at line 8 in pkg1.ads

-- Assume the following file contents:
--   gen_pkg.ads
--   1: generic
--   2:   type T is private;
--   3: package Gen_Pkg is
--   4:   procedure Proc(N : T);
--  ...   ...
--  ... end Gen_Pkg;
--
--    q.adb
--   1: with Gen_Pkg;
--   2: procedure Q is
--   3:   package Inst_Pkg is new Gen_Pkg(Integer);
--  ...   -- No calls on Inst_Pkg.Proc
--  ... end Q;

-- The following pragma eliminates Inst_Pkg.Proc from Q
pragma Eliminate (Q, Proc,
                  Source_Location => "gen_pkg.ads:4[q.adb:3]");
@end example
@end quotation

@node Pragma Enable_Atomic_Synchronization,Pragma Exceptional_Cases,Pragma Eliminate,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-enable-atomic-synchronization}@anchor{62}
@section Pragma Enable_Atomic_Synchronization


@geindex Atomic Synchronization

Syntax:

@example
pragma Enable_Atomic_Synchronization [(Entity)];
@end example

Reenables atomic synchronization; see @code{pragma Disable_Atomic_Synchronization}
for details.

@node Pragma Exceptional_Cases,Pragma Exit_Cases,Pragma Enable_Atomic_Synchronization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-exceptional-cases}@anchor{63}
@section Pragma Exceptional_Cases


@geindex Exceptional_Cases

Syntax:

@example
pragma Exceptional_Cases (EXCEPTIONAL_CASE_LIST);

EXCEPTIONAL_CASE_LIST ::= EXCEPTIONAL_CASE @{, EXCEPTIONAL_CASE@}
EXCEPTIONAL_CASE      ::= exception_choice @{'|' exception_choice@} => CONSEQUENCE
CONSEQUENCE           ::= Boolean_expression
@end example

For the semantics of this aspect, see the SPARK 2014 Reference Manual, section
6.1.9.

@node Pragma Exit_Cases,Pragma Export_Function,Pragma Exceptional_Cases,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-exit-cases}@anchor{64}
@section Pragma Exit_Cases


@geindex Exit_Cases

Syntax:

@example
pragma Exit_Cases (EXIT_CASE_LIST);

EXIT_CASE_LIST ::= EXIT_CASE @{, EXIT_CASE@}
EXIT_CASE      ::= GUARD => EXIT_KIND
EXIT_KIND      ::= Normal_Return
                 | Exception_Raised
                 | (Exception_Raised => exception_name)
                 | Program_Exit
GUARD          ::= Boolean_expression
@end example

For the semantics of this aspect, see the SPARK 2014 Reference Manual, section
6.1.10.

@node Pragma Export_Function,Pragma Export_Object,Pragma Exit_Cases,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-function}@anchor{65}
@section Pragma Export_Function


@geindex Argument passing mechanisms

Syntax:

@example
pragma Export_Function (
     [Internal         =>] LOCAL_NAME
  [, [External         =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types  =>] PARAMETER_TYPES]
  [, [Result_Type      =>] result_SUBTYPE_MARK]
  [, [Mechanism        =>] MECHANISM]
  [, [Result_Mechanism =>] MECHANISM_NAME]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
| ""

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

Use this pragma to make a function externally callable and optionally
provide information on mechanisms to be used for passing parameter and
result values.  We recommend, for the purposes of improving portability,
this pragma always be used in conjunction with a separate pragma
@code{Export}, which must precede the pragma @code{Export_Function}.
GNAT does not require a separate pragma @code{Export}, but if none is
present, @code{Convention Ada} is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a @code{Export} or @code{Convention}
pragma that specifies the desired foreign convention.
Pragma @code{Export_Function}
(and @code{Export}, if present) must appear in the same declarative
region as the function to which they apply.

The @code{internal_name} must uniquely designate the function to which the
pragma applies.  If more than one function name exists of this name in
the declarative part you must use the @code{Parameter_Types} and
@code{Result_Type} parameters to achieve the required
unique designation.  The @cite{subtype_mark}s in these parameters must
exactly match the subtypes in the corresponding function specification,
using positional notation to match parameters with subtype marks.
The form with an @code{'Access} attribute can be used to match an
anonymous access parameter.

@geindex Suppressing external name

Special treatment is given if the EXTERNAL is an explicit null
string or a static string expressions that evaluates to the null
string. In this case, no external name is generated. This form
still allows the specification of parameter mechanisms.

@node Pragma Export_Object,Pragma Export_Procedure,Pragma Export_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-object}@anchor{66}
@section Pragma Export_Object


Syntax:

@example
pragma Export_Object (
      [Internal =>] LOCAL_NAME
   [, [External =>] EXTERNAL_SYMBOL]
   [, [Size     =>] EXTERNAL_SYMBOL]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma designates an object as exported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal @code{Export} pragma applied to an object.  You may use a
separate Export pragma (and you probably should from the point of view
of portability), but it is not required.  @code{Size} is syntax checked,
but otherwise ignored by GNAT.

@node Pragma Export_Procedure,Pragma Export_Valued_Procedure,Pragma Export_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-procedure}@anchor{67}
@section Pragma Export_Procedure


Syntax:

@example
pragma Export_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
| ""

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @code{Export_Function} except that it
applies to a procedure rather than a function and the parameters
@code{Result_Type} and @code{Result_Mechanism} are not permitted.
GNAT does not require a separate pragma @code{Export}, but if none is
present, @code{Convention Ada} is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a @code{Export} or @code{Convention}
pragma that specifies the desired foreign convention.

@geindex Suppressing external name

Special treatment is given if the EXTERNAL is an explicit null
string or a static string expressions that evaluates to the null
string. In this case, no external name is generated. This form
still allows the specification of parameter mechanisms.

@node Pragma Export_Valued_Procedure,Pragma Extend_System,Pragma Export_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-valued-procedure}@anchor{68}
@section Pragma Export_Valued_Procedure


Syntax:

@example
pragma Export_Valued_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
| ""

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @code{Export_Procedure} except that the
first parameter of @code{LOCAL_NAME}, which must be present, must be of
mode @code{out}, and externally the subprogram is treated as a function
with this parameter as the result of the function.  GNAT provides for
this capability to allow the use of @code{out} and @code{in out}
parameters in interfacing to external functions (which are not permitted
in Ada functions).
GNAT does not require a separate pragma @code{Export}, but if none is
present, @code{Convention Ada} is assumed, which is almost certainly
not what is wanted since the whole point of this pragma is to interface
with foreign language functions, so it is usually appropriate to use this
pragma in conjunction with a @code{Export} or @code{Convention}
pragma that specifies the desired foreign convention.

@geindex Suppressing external name

Special treatment is given if the EXTERNAL is an explicit null
string or a static string expressions that evaluates to the null
string. In this case, no external name is generated. This form
still allows the specification of parameter mechanisms.

@node Pragma Extend_System,Pragma Extensions_Allowed,Pragma Export_Valued_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-extend-system}@anchor{69}
@section Pragma Extend_System


@geindex System
@geindex extending

@geindex DEC Ada 83

Syntax:

@example
pragma Extend_System ([Name =>] IDENTIFIER);
@end example

This pragma is used to provide backwards compatibility with other
implementations that extend the facilities of package @code{System}.  In
GNAT, @code{System} contains only the definitions that are present in
the Ada RM.  However, other implementations, notably the DEC Ada 83
implementation, provide many extensions to package @code{System}.

For each such implementation accommodated by this pragma, GNAT provides a
package @code{Aux_@var{xxx}}, e.g., @code{Aux_DEC} for the DEC Ada 83
implementation, which provides the required additional definitions.  You
can use this package in two ways.  You can @code{with} it in the normal
way and access entities either by selection or using a @code{use}
clause.  In this case no special processing is required.

However, if existing code contains references such as
@code{System.@var{xxx}} where `xxx' is an entity in the extended
definitions provided in package @code{System}, you may use this pragma
to extend visibility in @code{System} in a non-standard way that
provides greater compatibility with the existing code.  Pragma
@code{Extend_System} is a configuration pragma whose single argument is
the name of the package containing the extended definition
(e.g., @code{Aux_DEC} for the DEC Ada case).  A unit compiled under
control of this pragma will be processed using special visibility
processing that looks in package @code{System.Aux_@var{xxx}} where
@code{Aux_@var{xxx}} is the pragma argument for any entity referenced in
package @code{System}, but not found in package @code{System}.

You can use this pragma either to access a predefined @code{System}
extension supplied with the compiler, for example @code{Aux_DEC} or
you can construct your own extension unit following the above
definition.  Note that such a package is a child of @code{System}
and thus is considered part of the implementation.
To compile it you will have to use the `-gnatg' switch
for compiling System units, as explained in the
GNAT User’s Guide.

@node Pragma Extensions_Allowed,Pragma Extensions_Visible,Pragma Extend_System,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id13}@anchor{6a}@anchor{gnat_rm/implementation_defined_pragmas pragma-extensions-allowed}@anchor{6b}
@section Pragma Extensions_Allowed


@geindex Ada Extensions

@geindex GNAT Extensions

Syntax:

@example
pragma Extensions_Allowed (On | Off | All_Extensions);
@end example

This configuration pragma enables (via the “On” or “All_Extensions” argument)
or disables (via the “Off” argument) the implementation extension mode; the
pragma takes precedence over the @code{-gnatX} and @code{-gnatX0} command switches.

If an argument of @code{"On"} is specified, the latest version of the Ada language
is implemented (currently Ada 2022) and, in addition, a curated set of GNAT
specific extensions are recognized. (See the list here
@ref{6c,,here})

An argument of @code{"All_Extensions"} has the same effect except that some extra
experimental extensions are enabled (See the list here
@ref{6d,,here})

@node Pragma Extensions_Visible,Pragma External,Pragma Extensions_Allowed,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id14}@anchor{6e}@anchor{gnat_rm/implementation_defined_pragmas pragma-extensions-visible}@anchor{6f}
@section Pragma Extensions_Visible


Syntax:

@example
pragma Extensions_Visible [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Extensions_Visible}
in the SPARK 2014 Reference Manual, section 6.1.7.

@node Pragma External,Pragma External_Name_Casing,Pragma Extensions_Visible,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-external}@anchor{70}
@section Pragma External


Syntax:

@example
pragma External (
  [   Convention    =>] convention_IDENTIFIER,
  [   Entity        =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION ]
  [, [Link_Name     =>] static_string_EXPRESSION ]);
@end example

This pragma is identical in syntax and semantics to pragma
@code{Export} as defined in the Ada Reference Manual.  It is
provided for compatibility with some Ada 83 compilers that
used this pragma for exactly the same purposes as pragma
@code{Export} before the latter was standardized.

@node Pragma External_Name_Casing,Pragma Fast_Math,Pragma External,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-external-name-casing}@anchor{71}
@section Pragma External_Name_Casing


@geindex Dec Ada 83 casing compatibility

@geindex External Names
@geindex casing

@geindex Casing of External names

Syntax:

@example
pragma External_Name_Casing (
  Uppercase | Lowercase
  [, Uppercase | Lowercase | As_Is]);
@end example

This pragma provides control over the casing of external names associated
with Import and Export pragmas.  There are two cases to consider:


@itemize *

@item 
Implicit external names

Implicit external names are derived from identifiers.  The most common case
arises when a standard Ada Import or Export pragma is used with only two
arguments, as in:

@example
pragma Import (C, C_Routine);
@end example

Since Ada is a case-insensitive language, the spelling of the identifier in
the Ada source program does not provide any information on the desired
casing of the external name, and so a convention is needed.  In GNAT the
default treatment is that such names are converted to all lower case
letters.  This corresponds to the normal C style in many environments.
The first argument of pragma @code{External_Name_Casing} can be used to
control this treatment.  If @code{Uppercase} is specified, then the name
will be forced to all uppercase letters.  If @code{Lowercase} is specified,
then the normal default of all lower case letters will be used.

This same implicit treatment is also used in the case of extended DEC Ada 83
compatible Import and Export pragmas where an external name is explicitly
specified using an identifier rather than a string.

@item 
Explicit external names

Explicit external names are given as string literals.  The most common case
arises when a standard Ada Import or Export pragma is used with three
arguments, as in:

@example
pragma Import (C, C_Routine, "C_routine");
@end example

In this case, the string literal normally provides the exact casing required
for the external name.  The second argument of pragma
@code{External_Name_Casing} may be used to modify this behavior.
If @code{Uppercase} is specified, then the name
will be forced to all uppercase letters.  If @code{Lowercase} is specified,
then the name will be forced to all lowercase letters.  A specification of
@code{As_Is} provides the normal default behavior in which the casing is
taken from the string provided.
@end itemize

This pragma may appear anywhere that a pragma is valid. In particular, it
can be used as a configuration pragma in the @code{gnat.adc} file, in which
case it applies to all subsequent compilations, or it can be used as a program
unit pragma, in which case it only applies to the current unit, or it can
be used more locally to control individual Import/Export pragmas.

It was primarily intended for use with OpenVMS systems, where many
compilers convert all symbols to upper case by default.  For interfacing to
such compilers (e.g., the DEC C compiler), it may be convenient to use
the pragma:

@example
pragma External_Name_Casing (Uppercase, Uppercase);
@end example

to enforce the upper casing of all external symbols.

@node Pragma Fast_Math,Pragma Favor_Top_Level,Pragma External_Name_Casing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-fast-math}@anchor{72}
@section Pragma Fast_Math


Syntax:

@example
pragma Fast_Math;
@end example

This is a configuration pragma which activates a mode in which speed is
considered more important for floating-point operations than absolutely
accurate adherence to the requirements of the standard. Currently the
following operations are affected:


@table @asis

@item `Complex Multiplication'

The normal simple formula for complex multiplication can result in intermediate
overflows for numbers near the end of the range. The Ada standard requires that
this situation be detected and corrected by scaling, but in Fast_Math mode such
cases will simply result in overflow. Note that to take advantage of this you
must instantiate your own version of @code{Ada.Numerics.Generic_Complex_Types}
under control of the pragma, rather than use the preinstantiated versions.
@end table

@node Pragma Favor_Top_Level,Pragma Finalize_Storage_Only,Pragma Fast_Math,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id15}@anchor{73}@anchor{gnat_rm/implementation_defined_pragmas pragma-favor-top-level}@anchor{74}
@section Pragma Favor_Top_Level


Syntax:

@example
pragma Favor_Top_Level (type_LOCAL_NAME);
@end example

The argument of pragma @code{Favor_Top_Level} must be a named access-to-subprogram
type. This pragma is an efficiency hint to the compiler, regarding the use of
@code{'Access} or @code{'Unrestricted_Access} on nested (non-library-level) subprograms.
The pragma means that nested subprograms are not used with this type, or are
rare, so that the generated code should be efficient in the top-level case.
When this pragma is used, dynamically generated trampolines may be used on some
targets for nested subprograms. See restriction @code{No_Implicit_Dynamic_Code}.

@node Pragma Finalize_Storage_Only,Pragma Float_Representation,Pragma Favor_Top_Level,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-finalize-storage-only}@anchor{75}
@section Pragma Finalize_Storage_Only


Syntax:

@example
pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);
@end example

The argument of pragma @code{Finalize_Storage_Only} must denote a local type which
is derived from @code{Ada.Finalization.Controlled} or @code{Limited_Controlled}. The
pragma suppresses the call to @code{Finalize} for declared library-level objects
of the argument type. This is mostly useful for types where finalization is
only used to deal with storage reclamation since in most environments it is
not necessary to reclaim memory just before terminating execution, hence the
name. Note that this pragma does not suppress Finalize calls for library-level
heap-allocated objects (see pragma @code{No_Heap_Finalization}).

@node Pragma Float_Representation,Pragma Ghost,Pragma Finalize_Storage_Only,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-float-representation}@anchor{76}
@section Pragma Float_Representation


Syntax:

@example
pragma Float_Representation (FLOAT_REP[, float_type_LOCAL_NAME]);

FLOAT_REP ::= VAX_Float | IEEE_Float
@end example

In the one argument form, this pragma is a configuration pragma which
allows control over the internal representation chosen for the predefined
floating point types declared in the packages @code{Standard} and
@code{System}. This pragma is only provided for compatibility and has no effect.

The two argument form specifies the representation to be used for
the specified floating-point type. The argument must
be @code{IEEE_Float} to specify the use of IEEE format, as follows:


@itemize *

@item 
For a digits value of 6, 32-bit IEEE short format will be used.

@item 
For a digits value of 15, 64-bit IEEE long format will be used.

@item 
No other value of digits is permitted.
@end itemize

@node Pragma Ghost,Pragma Global,Pragma Float_Representation,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id16}@anchor{77}@anchor{gnat_rm/implementation_defined_pragmas pragma-ghost}@anchor{78}
@section Pragma Ghost


Syntax:

@example
pragma Ghost [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Ghost} in the SPARK
2014 Reference Manual, section 6.9.

@node Pragma Global,Pragma Ident,Pragma Ghost,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id17}@anchor{79}@anchor{gnat_rm/implementation_defined_pragmas pragma-global}@anchor{7a}
@section Pragma Global


Syntax:

@example
pragma Global (GLOBAL_SPECIFICATION);

GLOBAL_SPECIFICATION ::=
     null
  | (GLOBAL_LIST)
  | (MODED_GLOBAL_LIST @{, MODED_GLOBAL_LIST@})

MODED_GLOBAL_LIST ::= MODE_SELECTOR => GLOBAL_LIST

MODE_SELECTOR ::= In_Out | Input | Output | Proof_In
GLOBAL_LIST   ::= GLOBAL_ITEM | (GLOBAL_ITEM @{, GLOBAL_ITEM@})
GLOBAL_ITEM   ::= NAME
@end example

For the semantics of this pragma, see the entry for aspect @code{Global} in the
SPARK 2014 Reference Manual, section 6.1.4.

@node Pragma Ident,Pragma Ignore_Pragma,Pragma Global,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ident}@anchor{7b}
@section Pragma Ident


Syntax:

@example
pragma Ident (static_string_EXPRESSION);
@end example

This pragma is identical in effect to pragma @code{Comment}. It is provided
for compatibility with other Ada compilers providing this pragma.

@node Pragma Ignore_Pragma,Pragma Implementation_Defined,Pragma Ident,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ignore-pragma}@anchor{7c}
@section Pragma Ignore_Pragma


Syntax:

@example
pragma Ignore_Pragma (pragma_IDENTIFIER);
@end example

This is a configuration pragma
that takes a single argument that is a simple identifier. Any subsequent
use of a pragma whose pragma identifier matches this argument will be
silently ignored. Any preceding use of a pragma whose pragma identifier
matches this argument will be parsed and then ignored.
This may be useful when legacy code or code intended
for compilation with some other compiler contains pragmas that match the
name, but not the exact implementation, of a GNAT pragma. The use of this
pragma allows such pragmas to be ignored, which may be useful in CodePeer
mode, or during porting of legacy code.

@node Pragma Implementation_Defined,Pragma Implemented,Pragma Ignore_Pragma,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-implementation-defined}@anchor{7d}
@section Pragma Implementation_Defined


Syntax:

@example
pragma Implementation_Defined (local_NAME);
@end example

This pragma marks a previously declared entity as implementation-defined.
For an overloaded entity, applies to the most recent homonym.

@example
pragma Implementation_Defined;
@end example

The form with no arguments appears anywhere within a scope, most
typically a package spec, and indicates that all entities that are
defined within the package spec are Implementation_Defined.

This pragma is used within the GNAT runtime library to identify
implementation-defined entities introduced in language-defined units,
for the purpose of implementing the No_Implementation_Identifiers
restriction.

@node Pragma Implemented,Pragma Implicit_Packing,Pragma Implementation_Defined,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-implemented}@anchor{7e}
@section Pragma Implemented


Syntax:

@example
pragma Implemented (procedure_LOCAL_NAME, implementation_kind);

implementation_kind ::= By_Entry | By_Protected_Procedure | By_Any
@end example

This is an Ada 2012 representation pragma which applies to protected, task
and synchronized interface primitives. The use of pragma Implemented provides
a way to impose a static requirement on the overriding operation by adhering
to one of the three implementation kinds: entry, protected procedure or any of
the above. This pragma is available in all earlier versions of Ada as an
implementation-defined pragma.

@example
type Synch_Iface is synchronized interface;
procedure Prim_Op (Obj : in out Iface) is abstract;
pragma Implemented (Prim_Op, By_Protected_Procedure);

protected type Prot_1 is new Synch_Iface with
   procedure Prim_Op;  --  Legal
end Prot_1;

protected type Prot_2 is new Synch_Iface with
   entry Prim_Op;      --  Illegal
end Prot_2;

task type Task_Typ is new Synch_Iface with
   entry Prim_Op;      --  Illegal
end Task_Typ;
@end example

When applied to the procedure_or_entry_NAME of a requeue statement, pragma
Implemented determines the runtime behavior of the requeue. Implementation kind
By_Entry guarantees that the action of requeueing will proceed from an entry to
another entry. Implementation kind By_Protected_Procedure transforms the
requeue into a dispatching call, thus eliminating the chance of blocking. Kind
By_Any shares the behavior of By_Entry and By_Protected_Procedure depending on
the target’s overriding subprogram kind.

@node Pragma Implicit_Packing,Pragma Import_Function,Pragma Implemented,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-implicit-packing}@anchor{7f}
@section Pragma Implicit_Packing


@geindex Rational Profile

Syntax:

@example
pragma Implicit_Packing;
@end example

This is a configuration pragma that requests implicit packing for packed
arrays for which a size clause is given but no explicit pragma Pack or
specification of Component_Size is present. It also applies to records
where no record representation clause is present. Consider this example:

@example
type R is array (0 .. 7) of Boolean;
for R'Size use 8;
@end example

In accordance with the recommendation in the RM (RM 13.3(53)), a Size clause
does not change the layout of a composite object. So the Size clause in the
above example is normally rejected, since the default layout of the array uses
8-bit components, and thus the array requires a minimum of 64 bits.

If this declaration is compiled in a region of code covered by an occurrence
of the configuration pragma Implicit_Packing, then the Size clause in this
and similar examples will cause implicit packing and thus be accepted. For
this implicit packing to occur, the type in question must be an array of small
components whose size is known at compile time, and the Size clause must
specify the exact size that corresponds to the number of elements in the array
multiplied by the size in bits of the component type (both single and
multi-dimensioned arrays can be controlled with this pragma).

@geindex Array packing

Similarly, the following example shows the use in the record case

@example
type r is record
   a, b, c, d, e, f, g, h : boolean;
   chr                    : character;
end record;
for r'size use 16;
@end example

Without a pragma Pack, each Boolean field requires 8 bits, so the
minimum size is 72 bits, but with a pragma Pack, 16 bits would be
sufficient. The use of pragma Implicit_Packing allows this record
declaration to compile without an explicit pragma Pack.

@node Pragma Import_Function,Pragma Import_Object,Pragma Implicit_Packing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-function}@anchor{80}
@section Pragma Import_Function


Syntax:

@example
pragma Import_Function (
     [Internal         =>] LOCAL_NAME,
  [, [External         =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types  =>] PARAMETER_TYPES]
  [, [Result_Type      =>] SUBTYPE_MARK]
  [, [Mechanism        =>] MECHANISM]
  [, [Result_Mechanism =>] MECHANISM_NAME]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
@end example

This pragma is used in conjunction with a pragma @code{Import} to
specify additional information for an imported function.  The pragma
@code{Import} (or equivalent pragma @code{Interface}) must precede the
@code{Import_Function} pragma and both must appear in the same
declarative part as the function specification.

The @code{Internal} argument must uniquely designate
the function to which the
pragma applies.  If more than one function name exists of this name in
the declarative part you must use the @code{Parameter_Types} and
@code{Result_Type} parameters to achieve the required unique
designation.  Subtype marks in these parameters must exactly match the
subtypes in the corresponding function specification, using positional
notation to match parameters with subtype marks.
The form with an @code{'Access} attribute can be used to match an
anonymous access parameter.

You may optionally use the @code{Mechanism} and @code{Result_Mechanism}
parameters to specify passing mechanisms for the
parameters and result.  If you specify a single mechanism name, it
applies to all parameters.  Otherwise you may specify a mechanism on a
parameter by parameter basis using either positional or named
notation.  If the mechanism is not specified, the default mechanism
is used.

@node Pragma Import_Object,Pragma Import_Procedure,Pragma Import_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-object}@anchor{81}
@section Pragma Import_Object


Syntax:

@example
pragma Import_Object (
     [Internal =>] LOCAL_NAME
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma designates an object as imported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal @code{Import} pragma applied to an object.  Unlike the
subprogram case, you need not use a separate @code{Import} pragma,
although you may do so (and probably should do so from a portability
point of view).  @code{size} is syntax checked, but otherwise ignored by
GNAT.

@node Pragma Import_Procedure,Pragma Import_Valued_Procedure,Pragma Import_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-procedure}@anchor{82}
@section Pragma Import_Procedure


Syntax:

@example
pragma Import_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @code{Import_Function} except that it
applies to a procedure rather than a function and the parameters
@code{Result_Type} and @code{Result_Mechanism} are not permitted.

@node Pragma Import_Valued_Procedure,Pragma Independent,Pragma Import_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-valued-procedure}@anchor{83}
@section Pragma Import_Valued_Procedure


Syntax:

@example
pragma Import_Valued_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @code{Import_Procedure} except that the
first parameter of @code{LOCAL_NAME}, which must be present, must be of
mode @code{out}, and externally the subprogram is treated as a function
with this parameter as the result of the function.  The purpose of this
capability is to allow the use of @code{out} and @code{in out}
parameters in interfacing to external functions (which are not permitted
in Ada functions).  You may optionally use the @code{Mechanism}
parameters to specify passing mechanisms for the parameters.
If you specify a single mechanism name, it applies to all parameters.
Otherwise you may specify a mechanism on a parameter by parameter
basis using either positional or named notation.  If the mechanism is not
specified, the default mechanism is used.

Note that it is important to use this pragma in conjunction with a separate
pragma Import that specifies the desired convention, since otherwise the
default convention is Ada, which is almost certainly not what is required.

@node Pragma Independent,Pragma Independent_Components,Pragma Import_Valued_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-independent}@anchor{84}
@section Pragma Independent


Syntax:

@example
pragma Independent (component_LOCAL_NAME);
@end example

This pragma is standard in Ada 2012 mode (which also provides an aspect
of the same name). It is also available as an implementation-defined
pragma in all earlier versions. It specifies that the
designated object or all objects of the designated type must be
independently addressable. This means that separate tasks can safely
manipulate such objects. For example, if two components of a record are
independent, then two separate tasks may access these two components.
This may place
constraints on the representation of the object (for instance prohibiting
tight packing).

@node Pragma Independent_Components,Pragma Initial_Condition,Pragma Independent,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-independent-components}@anchor{85}
@section Pragma Independent_Components


Syntax:

@example
pragma Independent_Components (Local_NAME);
@end example

This pragma is standard in Ada 2012 mode (which also provides an aspect
of the same name). It is also available as an implementation-defined
pragma in all earlier versions. It specifies that the components of the
designated object, or the components of each object of the designated
type, must be
independently addressable. This means that separate tasks can safely
manipulate separate components in the composite object. This may place
constraints on the representation of the object (for instance prohibiting
tight packing).

@node Pragma Initial_Condition,Pragma Initialize_Scalars,Pragma Independent_Components,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id18}@anchor{86}@anchor{gnat_rm/implementation_defined_pragmas pragma-initial-condition}@anchor{87}
@section Pragma Initial_Condition


Syntax:

@example
pragma Initial_Condition (boolean_EXPRESSION);
@end example

For the semantics of this pragma, see the entry for aspect @code{Initial_Condition}
in the SPARK 2014 Reference Manual, section 7.1.6.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Initialize_Scalars,Pragma Initializes,Pragma Initial_Condition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-initialize-scalars}@anchor{88}
@section Pragma Initialize_Scalars


@geindex debugging with Initialize_Scalars

Syntax:

@example
pragma Initialize_Scalars
  [ ( TYPE_VALUE_PAIR @{, TYPE_VALUE_PAIR@} ) ];

TYPE_VALUE_PAIR ::=
  SCALAR_TYPE => static_EXPRESSION

SCALAR_TYPE :=
  Short_Float
| Float
| Long_Float
| Long_Long_Flat
| Signed_8
| Signed_16
| Signed_32
| Signed_64
| Unsigned_8
| Unsigned_16
| Unsigned_32
| Unsigned_64
@end example

This pragma is similar to @code{Normalize_Scalars} conceptually but has two
important differences.

First, there is no requirement for the pragma to be used uniformly in all units
of a partition. In particular, it is fine to use this just for some or all of
the application units of a partition, without needing to recompile the run-time
library. In the case where some units are compiled with the pragma, and some
without, then a declaration of a variable where the type is defined in package
Standard or is locally declared will always be subject to initialization, as
will any declaration of a scalar variable. For composite variables, whether the
variable is initialized may also depend on whether the package in which the
type of the variable is declared is compiled with the pragma.

The other important difference is that the programmer can control the value
used for initializing scalar objects. This effect can be achieved in several
different ways:


@itemize *

@item 
At compile time, the programmer can specify the invalid value for a
particular family of scalar types using the optional arguments of the pragma.

The compile-time approach is intended to optimize the generated code for the
pragma, by possibly using fast operations such as @code{memset}. Note that such
optimizations require using values where the bytes all have the same binary
representation.

@item 
At bind time, the programmer has several options:


@itemize *

@item 
Initialization with invalid values (similar to Normalize_Scalars, though
for Initialize_Scalars it is not always possible to determine the invalid
values in complex cases like signed component fields with nonstandard
sizes).

@item 
Initialization with high values.

@item 
Initialization with low values.

@item 
Initialization with a specific bit pattern.
@end itemize

See the GNAT User’s Guide for binder options for specifying these cases.

The bind-time approach is intended to provide fast turnaround for testing
with different values, without having to recompile the program.

@item 
At execution time, the programmer can specify the invalid values using an
environment variable. See the GNAT User’s Guide for details.

The execution-time approach is intended to provide fast turnaround for
testing with different values, without having to recompile and rebind the
program.
@end itemize

Note that pragma @code{Initialize_Scalars} is particularly useful in conjunction
with the enhanced validity checking that is now provided in GNAT, which checks
for invalid values under more conditions. Using this feature (see description
of the `-gnatV' flag in the GNAT User’s Guide) in conjunction with pragma
@code{Initialize_Scalars} provides a powerful new tool to assist in the detection
of problems caused by uninitialized variables.

Note: the use of @code{Initialize_Scalars} has a fairly extensive effect on the
generated code. This may cause your code to be substantially larger. It may
also cause an increase in the amount of stack required, so it is probably a
good idea to turn on stack checking (see description of stack checking in the
GNAT User’s Guide) when using this pragma.

@node Pragma Initializes,Pragma Inline_Always,Pragma Initialize_Scalars,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id19}@anchor{89}@anchor{gnat_rm/implementation_defined_pragmas pragma-initializes}@anchor{8a}
@section Pragma Initializes


Syntax:

@example
pragma Initializes (INITIALIZATION_LIST);

INITIALIZATION_LIST ::=
     null
  | (INITIALIZATION_ITEM @{, INITIALIZATION_ITEM@})

INITIALIZATION_ITEM ::= name [=> INPUT_LIST]

INPUT_LIST ::=
     null
  |  INPUT
  | (INPUT @{, INPUT@})

INPUT ::= name
@end example

For the semantics of this pragma, see the entry for aspect @code{Initializes} in the
SPARK 2014 Reference Manual, section 7.1.5.

@node Pragma Inline_Always,Pragma Inline_Generic,Pragma Initializes,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id20}@anchor{8b}@anchor{gnat_rm/implementation_defined_pragmas pragma-inline-always}@anchor{8c}
@section Pragma Inline_Always


Syntax:

@example
pragma Inline_Always (NAME [, NAME]);
@end example

Similar to pragma @code{Inline} except that inlining is unconditional.
Inline_Always instructs the compiler to inline every direct call to the
subprogram or else to emit a compilation error, independently of any
option, in particular `-gnatn' or `-gnatN' or the optimization level.
It is an error to take the address or access of @code{NAME}. It is also an error to
apply this pragma to a primitive operation of a tagged type. Thanks to such
restrictions, the compiler is allowed to remove the out-of-line body of @code{NAME}.

@node Pragma Inline_Generic,Pragma Interface,Pragma Inline_Always,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-inline-generic}@anchor{8d}
@section Pragma Inline_Generic


Syntax:

@example
pragma Inline_Generic (GNAME @{, GNAME@});

GNAME ::= generic_unit_NAME | generic_instance_NAME
@end example

This pragma is provided for compatibility with Dec Ada 83. It has
no effect in GNAT (which always inlines generics), other
than to check that the given names are all names of generic units or
generic instances.

@node Pragma Interface,Pragma Interface_Name,Pragma Inline_Generic,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interface}@anchor{8e}
@section Pragma Interface


Syntax:

@example
pragma Interface (
     [Convention    =>] convention_identifier,
     [Entity        =>] local_NAME
  [, [External_Name =>] static_string_expression]
  [, [Link_Name     =>] static_string_expression]);
@end example

This pragma is identical in syntax and semantics to
the standard Ada pragma @code{Import}.  It is provided for compatibility
with Ada 83.  The definition is upwards compatible both with pragma
@code{Interface} as defined in the Ada 83 Reference Manual, and also
with some extended implementations of this pragma in certain Ada 83
implementations.  The only difference between pragma @code{Interface}
and pragma @code{Import} is that there is special circuitry to allow
both pragmas to appear for the same subprogram entity (normally it
is illegal to have multiple @code{Import} pragmas). This is useful in
maintaining Ada 83/Ada 95 compatibility and is compatible with other
Ada 83 compilers.

@node Pragma Interface_Name,Pragma Interrupt_Handler,Pragma Interface,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interface-name}@anchor{8f}
@section Pragma Interface_Name


Syntax:

@example
pragma Interface_Name (
     [Entity        =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION]
  [, [Link_Name     =>] static_string_EXPRESSION]);
@end example

This pragma provides an alternative way of specifying the interface name
for an interfaced subprogram, and is provided for compatibility with Ada
83 compilers that use the pragma for this purpose.  You must provide at
least one of @code{External_Name} or @code{Link_Name}.

@node Pragma Interrupt_Handler,Pragma Interrupt_State,Pragma Interface_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interrupt-handler}@anchor{90}
@section Pragma Interrupt_Handler


Syntax:

@example
pragma Interrupt_Handler (procedure_LOCAL_NAME);
@end example

This program unit pragma is supported for parameterless protected procedures
as described in Annex C of the Ada Reference Manual.

@node Pragma Interrupt_State,Pragma Interrupts_System_By_Default,Pragma Interrupt_Handler,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interrupt-state}@anchor{91}
@section Pragma Interrupt_State


Syntax:

@example
pragma Interrupt_State
 ([Name  =>] value,
  [State =>] SYSTEM | RUNTIME | USER);
@end example

Normally certain interrupts are reserved to the implementation.  Any attempt
to attach an interrupt causes Program_Error to be raised, as described in
RM C.3.2(22).  A typical example is the @code{SIGINT} interrupt used in
many systems for an @code{Ctrl}-@code{C} interrupt.  Normally this interrupt is
reserved to the implementation, so that @code{Ctrl}-@code{C} can be used to
interrupt execution.  Additionally, signals such as @code{SIGSEGV},
@code{SIGABRT}, @code{SIGFPE} and @code{SIGILL} are often mapped to specific
Ada exceptions, or used to implement run-time functions such as the
@code{abort} statement and stack overflow checking.

Pragma @code{Interrupt_State} provides a general mechanism for overriding
such uses of interrupts.  It subsumes the functionality of pragma
@code{Unreserve_All_Interrupts}.  Pragma @code{Interrupt_State} is not
available on Windows.  On all other platforms than VxWorks,
it applies to signals; on VxWorks, it applies to vectored hardware interrupts
and may be used to mark interrupts required by the board support package
as reserved.

Interrupts can be in one of three states:


@itemize *

@item 
System

The interrupt is reserved (no Ada handler can be installed), and the
Ada run-time may not install a handler. As a result you are guaranteed
standard system default action if this interrupt is raised. This also allows
installing a low level handler via C APIs such as sigaction(), outside
of Ada control.

@item 
Runtime

The interrupt is reserved (no Ada handler can be installed). The run time
is allowed to install a handler for internal control purposes, but is
not required to do so.

@item 
User

The interrupt is unreserved.  The user may install an Ada handler via
Ada.Interrupts and pragma Interrupt_Handler or Attach_Handler to provide
some other action.
@end itemize

These states are the allowed values of the @code{State} parameter of the
pragma.  The @code{Name} parameter is a value of the type
@code{Ada.Interrupts.Interrupt_ID}.  Typically, it is a name declared in
@code{Ada.Interrupts.Names}.

This is a configuration pragma, and the binder will check that there
are no inconsistencies between different units in a partition in how a
given interrupt is specified. It may appear anywhere a pragma is legal.

The effect is to move the interrupt to the specified state.

By declaring interrupts to be SYSTEM, you guarantee the standard system
action, such as a core dump.

By declaring interrupts to be USER, you guarantee that you can install
a handler.

Note that certain signals on many operating systems cannot be caught and
handled by applications.  In such cases, the pragma is ignored.  See the
operating system documentation, or the value of the array @code{Reserved}
declared in the spec of package @code{System.OS_Interface}.

Overriding the default state of signals used by the Ada runtime may interfere
with an application’s runtime behavior in the cases of the synchronous signals,
and in the case of the signal used to implement the @code{abort} statement.

@node Pragma Interrupts_System_By_Default,Pragma Invariant,Pragma Interrupt_State,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interrupts-system-by-default}@anchor{92}
@section Pragma Interrupts_System_By_Default


Syntax:

@example
pragma Interrupts_System_By_Default;
@end example

Default all interrupts to the System state as defined above in pragma
@code{Interrupt_State}. This is a configuration pragma.

@node Pragma Invariant,Pragma Keep_Names,Pragma Interrupts_System_By_Default,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id21}@anchor{93}@anchor{gnat_rm/implementation_defined_pragmas pragma-invariant}@anchor{94}
@section Pragma Invariant


Syntax:

@example
pragma Invariant
  ([Entity =>]    private_type_LOCAL_NAME,
   [Check  =>]    EXPRESSION
   [,[Message =>] String_Expression]);
@end example

This pragma provides exactly the same capabilities as the Type_Invariant aspect
defined in AI05-0146-1, and in the Ada 2012 Reference Manual. The
Type_Invariant aspect is fully implemented in Ada 2012 mode, but since it
requires the use of the aspect syntax, which is not available except in 2012
mode, it is not possible to use the Type_Invariant aspect in earlier versions
of Ada. However the Invariant pragma may be used in any version of Ada. Also
note that the aspect Invariant is a synonym in GNAT for the aspect
Type_Invariant, but there is no pragma Type_Invariant.

The pragma must appear within the visible part of the package specification,
after the type to which its Entity argument appears. As with the Invariant
aspect, the Check expression is not analyzed until the end of the visible
part of the package, so it may contain forward references. The Message
argument, if present, provides the exception message used if the invariant
is violated. If no Message parameter is provided, a default message that
identifies the line on which the pragma appears is used.

It is permissible to have multiple Invariants for the same type entity, in
which case they are and’ed together. It is permissible to use this pragma
in Ada 2012 mode, but you cannot have both an invariant aspect and an
invariant pragma for the same entity.

For further details on the use of this pragma, see the Ada 2012 documentation
of the Type_Invariant aspect.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Keep_Names,Pragma License,Pragma Invariant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-keep-names}@anchor{95}
@section Pragma Keep_Names


Syntax:

@example
pragma Keep_Names ([On =>] enumeration_first_subtype_LOCAL_NAME);
@end example

The @code{LOCAL_NAME} argument
must refer to an enumeration first subtype
in the current declarative part. The effect is to retain the enumeration
literal names for use by @code{Image} and @code{Value} even if a global
@code{Discard_Names} pragma applies. This is useful when you want to
generally suppress enumeration literal names and for example you therefore
use a @code{Discard_Names} pragma in the @code{gnat.adc} file, but you
want to retain the names for specific enumeration types.

@node Pragma License,Pragma Link_With,Pragma Keep_Names,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-license}@anchor{96}
@section Pragma License


@geindex License checking

Syntax:

@example
pragma License (Unrestricted | GPL | Modified_GPL | Restricted);
@end example

This pragma is provided to allow automated checking for appropriate license
conditions with respect to the standard and modified GPL.  A pragma
@code{License}, which is a configuration pragma that typically appears at
the start of a source file or in a separate @code{gnat.adc} file, specifies
the licensing conditions of a unit as follows:


@itemize *

@item 
Unrestricted
This is used for a unit that can be freely used with no license restrictions.
Examples of such units are public domain units, and units from the Ada
Reference Manual.

@item 
GPL
This is used for a unit that is licensed under the unmodified GPL, and which
therefore cannot be @code{with}ed by a restricted unit.

@item 
Modified_GPL
This is used for a unit licensed under the GNAT modified GPL that includes
a special exception paragraph that specifically permits the inclusion of
the unit in programs without requiring the entire program to be released
under the GPL.

@item 
Restricted
This is used for a unit that is restricted in that it is not permitted to
depend on units that are licensed under the GPL.  Typical examples are
proprietary code that is to be released under more restrictive license
conditions.  Note that restricted units are permitted to @code{with} units
which are licensed under the modified GPL (this is the whole point of the
modified GPL).
@end itemize

Normally a unit with no @code{License} pragma is considered to have an
unknown license, and no checking is done.  However, standard GNAT headers
are recognized, and license information is derived from them as follows.

A GNAT license header starts with a line containing 78 hyphens.  The following
comment text is searched for the appearance of any of the following strings.

If the string ‘GNU General Public License’ is found, then the unit is assumed
to have GPL license, unless the string ‘As a special exception’ follows, in
which case the license is assumed to be modified GPL.

If one of the strings
‘This specification is adapted from the Ada Semantic Interface’ or
‘This specification is derived from the Ada Reference Manual’ is found
then the unit is assumed to be unrestricted.

These default actions means that a program with a restricted license pragma
will automatically get warnings if a GPL unit is inappropriately
@code{with}ed.  For example, the program:

@example
with Sem_Ch3;
with GNAT.Sockets;
procedure Secret_Stuff is
  ...
end Secret_Stuff
@end example

if compiled with pragma @code{License} (@code{Restricted}) in a
@code{gnat.adc} file will generate the warning:

@example
1.  with Sem_Ch3;
        |
   >>> license of withed unit "Sem_Ch3" is incompatible

2.  with GNAT.Sockets;
3.  procedure Secret_Stuff is
@end example

Here we get a warning on @code{Sem_Ch3} since it is part of the GNAT
compiler and is licensed under the
GPL, but no warning for @code{GNAT.Sockets} which is part of the GNAT
run time, and is therefore licensed under the modified GPL.

@node Pragma Link_With,Pragma Linker_Alias,Pragma License,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-link-with}@anchor{97}
@section Pragma Link_With


Syntax:

@example
pragma Link_With (static_string_EXPRESSION @{,static_string_EXPRESSION@});
@end example

This pragma is provided for compatibility with certain Ada 83 compilers.
It has exactly the same effect as pragma @code{Linker_Options} except
that spaces occurring within one of the string expressions are treated
as separators. For example, in the following case:

@example
pragma Link_With ("-labc -ldef");
@end example

results in passing the strings @code{-labc} and @code{-ldef} as two
separate arguments to the linker. In addition pragma Link_With allows
multiple arguments, with the same effect as successive pragmas.

@node Pragma Linker_Alias,Pragma Linker_Constructor,Pragma Link_With,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-alias}@anchor{98}
@section Pragma Linker_Alias


Syntax:

@example
pragma Linker_Alias (
  [Entity =>] LOCAL_NAME,
  [Target =>] static_string_EXPRESSION);
@end example

@code{LOCAL_NAME} must refer to an object that is declared at the library
level. This pragma establishes the given entity as a linker alias for the
given target. It is equivalent to @code{__attribute__((alias))} in GNU C
and causes @code{LOCAL_NAME} to be emitted as an alias for the symbol
@code{static_string_EXPRESSION} in the object file, that is to say no space
is reserved for @code{LOCAL_NAME} by the assembler and it will be resolved
to the same address as @code{static_string_EXPRESSION} by the linker.

The actual linker name for the target must be used (e.g., the fully
encoded name with qualification in Ada, or the mangled name in C++),
or it must be declared using the C convention with @code{pragma Import}
or @code{pragma Export}.

Not all target machines support this pragma. On some of them it is accepted
only if @code{pragma Weak_External} has been applied to @code{LOCAL_NAME}.

@example
--  Example of the use of pragma Linker_Alias

package p is
  i : Integer := 1;
  pragma Export (C, i);

  new_name_for_i : Integer;
  pragma Linker_Alias (new_name_for_i, "i");
end p;
@end example

@node Pragma Linker_Constructor,Pragma Linker_Destructor,Pragma Linker_Alias,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-constructor}@anchor{99}
@section Pragma Linker_Constructor


Syntax:

@example
pragma Linker_Constructor (procedure_LOCAL_NAME);
@end example

@code{procedure_LOCAL_NAME} must refer to a parameterless procedure that
is declared at the library level. A procedure to which this pragma is
applied will be treated as an initialization routine by the linker.
It is equivalent to @code{__attribute__((constructor))} in GNU C and
causes @code{procedure_LOCAL_NAME} to be invoked before the entry point
of the executable is called (or immediately after the shared library is
loaded if the procedure is linked in a shared library), in particular
before the Ada run-time environment is set up.

Because of these specific contexts, the set of operations such a procedure
can perform is very limited and the type of objects it can manipulate is
essentially restricted to the elementary types. In particular, it must only
contain code to which pragma Restrictions (No_Elaboration_Code) applies.

This pragma is used by GNAT to implement auto-initialization of shared Stand
Alone Libraries, which provides a related capability without the restrictions
listed above. Where possible, the use of Stand Alone Libraries is preferable
to the use of this pragma.

@node Pragma Linker_Destructor,Pragma Linker_Section,Pragma Linker_Constructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-destructor}@anchor{9a}
@section Pragma Linker_Destructor


Syntax:

@example
pragma Linker_Destructor (procedure_LOCAL_NAME);
@end example

@code{procedure_LOCAL_NAME} must refer to a parameterless procedure that
is declared at the library level. A procedure to which this pragma is
applied will be treated as a finalization routine by the linker.
It is equivalent to @code{__attribute__((destructor))} in GNU C and
causes @code{procedure_LOCAL_NAME} to be invoked after the entry point
of the executable has exited (or immediately before the shared library
is unloaded if the procedure is linked in a shared library), in particular
after the Ada run-time environment is shut down.

See @code{pragma Linker_Constructor} for the set of restrictions that apply
because of these specific contexts.

@node Pragma Linker_Section,Pragma Lock_Free,Pragma Linker_Destructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id22}@anchor{9b}@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-section}@anchor{9c}
@section Pragma Linker_Section


Syntax:

@example
pragma Linker_Section (
  [Entity  =>] LOCAL_NAME,
  [Section =>] static_string_EXPRESSION);
@end example

@code{LOCAL_NAME} must refer to an object, type, or subprogram that is
declared at the library level. This pragma specifies the name of the
linker section for the given entity. It is equivalent to
@code{__attribute__((section))} in GNU C and causes @code{LOCAL_NAME} to
be placed in the @code{static_string_EXPRESSION} section of the
executable (assuming the linker doesn’t rename the section).
GNAT also provides an implementation defined aspect of the same name.

In the case of specifying this aspect for a type, the effect is to
specify the corresponding section for all library-level objects of
the type that do not have an explicit linker section set. Note that
this only applies to whole objects, not to components of composite objects.

In the case of a subprogram, the linker section applies to all previously
declared matching overloaded subprograms in the current declarative part
which do not already have a linker section assigned. The linker section
aspect is useful in this case for specifying different linker sections
for different elements of such an overloaded set.

Note that an empty string specifies that no linker section is specified.
This is not quite the same as omitting the pragma or aspect, since it
can be used to specify that one element of an overloaded set of subprograms
has the default linker section, or that one object of a type for which a
linker section is specified should has the default linker section.

The compiler normally places library-level entities in standard sections
depending on the class: procedures and functions generally go in the
@code{.text} section, initialized variables in the @code{.data} section
and uninitialized variables in the @code{.bss} section.

Other, special sections may exist on given target machines to map special
hardware, for example I/O ports or flash memory. This pragma is a means to
defer the final layout of the executable to the linker, thus fully working
at the symbolic level with the compiler.

Some file formats do not support arbitrary sections so not all target
machines support this pragma. The use of this pragma may cause a program
execution to be erroneous if it is used to place an entity into an
inappropriate section (e.g., a modified variable into the @code{.text}
section). See also @code{pragma Persistent_BSS}.

@example
--  Example of the use of pragma Linker_Section

package IO_Card is
  Port_A : Integer;
  pragma Volatile (Port_A);
  pragma Linker_Section (Port_A, ".bss.port_a");

  Port_B : Integer;
  pragma Volatile (Port_B);
  pragma Linker_Section (Port_B, ".bss.port_b");

  type Port_Type is new Integer with Linker_Section => ".bss";
  PA : Port_Type with Linker_Section => ".bss.PA";
  PB : Port_Type; --  ends up in linker section ".bss"

  procedure Q with Linker_Section => "Qsection";
end IO_Card;
@end example

@node Pragma Lock_Free,Pragma Loop_Invariant,Pragma Linker_Section,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id23}@anchor{9d}@anchor{gnat_rm/implementation_defined_pragmas pragma-lock-free}@anchor{9e}
@section Pragma Lock_Free


Syntax:

@example
pragma Lock_Free [ (static_boolean_EXPRESSION) ];
@end example

This pragma may be specified for protected types or objects. It specifies that
the implementation of protected operations must be implemented without locks.
Compilation fails if the compiler cannot generate lock-free code for the
operations.

The current conditions required to support this pragma are:


@itemize *

@item 
Protected type declarations may not contain entries

@item 
Protected subprogram declarations may not have nonelementary parameters
@end itemize

In addition, each protected subprogram body must satisfy:


@itemize *

@item 
May reference only one protected component

@item 
May not reference nonconstant entities outside the protected subprogram
scope

@item 
May not contain address representation items, allocators, or quantified
expressions

@item 
May not contain delay, goto, loop, or procedure-call statements

@item 
May not contain exported and imported entities

@item 
May not dereferenced access values

@item 
Function calls and attribute references must be static
@end itemize

If the Lock_Free aspect is specified to be True for a protected unit
and the Ceiling_Locking locking policy is in effect, then the run-time
actions associated with the Ceiling_Locking locking policy (described in
Ada RM D.3) are not performed when a protected operation of the protected
unit is executed.

@node Pragma Loop_Invariant,Pragma Loop_Optimize,Pragma Lock_Free,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-loop-invariant}@anchor{9f}
@section Pragma Loop_Invariant


Syntax:

@example
pragma Loop_Invariant ( boolean_EXPRESSION );
@end example

The effect of this pragma is similar to that of pragma @code{Assert},
except that in an @code{Assertion_Policy} pragma, the identifier
@code{Loop_Invariant} is used to control whether it is ignored or checked
(or disabled).

@code{Loop_Invariant} can only appear as one of the items in the sequence
of statements of a loop body, or nested inside block statements that
appear in the sequence of statements of a loop body.
The intention is that it be used to
represent a “loop invariant” assertion, i.e. something that is true each
time through the loop, and which can be used to show that the loop is
achieving its purpose.

Multiple @code{Loop_Invariant} and @code{Loop_Variant} pragmas that
apply to the same loop should be grouped in the same sequence of
statements.

To aid in writing such invariants, the special attribute @code{Loop_Entry}
may be used to refer to the value of an expression on entry to the loop. This
attribute can only be used within the expression of a @code{Loop_Invariant}
pragma. For full details, see documentation of attribute @code{Loop_Entry}.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Loop_Optimize,Pragma Loop_Variant,Pragma Loop_Invariant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-loop-optimize}@anchor{a0}
@section Pragma Loop_Optimize


Syntax:

@example
pragma Loop_Optimize (OPTIMIZATION_HINT @{, OPTIMIZATION_HINT@});

OPTIMIZATION_HINT ::= Ivdep | No_Unroll | Unroll | No_Vector | Vector
@end example

This pragma must appear immediately within a loop statement.  It allows the
programmer to specify optimization hints for the enclosing loop.  The hints
are not mutually exclusive and can be freely mixed, but not all combinations
will yield a sensible outcome.

There are five supported optimization hints for a loop:


@itemize *

@item 
Ivdep

The programmer asserts that there are no loop-carried dependencies
which would prevent consecutive iterations of the loop from being
executed simultaneously.

@item 
No_Unroll

The loop must not be unrolled.  This is a strong hint: the compiler will not
unroll a loop marked with this hint.

@item 
Unroll

The loop should be unrolled.  This is a weak hint: the compiler will try to
apply unrolling to this loop preferably to other optimizations, notably
vectorization, but there is no guarantee that the loop will be unrolled.

@item 
No_Vector

The loop must not be vectorized.  This is a strong hint: the compiler will not
vectorize a loop marked with this hint.

@item 
Vector

The loop should be vectorized.  This is a weak hint: the compiler will try to
apply vectorization to this loop preferably to other optimizations, notably
unrolling, but there is no guarantee that the loop will be vectorized.
@end itemize

These hints do not remove the need to pass the appropriate switches to the
compiler in order to enable the relevant optimizations, that is to say
`-funroll-loops' for unrolling and `-ftree-vectorize' for
vectorization.

@node Pragma Loop_Variant,Pragma Machine_Attribute,Pragma Loop_Optimize,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-loop-variant}@anchor{a1}
@section Pragma Loop_Variant


Syntax:

@example
pragma Loop_Variant ( LOOP_VARIANT_ITEM @{, LOOP_VARIANT_ITEM @} );
LOOP_VARIANT_ITEM ::= CHANGE_DIRECTION => discrete_EXPRESSION
CHANGE_DIRECTION ::= Increases | Decreases
@end example

@code{Loop_Variant} can only appear as one of the items in the sequence
of statements of a loop body, or nested inside block statements that
appear in the sequence of statements of a loop body.
It allows the specification of quantities which must always
decrease or increase in successive iterations of the loop. In its simplest
form, just one expression is specified, whose value must increase or decrease
on each iteration of the loop.

In a more complex form, multiple arguments can be given which are interpreted
in a nesting lexicographic manner. For example:

@example
pragma Loop_Variant (Increases => X, Decreases => Y);
@end example

specifies that each time through the loop either X increases, or X stays
the same and Y decreases. A @code{Loop_Variant} pragma ensures that the
loop is making progress. It can be useful in helping to show informally
or prove formally that the loop always terminates.

@code{Loop_Variant} is an assertion whose effect can be controlled using
an @code{Assertion_Policy} with a check name of @code{Loop_Variant}. The
policy can be @code{Check} to enable the loop variant check, @code{Ignore}
to ignore the check (in which case the pragma has no effect on the program),
or @code{Disable} in which case the pragma is not even checked for correct
syntax.

Multiple @code{Loop_Invariant} and @code{Loop_Variant} pragmas that
apply to the same loop should be grouped in the same sequence of
statements.

The @code{Loop_Entry} attribute may be used within the expressions of the
@code{Loop_Variant} pragma to refer to values on entry to the loop.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Machine_Attribute,Pragma Main,Pragma Loop_Variant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-machine-attribute}@anchor{a2}
@section Pragma Machine_Attribute


Syntax:

@example
pragma Machine_Attribute (
     [Entity         =>] LOCAL_NAME,
     [Attribute_Name =>] static_string_EXPRESSION
  [, [Info           =>] static_EXPRESSION @{, static_EXPRESSION@}] );
@end example

Machine-dependent attributes can be specified for types and/or
declarations.  This pragma is semantically equivalent to
@code{__attribute__((@var{attribute_name}))} (if @code{info} is not
specified) or @code{__attribute__((@var{attribute_name(info})))}
or @code{__attribute__((@var{attribute_name(info,...})))} in GNU C,
where `attribute_name' is recognized by the compiler middle-end
or the @code{TARGET_ATTRIBUTE_TABLE} machine specific macro.  Note
that a string literal for the optional parameter @code{info} or the
following ones is transformed by default into an identifier,
which may make this pragma unusable for some attributes.
For further information see @cite{GNU Compiler Collection (GCC) Internals}.

@node Pragma Main,Pragma Main_Storage,Pragma Machine_Attribute,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-main}@anchor{a3}
@section Pragma Main


Syntax:

@example
pragma Main
 (MAIN_OPTION [, MAIN_OPTION]);

MAIN_OPTION ::=
  [Stack_Size              =>] static_integer_EXPRESSION
| [Task_Stack_Size_Default =>] static_integer_EXPRESSION
| [Time_Slicing_Enabled    =>] static_boolean_EXPRESSION
@end example

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has
no effect in GNAT, other than being syntax checked.

@node Pragma Main_Storage,Pragma Max_Queue_Length,Pragma Main,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-main-storage}@anchor{a4}
@section Pragma Main_Storage


Syntax:

@example
pragma Main_Storage
  (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);

MAIN_STORAGE_OPTION ::=
  [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
| [TOP_GUARD       =>] static_SIMPLE_EXPRESSION
@end example

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has
no effect in GNAT, other than being syntax checked.

@node Pragma Max_Queue_Length,Pragma No_Body,Pragma Main_Storage,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id24}@anchor{a5}@anchor{gnat_rm/implementation_defined_pragmas pragma-max-queue-length}@anchor{a6}
@section Pragma Max_Queue_Length


Syntax:

@example
pragma Max_Queue_Length (static_integer_EXPRESSION);
@end example

This pragma is used to specify the maximum callers per entry queue for
individual protected entries and entry families. It accepts a single
integer (-1 or more) as a parameter and must appear after the declaration of an
entry.

A value of -1 represents no additional restriction on queue length.

@node Pragma No_Body,Pragma No_Caching,Pragma Max_Queue_Length,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-body}@anchor{a7}
@section Pragma No_Body


Syntax:

@example
pragma No_Body;
@end example

There are a number of cases in which a package spec does not require a body,
and in fact a body is not permitted. GNAT will not permit the spec to be
compiled if there is a body around. The pragma No_Body allows you to provide
a body file, even in a case where no body is allowed. The body file must
contain only comments and a single No_Body pragma. This is recognized by
the compiler as indicating that no body is logically present.

This is particularly useful during maintenance when a package is modified in
such a way that a body needed before is no longer needed. The provision of a
dummy body with a No_Body pragma ensures that there is no interference from
earlier versions of the package body.

@node Pragma No_Caching,Pragma No_Component_Reordering,Pragma No_Body,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id25}@anchor{a8}@anchor{gnat_rm/implementation_defined_pragmas pragma-no-caching}@anchor{a9}
@section Pragma No_Caching


Syntax:

@example
pragma No_Caching [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{No_Caching} in
the SPARK 2014 Reference Manual, section 7.1.2.

@node Pragma No_Component_Reordering,Pragma No_Elaboration_Code_All,Pragma No_Caching,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-component-reordering}@anchor{aa}
@section Pragma No_Component_Reordering


Syntax:

@example
pragma No_Component_Reordering [([Entity =>] type_LOCAL_NAME)];
@end example

@code{type_LOCAL_NAME} must refer to a record type declaration in the current
declarative part. The effect is to preclude any reordering of components
for the layout of the record, i.e. the record is laid out by the compiler
in the order in which the components are declared textually. The form with
no argument is a configuration pragma which applies to all record types
declared in units to which the pragma applies and there is a requirement
that this pragma be used consistently within a partition.

@node Pragma No_Elaboration_Code_All,Pragma No_Heap_Finalization,Pragma No_Component_Reordering,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id26}@anchor{ab}@anchor{gnat_rm/implementation_defined_pragmas pragma-no-elaboration-code-all}@anchor{ac}
@section Pragma No_Elaboration_Code_All


Syntax:

@example
pragma No_Elaboration_Code_All [(program_unit_NAME)];
@end example

This is a program unit pragma (there is also an equivalent aspect of the
same name) that establishes the restriction @code{No_Elaboration_Code} for
the current unit and any extended main source units (body and subunits).
It also has the effect of enforcing a transitive application of this
aspect, so that if any unit is implicitly or explicitly with’ed by the
current unit, it must also have the @cite{No_Elaboration_Code_All} aspect set.
It may be applied to package or subprogram specs or their generic versions.

@node Pragma No_Heap_Finalization,Pragma No_Inline,Pragma No_Elaboration_Code_All,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-heap-finalization}@anchor{ad}
@section Pragma No_Heap_Finalization


Syntax:

@example
pragma No_Heap_Finalization [ (first_subtype_LOCAL_NAME) ];
@end example

Pragma @code{No_Heap_Finalization} may be used as a configuration pragma or as a
type-specific pragma.

In its configuration form, the pragma must appear within a configuration file
such as gnat.adc, without an argument. The pragma suppresses the call to
@code{Finalize} for heap-allocated objects created through library-level named
access-to-object types in cases where the designated type requires finalization
actions.

In its type-specific form, the argument of the pragma must denote a
library-level named access-to-object type. The pragma suppresses the call to
@code{Finalize} for heap-allocated objects created through the specific access type
in cases where the designated type requires finalization actions.

It is still possible to finalize such heap-allocated objects by explicitly
deallocating them.

A library-level named access-to-object type declared within a generic unit will
lose its @code{No_Heap_Finalization} pragma when the corresponding instance does not
appear at the library level.

@node Pragma No_Inline,Pragma No_Raise,Pragma No_Heap_Finalization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id27}@anchor{ae}@anchor{gnat_rm/implementation_defined_pragmas pragma-no-inline}@anchor{af}
@section Pragma No_Inline


Syntax:

@example
pragma No_Inline (NAME @{, NAME@});
@end example

This pragma suppresses inlining for the callable entity or the instances of
the generic subprogram designated by @code{NAME}, including inlining that
results from the use of pragma @code{Inline}.  This pragma is always active,
in particular it is not subject to the use of option `-gnatn' or
`-gnatN'.  It is illegal to specify both pragma @code{No_Inline} and
pragma @code{Inline_Always} for the same @code{NAME}.

@node Pragma No_Raise,Pragma No_Return,Pragma No_Inline,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id28}@anchor{b0}@anchor{gnat_rm/implementation_defined_pragmas pragma-no-raise}@anchor{b1}
@section Pragma No_Raise


Syntax:

@example
pragma No_Raise (subprogram_LOCAL_NAME @{, subprogram_LOCAL_NAME@});
@end example

Each @code{subprogram_LOCAL_NAME} argument must refer to one or more subprogram
declarations in the current declarative part.  A subprogram to which this
pragma is applied may not raise an exception that is not caught within it.
An implementation-defined check named @cite{Raise_Check} is associated with the
pragma, and @cite{Program_Error} is raised upon its failure (see RM 11.5(19/5)).

@node Pragma No_Return,Pragma No_Strict_Aliasing,Pragma No_Raise,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-return}@anchor{b2}
@section Pragma No_Return


Syntax:

@example
pragma No_Return (procedure_LOCAL_NAME @{, procedure_LOCAL_NAME@});
@end example

Each @code{procedure_LOCAL_NAME} argument must refer to one or more procedure
declarations in the current declarative part.  A procedure to which this
pragma is applied may not contain any explicit @code{return} statements.
In addition, if the procedure contains any implicit returns from falling
off the end of a statement sequence, then execution of that implicit
return will cause Program_Error to be raised.

One use of this pragma is to identify procedures whose only purpose is to raise
an exception. Another use of this pragma is to suppress incorrect warnings
about missing returns in functions, where the last statement of a function
statement sequence is a call to such a procedure.

Note that in Ada 2005 mode, this pragma is part of the language. It is
available in all earlier versions of Ada as an implementation-defined
pragma.

@node Pragma No_Strict_Aliasing,Pragma No_Tagged_Streams,Pragma No_Return,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-strict-aliasing}@anchor{b3}
@section Pragma No_Strict_Aliasing


Syntax:

@example
pragma No_Strict_Aliasing [([Entity =>] type_LOCAL_NAME)];
@end example

@code{type_LOCAL_NAME} must refer to an access type
declaration in the current declarative part.  The effect is to inhibit
strict aliasing optimization for the given type.  The form with no
arguments is a configuration pragma which applies to all access types
declared in units to which the pragma applies. For a detailed
description of the strict aliasing optimization, and the situations
in which it must be suppressed, see the section on Optimization and Strict Aliasing
in the @cite{GNAT User’s Guide}.

This pragma currently has no effects on access to unconstrained array types.

@node Pragma No_Tagged_Streams,Pragma Normalize_Scalars,Pragma No_Strict_Aliasing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id29}@anchor{b4}@anchor{gnat_rm/implementation_defined_pragmas pragma-no-tagged-streams}@anchor{b5}
@section Pragma No_Tagged_Streams


Syntax:

@example
pragma No_Tagged_Streams [([Entity =>] tagged_type_LOCAL_NAME)];
@end example

Normally when a tagged type is introduced using a full type declaration,
part of the processing includes generating stream access routines to be
used by stream attributes referencing the type (or one of its subtypes
or derived types). This can involve the generation of significant amounts
of code which is wasted space if stream routines are not needed for the
type in question.

The @code{No_Tagged_Streams} pragma causes the generation of these stream
routines to be skipped, and any attempt to use stream operations on
types subject to this pragma will be statically rejected as illegal.

There are two forms of the pragma. The form with no arguments must appear
in a declarative sequence or in the declarations of a package spec. This
pragma affects all subsequent root tagged types declared in the declaration
sequence, and specifies that no stream routines be generated. The form with
an argument (for which there is also a corresponding aspect) specifies a
single root tagged type for which stream routines are not to be generated.

Once the pragma has been given for a particular root tagged type, all subtypes
and derived types of this type inherit the pragma automatically, so the effect
applies to a complete hierarchy (this is necessary to deal with the class-wide
dispatching versions of the stream routines).

When pragmas @code{Discard_Names} and @code{No_Tagged_Streams} are simultaneously
applied to a tagged type its Expanded_Name and External_Tag are initialized
with empty strings. This is useful to avoid exposing entity names at binary
level but has a negative impact on the debuggability of tagged types.

@node Pragma Normalize_Scalars,Pragma Obsolescent,Pragma No_Tagged_Streams,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-normalize-scalars}@anchor{b6}
@section Pragma Normalize_Scalars


Syntax:

@example
pragma Normalize_Scalars;
@end example

This is a language defined pragma which is fully implemented in GNAT.  The
effect is to cause all scalar objects that are not otherwise initialized
to be initialized.  The initial values are implementation dependent and
are as follows:


@table @asis

@item `Standard.Character'

Objects whose root type is Standard.Character are initialized to
Character’Last unless the subtype range excludes NUL (in which case
NUL is used). This choice will always generate an invalid value if
one exists.

@item `Standard.Wide_Character'

Objects whose root type is Standard.Wide_Character are initialized to
Wide_Character’Last unless the subtype range excludes NUL (in which case
NUL is used). This choice will always generate an invalid value if
one exists.

@item `Standard.Wide_Wide_Character'

Objects whose root type is Standard.Wide_Wide_Character are initialized to
the invalid value 16#FFFF_FFFF# unless the subtype range excludes NUL (in
which case NUL is used). This choice will always generate an invalid value if
one exists.

@item `Integer types'

Objects of an integer type are treated differently depending on whether
negative values are present in the subtype. If no negative values are
present, then all one bits is used as the initial value except in the
special case where zero is excluded from the subtype, in which case
all zero bits are used. This choice will always generate an invalid
value if one exists.

For subtypes with negative values present, the largest negative number
is used, except in the unusual case where this largest negative number
is in the subtype, and the largest positive number is not, in which case
the largest positive value is used. This choice will always generate
an invalid value if one exists.

@item `Floating-Point Types'

Objects of all floating-point types are initialized to all 1-bits. For
standard IEEE format, this corresponds to a NaN (not a number) which is
indeed an invalid value.

@item `Fixed-Point Types'

Objects of all fixed-point types are treated as described above for integers,
with the rules applying to the underlying integer value used to represent
the fixed-point value.

@item `Modular types'

Objects of a modular type are initialized to all one bits, except in
the special case where zero is excluded from the subtype, in which
case all zero bits are used. This choice will always generate an
invalid value if one exists.

@item `Enumeration types'

Objects of an enumeration type are initialized to all one-bits, i.e., to
the value @code{2 ** typ'Size - 1} unless the subtype excludes the literal
whose Pos value is zero, in which case a code of zero is used. This choice
will always generate an invalid value if one exists.
@end table

@node Pragma Obsolescent,Pragma Optimize_Alignment,Pragma Normalize_Scalars,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id30}@anchor{b7}@anchor{gnat_rm/implementation_defined_pragmas pragma-obsolescent}@anchor{b8}
@section Pragma Obsolescent


Syntax:

@example
pragma Obsolescent;

pragma Obsolescent (
  [Message =>] static_string_EXPRESSION
[,[Version =>] Ada_05]);

pragma Obsolescent (
  [Entity  =>] NAME
[,[Message =>] static_string_EXPRESSION
[,[Version =>] Ada_05]]);
@end example

This pragma can occur immediately following a declaration of an entity,
including the case of a record component. If no Entity argument is present,
then this declaration is the one to which the pragma applies. If an Entity
parameter is present, it must either match the name of the entity in this
declaration, or alternatively, the pragma can immediately follow an enumeration
type declaration, where the Entity argument names one of the enumeration
literals.

This pragma is used to indicate that the named entity
is considered obsolescent and should not be used. Typically this is
used when an API must be modified by eventually removing or modifying
existing subprograms or other entities. The pragma can be used at an
intermediate stage when the entity is still present, but will be
removed later.

The effect of this pragma is to output a warning message on a reference to
an entity thus marked that the subprogram is obsolescent if the appropriate
warning option in the compiler is activated. If the @code{Message} parameter is
present, then a second warning message is given containing this text. In
addition, a reference to the entity is considered to be a violation of pragma
@code{Restrictions (No_Obsolescent_Features)}.

This pragma can also be used as a program unit pragma for a package,
in which case the entity name is the name of the package, and the
pragma indicates that the entire package is considered
obsolescent. In this case a client @code{with}ing such a package
violates the restriction, and the @code{with} clause is
flagged with warnings if the warning option is set.

If the @code{Version} parameter is present (which must be exactly
the identifier @code{Ada_05}, no other argument is allowed), then the
indication of obsolescence applies only when compiling in Ada 2005
mode. This is primarily intended for dealing with the situations
in the predefined library where subprograms or packages
have become defined as obsolescent in Ada 2005
(e.g., in @code{Ada.Characters.Handling}), but may be used anywhere.

The following examples show typical uses of this pragma:

@example
package p is
   pragma Obsolescent (p, Message => "use pp instead of p");
end p;

package q is
   procedure q2;
   pragma Obsolescent ("use q2new instead");

   type R is new integer;
   pragma Obsolescent
     (Entity  => R,
      Message => "use RR in Ada 2005",
      Version => Ada_05);

   type M is record
      F1 : Integer;
      F2 : Integer;
      pragma Obsolescent;
      F3 : Integer;
   end record;

   type E is (a, bc, 'd', quack);
   pragma Obsolescent (Entity => bc)
   pragma Obsolescent (Entity => 'd')

   function "+"
     (a, b : character) return character;
   pragma Obsolescent (Entity => "+");
end;
@end example

Note that, as for all pragmas, if you use a pragma argument identifier,
then all subsequent parameters must also use a pragma argument identifier.
So if you specify @code{Entity =>} for the @code{Entity} argument, and a @code{Message}
argument is present, it must be preceded by @code{Message =>}.

@node Pragma Optimize_Alignment,Pragma Ordered,Pragma Obsolescent,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-optimize-alignment}@anchor{b9}
@section Pragma Optimize_Alignment


@geindex Alignment
@geindex default settings

Syntax:

@example
pragma Optimize_Alignment (TIME | SPACE | OFF);
@end example

This is a configuration pragma which affects the choice of default alignments
for types and objects where no alignment is explicitly specified. There is a
time/space trade-off in the selection of these values. Large alignments result
in more efficient code, at the expense of larger data space, since sizes have
to be increased to match these alignments. Smaller alignments save space, but
the access code is slower. The normal choice of default alignments for types
and individual alignment promotions for objects (which is what you get if you
do not use this pragma, or if you use an argument of OFF), tries to balance
these two requirements.

Specifying SPACE causes smaller default alignments to be chosen in two cases.
First any packed record is given an alignment of 1. Second, if a size is given
for the type, then the alignment is chosen to avoid increasing this size. For
example, consider:

@example
type R is record
   X : Integer;
   Y : Character;
end record;

for R'Size use 5*8;
@end example

In the default mode, this type gets an alignment of 4, so that access to the
Integer field X are efficient. But this means that objects of the type end up
with a size of 8 bytes. This is a valid choice, since sizes of objects are
allowed to be bigger than the size of the type, but it can waste space if for
example fields of type R appear in an enclosing record. If the above type is
compiled in @code{Optimize_Alignment (Space)} mode, the alignment is set to 1.

However, there is one case in which SPACE is ignored. If a variable length
record (that is a discriminated record with a component which is an array
whose length depends on a discriminant), has a pragma Pack, then it is not
in general possible to set the alignment of such a record to one, so the
pragma is ignored in this case (with a warning).

Specifying SPACE also disables alignment promotions for standalone objects,
which occur when the compiler increases the alignment of a specific object
without changing the alignment of its type.

Specifying SPACE also disables component reordering in unpacked record types,
which can result in larger sizes in order to meet alignment requirements.

Specifying TIME causes larger default alignments to be chosen in the case of
small types with sizes that are not a power of 2. For example, consider:

@example
type R is record
   A : Character;
   B : Character;
   C : Boolean;
end record;

pragma Pack (R);
for R'Size use 17;
@end example

The default alignment for this record is normally 1, but if this type is
compiled in @code{Optimize_Alignment (Time)} mode, then the alignment is set
to 4, which wastes space for objects of the type, since they are now 4 bytes
long, but results in more efficient access when the whole record is referenced.

As noted above, this is a configuration pragma, and there is a requirement
that all units in a partition be compiled with a consistent setting of the
optimization setting. This would normally be achieved by use of a configuration
pragma file containing the appropriate setting. The exception to this rule is
that units with an explicit configuration pragma in the same file as the source
unit are excluded from the consistency check, as are all predefined units. The
latter are compiled by default in pragma Optimize_Alignment (Off) mode if no
pragma appears at the start of the file.

@node Pragma Ordered,Pragma Overflow_Mode,Pragma Optimize_Alignment,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ordered}@anchor{ba}
@section Pragma Ordered


Syntax:

@example
pragma Ordered (enumeration_first_subtype_LOCAL_NAME);
@end example

Most enumeration types are from a conceptual point of view unordered.
For example, consider:

@example
type Color is (Red, Blue, Green, Yellow);
@end example

By Ada semantics @code{Blue > Red} and @code{Green > Blue},
but really these relations make no sense; the enumeration type merely
specifies a set of possible colors, and the order is unimportant.

For unordered enumeration types, it is generally a good idea if
clients avoid comparisons (other than equality or inequality) and
explicit ranges. (A `client' is a unit where the type is referenced,
other than the unit where the type is declared, its body, and its subunits.)
For example, if code buried in some client says:

@example
if Current_Color < Yellow then ...
if Current_Color in Blue .. Green then ...
@end example

then the client code is relying on the order, which is undesirable.
It makes the code hard to read and creates maintenance difficulties if
entries have to be added to the enumeration type. Instead,
the code in the client should list the possibilities, or an
appropriate subtype should be declared in the unit that declares
the original enumeration type. E.g., the following subtype could
be declared along with the type @code{Color}:

@example
subtype RBG is Color range Red .. Green;
@end example

and then the client could write:

@example
if Current_Color in RBG then ...
if Current_Color = Blue or Current_Color = Green then ...
@end example

However, some enumeration types are legitimately ordered from a conceptual
point of view. For example, if you declare:

@example
type Day is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
@end example

then the ordering imposed by the language is reasonable, and
clients can depend on it, writing for example:

@example
if D in Mon .. Fri then ...
if D < Wed then ...
@end example

The pragma `Ordered' is provided to mark enumeration types that
are conceptually ordered, alerting the reader that clients may depend
on the ordering. GNAT provides a pragma to mark enumerations as ordered
rather than one to mark them as unordered, since in our experience,
the great majority of enumeration types are conceptually unordered.

The types @code{Boolean}, @code{Character}, @code{Wide_Character},
and @code{Wide_Wide_Character}
are considered to be ordered types, so each is declared with a
pragma @code{Ordered} in package @code{Standard}.

Normally pragma @code{Ordered} serves only as documentation and a guide for
coding standards, but GNAT provides a warning switch `-gnatw.u' that
requests warnings for inappropriate uses (comparisons and explicit
subranges) for unordered types. If this switch is used, then any
enumeration type not marked with pragma @code{Ordered} will be considered
as unordered, and will generate warnings for inappropriate uses.

Note that generic types are not considered ordered or unordered (since the
template can be instantiated for both cases), so we never generate warnings
for the case of generic enumerated types.

For additional information please refer to the description of the
`-gnatw.u' switch in the GNAT User’s Guide.

@node Pragma Overflow_Mode,Pragma Overriding_Renamings,Pragma Ordered,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-overflow-mode}@anchor{bb}
@section Pragma Overflow_Mode


Syntax:

@example
pragma Overflow_Mode
 (  [General    =>] MODE
  [,[Assertions =>] MODE]);

MODE ::= STRICT | MINIMIZED | ELIMINATED
@end example

This pragma sets the current overflow mode to the given setting. For details
of the meaning of these modes, please refer to the
‘Overflow Check Handling in GNAT’ appendix in the
GNAT User’s Guide. If only the @code{General} parameter is present,
the given mode applies to all expressions. If both parameters are present,
the @code{General} mode applies to expressions outside assertions, and
the @code{Eliminated} mode applies to expressions within assertions.

The case of the @code{MODE} parameter is ignored,
so @code{MINIMIZED}, @code{Minimized} and
@code{minimized} all have the same effect.

The @code{Overflow_Mode} pragma has the same scoping and placement
rules as pragma @code{Suppress}, so it can occur either as a
configuration pragma, specifying a default for the whole
program, or in a declarative scope, where it applies to the
remaining declarations and statements in that scope.

The pragma @code{Suppress (Overflow_Check)} suppresses
overflow checking, but does not affect the overflow mode.

The pragma @code{Unsuppress (Overflow_Check)} unsuppresses (enables)
overflow checking, but does not affect the overflow mode.

@node Pragma Overriding_Renamings,Pragma Part_Of,Pragma Overflow_Mode,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-overriding-renamings}@anchor{bc}
@section Pragma Overriding_Renamings


@geindex Rational profile

@geindex Rational compatibility

Syntax:

@example
pragma Overriding_Renamings;
@end example

This is a GNAT configuration pragma to simplify porting
legacy code accepted by the Rational
Ada compiler. In the presence of this pragma, a renaming declaration that
renames an inherited operation declared in the same scope is legal if selected
notation is used as in:

@example
pragma Overriding_Renamings;
...
package R is
  function F (..);
  ...
  function F (..) renames R.F;
end R;
@end example

even though
RM 8.3 (15) stipulates that an overridden operation is not visible within the
declaration of the overriding operation.

@node Pragma Part_Of,Pragma Partition_Elaboration_Policy,Pragma Overriding_Renamings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id31}@anchor{bd}@anchor{gnat_rm/implementation_defined_pragmas pragma-part-of}@anchor{be}
@section Pragma Part_Of


Syntax:

@example
pragma Part_Of (ABSTRACT_STATE);

ABSTRACT_STATE ::= NAME
@end example

For the semantics of this pragma, see the entry for aspect @code{Part_Of} in the
SPARK 2014 Reference Manual, section 7.2.6.

@node Pragma Partition_Elaboration_Policy,Pragma Passive,Pragma Part_Of,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-partition-elaboration-policy}@anchor{bf}
@section Pragma Partition_Elaboration_Policy


Syntax:

@example
pragma Partition_Elaboration_Policy (POLICY_IDENTIFIER);

POLICY_IDENTIFIER ::= Concurrent | Sequential
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Passive,Pragma Persistent_BSS,Pragma Partition_Elaboration_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-passive}@anchor{c0}
@section Pragma Passive


Syntax:

@example
pragma Passive [(Semaphore | No)];
@end example

Syntax checked, but otherwise ignored by GNAT.  This is recognized for
compatibility with DEC Ada 83 implementations, where it is used within a
task definition to request that a task be made passive.  If the argument
@code{Semaphore} is present, or the argument is omitted, then DEC Ada 83
treats the pragma as an assertion that the containing task is passive
and that optimization of context switch with this task is permitted and
desired.  If the argument @code{No} is present, the task must not be
optimized.  GNAT does not attempt to optimize any tasks in this manner
(since protected objects are available in place of passive tasks).

For more information on the subject of passive tasks, see the section
‘Passive Task Optimization’ in the GNAT Users Guide.

@node Pragma Persistent_BSS,Pragma Post,Pragma Passive,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id32}@anchor{c1}@anchor{gnat_rm/implementation_defined_pragmas pragma-persistent-bss}@anchor{c2}
@section Pragma Persistent_BSS


Syntax:

@example
pragma Persistent_BSS [(object_LOCAL_NAME)]
@end example

This pragma allows selected objects to be placed in the @code{.persistent_bss}
section. On some targets the linker and loader provide for special
treatment of this section, allowing a program to be reloaded without
affecting the contents of this data (hence the name persistent).

There are two forms of usage. If an argument is given, it must be the
local name of a library-level object, with no explicit initialization
and whose type is potentially persistent. If no argument is given, then
the pragma is a configuration pragma, and applies to all library-level
objects with no explicit initialization of potentially persistent types.

A potentially persistent type is a scalar type, or an untagged,
non-discriminated record, all of whose components have no explicit
initialization and are themselves of a potentially persistent type,
or an array, all of whose constraints are static, and whose component
type is potentially persistent.

If this pragma is used on a target where this feature is not supported,
then the pragma will be ignored. See also @code{pragma Linker_Section}.

@node Pragma Post,Pragma Postcondition,Pragma Persistent_BSS,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-post}@anchor{c3}
@section Pragma Post


@geindex Post

@geindex Checks
@geindex postconditions

Syntax:

@example
pragma Post (Boolean_Expression);
@end example

The @code{Post} pragma is intended to be an exact replacement for
the language-defined
@code{Post} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Postcondition,Pragma Post_Class,Pragma Post,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-postcondition}@anchor{c4}
@section Pragma Postcondition


@geindex Postcondition

@geindex Checks
@geindex postconditions

Syntax:

@example
pragma Postcondition (
   [Check   =>] Boolean_Expression
 [,[Message =>] String_Expression]);
@end example

The @code{Postcondition} pragma allows specification of automatic
postcondition checks for subprograms. These checks are similar to
assertions, but are automatically inserted just prior to the return
statements of the subprogram with which they are associated (including
implicit returns at the end of procedure bodies and associated
exception handlers).

In addition, the boolean expression which is the condition which
must be true may contain references to function’Result in the case
of a function to refer to the returned value.

@code{Postcondition} pragmas may appear either immediately following the
(separate) declaration of a subprogram, or at the start of the
declarations of a subprogram body. Only other pragmas may intervene
(that is appear between the subprogram declaration and its
postconditions, or appear before the postcondition in the
declaration sequence in a subprogram body). In the case of a
postcondition appearing after a subprogram declaration, the
formal arguments of the subprogram are visible, and can be
referenced in the postcondition expressions.

The postconditions are collected and automatically tested just
before any return (implicit or explicit) in the subprogram body.
A postcondition is only recognized if postconditions are active
at the time the pragma is encountered. The compiler switch `gnata'
turns on all postconditions by default, and pragma @code{Check_Policy}
with an identifier of @code{Postcondition} can also be used to
control whether postconditions are active.

The general approach is that postconditions are placed in the spec
if they represent functional aspects which make sense to the client.
For example we might have:

@example
function Direction return Integer;
pragma Postcondition
 (Direction'Result = +1
    or else
  Direction'Result = -1);
@end example

which serves to document that the result must be +1 or -1, and
will test that this is the case at run time if postcondition
checking is active.

Postconditions within the subprogram body can be used to
check that some internal aspect of the implementation,
not visible to the client, is operating as expected.
For instance if a square root routine keeps an internal
counter of the number of times it is called, then we
might have the following postcondition:

@example
Sqrt_Calls : Natural := 0;

function Sqrt (Arg : Float) return Float is
  pragma Postcondition
    (Sqrt_Calls = Sqrt_Calls'Old + 1);
  ...
end Sqrt
@end example

As this example, shows, the use of the @code{Old} attribute
is often useful in postconditions to refer to the state on
entry to the subprogram.

Note that postconditions are only checked on normal returns
from the subprogram. If an abnormal return results from
raising an exception, then the postconditions are not checked.

If a postcondition fails, then the exception
@code{System.Assertions.Assert_Failure} is raised. If
a message argument was supplied, then the given string
will be used as the exception message. If no message
argument was supplied, then the default message has
the form “Postcondition failed at file_name:line”. The
exception is raised in the context of the subprogram
body, so it is possible to catch postcondition failures
within the subprogram body itself.

Within a package spec, normal visibility rules
in Ada would prevent forward references within a
postcondition pragma to functions defined later in
the same package. This would introduce undesirable
ordering constraints. To avoid this problem, all
postcondition pragmas are analyzed at the end of
the package spec, allowing forward references.

The following example shows that this even allows
mutually recursive postconditions as in:

@example
package Parity_Functions is
   function Odd  (X : Natural) return Boolean;
   pragma Postcondition
     (Odd'Result =
        (x = 1
          or else
        (x /= 0 and then Even (X - 1))));

   function Even (X : Natural) return Boolean;
   pragma Postcondition
     (Even'Result =
        (x = 0
          or else
        (x /= 1 and then Odd (X - 1))));

end Parity_Functions;
@end example

There are no restrictions on the complexity or form of
conditions used within @code{Postcondition} pragmas.
The following example shows that it is even possible
to verify performance behavior.

@example
package Sort is

   Performance : constant Float;
   --  Performance constant set by implementation
   --  to match target architecture behavior.

   procedure Treesort (Arg : String);
   --  Sorts characters of argument using N*logN sort
   pragma Postcondition
     (Float (Clock - Clock'Old) <=
        Float (Arg'Length) *
        log (Float (Arg'Length)) *
        Performance);
end Sort;
@end example

Note: postcondition pragmas associated with subprograms that are
marked as Inline_Always, or those marked as Inline with front-end
inlining (-gnatN option set) are accepted and legality-checked
by the compiler, but are ignored at run-time even if postcondition
checking is enabled.

Note that pragma @code{Postcondition} differs from the language-defined
@code{Post} aspect (and corresponding @code{Post} pragma) in allowing
multiple occurrences, allowing occurrences in the body even if there
is a separate spec, and allowing a second string parameter, and the
use of the pragma identifier @code{Check}. Historically, pragma
@code{Postcondition} was implemented prior to the development of
Ada 2012, and has been retained in its original form for
compatibility purposes.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Post_Class,Pragma Pre,Pragma Postcondition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-post-class}@anchor{c5}
@section Pragma Post_Class


@geindex Post

@geindex Checks
@geindex postconditions

Syntax:

@example
pragma Post_Class (Boolean_Expression);
@end example

The @code{Post_Class} pragma is intended to be an exact replacement for
the language-defined
@code{Post'Class} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

Note: This pragma is called @code{Post_Class} rather than
@code{Post'Class} because the latter would not be strictly
conforming to the allowed syntax for pragmas. The motivation
for providing pragmas equivalent to the aspects is to allow a program
to be written using the pragmas, and then compiled if necessary
using an Ada compiler that does not recognize the pragmas or
aspects, but is prepared to ignore the pragmas. The assertion
policy that controls this pragma is @code{Post'Class}, not
@code{Post_Class}.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Pre,Pragma Precondition,Pragma Post_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-pre}@anchor{c6}
@section Pragma Pre


@geindex Pre

@geindex Checks
@geindex preconditions

Syntax:

@example
pragma Pre (Boolean_Expression);
@end example

The @code{Pre} pragma is intended to be an exact replacement for
the language-defined
@code{Pre} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Precondition,Pragma Predicate,Pragma Pre,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-precondition}@anchor{c7}
@section Pragma Precondition


@geindex Preconditions

@geindex Checks
@geindex preconditions

Syntax:

@example
pragma Precondition (
   [Check   =>] Boolean_Expression
 [,[Message =>] String_Expression]);
@end example

The @code{Precondition} pragma is similar to @code{Postcondition}
except that the corresponding checks take place immediately upon
entry to the subprogram, and if a precondition fails, the exception
is raised in the context of the caller, and the attribute ‘Result
cannot be used within the precondition expression.

Otherwise, the placement and visibility rules are identical to those
described for postconditions. The following is an example of use
within a package spec:

@example
package Math_Functions is
   ...
   function Sqrt (Arg : Float) return Float;
   pragma Precondition (Arg >= 0.0)
   ...
end Math_Functions;
@end example

@code{Precondition} pragmas may appear either immediately following the
(separate) declaration of a subprogram, or at the start of the
declarations of a subprogram body. Only other pragmas may intervene
(that is appear between the subprogram declaration and its
postconditions, or appear before the postcondition in the
declaration sequence in a subprogram body).

Note: precondition pragmas associated with subprograms that are
marked as Inline_Always, or those marked as Inline with front-end
inlining (-gnatN option set) are accepted and legality-checked
by the compiler, but are ignored at run-time even if precondition
checking is enabled.

Note that pragma @code{Precondition} differs from the language-defined
@code{Pre} aspect (and corresponding @code{Pre} pragma) in allowing
multiple occurrences, allowing occurrences in the body even if there
is a separate spec, and allowing a second string parameter, and the
use of the pragma identifier @code{Check}. Historically, pragma
@code{Precondition} was implemented prior to the development of
Ada 2012, and has been retained in its original form for
compatibility purposes.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Predicate,Pragma Predicate_Failure,Pragma Precondition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id33}@anchor{c8}@anchor{gnat_rm/implementation_defined_pragmas pragma-predicate}@anchor{c9}
@section Pragma Predicate


Syntax:

@example
pragma Predicate
  ([Entity =>] type_LOCAL_NAME,
   [Check  =>] EXPRESSION);
@end example

This pragma (available in all versions of Ada in GNAT) encompasses both
the @code{Static_Predicate} and @code{Dynamic_Predicate} aspects in
Ada 2012. A predicate is regarded as static if it has an allowed form
for @code{Static_Predicate} and is otherwise treated as a
@code{Dynamic_Predicate}. Otherwise, predicates specified by this
pragma behave exactly as described in the Ada 2012 reference manual.
For example, if we have

@example
type R is range 1 .. 10;
subtype S is R;
pragma Predicate (Entity => S, Check => S not in 4 .. 6);
subtype Q is R
pragma Predicate (Entity => Q, Check => F(Q) or G(Q));
@end example

the effect is identical to the following Ada 2012 code:

@example
type R is range 1 .. 10;
subtype S is R with
  Static_Predicate => S not in 4 .. 6;
subtype Q is R with
  Dynamic_Predicate => F(Q) or G(Q);
@end example

Note that there are no pragmas @code{Dynamic_Predicate}
or @code{Static_Predicate}. That is
because these pragmas would affect legality and semantics of
the program and thus do not have a neutral effect if ignored.
The motivation behind providing pragmas equivalent to
corresponding aspects is to allow a program to be written
using the pragmas, and then compiled with a compiler that
will ignore the pragmas. That doesn’t work in the case of
static and dynamic predicates, since if the corresponding
pragmas are ignored, then the behavior of the program is
fundamentally changed (for example a membership test
@code{A in B} would not take into account a predicate
defined for subtype B). When following this approach, the
use of predicates should be avoided.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Predicate_Failure,Pragma Preelaborable_Initialization,Pragma Predicate,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-predicate-failure}@anchor{ca}
@section Pragma Predicate_Failure


Syntax:

@example
pragma Predicate_Failure
  ([Entity  =>] type_LOCAL_NAME,
   [Message =>] String_Expression);
@end example

The @code{Predicate_Failure} pragma is intended to be an exact replacement for
the language-defined
@code{Predicate_Failure} aspect, and shares its restrictions and semantics.

@node Pragma Preelaborable_Initialization,Pragma Prefix_Exception_Messages,Pragma Predicate_Failure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-preelaborable-initialization}@anchor{cb}
@section Pragma Preelaborable_Initialization


Syntax:

@example
pragma Preelaborable_Initialization (DIRECT_NAME);
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Prefix_Exception_Messages,Pragma Pre_Class,Pragma Preelaborable_Initialization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-prefix-exception-messages}@anchor{cc}
@section Pragma Prefix_Exception_Messages


@geindex Prefix_Exception_Messages

@geindex exception

@geindex Exception_Message

Syntax:

@example
pragma Prefix_Exception_Messages;
@end example

This is an implementation-defined configuration pragma that affects the
behavior of raise statements with a message given as a static string
constant (typically a string literal). In such cases, the string will
be automatically prefixed by the name of the enclosing entity (giving
the package and subprogram containing the raise statement). This helps
to identify where messages are coming from, and this mode is automatic
for the run-time library.

The pragma has no effect if the message is computed with an expression other
than a static string constant, since the assumption in this case is that
the program computes exactly the string it wants. If you still want the
prefixing in this case, you can always call
@code{GNAT.Source_Info.Enclosing_Entity} and prepend the string manually.

@node Pragma Pre_Class,Pragma Priority_Specific_Dispatching,Pragma Prefix_Exception_Messages,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-pre-class}@anchor{cd}
@section Pragma Pre_Class


@geindex Pre_Class

@geindex Checks
@geindex preconditions

Syntax:

@example
pragma Pre_Class (Boolean_Expression);
@end example

The @code{Pre_Class} pragma is intended to be an exact replacement for
the language-defined
@code{Pre'Class} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

Note: This pragma is called @code{Pre_Class} rather than
@code{Pre'Class} because the latter would not be strictly
conforming to the allowed syntax for pragmas. The motivation
for providing pragmas equivalent to the aspects is to allow a program
to be written using the pragmas, and then compiled if necessary
using an Ada compiler that does not recognize the pragmas or
aspects, but is prepared to ignore the pragmas. The assertion
policy that controls this pragma is @code{Pre'Class}, not
@code{Pre_Class}.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Priority_Specific_Dispatching,Pragma Profile,Pragma Pre_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-priority-specific-dispatching}@anchor{ce}
@section Pragma Priority_Specific_Dispatching


Syntax:

@example
pragma Priority_Specific_Dispatching (
   POLICY_IDENTIFIER,
   first_priority_EXPRESSION,
   last_priority_EXPRESSION)

POLICY_IDENTIFIER ::=
   EDF_Across_Priorities            |
   FIFO_Within_Priorities           |
   Non_Preemptive_Within_Priorities |
   Round_Robin_Within_Priorities
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Profile,Pragma Profile_Warnings,Pragma Priority_Specific_Dispatching,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-profile}@anchor{cf}
@section Pragma Profile


Syntax:

@example
pragma Profile (Ravenscar | Restricted | Rational | Jorvik |
                GNAT_Extended_Ravenscar | GNAT_Ravenscar_EDF );
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma. This is a
configuration pragma that establishes a set of configuration pragmas
that depend on the argument. @code{Ravenscar} is standard in Ada 2005.
@code{Jorvik} is standard in Ada 202x.
The other possibilities (@code{Restricted}, @code{Rational},
@code{GNAT_Extended_Ravenscar}, @code{GNAT_Ravenscar_EDF})
are implementation-defined.  @code{GNAT_Extended_Ravenscar} is an alias for @code{Jorvik}.

The set of configuration pragmas is defined in the following sections.


@itemize *

@item 
Pragma Profile (Ravenscar)

The @code{Ravenscar} profile is standard in Ada 2005,
but is available in all earlier
versions of Ada as an implementation-defined pragma. This profile
establishes the following set of configuration pragmas:


@itemize *

@item 
@code{Task_Dispatching_Policy (FIFO_Within_Priorities)}

[RM D.2.2] Tasks are dispatched following a preemptive
priority-ordered scheduling policy.

@item 
@code{Locking_Policy (Ceiling_Locking)}

[RM D.3] While tasks and interrupts execute a protected action, they inherit
the ceiling priority of the corresponding protected object.

@item 
@code{Detect_Blocking}

This pragma forces the detection of potentially blocking operations within a
protected operation, and to raise Program_Error if that happens.
@end itemize

plus the following set of restrictions:


@itemize *

@item 
@code{Max_Entry_Queue_Length => 1}

No task can be queued on a protected entry.

@item 
@code{Max_Protected_Entries => 1}

@item 
@code{Max_Task_Entries => 0}

No rendezvous statements are allowed.

@item 
@code{No_Abort_Statements}

@item 
@code{No_Dynamic_Attachment}

@item 
@code{No_Dynamic_Priorities}

@item 
@code{No_Implicit_Heap_Allocations}

@item 
@code{No_Local_Protected_Objects}

@item 
@code{No_Local_Timing_Events}

@item 
@code{No_Protected_Type_Allocators}

@item 
@code{No_Relative_Delay}

@item 
@code{No_Requeue_Statements}

@item 
@code{No_Select_Statements}

@item 
@code{No_Specific_Termination_Handlers}

@item 
@code{No_Task_Allocators}

@item 
@code{No_Task_Hierarchy}

@item 
@code{No_Task_Termination}

@item 
@code{Simple_Barriers}
@end itemize

The Ravenscar profile also includes the following restrictions that specify
that there are no semantic dependencies on the corresponding predefined
packages:


@itemize *

@item 
@code{No_Dependence => Ada.Asynchronous_Task_Control}

@item 
@code{No_Dependence => Ada.Calendar}

@item 
@code{No_Dependence => Ada.Execution_Time.Group_Budget}

@item 
@code{No_Dependence => Ada.Execution_Time.Timers}

@item 
@code{No_Dependence => Ada.Task_Attributes}

@item 
@code{No_Dependence => System.Multiprocessors.Dispatching_Domains}
@end itemize

This set of configuration pragmas and restrictions correspond to the
definition of the ‘Ravenscar Profile’ for limited tasking, devised and
published by the @cite{International Real-Time Ada Workshop@comma{} 1997}.
A description is also available at
@indicateurl{http://www-users.cs.york.ac.uk/~burns/ravenscar.ps}.

The original definition of the profile was revised at subsequent IRTAW
meetings. It has been included in the ISO
@cite{Guide for the Use of the Ada Programming Language in High Integrity Systems},
and was made part of the Ada 2005 standard.
The formal definition given by
the Ada Rapporteur Group (ARG) can be found in two Ada Issues (AI-249 and
AI-305) available at
@indicateurl{http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00249.txt} and
@indicateurl{http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00305.txt}.

The above set is a superset of the restrictions provided by pragma
@code{Profile (Restricted)}, it includes six additional restrictions
(@code{Simple_Barriers}, @code{No_Select_Statements},
@code{No_Calendar}, @code{No_Implicit_Heap_Allocations},
@code{No_Relative_Delay} and @code{No_Task_Termination}).  This means
that pragma @code{Profile (Ravenscar)}, like the pragma
@code{Profile (Restricted)},
automatically causes the use of a simplified,
more efficient version of the tasking run-time library.

@item 
Pragma Profile (Jorvik)

@code{Jorvik} is the new profile added to the Ada 202x draft standard,
previously implemented under the name @code{GNAT_Extended_Ravenscar}.

The @code{No_Implicit_Heap_Allocations} restriction has been replaced
by @code{No_Implicit_Task_Allocations} and
@code{No_Implicit_Protected_Object_Allocations}.

The @code{Simple_Barriers} restriction has been replaced by
@code{Pure_Barriers}.

The @code{Max_Protected_Entries}, @code{Max_Entry_Queue_Length}, and
@code{No_Relative_Delay} restrictions have been removed.

Details on the rationale for @code{Jorvik} and implications for use may be
found in @cite{A New Ravenscar-Based Profile} by P. Rogers, J. Ruiz,
T. Gingold and P. Bernardi, in @cite{Reliable Software Technologies – Ada Europe 2017}, Springer-Verlag Lecture Notes in Computer Science,
Number 10300.

@item 
Pragma Profile (GNAT_Ravenscar_EDF)

This profile corresponds to the Ravenscar profile but using
EDF_Across_Priority as the Task_Scheduling_Policy.

@item 
Pragma Profile (Restricted)

This profile corresponds to the GNAT restricted run time. It
establishes the following set of restrictions:


@itemize *

@item 
@code{No_Abort_Statements}

@item 
@code{No_Entry_Queue}

@item 
@code{No_Task_Hierarchy}

@item 
@code{No_Task_Allocators}

@item 
@code{No_Dynamic_Priorities}

@item 
@code{No_Terminate_Alternatives}

@item 
@code{No_Dynamic_Attachment}

@item 
@code{No_Protected_Type_Allocators}

@item 
@code{No_Local_Protected_Objects}

@item 
@code{No_Requeue_Statements}

@item 
@code{No_Task_Attributes_Package}

@item 
@code{Max_Asynchronous_Select_Nesting =  0}

@item 
@code{Max_Task_Entries =  0}

@item 
@code{Max_Protected_Entries = 1}

@item 
@code{Max_Select_Alternatives = 0}
@end itemize

This set of restrictions causes the automatic selection of a simplified
version of the run time that provides improved performance for the
limited set of tasking functionality permitted by this set of restrictions.

@item 
Pragma Profile (Rational)

The Rational profile is intended to facilitate porting legacy code that
compiles with the Rational APEX compiler, even when the code includes non-
conforming Ada constructs.  The profile enables the following three pragmas:


@itemize *

@item 
@code{pragma Implicit_Packing}

@item 
@code{pragma Overriding_Renamings}

@item 
@code{pragma Use_VADS_Size}
@end itemize
@end itemize

@node Pragma Profile_Warnings,Pragma Program_Exit,Pragma Profile,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-profile-warnings}@anchor{d0}
@section Pragma Profile_Warnings


Syntax:

@example
pragma Profile_Warnings (Ravenscar | Restricted | Rational);
@end example

This is an implementation-defined pragma that is similar in
effect to @code{pragma Profile} except that instead of
generating @code{Restrictions} pragmas, it generates
@code{Restriction_Warnings} pragmas. The result is that
violations of the profile generate warning messages instead
of error messages.

@node Pragma Program_Exit,Pragma Propagate_Exceptions,Pragma Profile_Warnings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id34}@anchor{d1}@anchor{gnat_rm/implementation_defined_pragmas pragma-program-exit}@anchor{d2}
@section Pragma Program_Exit


Syntax:

@example
pragma Program_Exit [ (boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Program_Exit}
in the SPARK 2014 Reference Manual, section 6.1.10.

@node Pragma Propagate_Exceptions,Pragma Provide_Shift_Operators,Pragma Program_Exit,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-propagate-exceptions}@anchor{d3}
@section Pragma Propagate_Exceptions


@geindex Interfacing to C++

Syntax:

@example
pragma Propagate_Exceptions;
@end example

This pragma is now obsolete and, other than generating a warning if warnings
on obsolescent features are enabled, is ignored.
It is retained for compatibility
purposes. It used to be used in connection with optimization of
a now-obsolete mechanism for implementation of exceptions.

@node Pragma Provide_Shift_Operators,Pragma Psect_Object,Pragma Propagate_Exceptions,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-provide-shift-operators}@anchor{d4}
@section Pragma Provide_Shift_Operators


@geindex Shift operators

Syntax:

@example
pragma Provide_Shift_Operators (integer_first_subtype_LOCAL_NAME);
@end example

This pragma can be applied to a first subtype local name that specifies
either an unsigned or signed type. It has the effect of providing the
five shift operators (Shift_Left, Shift_Right, Shift_Right_Arithmetic,
Rotate_Left and Rotate_Right) for the given type. It is similar to
including the function declarations for these five operators, together
with the pragma Import (Intrinsic, …) statements.

@node Pragma Psect_Object,Pragma Pure_Function,Pragma Provide_Shift_Operators,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-psect-object}@anchor{d5}
@section Pragma Psect_Object


Syntax:

@example
pragma Psect_Object (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma is identical in effect to pragma @code{Common_Object}.

@node Pragma Pure_Function,Pragma Rational,Pragma Psect_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id35}@anchor{d6}@anchor{gnat_rm/implementation_defined_pragmas pragma-pure-function}@anchor{d7}
@section Pragma Pure_Function


Syntax:

@example
pragma Pure_Function ([Entity =>] function_LOCAL_NAME);
@end example

This pragma appears in the same declarative part as a function
declaration (or a set of function declarations if more than one
overloaded declaration exists, in which case the pragma applies
to all entities).  It specifies that the function @code{Entity} is
to be considered pure for the purposes of code generation.  This means
that the compiler can assume that there are no side effects, and
in particular that two identical calls produce the same result in
the same context. It also means that the function can be used in
an address clause.

Note that, quite deliberately, there are no static checks to try
to ensure that this promise is met, so @code{Pure_Function} can be used
with functions that are conceptually pure, even if they do modify
global variables.  For example, a square root function that is
instrumented to count the number of times it is called is still
conceptually pure, and can still be optimized, even though it
modifies a global variable (the count).  Memo functions are another
example (where a table of previous calls is kept and consulted to
avoid re-computation).

Note also that the normal rules excluding optimization of subprograms
in pure units (when parameter types are descended from System.Address,
or when the full view of a parameter type is limited), do not apply
for the Pure_Function case. If you explicitly specify Pure_Function,
the compiler may optimize away calls with identical arguments, and
if that results in unexpected behavior, the proper action is not to
use the pragma for subprograms that are not (conceptually) pure.

Note: Most functions in a @code{Pure} package are automatically pure, and
there is no need to use pragma @code{Pure_Function} for such functions.  One
exception is any function that has at least one formal of type
@code{System.Address} or a type derived from it.  Such functions are not
considered pure by default, since the compiler assumes that the
@code{Address} parameter may be functioning as a pointer and that the
referenced data may change even if the address value does not.
Similarly, imported functions are not considered to be pure by default,
since there is no way of checking that they are in fact pure.  The use
of pragma @code{Pure_Function} for such a function will override these default
assumption, and cause the compiler to treat a designated subprogram as pure
in these cases.

Note: If pragma @code{Pure_Function} is applied to a renamed function, it
applies to the underlying renamed function.  This can be used to
disambiguate cases of overloading where some but not all functions
in a set of overloaded functions are to be designated as pure.

If pragma @code{Pure_Function} is applied to a library-level function, the
function is also considered pure from an optimization point of view, but the
unit is not a Pure unit in the categorization sense. So for example, a function
thus marked is free to @code{with} non-pure units.

@node Pragma Rational,Pragma Ravenscar,Pragma Pure_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-rational}@anchor{d8}
@section Pragma Rational


Syntax:

@example
pragma Rational;
@end example

This pragma is considered obsolescent, but is retained for
compatibility purposes. It is equivalent to:

@example
pragma Profile (Rational);
@end example

@node Pragma Ravenscar,Pragma Refined_Depends,Pragma Rational,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ravenscar}@anchor{d9}
@section Pragma Ravenscar


Syntax:

@example
pragma Ravenscar;
@end example

This pragma is considered obsolescent, but is retained for
compatibility purposes. It is equivalent to:

@example
pragma Profile (Ravenscar);
@end example

which is the preferred method of setting the @code{Ravenscar} profile.

@node Pragma Refined_Depends,Pragma Refined_Global,Pragma Ravenscar,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id36}@anchor{da}@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-depends}@anchor{db}
@section Pragma Refined_Depends


Syntax:

@example
pragma Refined_Depends (DEPENDENCY_RELATION);

DEPENDENCY_RELATION ::=
     null
  | (DEPENDENCY_CLAUSE @{, DEPENDENCY_CLAUSE@})

DEPENDENCY_CLAUSE ::=
    OUTPUT_LIST =>[+] INPUT_LIST
  | NULL_DEPENDENCY_CLAUSE

NULL_DEPENDENCY_CLAUSE ::= null => INPUT_LIST

OUTPUT_LIST ::= OUTPUT | (OUTPUT @{, OUTPUT@})

INPUT_LIST ::= null | INPUT | (INPUT @{, INPUT@})

OUTPUT ::= NAME | FUNCTION_RESULT
INPUT  ::= NAME

where FUNCTION_RESULT is a function Result attribute_reference
@end example

For the semantics of this pragma, see the entry for aspect @code{Refined_Depends} in
the SPARK 2014 Reference Manual, section 6.1.5.

@node Pragma Refined_Global,Pragma Refined_Post,Pragma Refined_Depends,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id37}@anchor{dc}@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-global}@anchor{dd}
@section Pragma Refined_Global


Syntax:

@example
pragma Refined_Global (GLOBAL_SPECIFICATION);

GLOBAL_SPECIFICATION ::=
     null
  | (GLOBAL_LIST)
  | (MODED_GLOBAL_LIST @{, MODED_GLOBAL_LIST@})

MODED_GLOBAL_LIST ::= MODE_SELECTOR => GLOBAL_LIST

MODE_SELECTOR ::= In_Out | Input | Output | Proof_In
GLOBAL_LIST   ::= GLOBAL_ITEM | (GLOBAL_ITEM @{, GLOBAL_ITEM@})
GLOBAL_ITEM   ::= NAME
@end example

For the semantics of this pragma, see the entry for aspect @code{Refined_Global} in
the SPARK 2014 Reference Manual, section 6.1.4.

@node Pragma Refined_Post,Pragma Refined_State,Pragma Refined_Global,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id38}@anchor{de}@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-post}@anchor{df}
@section Pragma Refined_Post


Syntax:

@example
pragma Refined_Post (boolean_EXPRESSION);
@end example

For the semantics of this pragma, see the entry for aspect @code{Refined_Post} in
the SPARK 2014 Reference Manual, section 7.2.7.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Refined_State,Pragma Relative_Deadline,Pragma Refined_Post,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id39}@anchor{e0}@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-state}@anchor{e1}
@section Pragma Refined_State


Syntax:

@example
pragma Refined_State (REFINEMENT_LIST);

REFINEMENT_LIST ::=
  (REFINEMENT_CLAUSE @{, REFINEMENT_CLAUSE@})

REFINEMENT_CLAUSE ::= state_NAME => CONSTITUENT_LIST

CONSTITUENT_LIST ::=
     null
  |  CONSTITUENT
  | (CONSTITUENT @{, CONSTITUENT@})

CONSTITUENT ::= object_NAME | state_NAME
@end example

For the semantics of this pragma, see the entry for aspect @code{Refined_State} in
the SPARK 2014 Reference Manual, section 7.2.2.

@node Pragma Relative_Deadline,Pragma Remote_Access_Type,Pragma Refined_State,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-relative-deadline}@anchor{e2}
@section Pragma Relative_Deadline


Syntax:

@example
pragma Relative_Deadline (time_span_EXPRESSION);
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Remote_Access_Type,Pragma Rename_Pragma,Pragma Relative_Deadline,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id40}@anchor{e3}@anchor{gnat_rm/implementation_defined_pragmas pragma-remote-access-type}@anchor{e4}
@section Pragma Remote_Access_Type


Syntax:

@example
pragma Remote_Access_Type ([Entity =>] formal_access_type_LOCAL_NAME);
@end example

This pragma appears in the formal part of a generic declaration.
It specifies an exception to the RM rule from E.2.2(17/2), which forbids
the use of a remote access to class-wide type as actual for a formal
access type.

When this pragma applies to a formal access type @code{Entity}, that
type is treated as a remote access to class-wide type in the generic.
It must be a formal general access type, and its designated type must
be the class-wide type of a formal tagged limited private type from the
same generic declaration.

In the generic unit, the formal type is subject to all restrictions
pertaining to remote access to class-wide types. At instantiation, the
actual type must be a remote access to class-wide type.

@node Pragma Rename_Pragma,Pragma Restricted_Run_Time,Pragma Remote_Access_Type,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-rename-pragma}@anchor{e5}
@section Pragma Rename_Pragma


@geindex Pragmas
@geindex synonyms

Syntax:

@example
pragma Rename_Pragma (
         [New_Name =>] IDENTIFIER,
         [Renamed  =>] pragma_IDENTIFIER);
@end example

This pragma provides a mechanism for supplying new names for existing
pragmas. The @code{New_Name} identifier can subsequently be used as a synonym for
the Renamed pragma. For example, suppose you have code that was originally
developed on a compiler that supports Inline_Only as an implementation defined
pragma. And suppose the semantics of pragma Inline_Only are identical to (or at
least very similar to) the GNAT implementation defined pragma
Inline_Always. You could globally replace Inline_Only with Inline_Always.

However, to avoid that source modification, you could instead add a
configuration pragma:

@example
pragma Rename_Pragma (
         New_Name => Inline_Only,
         Renamed  => Inline_Always);
@end example

Then GNAT will treat “pragma Inline_Only …” as if you had written
“pragma Inline_Always …”.

Pragma Inline_Only will not necessarily mean the same thing as the other Ada
compiler; it’s up to you to make sure the semantics are close enough.

@node Pragma Restricted_Run_Time,Pragma Restriction_Warnings,Pragma Rename_Pragma,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-restricted-run-time}@anchor{e6}
@section Pragma Restricted_Run_Time


Syntax:

@example
pragma Restricted_Run_Time;
@end example

This pragma is considered obsolescent, but is retained for
compatibility purposes. It is equivalent to:

@example
pragma Profile (Restricted);
@end example

which is the preferred method of setting the restricted run time
profile.

@node Pragma Restriction_Warnings,Pragma Reviewable,Pragma Restricted_Run_Time,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-restriction-warnings}@anchor{e7}
@section Pragma Restriction_Warnings


Syntax:

@example
pragma Restriction_Warnings
  (restriction_IDENTIFIER @{, restriction_IDENTIFIER@});
@end example

This pragma allows a series of restriction identifiers to be
specified (the list of allowed identifiers is the same as for
pragma @code{Restrictions}). For each of these identifiers
the compiler checks for violations of the restriction, but
generates a warning message rather than an error message
if the restriction is violated.

One use of this is in situations where you want to know
about violations of a restriction, but you want to ignore some of
these violations. Consider this example, where you want to set
Ada_95 mode and enable style checks, but you want to know about
any other use of implementation pragmas:

@example
pragma Restriction_Warnings (No_Implementation_Pragmas);
pragma Warnings (Off, "violation of No_Implementation_Pragmas");
pragma Ada_95;
pragma Style_Checks ("2bfhkM160");
pragma Warnings (On, "violation of No_Implementation_Pragmas");
@end example

By including the above lines in a configuration pragmas file,
the Ada_95 and Style_Checks pragmas are accepted without
generating a warning, but any other use of implementation
defined pragmas will cause a warning to be generated.

@node Pragma Reviewable,Pragma Secondary_Stack_Size,Pragma Restriction_Warnings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-reviewable}@anchor{e8}
@section Pragma Reviewable


Syntax:

@example
pragma Reviewable;
@end example

This pragma is an RM-defined standard pragma, but has no effect on the
program being compiled, or on the code generated for the program.

To obtain the required output specified in RM H.3.1, the compiler must be
run with various special switches as follows:


@itemize *

@item 
`Where compiler-generated run-time checks remain'

The switch `-gnatGL'
may be used to list the expanded code in pseudo-Ada form.
Runtime checks show up in the listing either as explicit
checks or operators marked with @{@} to indicate a check is present.

@item 
`An identification of known exceptions at compile time'

If the program is compiled with `-gnatwa',
the compiler warning messages will indicate all cases where the compiler
detects that an exception is certain to occur at run time.

@item 
`Possible reads of uninitialized variables'

The compiler warns of many such cases, but its output is incomplete.
@end itemize


A supplemental static analysis tool
may be used to obtain a comprehensive list of all
possible points at which uninitialized data may be read.


@itemize *

@item 
`Where run-time support routines are implicitly invoked'

In the output from `-gnatGL',
run-time calls are explicitly listed as calls to the relevant
run-time routine.

@item 
`Object code listing'

This may be obtained either by using the `-S' switch,
or the objdump utility.

@item 
`Constructs known to be erroneous at compile time'

These are identified by warnings issued by the compiler (use `-gnatwa').

@item 
`Stack usage information'

Static stack usage data (maximum per-subprogram) can be obtained via the
`-fstack-usage' switch to the compiler.
Dynamic stack usage data (per task) can be obtained via the `-u' switch
to gnatbind
@end itemize



@itemize *

@item 
`Object code listing of entire partition'

This can be obtained by compiling the partition with `-S',
or by applying objdump
to all the object files that are part of the partition.

@item 
`A description of the run-time model'

The full sources of the run-time are available, and the documentation of
these routines describes how these run-time routines interface to the
underlying operating system facilities.

@item 
`Control and data-flow information'
@end itemize


A supplemental static analysis tool
may be used to obtain complete control and data-flow information, as well as
comprehensive messages identifying possible problems based on this
information.

@node Pragma Secondary_Stack_Size,Pragma Share_Generic,Pragma Reviewable,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id41}@anchor{e9}@anchor{gnat_rm/implementation_defined_pragmas pragma-secondary-stack-size}@anchor{ea}
@section Pragma Secondary_Stack_Size


Syntax:

@example
pragma Secondary_Stack_Size (integer_EXPRESSION);
@end example

This pragma appears within the task definition of a single task declaration
or a task type declaration (like pragma @code{Storage_Size}) and applies to all
task objects of that type. The argument specifies the size of the secondary
stack to be used by these task objects, and must be of an integer type. The
secondary stack is used to handle functions that return a variable-sized
result, for example a function returning an unconstrained String.

Note this pragma only applies to targets using fixed secondary stacks, like
VxWorks 653 and bare board targets, where a fixed block for the
secondary stack is allocated from the primary stack of the task. By default,
these targets assign a percentage of the primary stack for the secondary stack,
as defined by @code{System.Parameter.Sec_Stack_Percentage}. With this pragma,
an @code{integer_EXPRESSION} of bytes is assigned from the primary stack instead.

For most targets, the pragma does not apply as the secondary stack grows on
demand: allocated as a chain of blocks in the heap. The default size of these
blocks can be modified via the @code{-D} binder option as described in
@cite{GNAT User’s Guide}.

Note that no check is made to see if the secondary stack can fit inside the
primary stack.

Note the pragma cannot appear when the restriction @code{No_Secondary_Stack}
is in effect.

@node Pragma Share_Generic,Pragma Shared,Pragma Secondary_Stack_Size,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-share-generic}@anchor{eb}
@section Pragma Share_Generic


Syntax:

@example
pragma Share_Generic (GNAME @{, GNAME@});

GNAME ::= generic_unit_NAME | generic_instance_NAME
@end example

This pragma is provided for compatibility with Dec Ada 83. It has
no effect in GNAT (which does not implement shared generics), other
than to check that the given names are all names of generic units or
generic instances.

@node Pragma Shared,Pragma Short_Circuit_And_Or,Pragma Share_Generic,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id42}@anchor{ec}@anchor{gnat_rm/implementation_defined_pragmas pragma-shared}@anchor{ed}
@section Pragma Shared


This pragma is provided for compatibility with Ada 83. The syntax and
semantics are identical to pragma Atomic.

@node Pragma Short_Circuit_And_Or,Pragma Short_Descriptors,Pragma Shared,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-short-circuit-and-or}@anchor{ee}
@section Pragma Short_Circuit_And_Or


Syntax:

@example
pragma Short_Circuit_And_Or;
@end example

This configuration pragma causes the predefined AND and OR operators of
type Standard.Boolean to have short-circuit semantics. That is, they
behave like AND THEN and OR ELSE; the right-hand side is not evaluated
if the left-hand side determines the result. This may be useful in the
context of certification protocols requiring the use of short-circuited
logical operators.

There is no requirement that all units in a partition use this option.
However, mixing of short-circuit and non-short-circuit semantics can be
confusing. Therefore, the recommended use is to put the pragma in a
configuration file that applies to the whole program. Alternatively, if
you have a legacy library that should not use this pragma, you can put
it in a separate library project that does not use the pragma.
In any case, fine-grained mixing of the different semantics is not
recommended. If pragma @code{Short_Circuit_And_Or} is specified, then it
is illegal to rename the predefined Boolean AND and OR, or to pass
them to generic formal functions; this corresponds to the fact that
AND THEN and OR ELSE cannot be renamed nor passed as generic formal
functions.

Note that this pragma has no effect on other logical operators –
predefined operators of modular types, array-of-boolean types and types
derived from Standard.Boolean, nor user-defined operators.

See also the pragma @code{Unevaluated_Use_Of_Old} and the restriction
@code{No_Direct_Boolean_Operators}, which may be useful in conjunction
with @code{Short_Circuit_And_Or}.

@node Pragma Short_Descriptors,Pragma Side_Effects,Pragma Short_Circuit_And_Or,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-short-descriptors}@anchor{ef}
@section Pragma Short_Descriptors


Syntax:

@example
pragma Short_Descriptors;
@end example

This pragma is provided for compatibility with other Ada implementations. It
is recognized but ignored by all current versions of GNAT.

@node Pragma Side_Effects,Pragma Simple_Storage_Pool_Type,Pragma Short_Descriptors,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id43}@anchor{f0}@anchor{gnat_rm/implementation_defined_pragmas pragma-side-effects}@anchor{f1}
@section Pragma Side_Effects


Syntax:

@example
pragma Side_Effects [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect
@code{Side_Effects} in the SPARK Reference Manual, section 6.1.12.

@node Pragma Simple_Storage_Pool_Type,Pragma Source_File_Name,Pragma Side_Effects,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id44}@anchor{f2}@anchor{gnat_rm/implementation_defined_pragmas pragma-simple-storage-pool-type}@anchor{f3}
@section Pragma Simple_Storage_Pool_Type


@geindex Storage pool
@geindex simple

@geindex Simple storage pool

Syntax:

@example
pragma Simple_Storage_Pool_Type (type_LOCAL_NAME);
@end example

A type can be established as a ‘simple storage pool type’ by applying
the representation pragma @code{Simple_Storage_Pool_Type} to the type.
A type named in the pragma must be a library-level immutably limited record
type or limited tagged type declared immediately within a package declaration.
The type can also be a limited private type whose full type is allowed as
a simple storage pool type.

For a simple storage pool type @code{SSP}, nonabstract primitive subprograms
@code{Allocate}, @code{Deallocate}, and @code{Storage_Size} can be declared that
are subtype conformant with the following subprogram declarations:

@example
procedure Allocate
  (Pool                     : in out SSP;
   Storage_Address          : out System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count);

procedure Deallocate
  (Pool : in out SSP;
   Storage_Address          : System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count);

function Storage_Size (Pool : SSP)
  return System.Storage_Elements.Storage_Count;
@end example

Procedure @code{Allocate} must be declared, whereas @code{Deallocate} and
@code{Storage_Size} are optional. If @code{Deallocate} is not declared, then
applying an unchecked deallocation has no effect other than to set its actual
parameter to null. If @code{Storage_Size} is not declared, then the
@code{Storage_Size} attribute applied to an access type associated with
a pool object of type SSP returns zero. Additional operations can be declared
for a simple storage pool type (such as for supporting a mark/release
storage-management discipline).

An object of a simple storage pool type can be associated with an access
type by specifying the attribute
@ref{f4,,Simple_Storage_Pool}. For example:

@example
My_Pool : My_Simple_Storage_Pool_Type;

type Acc is access My_Data_Type;

for Acc'Simple_Storage_Pool use My_Pool;
@end example

See attribute @ref{f4,,Simple_Storage_Pool}
for further details.

@node Pragma Source_File_Name,Pragma Source_File_Name_Project,Pragma Simple_Storage_Pool_Type,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id45}@anchor{f5}@anchor{gnat_rm/implementation_defined_pragmas pragma-source-file-name}@anchor{f6}
@section Pragma Source_File_Name


Syntax:

@example
pragma Source_File_Name (
  [Unit_Name     =>] unit_NAME,
  Spec_File_Name =>  STRING_LITERAL,
  [Index => INTEGER_LITERAL]);

pragma Source_File_Name (
  [Unit_Name     =>] unit_NAME,
  Body_File_Name =>  STRING_LITERAL,
  [Index => INTEGER_LITERAL]);
@end example

Use this to override the normal naming convention.  It is a configuration
pragma, and so has the usual applicability of configuration pragmas
(i.e., it applies to either an entire partition, or to all units in a
compilation, or to a single unit, depending on how it is used).
@code{unit_name} is mapped to @code{file_name_literal}.  The identifier for
the second argument is required, and indicates whether this is the file
name for the spec or for the body.

The optional Index argument should be used when a file contains multiple
units, and when you do not want to use @code{gnatchop} to separate then
into multiple files (which is the recommended procedure to limit the
number of recompilations that are needed when some sources change).
For instance, if the source file @code{source.ada} contains

@example
package B is
...
end B;

with B;
procedure A is
begin
   ..
end A;
@end example

you could use the following configuration pragmas:

@example
pragma Source_File_Name
  (B, Spec_File_Name => "source.ada", Index => 1);
pragma Source_File_Name
  (A, Body_File_Name => "source.ada", Index => 2);
@end example

Note that the @code{gnatname} utility can also be used to generate those
configuration pragmas.

Another form of the @code{Source_File_Name} pragma allows
the specification of patterns defining alternative file naming schemes
to apply to all files.

@example
pragma Source_File_Name
  (  [Spec_File_Name  =>] STRING_LITERAL
   [,[Casing          =>] CASING_SPEC]
   [,[Dot_Replacement =>] STRING_LITERAL]);

pragma Source_File_Name
  (  [Body_File_Name  =>] STRING_LITERAL
   [,[Casing          =>] CASING_SPEC]
   [,[Dot_Replacement =>] STRING_LITERAL]);

pragma Source_File_Name
  (  [Subunit_File_Name =>] STRING_LITERAL
   [,[Casing            =>] CASING_SPEC]
   [,[Dot_Replacement   =>] STRING_LITERAL]);

CASING_SPEC ::= Lowercase | Uppercase | Mixedcase
@end example

The first argument is a pattern that contains a single asterisk indicating
the point at which the unit name is to be inserted in the pattern string
to form the file name.  The second argument is optional.  If present it
specifies the casing of the unit name in the resulting file name string.
The default is lower case.  Finally the third argument allows for systematic
replacement of any dots in the unit name by the specified string literal.

Note that Source_File_Name pragmas should not be used if you are using
project files. The reason for this rule is that the project manager is not
aware of these pragmas, and so other tools that use the project file would not
be aware of the intended naming conventions. If you are using project files,
file naming is controlled by Source_File_Name_Project pragmas, which are
usually supplied automatically by the project manager. A pragma
Source_File_Name cannot appear after a @ref{f7,,Pragma Source_File_Name_Project}.

For more details on the use of the @code{Source_File_Name} pragma, see the
sections on @cite{Using Other File Names} and @cite{Alternative File Naming Schemes}
in the @cite{GNAT User’s Guide}.

@node Pragma Source_File_Name_Project,Pragma Source_Reference,Pragma Source_File_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id46}@anchor{f8}@anchor{gnat_rm/implementation_defined_pragmas pragma-source-file-name-project}@anchor{f7}
@section Pragma Source_File_Name_Project


This pragma has the same syntax and semantics as pragma Source_File_Name.
It is only allowed as a stand-alone configuration pragma.
It cannot appear after a @ref{f6,,Pragma Source_File_Name}, and
most importantly, once pragma Source_File_Name_Project appears,
no further Source_File_Name pragmas are allowed.

The intention is that Source_File_Name_Project pragmas are always
generated by the Project Manager in a manner consistent with the naming
specified in a project file, and when naming is controlled in this manner,
it is not permissible to attempt to modify this naming scheme using
Source_File_Name or Source_File_Name_Project pragmas (which would not be
known to the project manager).

@node Pragma Source_Reference,Pragma SPARK_Mode,Pragma Source_File_Name_Project,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-source-reference}@anchor{f9}
@section Pragma Source_Reference


Syntax:

@example
pragma Source_Reference (INTEGER_LITERAL, STRING_LITERAL);
@end example

This pragma must appear as the first line of a source file.
@code{integer_literal} is the logical line number of the line following
the pragma line (for use in error messages and debugging
information).  @code{string_literal} is a static string constant that
specifies the file name to be used in error messages and debugging
information.  This is most notably used for the output of @code{gnatchop}
with the `-r' switch, to make sure that the original unchopped
source file is the one referred to.

The second argument must be a string literal, it cannot be a static
string expression other than a string literal.  This is because its value
is needed for error messages issued by all phases of the compiler.

@node Pragma SPARK_Mode,Pragma Static_Elaboration_Desired,Pragma Source_Reference,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id47}@anchor{fa}@anchor{gnat_rm/implementation_defined_pragmas pragma-spark-mode}@anchor{fb}
@section Pragma SPARK_Mode


Syntax:

@example
pragma SPARK_Mode [(On | Off)] ;
@end example

In general a program can have some parts that are in SPARK 2014 (and
follow all the rules in the SPARK Reference Manual), and some parts
that are full Ada 2012.

The SPARK_Mode pragma is used to identify which parts are in SPARK
2014 (by default programs are in full Ada). The SPARK_Mode pragma can
be used in the following places:


@itemize *

@item 
As a configuration pragma, in which case it sets the default mode for
all units compiled with this pragma.

@item 
Immediately following a library-level subprogram spec

@item 
Immediately within a library-level package body

@item 
Immediately following the @code{private} keyword of a library-level
package spec

@item 
Immediately following the @code{begin} keyword of a library-level
package body

@item 
Immediately within a library-level subprogram body
@end itemize

Normally a subprogram or package spec/body inherits the current mode
that is active at the point it is declared. But this can be overridden
by pragma within the spec or body as above.

The basic consistency rule is that you can’t turn SPARK_Mode back
@code{On}, once you have explicitly (with a pragma) turned if
@code{Off}. So the following rules apply:

If a subprogram spec has SPARK_Mode @code{Off}, then the body must
also have SPARK_Mode @code{Off}.

For a package, we have four parts:


@itemize *

@item 
the package public declarations

@item 
the package private part

@item 
the body of the package

@item 
the elaboration code after @code{begin}
@end itemize

For a package, the rule is that if you explicitly turn SPARK_Mode
@code{Off} for any part, then all the following parts must have
SPARK_Mode @code{Off}. Note that this may require repeating a pragma
SPARK_Mode (@code{Off}) in the body. For example, if we have a
configuration pragma SPARK_Mode (@code{On}) that turns the mode on by
default everywhere, and one particular package spec has pragma
SPARK_Mode (@code{Off}), then that pragma will need to be repeated in
the package body.

@node Pragma Static_Elaboration_Desired,Pragma Stream_Convert,Pragma SPARK_Mode,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-static-elaboration-desired}@anchor{fc}
@section Pragma Static_Elaboration_Desired


Syntax:

@example
pragma Static_Elaboration_Desired;
@end example

This pragma is used to indicate that the compiler should attempt to initialize
statically the objects declared in the library unit to which the pragma applies,
when these objects are initialized (explicitly or implicitly) by an aggregate.
In the absence of this pragma, aggregates in object declarations are expanded
into assignments and loops, even when the aggregate components are static
constants. When the aggregate is present the compiler builds a static expression
that requires no run-time code, so that the initialized object can be placed in
read-only data space. If the components are not static, or the aggregate has
more that 100 components, the compiler emits a warning that the pragma cannot
be obeyed. (See also the restriction No_Implicit_Loops, which supports static
construction of larger aggregates with static components that include an others
choice.)

@node Pragma Stream_Convert,Pragma Style_Checks,Pragma Static_Elaboration_Desired,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-stream-convert}@anchor{fd}
@section Pragma Stream_Convert


Syntax:

@example
pragma Stream_Convert (
  [Entity =>] type_LOCAL_NAME,
  [Read   =>] function_NAME,
  [Write  =>] function_NAME);
@end example

This pragma provides an efficient way of providing user-defined stream
attributes.  Not only is it simpler to use than specifying the attributes
directly, but more importantly, it allows the specification to be made in such
a way that the predefined unit Ada.Streams is not loaded unless it is actually
needed (i.e. unless the stream attributes are actually used); the use of
the Stream_Convert pragma adds no overhead at all, unless the stream
attributes are actually used on the designated type.

The first argument specifies the type for which stream functions are
provided.  The second parameter provides a function used to read values
of this type.  It must name a function whose argument type may be any
subtype, and whose returned type must be the type given as the first
argument to the pragma.

The meaning of the @code{Read} parameter is that if a stream attribute directly
or indirectly specifies reading of the type given as the first parameter,
then a value of the type given as the argument to the Read function is
read from the stream, and then the Read function is used to convert this
to the required target type.

Similarly the @code{Write} parameter specifies how to treat write attributes
that directly or indirectly apply to the type given as the first parameter.
It must have an input parameter of the type specified by the first parameter,
and the return type must be the same as the input type of the Read function.
The effect is to first call the Write function to convert to the given stream
type, and then write the result type to the stream.

The Read and Write functions must not be overloaded subprograms.  If necessary
renamings can be supplied to meet this requirement.
The usage of this attribute is best illustrated by a simple example, taken
from the GNAT implementation of package Ada.Strings.Unbounded:

@example
function To_Unbounded (S : String) return Unbounded_String
  renames To_Unbounded_String;

pragma Stream_Convert
  (Unbounded_String, To_Unbounded, To_String);
@end example

The specifications of the referenced functions, as given in the Ada
Reference Manual are:

@example
function To_Unbounded_String (Source : String)
  return Unbounded_String;

function To_String (Source : Unbounded_String)
  return String;
@end example

The effect is that if the value of an unbounded string is written to a stream,
then the representation of the item in the stream is in the same format that
would be used for @code{Standard.String'Output}, and this same representation
is expected when a value of this type is read from the stream. Note that the
value written always includes the bounds, even for Unbounded_String’Write,
since Unbounded_String is not an array type.

Note that the @code{Stream_Convert} pragma is not effective in the case of
a derived type of a non-limited tagged type. If such a type is specified then
the pragma is silently ignored, and the default implementation of the stream
attributes is used instead.

@node Pragma Style_Checks,Pragma Subprogram_Variant,Pragma Stream_Convert,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-style-checks}@anchor{fe}
@section Pragma Style_Checks


Syntax:

@example
pragma Style_Checks (string_LITERAL | ALL_CHECKS |
                     On | Off [, LOCAL_NAME]);
@end example

This pragma is used in conjunction with compiler switches to control the
built in style checking provided by GNAT.  The compiler switches, if set,
provide an initial setting for the switches, and this pragma may be used
to modify these settings, or the settings may be provided entirely by
the use of the pragma.  This pragma can be used anywhere that a pragma
is legal, including use as a configuration pragma (including use in
the @code{gnat.adc} file).

The form with a string literal specifies which style options are to be
activated.  These are additive, so they apply in addition to any previously
set style check options.  The codes for the options are the same as those
used in the `-gnaty' switch to `gcc' or `gnatmake'.
For example the following two methods can be used to enable
layout checking and to change the maximum nesting level value:


@itemize *

@item 
@example
--  switch on layout checks
pragma Style_Checks ("l");
--  set the number of maximum allowed nesting levels to 15
pragma Style_Checks ("L15");
@end example

@item 
@example
gcc -c -gnatyl -gnatyL15 ...
@end example
@end itemize

The string literal values can be cumulatively switched on and off by prefixing
the value with @code{+} or @code{-}, where:


@itemize *

@item 
@code{+} is equivalent to no prefix. It applies the check referenced by the
literal value;

@item 
@code{-} switches the referenced check off.
@end itemize

@example
--  allow misaligned block by disabling layout check
pragma Style_Checks ("-l");
declare
    msg : constant String := "Hello";
begin
    Put_Line (msg);
    end;

--  enable the layout check again
pragma Style_Checks ("l");
declare
    msg : constant String := "Hello";
begin
    Put_Line (msg);
    end;
@end example

The code above contains two layout errors, however, only
the last line is picked up by the compiler.

Similarly, the switches containing a numeric value can be applied in sequence.
In the example below, the permitted nesting level is reduced in in the middle
block and the compiler raises a warning on the highlighted line.

@example
-- Permit 3 levels of nesting
pragma Style_Checks ("L3");

procedure Main is
begin
    if True then
      if True then
          null;
      end if;
    end if;
    --  Reduce permitted nesting levels to 2.
    --  Note that "+L2" and "L2" are equivalent.
    pragma Style_Checks ("+L2");
    if True then
      if True then
          null;
      end if;
    end if;
    --  Disable checking permitted nesting levels.
    --  Note that the number after "-L" is insignificant,
    --  "-L", "-L3" and "-Lx" are all equivalent.
    pragma Style_Checks ("-L3");
    if True then
      if True then
          null;
      end if;
    end if;
end Main;
@end example

The form @code{ALL_CHECKS} activates all standard checks (its use is equivalent
to the use of the @code{gnaty} switch with no options.
See the @cite{GNAT User’s Guide} for details.)

Note: the behavior is slightly different in GNAT mode (@code{-gnatg} used).
In this case, @code{ALL_CHECKS} implies the standard set of GNAT mode style check
options (i.e. equivalent to @code{-gnatyg}).

The forms with @code{Off} and @code{On}
can be used to temporarily disable style checks
as shown in the following example:

@example
pragma Style_Checks ("k"); -- requires keywords in lower case
pragma Style_Checks (Off); -- turn off style checks
NULL;                      -- this will not generate an error message
pragma Style_Checks (On);  -- turn style checks back on
NULL;                      -- this will generate an error message
@end example

Finally the two argument form is allowed only if the first argument is
@code{On} or @code{Off}.  The effect is to turn of semantic style checks
for the specified entity, as shown in the following example:

@example
pragma Style_Checks ("r"); -- require consistency of identifier casing
Arg : Integer;
Rf1 : Integer := ARG;      -- incorrect, wrong case
pragma Style_Checks (Off, Arg);
Rf2 : Integer := ARG;      -- OK, no error
@end example

@node Pragma Subprogram_Variant,Pragma Subtitle,Pragma Style_Checks,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-subprogram-variant}@anchor{ff}
@section Pragma Subprogram_Variant


@geindex Subprogram_Variant

Syntax:

@example
pragma Subprogram_Variant (SUBPROGRAM_VARIANT_LIST);

SUBPROGRAM_VARIANT_LIST ::=
  STRUCTURAL_SUBPROGRAM_VARIANT_ITEM
| NUMERIC_SUBPROGRAM_VARIANT_ITEMS

NUMERIC_SUBPROGRAM_VARIANT_ITEMS ::=
  NUMERIC_SUBPROGRAM_VARIANT_ITEM @{, NUMERIC_SUBPROGRAM_VARIANT_ITEM@}

NUMERIC_SUBPROGRAM_VARIANT_ITEM ::=
  CHANGE_DIRECTION => EXPRESSION

STRUCTURAL_SUBPROGRAM_VARIANT_ITEM ::=
  STRUCTURAL => EXPRESSION

CHANGE_DIRECTION ::= Increases | Decreases
@end example

The @code{Subprogram_Variant} pragma is intended to be an exact replacement for
the implementation-defined @code{Subprogram_Variant} aspect, and shares its
restrictions and semantics.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Subtitle,Pragma Suppress,Pragma Subprogram_Variant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-subtitle}@anchor{100}
@section Pragma Subtitle


Syntax:

@example
pragma Subtitle ([Subtitle =>] STRING_LITERAL);
@end example

This pragma is recognized for compatibility with other Ada compilers
but is ignored by GNAT.

@node Pragma Suppress,Pragma Suppress_All,Pragma Subtitle,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress}@anchor{101}
@section Pragma Suppress


Syntax:

@example
pragma Suppress (Identifier [, [On =>] Name]);
@end example

This is a standard pragma, and supports all the check names required in
the RM. It is included here because GNAT recognizes some additional check
names that are implementation defined (as permitted by the RM):


@itemize *

@item 
@code{Alignment_Check} can be used to suppress alignment checks on addresses
used in address clauses. Note that @code{Alignment_Check} is suppressed by
default on non-strict alignment machines (such as the x86).

@item 
@code{Atomic_Synchronization} can be used to suppress the special memory
synchronization instructions that are normally generated for access to
@code{Atomic} variables to ensure correct synchronization between tasks
that use such variables for synchronization purposes.

@item 
@code{Duplicated_Tag_Check} Can be used to suppress the check that is generated
for a duplicated tag value when a tagged type is declared.

@item 
@code{Container_Checks} Can be used to suppress all checks within Ada.Containers
and instances of its children, including Tampering_Check.

@item 
@code{Tampering_Check} Can be used to suppress tampering check in the containers.

@item 
@code{Predicate_Check} can be used to control whether predicate checks are
active. It is applicable only to predicates for which the policy is
@code{Check}. Unlike @code{Assertion_Policy}, which determines if a given
predicate is ignored or checked for the whole program, the use of
@code{Suppress} and @code{Unsuppress} with this check name allows a given
predicate to be turned on and off at specific points in the program.

@item 
@code{Validity_Check} can be used specifically to control validity checks.
If @code{Suppress} is used to suppress validity checks, then no validity
checks are performed, including those specified by the appropriate compiler
switch or the @code{Validity_Checks} pragma.

@item 
Additional check names previously introduced by use of the @code{Check_Name}
pragma are also allowed.
@end itemize

Note that pragma Suppress gives the compiler permission to omit
checks, but does not require the compiler to omit checks. The compiler
will generate checks if they are essentially free, even when they are
suppressed. In particular, if the compiler can prove that a certain
check will necessarily fail, it will generate code to do an
unconditional ‘raise’, even if checks are suppressed. The compiler
warns in this case.

Of course, run-time checks are omitted whenever the compiler can prove
that they will not fail, whether or not checks are suppressed.

@node Pragma Suppress_All,Pragma Suppress_Debug_Info,Pragma Suppress,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-all}@anchor{102}
@section Pragma Suppress_All


Syntax:

@example
pragma Suppress_All;
@end example

This pragma can appear anywhere within a unit.
The effect is to apply @code{Suppress (All_Checks)} to the unit
in which it appears.  This pragma is implemented for compatibility with DEC
Ada 83 usage where it appears at the end of a unit, and for compatibility
with Rational Ada, where it appears as a program unit pragma.
The use of the standard Ada pragma @code{Suppress (All_Checks)}
as a normal configuration pragma is the preferred usage in GNAT.

@node Pragma Suppress_Debug_Info,Pragma Suppress_Exception_Locations,Pragma Suppress_All,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id48}@anchor{103}@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-debug-info}@anchor{104}
@section Pragma Suppress_Debug_Info


Syntax:

@example
pragma Suppress_Debug_Info ([Entity =>] LOCAL_NAME);
@end example

This pragma can be used to suppress generation of debug information
for the specified entity. It is intended primarily for use in debugging
the debugger, and navigating around debugger problems.

@node Pragma Suppress_Exception_Locations,Pragma Suppress_Initialization,Pragma Suppress_Debug_Info,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-exception-locations}@anchor{105}
@section Pragma Suppress_Exception_Locations


Syntax:

@example
pragma Suppress_Exception_Locations;
@end example

In normal mode, a raise statement for an exception by default generates
an exception message giving the file name and line number for the location
of the raise. This is useful for debugging and logging purposes, but this
entails extra space for the strings for the messages. The configuration
pragma @code{Suppress_Exception_Locations} can be used to suppress the
generation of these strings, with the result that space is saved, but the
exception message for such raises is null. This configuration pragma may
appear in a global configuration pragma file, or in a specific unit as
usual. It is not required that this pragma be used consistently within
a partition, so it is fine to have some units within a partition compiled
with this pragma and others compiled in normal mode without it.

@node Pragma Suppress_Initialization,Pragma Task_Name,Pragma Suppress_Exception_Locations,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id49}@anchor{106}@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-initialization}@anchor{107}
@section Pragma Suppress_Initialization


@geindex Suppressing initialization

@geindex Initialization
@geindex suppression of

Syntax:

@example
pragma Suppress_Initialization ([Entity =>] variable_or_subtype_LOCAL_NAME);
@end example

Here variable_or_subtype_LOCAL_NAME is the name introduced by a type
declaration or subtype declaration or the name of a variable introduced by
an object declaration.

In the case of a type or subtype
this pragma suppresses any implicit or explicit initialization
for all variables of the given type or subtype,
including initialization resulting from the use of pragmas
Normalize_Scalars or Initialize_Scalars.

This is considered a representation item, so it cannot be given after
the type is frozen. It applies to all subsequent object declarations,
and also any allocator that creates objects of the type.

If the pragma is given for the first subtype, then it is considered
to apply to the base type and all its subtypes. If the pragma is given
for other than a first subtype, then it applies only to the given subtype.
The pragma may not be given after the type is frozen.

Note that this includes eliminating initialization of discriminants
for discriminated types, and tags for tagged types. In these cases,
you will have to use some non-portable mechanism (e.g. address
overlays or unchecked conversion) to achieve required initialization
of these fields before accessing any object of the corresponding type.

For the variable case, implicit initialization for the named variable
is suppressed, just as though its subtype had been given in a pragma
Suppress_Initialization, as described above.

@node Pragma Task_Name,Pragma Task_Storage,Pragma Suppress_Initialization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-task-name}@anchor{108}
@section Pragma Task_Name


Syntax

@example
pragma Task_Name (string_EXPRESSION);
@end example

This pragma appears within a task definition (like pragma
@code{Priority}) and applies to the task in which it appears.  The
argument must be of type String, and provides a name to be used for
the task instance when the task is created.  Note that this expression
is not required to be static, and in particular, it can contain
references to task discriminants.  This facility can be used to
provide different names for different tasks as they are created,
as illustrated in the example below.

The task name is recorded internally in the run-time structures
and is accessible to tools like the debugger.  In addition the
routine @code{Ada.Task_Identification.Image} will return this
string, with a unique task address appended.

@example
--  Example of the use of pragma Task_Name

with Ada.Task_Identification;
use Ada.Task_Identification;
with Text_IO; use Text_IO;
procedure t3 is

   type Astring is access String;

   task type Task_Typ (Name : access String) is
      pragma Task_Name (Name.all);
   end Task_Typ;

   task body Task_Typ is
      Nam : constant String := Image (Current_Task);
   begin
      Put_Line ("-->" & Nam (1 .. 14) & "<--");
   end Task_Typ;

   type Ptr_Task is access Task_Typ;
   Task_Var : Ptr_Task;

begin
   Task_Var :=
     new Task_Typ (new String'("This is task 1"));
   Task_Var :=
     new Task_Typ (new String'("This is task 2"));
end;
@end example

@node Pragma Task_Storage,Pragma Test_Case,Pragma Task_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-task-storage}@anchor{109}
@section Pragma Task_Storage


Syntax:

@example
pragma Task_Storage (
  [Task_Type =>] LOCAL_NAME,
  [Top_Guard =>] static_integer_EXPRESSION);
@end example

This pragma specifies the length of the guard area for tasks.  The guard
area is an additional storage area allocated to a task.  A value of zero
means that either no guard area is created or a minimal guard area is
created, depending on the target.  This pragma can appear anywhere a
@code{Storage_Size} attribute definition clause is allowed for a task
type.

@node Pragma Test_Case,Pragma Thread_Local_Storage,Pragma Task_Storage,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id50}@anchor{10a}@anchor{gnat_rm/implementation_defined_pragmas pragma-test-case}@anchor{10b}
@section Pragma Test_Case


@geindex Test cases

Syntax:

@example
pragma Test_Case (
   [Name     =>] static_string_Expression
  ,[Mode     =>] (Nominal | Robustness)
 [, Requires =>  Boolean_Expression]
 [, Ensures  =>  Boolean_Expression]);
@end example

The @code{Test_Case} pragma allows defining fine-grain specifications
for use by testing tools.
The compiler checks the validity of the @code{Test_Case} pragma, but its
presence does not lead to any modification of the code generated by the
compiler.

@code{Test_Case} pragmas may only appear immediately following the
(separate) declaration of a subprogram in a package declaration, inside
a package spec unit. Only other pragmas may intervene (that is appear
between the subprogram declaration and a test case).

The compiler checks that boolean expressions given in @code{Requires} and
@code{Ensures} are valid, where the rules for @code{Requires} are the
same as the rule for an expression in @code{Precondition} and the rules
for @code{Ensures} are the same as the rule for an expression in
@code{Postcondition}. In particular, attributes @code{'Old} and
@code{'Result} can only be used within the @code{Ensures}
expression. The following is an example of use within a package spec:

@example
package Math_Functions is
   ...
   function Sqrt (Arg : Float) return Float;
   pragma Test_Case (Name     => "Test 1",
                     Mode     => Nominal,
                     Requires => Arg < 10000.0,
                     Ensures  => Sqrt'Result < 10.0);
   ...
end Math_Functions;
@end example

The meaning of a test case is that there is at least one context where
@code{Requires} holds such that, if the associated subprogram is executed in
that context, then @code{Ensures} holds when the subprogram returns.
Mode @code{Nominal} indicates that the input context should also satisfy the
precondition of the subprogram, and the output context should also satisfy its
postcondition. Mode @code{Robustness} indicates that the precondition and
postcondition of the subprogram should be ignored for this test case.

@node Pragma Thread_Local_Storage,Pragma Time_Slice,Pragma Test_Case,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id51}@anchor{10c}@anchor{gnat_rm/implementation_defined_pragmas pragma-thread-local-storage}@anchor{10d}
@section Pragma Thread_Local_Storage


@geindex Task specific storage

@geindex TLS (Thread Local Storage)

@geindex Task_Attributes

Syntax:

@example
pragma Thread_Local_Storage ([Entity =>] LOCAL_NAME);
@end example

This pragma specifies that the specified entity, which must be
a variable declared in a library-level package, is to be marked as
“Thread Local Storage” (@code{TLS}). On systems supporting this (which
include Windows, Solaris, GNU/Linux, and VxWorks), this causes each
thread (and hence each Ada task) to see a distinct copy of the variable.

The variable must not have default initialization, and if there is
an explicit initialization, it must be either @code{null} for an
access variable, a static expression for a scalar variable, or a fully
static aggregate for a composite type, that is to say, an aggregate all
of whose components are static, and which does not include packed or
discriminated components.

This provides a low-level mechanism similar to that provided by
the @code{Ada.Task_Attributes} package, but much more efficient
and is also useful in writing interface code that will interact
with foreign threads.

If this pragma is used on a system where @code{TLS} is not supported,
then an error message will be generated and the program will be rejected.

@node Pragma Time_Slice,Pragma Title,Pragma Thread_Local_Storage,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-time-slice}@anchor{10e}
@section Pragma Time_Slice


Syntax:

@example
pragma Time_Slice (static_duration_EXPRESSION);
@end example

For implementations of GNAT on operating systems where it is possible
to supply a time slice value, this pragma may be used for this purpose.
It is ignored if it is used in a system that does not allow this control,
or if it appears in other than the main program unit.

@node Pragma Title,Pragma Type_Invariant,Pragma Time_Slice,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-title}@anchor{10f}
@section Pragma Title


Syntax:

@example
pragma Title (TITLING_OPTION [, TITLING OPTION]);

TITLING_OPTION ::=
  [Title    =>] STRING_LITERAL,
| [Subtitle =>] STRING_LITERAL
@end example

Syntax checked but otherwise ignored by GNAT.  This is a listing control
pragma used in DEC Ada 83 implementations to provide a title and/or
subtitle for the program listing.  The program listing generated by GNAT
does not have titles or subtitles.

Unlike other pragmas, the full flexibility of named notation is allowed
for this pragma, i.e., the parameters may be given in any order if named
notation is used, and named and positional notation can be mixed
following the normal rules for procedure calls in Ada.

@node Pragma Type_Invariant,Pragma Type_Invariant_Class,Pragma Title,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-type-invariant}@anchor{110}
@section Pragma Type_Invariant


Syntax:

@example
pragma Type_Invariant
  ([Entity =>] type_LOCAL_NAME,
   [Check  =>] EXPRESSION);
@end example

The @code{Type_Invariant} pragma is intended to be an exact
replacement for the language-defined @code{Type_Invariant}
aspect, and shares its restrictions and semantics. It differs
from the language defined @code{Invariant} pragma in that it
does not permit a string parameter, and it is
controlled by the assertion identifier @code{Type_Invariant}
rather than @code{Invariant}.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Type_Invariant_Class,Pragma Unchecked_Union,Pragma Type_Invariant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id52}@anchor{111}@anchor{gnat_rm/implementation_defined_pragmas pragma-type-invariant-class}@anchor{112}
@section Pragma Type_Invariant_Class


Syntax:

@example
pragma Type_Invariant_Class
  ([Entity =>] type_LOCAL_NAME,
   [Check  =>] EXPRESSION);
@end example

The @code{Type_Invariant_Class} pragma is intended to be an exact
replacement for the language-defined @code{Type_Invariant'Class}
aspect, and shares its restrictions and semantics.

Note: This pragma is called @code{Type_Invariant_Class} rather than
@code{Type_Invariant'Class} because the latter would not be strictly
conforming to the allowed syntax for pragmas. The motivation
for providing pragmas equivalent to the aspects is to allow a program
to be written using the pragmas, and then compiled if necessary
using an Ada compiler that does not recognize the pragmas or
aspects, but is prepared to ignore the pragmas. The assertion
policy that controls this pragma is @code{Type_Invariant'Class},
not @code{Type_Invariant_Class}.

This is an assertion kind pragma that can associate a set of its arguments
with an assertion level. See SPARK 2014 Reference Manual, section
11.4.2.

@node Pragma Unchecked_Union,Pragma Unevaluated_Use_Of_Old,Pragma Type_Invariant_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unchecked-union}@anchor{113}
@section Pragma Unchecked_Union


@geindex Unions in C

Syntax:

@example
pragma Unchecked_Union (first_subtype_LOCAL_NAME);
@end example

This pragma is used to specify a representation of a record type that is
equivalent to a C union. It was introduced as a GNAT implementation defined
pragma in the GNAT Ada 95 mode. Ada 2005 includes an extended version of this
pragma, making it language defined, and GNAT fully implements this extended
version in all language modes (Ada 83, Ada 95, and Ada 2005). For full
details, consult the Ada 2012 Reference Manual, section B.3.3.

@node Pragma Unevaluated_Use_Of_Old,Pragma User_Aspect_Definition,Pragma Unchecked_Union,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unevaluated-use-of-old}@anchor{114}
@section Pragma Unevaluated_Use_Of_Old


@geindex Attribute Old

@geindex Attribute Loop_Entry

@geindex Unevaluated_Use_Of_Old

Syntax:

@example
pragma Unevaluated_Use_Of_Old (Error | Warn | Allow);
@end example

This pragma controls the processing of attributes Old and Loop_Entry.
If either of these attributes is used in a potentially unevaluated
expression  (e.g. the then or else parts of an if expression), then
normally this usage is considered illegal if the prefix of the attribute
is other than an entity name. The language requires this
behavior for Old, and GNAT copies the same rule for Loop_Entry.

The reason for this rule is that otherwise, we can have a situation
where we save the Old value, and this results in an exception, even
though we might not evaluate the attribute. Consider this example:

@example
package UnevalOld is
   K : Character;
   procedure U (A : String; C : Boolean)  -- ERROR
     with Post => (if C then A(1)'Old = K else True);
end;
@end example

If procedure U is called with a string with a lower bound of 2, and
C false, then an exception would be raised trying to evaluate A(1)
on entry even though the value would not be actually used.

Although the rule guarantees against this possibility, it is sometimes
too restrictive. For example if we know that the string has a lower
bound of 1, then we will never raise an exception.
The pragma @code{Unevaluated_Use_Of_Old} can be
used to modify this behavior. If the argument is @code{Error} then an
error is given (this is the default RM behavior). If the argument is
@code{Warn} then the usage is allowed as legal but with a warning
that an exception might be raised. If the argument is @code{Allow}
then the usage is allowed as legal without generating a warning.

This pragma may appear as a configuration pragma, or in a declarative
part or package specification. In the latter case it applies to
uses up to the end of the corresponding statement sequence or
sequence of package declarations.

@node Pragma User_Aspect_Definition,Pragma Unimplemented_Unit,Pragma Unevaluated_Use_Of_Old,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-user-aspect-definition}@anchor{115}
@section Pragma User_Aspect_Definition


Syntax:

@example
pragma User_Aspect_Definition
  (Identifier @{, Identifier [(Identifier @{, Identifier@})]@});
@end example

This configuration pragma defines a new aspect, making it available for
subsequent use in a @cite{User_Aspect} aspect specification. The first identifier
is the name of the new aspect. Any subsequent arguments specify the names
of other aspects. A subsequent name for which no parenthesized arguments
are given shall denote either a Boolean-valued non-representation aspect
or an aspect that has been defined by another @cite{User_Aspect_Definition}
pragma. A name for which one or more arguments are given shall be either
@cite{Annotate} or @cite{Local_Restrictions} (and the arguments shall be appropriate
for the named aspect).

This pragma, together with the @cite{User_Aspect} aspect, provides a mechanism
for avoiding textual duplication if some set of aspect specifications
is needed in multiple places. This is somewhat analogous to how profiles
allow avoiding duplication of @cite{Restrictions} pragmas.

The visibility rules for an aspect defined by a @cite{User_Aspect_Definition}
pragma are the same as for a check name introduced by a @cite{Check_Name}
pragma. If multiple definitions are visible for some aspect at some point,
then the definitions must agree. A predefined aspect cannot be redefined.

@node Pragma Unimplemented_Unit,Pragma Universal_Aliasing,Pragma User_Aspect_Definition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unimplemented-unit}@anchor{116}
@section Pragma Unimplemented_Unit


Syntax:

@example
pragma Unimplemented_Unit;
@end example

If this pragma occurs in a unit that is processed by the compiler, GNAT
aborts with the message @code{xxx is not supported in this configuration},
where @code{xxx} is the name of the current compilation unit.  This pragma
is intended to allow the compiler to handle unimplemented library units in
a clean manner.

The abort only happens if code is being generated.  Thus you can use
specs of unimplemented packages in syntax or semantic checking mode.

@node Pragma Universal_Aliasing,Pragma Unmodified,Pragma Unimplemented_Unit,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id53}@anchor{117}@anchor{gnat_rm/implementation_defined_pragmas pragma-universal-aliasing}@anchor{118}
@section Pragma Universal_Aliasing


Syntax:

@example
pragma Universal_Aliasing [([Entity =>] type_LOCAL_NAME)];
@end example

@code{type_LOCAL_NAME} must refer to a type declaration in the current
declarative part.  The effect is to inhibit strict type-based aliasing
optimizations for the given type.  For a detailed description of the
strict type-based aliasing optimizations and the situations in which
they need to be suppressed, see the section on
@code{Optimization and Strict Aliasing} in the @cite{GNAT User’s Guide}.

@node Pragma Unmodified,Pragma Unreferenced,Pragma Universal_Aliasing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id54}@anchor{119}@anchor{gnat_rm/implementation_defined_pragmas pragma-unmodified}@anchor{11a}
@section Pragma Unmodified


@geindex Warnings
@geindex unmodified

Syntax:

@example
pragma Unmodified (LOCAL_NAME @{, LOCAL_NAME@});
@end example

This pragma signals that the assignable entities (variables,
@code{out} parameters, @code{in out} parameters) whose names are listed are
deliberately not assigned in the current source unit. This
suppresses warnings about the
entities being referenced but not assigned, and in addition a warning will be
generated if one of these entities is in fact assigned in the
same unit as the pragma (or in the corresponding body, or one
of its subunits).

This is particularly useful for clearly signaling that a particular
parameter is not modified, even though the spec suggests that it might
be.

For the variable case, warnings are never given for unreferenced variables
whose name contains one of the substrings
@code{DISCARD, DUMMY, IGNORE, JUNK, UNUSE, TMP, TEMP} in any casing. Such names
are typically to be used in cases where such warnings are expected.
Thus it is never necessary to use @code{pragma Unmodified} for such
variables, though it is harmless to do so.

@node Pragma Unreferenced,Pragma Unreferenced_Objects,Pragma Unmodified,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id55}@anchor{11b}@anchor{gnat_rm/implementation_defined_pragmas pragma-unreferenced}@anchor{11c}
@section Pragma Unreferenced


@geindex Warnings
@geindex unreferenced

Syntax:

@example
pragma Unreferenced (LOCAL_NAME @{, LOCAL_NAME@});
pragma Unreferenced (library_unit_NAME @{, library_unit_NAME@});
@end example

This pragma signals that the entities whose names are listed are
deliberately not referenced in the current source unit after the
occurrence of the pragma. This
suppresses warnings about the
entities being unreferenced, and in addition a warning will be
generated if one of these entities is in fact subsequently referenced in the
same unit as the pragma (or in the corresponding body, or one
of its subunits).

This is particularly useful for clearly signaling that a particular
parameter is not referenced in some particular subprogram implementation
and that this is deliberate. It can also be useful in the case of
objects declared only for their initialization or finalization side
effects.

If @code{LOCAL_NAME} identifies more than one matching homonym in the
current scope, then the entity most recently declared is the one to which
the pragma applies. Note that in the case of accept formals, the pragma
Unreferenced may appear immediately after the keyword @code{do} which
allows the indication of whether or not accept formals are referenced
or not to be given individually for each accept statement.

The left hand side of an assignment does not count as a reference for the
purpose of this pragma. Thus it is fine to assign to an entity for which
pragma Unreferenced is given. However, use of an entity as an actual for
an out parameter does count as a reference unless warnings for unread output
parameters are enabled via @code{-gnatw.o}.

Note that if a warning is desired for all calls to a given subprogram,
regardless of whether they occur in the same unit as the subprogram
declaration, then this pragma should not be used (calls from another
unit would not be flagged); pragma Obsolescent can be used instead
for this purpose, see @ref{b8,,Pragma Obsolescent}.

The second form of pragma @code{Unreferenced} is used within a context
clause. In this case the arguments must be unit names of units previously
mentioned in @code{with} clauses (similar to the usage of pragma
@code{Elaborate_All}). The effect is to suppress warnings about unreferenced
units and unreferenced entities within these units.

For the variable case, warnings are never given for unreferenced variables
whose name contains one of the substrings
@code{DISCARD, DUMMY, IGNORE, JUNK, UNUSE, TMP, TEMP} in any casing. Such names
are typically to be used in cases where such warnings are expected.
Thus it is never necessary to use @code{pragma Unreferenced} for such
variables, though it is harmless to do so.

@node Pragma Unreferenced_Objects,Pragma Unreserve_All_Interrupts,Pragma Unreferenced,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id56}@anchor{11d}@anchor{gnat_rm/implementation_defined_pragmas pragma-unreferenced-objects}@anchor{11e}
@section Pragma Unreferenced_Objects


@geindex Warnings
@geindex unreferenced

Syntax:

@example
pragma Unreferenced_Objects (local_subtype_NAME @{, local_subtype_NAME@});
@end example

This pragma signals that for the types or subtypes whose names are
listed, objects which are declared with one of these types or subtypes may
not be referenced, and if no references appear, no warnings are given.

This is particularly useful for objects which are declared solely for their
initialization and finalization effect. Such variables are sometimes referred
to as RAII variables (Resource Acquisition Is Initialization). Using this
pragma on the relevant type (most typically a limited controlled type), the
compiler will automatically suppress unwanted warnings about these variables
not being referenced.

@node Pragma Unreserve_All_Interrupts,Pragma Unsuppress,Pragma Unreferenced_Objects,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unreserve-all-interrupts}@anchor{11f}
@section Pragma Unreserve_All_Interrupts


Syntax:

@example
pragma Unreserve_All_Interrupts;
@end example

Normally certain interrupts are reserved to the implementation.  Any attempt
to attach an interrupt causes Program_Error to be raised, as described in
RM C.3.2(22).  A typical example is the @code{SIGINT} interrupt used in
many systems for a @code{Ctrl}-@code{C} interrupt.  Normally this interrupt is
reserved to the implementation, so that @code{Ctrl}-@code{C} can be used to
interrupt execution.

If the pragma @code{Unreserve_All_Interrupts} appears anywhere in any unit in
a program, then all such interrupts are unreserved.  This allows the
program to handle these interrupts, but disables their standard
functions.  For example, if this pragma is used, then pressing
@code{Ctrl}-@code{C} will not automatically interrupt execution.  However,
a program can then handle the @code{SIGINT} interrupt as it chooses.

For a full list of the interrupts handled in a specific implementation,
see the source code for the spec of @code{Ada.Interrupts.Names} in
file @code{a-intnam.ads}.  This is a target dependent file that contains the
list of interrupts recognized for a given target.  The documentation in
this file also specifies what interrupts are affected by the use of
the @code{Unreserve_All_Interrupts} pragma.

For a more general facility for controlling what interrupts can be
handled, see pragma @code{Interrupt_State}, which subsumes the functionality
of the @code{Unreserve_All_Interrupts} pragma.

@node Pragma Unsuppress,Pragma Unused,Pragma Unreserve_All_Interrupts,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unsuppress}@anchor{120}
@section Pragma Unsuppress


Syntax:

@example
pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);
@end example

This pragma undoes the effect of a previous pragma @code{Suppress}.  If
there is no corresponding pragma @code{Suppress} in effect, it has no
effect.  The range of the effect is the same as for pragma
@code{Suppress}.  The meaning of the arguments is identical to that used
in pragma @code{Suppress}.

One important application is to ensure that checks are on in cases where
code depends on the checks for its correct functioning, so that the code
will compile correctly even if the compiler switches are set to suppress
checks. For example, in a program that depends on external names of tagged
types and wants to ensure that the duplicated tag check occurs even if all
run-time checks are suppressed by a compiler switch, the following
configuration pragma will ensure this test is not suppressed:

@example
pragma Unsuppress (Duplicated_Tag_Check);
@end example

This pragma is standard in Ada 2005. It is available in all earlier versions
of Ada as an implementation-defined pragma.

Note that in addition to the checks defined in the Ada RM, GNAT recognizes a
number of implementation-defined check names. See the description of pragma
@code{Suppress} for full details.

@node Pragma Unused,Pragma Use_VADS_Size,Pragma Unsuppress,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id57}@anchor{121}@anchor{gnat_rm/implementation_defined_pragmas pragma-unused}@anchor{122}
@section Pragma Unused


@geindex Warnings
@geindex unused

Syntax:

@example
pragma Unused (LOCAL_NAME @{, LOCAL_NAME@});
@end example

This pragma signals that the assignable entities (variables,
@code{out} parameters, and @code{in out} parameters) whose names are listed
deliberately do not get assigned or referenced in the current source unit
after the occurrence of the pragma in the current source unit. This
suppresses warnings about the entities that are unreferenced and/or not
assigned, and, in addition, a warning will be generated if one of these
entities gets assigned or subsequently referenced in the same unit as the
pragma (in the corresponding body or one of its subunits).

This is particularly useful for clearly signaling that a particular
parameter is not modified or referenced, even though the spec suggests
that it might be.

For the variable case, warnings are never given for unreferenced
variables whose name contains one of the substrings
@code{DISCARD, DUMMY, IGNORE, JUNK, UNUSE, TMP, TEMP} in any casing. Such names
are typically to be used in cases where such warnings are expected.
Thus it is never necessary to use @code{pragma Unused} for such
variables, though it is harmless to do so.

@node Pragma Use_VADS_Size,Pragma Validity_Checks,Pragma Unused,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-use-vads-size}@anchor{123}
@section Pragma Use_VADS_Size


@geindex Size
@geindex VADS compatibility

@geindex Rational profile

Syntax:

@example
pragma Use_VADS_Size;
@end example

This is a configuration pragma.  In a unit to which it applies, any use
of the ‘Size attribute is automatically interpreted as a use of the
‘VADS_Size attribute.  Note that this may result in incorrect semantic
processing of valid Ada 95 or Ada 2005 programs.  This is intended to aid in
the handling of existing code which depends on the interpretation of Size
as implemented in the VADS compiler.  See description of the VADS_Size
attribute for further details.

@node Pragma Validity_Checks,Pragma Volatile,Pragma Use_VADS_Size,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-validity-checks}@anchor{124}
@section Pragma Validity_Checks


Syntax:

@example
pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off);
@end example

This pragma is used in conjunction with compiler switches to control the
built-in validity checking provided by GNAT.  The compiler switches, if set
provide an initial setting for the switches, and this pragma may be used
to modify these settings, or the settings may be provided entirely by
the use of the pragma.  This pragma can be used anywhere that a pragma
is legal, including use as a configuration pragma (including use in
the @code{gnat.adc} file).

The form with a string literal specifies which validity options are to be
activated.  The validity checks are first set to include only the default
reference manual settings, and then a string of letters in the string
specifies the exact set of options required.  The form of this string
is exactly as described for the `-gnatVx' compiler switch (see the
GNAT User’s Guide for details).  For example the following two
methods can be used to enable validity checking for mode @code{in} and
@code{in out} subprogram parameters:


@itemize *

@item 
@example
pragma Validity_Checks ("im");
@end example

@item 
@example
$ gcc -c -gnatVim ...
@end example
@end itemize

The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the @code{gnatVa} switch).

The forms with @code{Off} and @code{On} can be used to temporarily disable
validity checks as shown in the following example:

@example
pragma Validity_Checks ("c"); -- validity checks for copies
pragma Validity_Checks (Off); -- turn off validity checks
A := B;                       -- B will not be validity checked
pragma Validity_Checks (On);  -- turn validity checks back on
A := C;                       -- C will be validity checked
@end example

@node Pragma Volatile,Pragma Volatile_Full_Access,Pragma Validity_Checks,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id58}@anchor{125}@anchor{gnat_rm/implementation_defined_pragmas pragma-volatile}@anchor{126}
@section Pragma Volatile


Syntax:

@example
pragma Volatile (LOCAL_NAME);
@end example

This pragma is defined by the Ada Reference Manual, and the GNAT
implementation is fully conformant with this definition.  The reason it
is mentioned in this section is that a pragma of the same name was supplied
in some Ada 83 compilers, including DEC Ada 83.  The Ada 95 / Ada 2005
implementation of pragma Volatile is upwards compatible with the
implementation in DEC Ada 83.

@node Pragma Volatile_Full_Access,Pragma Volatile_Function,Pragma Volatile,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id59}@anchor{127}@anchor{gnat_rm/implementation_defined_pragmas pragma-volatile-full-access}@anchor{128}
@section Pragma Volatile_Full_Access


Syntax:

@example
pragma Volatile_Full_Access (LOCAL_NAME);
@end example

This is similar in effect to pragma Volatile, except that any reference to the
object is guaranteed to be done only with instructions that read or write all
the bits of the object. Furthermore, if the object is of a composite type,
then any reference to a subcomponent of the object is guaranteed to read
and/or write all the bits of the object.

The intention is that this be suitable for use with memory-mapped I/O devices
on some machines. Note that there are two important respects in which this is
different from @code{pragma Atomic}. First a reference to a @code{Volatile_Full_Access}
object is not a sequential action in the RM 9.10 sense and, therefore, does
not create a synchronization point. Second, in the case of @code{pragma Atomic},
there is no guarantee that all the bits will be accessed if the reference
is not to the whole object; the compiler is allowed (and generally will)
access only part of the object in this case.

@node Pragma Volatile_Function,Pragma Warning_As_Error,Pragma Volatile_Full_Access,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id60}@anchor{129}@anchor{gnat_rm/implementation_defined_pragmas pragma-volatile-function}@anchor{12a}
@section Pragma Volatile_Function


Syntax:

@example
pragma Volatile_Function [ (static_boolean_EXPRESSION) ];
@end example

For the semantics of this pragma, see the entry for aspect @code{Volatile_Function}
in the SPARK 2014 Reference Manual, section 7.1.2.

@node Pragma Warning_As_Error,Pragma Warnings,Pragma Volatile_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-warning-as-error}@anchor{12b}
@section Pragma Warning_As_Error


Syntax:

@example
pragma Warning_As_Error (static_string_EXPRESSION);
@end example

This configuration pragma allows the programmer to specify a set
of warnings that will be treated as errors. Any warning that
matches the pattern given by the pragma argument will be treated
as an error. This gives more precise control than -gnatwe,
which treats warnings as errors.

This pragma can apply to regular warnings (messages enabled by -gnatw)
and to style warnings (messages that start with “(style)”,
enabled by -gnaty).

The pattern may contain asterisks, which match zero or more characters
in the message. For example, you can use @code{pragma Warning_As_Error
("bits of*unused")} to treat the warning message @code{warning: 960 bits of
"a" unused} as an error. All characters other than asterisk are treated
as literal characters in the match. The match is case insensitive; for
example XYZ matches xyz.

Note that the pattern matches if it occurs anywhere within the warning
message string (it is not necessary to put an asterisk at the start and
the end of the message, since this is implied).

Another possibility for the static_string_EXPRESSION which works whether
or not error tags are enabled (`-gnatw.d') is to use a single
`-gnatw' tag string, enclosed in brackets,
as shown in the example below, to treat one category of warnings as errors.
Note that if you want to treat multiple categories of warnings as errors,
you can use multiple pragma Warning_As_Error.

The above use of patterns to match the message applies only to warning
messages generated by the front end. This pragma can also be applied to
warnings provided by the back end and mentioned in @ref{12c,,Pragma Warnings}.
By using a single full `-Wxxx' switch in the pragma, such warnings
can also be treated as errors.

The pragma can appear either in a global configuration pragma file
(e.g. @code{gnat.adc}), or at the start of a file. Given a global
configuration pragma file containing:

@example
pragma Warning_As_Error ("[-gnatwj]");
@end example

which will treat all obsolescent feature warnings as errors, the
following program compiles as shown (compile options here are
`-gnatwa.d -gnatl -gnatj55').

@example
    1. pragma Warning_As_Error ("*never assigned*");
    2. function Warnerr return String is
    3.    X : Integer;
          |
       >>> error: variable "X" is never read and
           never assigned [-gnatwv] [warning-as-error]

    4.    Y : Integer;
          |
       >>> warning: variable "Y" is assigned but
           never read [-gnatwu]

    5. begin
    6.    Y := 0;
    7.    return %ABC%;
                 |
       >>> error: use of "%" is an obsolescent
           feature (RM J.2(4)), use """ instead
           [-gnatwj] [warning-as-error]

    8. end;

8 lines: No errors, 3 warnings (2 treated as errors)
@end example

Note that this pragma does not affect the set of warnings issued in
any way, it merely changes the effect of a matching warning if one
is produced as a result of other warnings options. As shown in this
example, if the pragma results in a warning being treated as an error,
the tag is changed from “warning:” to “error:” and the string
“[warning-as-error]” is appended to the end of the message.

@node Pragma Warnings,Pragma Weak_External,Pragma Warning_As_Error,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id61}@anchor{12d}@anchor{gnat_rm/implementation_defined_pragmas pragma-warnings}@anchor{12c}
@section Pragma Warnings


Syntax:

@example
pragma Warnings ([TOOL_NAME,] DETAILS [, REASON]);

DETAILS ::= On | Off
DETAILS ::= On | Off, local_NAME
DETAILS ::= static_string_EXPRESSION
DETAILS ::= On | Off, static_string_EXPRESSION

TOOL_NAME ::= GNAT | GNATprove

REASON ::= Reason => STRING_LITERAL @{& STRING_LITERAL@}
@end example

Note: in Ada 83 mode, a string literal may be used in place of a static string
expression (which does not exist in Ada 83).

Note if the second argument of @code{DETAILS} is a @code{local_NAME} then the
second form is always understood. If the intention is to use
the fourth form, then you can write @code{NAME & ""} to force the
interpretation as a `static_string_EXPRESSION'.

Note: if the first argument is a valid @code{TOOL_NAME}, it will be interpreted
that way. The use of the @code{TOOL_NAME} argument is relevant only to users
of SPARK and GNATprove, see last part of this section for details.

Normally warnings are enabled, with the output being controlled by
the command line switch.  Warnings (@code{Off}) turns off generation of
warnings until a Warnings (@code{On}) is encountered or the end of the
current unit.  If generation of warnings is turned off using this
pragma, then some or all of the warning messages are suppressed,
regardless of the setting of the command line switches.

The @code{Reason} parameter may optionally appear as the last argument
in any of the forms of this pragma. It is intended purely for the
purposes of documenting the reason for the @code{Warnings} pragma.
The compiler will check that the argument is a static string but
otherwise ignore this argument. Other tools may provide specialized
processing for this string.

The form with a single argument (or two arguments if Reason present),
where the first argument is @code{ON} or @code{OFF}
may be used as a configuration pragma.

If the @code{LOCAL_NAME} parameter is present, warnings are suppressed for
the specified entity.  This suppression is effective from the point where
it occurs till the end of the extended scope of the variable (similar to
the scope of @code{Suppress}). This form cannot be used as a configuration
pragma.

In the case where the first argument is other than @code{ON} or
@code{OFF},
the third form with a single static_string_EXPRESSION argument (and possible
reason) provides more precise
control over which warnings are active. The string is a list of letters
specifying which warnings are to be activated and which deactivated. The
code for these letters is the same as the string used in the command
line switch controlling warnings. For a brief summary, use the gnatmake
command with no arguments, which will generate usage information containing
the list of warnings switches supported. For
full details see the section on @code{Warning Message Control} in the
@cite{GNAT User’s Guide}.
This form can also be used as a configuration pragma.

The warnings controlled by the @code{-gnatw} switch are generated by the
front end of the compiler. The GCC back end can provide additional warnings
and they are controlled by the @code{-W} switch. Such warnings can be
identified by the appearance of a string of the form @code{[-W@{xxx@}]} in the
message which designates the @code{-W`xxx'} switch that controls the message.
The form with a single `static_string_EXPRESSION' argument also works for these
warnings, but the string must be a single full @code{-W`xxx'} switch in this
case. The above reference lists a few examples of these additional warnings.

The specified warnings will be in effect until the end of the program
or another pragma @code{Warnings} is encountered. The effect of the pragma is
cumulative. Initially the set of warnings is the standard default set
as possibly modified by compiler switches. Then each pragma Warning
modifies this set of warnings as specified. This form of the pragma may
also be used as a configuration pragma.

The fourth form, with an @code{On|Off} parameter and a string, is used to
control individual messages, based on their text. The string argument
is a pattern that is used to match against the text of individual
warning messages (not including the initial “warning: “ tag).

The pattern may contain asterisks, which match zero or more characters in
the message. For example, you can use
@code{pragma Warnings (Off, "bits of*unused")} to suppress the warning
message @code{warning: 960 bits of "a" unused}. No other regular
expression notations are permitted. All characters other than asterisk in
these three specific cases are treated as literal characters in the match.
The match is case insensitive, for example XYZ matches xyz.

Note that the pattern matches if it occurs anywhere within the warning
message string (it is not necessary to put an asterisk at the start and
the end of the message, since this is implied).

The above use of patterns to match the message applies only to warning
messages generated by the front end. This form of the pragma with a string
argument can also be used to control warnings provided by the back end and
mentioned above. By using a single full @code{-W`xxx'} switch in the pragma,
such warnings can be turned on and off.

There are two ways to use the pragma in this form. The OFF form can be used
as a configuration pragma. The effect is to suppress all warnings (if any)
that match the pattern string throughout the compilation (or match the
-W switch in the back end case).

The second usage is to suppress a warning locally, and in this case, two
pragmas must appear in sequence:

@example
pragma Warnings (Off, Pattern);
... code where given warning is to be suppressed
pragma Warnings (On, Pattern);
@end example

In this usage, the pattern string must match in the Off and On
pragmas, and (if `-gnatw.w' is given) at least one matching
warning must be suppressed.

Note: if the ON form is not found, then the effect of the OFF form extends
until the end of the file (pragma Warnings is purely textual, so its effect
does not stop at the end of the enclosing scope).

Note: to write a string that will match any warning, use the string
@code{"***"}. It will not work to use a single asterisk or two
asterisks since this looks like an operator name. This form with three
asterisks is similar in effect to specifying @code{pragma Warnings (Off)} except (if @code{-gnatw.w} is given) that a matching
@code{pragma Warnings (On, "***")} will be required. This can be
helpful in avoiding forgetting to turn warnings back on.

Note: the debug flag @code{-gnatd.i} can be
used to cause the compiler to entirely ignore all WARNINGS pragmas. This can
be useful in checking whether obsolete pragmas in existing programs are hiding
real problems.

Note: pragma Warnings does not affect the processing of style messages. See
separate entry for pragma Style_Checks for control of style messages.

Users of the formal verification tool GNATprove for the SPARK subset of Ada may
use the version of the pragma with a @code{TOOL_NAME} parameter.

If present, @code{TOOL_NAME} is the name of a tool, currently either @code{GNAT} for the
compiler or @code{GNATprove} for the formal verification tool. A given tool only
takes into account pragma Warnings that do not specify a tool name, or that
specify the matching tool name. This makes it possible to disable warnings
selectively for each tool, and as a consequence to detect useless pragma
Warnings with switch @code{-gnatw.w}.

@node Pragma Weak_External,Pragma Wide_Character_Encoding,Pragma Warnings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-weak-external}@anchor{12e}
@section Pragma Weak_External


Syntax:

@example
pragma Weak_External ([Entity =>] LOCAL_NAME);
@end example

@code{LOCAL_NAME} must refer to an object that is declared at the library
level. This pragma specifies that the given entity should be marked as a
weak symbol for the linker. It is equivalent to @code{__attribute__((weak))}
in GNU C and causes @code{LOCAL_NAME} to be emitted as a weak symbol instead
of a regular symbol, that is to say a symbol that does not have to be
resolved by the linker if used in conjunction with a pragma Import.

When a weak symbol is not resolved by the linker, its address is set to
zero. This is useful in writing interfaces to external modules that may
or may not be linked in the final executable, for example depending on
configuration settings.

If a program references at run time an entity to which this pragma has been
applied, and the corresponding symbol was not resolved at link time, then
the execution of the program is erroneous. It is not erroneous to take the
Address of such an entity, for example to guard potential references,
as shown in the example below.

Some file formats do not support weak symbols so not all target machines
support this pragma.

@example
--  Example of the use of pragma Weak_External

package External_Module is
  key : Integer;
  pragma Import (C, key);
  pragma Weak_External (key);
  function Present return boolean;
end External_Module;

with System; use System;
package body External_Module is
  function Present return boolean is
  begin
    return key'Address /= System.Null_Address;
  end Present;
end External_Module;
@end example

@node Pragma Wide_Character_Encoding,,Pragma Weak_External,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-wide-character-encoding}@anchor{12f}
@section Pragma Wide_Character_Encoding


Syntax:

@example
pragma Wide_Character_Encoding (IDENTIFIER | CHARACTER_LITERAL);
@end example

This pragma specifies the wide character encoding to be used in program
source text appearing subsequently. It is a configuration pragma, but may
also be used at any point that a pragma is allowed, and it is permissible
to have more than one such pragma in a file, allowing multiple encodings
to appear within the same file.

However, note that the pragma cannot immediately precede the relevant
wide character, because then the previous encoding will still be in
effect, causing “illegal character” errors.

The argument can be an identifier or a character literal. In the identifier
case, it is one of @code{HEX}, @code{UPPER}, @code{SHIFT_JIS},
@code{EUC}, @code{UTF8}, or @code{BRACKETS}. In the character literal
case it is correspondingly one of the characters @code{h}, @code{u},
@code{s}, @code{e}, @code{8}, or @code{b}.

Note that when the pragma is used within a file, it affects only the
encoding within that file, and does not affect withed units, specs,
or subunits.

@node Implementation Defined Aspects,Implementation Defined Attributes,Implementation Defined Pragmas,Top
@anchor{gnat_rm/implementation_defined_aspects doc}@anchor{130}@anchor{gnat_rm/implementation_defined_aspects id1}@anchor{131}@anchor{gnat_rm/implementation_defined_aspects implementation-defined-aspects}@anchor{132}
@chapter Implementation Defined Aspects


Ada defines (throughout the Ada 2012 reference manual, summarized
in Annex K) a set of aspects that can be specified for certain entities.
These language defined aspects are implemented in GNAT in Ada 2012 mode
and work as described in the Ada 2012 Reference Manual.

In addition, Ada 2012 allows implementations to define additional aspects
whose meaning is defined by the implementation.  GNAT provides
a number of these implementation-defined aspects which can be used
to extend and enhance the functionality of the compiler.  This section of
the GNAT reference manual describes these additional aspects.

Note that any program using these aspects may not be portable to
other compilers (although GNAT implements this set of aspects on all
platforms).  Therefore if portability to other compilers is an important
consideration, you should minimize the use of these aspects.

Note that for many of these aspects, the effect is essentially similar
to the use of a pragma or attribute specification with the same name
applied to the entity. For example, if we write:

@example
type R is range 1 .. 100
  with Value_Size => 10;
@end example

then the effect is the same as:

@example
type R is range 1 .. 100;
for R'Value_Size use 10;
@end example

and if we write:

@example
type R is new Integer
  with Shared => True;
@end example

then the effect is the same as:

@example
type R is new Integer;
pragma Shared (R);
@end example

In the documentation below, such cases are simply marked
as being boolean aspects equivalent to the corresponding pragma
or attribute definition clause.

@menu
* Aspect Abstract_State:: 
* Aspect Always_Terminates:: 
* Aspect Annotate:: 
* Aspect Async_Readers:: 
* Aspect Async_Writers:: 
* Aspect Constant_After_Elaboration:: 
* Aspect Contract_Cases:: 
* Aspect Depends:: 
* Aspect Default_Initial_Condition:: 
* Aspect Dimension:: 
* Aspect Dimension_System:: 
* Aspect Disable_Controlled:: 
* Aspect Effective_Reads:: 
* Aspect Effective_Writes:: 
* Aspect Exceptional_Cases:: 
* Aspect Exit_Cases:: 
* Aspect Extended_Access:: 
* Aspect Extensions_Visible:: 
* Aspect Favor_Top_Level:: 
* Aspect Ghost:: 
* Aspect Ghost_Predicate:: 
* Aspect Global:: 
* Aspect Initial_Condition:: 
* Aspect Initializes:: 
* Aspect Inline_Always:: 
* Aspect Invariant:: 
* Aspect Invariant’Class:: 
* Aspect Iterable:: 
* Aspect Linker_Section:: 
* Aspect Local_Restrictions:: 
* Aspect Lock_Free:: 
* Aspect Max_Queue_Length:: 
* Aspect No_Caching:: 
* Aspect No_Elaboration_Code_All:: 
* Aspect No_Inline:: 
* Aspect No_Raise:: 
* Aspect No_Tagged_Streams:: 
* Aspect No_Task_Parts:: 
* Aspect Object_Size:: 
* Aspect Obsolescent:: 
* Aspect Part_Of:: 
* Aspect Persistent_BSS:: 
* Aspect Potentially_Invalid:: 
* Aspect Predicate:: 
* Aspect Program_Exit:: 
* Aspect Pure_Function:: 
* Aspect Refined_Depends:: 
* Aspect Refined_Global:: 
* Aspect Refined_Post:: 
* Aspect Refined_State:: 
* Aspect Relaxed_Initialization:: 
* Aspect Remote_Access_Type:: 
* Aspect Scalar_Storage_Order:: 
* Aspect Secondary_Stack_Size:: 
* Aspect Shared:: 
* Aspect Side_Effects:: 
* Aspect Simple_Storage_Pool:: 
* Aspect Simple_Storage_Pool_Type:: 
* Aspect SPARK_Mode:: 
* Aspect Subprogram_Variant:: 
* Aspect Suppress_Debug_Info:: 
* Aspect Suppress_Initialization:: 
* Aspect Test_Case:: 
* Aspect Thread_Local_Storage:: 
* Aspect Universal_Aliasing:: 
* Aspect Unmodified:: 
* Aspect Unreferenced:: 
* Aspect Unreferenced_Objects:: 
* Aspect User_Aspect:: 
* Aspect Value_Size:: 
* Aspect Volatile_Full_Access:: 
* Aspect Volatile_Function:: 
* Aspect Warnings:: 

@end menu

@node Aspect Abstract_State,Aspect Always_Terminates,,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-abstract-state}@anchor{133}
@section Aspect Abstract_State


@geindex Abstract_State

This aspect is equivalent to @ref{1f,,pragma Abstract_State}.

@node Aspect Always_Terminates,Aspect Annotate,Aspect Abstract_State,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-always-terminates}@anchor{134}
@section Aspect Always_Terminates


@geindex Always_Terminates

This boolean aspect is equivalent to @ref{2a,,pragma Always_Terminates}.

@node Aspect Annotate,Aspect Async_Readers,Aspect Always_Terminates,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-annotate}@anchor{135}
@section Aspect Annotate


@geindex Annotate

There are three forms of this aspect (where ID is an identifier,
and ARG is a general expression),
corresponding to @ref{2c,,pragma Annotate}.


@table @asis

@item `Annotate => ID'

Equivalent to @code{pragma Annotate (ID, Entity => Name);}

@item `Annotate => (ID)'

Equivalent to @code{pragma Annotate (ID, Entity => Name);}

@item `Annotate => (ID ,ID @{, ARG@})'

Equivalent to @code{pragma Annotate (ID, ID @{, ARG@}, Entity => Name);}
@end table

@node Aspect Async_Readers,Aspect Async_Writers,Aspect Annotate,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-async-readers}@anchor{136}
@section Aspect Async_Readers


@geindex Async_Readers

This boolean aspect is equivalent to @ref{34,,pragma Async_Readers}.

@node Aspect Async_Writers,Aspect Constant_After_Elaboration,Aspect Async_Readers,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-async-writers}@anchor{137}
@section Aspect Async_Writers


@geindex Async_Writers

This boolean aspect is equivalent to @ref{36,,pragma Async_Writers}.

@node Aspect Constant_After_Elaboration,Aspect Contract_Cases,Aspect Async_Writers,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-constant-after-elaboration}@anchor{138}
@section Aspect Constant_After_Elaboration


@geindex Constant_After_Elaboration

This aspect is equivalent to @ref{46,,pragma Constant_After_Elaboration}.

@node Aspect Contract_Cases,Aspect Depends,Aspect Constant_After_Elaboration,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-contract-cases}@anchor{139}
@section Aspect Contract_Cases


@geindex Contract_Cases

This aspect is equivalent to @ref{48,,pragma Contract_Cases}, the sequence
of clauses being enclosed in parentheses so that syntactically it is an
aggregate.

@node Aspect Depends,Aspect Default_Initial_Condition,Aspect Contract_Cases,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-depends}@anchor{13a}
@section Aspect Depends


@geindex Depends

This aspect is equivalent to @ref{58,,pragma Depends}.

@node Aspect Default_Initial_Condition,Aspect Dimension,Aspect Depends,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-default-initial-condition}@anchor{13b}
@section Aspect Default_Initial_Condition


@geindex Default_Initial_Condition

This aspect is equivalent to @ref{54,,pragma Default_Initial_Condition}.

@node Aspect Dimension,Aspect Dimension_System,Aspect Default_Initial_Condition,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-dimension}@anchor{13c}
@section Aspect Dimension


@geindex Dimension

The @code{Dimension} aspect is used to specify the dimensions of a given
subtype of a dimensioned numeric type. The aspect also specifies a symbol
used when doing formatted output of dimensioned quantities. The syntax is:

@example
with Dimension =>
  ([Symbol =>] SYMBOL, DIMENSION_VALUE @{, DIMENSION_Value@})

SYMBOL ::= STRING_LITERAL | CHARACTER_LITERAL

DIMENSION_VALUE ::=
  RATIONAL
| others               => RATIONAL
| DISCRETE_CHOICE_LIST => RATIONAL

RATIONAL ::= [-] NUMERIC_LITERAL [/ NUMERIC_LITERAL]
@end example

This aspect can only be applied to a subtype whose parent type has
a @code{Dimension_System} aspect. The aspect must specify values for
all dimensions of the system. The rational values are the powers of the
corresponding dimensions that are used by the compiler to verify that
physical (numeric) computations are dimensionally consistent. For example,
the computation of a force must result in dimensions (L => 1, M => 1, T => -2).
For further examples of the usage
of this aspect, see package @code{System.Dim.Mks}.
Note that when the dimensioned type is an integer type, then any
dimension value must be an integer literal.

@node Aspect Dimension_System,Aspect Disable_Controlled,Aspect Dimension,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-dimension-system}@anchor{13d}
@section Aspect Dimension_System


@geindex Dimension_System

The @code{Dimension_System} aspect is used to define a system of
dimensions that will be used in subsequent subtype declarations with
@code{Dimension} aspects that reference this system. The syntax is:

@example
with Dimension_System => (DIMENSION @{, DIMENSION@});

DIMENSION ::= ([Unit_Name   =>] IDENTIFIER,
               [Unit_Symbol =>] SYMBOL,
               [Dim_Symbol  =>] SYMBOL)

SYMBOL ::= CHARACTER_LITERAL | STRING_LITERAL
@end example

This aspect is applied to a type, which must be a numeric derived type
(typically a floating-point type), that
will represent values within the dimension system. Each @code{DIMENSION}
corresponds to one particular dimension. A maximum of 7 dimensions may
be specified. @code{Unit_Name} is the name of the dimension (for example
@code{Meter}). @code{Unit_Symbol} is the shorthand used for quantities
of this dimension (for example @code{m} for @code{Meter}).
@code{Dim_Symbol} gives
the identification within the dimension system (typically this is a
single letter, e.g. @code{L} standing for length for unit name @code{Meter}).
The @code{Unit_Symbol} is used in formatted output of dimensioned quantities.
The @code{Dim_Symbol} is used in error messages when numeric operations have
inconsistent dimensions.

GNAT provides the standard definition of the International MKS system in
the run-time package @code{System.Dim.Mks}. You can easily define
similar packages for cgs units or British units, and define conversion factors
between values in different systems. The MKS system is characterized by the
following aspect:

@example
type Mks_Type is new Long_Long_Float with
  Dimension_System => (
    (Unit_Name => Meter,    Unit_Symbol => 'm',   Dim_Symbol => 'L'),
    (Unit_Name => Kilogram, Unit_Symbol => "kg",  Dim_Symbol => 'M'),
    (Unit_Name => Second,   Unit_Symbol => 's',   Dim_Symbol => 'T'),
    (Unit_Name => Ampere,   Unit_Symbol => 'A',   Dim_Symbol => 'I'),
    (Unit_Name => Kelvin,   Unit_Symbol => 'K',   Dim_Symbol => '@@'),
    (Unit_Name => Mole,     Unit_Symbol => "mol", Dim_Symbol => 'N'),
    (Unit_Name => Candela,  Unit_Symbol => "cd",  Dim_Symbol => 'J'));
@end example

Note that in the above type definition, we use the @code{at} symbol (@code{@@}) to
represent a theta character (avoiding the use of extended Latin-1
characters in this context).

See section ‘Performing Dimensionality Analysis in GNAT’ in the GNAT Users
Guide for detailed examples of use of the dimension system.

@node Aspect Disable_Controlled,Aspect Effective_Reads,Aspect Dimension_System,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-disable-controlled}@anchor{13e}
@section Aspect Disable_Controlled


@geindex Disable_Controlled

The aspect  @code{Disable_Controlled} is defined for controlled record types. If
active, this aspect causes suppression of all related calls to @code{Initialize},
@code{Adjust}, and @code{Finalize}. The intended use is for conditional compilation,
where for example you might want a record to be controlled or not depending on
whether some run-time check is enabled or suppressed.

@node Aspect Effective_Reads,Aspect Effective_Writes,Aspect Disable_Controlled,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-effective-reads}@anchor{13f}
@section Aspect Effective_Reads


@geindex Effective_Reads

This aspect is equivalent to @ref{5d,,pragma Effective_Reads}.

@node Aspect Effective_Writes,Aspect Exceptional_Cases,Aspect Effective_Reads,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-effective-writes}@anchor{140}
@section Aspect Effective_Writes


@geindex Effective_Writes

This aspect is equivalent to @ref{5f,,pragma Effective_Writes}.

@node Aspect Exceptional_Cases,Aspect Exit_Cases,Aspect Effective_Writes,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-exceptional-cases}@anchor{141}
@section Aspect Exceptional_Cases


@geindex Exceptional_Cases

This aspect may be specified for procedures and functions with side effects;
it can be used to list exceptions that might be propagated by the subprogram
with side effects in the context of its precondition, and associate them
with a specific postcondition.

For the syntax and semantics of this aspect, see the SPARK 2014 Reference
Manual, section 6.1.9.

@node Aspect Exit_Cases,Aspect Extended_Access,Aspect Exceptional_Cases,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-exit-cases}@anchor{142}
@section Aspect Exit_Cases


@geindex Exit_Cases

This aspect may be specified for procedures and functions with side effects;
it can be used to partition the input state into a list of cases and specify,
for each case, how the subprogram is allowed to terminate (i.e. return normally
or propagate an exception).

For the syntax and semantics of this aspect, see the SPARK 2014 Reference
Manual, section 6.1.10.

@node Aspect Extended_Access,Aspect Extensions_Visible,Aspect Exit_Cases,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-extended-access}@anchor{143}
@section Aspect Extended_Access


This nonoverridable boolean-valued type-related representation aspect can be
specified as part of a full_type_declaration for a general access type
designating an unconstrained array subtype.

The absence of an Extended_Access aspect specification for such a
full_type_declaration is equivalent to an explicit
“Extended_Access => False” specification. This implies
that the aspect is never unspecified for an eligible access type.
An access type for which this aspect is True is said to be an extended access
type; this includes the case of a type derived from an extended access type.
Similarly, a value of such a type is said to be an extended access value.

The representation of an extended access value is different than that of
other access values. This representation makes it possible to designate
objects that cannot be designated using the usual “thin” or “fat” access
representations for an access type designating an unconstrained array
subtype (notably slices and array objects imported from other languages).

In particular, two rules are modified in determining the legality of an Access
or Unchecked_Access attribute reference if the expected access type is
an extended access type:


@itemize *

@item 
A slice of an aliased array object of a non-bitpacked type (more precisely,
of an array type having independently addressable components) is considered
to be aliased (and the accessibility level of a slice of an array object is
defined to be that of the array object); this also applies to renamings
of such slices, slices of such renamings, etc.

@item 
The requirement that the nominal subtype of the prefix shall statically
match the designated subtype of the access type need not be met.
@end itemize

The Size aspect (and other aspects including Stream_Size, Object_Size,
and Alignment) of an extended access type may depend on the properties of the
designated type. Further details of this dependence are not documented.

An extended access value is not convertible to a non-extended access type,
although conversions in the opposite direction are allowed. We don’t want
to allow

@example
type Big_Ref is access all String with Extended_Access;
type Small_Ref is access all String;
Obj : aliased String := "abcde";
Big_Ptr : Big_Ref := Obj (2 .. 4)'Access; -- OK
Small_Ptr : Small_Ref := Small_Ref (Big_Ptr); -- ERROR: illegal conversion
@end example

because there is no way to represent the result of such a conversion.

A dereference of an extended access value (or a reference to a renaming
thereof) shall not occur in any of the following contexts:


@itemize *

@item 
as an operative constituent of the prefix of an Access or
Unchecked_Access attribute reference whose expected type is not extended; or

@item 
as an operative constituent of an actual parameter in a call where
the corresponding formal parameter is explicitly aliased.
@end itemize

For the same reasons that explicit conversions from an extended access type to a
non-extended access type are forbidden, we also need to disallow getting the
same effect via a Extended_Ptr.all’Access reference; this includes the case
of passing Extended_Ptr.all as an actual parameter in a call where the
corresponding formal parameter is explicitly aliased (because the callee
could evaluate Formal_Parameter’Access). This goal is accomplished by
adjusting the definition of the term “aliased”. A dereference of an extended
value occurring in one of these contexts is defined to denote
a nonaliased view. This has the desired effect because these contexts require
an aliased view. Continuing the preceding example, this rule disallows

@example
Sneaky_1 : Small_Ptr := Big_Ptr.all'Access; -- ERROR: illegal 'Access prefix

function Make (Str : aliased in out String) return Small_Ptr
  is (Str'Access); -- OK

Sneaky_2 : Small_Ptr := Make (Str => Big_Ptr.all); -- ERROR: bad parameter
@end example

for the same reason given above in the case of an explicit type conversion.

@node Aspect Extensions_Visible,Aspect Favor_Top_Level,Aspect Extended_Access,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-extensions-visible}@anchor{144}
@section Aspect Extensions_Visible


@geindex Extensions_Visible

This aspect is equivalent to @ref{6f,,pragma Extensions_Visible}.

@node Aspect Favor_Top_Level,Aspect Ghost,Aspect Extensions_Visible,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-favor-top-level}@anchor{145}
@section Aspect Favor_Top_Level


@geindex Favor_Top_Level

This boolean aspect is equivalent to @ref{74,,pragma Favor_Top_Level}.

@node Aspect Ghost,Aspect Ghost_Predicate,Aspect Favor_Top_Level,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-ghost}@anchor{146}
@section Aspect Ghost


@geindex Ghost

This aspect is equivalent to @ref{78,,pragma Ghost}.

@node Aspect Ghost_Predicate,Aspect Global,Aspect Ghost,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-ghost-predicate}@anchor{147}
@section Aspect Ghost_Predicate


@geindex Ghost_Predicate

This aspect introduces a subtype predicate that can reference ghost
entities. The subtype cannot appear as a subtype_mark in a membership test.

For the detailed semantics of this aspect, see the entry for subtype predicates
in the SPARK Reference Manual, section 3.2.4.

@node Aspect Global,Aspect Initial_Condition,Aspect Ghost_Predicate,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-global}@anchor{148}
@section Aspect Global


@geindex Global

This aspect is equivalent to @ref{7a,,pragma Global}.

@node Aspect Initial_Condition,Aspect Initializes,Aspect Global,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-initial-condition}@anchor{149}
@section Aspect Initial_Condition


@geindex Initial_Condition

This aspect is equivalent to @ref{87,,pragma Initial_Condition}.

@node Aspect Initializes,Aspect Inline_Always,Aspect Initial_Condition,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-initializes}@anchor{14a}
@section Aspect Initializes


@geindex Initializes

This aspect is equivalent to @ref{8a,,pragma Initializes}.

@node Aspect Inline_Always,Aspect Invariant,Aspect Initializes,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-inline-always}@anchor{14b}
@section Aspect Inline_Always


@geindex Inline_Always

This boolean aspect is equivalent to @ref{8c,,pragma Inline_Always}.

@node Aspect Invariant,Aspect Invariant’Class,Aspect Inline_Always,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-invariant}@anchor{14c}
@section Aspect Invariant


@geindex Invariant

This aspect is equivalent to @ref{94,,pragma Invariant}. It is a
synonym for the language defined aspect @code{Type_Invariant} except
that it is separately controllable using pragma @code{Assertion_Policy}.

@node Aspect Invariant’Class,Aspect Iterable,Aspect Invariant,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-invariant-class}@anchor{14d}
@section Aspect Invariant’Class


@geindex Invariant'Class

This aspect is equivalent to @ref{112,,pragma Type_Invariant_Class}. It is a
synonym for the language defined aspect @code{Type_Invariant'Class} except
that it is separately controllable using pragma @code{Assertion_Policy}.

@node Aspect Iterable,Aspect Linker_Section,Aspect Invariant’Class,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-iterable}@anchor{14e}
@section Aspect Iterable


@geindex Iterable

This aspect provides a light-weight mechanism for loops and quantified
expressions over container types, without the overhead imposed by the tampering
checks of standard Ada 2012 iterators. The value of the aspect is an aggregate
with six named components, of which the last three are optional: @code{First},
@code{Next}, @code{Has_Element}, @code{Element}, @code{Last}, and @code{Previous}.
When only the first three components are specified, only the
@code{for .. in} form of iteration over cursors is available. When @code{Element}
is specified, both this form and the @code{for .. of} form of iteration over
elements are available. If the last two components are specified, reverse
iterations over the container can be specified (analogous to what can be done
over predefined containers that support the @code{Reverse_Iterator} interface).
The following is a typical example of use:

@example
type List is private with
    Iterable => (First       => First_Cursor,
                 Next        => Advance,
                 Has_Element => Cursor_Has_Element
               [,Element     => Get_Element]
               [,Last        => Last_Cursor]
               [,Previous    => Retreat]);
@end example


@itemize *

@item 
The values of @code{First} and @code{Last} are primitive operations of the
container type that return a @code{Cursor}, which must be a type declared in
the container package or visible from it. For example:
@end itemize

@example
function First_Cursor (Cont : Container) return Cursor;
function Last_Cursor  (Cont : Container) return Cursor;
@end example


@itemize *

@item 
The values of @code{Next} and @code{Previous} are primitive operations of the container type that take
both a container and a cursor and yield a cursor. For example:
@end itemize

@example
function Advance (Cont : Container; Position : Cursor) return Cursor;
function Retreat (Cont : Container; Position : Cursor) return Cursor;
@end example


@itemize *

@item 
The value of @code{Has_Element} is a primitive operation of the container type
that takes both a container and a cursor and yields a boolean. For example:
@end itemize

@example
function Cursor_Has_Element (Cont : Container; Position : Cursor) return Boolean;
@end example


@itemize *

@item 
The value of @code{Element} is a primitive operation of the container type that
takes both a container and a cursor and yields an @code{Element_Type}, which must
be a type declared in the container package or visible from it. For example:
@end itemize

@example
function Get_Element (Cont : Container; Position : Cursor) return Element_Type;
@end example

This aspect is used in the GNAT-defined formal container packages.

@node Aspect Linker_Section,Aspect Local_Restrictions,Aspect Iterable,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-linker-section}@anchor{14f}
@section Aspect Linker_Section


@geindex Linker_Section

This aspect is equivalent to @ref{9c,,pragma Linker_Section}.

@node Aspect Local_Restrictions,Aspect Lock_Free,Aspect Linker_Section,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-local-restrictions}@anchor{150}
@section Aspect Local_Restrictions


@geindex Local_Restrictions

This aspect may be specified for a subprogram (and for other declarations
as described below). It is used to specify that a particular subprogram does
not violate one or more local restrictions, nor can it call a subprogram
that is not subject to the same requirement. Positional aggregate syntax
(with parentheses, not square brackets) may be used to specify more than one
local restriction, as in

@example
procedure Do_Something
  with Local_Restrictions => (Some_Restriction, Another_Restriction);
@end example

Parentheses are currently required even in the case of specifying a single
local restriction (this requirement may be relaxed in the future).
Supported local restrictions currently include (only) No_Heap_Allocations and
No_Secondary_Stack.
No_Secondary_Stack corresponds to the GNAT-defined (global) restriction
of the same name. No_Heap_Allocations corresponds to the conjunction of the
Ada-defined restrictions No_Allocators and No_Implicit_Heap_Allocations.

Additional requirements are imposed in order to ensure that restriction
violations cannot be achieved via overriding dispatching operations,
calling through an access-to-subprogram value, calling a generic formal
subprogram, or calling through a subprogram renaming.
For a dispatching operation, an overrider must be subject to (at least) the
same restrictions as the overridden inherited subprogram; similarly, the
actual subprogram corresponding to a generic formal subprogram
in an instantiation must be subject to (at least) the same restrictions
as the formal subprogram. A call through an access-to-subprogram value
is conservatively assumed to violate all local restrictions; tasking-related
constructs (notably entry calls) are treated similarly. A renaming-as-body is
treated like a subprogram body containing a call to the renamed subprogram.

The Local_Restrictions aspect can be specified for a package specification,
in which case the aspect specification also applies to all eligible entities
declared with the package. This includes types. Default initialization of an
object of a given type is treated like a call to an implicitly-declared
initialization subprogram. Such a “call” is subject to the same local
restriction checks as any other call. If a type is subject to a local
restriction, then any violations of that restriction within the default
initialization expressions (if any) of the type are rejected. This may
include “calls” to the default initialization subprograms of other types.

Local_Restrictions aspect specifications are additive (for example, in the
case of a declaration that occurs within nested packages that each have
a Local_Restrictions specification).

@node Aspect Lock_Free,Aspect Max_Queue_Length,Aspect Local_Restrictions,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-lock-free}@anchor{151}
@section Aspect Lock_Free


@geindex Lock_Free

This boolean aspect is equivalent to @ref{9e,,pragma Lock_Free}.

@node Aspect Max_Queue_Length,Aspect No_Caching,Aspect Lock_Free,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-max-queue-length}@anchor{152}
@section Aspect Max_Queue_Length


@geindex Max_Queue_Length

This aspect is equivalent to @ref{a6,,pragma Max_Queue_Length}.

@node Aspect No_Caching,Aspect No_Elaboration_Code_All,Aspect Max_Queue_Length,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-caching}@anchor{153}
@section Aspect No_Caching


@geindex No_Caching

This boolean aspect is equivalent to @ref{a9,,pragma No_Caching}.

@node Aspect No_Elaboration_Code_All,Aspect No_Inline,Aspect No_Caching,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-elaboration-code-all}@anchor{154}
@section Aspect No_Elaboration_Code_All


@geindex No_Elaboration_Code_All

This aspect is equivalent to @ref{ac,,pragma No_Elaboration_Code_All}
for a program unit.

@node Aspect No_Inline,Aspect No_Raise,Aspect No_Elaboration_Code_All,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-inline}@anchor{155}
@section Aspect No_Inline


@geindex No_Inline

This boolean aspect is equivalent to @ref{af,,pragma No_Inline}.

@node Aspect No_Raise,Aspect No_Tagged_Streams,Aspect No_Inline,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-raise}@anchor{156}
@section Aspect No_Raise


@geindex No_Raise

This boolean aspect is equivalent to @ref{b1,,pragma No_Raise}.

@node Aspect No_Tagged_Streams,Aspect No_Task_Parts,Aspect No_Raise,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-tagged-streams}@anchor{157}
@section Aspect No_Tagged_Streams


@geindex No_Tagged_Streams

This aspect is equivalent to @ref{b5,,pragma No_Tagged_Streams} with an
argument specifying a root tagged type (thus this aspect can only be
applied to such a type).

@node Aspect No_Task_Parts,Aspect Object_Size,Aspect No_Tagged_Streams,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-task-parts}@anchor{158}
@section Aspect No_Task_Parts


@geindex No_Task_Parts

Applies to a type. If True, requires that the type and any descendants
do not have any task parts. The rules for this aspect are the same as
for the language-defined No_Controlled_Parts aspect (see RM-H.4.1),
replacing “controlled” with “task”.

If No_Task_Parts is True for a type T, then the compiler can optimize
away certain tasking-related code that would otherwise be needed
for T’Class, because descendants of T might contain tasks.

@node Aspect Object_Size,Aspect Obsolescent,Aspect No_Task_Parts,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-object-size}@anchor{159}
@section Aspect Object_Size


@geindex Object_Size

This aspect is equivalent to @ref{15a,,attribute Object_Size}.

@node Aspect Obsolescent,Aspect Part_Of,Aspect Object_Size,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-obsolescent}@anchor{15b}
@section Aspect Obsolescent


@geindex Obsolescent

This aspect is equivalent to @ref{b8,,pragma Obsolescent}. Note that the
evaluation of this aspect happens at the point of occurrence, it is not
delayed until the freeze point.

@node Aspect Part_Of,Aspect Persistent_BSS,Aspect Obsolescent,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-part-of}@anchor{15c}
@section Aspect Part_Of


@geindex Part_Of

This aspect is equivalent to @ref{be,,pragma Part_Of}.

@node Aspect Persistent_BSS,Aspect Potentially_Invalid,Aspect Part_Of,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-persistent-bss}@anchor{15d}
@section Aspect Persistent_BSS


@geindex Persistent_BSS

This boolean aspect is equivalent to @ref{c2,,pragma Persistent_BSS}.

@node Aspect Potentially_Invalid,Aspect Predicate,Aspect Persistent_BSS,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-potentially-invalid}@anchor{15e}
@section Aspect Potentially_Invalid


@geindex Potentially_Invalid

For the syntax and semantics of this aspect, see the SPARK 2014 Reference
Manual, section 13.9.1.

@node Aspect Predicate,Aspect Program_Exit,Aspect Potentially_Invalid,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-predicate}@anchor{15f}
@section Aspect Predicate


@geindex Predicate

This aspect is equivalent to @ref{c9,,pragma Predicate}. It is thus
similar to the language defined aspects @code{Dynamic_Predicate}
and @code{Static_Predicate} except that whether the resulting
predicate is static or dynamic is controlled by the form of the
expression. It is also separately controllable using pragma
@code{Assertion_Policy}.

@node Aspect Program_Exit,Aspect Pure_Function,Aspect Predicate,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-program-exit}@anchor{160}
@section Aspect Program_Exit


@geindex Program_Exit

This boolean aspect is equivalent to @ref{d2,,pragma Program_Exit}.

@node Aspect Pure_Function,Aspect Refined_Depends,Aspect Program_Exit,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-pure-function}@anchor{161}
@section Aspect Pure_Function


@geindex Pure_Function

This boolean aspect is equivalent to @ref{d7,,pragma Pure_Function}.

@node Aspect Refined_Depends,Aspect Refined_Global,Aspect Pure_Function,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-depends}@anchor{162}
@section Aspect Refined_Depends


@geindex Refined_Depends

This aspect is equivalent to @ref{db,,pragma Refined_Depends}.

@node Aspect Refined_Global,Aspect Refined_Post,Aspect Refined_Depends,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-global}@anchor{163}
@section Aspect Refined_Global


@geindex Refined_Global

This aspect is equivalent to @ref{dd,,pragma Refined_Global}.

@node Aspect Refined_Post,Aspect Refined_State,Aspect Refined_Global,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-post}@anchor{164}
@section Aspect Refined_Post


@geindex Refined_Post

This aspect is equivalent to @ref{df,,pragma Refined_Post}.

@node Aspect Refined_State,Aspect Relaxed_Initialization,Aspect Refined_Post,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-state}@anchor{165}
@section Aspect Refined_State


@geindex Refined_State

This aspect is equivalent to @ref{e1,,pragma Refined_State}.

@node Aspect Relaxed_Initialization,Aspect Remote_Access_Type,Aspect Refined_State,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-relaxed-initialization}@anchor{166}
@section Aspect Relaxed_Initialization


@geindex Refined_Initialization

For the syntax and semantics of this aspect, see the SPARK 2014 Reference
Manual, section 6.10.

@node Aspect Remote_Access_Type,Aspect Scalar_Storage_Order,Aspect Relaxed_Initialization,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-remote-access-type}@anchor{167}
@section Aspect Remote_Access_Type


@geindex Remote_Access_Type

This aspect is equivalent to @ref{e4,,pragma Remote_Access_Type}.

@node Aspect Scalar_Storage_Order,Aspect Secondary_Stack_Size,Aspect Remote_Access_Type,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-scalar-storage-order}@anchor{168}
@section Aspect Scalar_Storage_Order


@geindex Scalar_Storage_Order

This aspect is equivalent to a @ref{169,,attribute Scalar_Storage_Order}.

@node Aspect Secondary_Stack_Size,Aspect Shared,Aspect Scalar_Storage_Order,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-secondary-stack-size}@anchor{16a}
@section Aspect Secondary_Stack_Size


@geindex Secondary_Stack_Size

This aspect is equivalent to @ref{ea,,pragma Secondary_Stack_Size}.

@node Aspect Shared,Aspect Side_Effects,Aspect Secondary_Stack_Size,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-shared}@anchor{16b}
@section Aspect Shared


@geindex Shared

This boolean aspect is equivalent to @ref{ed,,pragma Shared}
and is thus a synonym for aspect @code{Atomic}.

@node Aspect Side_Effects,Aspect Simple_Storage_Pool,Aspect Shared,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-side-effects}@anchor{16c}
@section Aspect Side_Effects


@geindex Side_Effects

This aspect is equivalent to @ref{f1,,pragma Side_Effects}.

@node Aspect Simple_Storage_Pool,Aspect Simple_Storage_Pool_Type,Aspect Side_Effects,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-simple-storage-pool}@anchor{16d}
@section Aspect Simple_Storage_Pool


@geindex Simple_Storage_Pool

This aspect is equivalent to @ref{f4,,attribute Simple_Storage_Pool}.

@node Aspect Simple_Storage_Pool_Type,Aspect SPARK_Mode,Aspect Simple_Storage_Pool,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-simple-storage-pool-type}@anchor{16e}
@section Aspect Simple_Storage_Pool_Type


@geindex Simple_Storage_Pool_Type

This boolean aspect is equivalent to @ref{f3,,pragma Simple_Storage_Pool_Type}.

@node Aspect SPARK_Mode,Aspect Subprogram_Variant,Aspect Simple_Storage_Pool_Type,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-spark-mode}@anchor{16f}
@section Aspect SPARK_Mode


@geindex SPARK_Mode

This aspect is equivalent to @ref{fb,,pragma SPARK_Mode} and
may be specified for either or both of the specification and body
of a subprogram or package.

@node Aspect Subprogram_Variant,Aspect Suppress_Debug_Info,Aspect SPARK_Mode,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-subprogram-variant}@anchor{170}
@section Aspect Subprogram_Variant


@geindex Subprogram_Variant

For the syntax and semantics of this aspect, see the SPARK 2014 Reference
Manual, section 6.1.8.

@node Aspect Suppress_Debug_Info,Aspect Suppress_Initialization,Aspect Subprogram_Variant,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-suppress-debug-info}@anchor{171}
@section Aspect Suppress_Debug_Info


@geindex Suppress_Debug_Info

This boolean aspect is equivalent to @ref{104,,pragma Suppress_Debug_Info}.

@node Aspect Suppress_Initialization,Aspect Test_Case,Aspect Suppress_Debug_Info,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-suppress-initialization}@anchor{172}
@section Aspect Suppress_Initialization


@geindex Suppress_Initialization

This boolean aspect is equivalent to @ref{107,,pragma Suppress_Initialization}.

@node Aspect Test_Case,Aspect Thread_Local_Storage,Aspect Suppress_Initialization,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-test-case}@anchor{173}
@section Aspect Test_Case


@geindex Test_Case

This aspect is equivalent to @ref{10b,,pragma Test_Case}.

@node Aspect Thread_Local_Storage,Aspect Universal_Aliasing,Aspect Test_Case,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-thread-local-storage}@anchor{174}
@section Aspect Thread_Local_Storage


@geindex Thread_Local_Storage

This boolean aspect is equivalent to @ref{10d,,pragma Thread_Local_Storage}.

@node Aspect Universal_Aliasing,Aspect Unmodified,Aspect Thread_Local_Storage,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-universal-aliasing}@anchor{175}
@section Aspect Universal_Aliasing


@geindex Universal_Aliasing

This boolean aspect is equivalent to @ref{118,,pragma Universal_Aliasing}.

@node Aspect Unmodified,Aspect Unreferenced,Aspect Universal_Aliasing,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-unmodified}@anchor{176}
@section Aspect Unmodified


@geindex Unmodified

This boolean aspect is equivalent to @ref{11a,,pragma Unmodified}.

@node Aspect Unreferenced,Aspect Unreferenced_Objects,Aspect Unmodified,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-unreferenced}@anchor{177}
@section Aspect Unreferenced


@geindex Unreferenced

This boolean aspect is equivalent to @ref{11c,,pragma Unreferenced}.

When using the @code{-gnat2022} switch, this aspect is also supported on formal
parameters, which is in particular the only form possible for expression
functions.

@node Aspect Unreferenced_Objects,Aspect User_Aspect,Aspect Unreferenced,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-unreferenced-objects}@anchor{178}
@section Aspect Unreferenced_Objects


@geindex Unreferenced_Objects

This boolean aspect is equivalent to @ref{11e,,pragma Unreferenced_Objects}.

@node Aspect User_Aspect,Aspect Value_Size,Aspect Unreferenced_Objects,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-user-aspect}@anchor{179}
@section Aspect User_Aspect


@geindex User_Aspect

This aspect takes an argument that is the name of an aspect defined by a
User_Aspect_Definition configuration pragma.
A User_Aspect aspect specification is semantically equivalent to
replicating the set of aspect specifications associated with the named
pragma-defined aspect.

@node Aspect Value_Size,Aspect Volatile_Full_Access,Aspect User_Aspect,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-value-size}@anchor{17a}
@section Aspect Value_Size


@geindex Value_Size

This aspect is equivalent to @ref{17b,,attribute Value_Size}.

@node Aspect Volatile_Full_Access,Aspect Volatile_Function,Aspect Value_Size,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-volatile-full-access}@anchor{17c}
@section Aspect Volatile_Full_Access


@geindex Volatile_Full_Access

This boolean aspect is equivalent to @ref{128,,pragma Volatile_Full_Access}.

@node Aspect Volatile_Function,Aspect Warnings,Aspect Volatile_Full_Access,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-volatile-function}@anchor{17d}
@section Aspect Volatile_Function


@geindex Volatile_Function

This boolean aspect is equivalent to @ref{12a,,pragma Volatile_Function}.

@node Aspect Warnings,,Aspect Volatile_Function,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-warnings}@anchor{17e}
@section Aspect Warnings


@geindex Warnings

This aspect is equivalent to the two argument form of @ref{12c,,pragma Warnings},
where the first argument is @code{ON} or @code{OFF} and the second argument
is the entity.

@node Implementation Defined Attributes,Standard and Implementation Defined Restrictions,Implementation Defined Aspects,Top
@anchor{gnat_rm/implementation_defined_attributes doc}@anchor{17f}@anchor{gnat_rm/implementation_defined_attributes id1}@anchor{180}@anchor{gnat_rm/implementation_defined_attributes implementation-defined-attributes}@anchor{8}
@chapter Implementation Defined Attributes


Ada defines (throughout the Ada reference manual,
summarized in Annex K),
a set of attributes that provide useful additional functionality in all
areas of the language.  These language defined attributes are implemented
in GNAT and work as described in the Ada Reference Manual.

In addition, Ada allows implementations to define additional
attributes whose meaning is defined by the implementation.  GNAT provides
a number of these implementation-dependent attributes which can be used
to extend and enhance the functionality of the compiler.  This section of
the GNAT reference manual describes these additional attributes.  It also
describes additional implementation-dependent features of standard
language-defined attributes.

Note that any program using these attributes may not be portable to
other compilers (although GNAT implements this set of attributes on all
platforms).  Therefore if portability to other compilers is an important
consideration, you should minimize the use of these attributes.

@menu
* Attribute Abort_Signal:: 
* Attribute Address_Size:: 
* Attribute Asm_Input:: 
* Attribute Asm_Output:: 
* Attribute Atomic_Always_Lock_Free:: 
* Attribute Bit:: 
* Attribute Bit_Position:: 
* Attribute Code_Address:: 
* Attribute Compiler_Version:: 
* Attribute Constrained:: 
* Attribute Default_Bit_Order:: 
* Attribute Default_Scalar_Storage_Order:: 
* Attribute Deref:: 
* Attribute Descriptor_Size:: 
* Attribute Elaborated:: 
* Attribute Elab_Body:: 
* Attribute Elab_Spec:: 
* Attribute Elab_Subp_Body:: 
* Attribute Emax:: 
* Attribute Enabled:: 
* Attribute Enum_Rep:: 
* Attribute Enum_Val:: 
* Attribute Epsilon:: 
* Attribute Fast_Math:: 
* Attribute Finalization_Size:: 
* Attribute Fixed_Value:: 
* Attribute From_Address:: 
* Attribute From_Any:: 
* Attribute Has_Access_Values:: 
* Attribute Has_Discriminants:: 
* Attribute Has_Tagged_Values:: 
* Attribute Img:: 
* Attribute Initialized:: 
* Attribute Integer_Value:: 
* Attribute Invalid_Value:: 
* Attribute Large:: 
* Attribute Library_Level:: 
* Attribute Loop_Entry:: 
* Attribute Machine_Size:: 
* Attribute Mantissa:: 
* Attribute Maximum_Alignment:: 
* Attribute Max_Integer_Size:: 
* Attribute Mechanism_Code:: 
* Attribute Null_Parameter:: 
* Attribute Object_Size:: 
* Attribute Old:: 
* Attribute Passed_By_Reference:: 
* Attribute Pool_Address:: 
* Attribute Range_Length:: 
* Attribute Restriction_Set:: 
* Attribute Result:: 
* Attribute Round:: 
* Attribute Safe_Emax:: 
* Attribute Safe_Large:: 
* Attribute Safe_Small:: 
* Attribute Scalar_Storage_Order:: 
* Attribute Simple_Storage_Pool:: 
* Attribute Small:: 
* Attribute Small_Denominator:: 
* Attribute Small_Numerator:: 
* Attribute Storage_Unit:: 
* Attribute Stub_Type:: 
* Attribute System_Allocator_Alignment:: 
* Attribute Target_Name:: 
* Attribute To_Address:: 
* Attribute To_Any:: 
* Attribute Type_Class:: 
* Attribute Type_Key:: 
* Attribute TypeCode:: 
* Attribute Unconstrained_Array:: 
* Attribute Universal_Literal_String:: 
* Attribute Unrestricted_Access:: 
* Attribute Update:: 
* Attribute Valid_Value:: 
* Attribute Valid_Scalars:: 
* Attribute VADS_Size:: 
* Attribute Value_Size:: 
* Attribute Wchar_T_Size:: 
* Attribute Word_Size:: 

@end menu

@node Attribute Abort_Signal,Attribute Address_Size,,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-abort-signal}@anchor{181}
@section Attribute Abort_Signal


@geindex Abort_Signal

@code{Standard'Abort_Signal} (@code{Standard} is the only allowed
prefix) provides the entity for the special exception used to signal
task abort or asynchronous transfer of control.  Normally this attribute
should only be used in the tasking runtime (it is highly peculiar, and
completely outside the normal semantics of Ada, for a user program to
intercept the abort exception).

@node Attribute Address_Size,Attribute Asm_Input,Attribute Abort_Signal,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-address-size}@anchor{182}
@section Attribute Address_Size


@geindex Size of `@w{`}Address`@w{`}

@geindex Address_Size

@code{Standard'Address_Size} (@code{Standard} is the only allowed
prefix) is a static constant giving the number of bits in an
@code{Address}. It is the same value as System.Address’Size,
but has the advantage of being static, while a direct
reference to System.Address’Size is nonstatic because Address
is a private type.

@node Attribute Asm_Input,Attribute Asm_Output,Attribute Address_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-asm-input}@anchor{183}
@section Attribute Asm_Input


@geindex Asm_Input

The @code{Asm_Input} attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix.  The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g., what kind of register is required).  The second argument is the
value to be used as the input argument.  The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
@ref{184,,Machine Code Insertions}

@node Attribute Asm_Output,Attribute Atomic_Always_Lock_Free,Attribute Asm_Input,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-asm-output}@anchor{185}
@section Attribute Asm_Output


@geindex Asm_Output

The @code{Asm_Output} attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix.  The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g., what kind of register is
required).  The second argument is the variable to be updated with the
result.  The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end.  If there are no output operands, then this argument may
either be omitted, or explicitly given as @code{No_Output_Operands}.
@ref{184,,Machine Code Insertions}

@node Attribute Atomic_Always_Lock_Free,Attribute Bit,Attribute Asm_Output,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-atomic-always-lock-free}@anchor{186}
@section Attribute Atomic_Always_Lock_Free


@geindex Atomic_Always_Lock_Free

The prefix of the @code{Atomic_Always_Lock_Free} attribute is a type. The
result indicates whether atomic operations are supported by the target
for the given type.

@node Attribute Bit,Attribute Bit_Position,Attribute Atomic_Always_Lock_Free,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-bit}@anchor{187}
@section Attribute Bit


@geindex Bit

@code{obj'Bit}, where @code{obj} is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object.  The value of this attribute is of the
type `universal_integer' and is always a nonnegative number smaller
than @code{System.Storage_Unit}.

For an object that is a variable or a constant allocated in a register,
the value is zero.  (The use of this attribute does not force the
allocation of a variable to memory).

For an object that is a formal parameter, this attribute applies
to either the matching actual parameter or to a copy of the
matching actual parameter.

For an access object the value is zero.  Note that
@code{obj.all'Bit} is subject to an @code{Access_Check} for the
designated object.  Similarly for a record component
@code{X.C'Bit} is subject to a discriminant check and
@code{X(I).Bit} and @code{X(I1..I2)'Bit}
are subject to index checks.

This attribute is designed to be compatible with the DEC Ada 83 definition
and implementation of the @code{Bit} attribute.

@node Attribute Bit_Position,Attribute Code_Address,Attribute Bit,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-bit-position}@anchor{188}
@section Attribute Bit_Position


@geindex Bit_Position

@code{R.C'Bit_Position}, where @code{R} is a record object and @code{C} is one
of the fields of the record type, yields the bit
offset within the record contains the first bit of
storage allocated for the object.  The value of this attribute is of the
type `universal_integer'.  The value depends only on the field
@code{C} and is independent of the alignment of
the containing record @code{R}.

@node Attribute Code_Address,Attribute Compiler_Version,Attribute Bit_Position,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-code-address}@anchor{189}
@section Attribute Code_Address


@geindex Code_Address

@geindex Subprogram address

@geindex Address of subprogram code

The @code{'Address}
attribute may be applied to subprograms in Ada 95 and Ada 2005, but the
intended effect seems to be to provide
an address value which can be used to call the subprogram by means of
an address clause as in the following example:

@example
procedure K is ...

procedure L;
for L'Address use K'Address;
pragma Import (Ada, L);
@end example

A call to @code{L} is then expected to result in a call to @code{K}.
In Ada 83, where there were no access-to-subprogram values, this was
a common work-around for getting the effect of an indirect call.
GNAT implements the above use of @code{Address} and the technique
illustrated by the example code works correctly.

However, for some purposes, it is useful to have the address of the start
of the generated code for the subprogram.  On some architectures, this is
not necessarily the same as the @code{Address} value described above.
For example, the @code{Address} value may reference a subprogram
descriptor rather than the subprogram itself.

The @code{'Code_Address} attribute, which can only be applied to
subprogram entities, always returns the address of the start of the
generated code of the specified subprogram, which may or may not be
the same value as is returned by the corresponding @code{'Address}
attribute.

@node Attribute Compiler_Version,Attribute Constrained,Attribute Code_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-compiler-version}@anchor{18a}
@section Attribute Compiler_Version


@geindex Compiler_Version

@code{Standard'Compiler_Version} (@code{Standard} is the only allowed
prefix) yields a static string identifying the version of the compiler
being used to compile the unit containing the attribute reference.

@node Attribute Constrained,Attribute Default_Bit_Order,Attribute Compiler_Version,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-constrained}@anchor{18b}
@section Attribute Constrained


@geindex Constrained

In addition to the usage of this attribute in the Ada RM, GNAT
also permits the use of the @code{'Constrained} attribute
in a generic template
for any type, including types without discriminants. The value of this
attribute in the generic instance when applied to a scalar type or a
record type without discriminants is always @code{True}. This usage is
compatible with older Ada compilers, including notably DEC Ada.

@node Attribute Default_Bit_Order,Attribute Default_Scalar_Storage_Order,Attribute Constrained,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-default-bit-order}@anchor{18c}
@section Attribute Default_Bit_Order


@geindex Big endian

@geindex Little endian

@geindex Default_Bit_Order

@code{Standard'Default_Bit_Order} (@code{Standard} is the only
allowed prefix), provides the value @code{System.Default_Bit_Order}
as a @code{Pos} value (0 for @code{High_Order_First}, 1 for
@code{Low_Order_First}).  This is used to construct the definition of
@code{Default_Bit_Order} in package @code{System}.

@node Attribute Default_Scalar_Storage_Order,Attribute Deref,Attribute Default_Bit_Order,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-default-scalar-storage-order}@anchor{18d}
@section Attribute Default_Scalar_Storage_Order


@geindex Big endian

@geindex Little endian

@geindex Default_Scalar_Storage_Order

@code{Standard'Default_Scalar_Storage_Order} (@code{Standard} is the only
allowed prefix), provides the current value of the default scalar storage
order (as specified using pragma @code{Default_Scalar_Storage_Order}, or
equal to @code{Default_Bit_Order} if unspecified) as a
@code{System.Bit_Order} value. This is a static attribute.

@node Attribute Deref,Attribute Descriptor_Size,Attribute Default_Scalar_Storage_Order,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-deref}@anchor{18e}
@section Attribute Deref


@geindex Deref

The attribute @code{typ'Deref(expr)} where @code{expr} is of type @code{System.Address} yields
the variable of type @code{typ} that is located at the given address. It is similar
to @code{(totyp (expr).all)}, where @code{totyp} is an unchecked conversion from address to
a named access-to-@cite{typ} type, except that it yields a variable, so it can be
used on the left side of an assignment.

@node Attribute Descriptor_Size,Attribute Elaborated,Attribute Deref,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-descriptor-size}@anchor{18f}
@section Attribute Descriptor_Size


@geindex Descriptor

@geindex Dope vector

@geindex Descriptor_Size

Nonstatic attribute @code{Descriptor_Size} returns the size in bits of the
descriptor allocated for a type.  The result is non-zero only for unconstrained
array types and the returned value is of type universal integer.  In GNAT, an
array descriptor contains bounds information and is located immediately before
the first element of the array.

@example
type Unconstr_Array is array (Short_Short_Integer range <>) of Positive;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);
@end example

The attribute takes into account any padding due to the alignment of the
component type. In the example above, the descriptor contains two values
of type @code{Short_Short_Integer} representing the low and high bound. But,
since @code{Positive} has an alignment of 4, the size of the descriptor is
@code{2 * Short_Short_Integer'Size} rounded up to the next multiple of 32,
which yields a size of 32 bits, i.e. including 16 bits of padding.

@node Attribute Elaborated,Attribute Elab_Body,Attribute Descriptor_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elaborated}@anchor{190}
@section Attribute Elaborated


@geindex Elaborated

The prefix of the @code{'Elaborated} attribute must be a unit name.  The
value is a Boolean which indicates whether or not the given unit has been
elaborated.  This attribute is primarily intended for internal use by the
generated code for dynamic elaboration checking, but it can also be used
in user programs.  The value will always be True once elaboration of all
units has been completed.  An exception is for units which need no
elaboration, the value is always False for such units.

@node Attribute Elab_Body,Attribute Elab_Spec,Attribute Elaborated,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elab-body}@anchor{191}
@section Attribute Elab_Body


@geindex Elab_Body

This attribute can only be applied to a program unit name.  It returns
the entity for the corresponding elaboration procedure for elaborating
the body of the referenced unit.  This is used in the main generated
elaboration procedure by the binder and is not normally used in any
other context.  However, there may be specialized situations in which it
is useful to be able to call this elaboration procedure from Ada code,
e.g., if it is necessary to do selective re-elaboration to fix some
error.

@node Attribute Elab_Spec,Attribute Elab_Subp_Body,Attribute Elab_Body,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elab-spec}@anchor{192}
@section Attribute Elab_Spec


@geindex Elab_Spec

This attribute can only be applied to a program unit name.  It returns
the entity for the corresponding elaboration procedure for elaborating
the spec of the referenced unit.  This is used in the main
generated elaboration procedure by the binder and is not normally used
in any other context.  However, there may be specialized situations in
which it is useful to be able to call this elaboration procedure from
Ada code, e.g., if it is necessary to do selective re-elaboration to fix
some error.

@node Attribute Elab_Subp_Body,Attribute Emax,Attribute Elab_Spec,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elab-subp-body}@anchor{193}
@section Attribute Elab_Subp_Body


@geindex Elab_Subp_Body

This attribute can only be applied to a library level subprogram
name and is only allowed in CodePeer mode. It returns the entity
for the corresponding elaboration procedure for elaborating the body
of the referenced subprogram unit. This is used in the main generated
elaboration procedure by the binder in CodePeer mode only and is unrecognized
otherwise.

@node Attribute Emax,Attribute Enabled,Attribute Elab_Subp_Body,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-emax}@anchor{194}
@section Attribute Emax


@geindex Ada 83 attributes

@geindex Emax

The @code{Emax} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Enabled,Attribute Enum_Rep,Attribute Emax,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-enabled}@anchor{195}
@section Attribute Enabled


@geindex Enabled

The @code{Enabled} attribute allows an application program to check at compile
time to see if the designated check is currently enabled. The prefix is a
simple identifier, referencing any predefined check name (other than
@code{All_Checks}) or a check name introduced by pragma Check_Name. If
no argument is given for the attribute, the check is for the general state
of the check, if an argument is given, then it is an entity name, and the
check indicates whether an @code{Suppress} or @code{Unsuppress} has been
given naming the entity (if not, then the argument is ignored).

Note that instantiations inherit the check status at the point of the
instantiation, so a useful idiom is to have a library package that
introduces a check name with @code{pragma Check_Name}, and then contains
generic packages or subprograms which use the @code{Enabled} attribute
to see if the check is enabled. A user of this package can then issue
a @code{pragma Suppress} or @code{pragma Unsuppress} before instantiating
the package or subprogram, controlling whether the check will be present.

@node Attribute Enum_Rep,Attribute Enum_Val,Attribute Enabled,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-enum-rep}@anchor{196}
@section Attribute Enum_Rep


@geindex Representation of enums

@geindex Enum_Rep

Note that this attribute is now standard in Ada 202x and is available
as an implementation defined attribute for earlier Ada versions.

For every enumeration subtype @code{S}, @code{S'Enum_Rep} denotes a
function with the following spec:

@example
function S'Enum_Rep (Arg : S'Base) return <Universal_Integer>;
@end example

It is also allowable to apply @code{Enum_Rep} directly to an object of an
enumeration type or to a non-overloaded enumeration
literal.  In this case @code{S'Enum_Rep} is equivalent to
@code{typ'Enum_Rep(S)} where @code{typ} is the type of the
enumeration literal or object.

The function returns the representation value for the given enumeration
value.  This will be equal to value of the @code{Pos} attribute in the
absence of an enumeration representation clause.  This is a static
attribute (i.e., the result is static if the argument is static).

@code{S'Enum_Rep} can also be used with integer types and objects,
in which case it simply returns the integer value.  The reason for this
is to allow it to be used for @code{(<>)} discrete formal arguments in
a generic unit that can be instantiated with either enumeration types
or integer types.  Note that if @code{Enum_Rep} is used on a modular
type whose upper bound exceeds the upper bound of the largest signed
integer type, and the argument is a variable, so that the universal
integer calculation is done at run time, then the call to @code{Enum_Rep}
may raise @code{Constraint_Error}.

@node Attribute Enum_Val,Attribute Epsilon,Attribute Enum_Rep,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-enum-val}@anchor{197}
@section Attribute Enum_Val


@geindex Representation of enums

@geindex Enum_Val

Note that this attribute is now standard in Ada 202x and is available
as an implementation defined attribute for earlier Ada versions.

For every enumeration subtype @code{S}, @code{S'Enum_Val} denotes a
function with the following spec:

@example
function S'Enum_Val (Arg : <Universal_Integer>) return S'Base;
@end example

The function returns the enumeration value whose representation matches the
argument, or raises Constraint_Error if no enumeration literal of the type
has the matching value.
This will be equal to value of the @code{Val} attribute in the
absence of an enumeration representation clause.  This is a static
attribute (i.e., the result is static if the argument is static).

@node Attribute Epsilon,Attribute Fast_Math,Attribute Enum_Val,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-epsilon}@anchor{198}
@section Attribute Epsilon


@geindex Ada 83 attributes

@geindex Epsilon

The @code{Epsilon} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Fast_Math,Attribute Finalization_Size,Attribute Epsilon,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-fast-math}@anchor{199}
@section Attribute Fast_Math


@geindex Fast_Math

@code{Standard'Fast_Math} (@code{Standard} is the only allowed
prefix) yields a static Boolean value that is True if pragma
@code{Fast_Math} is active, and False otherwise.

@node Attribute Finalization_Size,Attribute Fixed_Value,Attribute Fast_Math,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-finalization-size}@anchor{19a}
@section Attribute Finalization_Size


@geindex Finalization_Size

The prefix of attribute @code{Finalization_Size} must be an object or
a type. This attribute returns the size of any hidden data
reserved by the compiler to handle finalization-related actions. The type of
the attribute is `universal_integer'.

@code{Finalization_Size} yields a value of zero for a type with no controlled
parts, an object whose type has no controlled parts, or an object of a
class-wide type whose tag denotes a type with no controlled parts.
For a class-wide type, @code{Finalization_Size} yields a non-zero value except
if a No_Finalization restriction is in effect, in which case it yields zero.

Note that only heap-allocated objects contain finalization data.

@node Attribute Fixed_Value,Attribute From_Address,Attribute Finalization_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-fixed-value}@anchor{19b}
@section Attribute Fixed_Value


@geindex Fixed_Value

For every fixed-point type @code{S}, @code{S'Fixed_Value} denotes a
function with the following specification:

@example
function S'Fixed_Value (Arg : <Universal_Integer>) return S;
@end example

The value returned is the fixed-point value @code{V} such that:

@example
V = Arg * S'Small
@end example

The effect is thus similar to first converting the argument to the
integer type used to represent @code{S}, and then doing an unchecked
conversion to the fixed-point type.  The difference is
that there are full range checks, to ensure that the result is in range.
This attribute is primarily intended for use in implementation of the
input-output functions for fixed-point values.

@node Attribute From_Address,Attribute From_Any,Attribute Fixed_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-from-address}@anchor{19c}
@section Attribute From_Address


@geindex From_Address

The prefix of this attribute must be a general access-to-array type (or
subtype); the attribute takes a System.Address argument and possibly some
additional arguments (described below) and yields a value of the given
access type that designates an array object located at the given address.
In the case of a non-null array this means that the given address is the
address of the first element of the array object (not the address of any sort
of bounds descriptor). This allows associating bounds with an address that is,
for example, passed in from C code.

If the designated array subtype is unconstrained (which is the usual case),
then for each dimension (in order) the attribute takes either one or two
additional arguments of the corresponding index type - one if the index
subtype is a fixed lower bound subtype, two (low bound first) otherwise.
In this case, the access type shall be an extended access type (see the
description of the Extended_Access aspect). These additional arguments
specify the bounds of the designated array object.

If the designated array subtype is constrained then no additional arguments
are provided and the bounds of the designated object are those of the
designated subtype.

Roughly speaking, My_Access_Type’From_Address (Addr, Lo, Hi) is equivalent to a
declare expression:

@example
(declare
   Obj : aliased Designated_Array_Type (Lo .. Hi)
     with Import, Address => Addr;
begin
   My_Access_Type'(Obj'Unchecked_Access)
end)
@end example

@node Attribute From_Any,Attribute Has_Access_Values,Attribute From_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-from-any}@anchor{19d}
@section Attribute From_Any


@geindex From_Any

This internal attribute is used for the generation of remote subprogram
stubs in the context of the Distributed Systems Annex.

@node Attribute Has_Access_Values,Attribute Has_Discriminants,Attribute From_Any,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-has-access-values}@anchor{19e}
@section Attribute Has_Access_Values


@geindex Access values
@geindex testing for

@geindex Has_Access_Values

The prefix of the @code{Has_Access_Values} attribute is a type.  The result
is a Boolean value which is True if the is an access type, or is a composite
type with a component (at any nesting depth) that is an access type, and is
False otherwise.
The intended use of this attribute is in conjunction with generic
definitions.  If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has access values.

@node Attribute Has_Discriminants,Attribute Has_Tagged_Values,Attribute Has_Access_Values,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-has-discriminants}@anchor{19f}
@section Attribute Has_Discriminants


@geindex Discriminants
@geindex testing for

@geindex Has_Discriminants

The prefix of the @code{Has_Discriminants} attribute is a type.  The result
is a Boolean value which is True if the type has discriminants, and False
otherwise.  The intended use of this attribute is in conjunction with generic
definitions.  If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has discriminants.

@node Attribute Has_Tagged_Values,Attribute Img,Attribute Has_Discriminants,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-has-tagged-values}@anchor{1a0}
@section Attribute Has_Tagged_Values


@geindex Tagged values
@geindex testing for

@geindex Has_Tagged_Values

The prefix of the @code{Has_Tagged_Values} attribute is a type. The result is a
Boolean value which is True if the type is a composite type (array or record)
that is either a tagged type or has a subcomponent that is tagged, and is False
otherwise. The intended use of this attribute is in conjunction with generic
definitions. If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has access values.

@node Attribute Img,Attribute Initialized,Attribute Has_Tagged_Values,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-img}@anchor{1a1}
@section Attribute Img


@geindex Img

The @code{Img} attribute differs from @code{Image} in that, while both can be
applied directly to an object, @code{Img} cannot be applied to types.

Example usage of the attribute:

@example
Put_Line ("X = " & X'Img);
@end example

which has the same meaning as the more verbose:

@example
Put_Line ("X = " & T'Image (X));
@end example

where @code{T} is the (sub)type of the object @code{X}.

Note that technically, in analogy to @code{Image},
@code{X'Img} returns a parameterless function
that returns the appropriate string when called. This means that
@code{X'Img} can be renamed as a function-returning-string, or used
in an instantiation as a function parameter.

@node Attribute Initialized,Attribute Integer_Value,Attribute Img,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-initialized}@anchor{1a2}
@section Attribute Initialized


@geindex Initialized

For the syntax and semantics of this attribute, see the SPARK 2014 Reference
Manual, section 6.10.

@node Attribute Integer_Value,Attribute Invalid_Value,Attribute Initialized,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-integer-value}@anchor{1a3}
@section Attribute Integer_Value


@geindex Integer_Value

For every integer type @code{S}, @code{S'Integer_Value} denotes a
function with the following spec:

@example
function S'Integer_Value (Arg : <Universal_Fixed>) return S;
@end example

The value returned is the integer value @code{V}, such that:

@example
Arg = V * T'Small
@end example

where @code{T} is the type of @code{Arg}.
The effect is thus similar to first doing an unchecked conversion from
the fixed-point type to its corresponding implementation type, and then
converting the result to the target integer type.  The difference is
that there are full range checks, to ensure that the result is in range.
This attribute is primarily intended for use in implementation of the
standard input-output functions for fixed-point values.

@node Attribute Invalid_Value,Attribute Large,Attribute Integer_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-invalid-value}@anchor{1a4}
@section Attribute Invalid_Value


@geindex Invalid_Value

For every scalar type S, S’Invalid_Value returns an undefined value of the
type. If possible this value is an invalid representation for the type. The
value returned is identical to the value used to initialize an otherwise
uninitialized value of the type if pragma Initialize_Scalars is used,
including the ability to modify the value with the binder -Sxx flag and
relevant environment variables at run time.

@node Attribute Large,Attribute Library_Level,Attribute Invalid_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-large}@anchor{1a5}
@section Attribute Large


@geindex Ada 83 attributes

@geindex Large

The @code{Large} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Library_Level,Attribute Loop_Entry,Attribute Large,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-library-level}@anchor{1a6}
@section Attribute Library_Level


@geindex Library_Level

@code{P'Library_Level}, where P is an entity name,
returns a Boolean value which is True if the entity is declared
at the library level, and False otherwise. Note that within a
generic instantiation, the name of the generic unit denotes the
instance, which means that this attribute can be used to test
if a generic is instantiated at the library level, as shown
in this example:

@example
generic
  ...
package Gen is
  pragma Compile_Time_Error
    (not Gen'Library_Level,
     "Gen can only be instantiated at library level");
  ...
end Gen;
@end example

@node Attribute Loop_Entry,Attribute Machine_Size,Attribute Library_Level,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-loop-entry}@anchor{1a7}
@section Attribute Loop_Entry


@geindex Loop_Entry

Syntax:

@example
X'Loop_Entry [(loop_name)]
@end example

The @code{Loop_Entry} attribute is used to refer to the value that an
expression had upon entry to a given loop in much the same way that the
@code{Old} attribute in a subprogram postcondition can be used to refer
to the value an expression had upon entry to the subprogram. The
relevant loop is either identified by the given loop name, or it is the
innermost enclosing loop when no loop name is given.

A @code{Loop_Entry} attribute can only occur within an @code{Assert},
@code{Assert_And_Cut}, @code{Assume}, @code{Loop_Variant} or @code{Loop_Invariant} pragma.
In addition, such a pragma must be one of the items in the sequence
of statements of a loop body, or nested inside block statements that
appear in the sequence of statements of a loop body.
A common use of @code{Loop_Entry} is to compare the current value of objects with
their initial value at loop entry, in a @code{Loop_Invariant} pragma.

The effect of using @code{X'Loop_Entry} is the same as declaring
a constant initialized with the initial value of @code{X} at loop
entry. This copy is not performed if the loop is not entered, or if the
corresponding pragmas are ignored or disabled.

@node Attribute Machine_Size,Attribute Mantissa,Attribute Loop_Entry,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-machine-size}@anchor{1a8}
@section Attribute Machine_Size


@geindex Machine_Size

This attribute is identical to the @code{Object_Size} attribute.  It is
provided for compatibility with the DEC Ada 83 attribute of this name.

@node Attribute Mantissa,Attribute Maximum_Alignment,Attribute Machine_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-mantissa}@anchor{1a9}
@section Attribute Mantissa


@geindex Ada 83 attributes

@geindex Mantissa

The @code{Mantissa} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Maximum_Alignment,Attribute Max_Integer_Size,Attribute Mantissa,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-maximum-alignment}@anchor{1aa}@anchor{gnat_rm/implementation_defined_attributes id2}@anchor{1ab}
@section Attribute Maximum_Alignment


@geindex Alignment
@geindex maximum

@geindex Maximum_Alignment

@code{Standard'Maximum_Alignment} (@code{Standard} is the only allowed prefix)
provides the maximum default alignment value for the target, that is to
say the maximum alignment that the compiler may choose by default for a
type or an object. Larger alignments are supported up to some maximum
value dependent on the target, but may require specific mechanisms that
are not needed up to @code{Standard'Maximum_Alignment}.

@node Attribute Max_Integer_Size,Attribute Mechanism_Code,Attribute Maximum_Alignment,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-max-integer-size}@anchor{1ac}
@section Attribute Max_Integer_Size


@geindex Max_Integer_Size

@code{Standard'Max_Integer_Size} (@code{Standard} is the only allowed
prefix) provides the size of the largest supported integer type for
the target. The result is a static constant.

@node Attribute Mechanism_Code,Attribute Null_Parameter,Attribute Max_Integer_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-mechanism-code}@anchor{1ad}
@section Attribute Mechanism_Code


@geindex Return values
@geindex passing mechanism

@geindex Parameters
@geindex passing mechanism

@geindex Mechanism_Code

@code{func'Mechanism_Code} yields an integer code for the
mechanism used for the result of function @code{func}, and
@code{subprog'Mechanism_Code (n)} yields the mechanism
used for formal parameter number `n' (a static integer value, with 1
meaning the first parameter) of subprogram @code{subprog}.  The code returned is:


@table @asis

@item `1'

by copy (value)

@item `2'

by reference
@end table

@node Attribute Null_Parameter,Attribute Object_Size,Attribute Mechanism_Code,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-null-parameter}@anchor{1ae}
@section Attribute Null_Parameter


@geindex Zero address
@geindex passing

@geindex Null_Parameter

A reference @code{T'Null_Parameter} denotes an imaginary object of
type or subtype @code{T} allocated at machine address zero.  The attribute
is allowed only as the default expression of a formal parameter, or as
an actual expression of a subprogram call.  In either case, the
subprogram must be imported.

The identity of the object is represented by the address zero in the
argument list, independent of the passing mechanism (explicit or
default).

This capability is needed to specify that a zero address should be
passed for a record or other composite object passed by reference.
There is no way of indicating this without the @code{Null_Parameter}
attribute.

@node Attribute Object_Size,Attribute Old,Attribute Null_Parameter,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-object-size}@anchor{15a}@anchor{gnat_rm/implementation_defined_attributes id3}@anchor{1af}
@section Attribute Object_Size


@geindex Size
@geindex used for objects

@geindex Object_Size

The size of an object is not necessarily the same as the size of the type
of an object.  This is because by default object sizes are increased to be
a multiple of the alignment of the object.  For example,
@code{Natural'Size} is
31, but by default objects of type @code{Natural} will have a size of 32 bits.
Similarly, a record containing an integer and a character:

@example
type Rec is record
   I : Integer;
   C : Character;
end record;
@end example

will have a size of 40 (that is @code{Rec'Size} will be 40).  The
alignment will be 4, because of the
integer field, and so the default size of record objects for this type
will be 64 (8 bytes).

If the alignment of the above record is specified to be 1, then the
object size will be 40 (5 bytes). This is true by default, and also
an object size of 40 can be explicitly specified in this case.

A consequence of this capability is that different object sizes can be
given to subtypes that would otherwise be considered in Ada to be
statically matching.  But it makes no sense to consider such subtypes
as statically matching.  Consequently, GNAT adds a rule
to the static matching rules that requires object sizes to match.
Consider this example:

@example
 1. procedure BadAVConvert is
 2.    type R is new Integer;
 3.    subtype R1 is R range 1 .. 10;
 4.    subtype R2 is R range 1 .. 10;
 5.    for R1'Object_Size use 8;
 6.    for R2'Object_Size use 16;
 7.    type R1P is access all R1;
 8.    type R2P is access all R2;
 9.    R1PV : R1P := new R1'(4);
10.    R2PV : R2P;
11. begin
12.    R2PV := R2P (R1PV);
               |
       >>> target designated subtype not compatible with
           type "R1" defined at line 3

13. end;
@end example

In the absence of lines 5 and 6,
types @code{R1} and @code{R2} statically match and
hence the conversion on line 12 is legal. But since lines 5 and 6
cause the object sizes to differ, GNAT considers that types
@code{R1} and @code{R2} are not statically matching, and line 12
generates the diagnostic shown above.

Similar additional checks are performed in other contexts requiring
statically matching subtypes.

@node Attribute Old,Attribute Passed_By_Reference,Attribute Object_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-old}@anchor{1b0}
@section Attribute Old


@geindex Old

In addition to the usage of @code{Old} defined in the Ada 2012 RM (usage
within @code{Post} aspect), GNAT also permits the use of this attribute
in implementation defined pragmas @code{Postcondition},
@code{Contract_Cases} and @code{Test_Case}. Also usages of
@code{Old} which would be illegal according to the Ada 2012 RM
definition are allowed under control of
implementation defined pragma @code{Unevaluated_Use_Of_Old}.

@node Attribute Passed_By_Reference,Attribute Pool_Address,Attribute Old,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-passed-by-reference}@anchor{1b1}
@section Attribute Passed_By_Reference


@geindex Parameters
@geindex when passed by reference

@geindex Passed_By_Reference

@code{typ'Passed_By_Reference} for any subtype @cite{typ} returns
a value of type @code{Boolean} value that is @code{True} if the type is
normally passed by reference and @code{False} if the type is normally
passed by copy in calls.  For scalar types, the result is always @code{False}
and is static.  For non-scalar types, the result is nonstatic.

@node Attribute Pool_Address,Attribute Range_Length,Attribute Passed_By_Reference,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-pool-address}@anchor{1b2}
@section Attribute Pool_Address


@geindex Pool_Address

@code{X'Pool_Address} for any object @code{X} returns the address
of X within its storage pool. This is the same as
@code{X'Address}, except that for an unconstrained array whose
bounds are allocated just before the first component,
@code{X'Pool_Address} returns the address of those bounds,
whereas @code{X'Address} returns the address of the first
component.

Here, we are interpreting ‘storage pool’ broadly to mean
@code{wherever the object is allocated}, which could be a
user-defined storage pool,
the global heap, on the stack, or in a static memory area.
For an object created by @code{new}, @code{Ptr.all'Pool_Address} is
what is passed to @code{Allocate} and returned from @code{Deallocate}.

@node Attribute Range_Length,Attribute Restriction_Set,Attribute Pool_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-range-length}@anchor{1b3}
@section Attribute Range_Length


@geindex Range_Length

@code{typ'Range_Length} for any discrete type @cite{typ} yields
the number of values represented by the subtype (zero for a null
range).  The result is static for static subtypes.  @code{Range_Length}
applied to the index subtype of a one dimensional array always gives the
same result as @code{Length} applied to the array itself.

@node Attribute Restriction_Set,Attribute Result,Attribute Range_Length,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-restriction-set}@anchor{1b4}
@section Attribute Restriction_Set


@geindex Restriction_Set

@geindex Restrictions

This attribute allows compile time testing of restrictions that
are currently in effect. It is primarily intended for specializing
code in the run-time based on restrictions that are active (e.g.
don’t need to save fpt registers if restriction No_Floating_Point
is known to be in effect), but can be used anywhere.

There are two forms:

@example
System'Restriction_Set (partition_boolean_restriction_NAME)
System'Restriction_Set (No_Dependence => library_unit_NAME);
@end example

In the case of the first form, the only restriction names
allowed are parameterless restrictions that are checked
for consistency at bind time. For a complete list see the
subtype @code{System.Rident.Partition_Boolean_Restrictions}.

The result returned is True if the restriction is known to
be in effect, and False if the restriction is known not to
be in effect. An important guarantee is that the value of
a Restriction_Set attribute is known to be consistent throughout
all the code of a partition.

This is trivially achieved if the entire partition is compiled
with a consistent set of restriction pragmas. However, the
compilation model does not require this. It is possible to
compile one set of units with one set of pragmas, and another
set of units with another set of pragmas. It is even possible
to compile a spec with one set of pragmas, and then WITH the
same spec with a different set of pragmas. Inconsistencies
in the actual use of the restriction are checked at bind time.

In order to achieve the guarantee of consistency for the
Restriction_Set pragma, we consider that a use of the pragma
that yields False is equivalent to a violation of the
restriction.

So for example if you write

@example
if System'Restriction_Set (No_Floating_Point) then
   ...
else
   ...
end if;
@end example

And the result is False, so that the else branch is executed,
you can assume that this restriction is not set for any unit
in the partition. This is checked by considering this use of
the restriction pragma to be a violation of the restriction
No_Floating_Point. This means that no other unit can attempt
to set this restriction (if some unit does attempt to set it,
the binder will refuse to bind the partition).

Technical note: The restriction name and the unit name are
interpreted entirely syntactically, as in the corresponding
Restrictions pragma, they are not analyzed semantically,
so they do not have a type.

@node Attribute Result,Attribute Round,Attribute Restriction_Set,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-result}@anchor{1b5}
@section Attribute Result


@geindex Result

@code{function'Result} can only be used with in a Postcondition pragma
for a function. The prefix must be the name of the corresponding function. This
is used to refer to the result of the function in the postcondition expression.
For a further discussion of the use of this attribute and examples of its use,
see the description of pragma Postcondition.

@node Attribute Round,Attribute Safe_Emax,Attribute Result,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-round}@anchor{1b6}
@section Attribute Round


@geindex Round

In addition to the usage of this attribute in the Ada RM, GNAT
also permits the use of the @code{'Round} attribute for ordinary
fixed point types.

@node Attribute Safe_Emax,Attribute Safe_Large,Attribute Round,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-safe-emax}@anchor{1b7}
@section Attribute Safe_Emax


@geindex Ada 83 attributes

@geindex Safe_Emax

The @code{Safe_Emax} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Safe_Large,Attribute Safe_Small,Attribute Safe_Emax,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-safe-large}@anchor{1b8}
@section Attribute Safe_Large


@geindex Ada 83 attributes

@geindex Safe_Large

The @code{Safe_Large} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Safe_Small,Attribute Scalar_Storage_Order,Attribute Safe_Large,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-safe-small}@anchor{1b9}
@section Attribute Safe_Small


@geindex Ada 83 attributes

@geindex Safe_Small

The @code{Safe_Small} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Scalar_Storage_Order,Attribute Simple_Storage_Pool,Attribute Safe_Small,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-scalar-storage-order}@anchor{169}@anchor{gnat_rm/implementation_defined_attributes id4}@anchor{1ba}
@section Attribute Scalar_Storage_Order


@geindex Endianness

@geindex Scalar storage order

@geindex Scalar_Storage_Order

For every array or record type @code{S}, the representation attribute
@code{Scalar_Storage_Order} denotes the order in which storage elements
that make up scalar components are ordered within S. The value given must
be a static expression of type System.Bit_Order. The following is an example
of the use of this feature:

@example
--  Component type definitions

subtype Yr_Type is Natural range 0 .. 127;
subtype Mo_Type is Natural range 1 .. 12;
subtype Da_Type is Natural range 1 .. 31;

--  Record declaration

type Date is record
   Years_Since_1980 : Yr_Type;
   Month            : Mo_Type;
   Day_Of_Month     : Da_Type;
end record;

--  Record representation clause

for Date use record
   Years_Since_1980 at 0 range 0  ..  6;
   Month            at 0 range 7  .. 10;
   Day_Of_Month     at 0 range 11 .. 15;
end record;

--  Attribute definition clauses

for Date'Bit_Order use System.High_Order_First;
for Date'Scalar_Storage_Order use System.High_Order_First;
--  If Scalar_Storage_Order is specified, it must be consistent with
--  Bit_Order, so it's best to always define the latter explicitly if
--  the former is used.
@end example

Other properties are as for the standard representation attribute @code{Bit_Order}
defined by Ada RM 13.5.3(4). The default is @code{System.Default_Bit_Order}.

For a record type @code{T}, if @code{T'Scalar_Storage_Order} is
specified explicitly, it shall be equal to @code{T'Bit_Order}. Note:
this means that if a @code{Scalar_Storage_Order} attribute definition
clause is not confirming, then the type’s @code{Bit_Order} shall be
specified explicitly and set to the same value.

Derived types inherit an explicitly set scalar storage order from their parent
types. This may be overridden for the derived type by giving an explicit scalar
storage order for it. However, for a record extension, the derived type must
have the same scalar storage order as the parent type.

A component of a record type that is itself a record or an array and that does
not start and end on a byte boundary must have have the same scalar storage
order as the record type. A component of a bit-packed array type that is itself
a record or an array must have the same scalar storage order as the array type.

No component of a type that has an explicit @code{Scalar_Storage_Order}
attribute definition may be aliased.

A confirming @code{Scalar_Storage_Order} attribute definition clause (i.e.
with a value equal to @code{System.Default_Bit_Order}) has no effect.

If the opposite storage order is specified, then whenever the value of
a scalar component of an object of type @code{S} is read, the storage
elements of the enclosing machine scalar are first reversed (before
retrieving the component value, possibly applying some shift and mask
operatings on the enclosing machine scalar), and the opposite operation
is done for writes.

In that case, the restrictions set forth in 13.5.1(10.3/2) for scalar components
are relaxed. Instead, the following rules apply:


@itemize *

@item 
the underlying storage elements are those at positions
@code{(position + first_bit / storage_element_size) .. (position + (last_bit + storage_element_size - 1) / storage_element_size)}

@item 
the sequence of underlying storage elements shall have
a size no greater than the largest machine scalar

@item 
the enclosing machine scalar is defined as the smallest machine
scalar starting at a position no greater than
@code{position + first_bit / storage_element_size} and covering
storage elements at least up to @code{position + (last_bit + storage_element_size - 1) / storage_element_size}

@item 
the position of the component is interpreted relative to that machine
scalar.
@end itemize

If no scalar storage order is specified for a type (either directly, or by
inheritance in the case of a derived type), then the default is normally
the native ordering of the target, but this default can be overridden using
pragma @code{Default_Scalar_Storage_Order}.

If a component of @code{T} is itself of a record or array type, the specified
@code{Scalar_Storage_Order} does `not' apply to that nested type: an explicit
attribute definition clause must be provided for the component type as well
if desired.

Representation changes that explicitly or implicitly toggle the scalar storage
order are not supported and may result in erroneous execution of the program,
except when performed by means of an instance of @code{Ada.Unchecked_Conversion}.

In particular, overlays are not supported and a warning is given for them:

@example
type Rec_LE is record
   I : Integer;
end record;

for Rec_LE use record
   I at 0 range 0 .. 31;
end record;

for Rec_LE'Bit_Order use System.Low_Order_First;
for Rec_LE'Scalar_Storage_Order use System.Low_Order_First;

type Rec_BE is record
   I : Integer;
end record;

for Rec_BE use record
   I at 0 range 0 .. 31;
end record;

for Rec_BE'Bit_Order use System.High_Order_First;
for Rec_BE'Scalar_Storage_Order use System.High_Order_First;

R_LE : Rec_LE;

R_BE : Rec_BE;
for R_BE'Address use R_LE'Address;
@end example

@code{warning: overlay changes scalar storage order [enabled by default]}

In most cases, such representation changes ought to be replaced by an
instantiation of a function or procedure provided by @code{GNAT.Byte_Swapping}.

Note that the scalar storage order only affects the in-memory data
representation. It has no effect on the representation used by stream
attributes.

Note that debuggers may be unable to display the correct value of scalar
components of a type for which the opposite storage order is specified.

@node Attribute Simple_Storage_Pool,Attribute Small,Attribute Scalar_Storage_Order,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-simple-storage-pool}@anchor{f4}@anchor{gnat_rm/implementation_defined_attributes id5}@anchor{1bb}
@section Attribute Simple_Storage_Pool


@geindex Storage pool
@geindex simple

@geindex Simple storage pool

@geindex Simple_Storage_Pool

For every nonformal, nonderived access-to-object type @code{Acc}, the
representation attribute @code{Simple_Storage_Pool} may be specified
via an attribute_definition_clause (or by specifying the equivalent aspect):

@example
My_Pool : My_Simple_Storage_Pool_Type;

type Acc is access My_Data_Type;

for Acc'Simple_Storage_Pool use My_Pool;
@end example

The name given in an attribute_definition_clause for the
@code{Simple_Storage_Pool} attribute shall denote a variable of
a ‘simple storage pool type’ (see pragma @cite{Simple_Storage_Pool_Type}).

The use of this attribute is only allowed for a prefix denoting a type
for which it has been specified. The type of the attribute is the type
of the variable specified as the simple storage pool of the access type,
and the attribute denotes that variable.

It is illegal to specify both @code{Storage_Pool} and @code{Simple_Storage_Pool}
for the same access type.

If the @code{Simple_Storage_Pool} attribute has been specified for an access
type, then applying the @code{Storage_Pool} attribute to the type is flagged
with a warning and its evaluation raises the exception @code{Program_Error}.

If the Simple_Storage_Pool attribute has been specified for an access
type @code{S}, then the evaluation of the attribute @code{S'Storage_Size}
returns the result of calling @code{Storage_Size (S'Simple_Storage_Pool)},
which is intended to indicate the number of storage elements reserved for
the simple storage pool. If the Storage_Size function has not been defined
for the simple storage pool type, then this attribute returns zero.

If an access type @code{S} has a specified simple storage pool of type
@code{SSP}, then the evaluation of an allocator for that access type calls
the primitive @code{Allocate} procedure for type @code{SSP}, passing
@code{S'Simple_Storage_Pool} as the pool parameter. The detailed
semantics of such allocators is the same as those defined for allocators
in section 13.11 of the @cite{Ada Reference Manual}, with the term
`simple storage pool' substituted for `storage pool'.

If an access type @code{S} has a specified simple storage pool of type
@code{SSP}, then a call to an instance of the @code{Ada.Unchecked_Deallocation}
for that access type invokes the primitive @code{Deallocate} procedure
for type @code{SSP}, passing @code{S'Simple_Storage_Pool} as the pool
parameter. The detailed semantics of such unchecked deallocations is the same
as defined in section 13.11.2 of the Ada Reference Manual, except that the
term `simple storage pool' is substituted for `storage pool'.

@node Attribute Small,Attribute Small_Denominator,Attribute Simple_Storage_Pool,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-small}@anchor{1bc}
@section Attribute Small


@geindex Ada 83 attributes

@geindex Small

The @code{Small} attribute is defined in Ada 95 (and Ada 2005) only for
fixed-point types.
GNAT also allows this attribute to be applied to floating-point types
for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute when applied to floating-point types.

@node Attribute Small_Denominator,Attribute Small_Numerator,Attribute Small,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-small-denominator}@anchor{1bd}
@section Attribute Small_Denominator


@geindex Small

@geindex Small_Denominator

@code{typ'Small_Denominator} for any fixed-point subtype @cite{typ} yields the
denominator in the representation of @code{typ'Small} as a rational number
with coprime factors (i.e. as an irreducible fraction).

@node Attribute Small_Numerator,Attribute Storage_Unit,Attribute Small_Denominator,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-small-numerator}@anchor{1be}
@section Attribute Small_Numerator


@geindex Small

@geindex Small_Numerator

@code{typ'Small_Numerator} for any fixed-point subtype @cite{typ} yields the
numerator in the representation of @code{typ'Small} as a rational number
with coprime factors (i.e. as an irreducible fraction).

@node Attribute Storage_Unit,Attribute Stub_Type,Attribute Small_Numerator,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-storage-unit}@anchor{1bf}
@section Attribute Storage_Unit


@geindex Storage_Unit

@code{Standard'Storage_Unit} (@code{Standard} is the only allowed
prefix) provides the same value as @code{System.Storage_Unit}.

@node Attribute Stub_Type,Attribute System_Allocator_Alignment,Attribute Storage_Unit,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-stub-type}@anchor{1c0}
@section Attribute Stub_Type


@geindex Stub_Type

The GNAT implementation of remote access-to-classwide types is
organized as described in AARM section E.4 (20.t): a value of an RACW type
(designating a remote object) is represented as a normal access
value, pointing to a “stub” object which in turn contains the
necessary information to contact the designated remote object. A
call on any dispatching operation of such a stub object does the
remote call, if necessary, using the information in the stub object
to locate the target partition, etc.

For a prefix @code{T} that denotes a remote access-to-classwide type,
@code{T'Stub_Type} denotes the type of the corresponding stub objects.

By construction, the layout of @code{T'Stub_Type} is identical to that of
type @code{RACW_Stub_Type} declared in the internal implementation-defined
unit @code{System.Partition_Interface}. Use of this attribute will create
an implicit dependency on this unit.

@node Attribute System_Allocator_Alignment,Attribute Target_Name,Attribute Stub_Type,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-system-allocator-alignment}@anchor{1c1}
@section Attribute System_Allocator_Alignment


@geindex Alignment
@geindex allocator

@geindex System_Allocator_Alignment

@code{Standard'System_Allocator_Alignment} (@code{Standard} is the only
allowed prefix) provides the observable guaranteed to be honored by
the system allocator (malloc). This is a static value that can be used
in user storage pools based on malloc either to reject allocation
with alignment too large or to enable a realignment circuitry if the
alignment request is larger than this value.

@node Attribute Target_Name,Attribute To_Address,Attribute System_Allocator_Alignment,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-target-name}@anchor{1c2}
@section Attribute Target_Name


@geindex Target_Name

@code{Standard'Target_Name} (@code{Standard} is the only allowed
prefix) provides a static string value that identifies the target
for the current compilation. For GCC implementations, this is the
standard gcc target name without the terminating slash (for
example, GNAT 5.0 on windows yields “i586-pc-mingw32msv”).

@node Attribute To_Address,Attribute To_Any,Attribute Target_Name,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-to-address}@anchor{1c3}
@section Attribute To_Address


@geindex To_Address

The @code{System'To_Address}
(@code{System} is the only allowed prefix)
denotes a function identical to
@code{System.Storage_Elements.To_Address} except that
it is a static attribute.  This means that if its argument is
a static expression, then the result of the attribute is a
static expression.  This means that such an expression can be
used in contexts (e.g., preelaborable packages) which require a
static expression and where the function call could not be used
(since the function call is always nonstatic, even if its
argument is static). The argument must be in the range
-(2**(m-1)) .. 2**m-1, where m is the memory size
(typically 32 or 64). Negative values are interpreted in a
modular manner (e.g., -1 means the same as 16#FFFF_FFFF# on
a 32 bits machine).

@node Attribute To_Any,Attribute Type_Class,Attribute To_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-to-any}@anchor{1c4}
@section Attribute To_Any


@geindex To_Any

This internal attribute is used for the generation of remote subprogram
stubs in the context of the Distributed Systems Annex.

@node Attribute Type_Class,Attribute Type_Key,Attribute To_Any,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-type-class}@anchor{1c5}
@section Attribute Type_Class


@geindex Type_Class

@code{typ'Type_Class} for any type or subtype @cite{typ} yields
the value of the type class for the full type of @cite{typ}.  If
@cite{typ} is a generic formal type, the value is the value for the
corresponding actual subtype.  The value of this attribute is of type
@code{System.Aux_DEC.Type_Class}, which has the following definition:

@example
type Type_Class is
  (Type_Class_Enumeration,
   Type_Class_Integer,
   Type_Class_Fixed_Point,
   Type_Class_Floating_Point,
   Type_Class_Array,
   Type_Class_Record,
   Type_Class_Access,
   Type_Class_Task,
   Type_Class_Address);
@end example

Protected types yield the value @code{Type_Class_Task}, which thus
applies to all concurrent types.  This attribute is designed to
be compatible with the DEC Ada 83 attribute of the same name.

@node Attribute Type_Key,Attribute TypeCode,Attribute Type_Class,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-type-key}@anchor{1c6}
@section Attribute Type_Key


@geindex Type_Key

The @code{Type_Key} attribute is applicable to a type or subtype and
yields a value of type Standard.String containing encoded information
about the type or subtype. This provides improved compatibility with
other implementations that support this attribute.

@node Attribute TypeCode,Attribute Unconstrained_Array,Attribute Type_Key,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-typecode}@anchor{1c7}
@section Attribute TypeCode


@geindex TypeCode

This internal attribute is used for the generation of remote subprogram
stubs in the context of the Distributed Systems Annex.

@node Attribute Unconstrained_Array,Attribute Universal_Literal_String,Attribute TypeCode,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-unconstrained-array}@anchor{1c8}
@section Attribute Unconstrained_Array


@geindex Unconstrained_Array

The @code{Unconstrained_Array} attribute can be used with a prefix that
denotes any type or subtype. It is a static attribute that yields
@code{True} if the prefix designates an unconstrained array,
and @code{False} otherwise. In a generic instance, the result is
still static, and yields the result of applying this test to the
generic actual.

@node Attribute Universal_Literal_String,Attribute Unrestricted_Access,Attribute Unconstrained_Array,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-universal-literal-string}@anchor{1c9}
@section Attribute Universal_Literal_String


@geindex Named numbers
@geindex representation of

@geindex Universal_Literal_String

The prefix of @code{Universal_Literal_String} must be a named
number.  The static result is the string consisting of the characters of
the number as defined in the original source.  This allows the user
program to access the actual text of named numbers without intermediate
conversions and without the need to enclose the strings in quotes (which
would preclude their use as numbers).

For example, the following program prints the first 50 digits of pi:

@example
with Text_IO; use Text_IO;
with Ada.Numerics;
procedure Pi is
begin
   Put (Ada.Numerics.Pi'Universal_Literal_String);
end;
@end example

@node Attribute Unrestricted_Access,Attribute Update,Attribute Universal_Literal_String,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-unrestricted-access}@anchor{1ca}
@section Attribute Unrestricted_Access


@geindex Access
@geindex unrestricted

@geindex Unrestricted_Access

The @code{Unrestricted_Access} attribute is similar to @code{Access}
except that all accessibility and aliased view checks are omitted.  This
is a user-beware attribute.

For objects, it is similar to @code{Address}, for which it is a
desirable replacement where the value desired is an access type.
In other words, its effect is similar to first applying the
@code{Address} attribute and then doing an unchecked conversion to a
desired access type.

For subprograms, @code{P'Unrestricted_Access} may be used where
@code{P'Access} would be illegal, to construct a value of a
less-nested named access type that designates a more-nested
subprogram. This value may be used in indirect calls, so long as the
more-nested subprogram still exists; once the subprogram containing it
has returned, such calls are erroneous. For example:

@example
package body P is

   type Less_Nested is access procedure;
   Global : Less_Nested;

   procedure P1 is
   begin
      Global.all;
   end P1;

   procedure P2 is
      Local_Var : Integer;

      procedure More_Nested is
      begin
         ... Local_Var ...
      end More_Nested;
   begin
      Global := More_Nested'Unrestricted_Access;
      P1;
   end P2;

end P;
@end example

When P1 is called from P2, the call via Global is OK, but if P1 were
called after P2 returns, it would be an erroneous use of a dangling
pointer.

For objects, it is possible to use @code{Unrestricted_Access} for any
type. However, if the result is of an access-to-unconstrained array
subtype, then the resulting pointer has the same scope as the context
of the attribute, and must not be returned to some enclosing scope.
For instance, if a function uses @code{Unrestricted_Access} to create
an access-to-unconstrained-array and returns that value to the caller,
the result will involve dangling pointers. In addition, it is only
valid to create pointers to unconstrained arrays using this attribute
if the pointer has the normal default ‘fat’ representation where a
pointer has two components, one points to the array and one points to
the bounds. If a size clause is used to force ‘thin’ representation
for a pointer to unconstrained where there is only space for a single
pointer, then the resulting pointer is not usable.

In the simple case where a direct use of Unrestricted_Access attempts
to make a thin pointer for a non-aliased object, the compiler will
reject the use as illegal, as shown in the following example:

@example
with System; use System;
procedure SliceUA2 is
   type A is access all String;
   for A'Size use Standard'Address_Size;

   procedure P (Arg : A) is
   begin
      null;
   end P;

   X : String := "hello world!";
   X2 : aliased String := "hello world!";

   AV : A := X'Unrestricted_Access;    -- ERROR
             |
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object

begin
   P (X'Unrestricted_Access);          -- ERROR
      |
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object

   P (X(7 .. 12)'Unrestricted_Access); -- ERROR
      |
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object

   P (X2'Unrestricted_Access);         -- OK
end;
@end example

but other cases cannot be detected by the compiler, and are
considered to be erroneous. Consider the following example:

@example
with System; use System;
with System; use System;
procedure SliceUA is
   type AF is access all String;

   type A is access all String;
   for A'Size use Standard'Address_Size;

   procedure P (Arg : A) is
   begin
      if Arg'Length /= 6 then
         raise Program_Error;
      end if;
   end P;

   X : String := "hello world!";
   Y : AF := X (7 .. 12)'Unrestricted_Access;

begin
   P (A (Y));
end;
@end example

A normal unconstrained array value
or a constrained array object marked as aliased has the bounds in memory
just before the array, so a thin pointer can retrieve both the data and
the bounds.  But in this case, the non-aliased object @code{X} does not have the
bounds before the string.  If the size clause for type @code{A}
were not present, then the pointer
would be a fat pointer, where one component is a pointer to the bounds,
and all would be well.  But with the size clause present, the conversion from
fat pointer to thin pointer in the call loses the bounds, and so this
is erroneous, and the program likely raises a @code{Program_Error} exception.

In general, it is advisable to completely
avoid mixing the use of thin pointers and the use of
@code{Unrestricted_Access} where the designated type is an
unconstrained array.  The use of thin pointers should be restricted to
cases of porting legacy code that implicitly assumes the size of pointers,
and such code should not in any case be using this attribute.

Another erroneous situation arises if the attribute is
applied to a constant. The resulting pointer can be used to access the
constant, but the effect of trying to modify a constant in this manner
is not well-defined. Consider this example:

@example
P : constant Integer := 4;
type R is access all Integer;
RV : R := P'Unrestricted_Access;
..
RV.all := 3;
@end example

Here we attempt to modify the constant P from 4 to 3, but the compiler may
or may not notice this attempt, and subsequent references to P may yield
either the value 3 or the value 4 or the assignment may blow up if the
compiler decides to put P in read-only memory. One particular case where
@code{Unrestricted_Access} can be used in this way is to modify the
value of an @code{in} parameter:

@example
procedure K (S : in String) is
   type R is access all Character;
   RV : R := S (3)'Unrestricted_Access;
begin
   RV.all := 'a';
end;
@end example

In general this is a risky approach. It may appear to “work” but such uses of
@code{Unrestricted_Access} are potentially non-portable, even from one version
of GNAT to another, so are best avoided if possible.

@node Attribute Update,Attribute Valid_Value,Attribute Unrestricted_Access,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-update}@anchor{1cb}
@section Attribute Update


@geindex Update

The @code{Update} attribute creates a copy of an array or record value
with one or more modified components. The syntax is:

@example
PREFIX'Update ( RECORD_COMPONENT_ASSOCIATION_LIST )
PREFIX'Update ( ARRAY_COMPONENT_ASSOCIATION @{, ARRAY_COMPONENT_ASSOCIATION @} )
PREFIX'Update ( MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION
                @{, MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION @} )

MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION ::= INDEX_EXPRESSION_LIST_LIST => EXPRESSION
INDEX_EXPRESSION_LIST_LIST                   ::= INDEX_EXPRESSION_LIST @{| INDEX_EXPRESSION_LIST @}
INDEX_EXPRESSION_LIST                        ::= ( EXPRESSION @{, EXPRESSION @} )
@end example

where @code{PREFIX} is the name of an array or record object, the
association list in parentheses does not contain an @code{others}
choice and the box symbol @code{<>} may not appear in any
expression. The effect is to yield a copy of the array or record value
which is unchanged apart from the components mentioned in the
association list, which are changed to the indicated value. The
original value of the array or record value is not affected. For
example:

@example
type Arr is Array (1 .. 5) of Integer;
...
Avar1 : Arr := (1,2,3,4,5);
Avar2 : Arr := Avar1'Update (2 => 10, 3 .. 4 => 20);
@end example

yields a value for @code{Avar2} of 1,10,20,20,5 with @code{Avar1}
begin unmodified. Similarly:

@example
type Rec is A, B, C : Integer;
...
Rvar1 : Rec := (A => 1, B => 2, C => 3);
Rvar2 : Rec := Rvar1'Update (B => 20);
@end example

yields a value for @code{Rvar2} of (A => 1, B => 20, C => 3),
with @code{Rvar1} being unmodified.
Note that the value of the attribute reference is computed
completely before it is used. This means that if you write:

@example
Avar1 := Avar1'Update (1 => 10, 2 => Function_Call);
@end example

then the value of @code{Avar1} is not modified if @code{Function_Call}
raises an exception, unlike the effect of a series of direct assignments
to elements of @code{Avar1}. In general this requires that
two extra complete copies of the object are required, which should be
kept in mind when considering efficiency.

The @code{Update} attribute cannot be applied to prefixes of a limited
type, and cannot reference discriminants in the case of a record type.
The accessibility level of an Update attribute result object is defined
as for an aggregate.

In the record case, no component can be mentioned more than once. In
the array case, two overlapping ranges can appear in the association list,
in which case the modifications are processed left to right.

Multi-dimensional arrays can be modified, as shown by this example:

@example
A : array (1 .. 10, 1 .. 10) of Integer;
..
A := A'Update ((1, 2) => 20, (3, 4) => 30);
@end example

which changes element (1,2) to 20 and (3,4) to 30.

@node Attribute Valid_Value,Attribute Valid_Scalars,Attribute Update,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-valid-value}@anchor{1cc}
@section Attribute Valid_Value


@geindex Valid_Value

The @code{'Valid_Value} attribute is defined for enumeration types other than
those in package Standard or types derived from those types. This attribute is
a function that takes a String, and returns Boolean. @code{T'Valid_Value (S)}
returns True if and only if @code{T'Value (S)} would not raise Constraint_Error.

@node Attribute Valid_Scalars,Attribute VADS_Size,Attribute Valid_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-valid-scalars}@anchor{1cd}
@section Attribute Valid_Scalars


@geindex Valid_Scalars

The @code{'Valid_Scalars} attribute is intended to make it easier to check the
validity of scalar subcomponents of composite objects. The attribute is defined
for any prefix @code{P} which denotes an object. Prefix @code{P} can be any type
except for tagged private or @code{Unchecked_Union} types. The value of the
attribute is of type @code{Boolean}.

@code{P'Valid_Scalars} yields @code{True} if and only if the evaluation of
@code{C'Valid} yields @code{True} for every scalar subcomponent @code{C} of @code{P}, or if
@code{P} has no scalar subcomponents. Attribute @code{'Valid_Scalars} is equivalent
to attribute @code{'Valid} for scalar types.

It is not specified in what order the subcomponents are checked, nor whether
any more are checked after any one of them is determined to be invalid. If the
prefix @code{P} is of a class-wide type @code{T'Class} (where @code{T} is the associated
specific type), or if the prefix @code{P} is of a specific tagged type @code{T}, then
only the subcomponents of @code{T} are checked; in other words, components of
extensions of @code{T} are not checked even if @code{T'Class (P)'Tag /= T'Tag}.

The compiler will issue a warning if it can be determined at compile time that
the prefix of the attribute has no scalar subcomponents.

Note: @code{Valid_Scalars} can generate a lot of code, especially in the case of
a large variant record. If the attribute is called in many places in the same
program applied to objects of the same type, it can reduce program size to
write a function with a single use of the attribute, and then call that
function from multiple places.

@node Attribute VADS_Size,Attribute Value_Size,Attribute Valid_Scalars,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-vads-size}@anchor{1ce}
@section Attribute VADS_Size


@geindex Size
@geindex VADS compatibility

@geindex VADS_Size

The @code{'VADS_Size} attribute is intended to make it easier to port
legacy code which relies on the semantics of @code{'Size} as implemented
by the VADS Ada 83 compiler.  GNAT makes a best effort at duplicating the
same semantic interpretation.  In particular, @code{'VADS_Size} applied
to a predefined or other primitive type with no Size clause yields the
Object_Size (for example, @code{Natural'Size} is 32 rather than 31 on
typical machines).  In addition @code{'VADS_Size} applied to an object
gives the result that would be obtained by applying the attribute to
the corresponding type.

@node Attribute Value_Size,Attribute Wchar_T_Size,Attribute VADS_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-value-size}@anchor{17b}@anchor{gnat_rm/implementation_defined_attributes id6}@anchor{1cf}
@section Attribute Value_Size


@geindex Size
@geindex setting for not-first subtype

@geindex Value_Size

@code{type'Value_Size} is the number of bits required to represent
a value of the given subtype.  It is the same as @code{type'Size},
but, unlike @code{Size}, may be set for non-first subtypes.

@node Attribute Wchar_T_Size,Attribute Word_Size,Attribute Value_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-wchar-t-size}@anchor{1d0}
@section Attribute Wchar_T_Size


@geindex Wchar_T_Size

@code{Standard'Wchar_T_Size} (@code{Standard} is the only allowed
prefix) provides the size in bits of the C @code{wchar_t} type
primarily for constructing the definition of this type in
package @code{Interfaces.C}. The result is a static constant.

@node Attribute Word_Size,,Attribute Wchar_T_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-word-size}@anchor{1d1}
@section Attribute Word_Size


@geindex Word_Size

@code{Standard'Word_Size} (@code{Standard} is the only allowed
prefix) provides the value @code{System.Word_Size}. The result is
a static constant.

@node Standard and Implementation Defined Restrictions,Implementation Advice,Implementation Defined Attributes,Top
@anchor{gnat_rm/standard_and_implementation_defined_restrictions doc}@anchor{1d2}@anchor{gnat_rm/standard_and_implementation_defined_restrictions id1}@anchor{1d3}@anchor{gnat_rm/standard_and_implementation_defined_restrictions standard-and-implementation-defined-restrictions}@anchor{9}
@chapter Standard and Implementation Defined Restrictions


All Ada Reference Manual-defined Restriction identifiers are implemented:


@itemize *

@item 
language-defined restrictions (see 13.12.1)

@item 
tasking restrictions (see D.7)

@item 
high integrity restrictions (see H.4)
@end itemize

GNAT implements additional restriction identifiers. All restrictions, whether
language defined or GNAT-specific, are listed in the following.

@menu
* Partition-Wide Restrictions:: 
* Program Unit Level Restrictions:: 

@end menu

@node Partition-Wide Restrictions,Program Unit Level Restrictions,,Standard and Implementation Defined Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions id2}@anchor{1d4}@anchor{gnat_rm/standard_and_implementation_defined_restrictions partition-wide-restrictions}@anchor{1d5}
@section Partition-Wide Restrictions


There are two separate lists of restriction identifiers. The first
set requires consistency throughout a partition (in other words, if the
restriction identifier is used for any compilation unit in the partition,
then all compilation units in the partition must obey the restriction).

@menu
* Immediate_Reclamation:: 
* Max_Asynchronous_Select_Nesting:: 
* Max_Entry_Queue_Length:: 
* Max_Protected_Entries:: 
* Max_Select_Alternatives:: 
* Max_Storage_At_Blocking:: 
* Max_Task_Entries:: 
* Max_Tasks:: 
* No_Abort_Statements:: 
* No_Access_Parameter_Allocators:: 
* No_Access_Subprograms:: 
* No_Allocators:: 
* No_Anonymous_Allocators:: 
* No_Asynchronous_Control:: 
* No_Calendar:: 
* No_Coextensions:: 
* No_Default_Initialization:: 
* No_Delay:: 
* No_Dependence:: 
* No_Direct_Boolean_Operators:: 
* No_Dispatch:: 
* No_Dispatching_Calls:: 
* No_Dynamic_Attachment:: 
* No_Dynamic_Priorities:: 
* No_Entry_Calls_In_Elaboration_Code:: 
* No_Enumeration_Maps:: 
* No_Exception_Handlers:: 
* No_Exception_Propagation:: 
* No_Exception_Registration:: 
* No_Exceptions:: 
* No_Finalization:: 
* No_Fixed_Point:: 
* No_Floating_Point:: 
* No_Implicit_Conditionals:: 
* No_Implicit_Dynamic_Code:: 
* No_Implicit_Heap_Allocations:: 
* No_Implicit_Protected_Object_Allocations:: 
* No_Implicit_Task_Allocations:: 
* No_Initialize_Scalars:: 
* No_IO:: 
* No_Local_Allocators:: 
* No_Local_Protected_Objects:: 
* No_Local_Tagged_Types:: 
* No_Local_Timing_Events:: 
* No_Long_Long_Integers:: 
* No_Multiple_Elaboration:: 
* No_Nested_Finalization:: 
* No_Protected_Type_Allocators:: 
* No_Protected_Types:: 
* No_Recursion:: 
* No_Reentrancy:: 
* No_Relative_Delay:: 
* No_Requeue_Statements:: 
* No_Secondary_Stack:: 
* No_Select_Statements:: 
* No_Specific_Termination_Handlers:: 
* No_Specification_of_Aspect:: 
* No_Standard_Allocators_After_Elaboration:: 
* No_Standard_Storage_Pools:: 
* No_Stream_Optimizations:: 
* No_Streams:: 
* No_Tagged_Type_Registration:: 
* No_Task_Allocators:: 
* No_Task_At_Interrupt_Priority:: 
* No_Task_Attributes_Package:: 
* No_Task_Hierarchy:: 
* No_Task_Termination:: 
* No_Tasking:: 
* No_Terminate_Alternatives:: 
* No_Unchecked_Access:: 
* No_Unchecked_Conversion:: 
* No_Unchecked_Deallocation:: 
* No_Use_Of_Attribute:: 
* No_Use_Of_Entity:: 
* No_Use_Of_Pragma:: 
* Pure_Barriers:: 
* Simple_Barriers:: 
* Static_Priorities:: 
* Static_Storage_Size:: 

@end menu

@node Immediate_Reclamation,Max_Asynchronous_Select_Nesting,,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions immediate-reclamation}@anchor{1d6}
@subsection Immediate_Reclamation


@geindex Immediate_Reclamation

[RM H.4] This restriction ensures that, except for storage occupied by
objects created by allocators and not deallocated via unchecked
deallocation, any storage reserved at run time for an object is
immediately reclaimed when the object no longer exists.

@node Max_Asynchronous_Select_Nesting,Max_Entry_Queue_Length,Immediate_Reclamation,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-asynchronous-select-nesting}@anchor{1d7}
@subsection Max_Asynchronous_Select_Nesting


@geindex Max_Asynchronous_Select_Nesting

[RM D.7] Specifies the maximum dynamic nesting level of asynchronous
selects. Violations of this restriction with a value of zero are
detected at compile time. Violations of this restriction with values
other than zero cause Storage_Error to be raised.

@node Max_Entry_Queue_Length,Max_Protected_Entries,Max_Asynchronous_Select_Nesting,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-entry-queue-length}@anchor{1d8}
@subsection Max_Entry_Queue_Length


@geindex Max_Entry_Queue_Length

[RM D.7] This restriction is a declaration that any protected entry compiled in
the scope of the restriction has at most the specified number of
tasks waiting on the entry at any one time, and so no queue is required.
Note that this restriction is checked at run time. Violation of this
restriction results in the raising of Program_Error exception at the point of
the call.

@geindex Max_Entry_Queue_Depth

The restriction @code{Max_Entry_Queue_Depth} is recognized as a
synonym for @code{Max_Entry_Queue_Length}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node Max_Protected_Entries,Max_Select_Alternatives,Max_Entry_Queue_Length,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-protected-entries}@anchor{1d9}
@subsection Max_Protected_Entries


@geindex Max_Protected_Entries

[RM D.7] Specifies the maximum number of entries per protected type. The
bounds of every entry family of a protected unit shall be static, or shall be
defined by a discriminant of a subtype whose corresponding bound is static.

@node Max_Select_Alternatives,Max_Storage_At_Blocking,Max_Protected_Entries,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-select-alternatives}@anchor{1da}
@subsection Max_Select_Alternatives


@geindex Max_Select_Alternatives

[RM D.7] Specifies the maximum number of alternatives in a selective accept.

@node Max_Storage_At_Blocking,Max_Task_Entries,Max_Select_Alternatives,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-storage-at-blocking}@anchor{1db}
@subsection Max_Storage_At_Blocking


@geindex Max_Storage_At_Blocking

[RM D.7] Specifies the maximum portion (in storage elements) of a task’s
Storage_Size that can be retained by a blocked task. A violation of this
restriction causes Storage_Error to be raised.

@node Max_Task_Entries,Max_Tasks,Max_Storage_At_Blocking,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-task-entries}@anchor{1dc}
@subsection Max_Task_Entries


@geindex Max_Task_Entries

[RM D.7] Specifies the maximum number of entries
per task.  The bounds of every entry family
of a task unit shall be static, or shall be
defined by a discriminant of a subtype whose
corresponding bound is static.

@node Max_Tasks,No_Abort_Statements,Max_Task_Entries,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-tasks}@anchor{1dd}
@subsection Max_Tasks


@geindex Max_Tasks

[RM D.7] Specifies the maximum number of tasks that may be created, not
counting the creation of the environment task.  Violations of this
restriction with a value of zero are detected at compile time in all
contexts.  Violations of this restriction with a value greater than
zero are detected for library-level tasks at compile time, but are
not detected for local tasks at all.

@node No_Abort_Statements,No_Access_Parameter_Allocators,Max_Tasks,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-abort-statements}@anchor{1de}
@subsection No_Abort_Statements


@geindex No_Abort_Statements

[RM D.7] There are no abort_statements, and there are
no calls to Task_Identification.Abort_Task.

@node No_Access_Parameter_Allocators,No_Access_Subprograms,No_Abort_Statements,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-access-parameter-allocators}@anchor{1df}
@subsection No_Access_Parameter_Allocators


@geindex No_Access_Parameter_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator as the actual parameter to an access
parameter.

@node No_Access_Subprograms,No_Allocators,No_Access_Parameter_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-access-subprograms}@anchor{1e0}
@subsection No_Access_Subprograms


@geindex No_Access_Subprograms

[RM H.4] This restriction ensures at compile time that there are no
declarations of access-to-subprogram types.

@node No_Allocators,No_Anonymous_Allocators,No_Access_Subprograms,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-allocators}@anchor{1e1}
@subsection No_Allocators


@geindex No_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator.

@node No_Anonymous_Allocators,No_Asynchronous_Control,No_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-anonymous-allocators}@anchor{1e2}
@subsection No_Anonymous_Allocators


@geindex No_Anonymous_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator of anonymous access type.

@node No_Asynchronous_Control,No_Calendar,No_Anonymous_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-asynchronous-control}@anchor{1e3}
@subsection No_Asynchronous_Control


@geindex No_Asynchronous_Control

[RM J.13] This restriction ensures at compile time that there are no semantic
dependences on the predefined package Asynchronous_Task_Control.

@node No_Calendar,No_Coextensions,No_Asynchronous_Control,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-calendar}@anchor{1e4}
@subsection No_Calendar


@geindex No_Calendar

[GNAT] This restriction ensures at compile time that there are no semantic
dependences on package Calendar.

@node No_Coextensions,No_Default_Initialization,No_Calendar,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-coextensions}@anchor{1e5}
@subsection No_Coextensions


@geindex No_Coextensions

[RM H.4] This restriction ensures at compile time that there are no
coextensions. See 3.10.2.

@node No_Default_Initialization,No_Delay,No_Coextensions,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-default-initialization}@anchor{1e6}
@subsection No_Default_Initialization


@geindex No_Default_Initialization

[GNAT] This restriction prohibits any instance of default initialization
of variables or components. The binder implements a consistency check that
prevents any unit without the restriction from with’ing a unit with the
restriction (this allows the generation of initialization procedures to
be skipped, since you can be sure that no call is ever generated to an
initialization procedure in a unit with the restriction active). If used
in conjunction with Initialize_Scalars or Normalize_Scalars, the effect
is to prohibit all cases of variables declared without a specific
initializer (including the case of OUT scalar parameters).

@node No_Delay,No_Dependence,No_Default_Initialization,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-delay}@anchor{1e7}
@subsection No_Delay


@geindex No_Delay

[RM H.4] This restriction ensures at compile time that there are no
delay statements and no semantic dependences on package Calendar.

@node No_Dependence,No_Direct_Boolean_Operators,No_Delay,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dependence}@anchor{1e8}
@subsection No_Dependence


@geindex No_Dependence

[RM 13.12.1] This restriction ensures at compile time that there are no
dependences on a library unit. For GNAT, this includes implicit implementation
dependences on units of the runtime library that are created by the compiler
to support specific constructs of the language. Here are some examples:


@itemize *

@item 
@code{System.Arith_64}: 64-bit arithmetics for 32-bit platforms,

@item 
@code{System.Arith_128}: 128-bit arithmetics for 64-bit platforms,

@item 
@code{System.Memory}: heap memory allocation routines,

@item 
@code{System.Memory_Compare}: memory comparison routine (aka @code{memcmp} for C),

@item 
@code{System.Memory_Copy}: memory copy routine (aka @code{memcpy} for C),

@item 
@code{System.Memory_Move}: memory move routine (aka @code{memmove} for C),

@item 
@code{System.Memory_Set}: memory set routine (aka @code{memset} for C),

@item 
@code{System.Stack_Checking[.Operations]}: stack checking without MMU,

@item 
@code{System.GCC}: support routines from the GCC library.
@end itemize

@node No_Direct_Boolean_Operators,No_Dispatch,No_Dependence,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-direct-boolean-operators}@anchor{1e9}
@subsection No_Direct_Boolean_Operators


@geindex No_Direct_Boolean_Operators

[GNAT] This restriction ensures that no logical operators (and/or/xor)
are used on operands of type Boolean (or any type derived from Boolean).
This is intended for use in safety critical programs where the certification
protocol requires the use of short-circuit (and then, or else) forms for all
composite boolean operations.

@node No_Dispatch,No_Dispatching_Calls,No_Direct_Boolean_Operators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dispatch}@anchor{1ea}
@subsection No_Dispatch


@geindex No_Dispatch

[RM H.4] This restriction ensures at compile time that there are no
occurrences of @code{T'Class}, for any (tagged) subtype @code{T}.

@node No_Dispatching_Calls,No_Dynamic_Attachment,No_Dispatch,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dispatching-calls}@anchor{1eb}
@subsection No_Dispatching_Calls


@geindex No_Dispatching_Calls

[GNAT] This restriction ensures at compile time that the code generated by the
compiler involves no dispatching calls. The use of this restriction allows the
safe use of record extensions, classwide membership tests and other classwide
features not involving implicit dispatching. This restriction ensures that
the code contains no indirect calls through a dispatching mechanism. Note that
this includes internally-generated calls created by the compiler, for example
in the implementation of class-wide objects assignments. The
membership test is allowed in the presence of this restriction, because its
implementation requires no dispatching.
This restriction is comparable to the official Ada restriction
@code{No_Dispatch} except that it is a bit less restrictive in that it allows
all classwide constructs that do not imply dispatching.
The following example indicates constructs that violate this restriction.

@example
package Pkg is
  type T is tagged record
    Data : Natural;
  end record;
  procedure P (X : T);

  type DT is new T with record
    More_Data : Natural;
  end record;
  procedure Q (X : DT);
end Pkg;

with Pkg; use Pkg;
procedure Example is
  procedure Test (O : T'Class) is
    N : Natural := O'Size; --  Error: Dispatching call
    C : T'Class := O;      --  Error: implicit Dispatching Call
  begin
    if O in DT'Class then  --  OK   : Membership test
       Q (DT (O));         --  OK   : Type conversion plus direct call
    else
       P (O);              --  Error: Dispatching call
    end if;
  end Test;

  Obj : DT;
begin
  P (Obj);                 --  OK   : Direct call
  P (T (Obj));             --  OK   : Type conversion plus direct call
  P (T'Class (Obj));       --  Error: Dispatching call

  Test (Obj);              --  OK   : Type conversion

  if Obj in T'Class then   --  OK   : Membership test
     null;
  end if;
end Example;
@end example

@node No_Dynamic_Attachment,No_Dynamic_Priorities,No_Dispatching_Calls,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dynamic-attachment}@anchor{1ec}
@subsection No_Dynamic_Attachment


@geindex No_Dynamic_Attachment

[RM D.7] This restriction ensures that there is no call to any of the
operations defined in package Ada.Interrupts
(Is_Reserved, Is_Attached, Current_Handler, Attach_Handler, Exchange_Handler,
Detach_Handler, and Reference).

@geindex No_Dynamic_Interrupts

The restriction @code{No_Dynamic_Interrupts} is recognized as a
synonym for @code{No_Dynamic_Attachment}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node No_Dynamic_Priorities,No_Entry_Calls_In_Elaboration_Code,No_Dynamic_Attachment,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dynamic-priorities}@anchor{1ed}
@subsection No_Dynamic_Priorities


@geindex No_Dynamic_Priorities

[RM D.7] There are no semantic dependencies on the package Dynamic_Priorities.

@node No_Entry_Calls_In_Elaboration_Code,No_Enumeration_Maps,No_Dynamic_Priorities,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-entry-calls-in-elaboration-code}@anchor{1ee}
@subsection No_Entry_Calls_In_Elaboration_Code


@geindex No_Entry_Calls_In_Elaboration_Code

[GNAT] This restriction ensures at compile time that no task or protected entry
calls are made during elaboration code.  As a result of the use of this
restriction, the compiler can assume that no code past an accept statement
in a task can be executed at elaboration time.

@node No_Enumeration_Maps,No_Exception_Handlers,No_Entry_Calls_In_Elaboration_Code,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-enumeration-maps}@anchor{1ef}
@subsection No_Enumeration_Maps


@geindex No_Enumeration_Maps

[GNAT] This restriction ensures at compile time that no operations requiring
enumeration maps are used (that is Image and Value attributes applied
to enumeration types).

@node No_Exception_Handlers,No_Exception_Propagation,No_Enumeration_Maps,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exception-handlers}@anchor{1f0}
@subsection No_Exception_Handlers


@geindex No_Exception_Handlers

[GNAT] This restriction ensures at compile time that there are no explicit
exception handlers. It also indicates that no exception propagation will
be provided. In this mode, exceptions may be raised but will result in
an immediate call to the last chance handler, a routine that the user
must define with the following profile:

@example
procedure Last_Chance_Handler
  (Source_Location : System.Address; Line : Integer);
pragma Export (C, Last_Chance_Handler,
               "__gnat_last_chance_handler");
@end example

The @code{Source_Location} parameter is a C null-terminated string representing a
message to be associated with the exception (typically the source location of
the raise statement generated by the compiler). The Line parameter when nonzero
represents the line number in the source program where the raise occurs.

@node No_Exception_Propagation,No_Exception_Registration,No_Exception_Handlers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exception-propagation}@anchor{1f1}
@subsection No_Exception_Propagation


@geindex No_Exception_Propagation

[GNAT] This restriction guarantees that exceptions are never propagated
to an outer subprogram scope. The only case in which an exception may
be raised is when the handler is statically in the same subprogram, so
that the effect of a raise is essentially like a goto statement. Any
other raise statement (implicit or explicit) will be considered
unhandled. Exception handlers are allowed, but may not contain an
exception occurrence identifier (exception choice). In addition, use of
the package GNAT.Current_Exception is not permitted, and reraise
statements (raise with no operand) are not permitted.

@node No_Exception_Registration,No_Exceptions,No_Exception_Propagation,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exception-registration}@anchor{1f2}
@subsection No_Exception_Registration


@geindex No_Exception_Registration

[GNAT] This restriction ensures at compile time that no stream operations for
types Exception_Id or Exception_Occurrence are used. This also makes it
impossible to pass exceptions to or from a partition with this restriction
in a distributed environment. If this restriction is active, the generated
code is simplified by omitting the otherwise-required global registration
of exceptions when they are declared.

@node No_Exceptions,No_Finalization,No_Exception_Registration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exceptions}@anchor{1f3}
@subsection No_Exceptions


@geindex No_Exceptions

[RM H.4] This restriction ensures at compile time that there are no
raise statements and no exception handlers and also suppresses the
generation of language-defined run-time checks.

@node No_Finalization,No_Fixed_Point,No_Exceptions,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-finalization}@anchor{1f4}
@subsection No_Finalization


@geindex No_Finalization

[GNAT] This restriction disables the language features described in
chapter 7.6 of the Ada 2005 RM as well as all form of code generation
performed by the compiler to support these features. The following types
are no longer considered controlled when this restriction is in effect:


@itemize *

@item 
@code{Ada.Finalization.Controlled}

@item 
@code{Ada.Finalization.Limited_Controlled}

@item 
Derivations from @code{Controlled} or @code{Limited_Controlled}

@item 
Class-wide types

@item 
Protected types

@item 
Task types

@item 
Array and record types with controlled components
@end itemize

The compiler no longer generates code to initialize, finalize or adjust an
object or a nested component, either declared on the stack or on the heap. The
deallocation of a controlled object no longer finalizes its contents.

@node No_Fixed_Point,No_Floating_Point,No_Finalization,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-fixed-point}@anchor{1f5}
@subsection No_Fixed_Point


@geindex No_Fixed_Point

[RM H.4] This restriction ensures at compile time that there are no
occurrences of fixed point types and operations.

@node No_Floating_Point,No_Implicit_Conditionals,No_Fixed_Point,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-floating-point}@anchor{1f6}
@subsection No_Floating_Point


@geindex No_Floating_Point

[RM H.4] This restriction ensures at compile time that there are no
occurrences of floating point types and operations.

@node No_Implicit_Conditionals,No_Implicit_Dynamic_Code,No_Floating_Point,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-conditionals}@anchor{1f7}
@subsection No_Implicit_Conditionals


@geindex No_Implicit_Conditionals

[GNAT] This restriction ensures that the generated code does not contain any
implicit conditionals, either by modifying the generated code where possible,
or by rejecting any construct that would otherwise generate an implicit
conditional. Note that this check does not include run time constraint
checks, which on some targets may generate implicit conditionals as
well. To control the latter, constraint checks can be suppressed in the
normal manner. Constructs generating implicit conditionals include comparisons
of composite objects and the Max/Min attributes.

@node No_Implicit_Dynamic_Code,No_Implicit_Heap_Allocations,No_Implicit_Conditionals,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-dynamic-code}@anchor{1f8}
@subsection No_Implicit_Dynamic_Code


@geindex No_Implicit_Dynamic_Code

@geindex trampoline

[GNAT] This restriction prevents the compiler from building ‘trampolines’.
This is a structure that is built on the stack and contains dynamic
code to be executed at run time. On some targets, a trampoline is
built for the following features: @code{Access},
@code{Unrestricted_Access}, or @code{Address} of a nested subprogram;
nested task bodies; primitive operations of nested tagged types.
Trampolines do not work on machines that prevent execution of stack
data. For example, on windows systems, enabling DEP (data execution
protection) will cause trampolines to raise an exception.
Trampolines are also quite slow at run time.

On many targets, trampolines have been largely eliminated. Look at the
version of system.ads for your target — if it has
Always_Compatible_Rep equal to False, then trampolines are largely
eliminated. In particular, a trampoline is built for the following
features: @code{Address} of a nested subprogram;
@code{Access} or @code{Unrestricted_Access} of a nested subprogram,
but only if pragma Favor_Top_Level applies, or the access type has a
foreign-language convention; primitive operations of nested tagged
types.

@node No_Implicit_Heap_Allocations,No_Implicit_Protected_Object_Allocations,No_Implicit_Dynamic_Code,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-heap-allocations}@anchor{1f9}
@subsection No_Implicit_Heap_Allocations


@geindex No_Implicit_Heap_Allocations

[RM D.7] No constructs are allowed to cause implicit heap allocation.

@node No_Implicit_Protected_Object_Allocations,No_Implicit_Task_Allocations,No_Implicit_Heap_Allocations,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-protected-object-allocations}@anchor{1fa}
@subsection No_Implicit_Protected_Object_Allocations


@geindex No_Implicit_Protected_Object_Allocations

[GNAT] No constructs are allowed to cause implicit heap allocation of a
protected object.

@node No_Implicit_Task_Allocations,No_Initialize_Scalars,No_Implicit_Protected_Object_Allocations,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-task-allocations}@anchor{1fb}
@subsection No_Implicit_Task_Allocations


@geindex No_Implicit_Task_Allocations

[GNAT] No constructs are allowed to cause implicit heap allocation of a task.

@node No_Initialize_Scalars,No_IO,No_Implicit_Task_Allocations,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-initialize-scalars}@anchor{1fc}
@subsection No_Initialize_Scalars


@geindex No_Initialize_Scalars

[GNAT] This restriction ensures that no unit in the partition is compiled with
pragma Initialize_Scalars. This allows the generation of more efficient
code, and in particular eliminates dummy null initialization routines that
are otherwise generated for some record and array types.

@node No_IO,No_Local_Allocators,No_Initialize_Scalars,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-io}@anchor{1fd}
@subsection No_IO


@geindex No_IO

[RM H.4] This restriction ensures at compile time that there are no
dependences on any of the library units Sequential_IO, Direct_IO,
Text_IO, Wide_Text_IO, Wide_Wide_Text_IO, or Stream_IO.

@node No_Local_Allocators,No_Local_Protected_Objects,No_IO,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-allocators}@anchor{1fe}
@subsection No_Local_Allocators


@geindex No_Local_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator in subprograms, generic subprograms, tasks,
and entry bodies.

@node No_Local_Protected_Objects,No_Local_Tagged_Types,No_Local_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-protected-objects}@anchor{1ff}
@subsection No_Local_Protected_Objects


@geindex No_Local_Protected_Objects

[RM D.7] This restriction ensures at compile time that protected objects are
only declared at the library level.

@node No_Local_Tagged_Types,No_Local_Timing_Events,No_Local_Protected_Objects,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-tagged-types}@anchor{200}
@subsection No_Local_Tagged_Types


@geindex No_Local_Tagged_Types

[GNAT] This restriction ensures at compile time that tagged types are only
declared at the library level.

@node No_Local_Timing_Events,No_Long_Long_Integers,No_Local_Tagged_Types,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-timing-events}@anchor{201}
@subsection No_Local_Timing_Events


@geindex No_Local_Timing_Events

[RM D.7] All objects of type Ada.Real_Time.Timing_Events.Timing_Event are
declared at the library level.

@node No_Long_Long_Integers,No_Multiple_Elaboration,No_Local_Timing_Events,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-long-long-integers}@anchor{202}
@subsection No_Long_Long_Integers


@geindex No_Long_Long_Integers

[GNAT] This partition-wide restriction forbids any explicit reference to
type Standard.Long_Long_Integer, and also forbids declaring range types whose
implicit base type is Long_Long_Integer, and modular types whose size exceeds
Long_Integer’Size.

@node No_Multiple_Elaboration,No_Nested_Finalization,No_Long_Long_Integers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-multiple-elaboration}@anchor{203}
@subsection No_Multiple_Elaboration


@geindex No_Multiple_Elaboration

[GNAT] When this restriction is active and the static elaboration model is
used, and -fpreserve-control-flow is not used, the compiler is allowed to
suppress the elaboration counter normally associated with the unit, even if
the unit has elaboration code. This counter is typically used to check for
access before elaboration and to control multiple elaboration attempts. If the
restriction is used, then the situations in which multiple elaboration is
possible, including non-Ada main programs and Stand Alone libraries, are not
permitted and will be diagnosed by the binder.

@node No_Nested_Finalization,No_Protected_Type_Allocators,No_Multiple_Elaboration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-nested-finalization}@anchor{204}
@subsection No_Nested_Finalization


@geindex No_Nested_Finalization

[RM D.7] All objects requiring finalization are declared at the library level.

@node No_Protected_Type_Allocators,No_Protected_Types,No_Nested_Finalization,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-protected-type-allocators}@anchor{205}
@subsection No_Protected_Type_Allocators


@geindex No_Protected_Type_Allocators

[RM D.7] This restriction ensures at compile time that there are no allocator
expressions that attempt to allocate protected objects.

@node No_Protected_Types,No_Recursion,No_Protected_Type_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-protected-types}@anchor{206}
@subsection No_Protected_Types


@geindex No_Protected_Types

[RM H.4] This restriction ensures at compile time that there are no
declarations of protected types or protected objects.

@node No_Recursion,No_Reentrancy,No_Protected_Types,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-recursion}@anchor{207}
@subsection No_Recursion


@geindex No_Recursion

[RM H.4] A program execution is erroneous if a subprogram is invoked as
part of its execution.

@node No_Reentrancy,No_Relative_Delay,No_Recursion,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-reentrancy}@anchor{208}
@subsection No_Reentrancy


@geindex No_Reentrancy

[RM H.4] A program execution is erroneous if a subprogram is executed by
two tasks at the same time.

@node No_Relative_Delay,No_Requeue_Statements,No_Reentrancy,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-relative-delay}@anchor{209}
@subsection No_Relative_Delay


@geindex No_Relative_Delay

[RM D.7] This restriction ensures at compile time that there are no delay
relative statements and prevents expressions such as @code{delay 1.23;} from
appearing in source code.

@node No_Requeue_Statements,No_Secondary_Stack,No_Relative_Delay,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-requeue-statements}@anchor{20a}
@subsection No_Requeue_Statements


@geindex No_Requeue_Statements

[RM D.7] This restriction ensures at compile time that no requeue statements
are permitted and prevents keyword @code{requeue} from being used in source
code.

@geindex No_Requeue

The restriction @code{No_Requeue} is recognized as a
synonym for @code{No_Requeue_Statements}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on oNobsolescent features are activated).

@node No_Secondary_Stack,No_Select_Statements,No_Requeue_Statements,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-secondary-stack}@anchor{20b}
@subsection No_Secondary_Stack


@geindex No_Secondary_Stack

[GNAT] This restriction ensures at compile time that the generated code
does not contain any reference to the secondary stack.  The secondary
stack is used to implement functions returning unconstrained objects
(arrays or records) on some targets. Suppresses the allocation of
secondary stacks for tasks (excluding the environment task) at run time.

@node No_Select_Statements,No_Specific_Termination_Handlers,No_Secondary_Stack,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-select-statements}@anchor{20c}
@subsection No_Select_Statements


@geindex No_Select_Statements

[RM D.7] This restriction ensures at compile time no select statements of any
kind are permitted, that is the keyword @code{select} may not appear.

@node No_Specific_Termination_Handlers,No_Specification_of_Aspect,No_Select_Statements,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-specific-termination-handlers}@anchor{20d}
@subsection No_Specific_Termination_Handlers


@geindex No_Specific_Termination_Handlers

[RM D.7] There are no calls to Ada.Task_Termination.Set_Specific_Handler
or to Ada.Task_Termination.Specific_Handler.

@node No_Specification_of_Aspect,No_Standard_Allocators_After_Elaboration,No_Specific_Termination_Handlers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-specification-of-aspect}@anchor{20e}
@subsection No_Specification_of_Aspect


@geindex No_Specification_of_Aspect

[RM 13.12.1] This restriction checks at compile time that no aspect
specification, attribute definition clause, or pragma is given for a
given aspect.

@node No_Standard_Allocators_After_Elaboration,No_Standard_Storage_Pools,No_Specification_of_Aspect,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-standard-allocators-after-elaboration}@anchor{20f}
@subsection No_Standard_Allocators_After_Elaboration


@geindex No_Standard_Allocators_After_Elaboration

[RM D.7] Specifies that an allocator using a standard storage pool
should never be evaluated at run time after the elaboration of the
library items of the partition has completed. Otherwise, Storage_Error
is raised.

@node No_Standard_Storage_Pools,No_Stream_Optimizations,No_Standard_Allocators_After_Elaboration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-standard-storage-pools}@anchor{210}
@subsection No_Standard_Storage_Pools


@geindex No_Standard_Storage_Pools

[GNAT] This restriction ensures at compile time that no access types
use the standard default storage pool.  Any access type declared must
have an explicit Storage_Pool attribute defined specifying a
user-defined storage pool.

@node No_Stream_Optimizations,No_Streams,No_Standard_Storage_Pools,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-stream-optimizations}@anchor{211}
@subsection No_Stream_Optimizations


@geindex No_Stream_Optimizations

[GNAT] This restriction affects the performance of stream operations on types
@code{String}, @code{Wide_String} and @code{Wide_Wide_String}. By default, the
compiler uses block reads and writes when manipulating @code{String} objects
due to their superior performance. When this restriction is in effect, the
compiler performs all IO operations on a per-character basis.

@node No_Streams,No_Tagged_Type_Registration,No_Stream_Optimizations,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-streams}@anchor{212}
@subsection No_Streams


@geindex No_Streams

[GNAT] This restriction ensures at compile/bind time that there are no
stream objects created and no use of stream attributes.
This restriction does not forbid dependences on the package
@code{Ada.Streams}. So it is permissible to with
@code{Ada.Streams} (or another package that does so itself)
as long as no actual stream objects are created and no
stream attributes are used.

Note that the use of restriction allows optimization of tagged types,
since they do not need to worry about dispatching stream operations.
To take maximum advantage of this space-saving optimization, any
unit declaring a tagged type should be compiled with the restriction,
though this is not required.

When pragmas @code{Discard_Names} and @code{Restrictions (No_Streams)} simultaneously
apply to a tagged type, its Expanded_Name and External_Tag are also initialized
with empty strings. In particular, both these pragmas can be applied as
configuration pragmas to avoid exposing entity names at binary level for the
entire partition.

@node No_Tagged_Type_Registration,No_Task_Allocators,No_Streams,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-tagged-type-registration}@anchor{213}
@subsection No_Tagged_Type_Registration


@geindex No_Tagged_Type_Registration

[GNAT] If this restriction is active, then class-wide streaming
attributes are not supported. In addition, the subprograms in
Ada.Tags are not supported.
If this restriction is active, the generated code is simplified by
omitting the otherwise-required global registration of tagged types when they
are declared. This restriction may be necessary in order to also apply
the No_Elaboration_Code restriction.

@node No_Task_Allocators,No_Task_At_Interrupt_Priority,No_Tagged_Type_Registration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-allocators}@anchor{214}
@subsection No_Task_Allocators


@geindex No_Task_Allocators

[RM D.7] There are no allocators for task types
or types containing task subcomponents.

@node No_Task_At_Interrupt_Priority,No_Task_Attributes_Package,No_Task_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-at-interrupt-priority}@anchor{215}
@subsection No_Task_At_Interrupt_Priority


@geindex No_Task_At_Interrupt_Priority

[GNAT] This restriction ensures at compile time that there is no
Interrupt_Priority aspect or pragma for a task or a task type. As
a consequence, the tasks are always created with a priority below
that an interrupt priority.

@node No_Task_Attributes_Package,No_Task_Hierarchy,No_Task_At_Interrupt_Priority,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-attributes-package}@anchor{216}
@subsection No_Task_Attributes_Package


@geindex No_Task_Attributes_Package

[GNAT] This restriction ensures at compile time that there are no implicit or
explicit dependencies on the package @code{Ada.Task_Attributes}.

@geindex No_Task_Attributes

The restriction @code{No_Task_Attributes} is recognized as a synonym
for @code{No_Task_Attributes_Package}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node No_Task_Hierarchy,No_Task_Termination,No_Task_Attributes_Package,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-hierarchy}@anchor{217}
@subsection No_Task_Hierarchy


@geindex No_Task_Hierarchy

[RM D.7] All (non-environment) tasks depend
directly on the environment task of the partition.

@node No_Task_Termination,No_Tasking,No_Task_Hierarchy,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-termination}@anchor{218}
@subsection No_Task_Termination


@geindex No_Task_Termination

[RM D.7] Tasks that terminate are erroneous.

@node No_Tasking,No_Terminate_Alternatives,No_Task_Termination,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-tasking}@anchor{219}
@subsection No_Tasking


@geindex No_Tasking

[GNAT] This restriction prevents the declaration of tasks or task types
throughout the partition.  It is similar in effect to the use of
@code{Max_Tasks => 0} except that violations are caught at compile time
and cause an error message to be output either by the compiler or
binder.

@node No_Terminate_Alternatives,No_Unchecked_Access,No_Tasking,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-terminate-alternatives}@anchor{21a}
@subsection No_Terminate_Alternatives


@geindex No_Terminate_Alternatives

[RM D.7] There are no selective accepts with terminate alternatives.

@node No_Unchecked_Access,No_Unchecked_Conversion,No_Terminate_Alternatives,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-unchecked-access}@anchor{21b}
@subsection No_Unchecked_Access


@geindex No_Unchecked_Access

[RM H.4] This restriction ensures at compile time that there are no
occurrences of the Unchecked_Access attribute.

@node No_Unchecked_Conversion,No_Unchecked_Deallocation,No_Unchecked_Access,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-unchecked-conversion}@anchor{21c}
@subsection No_Unchecked_Conversion


@geindex No_Unchecked_Conversion

[RM J.13] This restriction ensures at compile time that there are no semantic
dependences on the predefined generic function Unchecked_Conversion.

@node No_Unchecked_Deallocation,No_Use_Of_Attribute,No_Unchecked_Conversion,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-unchecked-deallocation}@anchor{21d}
@subsection No_Unchecked_Deallocation


@geindex No_Unchecked_Deallocation

[RM J.13] This restriction ensures at compile time that there are no semantic
dependences on the predefined generic procedure Unchecked_Deallocation.

@node No_Use_Of_Attribute,No_Use_Of_Entity,No_Unchecked_Deallocation,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-use-of-attribute}@anchor{21e}
@subsection No_Use_Of_Attribute


@geindex No_Use_Of_Attribute

[RM 13.12.1] This is a standard Ada 2012 restriction that is GNAT defined in
earlier versions of Ada.

@node No_Use_Of_Entity,No_Use_Of_Pragma,No_Use_Of_Attribute,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-use-of-entity}@anchor{21f}
@subsection No_Use_Of_Entity


@geindex No_Use_Of_Entity

[GNAT] This restriction ensures at compile time that there are no references
to the entity given in the form

@example
No_Use_Of_Entity => Name
@end example

where @code{Name} is the fully qualified entity, for example

@example
No_Use_Of_Entity => Ada.Text_IO.Put_Line
@end example

@node No_Use_Of_Pragma,Pure_Barriers,No_Use_Of_Entity,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-use-of-pragma}@anchor{220}
@subsection No_Use_Of_Pragma


@geindex No_Use_Of_Pragma

[RM 13.12.1] This is a standard Ada 2012 restriction that is GNAT defined in
earlier versions of Ada.

@node Pure_Barriers,Simple_Barriers,No_Use_Of_Pragma,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions pure-barriers}@anchor{221}
@subsection Pure_Barriers


@geindex Pure_Barriers

[GNAT] This restriction ensures at compile time that protected entry
barriers are restricted to:


@itemize *

@item 
components of the protected object (excluding selection from dereferences),

@item 
constant declarations,

@item 
named numbers,

@item 
enumeration literals,

@item 
integer literals,

@item 
real literals,

@item 
character literals,

@item 
implicitly defined comparison operators,

@item 
uses of the Standard.”not” operator,

@item 
short-circuit operator,

@item 
the Count attribute
@end itemize

This restriction is a relaxation of the Simple_Barriers restriction,
but still ensures absence of side effects, exceptions, and recursion
during the evaluation of the barriers.

@node Simple_Barriers,Static_Priorities,Pure_Barriers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions simple-barriers}@anchor{222}
@subsection Simple_Barriers


@geindex Simple_Barriers

[RM D.7] This restriction ensures at compile time that barriers in entry
declarations for protected types are restricted to either static boolean
expressions or references to simple boolean variables defined in the private
part of the protected type.  No other form of entry barriers is permitted.

@geindex Boolean_Entry_Barriers

The restriction @code{Boolean_Entry_Barriers} is recognized as a
synonym for @code{Simple_Barriers}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node Static_Priorities,Static_Storage_Size,Simple_Barriers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions static-priorities}@anchor{223}
@subsection Static_Priorities


@geindex Static_Priorities

[GNAT] This restriction ensures at compile time that all priority expressions
are static, and that there are no dependences on the package
@code{Ada.Dynamic_Priorities}.

@node Static_Storage_Size,,Static_Priorities,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions static-storage-size}@anchor{224}
@subsection Static_Storage_Size


@geindex Static_Storage_Size

[GNAT] This restriction ensures at compile time that any expression appearing
in a Storage_Size pragma or attribute definition clause is static.

@node Program Unit Level Restrictions,,Partition-Wide Restrictions,Standard and Implementation Defined Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions id3}@anchor{225}@anchor{gnat_rm/standard_and_implementation_defined_restrictions program-unit-level-restrictions}@anchor{226}
@section Program Unit Level Restrictions


The second set of restriction identifiers
does not require partition-wide consistency.
The restriction may be enforced for a single
compilation unit without any effect on any of the
other compilation units in the partition.

@menu
* No_Elaboration_Code:: 
* No_Dynamic_Accessibility_Checks:: 
* No_Dynamic_Sized_Objects:: 
* No_Entry_Queue:: 
* No_Implementation_Aspect_Specifications:: 
* No_Implementation_Attributes:: 
* No_Implementation_Identifiers:: 
* No_Implementation_Pragmas:: 
* No_Implementation_Restrictions:: 
* No_Implementation_Units:: 
* No_Implicit_Aliasing:: 
* No_Implicit_Loops:: 
* No_Obsolescent_Features:: 
* No_Wide_Characters:: 
* Static_Dispatch_Tables:: 
* SPARK_05:: 

@end menu

@node No_Elaboration_Code,No_Dynamic_Accessibility_Checks,,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-elaboration-code}@anchor{227}
@subsection No_Elaboration_Code


@geindex No_Elaboration_Code

[GNAT] This restriction ensures at compile time that no elaboration code is
generated.  Note that this is not the same condition as is enforced
by pragma @code{Preelaborate}.  There are cases in which pragma
@code{Preelaborate} still permits code to be generated (e.g., code
to initialize a large array to all zeroes), and there are cases of units
which do not meet the requirements for pragma @code{Preelaborate},
but for which no elaboration code is generated.  Generally, it is
the case that preelaborable units will meet the restrictions, with
the exception of large aggregates initialized with an others_clause,
and exception declarations (which generate calls to a run-time
registry procedure).  This restriction is enforced on
a unit by unit basis, it need not be obeyed consistently
throughout a partition.

In the case of aggregates with others, if the aggregate has a dynamic
size, there is no way to eliminate the elaboration code (such dynamic
bounds would be incompatible with @code{Preelaborate} in any case). If
the bounds are static, then use of this restriction actually modifies
the code choice of the compiler to avoid generating a loop, and instead
generate the aggregate statically if possible, no matter how many times
the data for the others clause must be repeatedly generated.

It is not possible to precisely document
the constructs which are compatible with this restriction, since,
unlike most other restrictions, this is not a restriction on the
source code, but a restriction on the generated object code. For
example, if the source contains a declaration:

@example
Val : constant Integer := X;
@end example

where X is not a static constant, it may be possible, depending
on complex optimization circuitry, for the compiler to figure
out the value of X at compile time, in which case this initialization
can be done by the loader, and requires no initialization code. It
is not possible to document the precise conditions under which the
optimizer can figure this out.

Note that this the implementation of this restriction requires full
code generation. If it is used in conjunction with “semantics only”
checking, then some cases of violations may be missed.

When this restriction is active, we are not requesting control-flow
preservation with -fpreserve-control-flow, and the static elaboration model is
used, the compiler is allowed to suppress the elaboration counter normally
associated with the unit. This counter is typically used to check for access
before elaboration and to control multiple elaboration attempts.

@node No_Dynamic_Accessibility_Checks,No_Dynamic_Sized_Objects,No_Elaboration_Code,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dynamic-accessibility-checks}@anchor{228}
@subsection No_Dynamic_Accessibility_Checks


@geindex No_Dynamic_Accessibility_Checks

[GNAT] No dynamic accessibility checks are generated when this restriction is
in effect. Instead, dangling references are prevented via more conservative
compile-time checking. More specifically, existing compile-time checks are
enforced but with more conservative assumptions about the accessibility levels
of the relevant entities. These conservative assumptions eliminate the need for
dynamic accessibility checks.

These new rules for computing (at compile-time) the accessibility level of an
anonymous access type T are as follows:


@itemize *

@item 
If T is a function result type then, from the caller’s perspective, its level
is that of the innermost master enclosing the function call. From the callee’s
perspective, the level of parameters and local variables of the callee is
statically deeper than the level of T.

For any other accessibility level L such that the level of parameters and local
variables of the callee is statically deeper than L, the level of T (from the
callee’s perspective) is also statically deeper than L.

@item 
If T is the type of a formal parameter then, from the caller’s perspective,
its level is at least as deep as that of the type of the corresponding actual
parameter (whatever that actual parameter might be). From the callee’s
perspective, the level of parameters and local variables of the callee is
statically deeper than the level of T.

@item 
If T is the type of a discriminant then its level is that of the discriminated
type.

@item 
If T is the type of a stand-alone object then its level is the level of the
object.

@item 
In all other cases, the level of T is as defined by the existing rules of Ada.
@end itemize

@node No_Dynamic_Sized_Objects,No_Entry_Queue,No_Dynamic_Accessibility_Checks,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dynamic-sized-objects}@anchor{229}
@subsection No_Dynamic_Sized_Objects


@geindex No_Dynamic_Sized_Objects

[GNAT] This restriction disallows certain constructs that might lead to the
creation of dynamic-sized composite objects (or array or discriminated type).
An array subtype indication is illegal if the bounds are not static
or references to discriminants of an enclosing type.
A discriminated subtype indication is illegal if the type has
discriminant-dependent array components or a variant part, and the
discriminants are not static. In addition, array and record aggregates are
illegal in corresponding cases. Note that this restriction does not forbid
access discriminants. It is often a good idea to combine this restriction
with No_Secondary_Stack.

@node No_Entry_Queue,No_Implementation_Aspect_Specifications,No_Dynamic_Sized_Objects,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-entry-queue}@anchor{22a}
@subsection No_Entry_Queue


@geindex No_Entry_Queue

[GNAT] This restriction is a declaration that any protected entry compiled in
the scope of the restriction has at most one task waiting on the entry
at any one time, and so no queue is required.  This restriction is not
checked at compile time.  A program execution is erroneous if an attempt
is made to queue a second task on such an entry.

@node No_Implementation_Aspect_Specifications,No_Implementation_Attributes,No_Entry_Queue,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-aspect-specifications}@anchor{22b}
@subsection No_Implementation_Aspect_Specifications


@geindex No_Implementation_Aspect_Specifications

[RM 13.12.1] This restriction checks at compile time that no
GNAT-defined aspects are present.  With this restriction, the only
aspects that can be used are those defined in the Ada Reference Manual.

@node No_Implementation_Attributes,No_Implementation_Identifiers,No_Implementation_Aspect_Specifications,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-attributes}@anchor{22c}
@subsection No_Implementation_Attributes


@geindex No_Implementation_Attributes

[RM 13.12.1] This restriction checks at compile time that no
GNAT-defined attributes are present.  With this restriction, the only
attributes that can be used are those defined in the Ada Reference
Manual.

@node No_Implementation_Identifiers,No_Implementation_Pragmas,No_Implementation_Attributes,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-identifiers}@anchor{22d}
@subsection No_Implementation_Identifiers


@geindex No_Implementation_Identifiers

[RM 13.12.1] This restriction checks at compile time that no
implementation-defined identifiers (marked with pragma Implementation_Defined)
occur within language-defined packages.

@node No_Implementation_Pragmas,No_Implementation_Restrictions,No_Implementation_Identifiers,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-pragmas}@anchor{22e}
@subsection No_Implementation_Pragmas


@geindex No_Implementation_Pragmas

[RM 13.12.1] This restriction checks at compile time that no
GNAT-defined pragmas are present.  With this restriction, the only
pragmas that can be used are those defined in the Ada Reference Manual.

@node No_Implementation_Restrictions,No_Implementation_Units,No_Implementation_Pragmas,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-restrictions}@anchor{22f}
@subsection No_Implementation_Restrictions


@geindex No_Implementation_Restrictions

[GNAT] This restriction checks at compile time that no GNAT-defined restriction
identifiers (other than @code{No_Implementation_Restrictions} itself)
are present.  With this restriction, the only other restriction identifiers
that can be used are those defined in the Ada Reference Manual.

@node No_Implementation_Units,No_Implicit_Aliasing,No_Implementation_Restrictions,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-units}@anchor{230}
@subsection No_Implementation_Units


@geindex No_Implementation_Units

[RM 13.12.1] This restriction checks at compile time that there is no
mention in the context clause of any implementation-defined descendants
of packages Ada, Interfaces, or System.

@node No_Implicit_Aliasing,No_Implicit_Loops,No_Implementation_Units,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-aliasing}@anchor{231}
@subsection No_Implicit_Aliasing


@geindex No_Implicit_Aliasing

[GNAT] This restriction, which is not required to be partition-wide consistent,
requires an explicit aliased keyword for an object to which ‘Access,
‘Unchecked_Access, or ‘Address is applied, and forbids entirely the use of
the ‘Unrestricted_Access attribute for objects. Note: the reason that
Unrestricted_Access is forbidden is that it would require the prefix
to be aliased, and in such cases, it can always be replaced by
the standard attribute Unchecked_Access which is preferable.

@node No_Implicit_Loops,No_Obsolescent_Features,No_Implicit_Aliasing,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-loops}@anchor{232}
@subsection No_Implicit_Loops


@geindex No_Implicit_Loops

[GNAT] This restriction ensures that the generated code of the unit marked
with this restriction does not contain any implicit @code{for} loops, either by
modifying the generated code where possible, or by rejecting any construct
that would otherwise generate an implicit @code{for} loop. If this restriction is
active, it is possible to build large array aggregates with all static
components without generating an intermediate temporary, and without generating
a loop to initialize individual components. Otherwise, a loop is created for
arrays larger than about 5000 scalar components. Note that if this restriction
is set in the spec of a package, it will not apply to its body.

@node No_Obsolescent_Features,No_Wide_Characters,No_Implicit_Loops,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-obsolescent-features}@anchor{233}
@subsection No_Obsolescent_Features


@geindex No_Obsolescent_Features

[RM 13.12.1] This restriction checks at compile time that no obsolescent
features are used, as defined in Annex J of the Ada Reference Manual.

@node No_Wide_Characters,Static_Dispatch_Tables,No_Obsolescent_Features,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-wide-characters}@anchor{234}
@subsection No_Wide_Characters


@geindex No_Wide_Characters

[GNAT] This restriction ensures at compile time that no uses of the types
@code{Wide_Character} or @code{Wide_String} or corresponding wide
wide types
appear, and that no wide or wide wide string or character literals
appear in the program (that is literals representing characters not in
type @code{Character}).

@node Static_Dispatch_Tables,SPARK_05,No_Wide_Characters,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions static-dispatch-tables}@anchor{235}
@subsection Static_Dispatch_Tables


@geindex Static_Dispatch_Tables

[GNAT] This restriction checks at compile time that all the artifacts
associated with dispatch tables can be placed in read-only memory.

@node SPARK_05,,Static_Dispatch_Tables,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions spark-05}@anchor{236}
@subsection SPARK_05


@geindex SPARK_05

[GNAT] This restriction no longer has any effect and is superseded by
SPARK 2014, whose restrictions are checked by the tool GNATprove. To check that
a codebase respects SPARK 2014 restrictions, mark the code with pragma or
aspect @code{SPARK_Mode}, and run the tool GNATprove at Stone assurance level, as
follows:

@example
gnatprove -P project.gpr --mode=stone
@end example

or equivalently:

@example
gnatprove -P project.gpr --mode=check_all
@end example

@node Implementation Advice,GNAT Implementation Mode the -gnatg switch,Standard and Implementation Defined Restrictions,Top
@anchor{gnat_rm/implementation_advice doc}@anchor{237}@anchor{gnat_rm/implementation_advice id1}@anchor{238}@anchor{gnat_rm/implementation_advice implementation-advice}@anchor{a}
@chapter Implementation Advice


The main text of the Ada Reference Manual describes the required
behavior of all Ada compilers, and the GNAT compiler conforms to
these requirements.

In addition, there are sections throughout the Ada Reference Manual headed
by the phrase ‘Implementation advice’.  These sections are not normative,
i.e., they do not specify requirements that all compilers must
follow.  Rather they provide advice on generally desirable behavior.
They are not requirements, because they describe behavior that cannot
be provided on all systems, or may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice in
the Ada Reference Manual.  Each such RM section corresponds to a section
in this chapter whose title specifies the
RM section number and paragraph number and the subject of
the advice.  The contents of each section consists of the RM text within
quotation marks,
followed by the GNAT interpretation of the advice.  Most often, this simply says
‘followed’, which means that GNAT follows the advice.  However, in a
number of cases, GNAT deliberately deviates from this advice, in which
case the text describes what GNAT does and why.

@geindex Error detection

@menu
* RM 1.1.3(20); Error Detection: RM 1 1 3 20 Error Detection. 
* RM 1.1.3(31); Child Units: RM 1 1 3 31 Child Units. 
* RM 1.1.5(12); Bounded Errors: RM 1 1 5 12 Bounded Errors. 
* RM 2.8(16); Pragmas: RM 2 8 16 Pragmas. 
* RM 2.8(17-19); Pragmas: RM 2 8 17-19 Pragmas. 
* RM 3.5.2(5); Alternative Character Sets: RM 3 5 2 5 Alternative Character Sets. 
* RM 3.5.4(28); Integer Types: RM 3 5 4 28 Integer Types. 
* RM 3.5.4(29); Integer Types: RM 3 5 4 29 Integer Types. 
* RM 3.5.5(8); Enumeration Values: RM 3 5 5 8 Enumeration Values. 
* RM 3.5.7(17); Float Types: RM 3 5 7 17 Float Types. 
* RM 3.6.2(11); Multidimensional Arrays: RM 3 6 2 11 Multidimensional Arrays. 
* RM 9.6(30-31); Duration’Small: RM 9 6 30-31 Duration’Small. 
* RM 10.2.1(12); Consistent Representation: RM 10 2 1 12 Consistent Representation. 
* RM 11.4.1(19); Exception Information: RM 11 4 1 19 Exception Information. 
* RM 11.5(28); Suppression of Checks: RM 11 5 28 Suppression of Checks. 
* RM 13.1 (21-24); Representation Clauses: RM 13 1 21-24 Representation Clauses. 
* RM 13.2(6-8); Packed Types: RM 13 2 6-8 Packed Types. 
* RM 13.3(14-19); Address Clauses: RM 13 3 14-19 Address Clauses. 
* RM 13.3(29-35); Alignment Clauses: RM 13 3 29-35 Alignment Clauses. 
* RM 13.3(42-43); Size Clauses: RM 13 3 42-43 Size Clauses. 
* RM 13.3(50-56); Size Clauses: RM 13 3 50-56 Size Clauses. 
* RM 13.3(71-73); Component Size Clauses: RM 13 3 71-73 Component Size Clauses. 
* RM 13.4(9-10); Enumeration Representation Clauses: RM 13 4 9-10 Enumeration Representation Clauses. 
* RM 13.5.1(17-22); Record Representation Clauses: RM 13 5 1 17-22 Record Representation Clauses. 
* RM 13.5.2(5); Storage Place Attributes: RM 13 5 2 5 Storage Place Attributes. 
* RM 13.5.3(7-8); Bit Ordering: RM 13 5 3 7-8 Bit Ordering. 
* RM 13.7(37); Address as Private: RM 13 7 37 Address as Private. 
* RM 13.7.1(16); Address Operations: RM 13 7 1 16 Address Operations. 
* RM 13.9(14-17); Unchecked Conversion: RM 13 9 14-17 Unchecked Conversion. 
* RM 13.11(23-25); Implicit Heap Usage: RM 13 11 23-25 Implicit Heap Usage. 
* RM 13.11.2(17); Unchecked Deallocation: RM 13 11 2 17 Unchecked Deallocation. 
* RM 13.13.2(1.6); Stream Oriented Attributes: RM 13 13 2 1 6 Stream Oriented Attributes. 
* RM A.1(52); Names of Predefined Numeric Types: RM A 1 52 Names of Predefined Numeric Types. 
* RM A.3.2(49); Ada.Characters.Handling: RM A 3 2 49 Ada Characters Handling. 
* RM A.4.4(106); Bounded-Length String Handling: RM A 4 4 106 Bounded-Length String Handling. 
* RM A.5.2(46-47); Random Number Generation: RM A 5 2 46-47 Random Number Generation. 
* RM A.10.7(23); Get_Immediate: RM A 10 7 23 Get_Immediate. 
* RM A.18; Containers: RM A 18 Containers. 
* RM B.1(39-41); Pragma Export: RM B 1 39-41 Pragma Export. 
* RM B.2(12-13); Package Interfaces: RM B 2 12-13 Package Interfaces. 
* RM B.3(63-71); Interfacing with C: RM B 3 63-71 Interfacing with C. 
* RM B.4(95-98); Interfacing with COBOL: RM B 4 95-98 Interfacing with COBOL. 
* RM B.5(22-26); Interfacing with Fortran: RM B 5 22-26 Interfacing with Fortran. 
* RM C.1(3-5); Access to Machine Operations: RM C 1 3-5 Access to Machine Operations. 
* RM C.1(10-16); Access to Machine Operations: RM C 1 10-16 Access to Machine Operations. 
* RM C.3(28); Interrupt Support: RM C 3 28 Interrupt Support. 
* RM C.3.1(20-21); Protected Procedure Handlers: RM C 3 1 20-21 Protected Procedure Handlers. 
* RM C.3.2(25); Package Interrupts: RM C 3 2 25 Package Interrupts. 
* RM C.4(14); Pre-elaboration Requirements: RM C 4 14 Pre-elaboration Requirements. 
* RM C.5(8); Pragma Discard_Names: RM C 5 8 Pragma Discard_Names. 
* RM C.7.2(30); The Package Task_Attributes: RM C 7 2 30 The Package Task_Attributes. 
* RM D.3(17); Locking Policies: RM D 3 17 Locking Policies. 
* RM D.4(16); Entry Queuing Policies: RM D 4 16 Entry Queuing Policies. 
* RM D.6(9-10); Preemptive Abort: RM D 6 9-10 Preemptive Abort. 
* RM D.7(21); Tasking Restrictions: RM D 7 21 Tasking Restrictions. 
* RM D.8(47-49); Monotonic Time: RM D 8 47-49 Monotonic Time. 
* RM E.5(28-29); Partition Communication Subsystem: RM E 5 28-29 Partition Communication Subsystem. 
* RM F(7); COBOL Support: RM F 7 COBOL Support. 
* RM F.1(2); Decimal Radix Support: RM F 1 2 Decimal Radix Support. 
* RM G; Numerics: RM G Numerics. 
* RM G.1.1(56-58); Complex Types: RM G 1 1 56-58 Complex Types. 
* RM G.1.2(49); Complex Elementary Functions: RM G 1 2 49 Complex Elementary Functions. 
* RM G.2.4(19); Accuracy Requirements: RM G 2 4 19 Accuracy Requirements. 
* RM G.2.6(15); Complex Arithmetic Accuracy: RM G 2 6 15 Complex Arithmetic Accuracy. 
* RM H.6(15/2); Pragma Partition_Elaboration_Policy: RM H 6 15/2 Pragma Partition_Elaboration_Policy. 

@end menu

@node RM 1 1 3 20 Error Detection,RM 1 1 3 31 Child Units,,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-1-1-3-20-error-detection}@anchor{239}
@section RM 1.1.3(20): Error Detection


@quotation

“If an implementation detects the use of an unsupported Specialized Needs
Annex feature at run time, it should raise @code{Program_Error} if
feasible.”
@end quotation

Not relevant.  All specialized needs annex features are either supported,
or diagnosed at compile time.

@geindex Child Units

@node RM 1 1 3 31 Child Units,RM 1 1 5 12 Bounded Errors,RM 1 1 3 20 Error Detection,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-1-1-3-31-child-units}@anchor{23a}
@section RM 1.1.3(31): Child Units


@quotation

“If an implementation wishes to provide implementation-defined
extensions to the functionality of a language-defined library unit, it
should normally do so by adding children to the library unit.”
@end quotation

Followed.

@geindex Bounded errors

@node RM 1 1 5 12 Bounded Errors,RM 2 8 16 Pragmas,RM 1 1 3 31 Child Units,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-1-1-5-12-bounded-errors}@anchor{23b}
@section RM 1.1.5(12): Bounded Errors


@quotation

“If an implementation detects a bounded error or erroneous
execution, it should raise @code{Program_Error}.”
@end quotation

Followed in all cases in which the implementation detects a bounded
error or erroneous execution.  Not all such situations are detected at
runtime.

@geindex Pragmas

@node RM 2 8 16 Pragmas,RM 2 8 17-19 Pragmas,RM 1 1 5 12 Bounded Errors,Implementation Advice
@anchor{gnat_rm/implementation_advice id2}@anchor{23c}@anchor{gnat_rm/implementation_advice rm-2-8-16-pragmas}@anchor{23d}
@section RM 2.8(16): Pragmas


@quotation

“Normally, implementation-defined pragmas should have no semantic effect
for error-free programs; that is, if the implementation-defined pragmas
are removed from a working program, the program should still be legal,
and should still have the same semantics.”
@end quotation

The following implementation defined pragmas are exceptions to this
rule:


@multitable {xxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxx} 
@headitem

Pragma

@tab

Explanation

@item

`Abort_Defer'

@tab

Affects semantics

@item

`Ada_83'

@tab

Affects legality

@item

`Assert'

@tab

Affects semantics

@item

`CPP_Class'

@tab

Affects semantics

@item

`CPP_Constructor'

@tab

Affects semantics

@item

`Debug'

@tab

Affects semantics

@item

`Interface_Name'

@tab

Affects semantics

@item

`Machine_Attribute'

@tab

Affects semantics

@item

`Unimplemented_Unit'

@tab

Affects legality

@item

`Unchecked_Union'

@tab

Affects semantics

@end multitable


In each of the above cases, it is essential to the purpose of the pragma
that this advice not be followed.  For details see
@ref{7,,Implementation Defined Pragmas}.

@node RM 2 8 17-19 Pragmas,RM 3 5 2 5 Alternative Character Sets,RM 2 8 16 Pragmas,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-2-8-17-19-pragmas}@anchor{23e}
@section RM 2.8(17-19): Pragmas


@quotation

“Normally, an implementation should not define pragmas that can
make an illegal program legal, except as follows:


@itemize *

@item 
A pragma used to complete a declaration, such as a pragma @code{Import};

@item 
A pragma used to configure the environment by adding, removing, or
replacing @code{library_items}.”
@end itemize
@end quotation

See @ref{23d,,RM 2.8(16); Pragmas}.

@geindex Character Sets

@geindex Alternative Character Sets

@node RM 3 5 2 5 Alternative Character Sets,RM 3 5 4 28 Integer Types,RM 2 8 17-19 Pragmas,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-2-5-alternative-character-sets}@anchor{23f}
@section RM 3.5.2(5): Alternative Character Sets


@quotation

“If an implementation supports a mode with alternative interpretations
for @code{Character} and @code{Wide_Character}, the set of graphic
characters of @code{Character} should nevertheless remain a proper
subset of the set of graphic characters of @code{Wide_Character}.  Any
character set ‘localizations’ should be reflected in the results of
the subprograms defined in the language-defined package
@code{Characters.Handling} (see A.3) available in such a mode.  In a mode with
an alternative interpretation of @code{Character}, the implementation should
also support a corresponding change in what is a legal
@code{identifier_letter}.”
@end quotation

Not all wide character modes follow this advice, in particular the JIS
and IEC modes reflect standard usage in Japan, and in these encoding,
the upper half of the Latin-1 set is not part of the wide-character
subset, since the most significant bit is used for wide character
encoding.  However, this only applies to the external forms.  Internally
there is no such restriction.

@geindex Integer types

@node RM 3 5 4 28 Integer Types,RM 3 5 4 29 Integer Types,RM 3 5 2 5 Alternative Character Sets,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-4-28-integer-types}@anchor{240}
@section RM 3.5.4(28): Integer Types


@quotation

“An implementation should support @code{Long_Integer} in addition to
@code{Integer} if the target machine supports 32-bit (or longer)
arithmetic.  No other named integer subtypes are recommended for package
@code{Standard}.  Instead, appropriate named integer subtypes should be
provided in the library package @code{Interfaces} (see B.2).”
@end quotation

@code{Long_Integer} is supported.  Other standard integer types are supported
so this advice is not fully followed.  These types
are supported for convenient interface to C, and so that all hardware
types of the machine are easily available.

@node RM 3 5 4 29 Integer Types,RM 3 5 5 8 Enumeration Values,RM 3 5 4 28 Integer Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-4-29-integer-types}@anchor{241}
@section RM 3.5.4(29): Integer Types


@quotation

“An implementation for a two’s complement machine should support
modular types with a binary modulus up to @code{System.Max_Int*2+2}.  An
implementation should support a non-binary modules up to @code{Integer'Last}.”
@end quotation

Followed.

@geindex Enumeration values

@node RM 3 5 5 8 Enumeration Values,RM 3 5 7 17 Float Types,RM 3 5 4 29 Integer Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-5-8-enumeration-values}@anchor{242}
@section RM 3.5.5(8): Enumeration Values


@quotation

“For the evaluation of a call on @code{S'Pos} for an enumeration
subtype, if the value of the operand does not correspond to the internal
code for any enumeration literal of its type (perhaps due to an
un-initialized variable), then the implementation should raise
@code{Program_Error}.  This is particularly important for enumeration
types with noncontiguous internal codes specified by an
enumeration_representation_clause.”
@end quotation

Followed.

@geindex Float types

@node RM 3 5 7 17 Float Types,RM 3 6 2 11 Multidimensional Arrays,RM 3 5 5 8 Enumeration Values,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-7-17-float-types}@anchor{243}
@section RM 3.5.7(17): Float Types


@quotation

“An implementation should support @code{Long_Float} in addition to
@code{Float} if the target machine supports 11 or more digits of
precision.  No other named floating point subtypes are recommended for
package @code{Standard}.  Instead, appropriate named floating point subtypes
should be provided in the library package @code{Interfaces} (see B.2).”
@end quotation

@code{Short_Float} and @code{Long_Long_Float} are also provided.  The
former provides improved compatibility with other implementations
supporting this type.  The latter corresponds to the highest precision
floating-point type supported by the hardware.  On most machines, this
will be the same as @code{Long_Float}, but on some machines, it will
correspond to the IEEE extended form.  The notable case is all x86
implementations, where @code{Long_Long_Float} corresponds to the 80-bit
extended precision format supported in hardware on this processor.
Note that the 128-bit format on SPARC is not supported, since this
is a software rather than a hardware format.

@geindex Multidimensional arrays

@geindex Arrays
@geindex multidimensional

@node RM 3 6 2 11 Multidimensional Arrays,RM 9 6 30-31 Duration’Small,RM 3 5 7 17 Float Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-6-2-11-multidimensional-arrays}@anchor{244}
@section RM 3.6.2(11): Multidimensional Arrays


@quotation

“An implementation should normally represent multidimensional arrays in
row-major order, consistent with the notation used for multidimensional
array aggregates (see 4.3.3).  However, if a pragma @code{Convention}
(@code{Fortran}, …) applies to a multidimensional array type, then
column-major order should be used instead (see B.5, `Interfacing with Fortran').”
@end quotation

Followed.

@geindex Duration'Small

@node RM 9 6 30-31 Duration’Small,RM 10 2 1 12 Consistent Representation,RM 3 6 2 11 Multidimensional Arrays,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-9-6-30-31-duration-small}@anchor{245}
@section RM 9.6(30-31): Duration’Small


@quotation

“Whenever possible in an implementation, the value of @code{Duration'Small}
should be no greater than 100 microseconds.”
@end quotation

Followed.  (@code{Duration'Small} = 10**(-9)).

@quotation

“The time base for @code{delay_relative_statements} should be monotonic;
it need not be the same time base as used for @code{Calendar.Clock}.”
@end quotation

Followed.

@node RM 10 2 1 12 Consistent Representation,RM 11 4 1 19 Exception Information,RM 9 6 30-31 Duration’Small,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-10-2-1-12-consistent-representation}@anchor{246}
@section RM 10.2.1(12): Consistent Representation


@quotation

“In an implementation, a type declared in a pre-elaborated package should
have the same representation in every elaboration of a given version of
the package, whether the elaborations occur in distinct executions of
the same program, or in executions of distinct programs or partitions
that include the given version.”
@end quotation

Followed, except in the case of tagged types.  Tagged types involve
implicit pointers to a local copy of a dispatch table, and these pointers
have representations which thus depend on a particular elaboration of the
package.  It is not easy to see how it would be possible to follow this
advice without severely impacting efficiency of execution.

@geindex Exception information

@node RM 11 4 1 19 Exception Information,RM 11 5 28 Suppression of Checks,RM 10 2 1 12 Consistent Representation,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-11-4-1-19-exception-information}@anchor{247}
@section RM 11.4.1(19): Exception Information


@quotation

“@code{Exception_Message} by default and @code{Exception_Information}
should produce information useful for
debugging.  @code{Exception_Message} should be short, about one
line.  @code{Exception_Information} can be long.  @code{Exception_Message}
should not include the
@code{Exception_Name}.  @code{Exception_Information} should include both
the @code{Exception_Name} and the @code{Exception_Message}.”
@end quotation

Followed.  For each exception that doesn’t have a specified
@code{Exception_Message}, the compiler generates one containing the location
of the raise statement.  This location has the form ‘file_name:line’, where
file_name is the short file name (without path information) and line is the line
number in the file.  Note that in the case of the Zero Cost Exception
mechanism, these messages become redundant with the Exception_Information that
contains a full backtrace of the calling sequence, so they are disabled.
To disable explicitly the generation of the source location message, use the
Pragma @code{Discard_Names}.

@geindex Suppression of checks

@geindex Checks
@geindex suppression of

@node RM 11 5 28 Suppression of Checks,RM 13 1 21-24 Representation Clauses,RM 11 4 1 19 Exception Information,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-11-5-28-suppression-of-checks}@anchor{248}
@section RM 11.5(28): Suppression of Checks


@quotation

“The implementation should minimize the code executed for checks that
have been suppressed.”
@end quotation

Followed.

@geindex Representation clauses

@node RM 13 1 21-24 Representation Clauses,RM 13 2 6-8 Packed Types,RM 11 5 28 Suppression of Checks,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-1-21-24-representation-clauses}@anchor{249}
@section RM 13.1 (21-24): Representation Clauses


@quotation

“The recommended level of support for all representation items is
qualified as follows:

An implementation need not support representation items containing
nonstatic expressions, except that an implementation should support a
representation item for a given entity if each nonstatic expression in
the representation item is a name that statically denotes a constant
declared before the entity.”
@end quotation

Followed.  In fact, GNAT goes beyond the recommended level of support
by allowing nonstatic expressions in some representation clauses even
without the need to declare constants initialized with the values of
such expressions. For example:

@example
X : Integer;
Y : Float;
for Y'Address use X'Address;
@end example

is accepted directly by GNAT.

@quotation

“An implementation need not support a specification for the @code{Size}
for a given composite subtype, nor the size or storage place for an
object (including a component) of a given composite subtype, unless the
constraints on the subtype and its composite subcomponents (if any) are
all static constraints.”
@end quotation

Followed.  Size Clauses are not permitted on nonstatic components, as
described above.

@quotation

“An aliased component, or a component whose type is by-reference, should
always be allocated at an addressable location.”
@end quotation

Followed.

@geindex Packed types

@node RM 13 2 6-8 Packed Types,RM 13 3 14-19 Address Clauses,RM 13 1 21-24 Representation Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-2-6-8-packed-types}@anchor{24a}
@section RM 13.2(6-8): Packed Types


@quotation

“If a type is packed, then the implementation should try to minimize
storage allocated to objects of the type, possibly at the expense of
speed of accessing components, subject to reasonable complexity in
addressing calculations.

The recommended level of support pragma @code{Pack} is:

For a packed record type, the components should be packed as tightly as
possible subject to the Sizes of the component subtypes, and subject to
any `record_representation_clause' that applies to the type; the
implementation may, but need not, reorder components or cross aligned
word boundaries to improve the packing.  A component whose @code{Size} is
greater than the word size may be allocated an integral number of words.”
@end quotation

Followed.  Tight packing of arrays is supported for all component sizes
up to 64-bits. If the array component size is 1 (that is to say, if
the component is a boolean type or an enumeration type with two values)
then values of the type are implicitly initialized to zero. This
happens both for objects of the packed type, and for objects that have a
subcomponent of the packed type.

@geindex Address clauses

@node RM 13 3 14-19 Address Clauses,RM 13 3 29-35 Alignment Clauses,RM 13 2 6-8 Packed Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-14-19-address-clauses}@anchor{24b}
@section RM 13.3(14-19): Address Clauses


@quotation

“For an array @code{X}, @code{X'Address} should point at the first
component of the array, and not at the array bounds.”
@end quotation

Followed.

@quotation

“The recommended level of support for the @code{Address} attribute is:

@code{X'Address} should produce a useful result if @code{X} is an
object that is aliased or of a by-reference type, or is an entity whose
@code{Address} has been specified.”
@end quotation

Followed.  A valid address will be produced even if none of those
conditions have been met.  If necessary, the object is forced into
memory to ensure the address is valid.

@quotation

“An implementation should support @code{Address} clauses for imported
subprograms.”
@end quotation

Followed.

@quotation

“Objects (including subcomponents) that are aliased or of a by-reference
type should be allocated on storage element boundaries.”
@end quotation

Followed.

@quotation

“If the @code{Address} of an object is specified, or it is imported or exported,
then the implementation should not perform optimizations based on
assumptions of no aliases.”
@end quotation

Followed.

@geindex Alignment clauses

@node RM 13 3 29-35 Alignment Clauses,RM 13 3 42-43 Size Clauses,RM 13 3 14-19 Address Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-29-35-alignment-clauses}@anchor{24c}
@section RM 13.3(29-35): Alignment Clauses


@quotation

“The recommended level of support for the @code{Alignment} attribute for
subtypes is:

An implementation should support specified Alignments that are factors
and multiples of the number of storage elements per word, subject to the
following:”
@end quotation

Followed.

@quotation

“An implementation need not support specified Alignments for
combinations of Sizes and Alignments that cannot be easily
loaded and stored by available machine instructions.”
@end quotation

Followed.

@quotation

“An implementation need not support specified Alignments that are
greater than the maximum @code{Alignment} the implementation ever returns by
default.”
@end quotation

Followed.

@quotation

“The recommended level of support for the @code{Alignment} attribute for
objects is:

Same as above, for subtypes, but in addition:”
@end quotation

Followed.

@quotation

“For stand-alone library-level objects of statically constrained
subtypes, the implementation should support all alignments
supported by the target linker.  For example, page alignment is likely to
be supported for such objects, but not for subtypes.”
@end quotation

Followed.

@geindex Size clauses

@node RM 13 3 42-43 Size Clauses,RM 13 3 50-56 Size Clauses,RM 13 3 29-35 Alignment Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-42-43-size-clauses}@anchor{24d}
@section RM 13.3(42-43): Size Clauses


@quotation

“The recommended level of support for the @code{Size} attribute of
objects is:

A @code{Size} clause should be supported for an object if the specified
@code{Size} is at least as large as its subtype’s @code{Size}, and
corresponds to a size in storage elements that is a multiple of the
object’s @code{Alignment} (if the @code{Alignment} is nonzero).”
@end quotation

Followed.

@node RM 13 3 50-56 Size Clauses,RM 13 3 71-73 Component Size Clauses,RM 13 3 42-43 Size Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-50-56-size-clauses}@anchor{24e}
@section RM 13.3(50-56): Size Clauses


@quotation

“If the @code{Size} of a subtype is specified, and allows for efficient
independent addressability (see 9.10) on the target architecture, then
the @code{Size} of the following objects of the subtype should equal the
@code{Size} of the subtype:

Aliased objects (including components).”
@end quotation

Followed.

@quotation

“@cite{Size} clause on a composite subtype should not affect the
internal layout of components.”
@end quotation

Followed. But note that this can be overridden by use of the implementation
pragma Implicit_Packing in the case of packed arrays.

@quotation

“The recommended level of support for the @code{Size} attribute of subtypes is:

The @code{Size} (if not specified) of a static discrete or fixed point
subtype should be the number of bits needed to represent each value
belonging to the subtype using an unbiased representation, leaving space
for a sign bit only if the subtype contains negative values.  If such a
subtype is a first subtype, then an implementation should support a
specified @code{Size} for it that reflects this representation.”
@end quotation

Followed.

@quotation

“For a subtype implemented with levels of indirection, the @code{Size}
should include the size of the pointers, but not the size of what they
point at.”
@end quotation

Followed.

@geindex Component_Size clauses

@node RM 13 3 71-73 Component Size Clauses,RM 13 4 9-10 Enumeration Representation Clauses,RM 13 3 50-56 Size Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-71-73-component-size-clauses}@anchor{24f}
@section RM 13.3(71-73): Component Size Clauses


@quotation

“The recommended level of support for the @code{Component_Size}
attribute is:

An implementation need not support specified @code{Component_Sizes} that are
less than the @code{Size} of the component subtype.”
@end quotation

Followed.

@quotation

“An implementation should support specified Component_Sizes that
are factors and multiples of the word size.  For such
Component_Sizes, the array should contain no gaps between
components.  For other Component_Sizes (if supported), the array
should contain no gaps between components when packing is also
specified; the implementation should forbid this combination in cases
where it cannot support a no-gaps representation.”
@end quotation

Followed.

@geindex Enumeration representation clauses

@geindex Representation clauses
@geindex enumeration

@node RM 13 4 9-10 Enumeration Representation Clauses,RM 13 5 1 17-22 Record Representation Clauses,RM 13 3 71-73 Component Size Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-4-9-10-enumeration-representation-clauses}@anchor{250}
@section RM 13.4(9-10): Enumeration Representation Clauses


@quotation

“The recommended level of support for enumeration representation clauses
is:

An implementation need not support enumeration representation clauses
for boolean types, but should at minimum support the internal codes in
the range @code{System.Min_Int .. System.Max_Int}.”
@end quotation

Followed.

@geindex Record representation clauses

@geindex Representation clauses
@geindex records

@node RM 13 5 1 17-22 Record Representation Clauses,RM 13 5 2 5 Storage Place Attributes,RM 13 4 9-10 Enumeration Representation Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-5-1-17-22-record-representation-clauses}@anchor{251}
@section RM 13.5.1(17-22): Record Representation Clauses


@quotation

“The recommended level of support for
`record_representation_clause's is:

An implementation should support storage places that can be extracted
with a load, mask, shift sequence of machine code, and set with a load,
shift, mask, store sequence, given the available machine instructions
and run-time model.”
@end quotation

Followed.

@quotation

“A storage place should be supported if its size is equal to the
@code{Size} of the component subtype, and it starts and ends on a
boundary that obeys the @code{Alignment} of the component subtype.”
@end quotation

Followed.

@quotation

“If the default bit ordering applies to the declaration of a given type,
then for a component whose subtype’s @code{Size} is less than the word
size, any storage place that does not cross an aligned word boundary
should be supported.”
@end quotation

Followed.

@quotation

“An implementation may reserve a storage place for the tag field of a
tagged type, and disallow other components from overlapping that place.”
@end quotation

Followed.  The storage place for the tag field is the beginning of the tagged
record, and its size is Address’Size.  GNAT will reject an explicit component
clause for the tag field.

@quotation

“An implementation need not support a `component_clause' for a
component of an extension part if the storage place is not after the
storage places of all components of the parent type, whether or not
those storage places had been specified.”
@end quotation

Followed.  The above advice on record representation clauses is followed,
and all mentioned features are implemented.

@geindex Storage place attributes

@node RM 13 5 2 5 Storage Place Attributes,RM 13 5 3 7-8 Bit Ordering,RM 13 5 1 17-22 Record Representation Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-5-2-5-storage-place-attributes}@anchor{252}
@section RM 13.5.2(5): Storage Place Attributes


@quotation

“If a component is represented using some form of pointer (such as an
offset) to the actual data of the component, and this data is contiguous
with the rest of the object, then the storage place attributes should
reflect the place of the actual data, not the pointer.  If a component is
allocated discontinuously from the rest of the object, then a warning
should be generated upon reference to one of its storage place
attributes.”
@end quotation

Followed.  There are no such components in GNAT.

@geindex Bit ordering

@node RM 13 5 3 7-8 Bit Ordering,RM 13 7 37 Address as Private,RM 13 5 2 5 Storage Place Attributes,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-5-3-7-8-bit-ordering}@anchor{253}
@section RM 13.5.3(7-8): Bit Ordering


@quotation

“The recommended level of support for the non-default bit ordering is:

The implementation should support the nondefault bit ordering in addition
to the default bit ordering.”
@end quotation

Followed.

@geindex Address
@geindex as private type

@node RM 13 7 37 Address as Private,RM 13 7 1 16 Address Operations,RM 13 5 3 7-8 Bit Ordering,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-7-37-address-as-private}@anchor{254}
@section RM 13.7(37): Address as Private


@quotation

“@cite{Address} should be of a private type.”
@end quotation

Followed.

@geindex Operations
@geindex on `@w{`}Address`@w{`}

@geindex Address
@geindex operations of

@node RM 13 7 1 16 Address Operations,RM 13 9 14-17 Unchecked Conversion,RM 13 7 37 Address as Private,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-7-1-16-address-operations}@anchor{255}
@section RM 13.7.1(16): Address Operations


@quotation

“Operations in @code{System} and its children should reflect the target
environment semantics as closely as is reasonable.  For example, on most
machines, it makes sense for address arithmetic to ‘wrap around’.
Operations that do not make sense should raise @code{Program_Error}.”
@end quotation

Followed.  Address arithmetic is modular arithmetic that wraps around.  No
operation raises @code{Program_Error}, since all operations make sense.

@geindex Unchecked conversion

@node RM 13 9 14-17 Unchecked Conversion,RM 13 11 23-25 Implicit Heap Usage,RM 13 7 1 16 Address Operations,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-9-14-17-unchecked-conversion}@anchor{256}
@section RM 13.9(14-17): Unchecked Conversion


@quotation

“The @code{Size} of an array object should not include its bounds; hence,
the bounds should not be part of the converted data.”
@end quotation

Followed.

@quotation

“The implementation should not generate unnecessary run-time checks to
ensure that the representation of @code{S} is a representation of the
target type.  It should take advantage of the permission to return by
reference when possible.  Restrictions on unchecked conversions should be
avoided unless required by the target environment.”
@end quotation

Followed.  There are no restrictions on unchecked conversion.  A warning is
generated if the source and target types do not have the same size since
the semantics in this case may be target dependent.

@quotation

“The recommended level of support for unchecked conversions is:

Unchecked conversions should be supported and should be reversible in
the cases where this clause defines the result.  To enable meaningful use
of unchecked conversion, a contiguous representation should be used for
elementary subtypes, for statically constrained array subtypes whose
component subtype is one of the subtypes described in this paragraph,
and for record subtypes without discriminants whose component subtypes
are described in this paragraph.”
@end quotation

Followed.

@geindex Heap usage
@geindex implicit

@node RM 13 11 23-25 Implicit Heap Usage,RM 13 11 2 17 Unchecked Deallocation,RM 13 9 14-17 Unchecked Conversion,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-11-23-25-implicit-heap-usage}@anchor{257}
@section RM 13.11(23-25): Implicit Heap Usage


@quotation

“An implementation should document any cases in which it dynamically
allocates heap storage for a purpose other than the evaluation of an
allocator.”
@end quotation

Followed, the only other points at which heap storage is dynamically
allocated are as follows:


@itemize *

@item 
At initial elaboration time, to allocate dynamically sized global
objects.

@item 
To allocate space for a task when a task is created.

@item 
To extend the secondary stack dynamically when needed.  The secondary
stack is used for returning variable length results.
@end itemize


@quotation

“A default (implementation-provided) storage pool for an
access-to-constant type should not have overhead to support deallocation of
individual objects.”
@end quotation

Followed.

@quotation

“A storage pool for an anonymous access type should be created at the
point of an allocator for the type, and be reclaimed when the designated
object becomes inaccessible.”
@end quotation

Followed.

@geindex Unchecked deallocation

@node RM 13 11 2 17 Unchecked Deallocation,RM 13 13 2 1 6 Stream Oriented Attributes,RM 13 11 23-25 Implicit Heap Usage,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-11-2-17-unchecked-deallocation}@anchor{258}
@section RM 13.11.2(17): Unchecked Deallocation


@quotation

“For a standard storage pool, @code{Free} should actually reclaim the
storage.”
@end quotation

Followed.

@geindex Stream oriented attributes

@node RM 13 13 2 1 6 Stream Oriented Attributes,RM A 1 52 Names of Predefined Numeric Types,RM 13 11 2 17 Unchecked Deallocation,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-13-2-1-6-stream-oriented-attributes}@anchor{259}
@section RM 13.13.2(1.6): Stream Oriented Attributes


@quotation

“If not specified, the value of Stream_Size for an elementary type
should be the number of bits that corresponds to the minimum number of
stream elements required by the first subtype of the type, rounded up
to the nearest factor or multiple of the word size that is also a
multiple of the stream element size.”
@end quotation

Followed, except that the number of stream elements is 1, 2, 3, 4 or 8.
The Stream_Size may be used to override the default choice.

The default implementation is based on direct binary representations and is
therefore target- and endianness-dependent.  To address this issue, GNAT also
supplies an alternate implementation of the stream attributes @code{Read} and
@code{Write}, which uses the target-independent XDR standard representation for
scalar types. This XDR alternative can be enabled via the binder switch -xdr.

@geindex XDR representation

@geindex Read attribute

@geindex Write attribute

@geindex Stream oriented attributes

@node RM A 1 52 Names of Predefined Numeric Types,RM A 3 2 49 Ada Characters Handling,RM 13 13 2 1 6 Stream Oriented Attributes,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-1-52-names-of-predefined-numeric-types}@anchor{25a}
@section RM A.1(52): Names of Predefined Numeric Types


@quotation

“If an implementation provides additional named predefined integer types,
then the names should end with @code{Integer} as in
@code{Long_Integer}.  If an implementation provides additional named
predefined floating point types, then the names should end with
@code{Float} as in @code{Long_Float}.”
@end quotation

Followed.

@geindex Ada.Characters.Handling

@node RM A 3 2 49 Ada Characters Handling,RM A 4 4 106 Bounded-Length String Handling,RM A 1 52 Names of Predefined Numeric Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-3-2-49-ada-characters-handling}@anchor{25b}
@section RM A.3.2(49): @code{Ada.Characters.Handling}


@quotation

“If an implementation provides a localized definition of @code{Character}
or @code{Wide_Character}, then the effects of the subprograms in
@code{Characters.Handling} should reflect the localizations.
See also 3.5.2.”
@end quotation

Followed.  GNAT provides no such localized definitions.

@geindex Bounded-length strings

@node RM A 4 4 106 Bounded-Length String Handling,RM A 5 2 46-47 Random Number Generation,RM A 3 2 49 Ada Characters Handling,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-4-4-106-bounded-length-string-handling}@anchor{25c}
@section RM A.4.4(106): Bounded-Length String Handling


@quotation

“Bounded string objects should not be implemented by implicit pointers
and dynamic allocation.”
@end quotation

Followed.  No implicit pointers or dynamic allocation are used.

@geindex Random number generation

@node RM A 5 2 46-47 Random Number Generation,RM A 10 7 23 Get_Immediate,RM A 4 4 106 Bounded-Length String Handling,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-5-2-46-47-random-number-generation}@anchor{25d}
@section RM A.5.2(46-47): Random Number Generation


@quotation

“Any storage associated with an object of type @code{Generator} should be
reclaimed on exit from the scope of the object.”
@end quotation

Followed.

@quotation

“If the generator period is sufficiently long in relation to the number
of distinct initiator values, then each possible value of
@code{Initiator} passed to @code{Reset} should initiate a sequence of
random numbers that does not, in a practical sense, overlap the sequence
initiated by any other value.  If this is not possible, then the mapping
between initiator values and generator states should be a rapidly
varying function of the initiator value.”
@end quotation

Followed.  The generator period is sufficiently long for the first
condition here to hold true.

@geindex Get_Immediate

@node RM A 10 7 23 Get_Immediate,RM A 18 Containers,RM A 5 2 46-47 Random Number Generation,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-10-7-23-get-immediate}@anchor{25e}
@section RM A.10.7(23): @code{Get_Immediate}


@quotation

“The @code{Get_Immediate} procedures should be implemented with
unbuffered input.  For a device such as a keyboard, input should be
available if a key has already been typed, whereas for a disk
file, input should always be available except at end of file.  For a file
associated with a keyboard-like device, any line-editing features of the
underlying operating system should be disabled during the execution of
@code{Get_Immediate}.”
@end quotation

Followed on all targets except VxWorks. For VxWorks, there is no way to
provide this functionality that does not result in the input buffer being
flushed before the @code{Get_Immediate} call. A special unit
@code{Interfaces.Vxworks.IO} is provided that contains routines to enable
this functionality.

@geindex Containers

@node RM A 18 Containers,RM B 1 39-41 Pragma Export,RM A 10 7 23 Get_Immediate,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-18-containers}@anchor{25f}
@section RM A.18: @code{Containers}


All implementation advice pertaining to Ada.Containers and its
child units (that is, all implementation advice occurring within
section A.18 and its subsections) is followed except for A.18.24(17):

@quotation

“Bounded ordered set objects should be implemented without implicit pointers or dynamic allocation. “
@end quotation

The implementations of the two Reference_Preserving_Key functions of
the generic package Ada.Containers.Bounded_Ordered_Sets each currently make
use of dynamic allocation; other operations on bounded ordered set objects
follow the implementation advice.

@geindex Export

@node RM B 1 39-41 Pragma Export,RM B 2 12-13 Package Interfaces,RM A 18 Containers,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-1-39-41-pragma-export}@anchor{260}
@section RM B.1(39-41): Pragma @code{Export}


@quotation

“If an implementation supports pragma @code{Export} to a given language,
then it should also allow the main subprogram to be written in that
language.  It should support some mechanism for invoking the elaboration
of the Ada library units included in the system, and for invoking the
finalization of the environment task.  On typical systems, the
recommended mechanism is to provide two subprograms whose link names are
@code{adainit} and @code{adafinal}.  @code{adainit} should contain the
elaboration code for library units.  @code{adafinal} should contain the
finalization code.  These subprograms should have no effect the second
and subsequent time they are called.”
@end quotation

Followed.

@quotation

“Automatic elaboration of pre-elaborated packages should be
provided when pragma @code{Export} is supported.”
@end quotation

Followed when the main program is in Ada.  If the main program is in a
foreign language, then
@code{adainit} must be called to elaborate pre-elaborated
packages.

@quotation

“For each supported convention `L' other than @code{Intrinsic}, an
implementation should support @code{Import} and @code{Export} pragmas
for objects of `L'-compatible types and for subprograms, and pragma
@cite{Convention} for `L'-eligible types and for subprograms,
presuming the other language has corresponding features.  Pragma
@code{Convention} need not be supported for scalar types.”
@end quotation

Followed.

@geindex Package Interfaces

@geindex Interfaces

@node RM B 2 12-13 Package Interfaces,RM B 3 63-71 Interfacing with C,RM B 1 39-41 Pragma Export,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-2-12-13-package-interfaces}@anchor{261}
@section RM B.2(12-13): Package @code{Interfaces}


@quotation

“For each implementation-defined convention identifier, there should be a
child package of package Interfaces with the corresponding name.  This
package should contain any declarations that would be useful for
interfacing to the language (implementation) represented by the
convention.  Any declarations useful for interfacing to any language on
the given hardware architecture should be provided directly in
@code{Interfaces}.”
@end quotation

Followed.

@quotation

“An implementation supporting an interface to C, COBOL, or Fortran should
provide the corresponding package or packages described in the following
clauses.”
@end quotation

Followed.  GNAT provides all the packages described in this section.

@geindex C
@geindex interfacing with

@node RM B 3 63-71 Interfacing with C,RM B 4 95-98 Interfacing with COBOL,RM B 2 12-13 Package Interfaces,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-3-63-71-interfacing-with-c}@anchor{262}
@section RM B.3(63-71): Interfacing with C


@quotation

“An implementation should support the following interface correspondences
between Ada and C.”
@end quotation

Followed.

@quotation

“An Ada procedure corresponds to a void-returning C function.”
@end quotation

Followed.

@quotation

“An Ada function corresponds to a non-void C function.”
@end quotation

Followed.

@quotation

“An Ada @code{in} scalar parameter is passed as a scalar argument to a C
function.”
@end quotation

Followed.

@quotation

“An Ada @code{in} parameter of an access-to-object type with designated
type @code{T} is passed as a @code{t*} argument to a C function,
where @code{t} is the C type corresponding to the Ada type @code{T}.”
@end quotation

Followed.

@quotation

“An Ada access @code{T} parameter, or an Ada @code{out} or @code{in out}
parameter of an elementary type @code{T}, is passed as a @code{t*}
argument to a C function, where @code{t} is the C type corresponding to
the Ada type @code{T}.  In the case of an elementary @code{out} or
@code{in out} parameter, a pointer to a temporary copy is used to
preserve by-copy semantics.”
@end quotation

Followed.

@quotation

“An Ada parameter of a record type @code{T}, of any mode, is passed as a
@code{t*} argument to a C function, where @code{t} is the C
structure corresponding to the Ada type @code{T}.”
@end quotation

Followed.  This convention may be overridden by the use of the C_Pass_By_Copy
pragma, or Convention, or by explicitly specifying the mechanism for a given
call using an extended import or export pragma.

@quotation

“An Ada parameter of an array type with component type @code{T}, of any
mode, is passed as a @code{t*} argument to a C function, where
@code{t} is the C type corresponding to the Ada type @code{T}.”
@end quotation

Followed.

@quotation

“An Ada parameter of an access-to-subprogram type is passed as a pointer
to a C function whose prototype corresponds to the designated
subprogram’s specification.”
@end quotation

Followed.

@geindex COBOL
@geindex interfacing with

@node RM B 4 95-98 Interfacing with COBOL,RM B 5 22-26 Interfacing with Fortran,RM B 3 63-71 Interfacing with C,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-4-95-98-interfacing-with-cobol}@anchor{263}
@section RM B.4(95-98): Interfacing with COBOL


@quotation

“An Ada implementation should support the following interface
correspondences between Ada and COBOL.”
@end quotation

Followed.

@quotation

“An Ada access @code{T} parameter is passed as a @code{BY REFERENCE} data item of
the COBOL type corresponding to @code{T}.”
@end quotation

Followed.

@quotation

“An Ada in scalar parameter is passed as a @code{BY CONTENT} data item of
the corresponding COBOL type.”
@end quotation

Followed.

@quotation

“Any other Ada parameter is passed as a @code{BY REFERENCE} data item of the
COBOL type corresponding to the Ada parameter type; for scalars, a local
copy is used if necessary to ensure by-copy semantics.”
@end quotation

Followed.

@geindex Fortran
@geindex interfacing with

@node RM B 5 22-26 Interfacing with Fortran,RM C 1 3-5 Access to Machine Operations,RM B 4 95-98 Interfacing with COBOL,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-5-22-26-interfacing-with-fortran}@anchor{264}
@section RM B.5(22-26): Interfacing with Fortran


@quotation

“An Ada implementation should support the following interface
correspondences between Ada and Fortran:”
@end quotation

Followed.

@quotation

“An Ada procedure corresponds to a Fortran subroutine.”
@end quotation

Followed.

@quotation

“An Ada function corresponds to a Fortran function.”
@end quotation

Followed.

@quotation

“An Ada parameter of an elementary, array, or record type @code{T} is
passed as a @code{T} argument to a Fortran procedure, where @code{T} is
the Fortran type corresponding to the Ada type @code{T}, and where the
INTENT attribute of the corresponding dummy argument matches the Ada
formal parameter mode; the Fortran implementation’s parameter passing
conventions are used.  For elementary types, a local copy is used if
necessary to ensure by-copy semantics.”
@end quotation

Followed.

@quotation

“An Ada parameter of an access-to-subprogram type is passed as a
reference to a Fortran procedure whose interface corresponds to the
designated subprogram’s specification.”
@end quotation

Followed.

@geindex Machine operations

@node RM C 1 3-5 Access to Machine Operations,RM C 1 10-16 Access to Machine Operations,RM B 5 22-26 Interfacing with Fortran,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-1-3-5-access-to-machine-operations}@anchor{265}
@section RM C.1(3-5): Access to Machine Operations


@quotation

“The machine code or intrinsic support should allow access to all
operations normally available to assembly language programmers for the
target environment, including privileged instructions, if any.”
@end quotation

Followed.

@quotation

“The interfacing pragmas (see Annex B) should support interface to
assembler; the default assembler should be associated with the
convention identifier @code{Assembler}.”
@end quotation

Followed.

@quotation

“If an entity is exported to assembly language, then the implementation
should allocate it at an addressable location, and should ensure that it
is retained by the linking process, even if not otherwise referenced
from the Ada code.  The implementation should assume that any call to a
machine code or assembler subprogram is allowed to read or update every
object that is specified as exported.”
@end quotation

Followed.

@node RM C 1 10-16 Access to Machine Operations,RM C 3 28 Interrupt Support,RM C 1 3-5 Access to Machine Operations,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-1-10-16-access-to-machine-operations}@anchor{266}
@section RM C.1(10-16): Access to Machine Operations


@quotation

“The implementation should ensure that little or no overhead is
associated with calling intrinsic and machine-code subprograms.”
@end quotation

Followed for both intrinsics and machine-code subprograms.

@quotation

“It is recommended that intrinsic subprograms be provided for convenient
access to any machine operations that provide special capabilities or
efficiency and that are not otherwise available through the language
constructs.”
@end quotation

Followed.  A full set of machine operation intrinsic subprograms is provided.

@quotation

“Atomic read-modify-write operations—e.g., test and set, compare and
swap, decrement and test, enqueue/dequeue.”
@end quotation

Followed on any target supporting such operations.

@quotation

“Standard numeric functions—e.g.:, sin, log.”
@end quotation

Followed on any target supporting such operations.

@quotation

“String manipulation operations—e.g.:, translate and test.”
@end quotation

Followed on any target supporting such operations.

@quotation

“Vector operations—e.g.:, compare vector against thresholds.”
@end quotation

Followed on any target supporting such operations.

@quotation

“Direct operations on I/O ports.”
@end quotation

Followed on any target supporting such operations.

@geindex Interrupt support

@node RM C 3 28 Interrupt Support,RM C 3 1 20-21 Protected Procedure Handlers,RM C 1 10-16 Access to Machine Operations,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-3-28-interrupt-support}@anchor{267}
@section RM C.3(28): Interrupt Support


@quotation

“If the @code{Ceiling_Locking} policy is not in effect, the
implementation should provide means for the application to specify which
interrupts are to be blocked during protected actions, if the underlying
system allows for a finer-grain control of interrupt blocking.”
@end quotation

Followed.  The underlying system does not allow for finer-grain control
of interrupt blocking.

@geindex Protected procedure handlers

@node RM C 3 1 20-21 Protected Procedure Handlers,RM C 3 2 25 Package Interrupts,RM C 3 28 Interrupt Support,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-3-1-20-21-protected-procedure-handlers}@anchor{268}
@section RM C.3.1(20-21): Protected Procedure Handlers


@quotation

“Whenever possible, the implementation should allow interrupt handlers to
be called directly by the hardware.”
@end quotation

Followed on any target where the underlying operating system permits
such direct calls.

@quotation

“Whenever practical, violations of any
implementation-defined restrictions should be detected before run time.”
@end quotation

Followed.  Compile time warnings are given when possible.

@geindex Package `@w{`}Interrupts`@w{`}

@geindex Interrupts

@node RM C 3 2 25 Package Interrupts,RM C 4 14 Pre-elaboration Requirements,RM C 3 1 20-21 Protected Procedure Handlers,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-3-2-25-package-interrupts}@anchor{269}
@section RM C.3.2(25): Package @code{Interrupts}


@quotation

“If implementation-defined forms of interrupt handler procedures are
supported, such as protected procedures with parameters, then for each
such form of a handler, a type analogous to @code{Parameterless_Handler}
should be specified in a child package of @code{Interrupts}, with the
same operations as in the predefined package Interrupts.”
@end quotation

Followed.

@geindex Pre-elaboration requirements

@node RM C 4 14 Pre-elaboration Requirements,RM C 5 8 Pragma Discard_Names,RM C 3 2 25 Package Interrupts,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-4-14-pre-elaboration-requirements}@anchor{26a}
@section RM C.4(14): Pre-elaboration Requirements


@quotation

“It is recommended that pre-elaborated packages be implemented in such a
way that there should be little or no code executed at run time for the
elaboration of entities not already covered by the Implementation
Requirements.”
@end quotation

Followed.  Executable code is generated in some cases, e.g., loops
to initialize large arrays.

@node RM C 5 8 Pragma Discard_Names,RM C 7 2 30 The Package Task_Attributes,RM C 4 14 Pre-elaboration Requirements,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-5-8-pragma-discard-names}@anchor{26b}
@section RM C.5(8): Pragma @code{Discard_Names}


@quotation

“If the pragma applies to an entity, then the implementation should
reduce the amount of storage used for storing names associated with that
entity.”
@end quotation

Followed.

@geindex Package Task_Attributes

@geindex Task_Attributes

@node RM C 7 2 30 The Package Task_Attributes,RM D 3 17 Locking Policies,RM C 5 8 Pragma Discard_Names,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-7-2-30-the-package-task-attributes}@anchor{26c}
@section RM C.7.2(30): The Package Task_Attributes


@quotation

“Some implementations are targeted to domains in which memory use at run
time must be completely deterministic.  For such implementations, it is
recommended that the storage for task attributes will be pre-allocated
statically and not from the heap.  This can be accomplished by either
placing restrictions on the number and the size of the task’s
attributes, or by using the pre-allocated storage for the first @code{N}
attribute objects, and the heap for the others.  In the latter case,
@code{N} should be documented.”
@end quotation

Not followed.  This implementation is not targeted to such a domain.

@geindex Locking Policies

@node RM D 3 17 Locking Policies,RM D 4 16 Entry Queuing Policies,RM C 7 2 30 The Package Task_Attributes,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-3-17-locking-policies}@anchor{26d}
@section RM D.3(17): Locking Policies


@quotation

“The implementation should use names that end with @code{_Locking} for
locking policies defined by the implementation.”
@end quotation

Followed.  Two implementation-defined locking policies are defined,
whose names (@code{Inheritance_Locking} and
@code{Concurrent_Readers_Locking}) follow this suggestion.

@geindex Entry queuing policies

@node RM D 4 16 Entry Queuing Policies,RM D 6 9-10 Preemptive Abort,RM D 3 17 Locking Policies,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-4-16-entry-queuing-policies}@anchor{26e}
@section RM D.4(16): Entry Queuing Policies


@quotation

“Names that end with @code{_Queuing} should be used
for all implementation-defined queuing policies.”
@end quotation

Followed.  No such implementation-defined queuing policies exist.

@geindex Preemptive abort

@node RM D 6 9-10 Preemptive Abort,RM D 7 21 Tasking Restrictions,RM D 4 16 Entry Queuing Policies,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-6-9-10-preemptive-abort}@anchor{26f}
@section RM D.6(9-10): Preemptive Abort


@quotation

“Even though the `abort_statement' is included in the list of
potentially blocking operations (see 9.5.1), it is recommended that this
statement be implemented in a way that never requires the task executing
the `abort_statement' to block.”
@end quotation

Followed.

@quotation

“On a multi-processor, the delay associated with aborting a task on
another processor should be bounded; the implementation should use
periodic polling, if necessary, to achieve this.”
@end quotation

Followed.

@geindex Tasking restrictions

@node RM D 7 21 Tasking Restrictions,RM D 8 47-49 Monotonic Time,RM D 6 9-10 Preemptive Abort,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-7-21-tasking-restrictions}@anchor{270}
@section RM D.7(21): Tasking Restrictions


@quotation

“When feasible, the implementation should take advantage of the specified
restrictions to produce a more efficient implementation.”
@end quotation

GNAT currently takes advantage of these restrictions by providing an optimized
run time when the Ravenscar profile and the GNAT restricted run time set
of restrictions are specified.  See pragma @code{Profile (Ravenscar)} and
pragma @code{Profile (Restricted)} for more details.

@geindex Time
@geindex monotonic

@node RM D 8 47-49 Monotonic Time,RM E 5 28-29 Partition Communication Subsystem,RM D 7 21 Tasking Restrictions,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-8-47-49-monotonic-time}@anchor{271}
@section RM D.8(47-49): Monotonic Time


@quotation

“When appropriate, implementations should provide configuration
mechanisms to change the value of @code{Tick}.”
@end quotation

Such configuration mechanisms are not appropriate to this implementation
and are thus not supported.

@quotation

“It is recommended that @code{Calendar.Clock} and @code{Real_Time.Clock}
be implemented as transformations of the same time base.”
@end quotation

Followed.

@quotation

“It is recommended that the best time base which exists in
the underlying system be available to the application through
@code{Clock}.  @cite{Best} may mean highest accuracy or largest range.”
@end quotation

Followed.

@geindex Partition communication subsystem

@geindex PCS

@node RM E 5 28-29 Partition Communication Subsystem,RM F 7 COBOL Support,RM D 8 47-49 Monotonic Time,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-e-5-28-29-partition-communication-subsystem}@anchor{272}
@section RM E.5(28-29): Partition Communication Subsystem


@quotation

“Whenever possible, the PCS on the called partition should allow for
multiple tasks to call the RPC-receiver with different messages and
should allow them to block until the corresponding subprogram body
returns.”
@end quotation

A separately supplied PCS that can be used with GNAT when combined with the PolyORB product (NB! See the note in @ref{273,,PolyORB} regarding the lifetime
of this product).

@quotation

“The @code{Write} operation on a stream of type @code{Params_Stream_Type}
should raise @code{Storage_Error} if it runs out of space trying to
write the @code{Item} into the stream.”
@end quotation

@geindex COBOL support

@node RM F 7 COBOL Support,RM F 1 2 Decimal Radix Support,RM E 5 28-29 Partition Communication Subsystem,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-f-7-cobol-support}@anchor{274}
@section RM F(7): COBOL Support


@quotation

“If COBOL (respectively, C) is widely supported in the target
environment, implementations supporting the Information Systems Annex
should provide the child package @code{Interfaces.COBOL} (respectively,
@code{Interfaces.C}) specified in Annex B and should support a
@code{convention_identifier} of COBOL (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language.”
@end quotation

Followed.

@geindex Decimal radix support

@node RM F 1 2 Decimal Radix Support,RM G Numerics,RM F 7 COBOL Support,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-f-1-2-decimal-radix-support}@anchor{275}
@section RM F.1(2): Decimal Radix Support


@quotation

“Packed decimal should be used as the internal representation for objects
of subtype @code{S} when @code{S}’Machine_Radix = 10.”
@end quotation

Not followed.  GNAT ignores @code{S}’Machine_Radix and always uses binary
representations.

@geindex Numerics

@node RM G Numerics,RM G 1 1 56-58 Complex Types,RM F 1 2 Decimal Radix Support,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-numerics}@anchor{276}
@section RM G: Numerics


@quotation

“If Fortran (respectively, C) is widely supported in the target
environment, implementations supporting the Numerics Annex
should provide the child package @code{Interfaces.Fortran} (respectively,
@code{Interfaces.C}) specified in Annex B and should support a
@code{convention_identifier} of Fortran (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language.”
@end quotation

Followed.

@geindex Complex types

@node RM G 1 1 56-58 Complex Types,RM G 1 2 49 Complex Elementary Functions,RM G Numerics,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-1-1-56-58-complex-types}@anchor{277}
@section RM G.1.1(56-58): Complex Types


@quotation

“Because the usual mathematical meaning of multiplication of a complex
operand and a real operand is that of the scaling of both components of
the former by the latter, an implementation should not perform this
operation by first promoting the real operand to complex type and then
performing a full complex multiplication.  In systems that, in the
future, support an Ada binding to IEC 559:1989, the latter technique
will not generate the required result when one of the components of the
complex operand is infinite.  (Explicit multiplication of the infinite
component by the zero component obtained during promotion yields a NaN
that propagates into the final result.) Analogous advice applies in the
case of multiplication of a complex operand and a pure-imaginary
operand, and in the case of division of a complex operand by a real or
pure-imaginary operand.”
@end quotation

Not followed.

@quotation

“Similarly, because the usual mathematical meaning of addition of a
complex operand and a real operand is that the imaginary operand remains
unchanged, an implementation should not perform this operation by first
promoting the real operand to complex type and then performing a full
complex addition.  In implementations in which the @code{Signed_Zeros}
attribute of the component type is @code{True} (and which therefore
conform to IEC 559:1989 in regard to the handling of the sign of zero in
predefined arithmetic operations), the latter technique will not
generate the required result when the imaginary component of the complex
operand is a negatively signed zero.  (Explicit addition of the negative
zero to the zero obtained during promotion yields a positive zero.)
Analogous advice applies in the case of addition of a complex operand
and a pure-imaginary operand, and in the case of subtraction of a
complex operand and a real or pure-imaginary operand.”
@end quotation

Not followed.

@quotation

“Implementations in which @code{Real'Signed_Zeros} is @code{True} should
attempt to provide a rational treatment of the signs of zero results and
result components.  As one example, the result of the @code{Argument}
function should have the sign of the imaginary component of the
parameter @code{X} when the point represented by that parameter lies on
the positive real axis; as another, the sign of the imaginary component
of the @code{Compose_From_Polar} function should be the same as
(respectively, the opposite of) that of the @code{Argument} parameter when that
parameter has a value of zero and the @code{Modulus} parameter has a
nonnegative (respectively, negative) value.”
@end quotation

Followed.

@geindex Complex elementary functions

@node RM G 1 2 49 Complex Elementary Functions,RM G 2 4 19 Accuracy Requirements,RM G 1 1 56-58 Complex Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-1-2-49-complex-elementary-functions}@anchor{278}
@section RM G.1.2(49): Complex Elementary Functions


@quotation

“Implementations in which @code{Complex_Types.Real'Signed_Zeros} is
@code{True} should attempt to provide a rational treatment of the signs
of zero results and result components.  For example, many of the complex
elementary functions have components that are odd functions of one of
the parameter components; in these cases, the result component should
have the sign of the parameter component at the origin.  Other complex
elementary functions have zero components whose sign is opposite that of
a parameter component at the origin, or is always positive or always
negative.”
@end quotation

Followed.

@geindex Accuracy requirements

@node RM G 2 4 19 Accuracy Requirements,RM G 2 6 15 Complex Arithmetic Accuracy,RM G 1 2 49 Complex Elementary Functions,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-2-4-19-accuracy-requirements}@anchor{279}
@section RM G.2.4(19): Accuracy Requirements


@quotation

“The versions of the forward trigonometric functions without a
@code{Cycle} parameter should not be implemented by calling the
corresponding version with a @code{Cycle} parameter of
@code{2.0*Numerics.Pi}, since this will not provide the required
accuracy in some portions of the domain.  For the same reason, the
version of @code{Log} without a @code{Base} parameter should not be
implemented by calling the corresponding version with a @code{Base}
parameter of @code{Numerics.e}.”
@end quotation

Followed.

@geindex Complex arithmetic accuracy

@geindex Accuracy
@geindex complex arithmetic

@node RM G 2 6 15 Complex Arithmetic Accuracy,RM H 6 15/2 Pragma Partition_Elaboration_Policy,RM G 2 4 19 Accuracy Requirements,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-2-6-15-complex-arithmetic-accuracy}@anchor{27a}
@section RM G.2.6(15): Complex Arithmetic Accuracy


@quotation

“The version of the @code{Compose_From_Polar} function without a
@code{Cycle} parameter should not be implemented by calling the
corresponding version with a @code{Cycle} parameter of
@code{2.0*Numerics.Pi}, since this will not provide the required
accuracy in some portions of the domain.”
@end quotation

Followed.

@geindex Sequential elaboration policy

@node RM H 6 15/2 Pragma Partition_Elaboration_Policy,,RM G 2 6 15 Complex Arithmetic Accuracy,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-h-6-15-2-pragma-partition-elaboration-policy}@anchor{27b}
@section RM H.6(15/2): Pragma Partition_Elaboration_Policy


@quotation

“If the partition elaboration policy is @code{Sequential} and the
Environment task becomes permanently blocked during elaboration then the
partition is deadlocked and it is recommended that the partition be
immediately terminated.”
@end quotation

Not followed.

@node GNAT Implementation Mode the -gnatg switch,Implementation Defined Characteristics,Implementation Advice,Top
@anchor{gnat_rm/gnat_implementation_mode doc}@anchor{27c}@anchor{gnat_rm/gnat_implementation_mode gnat-implementation-mode}@anchor{b}@anchor{gnat_rm/gnat_implementation_mode gnat-implementation-mode-the-gnatg-switch}@anchor{27d}
@chapter GNAT Implementation Mode (the @code{-gnatg} switch)


@code{-gnatg} enables `GNAT implementation mode', the internal compilation mode
used when building GNAT itself and its runtime libraries. It is not intended
for user application programs.

It implies @code{-gnatw.ge} (extended warning set, treated as errors) and
@code{-gnatyg} (GNAT style checks).

@menu
* Language & Semantic Changes:: 

@end menu

@node Language & Semantic Changes,,,GNAT Implementation Mode the -gnatg switch
@anchor{gnat_rm/gnat_implementation_mode language-semantic-changes}@anchor{27e}
@section Language & Semantic Changes


@menu
* Ada Version and Identifier Rules:: 
* Limited Types:: 
* Preelaboration and Categorization:: 
* Overflow:: 
* Miscellaneous:: 
* ALI Files and Recompilation:: 

@end menu

@node Ada Version and Identifier Rules,Limited Types,,Language & Semantic Changes
@anchor{gnat_rm/gnat_implementation_mode ada-version-and-identifier-rules}@anchor{27f}
@subsection Ada Version and Identifier Rules



@itemize *

@item 
`Ada version is fixed at Ada 2012', regardless of other switches.

@item 
`Identifier character set is restricted to 7-bit ASCII';
extended characters are disallowed.

@item 
`Identifiers are allowed to start with underscore' to avoid conflicts
between global symbols in the runtime and those defined in the user code.
@end itemize

@node Limited Types,Preelaboration and Categorization,Ada Version and Identifier Rules,Language & Semantic Changes
@anchor{gnat_rm/gnat_implementation_mode limited-types}@anchor{280}
@subsection Limited Types



@itemize *

@item 
Copying limited objects is `permitted' (normally illegal).

@item 
Initializing limited types emits a `warning' rather than an error.

@item 
Returning objects of limited types is `permitted'.
@end itemize

@node Preelaboration and Categorization,Overflow,Limited Types,Language & Semantic Changes
@anchor{gnat_rm/gnat_implementation_mode preelaboration-and-categorization}@anchor{281}
@subsection Preelaboration and Categorization



@itemize *

@item 
Categorization errors (pure/preelaborated unit violations) are downgraded
to `warnings'.

@item 
Statements in preelaborated units emit a `warning' rather than an error.

@item 
Private objects and non-static constants/calls in preelaborated units emit
`warnings' rather than errors.

@item 
Calls to non-preelaborable subprograms in preelaborated packages are
demoted to `suppressible warnings' (RM 10.2.1), allowing the runtime to
include elaboration code that would otherwise be forbidden.

@item 
Assignment statements to library-level variables are `excluded' from
access-before-elaboration (ABE) analysis, on the assumption that the
predefined runtime manages its own elaboration order correctly.
@end itemize

@node Overflow,Miscellaneous,Preelaboration and Categorization,Language & Semantic Changes
@anchor{gnat_rm/gnat_implementation_mode overflow}@anchor{282}
@subsection Overflow


Overflow checks are `suppressed' but the overflow checking mode is set to
`Strict' (for assertion purposes).

@node Miscellaneous,ALI Files and Recompilation,Overflow,Language & Semantic Changes
@anchor{gnat_rm/gnat_implementation_mode miscellaneous}@anchor{283}
@subsection Miscellaneous



@itemize *

@item 
@code{Scalar_Storage_Order} attribute applies to `generic types' (normally
it does not).

@item 
Only the @code{--!} special comment sequence is recognized; all others are
disallowed.

@item 
The @code{Priority} aspect/attribute on protected types accepts the
unconstrained type @code{Any_Priority} rather than the subtype
@code{System.Priority} (a subrange of @code{Integer}), giving GNAT’s own
protected objects access to the full implementation-defined priority range.

@item 
@code{pragma Extend_System} is `ignored' (prevents circularities in System).
Exception: predefined renaming units (@code{Text_IO}, @code{Direct_IO}, etc.) are
compiled without GNAT implementation mode so that @code{Extend_System} still
works for them.

@item 
Importing a non-pure external function (@code{pragma Import}) into a @code{Pure}
unit does `not' produce a purity warning, on the assumption that the
runtime knows the purity properties of the C functions it imports.

@item 
@code{pragma Style_Checks ("All_Checks")} activates the stricter @code{-gnatyg}
rule set rather than the default @code{-gnaty} set.
@end itemize

@node ALI Files and Recompilation,,Miscellaneous,Language & Semantic Changes
@anchor{gnat_rm/gnat_implementation_mode ali-files-and-recompilation}@anchor{284}
@subsection ALI Files and Recompilation



@itemize *

@item 
ALI files are always generated for predefined generic packages.

@item 
Compiling a unit whose top-level name is @code{Ada}, @code{Interfaces}, or
@code{System} as the main unit is permitted; the RM reserves these as
hierarchical library-unit name prefixes, but the diagnostic is suppressed
to let GNAT compile its own predefined library (@code{Ada.Text_IO},
@code{System.Storage_Elements}, etc.) as main units.
@end itemize

@node Implementation Defined Characteristics,Intrinsic Subprograms,GNAT Implementation Mode the -gnatg switch,Top
@anchor{gnat_rm/implementation_defined_characteristics doc}@anchor{285}@anchor{gnat_rm/implementation_defined_characteristics id1}@anchor{286}@anchor{gnat_rm/implementation_defined_characteristics implementation-defined-characteristics}@anchor{c}
@chapter Implementation Defined Characteristics


In addition to the implementation dependent pragmas and attributes, and the
implementation advice, there are a number of other Ada features that are
potentially implementation dependent and are designated as
implementation-defined. These are mentioned throughout the Ada Reference
Manual, and are summarized in Annex M.

A requirement for conforming Ada compilers is that they provide
documentation describing how the implementation deals with each of these
issues.  In this chapter you will find each point in Annex M listed,
followed by a description of how GNAT handles the implementation dependence.

You can use this chapter as a guide to minimizing implementation
dependent features in your programs if portability to other compilers
and other operating systems is an important consideration.  The numbers
in each entry below correspond to the paragraph numbers in the Ada
Reference Manual.


@itemize *

@item 
“Whether or not each recommendation given in Implementation
Advice is followed.  See 1.1.2(37).”
@end itemize

See @ref{a,,Implementation Advice}.


@itemize *

@item 
“Capacity limitations of the implementation.  See 1.1.3(3).”
@end itemize

The complexity of programs that can be processed is limited only by the
total amount of available virtual memory, and disk space for the
generated object files.


@itemize *

@item 
“Variations from the standard that are impractical to avoid
given the implementation’s execution environment.  See 1.1.3(6).”
@end itemize

There are no variations from the standard.


@itemize *

@item 
“Which code_statements cause external
interactions.  See 1.1.3(10).”
@end itemize

Any `code_statement' can potentially cause external interactions.


@itemize *

@item 
“The coded representation for the text of an Ada
program.  See 2.1(4).”
@end itemize

See separate section on source representation.


@itemize *

@item 
“The semantics of an Ada program whose text is not in
Normalization Form C.  See 2.1(4).”
@end itemize

See separate section on source representation.


@itemize *

@item 
“The representation for an end of line.  See 2.2(2).”
@end itemize

See separate section on source representation.


@itemize *

@item 
“Maximum supported line length and lexical element
length.  See 2.2(15).”
@end itemize

The maximum line length is 255 characters and the maximum length of
a lexical element is also 255 characters. This is the default setting
if not overridden by the use of compiler switch `-gnaty' (which
sets the maximum to 79) or `-gnatyMnn' which allows the maximum
line length to be specified to be any value up to 32767. The maximum
length of a lexical element is the same as the maximum line length.


@itemize *

@item 
“Implementation defined pragmas.  See 2.8(14).”
@end itemize

See @ref{7,,Implementation Defined Pragmas}.


@itemize *

@item 
“Effect of pragma @code{Optimize}.  See 2.8(27).”
@end itemize

Pragma @code{Optimize}, if given with a @code{Time} or @code{Space}
parameter, checks that the optimization flag is set, and aborts if it is
not.


@itemize *

@item 
“The message string associated with the Assertion_Error exception raised
by the failure of a predicate check if there is no applicable
Predicate_Failure aspect.  See 3.2.4(31).”
@end itemize

In the case of a Dynamic_Predicate aspect, the string is
“Dynamic_Predicate failed at <source position>”, where
“<source position>” might be something like “foo.adb:123”.
The Static_Predicate case is handled analogously.


@itemize *

@item 
“The predefined integer types declared in
@code{Standard}.  See 3.5.4(25).”
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Type

@tab

Representation

@item

`Short_Short_Integer'

@tab

8-bit signed

@item

`Short_Integer'

@tab

16-bit signed

@item

`Integer'

@tab

32-bit signed

@item

`Long_Integer'

@tab

64-bit signed (on most 64-bit targets,
depending on the C definition of long)
32-bit signed (on all other targets)

@item

`Long_Long_Integer'

@tab

64-bit signed

@item

`Long_Long_Long_Integer'

@tab

128-bit signed (on 64-bit targets)
64-bit signed (on 32-bit targets)

@end multitable



@itemize *

@item 
“Any nonstandard integer types and the operators defined
for them.  See 3.5.4(26).”
@end itemize

There are no nonstandard integer types.


@itemize *

@item 
“Any nonstandard real types and the operators defined for
them.  See 3.5.6(8).”
@end itemize

There are no nonstandard real types.


@itemize *

@item 
“What combinations of requested decimal precision and range
are supported for floating point types.  See 3.5.7(7).”
@end itemize

The precision and range are defined by the IEEE Standard for Floating-Point
Arithmetic (IEEE 754-2019).


@itemize *

@item 
“The predefined floating point types declared in
@code{Standard}.  See 3.5.7(16).”
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Type

@tab

Representation

@item

`Short_Float'

@tab

IEEE Binary32 (Single)

@item

`Float'

@tab

IEEE Binary32 (Single)

@item

`Long_Float'

@tab

IEEE Binary64 (Double)

@item

`Long_Long_Float'

@tab

IEEE Binary64 (Double) on non-x86 architectures
IEEE 80-bit Extended on x86 architecture

@end multitable


The default rounding mode specified by the IEEE 754 Standard is assumed both
for static and dynamic computations (that is, round to nearest, ties to even).
The input routines yield correctly rounded values for Short_Float, Float, and
Long_Float at least. The output routines can compute up to twice as many exact
digits as the value of @code{T'Digits} for any type, for example 30 digits for
Long_Float; if more digits are requested, zeros are printed.


@itemize *

@item 
“The small of an ordinary fixed point type.  See 3.5.9(8).”
@end itemize

The small is the largest power of two that does not exceed the delta.


@itemize *

@item 
“What combinations of small, range, and digits are
supported for fixed point types.  See 3.5.9(10).”
@end itemize

For an ordinary fixed point type, on 32-bit platforms, the small must lie in
2.0**(-80) .. 2.0**80 and the range in -9.0E+36 .. 9.0E+36; any combination
is permitted that does not result in a mantissa larger than 63 bits.

On 64-bit platforms, the small must lie in 2.0**(-127) .. 2.0**127 and the
range in -1.0E+76 .. 1.0E+76; any combination is permitted that does not
result in a mantissa larger than 63 bits, and any combination is permitted
that results in a mantissa between 64 and 127 bits if the small is the
ratio of two integers that lie in 1 .. 2.0**127.

If the small is the ratio of two integers with 64-bit magnitude on 32-bit
platforms and 128-bit magnitude on 64-bit platforms, which is the case if
no @code{small} clause is provided, then the operations of the fixed point
type are entirely implemented by means of integer instructions.  In the
other cases, some operations, in particular input and output, may be
implemented by means of floating-point instructions and may be affected
by accuracy issues on architectures other than x86.

For a decimal fixed point type, on 32-bit platforms, the small must lie in
1.0E-18 .. 1.0E+18 and the digits in 1 .. 18.  On 64-bit platforms, the
small must lie in 1.0E-38 .. 1.0E+38 and the digits in 1 .. 38.


@itemize *

@item 
“The result of @code{Tags.Expanded_Name} for types declared
within an unnamed `block_statement'.  See 3.9(10).”
@end itemize

Block numbers of the form @code{B@var{nnn}}, where `nnn' is a
decimal integer are allocated.


@itemize *

@item 
“The sequence of characters of the value returned by Tags.Expanded_Name
(respectively, Tags.Wide_Expanded_Name) when some of the graphic
characters of Tags.Wide_Wide_Expanded_Name are not defined in Character
(respectively, Wide_Character).  See 3.9(10.1).”
@end itemize

This is handled in the same way as the implementation-defined behavior
referenced in A.4.12(34).


@itemize *

@item 
“Implementation-defined attributes.  See 4.1.4(12).”
@end itemize

See @ref{8,,Implementation Defined Attributes}.


@itemize *

@item 
“The value of the parameter to Empty for some container aggregates.
See 4.3.5(40).”
@end itemize

As per the suggestion given in the Annotated Ada RM, the default value
of the formal parameter is used if one exists and zero is used otherwise.


@itemize *

@item 
“The maximum number of chunks for a parallel reduction expression without
a chunk_specification.  See 4.5.10(21).”
@end itemize

Feature unimplemented.


@itemize *

@item 
“Rounding of real static expressions which are exactly half-way between
two machine numbers.  See 4.9(38).”
@end itemize

Round to even is used in all such cases.


@itemize *

@item 
“The maximum number of chunks for a parallel generalized iterator without
a chunk_specification.  See 5.5.2(10).”
@end itemize

Feature unimplemented.


@itemize *

@item 
“The number of chunks for an array component iterator.  See 5.5.2(11).”
@end itemize

Feature unimplemented.


@itemize *

@item 
“Any extensions of the Global aspect.  See 6.1.2(43).”
@end itemize

Feature unimplemented.


@itemize *

@item 
“The circumstances the implementation passes in the null value for a view
conversion of an access type used as an out parameter.  See 6.4.1(19).”
@end itemize

Difficult to characterize.


@itemize *

@item 
“Any extensions of the Default_Initial_Condition aspect.  See 7.3.3(11).”
@end itemize

SPARK allows specifying `null' as the Default_Initial_Condition
aspect of a type. See the SPARK reference manual for further details.


@itemize *

@item 
“Any implementation-defined time types.  See 9.6(6).”
@end itemize

There are no implementation-defined time types.


@itemize *

@item 
“The time base associated with relative delays.  See 9.6(20).”
@end itemize

See 9.6(20).  The time base used is that provided by the C library
function @code{gettimeofday}.


@itemize *

@item 
“The time base of the type @code{Calendar.Time}.  See 9.6(23).”
@end itemize

The time base used is that provided by the C library function
@code{gettimeofday}.


@itemize *

@item 
“The time zone used for package @code{Calendar}
operations.  See 9.6(24).”
@end itemize

The time zone used by package @code{Calendar} is the current system time zone
setting for local time, as accessed by the C library function
@code{localtime}.


@itemize *

@item 
“Any limit on `delay_until_statements' of
`select_statements'.  See 9.6(29).”
@end itemize

There are no such limits.


@itemize *

@item 
“The result of Calendar.Formatting.Image if its argument represents more
than 100 hours.  See 9.6.1(86).”
@end itemize

Calendar.Time_Error is raised.


@itemize *

@item 
“Implementation-defined conflict check policies.  See 9.10.1(5).”
@end itemize

There are no implementation-defined conflict check policies.


@itemize *

@item 
“The representation for a compilation.  See 10.1(2).”
@end itemize

A compilation is represented by a sequence of files presented to the
compiler in a single invocation of the `gcc' command.


@itemize *

@item 
“Any restrictions on compilations that contain multiple
compilation_units.  See 10.1(4).”
@end itemize

No single file can contain more than one compilation unit, but any
sequence of files can be presented to the compiler as a single
compilation.


@itemize *

@item 
“The mechanisms for creating an environment and for adding
and replacing compilation units.  See 10.1.4(3).”
@end itemize

See separate section on compilation model.


@itemize *

@item 
“The manner of explicitly assigning library units to a
partition.  See 10.2(2).”
@end itemize

If a unit contains an Ada main program, then the Ada units for the partition
are determined by recursive application of the rules in the Ada Reference
Manual section 10.2(2-6).  In other words, the Ada units will be those that
are needed by the main program, and then this definition of need is applied
recursively to those units, and the partition contains the transitive
closure determined by this relationship.  In short, all the necessary units
are included, with no need to explicitly specify the list.  If additional
units are required, e.g., by foreign language units, then all units must be
mentioned in the context clause of one of the needed Ada units.

If the partition contains no main program, or if the main program is in
a language other than Ada, then GNAT
provides the binder options `-z' and `-n' respectively, and in
this case a list of units can be explicitly supplied to the binder for
inclusion in the partition (all units needed by these units will also
be included automatically).  For full details on the use of these
options, refer to `GNAT Make Program gnatmake' in the
@cite{GNAT User’s Guide}.


@itemize *

@item 
“The implementation-defined means, if any, of specifying which compilation
units are needed by a given compilation unit.  See 10.2(2).”
@end itemize

The units needed by a given compilation unit are as defined in
the Ada Reference Manual section 10.2(2-6).  There are no
implementation-defined pragmas or other implementation-defined
means for specifying needed units.


@itemize *

@item 
“The manner of designating the main subprogram of a
partition.  See 10.2(7).”
@end itemize

The main program is designated by providing the name of the
corresponding @code{ALI} file as the input parameter to the binder.


@itemize *

@item 
“The order of elaboration of `library_items'.  See 10.2(18).”
@end itemize

The first constraint on ordering is that it meets the requirements of
Chapter 10 of the Ada Reference Manual.  This still leaves some
implementation-dependent choices, which are resolved by analyzing
the elaboration code of each unit and identifying implicit
elaboration-order dependencies.


@itemize *

@item 
“Parameter passing and function return for the main
subprogram.  See 10.2(21).”
@end itemize

The main program has no parameters.  It may be a procedure, or a function
returning an integer type.  In the latter case, the returned integer
value is the return code of the program (overriding any value that
may have been set by a call to @code{Ada.Command_Line.Set_Exit_Status}).


@itemize *

@item 
“The mechanisms for building and running partitions.  See 10.2(24).”
@end itemize

GNAT itself supports programs with only a single partition. The PolyORB product (which also includes an implementation of the PCS) provides a completely flexible method for building and running programs consisting of multiple partitions. `NB!' See the note in @ref{273,,PolyORB} regarding the lifetime
of this product.


@itemize *

@item 
“The details of program execution, including program
termination.  See 10.2(25).”
@end itemize

See separate section on compilation model.


@itemize *

@item 
“The semantics of any non-active partitions supported by the
implementation.  See 10.2(28).”
@end itemize

Passive partitions are supported on targets where shared memory is
provided by the operating system. `NB!' See the note in @ref{273,,PolyORB} regarding the lifetime of this product.


@itemize *

@item 
“The information returned by @code{Exception_Message}.  See 11.4.1(10).”
@end itemize

Exception message returns the null string unless a specific message has
been passed by the program.


@itemize *

@item 
“The result of @code{Exceptions.Exception_Name} for types
declared within an unnamed `block_statement'.  See 11.4.1(12).”
@end itemize

Blocks have implementation defined names of the form @code{B@var{nnn}}
where `nnn' is an integer.


@itemize *

@item 
“The information returned by
@code{Exception_Information}.  See 11.4.1(13).”
@end itemize

@code{Exception_Information} returns a string in the following format:

@example
*Exception_Name:* nnnnn
*Message:* mmmmm
*PID:* ppp
*Load address:* 0xhhhh
*Call stack traceback locations:*
0xhhhh 0xhhhh 0xhhhh ... 0xhhh
@end example

where

@quotation


@itemize *

@item 
@code{nnnn} is the fully qualified name of the exception in all upper
case letters. This line is always present.

@item 
@code{mmmm} is the message (this line present only if message is non-null)

@item 
@code{ppp} is the Process Id value as a decimal integer (this line is
present only if the Process Id is nonzero). Currently we are
not making use of this field.

@item 
The Load address line, the Call stack traceback locations line and the
following values are present only if at least one traceback location was
recorded. The Load address indicates the address at which the main executable
was loaded; this line may not be present if operating system hasn’t relocated
the main executable. The values are given in C style format, with lower case
letters for a-f, and only as many digits present as are necessary.
The line terminator sequence at the end of each line, including
the last line is a single @code{LF} character (@code{16#0A#}).
@end itemize
@end quotation


@itemize *

@item 
“The sequence of characters of the value returned by
Exceptions.Exception_Name (respectively, Exceptions.Wide_Exception_Name)
when some of the graphic characters of Exceptions.Wide_Wide_Exception_Name
are not defined in Character (respectively, Wide_Character).
See 11.4.1(12.1).”
@end itemize

This is handled in the same way as the implementation-defined behavior
referenced in A.4.12(34).


@itemize *

@item 
“The information returned by Exception_Information.  See 11.4.1(13).”
@end itemize

The exception name and the source location at which the exception was
raised are included.


@itemize *

@item 
“Implementation-defined policy_identifiers and assertion_aspect_marks
allowed in a pragma Assertion_Policy.  See 11.4.2(9).”
@end itemize

Implementation-defined assertion_aspect_marks include Assert_And_Cut,
Assume, Contract_Cases, Debug, Ghost, Initial_Condition, Loop_Invariant,
Loop_Variant, Postcondition, Precondition, Predicate, Refined_Post,
Statement_Assertions, and Subprogram_Variant. Implementation-defined
policy_identifiers include Disable and Suppressible.


@itemize *

@item 
“The default assertion policy.  See 11.4.2(10).”
@end itemize

The default assertion policy is Ignore, although this can be overridden
via compiler switches such as “-gnata”.


@itemize *

@item 
“Implementation-defined check names.  See 11.5(27).”
@end itemize

The implementation-defined check names include Alignment_Check,
Container_Checks, Duplicated_Tag_Check, Predicate_Check,
Raise_Check, Tampering_Check, and Validity_Check. In addition, a
user program can add implementation-defined check names by means
of the pragma Check_Name. See the description of pragma
@code{Suppress} for details.


@itemize *

@item 
“Existence and meaning of second parameter of pragma Unsuppress.
See 11.5(27.1).”
@end itemize

The legality rules for and semantics of the second parameter of pragma
Unsuppress match those for the second argument of pragma Suppress.


@itemize *

@item 
“The cases that cause conflicts between the representation of the
ancestors of a type_declaration.  See 13.1(13.1).”
@end itemize

No such cases exist.


@itemize *

@item 
“The interpretation of each representation aspect.  See 13.1(20).”
@end itemize

See separate section on data representations.


@itemize *

@item 
“Any restrictions placed upon the specification of representation aspects.
See 13.1(20).”
@end itemize

See separate section on data representations.


@itemize *

@item 
“Implementation-defined aspects, including the syntax for specifying
such aspects and the legality rules for such aspects.  See 13.1.1(38).”
@end itemize

See @ref{132,,Implementation Defined Aspects}.


@itemize *

@item 
“The set of machine scalars.  See 13.3(8.1).”
@end itemize

See separate section on data representations.


@itemize *

@item 
“The meaning of @code{Size} for indefinite subtypes.  See 13.3(48).”
@end itemize

The Size attribute of an indefinite subtype is not less than the Size
attribute of any object of that type.


@itemize *

@item 
“The meaning of Object_Size for indefinite subtypes.  See 13.3(58).”
@end itemize

The Object_Size attribute of an indefinite subtype is not less than the
Object_Size attribute of any object of that type.


@itemize *

@item 
“The default external representation for a type tag.  See 13.3(75).”
@end itemize

The default external representation for a type tag is the fully expanded
name of the type in upper case letters.


@itemize *

@item 
“What determines whether a compilation unit is the same in
two different partitions.  See 13.3(76).”
@end itemize

A compilation unit is the same in two different partitions if and only
if it derives from the same source file.


@itemize *

@item 
“Implementation-defined components.  See 13.5.1(15).”
@end itemize

The only implementation defined component is the tag for a tagged type,
which contains a pointer to the dispatching table.


@itemize *

@item 
“If @code{Word_Size} = @code{Storage_Unit}, the default bit
ordering.  See 13.5.3(5).”
@end itemize

@code{Word_Size} does not equal @code{Storage_Unit} in this implementation.


@itemize *

@item 
“The contents of the visible part of package @code{System}.  See 13.7(2).”
@end itemize

See the definition of package System in @code{system.ads}.
Note that two declarations are added to package System.

@example
Max_Priority           : constant Positive := Priority'Last;
Max_Interrupt_Priority : constant Positive := Interrupt_Priority'Last;
@end example


@itemize *

@item 
“The range of Storage_Elements.Storage_Offset, the modulus of
Storage_Elements.Storage_Element, and the declaration of
Storage_Elements.Integer_Address.  See 13.7.1(11).”
@end itemize

See the definition of package System.Storage_Elements in @code{s-stoele.ads}.


@itemize *

@item 
“The contents of the visible part of package @code{System.Machine_Code},
and the meaning of `code_statements'.  See 13.8(7).”
@end itemize

See the definition and documentation in file @code{s-maccod.ads}.


@itemize *

@item 
“The result of unchecked conversion for instances with scalar result
types whose result is not defined by the language.  See 13.9(11).”
@end itemize

Unchecked conversion between types of the same size
results in an uninterpreted transmission of the bits from one type
to the other.  If the types are of unequal sizes, then in the case of
discrete types, a shorter source is first zero or sign extended as
necessary, and a shorter target is simply truncated on the left.
For all non-discrete types, the source is first copied if necessary
to ensure that the alignment requirements of the target are met, then
a pointer is constructed to the source value, and the result is obtained
by dereferencing this pointer after converting it to be a pointer to the
target type. Unchecked conversions where the target subtype is an
unconstrained array are not permitted. If the target alignment is
greater than the source alignment, then a copy of the result is
made with appropriate alignment


@itemize *

@item 
“The result of unchecked conversion for instances with nonscalar result
types whose result is not defined by the language.  See 13.9(11).”
@end itemize

See preceding definition for the scalar result case.


@itemize *

@item 
“Whether or not the implementation provides user-accessible
names for the standard pool type(s).  See 13.11(17).”
@end itemize

There are 3 different standard pools used by the compiler when
@code{Storage_Pool} is not specified depending whether the type is local
to a subprogram or defined at the library level and whether
@code{Storage_Size`@w{`}is specified or not. See documentation in the runtime
library units `@w{`}System.Pool_Global}, @code{System.Pool_Size} and
@code{System.Pool_Local} in files @code{s-poosiz.ads},
@code{s-pooglo.ads} and @code{s-pooloc.ads} for full details on the
default pools used.  All these pools are accessible by means of @cite{with}ing
these units.


@itemize *

@item 
“The meaning of @code{Storage_Size} when neither the Storage_Size nor the
Storage_Pool is specified for an access type.  See 13.11(18).”
@end itemize

@code{Storage_Size} is measured in storage units, and refers to the
total space available for an access type collection, or to the primary
stack space for a task.


@itemize *

@item 
“The effect of specifying aspect Default_Storage_Pool on an instance
of a language-defined generic unit.  See 13.11.3(5).”
@end itemize

Instances of language-defined generic units are treated the same as other
instances with respect to the Default_Storage_Pool aspect.


@itemize *

@item 
“Implementation-defined restrictions allowed in a pragma
@code{Restrictions}.  See 13.12(8.7).”
@end itemize

See @ref{9,,Standard and Implementation Defined Restrictions}.


@itemize *

@item 
“The consequences of violating limitations on
@code{Restrictions} pragmas.  See 13.12(9).”
@end itemize

Restrictions that can be checked at compile time are enforced at
compile time; violations are illegal. For other restrictions, any
violation during program execution results in erroneous execution.


@itemize *

@item 
“Implementation-defined usage profiles allowed in a pragma Profile.
See 13.12(15).”
@end itemize

See @ref{7,,Implementation Defined Pragmas}.


@itemize *

@item 
“The contents of the stream elements read and written by the Read and
Write attributes of elementary types.  See 13.13.2(9).”
@end itemize

The representation is the in-memory representation of the base type of
the type, using the number of bits corresponding to the
@code{type'Size} value, and the natural ordering of the machine.


@itemize *

@item 
“The names and characteristics of the numeric subtypes
declared in the visible part of package @code{Standard}.  See A.1(3).”
@end itemize

See items describing the integer and floating-point types supported.


@itemize *

@item 
“The values returned by Strings.Hash.  See A.4.9(3).”
@end itemize

This hash function has predictable collisions and is subject to
equivalent substring attacks. It is not suitable for construction of a
hash table keyed on possibly malicious user input.


@itemize *

@item 
“The value returned by a call to a Text_Buffer Get procedure if any
character in the returned sequence is not defined in Character.
See A.4.12(34).”
@end itemize

The contents of a buffer is represented internally as a UTF_8 string.
The value return by Text_Buffer.Get is the result of passing that
UTF_8 string to UTF_Encoding.Strings.Decode.


@itemize *

@item 
“The value returned by a call to a Text_Buffer Wide_Get procedure if
any character in the returned sequence is not defined in Wide_Character.
See A.4.12(34).”
@end itemize

The contents of a buffer is represented internally as a UTF_8 string.
The value return by Text_Buffer.Wide_Get is the result of passing that
UTF_8 string to UTF_Encoding.Wide_Strings.Decode.


@itemize *

@item 
“The accuracy actually achieved by the elementary
functions.  See A.5.1(1).”
@end itemize

The elementary functions correspond to the functions available in the C
library.  Only fast math mode is implemented.


@itemize *

@item 
“The sign of a zero result from some of the operators or
functions in @code{Numerics.Generic_Elementary_Functions}, when
@code{Float_Type'Signed_Zeros} is @code{True}.  See A.5.1(46).”
@end itemize

The sign of zeroes follows the requirements of the IEEE 754 standard on
floating-point.


@itemize *

@item 
“The value of
@code{Numerics.Float_Random.Max_Image_Width}.  See A.5.2(27).”
@end itemize

Maximum image width is 6864, see library file @code{s-rannum.ads}.


@itemize *

@item 
“The value of
@code{Numerics.Discrete_Random.Max_Image_Width}.  See A.5.2(27).”
@end itemize

Maximum image width is 6864, see library file @code{s-rannum.ads}.


@itemize *

@item 
“The string representation of a random number generator’s
state.  See A.5.2(38).”
@end itemize

The value returned by the Image function is the concatenation of
the fixed-width decimal representations of the 624 32-bit integers
of the state vector.


@itemize *

@item 
“The values of the @code{Model_Mantissa},
@code{Model_Emin}, @code{Model_Epsilon}, @code{Model},
@code{Safe_First}, and @code{Safe_Last} attributes, if the Numerics
Annex is not supported.  See A.5.3(72).”
@end itemize

Running the compiler with `-gnatS' to produce a listing of package
@code{Standard} displays the values of these attributes.


@itemize *

@item 
“The value of @code{Buffer_Size} in @code{Storage_IO}.  See A.9(10).”
@end itemize

All type representations are contiguous, and the @code{Buffer_Size} is
the value of @code{type'Size} rounded up to the next storage unit
boundary.


@itemize *

@item 
“External files for standard input, standard output, and
standard error See A.10(5).”
@end itemize

These files are mapped onto the files provided by the C streams
libraries. See source file @code{i-cstrea.ads} for further details.


@itemize *

@item 
“The accuracy of the value produced by @code{Put}.  See A.10.9(36).”
@end itemize

If more digits are requested in the output than are represented by the
precision of the value, zeroes are output in the corresponding least
significant digit positions.


@itemize *

@item 
“Current size for a stream file for which positioning is not supported.
See A.12.1(1.1).”
@end itemize

Positioning is supported.


@itemize *

@item 
“The meaning of @code{Argument_Count}, @code{Argument}, and
@code{Command_Name}.  See A.15(1).”
@end itemize

These are mapped onto the @code{argv} and @code{argc} parameters of the
main program in the natural manner.


@itemize *

@item 
“The interpretation of file names and directory names.  See A.16(46).”
@end itemize

These names are interpreted consistently with the underlying file system.


@itemize *

@item 
“The maximum value for a file size in Directories.  See A.16(87).”
@end itemize

Directories.File_Size’Last is equal to Long_Long_Integer’Last .


@itemize *

@item 
“The result for Directories.Size for a directory or special file.
See A.16(93).”
@end itemize

Name_Error is raised.


@itemize *

@item 
“The result for Directories.Modification_Time for a directory or special file.
See A.16(93).”
@end itemize

Name_Error is raised.


@itemize *

@item 
“The interpretation of a nonnull search pattern in Directories.
See A.16(104).”
@end itemize

When the @code{Pattern} parameter is not the null string, it is interpreted
according to the syntax of regular expressions as defined in the
@code{GNAT.Regexp} package.

See @ref{287,,GNAT.Regexp (g-regexp.ads)}.


@itemize *

@item 
“The results of a Directories search if the contents of the directory are
altered while a search is in progress.  See A.16(110).”
@end itemize

The effect of a call to Get_Next_Entry is determined by the current
state of the directory.


@itemize *

@item 
“The definition and meaning of an environment variable.  See A.17(1).”
@end itemize

This definition is determined by the underlying operating system.


@itemize *

@item 
“The circumstances where an environment variable cannot be defined.
See A.17(16).”
@end itemize

There are no such implementation-defined circumstances.


@itemize *

@item 
“Environment names for which Set has the effect of Clear.  See A.17(17).”
@end itemize

There are no such names.


@itemize *

@item 
“The value of Containers.Hash_Type’Modulus. The value of
Containers.Count_Type’Last.  See A.18.1(7).”
@end itemize

Containers.Hash_Type’Modulus is 2**32.
Containers.Count_Type’Last is 2**31 - 1.


@itemize *

@item 
“Implementation-defined convention names.  See B.1(11).”
@end itemize

The following convention names are supported


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Convention Name

@tab

Interpretation

@item

`Ada'

@tab

Ada

@item

`Ada_Pass_By_Copy'

@tab

Allowed for any types except by-reference types such as limited
records. Compatible with convention Ada, but causes any parameters
with this convention to be passed by copy.

@item

`Ada_Pass_By_Reference'

@tab

Allowed for any types except by-copy types such as scalars.
Compatible with convention Ada, but causes any parameters
with this convention to be passed by reference.

@item

`Assembler'

@tab

Assembly language

@item

`Asm'

@tab

Synonym for Assembler

@item

`Assembly'

@tab

Synonym for Assembler

@item

`C'

@tab

C

@item

`C_Pass_By_Copy'

@tab

Allowed only for record types, like C, but also notes that record
is to be passed by copy rather than reference.

@item

`COBOL'

@tab

COBOL

@item

`C_Plus_Plus (or CPP)'

@tab

C++

@item

`Default'

@tab

Treated the same as C

@item

`External'

@tab

Treated the same as C

@item

`Fortran'

@tab

Fortran

@item

`Intrinsic'

@tab

For support of pragma @code{Import} with convention Intrinsic, see
separate section on Intrinsic Subprograms.

@item

`Stdcall'

@tab

Stdcall (used for Windows implementations only).  This convention correspond
to the WINAPI (previously called Pascal convention) C/C++ convention under
Windows.  A routine with this convention cleans the stack before
exit. This pragma cannot be applied to a dispatching call.

@item

`DLL'

@tab

Synonym for Stdcall

@item

`Win32'

@tab

Synonym for Stdcall

@item

`Stubbed'

@tab

Stubbed is a special convention used to indicate that the body of the
subprogram will be entirely ignored.  Any call to the subprogram
is converted into a raise of the @code{Program_Error} exception.  If a
pragma @code{Import} specifies convention @code{stubbed} then no body need
be present at all.  This convention is useful during development for the
inclusion of subprograms whose body has not yet been written.
In addition, all otherwise unrecognized convention names are also
treated as being synonymous with convention C.  In all implementations,
use of such other names results in a warning.

@end multitable



@itemize *

@item 
“The meaning of link names.  See B.1(36).”
@end itemize

Link names are the actual names used by the linker.


@itemize *

@item 
“The manner of choosing link names when neither the link name nor the
address of an imported or exported entity is specified.  See B.1(36).”
@end itemize

The default linker name is that which would be assigned by the relevant
external language, interpreting the Ada name as being in all lower case
letters.


@itemize *

@item 
“The effect of pragma @code{Linker_Options}.  See B.1(37).”
@end itemize

The string passed to @code{Linker_Options} is presented uninterpreted as
an argument to the link command, unless it contains ASCII.NUL characters.
NUL characters if they appear act as argument separators, so for example

@example
pragma Linker_Options ("-labc" & ASCII.NUL & "-ldef");
@end example

causes two separate arguments @code{-labc} and @code{-ldef} to be passed to the
linker. The order of linker options is preserved for a given unit. The final
list of options passed to the linker is in reverse order of the elaboration
order. For example, linker options for a body always appear before the options
from the corresponding package spec.


@itemize *

@item 
“The contents of the visible part of package
@code{Interfaces} and its language-defined descendants.  See B.2(1).”
@end itemize

See files with prefix @code{i-} in the distributed library.


@itemize *

@item 
“Implementation-defined children of package
@code{Interfaces}.  The contents of the visible part of package
@code{Interfaces}.  See B.2(11).”
@end itemize

See files with prefix @code{i-} in the distributed library.


@itemize *

@item 
“The definitions of certain types and constants in Interfaces.C.
See B.3(41).”
@end itemize

See source file @code{i-c.ads}.


@itemize *

@item 
“The types @code{Floating}, @code{Long_Floating},
@code{Binary}, @code{Long_Binary}, @code{Decimal_ Element}, and
@code{COBOL_Character}; and the initialization of the variables
@code{Ada_To_COBOL} and @code{COBOL_To_Ada}, in
@code{Interfaces.COBOL}.  See B.4(50).”
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

COBOL

@tab

Ada

@item

`Floating'

@tab

Float

@item

`Long_Floating'

@tab

(Floating) Long_Float

@item

`Binary'

@tab

Integer

@item

`Long_Binary'

@tab

Long_Long_Integer

@item

`Decimal_Element'

@tab

Character

@item

`COBOL_Character'

@tab

Character

@end multitable


For initialization, see the file @code{i-cobol.ads} in the distributed library.


@itemize *

@item 
“The types Fortran_Integer, Real, Double_Precision, and Character_Set
in Interfaces.Fortran.  See B.5(17).”
@end itemize

See source file @code{i-fortra.ads}. These types are derived, respectively,
from Integer, Float, Long_Float, and Character.


@itemize *

@item 
“Implementation-defined intrinsic subprograms.  See C.1(1).”
@end itemize

See separate section on Intrinsic Subprograms.


@itemize *

@item 
“Any restrictions on a protected procedure or its containing type when an
aspect Attach_handler or Interrupt_Handler is specified.  See C.3.1(17).”
@end itemize

There are no such restrictions.


@itemize *

@item 
“Any other forms of interrupt handler supported by the Attach_Handler and
Interrupt_Handler aspects.  See C.3.1(19).”
@end itemize

There are no such forms.


@itemize *

@item 
“The semantics of some attributes and functions of an entity for which
aspect Discard_Names is True.  See C.5(7).”
@end itemize

If Discard_Names is True for an enumeration type, the Image attribute
provides the image of the Pos of the literal, and Value accepts
Pos values.

If both of the aspects`@w{`}Discard_Names`@w{`} and @code{No_Tagged_Streams} are true
for a tagged type, its Expanded_Name and External_Tag values are
empty strings. This is useful to avoid exposing entity names at binary
level.


@itemize *

@item 
“The modulus and size of Test_and_Set_Flag.  See C.6.3(8).”
@end itemize

The modulus is 2**8. The size is 8.


@itemize *

@item 
“The value used to represent the set value for Atomic_Test_and_Set.
See C.6.3(10).”
@end itemize

The value is 1.


@itemize *

@item 
“The result of the @code{Task_Identification.Image}
attribute.  See C.7.1(7).”
@end itemize

The result of this attribute is a string that identifies
the object or component that denotes a given task. If a variable @code{Var}
has a task type, the image for this task will have the form @code{Var_@var{XXXXXXXX}},
where the suffix `XXXXXXXX'
is the hexadecimal representation of the virtual address of the corresponding
task control block. If the variable is an array of tasks, the image of each
task will have the form of an indexed component indicating the position of a
given task in the array, e.g., @code{Group(5)_@var{XXXXXXX}}. If the task is a
component of a record, the image of the task will have the form of a selected
component. These rules are fully recursive, so that the image of a task that
is a subcomponent of a composite object corresponds to the expression that
designates this task.

If a task is created by an allocator, its image depends on the context. If the
allocator is part of an object declaration, the rules described above are used
to construct its image, and this image is not affected by subsequent
assignments. If the allocator appears within an expression, the image
includes only the name of the task type.

If the configuration pragma Discard_Names is present, or if the restriction
No_Implicit_Heap_Allocation is in effect,  the image reduces to
the numeric suffix, that is to say the hexadecimal representation of the
virtual address of the control block of the task.


@itemize *

@item 
“The value of @code{Current_Task} when in a protected entry
or interrupt handler.  See C.7.1(17).”
@end itemize

Protected entries or interrupt handlers can be executed by any
convenient thread, so the value of @code{Current_Task} is undefined.


@itemize *

@item 
“Granularity of locking for Task_Attributes.  See C.7.2(16).”
@end itemize

No locking is needed if the formal type Attribute has the size and
alignment of either Integer or System.Address and the bit representation
of Initial_Value is all zeroes. Otherwise, locking is performed.


@itemize *

@item 
“The declarations of @code{Any_Priority} and
@code{Priority}.  See D.1(11).”
@end itemize

See declarations in file @code{system.ads}.


@itemize *

@item 
“Implementation-defined execution resources.  See D.1(15).”
@end itemize

There are no implementation-defined execution resources.


@itemize *

@item 
“Whether, on a multiprocessor, a task that is waiting for
access to a protected object keeps its processor busy.  See D.2.1(3).”
@end itemize

On a multi-processor, a task that is waiting for access to a protected
object does not keep its processor busy.


@itemize *

@item 
“The affect of implementation defined execution resources
on task dispatching.  See D.2.1(9).”
@end itemize

Tasks map to threads in the threads package used by GNAT.  Where possible
and appropriate, these threads correspond to native threads of the
underlying operating system.


@itemize *

@item 
“Implementation-defined task dispatching policies.  See D.2.2(3).”
@end itemize

There are no implementation-defined task dispatching policies.


@itemize *

@item 
“The value of Default_Quantum in Dispatching.Round_Robin.  See D.2.5(4).”
@end itemize

The value is 10 milliseconds.


@itemize *

@item 
“Implementation-defined `policy_identifiers' allowed
in a pragma @code{Locking_Policy}.  See D.3(4).”
@end itemize

The two implementation defined policies permitted in GNAT are
@code{Inheritance_Locking} and  @code{Concurrent_Readers_Locking}. On
targets that support the @code{Inheritance_Locking} policy, locking is
implemented by inheritance, i.e., the task owning the lock operates
at a priority equal to the highest priority of any task currently
requesting the lock. On targets that support the
@code{Concurrent_Readers_Locking} policy, locking is implemented with a
read/write lock allowing multiple protected object functions to enter
concurrently.


@itemize *

@item 
“Default ceiling priorities.  See D.3(10).”
@end itemize

The ceiling priority of protected objects of the type
@code{System.Interrupt_Priority'Last} as described in the Ada
Reference Manual D.3(10),


@itemize *

@item 
“The ceiling of any protected object used internally by
the implementation.  See D.3(16).”
@end itemize

The ceiling priority of internal protected objects is
@code{System.Priority'Last}.


@itemize *

@item 
“Implementation-defined queuing policies.  See D.4(1).”
@end itemize

There are no implementation-defined queuing policies.


@itemize *

@item 
“Implementation-defined admission policies.  See D.4.1(1).”
@end itemize

There are no implementation-defined admission policies.


@itemize *

@item 
“Any operations that implicitly require heap storage
allocation.  See D.7(8).”
@end itemize

The only operation that implicitly requires heap storage allocation is
task creation.


@itemize *

@item 
“When restriction No_Dynamic_CPU_Assignment applies to a partition, the
processor on which a task with a CPU value of a Not_A_Specific_CPU will
execute.  See D.7(10).”
@end itemize

Unknown.


@itemize *

@item 
“When restriction No_Task_Termination applies to a partition, what happens
when a task terminates.  See D.7(15.1).”
@end itemize

Execution is erroneous in that case.


@itemize *

@item 
“The behavior when restriction Max_Storage_At_Blocking is violated.
See D.7(17).”
@end itemize

Execution is erroneous in that case.


@itemize *

@item 
“The behavior when restriction Max_Asynchronous_Select_Nesting is violated.
See D.7(18).”
@end itemize

Execution is erroneous in that case.


@itemize *

@item 
“The behavior when restriction Max_Tasks is violated.  See D.7(19).”
@end itemize

Execution is erroneous in that case.


@itemize *

@item 
“Whether the use of pragma Restrictions results in a reduction in program
code or data size or execution time.  See D.7(20).”
@end itemize

Yes it can, but the precise circumstances and properties of such reductions
are difficult to characterize.


@itemize *

@item 
“The value of Barrier_Limit’Last in Synchronous_Barriers.  See D.10.1(4).”
@end itemize

Synchronous_Barriers.Barrier_Limit’Last is Integer’Last .


@itemize *

@item 
“When an aborted task that is waiting on a Synchronous_Barrier is aborted.
See D.10.1(13).”
@end itemize

Difficult to characterize.


@itemize *

@item 
“The value of Min_Handler_Ceiling in Execution_Time.Group_Budgets.
See D.14.2(7).”
@end itemize

See source file @code{a-etgrbu.ads}.


@itemize *

@item 
“The value of CPU_Range’Last in System.Multiprocessors.  See D.16(4).”
@end itemize

See source file @code{s-multip.ads}.


@itemize *

@item 
“The processor on which the environment task executes in the absence
of a value for the aspect CPU.  See D.16(13).”
@end itemize

Unknown.


@itemize *

@item 
“The means for creating and executing distributed
programs.  See E(5).”
@end itemize

The PolyORB product provides means creating and executing
distributed programs. `NB!' See the note in @ref{273,,PolyORB} regarding the lifetime of this product.


@itemize *

@item 
“Any events that can result in a partition becoming
inaccessible.  See E.1(7).”
@end itemize

See the PolyORB user guide for full details on such events.
`NB!' Consider the note in @ref{273,,PolyORB} regarding the lifetime
of this product.


@itemize *

@item 
“The scheduling policies, treatment of priorities, and management of
shared resources between partitions in certain cases.  See E.1(11).”
@end itemize

See the PolyORB user guide for full details on these aspects of
multi-partition execution. `NB!' Consider the note in @ref{273,,PolyORB} regarding the lifetime of this product.


@itemize *

@item 
“Whether the execution of the remote subprogram is
immediately aborted as a result of cancellation.  See E.4(13).”
@end itemize

See the PolyORB user guide for details on the effect of abort in
a distributed application.  `NB!' Consider the note in @ref{273,,PolyORB} regarding the lifetime of this product.


@itemize *

@item 
“The range of type System.RPC.Partition_Id.  See E.5(14).”
@end itemize

System.RPC.Partition_ID’Last is Integer’Last. See source file @code{s-rpc.ads}.


@itemize *

@item 
“Implementation-defined interfaces in the PCS.  See E.5(26).”
@end itemize

See the PolyORB user guide for a full description of all
implementation defined interfaces.  `NB!' See the note in @ref{273,,PolyORB} regarding the lifetime of this product.


@itemize *

@item 
“The values of named numbers in the package
@code{Decimal}.  See F.2(7).”
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxx} 
@headitem

Named Number

@tab

Value

@item

`Max_Scale'

@tab

+18

@item

`Min_Scale'

@tab

-18

@item

`Min_Delta'

@tab

1.0E-18

@item

`Max_Delta'

@tab

1.0E+18

@item

`Max_Decimal_Digits'

@tab

18

@end multitable



@itemize *

@item 
“The value of @code{Max_Picture_Length} in the package
@code{Text_IO.Editing}.  See F.3.3(16).”

64

@item 
“The value of @code{Max_Picture_Length} in the package
@code{Wide_Text_IO.Editing}.  See F.3.4(5).”

64

@item 
“The accuracy actually achieved by the complex elementary
functions and by other complex arithmetic operations.  See G.1(1).”
@end itemize

Standard library functions are used for the complex arithmetic
operations.  Only fast math mode is currently supported.


@itemize *

@item 
“The sign of a zero result (or a component thereof) from
any operator or function in @code{Numerics.Generic_Complex_Types}, when
@code{Real'Signed_Zeros} is True.  See G.1.1(53).”
@end itemize

The signs of zero values are as recommended by the relevant
implementation advice.


@itemize *

@item 
“The sign of a zero result (or a component thereof) from
any operator or function in
@code{Numerics.Generic_Complex_Elementary_Functions}, when
@code{Real'Signed_Zeros} is @code{True}.  See G.1.2(45).”
@end itemize

The signs of zero values are as recommended by the relevant
implementation advice.


@itemize *

@item 
“Whether the strict mode or the relaxed mode is the
default.  See G.2(2).”
@end itemize

The strict mode is the default.  There is no separate relaxed mode.  GNAT
provides a highly efficient implementation of strict mode.


@itemize *

@item 
“The result interval in certain cases of fixed-to-float
conversion.  See G.2.1(10).”
@end itemize

For cases where the result interval is implementation dependent, the
accuracy is that provided by performing all operations in 64-bit IEEE
floating-point format.


@itemize *

@item 
“The result of a floating point arithmetic operation in
overflow situations, when the @code{Machine_Overflows} attribute of the
result type is @code{False}.  See G.2.1(13).”
@end itemize

Infinite and NaN values are produced as dictated by the IEEE
floating-point standard.
Note that on machines that are not fully compliant with the IEEE
floating-point standard, such as Alpha, the `-mieee' compiler flag
must be used for achieving IEEE conforming behavior (although at the cost
of a significant performance penalty), so infinite and NaN values are
properly generated.


@itemize *

@item 
“The result interval for division (or exponentiation by a
negative exponent), when the floating point hardware implements division
as multiplication by a reciprocal.  See G.2.1(16).”
@end itemize

Not relevant, division is IEEE exact.


@itemize *

@item 
“The definition of close result set, which determines the accuracy of
certain fixed point multiplications and divisions.  See G.2.3(5).”
@end itemize

Operations in the close result set are performed using IEEE long format
floating-point arithmetic.  The input operands are converted to
floating-point, the operation is done in floating-point, and the result
is converted to the target type.


@itemize *

@item 
“Conditions on a `universal_real' operand of a fixed
point multiplication or division for which the result shall be in the
perfect result set.  See G.2.3(22).”
@end itemize

The result is only defined to be in the perfect result set if the result
can be computed by a single scaling operation involving a scale factor
representable in 64 bits.


@itemize *

@item 
“The result of a fixed point arithmetic operation in
overflow situations, when the @code{Machine_Overflows} attribute of the
result type is @code{False}.  See G.2.3(27).”
@end itemize

Not relevant, @code{Machine_Overflows} is @code{True} for fixed-point
types.


@itemize *

@item 
“The result of an elementary function reference in
overflow situations, when the @code{Machine_Overflows} attribute of the
result type is @code{False}.  See G.2.4(4).”
@end itemize

IEEE infinite and Nan values are produced as appropriate.


@itemize *

@item 
“The value of the angle threshold, within which certain
elementary functions, complex arithmetic operations, and complex
elementary functions yield results conforming to a maximum relative
error bound.  See G.2.4(10).”
@end itemize

Information on this subject is not yet available.


@itemize *

@item 
“The accuracy of certain elementary functions for
parameters beyond the angle threshold.  See G.2.4(10).”
@end itemize

Information on this subject is not yet available.


@itemize *

@item 
“The result of a complex arithmetic operation or complex
elementary function reference in overflow situations, when the
@code{Machine_Overflows} attribute of the corresponding real type is
@code{False}.  See G.2.6(5).”
@end itemize

IEEE infinite and Nan values are produced as appropriate.


@itemize *

@item 
“The accuracy of certain complex arithmetic operations and
certain complex elementary functions for parameters (or components
thereof) beyond the angle threshold.  See G.2.6(8).”
@end itemize

Information on those subjects is not yet available.


@itemize *

@item 
“The accuracy requirements for the subprograms Solve, Inverse,
Determinant, Eigenvalues and Eigensystem for type Real_Matrix.
See G.3.1(81).”
@end itemize

Information on those subjects is not yet available.


@itemize *

@item 
“The accuracy requirements for the subprograms Solve, Inverse,
Determinant, Eigenvalues and Eigensystem for type Complex_Matrix.
See G.3.2(149).”
@end itemize

Information on those subjects is not yet available.


@itemize *

@item 
“The consequences of violating No_Hidden_Indirect_Globals.  See H.4(23.9).”
@end itemize

Execution is erroneous in that case.

@node Intrinsic Subprograms,Representation Clauses and Pragmas,Implementation Defined Characteristics,Top
@anchor{gnat_rm/intrinsic_subprograms doc}@anchor{288}@anchor{gnat_rm/intrinsic_subprograms id1}@anchor{289}@anchor{gnat_rm/intrinsic_subprograms intrinsic-subprograms}@anchor{d}
@chapter Intrinsic Subprograms


@geindex Intrinsic Subprograms

GNAT allows a user application program to write the declaration:

@example
pragma Import (Intrinsic, name);
@end example

providing that the name corresponds to one of the implemented intrinsic
subprograms in GNAT, and that the parameter profile of the referenced
subprogram meets the requirements.  This chapter describes the set of
implemented intrinsic subprograms, and the requirements on parameter profiles.
Note that no body is supplied; as with other uses of pragma Import, the
body is supplied elsewhere (in this case by the compiler itself).  Note
that any use of this feature is potentially non-portable, since the
Ada standard does not require Ada compilers to implement this feature.

@menu
* Intrinsic Operators:: 
* Compilation_ISO_Date:: 
* Compilation_Date:: 
* Compilation_Time:: 
* Enclosing_Entity:: 
* Exception_Information:: 
* Exception_Message:: 
* Exception_Name:: 
* File:: 
* Line:: 
* Shifts and Rotates:: 
* Source_Location:: 

@end menu

@node Intrinsic Operators,Compilation_ISO_Date,,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id2}@anchor{28a}@anchor{gnat_rm/intrinsic_subprograms intrinsic-operators}@anchor{28b}
@section Intrinsic Operators


@geindex Intrinsic operator

All the predefined numeric operators in package Standard
in @code{pragma Import (Intrinsic,..)}
declarations.  In the binary operator case, the operands must have the same
size.  The operand or operands must also be appropriate for
the operator.  For example, for addition, the operands must
both be floating-point or both be fixed-point, and the
right operand for @code{"**"} must have a root type of
@code{Standard.Integer'Base}.
You can use an intrinsic operator declaration as in the following example:

@example
type Int1 is new Integer;
type Int2 is new Integer;

function "+" (X1 : Int1; X2 : Int2) return Int1;
function "+" (X1 : Int1; X2 : Int2) return Int2;
pragma Import (Intrinsic, "+");
@end example

This declaration would permit ‘mixed mode’ arithmetic on items
of the differing types @code{Int1} and @code{Int2}.
It is also possible to specify such operators for private types, if the
full views are appropriate arithmetic types.

@node Compilation_ISO_Date,Compilation_Date,Intrinsic Operators,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms compilation-iso-date}@anchor{28c}@anchor{gnat_rm/intrinsic_subprograms id3}@anchor{28d}
@section Compilation_ISO_Date


@geindex Compilation_ISO_Date

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Compilation_ISO_Date} to obtain the date of
the current compilation (in local time format YYYY-MM-DD).

@node Compilation_Date,Compilation_Time,Compilation_ISO_Date,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms compilation-date}@anchor{28e}@anchor{gnat_rm/intrinsic_subprograms id4}@anchor{28f}
@section Compilation_Date


@geindex Compilation_Date

Same as Compilation_ISO_Date, except the string is in the form
MMM DD YYYY.

@node Compilation_Time,Enclosing_Entity,Compilation_Date,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms compilation-time}@anchor{290}@anchor{gnat_rm/intrinsic_subprograms id5}@anchor{291}
@section Compilation_Time


@geindex Compilation_Time

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Compilation_Time} to obtain the time of
the current compilation (in local time format HH:MM:SS).

@node Enclosing_Entity,Exception_Information,Compilation_Time,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms enclosing-entity}@anchor{292}@anchor{gnat_rm/intrinsic_subprograms id6}@anchor{293}
@section Enclosing_Entity


@geindex Enclosing_Entity

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Enclosing_Entity} to obtain the name of
the current subprogram, package, task, entry, or protected subprogram.

@node Exception_Information,Exception_Message,Enclosing_Entity,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms exception-information}@anchor{294}@anchor{gnat_rm/intrinsic_subprograms id7}@anchor{295}
@section Exception_Information


@geindex Exception_Information'

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Current_Exception}.  The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@code{GNAT.Current_Exception.Exception_Information} to obtain
the exception information associated with the current exception.

@node Exception_Message,Exception_Name,Exception_Information,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms exception-message}@anchor{296}@anchor{gnat_rm/intrinsic_subprograms id8}@anchor{297}
@section Exception_Message


@geindex Exception_Message

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Current_Exception}.  The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@code{GNAT.Current_Exception.Exception_Message} to obtain
the message associated with the current exception.

@node Exception_Name,File,Exception_Message,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms exception-name}@anchor{298}@anchor{gnat_rm/intrinsic_subprograms id9}@anchor{299}
@section Exception_Name


@geindex Exception_Name

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Current_Exception}.  The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@code{GNAT.Current_Exception.Exception_Name} to obtain
the name of the current exception.

@node File,Line,Exception_Name,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms file}@anchor{29a}@anchor{gnat_rm/intrinsic_subprograms id10}@anchor{29b}
@section File


@geindex File

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.File} to obtain the name of the current
file.

@node Line,Shifts and Rotates,File,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id11}@anchor{29c}@anchor{gnat_rm/intrinsic_subprograms line}@anchor{29d}
@section Line


@geindex Line

This intrinsic subprogram is used in the implementation of the
library package @code{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Line} to obtain the number of the current
source line.

@node Shifts and Rotates,Source_Location,Line,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id12}@anchor{29e}@anchor{gnat_rm/intrinsic_subprograms shifts-and-rotates}@anchor{29f}
@section Shifts and Rotates


@geindex Shift_Left

@geindex Shift_Right

@geindex Shift_Right_Arithmetic

@geindex Rotate_Left

@geindex Rotate_Right

In standard Ada, the shift and rotate functions are available only
for the predefined modular types in package @code{Interfaces}.  However, in
GNAT it is possible to define these functions for any integer
type (signed or modular), as in this example:

@example
function Shift_Left
  (Value  : T;
   Amount : Natural) return T
with Import, Convention => Intrinsic;
@end example

The function name must be one of
Shift_Left, Shift_Right, Shift_Right_Arithmetic, Rotate_Left, or
Rotate_Right. T must be an integer type. T’Size must be
8, 16, 32 or 64 bits; if T is modular, the modulus
must be 2**8, 2**16, 2**32 or 2**64.
The result type must be the same as the type of @code{Value}.
The shift amount must be Natural.
The formal parameter names can be anything.

A more convenient way of providing these shift operators is to use the
Provide_Shift_Operators pragma, which provides the function declarations and
corresponding pragma Import’s for all five shift functions. For signed types
the semantics of these operators is to interpret the bitwise result of the
corresponding operator for modular type. In particular, shifting a negative
number may change its sign bit to positive.

@node Source_Location,,Shifts and Rotates,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id13}@anchor{2a0}@anchor{gnat_rm/intrinsic_subprograms source-location}@anchor{2a1}
@section Source_Location


@geindex Source_Location

This intrinsic subprogram is used in the implementation of the
library routine @code{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@code{GNAT.Source_Info.Source_Location} to obtain the current
source file location.

@node Representation Clauses and Pragmas,Standard Library Routines,Intrinsic Subprograms,Top
@anchor{gnat_rm/representation_clauses_and_pragmas doc}@anchor{2a2}@anchor{gnat_rm/representation_clauses_and_pragmas id1}@anchor{2a3}@anchor{gnat_rm/representation_clauses_and_pragmas representation-clauses-and-pragmas}@anchor{e}
@chapter Representation Clauses and Pragmas


@geindex Representation Clauses

@geindex Representation Clause

@geindex Representation Pragma

@geindex Pragma
@geindex representation

This section describes the representation clauses accepted by GNAT, and
their effect on the representation of corresponding data objects.

GNAT fully implements Annex C (Systems Programming).  This means that all
the implementation advice sections in chapter 13 are fully implemented.
However, these sections only require a minimal level of support for
representation clauses.  GNAT provides much more extensive capabilities,
and this section describes the additional capabilities provided.

@menu
* Alignment Clauses:: 
* Size Clauses:: 
* Storage_Size Clauses:: 
* Size of Variant Record Objects:: 
* Biased Representation:: 
* Value_Size and Object_Size Clauses:: 
* Component_Size Clauses:: 
* Bit_Order Clauses:: 
* Effect of Bit_Order on Byte Ordering:: 
* Pragma Pack for Arrays:: 
* Pragma Pack for Records:: 
* Record Representation Clauses:: 
* Handling of Records with Holes:: 
* Enumeration Clauses:: 
* Address Clauses:: 
* Use of Address Clauses for Memory-Mapped I/O:: 
* Effect of Convention on Representation:: 
* Conventions and Anonymous Access Types:: 
* Determining the Representations chosen by GNAT:: 

@end menu

@node Alignment Clauses,Size Clauses,,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas alignment-clauses}@anchor{2a4}@anchor{gnat_rm/representation_clauses_and_pragmas id2}@anchor{2a5}
@section Alignment Clauses


@geindex Alignment Clause

GNAT requires that all alignment clauses specify 0 or a power of 2, and
all default alignments are always a power of 2. Specifying 0 is the
same as specifying 1.

The default alignment values are as follows:


@itemize *

@item 
`Elementary Types'.

For elementary types, the alignment is the minimum of the actual size of
objects of the type divided by @code{Storage_Unit},
and the maximum default alignment supported by the target.
(This maximum default alignment is given by the GNAT-specific attribute
@code{Standard'Maximum_Alignment}; see @ref{1aa,,Attribute Maximum_Alignment}.)

@geindex Maximum_Alignment attribute

For example, for type @code{Long_Float}, the object size is 8 bytes, and the
default alignment will be 8 on any target that supports alignments
this large, but on some targets, the maximum alignment may be smaller
than 8, in which case objects of type @code{Long_Float} will be maximally
aligned.

@item 
`Arrays'.

For arrays, the alignment is equal to the alignment of the component type
for the normal case where no packing or component size is given.  If the
array is packed, and the packing is effective (see separate section on
packed arrays), then the alignment will be either 4, 2, or 1 for long packed
arrays or arrays whose length is not known at compile time, depending on
whether the component size is divisible by 4, 2, or is odd.  For short packed
arrays, which are handled internally as modular types, the alignment
will be as described for elementary types, e.g. a packed array of length
31 bits will have an object size of four bytes, and an alignment of 4.

@item 
`Records'.

For the normal unpacked case, the alignment of a record is equal to
the maximum alignment of any of its components.  For tagged records, this
includes the implicit access type used for the tag.  If a pragma @code{Pack}
is used and all components are packable (see separate section on pragma
@code{Pack}), then the resulting alignment is 1, unless the layout of the
record makes it profitable to increase it.

A special case is when:


@itemize *

@item 
the size of the record is given explicitly, or a
full record representation clause is given, and

@item 
the size of the record is 2, 4, or 8 bytes.
@end itemize

In this case, an alignment is chosen to match the
size of the record. For example, if we have:

@example
type Small is record
   A, B : Character;
end record;
for Small'Size use 16;
@end example

then the default alignment of the record type @code{Small} is 2, not 1. This
leads to more efficient code when the record is treated as a unit, and also
allows the type to specified as @code{Atomic} on architectures requiring
strict alignment.
@end itemize

An alignment clause may specify a larger alignment than the default value
up to some maximum value dependent on the target. It may also specify
a smaller alignment than the default value for enumeration, integer and
fixed point types, as well as for record types, for example

@example
type V is record
   A : Integer;
end record;

for V'alignment use 1;
@end example

@geindex Alignment
@geindex default

The default alignment for the type @code{V} is 4, as a result of the
Integer field in the record, but it is permissible, as shown, to
override the default alignment of the record with a smaller value.

@geindex Alignment
@geindex subtypes

Note that according to the Ada standard, an alignment clause applies only
to the first named subtype. If additional subtypes are declared, then the
compiler is allowed to choose any alignment it likes, and there is no way
to control this choice. Consider:

@example
type R is range 1 .. 10_000;
for R'Alignment use 1;
subtype RS is R range 1 .. 1000;
@end example

The alignment clause specifies an alignment of 1 for the first named subtype
@code{R} but this does not necessarily apply to @code{RS}. When writing
portable Ada code, you should avoid writing code that explicitly or
implicitly relies on the alignment of such subtypes.

For the GNAT compiler, if an explicit alignment clause is given, this
value is also used for any subsequent subtypes. So for GNAT, in the
above example, you can count on the alignment of @code{RS} being 1. But this
assumption is non-portable, and other compilers may choose different
alignments for the subtype @code{RS}.

@node Size Clauses,Storage_Size Clauses,Alignment Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id3}@anchor{2a6}@anchor{gnat_rm/representation_clauses_and_pragmas size-clauses}@anchor{2a7}
@section Size Clauses


@geindex Size Clause

The default size for a type @code{T} is obtainable through the
language-defined attribute @code{T'Size} and also through the
equivalent GNAT-defined attribute @code{T'Value_Size}.
For objects of type @code{T}, GNAT will generally increase the type size
so that the object size (obtainable through the GNAT-defined attribute
@code{T'Object_Size})
is a multiple of @code{T'Alignment * Storage_Unit}.

For example:

@example
type Smallint is range 1 .. 6;

type Rec is record
   Y1 : integer;
   Y2 : boolean;
end record;
@end example

In this example, @code{Smallint'Size} = @code{Smallint'Value_Size} = 3,
as specified by the RM rules,
but objects of this type will have a size of 8
(@code{Smallint'Object_Size} = 8),
since objects by default occupy an integral number
of storage units.  On some targets, notably older
versions of the Digital Alpha, the size of stand
alone objects of this type may be 32, reflecting
the inability of the hardware to do byte load/stores.

Similarly, the size of type @code{Rec} is 40 bits
(@code{Rec'Size} = @code{Rec'Value_Size} = 40), but
the alignment is 4, so objects of this type will have
their size increased to 64 bits so that it is a multiple
of the alignment (in bits).  This decision is
in accordance with the specific Implementation Advice in RM 13.3(43):

@quotation

“A @code{Size} clause should be supported for an object if the specified
@code{Size} is at least as large as its subtype’s @code{Size}, and corresponds
to a size in storage elements that is a multiple of the object’s
@code{Alignment} (if the @code{Alignment} is nonzero).”
@end quotation

An explicit size clause may be used to override the default size by
increasing it.  For example, if we have:

@example
type My_Boolean is new Boolean;
for My_Boolean'Size use 32;
@end example

then values of this type will always be 32-bit long.  In the case of discrete
types, the size can be increased up to 64 bits on 32-bit targets and 128 bits
on 64-bit targets, with the effect that the entire specified field is used to
hold the value, sign- or zero-extended as appropriate.  If more than 64 bits
or 128 bits resp. is specified, then padding space is allocated after the
value, and a warning is issued that there are unused bits.

Similarly the size of records and arrays may be increased, and the effect
is to add padding bits after the value.  This also causes a warning message
to be generated.

The largest Size value permitted in GNAT is 2**31-1.  Since this is a
Size in bits, this corresponds to an object of size 256 megabytes (minus
one).  This limitation is true on all targets.  The reason for this
limitation is that it improves the quality of the code in many cases
if it is known that a Size value can be accommodated in an object of
type Integer.

@node Storage_Size Clauses,Size of Variant Record Objects,Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id4}@anchor{2a8}@anchor{gnat_rm/representation_clauses_and_pragmas storage-size-clauses}@anchor{2a9}
@section Storage_Size Clauses


@geindex Storage_Size Clause

For tasks, the @code{Storage_Size} clause specifies the amount of space
to be allocated for the task stack.  This cannot be extended, and if the
stack is exhausted, then @code{Storage_Error} will be raised (if stack
checking is enabled).  Use a @code{Storage_Size} attribute definition clause,
or a @code{Storage_Size} pragma in the task definition to set the
appropriate required size.  A useful technique is to include in every
task definition a pragma of the form:

@example
pragma Storage_Size (Default_Stack_Size);
@end example

Then @code{Default_Stack_Size} can be defined in a global package, and
modified as required. Any tasks requiring stack sizes different from the
default can have an appropriate alternative reference in the pragma.

You can also use the `-d' binder switch to modify the default stack
size.

For access types, the @code{Storage_Size} clause specifies the maximum
space available for allocation of objects of the type.  If this space is
exceeded then @code{Storage_Error} will be raised by an allocation attempt.
In the case where the access type is declared local to a subprogram, the
use of a @code{Storage_Size} clause triggers automatic use of a special
predefined storage pool (@code{System.Pool_Size}) that ensures that all
space for the pool is automatically reclaimed on exit from the scope in
which the type is declared.

A special case recognized by the compiler is the specification of a
@code{Storage_Size} of zero for an access type.  This means that no
items can be allocated from the pool, and this is recognized at compile
time, and all the overhead normally associated with maintaining a fixed
size storage pool is eliminated.  Consider the following example:

@example
procedure p is
   type R is array (Natural) of Character;
   type P is access all R;
   for P'Storage_Size use 0;
   --  Above access type intended only for interfacing purposes

   y : P;

   procedure g (m : P);
   pragma Import (C, g);

   --  ...

begin
   --  ...
   y := new R;
end;
@end example

As indicated in this example, these dummy storage pools are often useful in
connection with interfacing where no object will ever be allocated.  If you
compile the above example, you get the warning:

@example
p.adb:16:09: warning: allocation from empty storage pool
p.adb:16:09: warning: Storage_Error will be raised at run time
@end example

Of course in practice, there will not be any explicit allocators in the
case of such an access declaration.

@node Size of Variant Record Objects,Biased Representation,Storage_Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id5}@anchor{2aa}@anchor{gnat_rm/representation_clauses_and_pragmas size-of-variant-record-objects}@anchor{2ab}
@section Size of Variant Record Objects


@geindex Size
@geindex variant record objects

@geindex Variant record objects
@geindex size

In the case of variant record objects, there is a question whether Size gives
information about a particular variant, or the maximum size required
for any variant.  Consider the following program

@example
with Text_IO; use Text_IO;
procedure q is
   type R1 (A : Boolean := False) is record
     case A is
       when True  => X : Character;
       when False => null;
     end case;
   end record;

   V1 : R1 (False);
   V2 : R1;

begin
   Put_Line (Integer'Image (V1'Size));
   Put_Line (Integer'Image (V2'Size));
end q;
@end example

Here we are dealing with a variant record, where the True variant
requires 16 bits, and the False variant requires 8 bits.
In the above example, both V1 and V2 contain the False variant,
which is only 8 bits long.  However, the result of running the
program is:

@example
8
16
@end example

The reason for the difference here is that the discriminant value of
V1 is fixed, and will always be False.  It is not possible to assign
a True variant value to V1, therefore 8 bits is sufficient.  On the
other hand, in the case of V2, the initial discriminant value is
False (from the default), but it is possible to assign a True
variant value to V2, therefore 16 bits must be allocated for V2
in the general case, even fewer bits may be needed at any particular
point during the program execution.

As can be seen from the output of this program, the @code{'Size}
attribute applied to such an object in GNAT gives the actual allocated
size of the variable, which is the largest size of any of the variants.
The Ada Reference Manual is not completely clear on what choice should
be made here, but the GNAT behavior seems most consistent with the
language in the RM.

In some cases, it may be desirable to obtain the size of the current
variant, rather than the size of the largest variant.  This can be
achieved in GNAT by making use of the fact that in the case of a
subprogram parameter, GNAT does indeed return the size of the current
variant (because a subprogram has no way of knowing how much space
is actually allocated for the actual).

Consider the following modified version of the above program:

@example
with Text_IO; use Text_IO;
procedure q is
   type R1 (A : Boolean := False) is record
     case A is
       when True  => X : Character;
       when False => null;
     end case;
   end record;

   V2 : R1;

   function Size (V : R1) return Integer is
   begin
      return V'Size;
   end Size;

begin
   Put_Line (Integer'Image (V2'Size));
   Put_Line (Integer'Image (Size (V2)));
   V2 := (True, 'x');
   Put_Line (Integer'Image (V2'Size));
   Put_Line (Integer'Image (Size (V2)));
end q;
@end example

The output from this program is

@example
16
8
16
16
@end example

Here we see that while the @code{'Size} attribute always returns
the maximum size, regardless of the current variant value, the
@code{Size} function does indeed return the size of the current
variant value.

@node Biased Representation,Value_Size and Object_Size Clauses,Size of Variant Record Objects,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas biased-representation}@anchor{2ac}@anchor{gnat_rm/representation_clauses_and_pragmas id6}@anchor{2ad}
@section Biased Representation


@geindex Size for biased representation

@geindex Biased representation

In the case of scalars with a range starting at other than zero, it is
possible in some cases to specify a size smaller than the default minimum
value, and in such cases, GNAT uses an unsigned biased representation,
in which zero is used to represent the lower bound, and successive values
represent successive values of the type.

For example, suppose we have the declaration:

@example
type Small is range -7 .. -4;
for Small'Size use 2;
@end example

Although the default size of type @code{Small} is 4, the @code{Size}
clause is accepted by GNAT and results in the following representation
scheme:

@example
-7 is represented as 2#00#
-6 is represented as 2#01#
-5 is represented as 2#10#
-4 is represented as 2#11#
@end example

Biased representation is only used if the specified @code{Size} clause
cannot be accepted in any other manner.  These reduced sizes that force
biased representation can be used for all discrete types except for
enumeration types for which a representation clause is given.

@node Value_Size and Object_Size Clauses,Component_Size Clauses,Biased Representation,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id7}@anchor{2ae}@anchor{gnat_rm/representation_clauses_and_pragmas value-size-and-object-size-clauses}@anchor{2af}
@section Value_Size and Object_Size Clauses


@geindex Value_Size

@geindex Object_Size

@geindex Size
@geindex of objects

In Ada 95 and Ada 2005, @code{T'Size} for a type @code{T} is the minimum
number of bits required to hold values of type @code{T}.
Although this interpretation was allowed in Ada 83, it was not required,
and this requirement in practice can cause some significant difficulties.
For example, in most Ada 83 compilers, @code{Natural'Size} was 32.
However, in Ada 95 and Ada 2005,
@code{Natural'Size} is
typically 31.  This means that code may change in behavior when moving
from Ada 83 to Ada 95 or Ada 2005.  For example, consider:

@example
type Rec is record
   A : Natural;
   B : Natural;
end record;

for Rec use record
   A at 0 range 0 .. Natural'Size - 1;
   B at 0 range Natural'Size .. 2 * Natural'Size - 1;
end record;
@end example

In the above code, since the typical size of @code{Natural} objects
is 32 bits and @code{Natural'Size} is 31, the above code can cause
unexpected inefficient packing in Ada 95 and Ada 2005, and in general
there are cases where the fact that the object size can exceed the
size of the type causes surprises.

To help get around this problem GNAT provides two implementation
defined attributes, @code{Value_Size} and @code{Object_Size}.  When
applied to a type, these attributes yield the size of the type
(corresponding to the RM defined size attribute), and the size of
objects of the type respectively.

The @code{Object_Size} is used for determining the default size of
objects and components.  This size value can be referred to using the
@code{Object_Size} attribute.  The phrase ‘is used’ here means that it is
the basis of the determination of the size.  The backend is free to
pad this up if necessary for efficiency, e.g., an 8-bit stand-alone
character might be stored in 32 bits on a machine with no efficient
byte access instructions such as the Alpha.

The default rules for the value of @code{Object_Size} for
discrete types are as follows:


@itemize *

@item 
The @code{Object_Size} for base subtypes reflect the natural hardware
size in bits (run the compiler with `-gnatS' to find those values
for numeric types). Enumeration types and fixed-point base subtypes have
8, 16, 32, or 64 bits for this size, depending on the range of values
to be stored.

@item 
The @code{Object_Size} of a subtype is the same as the
@code{Object_Size} of
the type from which it is obtained.

@item 
The @code{Object_Size} of a derived base type is copied from the parent
base type, and the @code{Object_Size} of a derived first subtype is copied
from the parent first subtype.
@end itemize

The @code{Value_Size} attribute
is the (minimum) number of bits required to store a value
of the type.
This value is used to determine how tightly to pack
records or arrays with components of this type, and also affects
the semantics of unchecked conversion (unchecked conversions where
the @code{Value_Size} values differ generate a warning, and are potentially
target dependent).

The default rules for the value of @code{Value_Size} are as follows:


@itemize *

@item 
The @code{Value_Size} for a base subtype is the minimum number of bits
required to store all values of the type (including the sign bit
only if negative values are possible).

@item 
If a subtype statically matches the first subtype of a given type, then it has
by default the same @code{Value_Size} as the first subtype.  (This is a
consequence of RM 13.1(14): “if two subtypes statically match,
then their subtype-specific aspects are the same”.)

@item 
All other subtypes have a @code{Value_Size} corresponding to the minimum
number of bits required to store all values of the subtype.  For
dynamic bounds, it is assumed that the value can range down or up
to the corresponding bound of the ancestor
@end itemize

The RM defined attribute @code{Size} corresponds to the
@code{Value_Size} attribute.

The @code{Size} attribute may be defined for a first-named subtype.  This sets
the @code{Value_Size} of
the first-named subtype to the given value, and the
@code{Object_Size} of this first-named subtype to the given value padded up
to an appropriate boundary.  It is a consequence of the default rules
above that this @code{Object_Size} will apply to all further subtypes.  On the
other hand, @code{Value_Size} is affected only for the first subtype, any
dynamic subtypes obtained from it directly, and any statically matching
subtypes.  The @code{Value_Size} of any other static subtypes is not affected.

@code{Value_Size} and
@code{Object_Size} may be explicitly set for any subtype using
an attribute definition clause.  Note that the use of these attributes
can cause the RM 13.1(14) rule to be violated.  If two access types
reference aliased objects whose subtypes have differing @code{Object_Size}
values as a result of explicit attribute definition clauses, then it
is illegal to convert from one access subtype to the other. For a more
complete description of this additional legality rule, see the
description of the @code{Object_Size} attribute.

To get a feel for the difference, consider the following examples (note
that in each case the base is @code{Short_Short_Integer} with a size of 8):


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxx} 
@headitem

Type or subtype declaration

@tab

Object_Size

@tab

Value_Size

@item

@code{type X1 is range 0 .. 5;}

@tab

8

@tab

3

@item

@code{type X2 is range 0 .. 5;
for X2'Size use 12;}

@tab

16

@tab

12

@item

@code{subtype X3 is X2 range 0 .. 3;}

@tab

16

@tab

2

@item

@code{subtype X4 is X2'Base range 0 .. 10;}

@tab

8

@tab

4

@item

@code{Dynamic : X2'Base range -64 .. +63;}

@tab

@tab

@item

@code{subtype X5 is X2 range 0 .. Dynamic;}

@tab

16

@tab

3*

@item

@code{subtype X6 is X2'Base range 0 .. Dynamic;}

@tab

8

@tab

7*

@end multitable


Note: the entries marked ‘*’ are not actually specified by the Ada
Reference Manual, which has nothing to say about size in the dynamic
case. What GNAT does is to allocate sufficient bits to accommodate any
possible dynamic values for the bounds at run-time.

So far, so good, but GNAT has to obey the RM rules, so the question is
under what conditions must the RM @code{Size} be used.
The following is a list
of the occasions on which the RM @code{Size} must be used:


@itemize *

@item 
Component size for packed arrays or records

@item 
Value of the attribute @code{Size} for a type

@item 
Warning about sizes not matching for unchecked conversion
@end itemize

For record types, the @code{Object_Size} is always a multiple of the
alignment of the type (this is true for all types). In some cases the
@code{Value_Size} can be smaller. Consider:

@example
type R is record
  X : Integer;
  Y : Character;
end record;
@end example

On a typical 32-bit architecture, the X component will occupy four bytes
and the Y component will occupy one byte, for a total of 5 bytes. As a
result @code{R'Value_Size} will be 40 (bits) since this is the minimum size
required to store a value of this type. For example, it is permissible
to have a component of type R in an array whose component size is
specified to be 40 bits.

However, @code{R'Object_Size} will be 64 (bits). The difference is due to
the alignment requirement for objects of the record type. The X
component will require four-byte alignment because that is what type
Integer requires, whereas the Y component, a Character, will only
require 1-byte alignment. Since the alignment required for X is the
greatest of all the components’ alignments, that is the alignment
required for the enclosing record type, i.e., 4 bytes or 32 bits. As
indicated above, the actual object size must be rounded up so that it is
a multiple of the alignment value. Therefore, 40 bits rounded up to the
next multiple of 32 yields 64 bits.

For all other types, the @code{Object_Size}
and @code{Value_Size} are the same (and equivalent to the RM attribute @code{Size}).
Only @code{Size} may be specified for such types.

Note that @code{Value_Size} can be used to force biased representation
for a particular subtype. Consider this example:

@example
type R is (A, B, C, D, E, F);
subtype RAB is R range A .. B;
subtype REF is R range E .. F;
@end example

By default, @code{RAB}
has a size of 1 (sufficient to accommodate the representation
of @code{A} and @code{B}, 0 and 1), and @code{REF}
has a size of 3 (sufficient to accommodate the representation
of @code{E} and @code{F}, 4 and 5). But if we add the
following @code{Value_Size} attribute definition clause:

@example
for REF'Value_Size use 1;
@end example

then biased representation is forced for @code{REF},
and 0 will represent @code{E} and 1 will represent @code{F}.
A warning is issued when a @code{Value_Size} attribute
definition clause forces biased representation. This
warning can be turned off using @code{-gnatw.B}.

@node Component_Size Clauses,Bit_Order Clauses,Value_Size and Object_Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas component-size-clauses}@anchor{2b0}@anchor{gnat_rm/representation_clauses_and_pragmas id8}@anchor{2b1}
@section Component_Size Clauses


@geindex Component_Size Clause

Normally, the value specified in a component size clause must be consistent
with the subtype of the array component with regard to size and alignment.
In other words, the value specified must be at least equal to the size
of this subtype, and must be a multiple of the alignment value.

In addition, component size clauses are allowed which cause the array
to be packed, by specifying a smaller value.  A first case is for
component size values in the range 1 through 63 on 32-bit targets,
and 1 through 127 on 64-bit targets.  The value specified may not
be smaller than the Size of the subtype.  GNAT will accurately
honor all packing requests in this range.  For example, if we have:

@example
type r is array (1 .. 8) of Natural;
for r'Component_Size use 31;
@end example

then the resulting array has a length of 31 bytes (248 bits = 8 * 31).
Of course access to the components of such an array is considerably
less efficient than if the natural component size of 32 is used.
A second case is when the subtype of the component is a record type
padded because of its default alignment.  For example, if we have:

@example
type r is record
  i : Integer;
  j : Integer;
  b : Boolean;
end record;

type a is array (1 .. 8) of r;
for a'Component_Size use 72;
@end example

then the resulting array has a length of 72 bytes, instead of 96 bytes
if the alignment of the record (4) was obeyed.

Note that there is no point in giving both a component size clause
and a pragma Pack for the same array type. if such duplicate
clauses are given, the pragma Pack will be ignored.

@node Bit_Order Clauses,Effect of Bit_Order on Byte Ordering,Component_Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas bit-order-clauses}@anchor{2b2}@anchor{gnat_rm/representation_clauses_and_pragmas id9}@anchor{2b3}
@section Bit_Order Clauses


@geindex Bit_Order Clause

@geindex bit ordering

@geindex ordering
@geindex of bits

For record subtypes, GNAT permits the specification of the @code{Bit_Order}
attribute.  The specification may either correspond to the default bit
order for the target, in which case the specification has no effect and
places no additional restrictions, or it may be for the non-standard
setting (that is the opposite of the default).

In the case where the non-standard value is specified, the effect is
to renumber bits within each byte, but the ordering of bytes is not
affected.  There are certain
restrictions placed on component clauses as follows:


@itemize *

@item 
Components fitting within a single storage unit.

These are unrestricted, and the effect is merely to renumber bits.  For
example if we are on a little-endian machine with @code{Low_Order_First}
being the default, then the following two declarations have exactly
the same effect:

@example
type R1 is record
   A : Boolean;
   B : Integer range 1 .. 120;
end record;

for R1 use record
   A at 0 range 0 .. 0;
   B at 0 range 1 .. 7;
end record;

type R2 is record
   A : Boolean;
   B : Integer range 1 .. 120;
end record;

for R2'Bit_Order use High_Order_First;

for R2 use record
   A at 0 range 7 .. 7;
   B at 0 range 0 .. 6;
end record;
@end example

The useful application here is to write the second declaration with the
@code{Bit_Order} attribute definition clause, and know that it will be treated
the same, regardless of whether the target is little-endian or big-endian.

@item 
Components occupying an integral number of bytes.

These are components that exactly fit in two or more bytes.  Such component
declarations are allowed, but have no effect, since it is important to realize
that the @code{Bit_Order} specification does not affect the ordering of bytes.
In particular, the following attempt at getting an endian-independent integer
does not work:

@example
type R2 is record
   A : Integer;
end record;

for R2'Bit_Order use High_Order_First;

for R2 use record
   A at 0 range 0 .. 31;
end record;
@end example

This declaration will result in a little-endian integer on a
little-endian machine, and a big-endian integer on a big-endian machine.
If byte flipping is required for interoperability between big- and
little-endian machines, this must be explicitly programmed.  This capability
is not provided by @code{Bit_Order}.

@item 
Components that are positioned across byte boundaries.

but do not occupy an integral number of bytes.  Given that bytes are not
reordered, such fields would occupy a non-contiguous sequence of bits
in memory, requiring non-trivial code to reassemble.  They are for this
reason not permitted, and any component clause specifying such a layout
will be flagged as illegal by GNAT.
@end itemize

Since the misconception that Bit_Order automatically deals with all
endian-related incompatibilities is a common one, the specification of
a component field that is an integral number of bytes will always
generate a warning.  This warning may be suppressed using @code{pragma Warnings (Off)}
if desired.  The following section contains additional
details regarding the issue of byte ordering.

@node Effect of Bit_Order on Byte Ordering,Pragma Pack for Arrays,Bit_Order Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas effect-of-bit-order-on-byte-ordering}@anchor{2b4}@anchor{gnat_rm/representation_clauses_and_pragmas id10}@anchor{2b5}
@section Effect of Bit_Order on Byte Ordering


@geindex byte ordering

@geindex ordering
@geindex of bytes

In this section we will review the effect of the @code{Bit_Order} attribute
definition clause on byte ordering.  Briefly, it has no effect at all, but
a detailed example will be helpful.  Before giving this
example, let us review the precise
definition of the effect of defining @code{Bit_Order}.  The effect of a
non-standard bit order is described in section 13.5.3 of the Ada
Reference Manual:

@quotation

“2   A bit ordering is a method of interpreting the meaning of
the storage place attributes.”
@end quotation

To understand the precise definition of storage place attributes in
this context, we visit section 13.5.1 of the manual:

@quotation

“13   A record_representation_clause (without the mod_clause)
specifies the layout.  The storage place attributes (see 13.5.2)
are taken from the values of the position, first_bit, and last_bit
expressions after normalizing those values so that first_bit is
less than Storage_Unit.”
@end quotation

The critical point here is that storage places are taken from
the values after normalization, not before.  So the @code{Bit_Order}
interpretation applies to normalized values.  The interpretation
is described in the later part of the 13.5.3 paragraph:

@quotation

“2   A bit ordering is a method of interpreting the meaning of
the storage place attributes.  High_Order_First (known in the
vernacular as ‘big endian’) means that the first bit of a
storage element (bit 0) is the most significant bit (interpreting
the sequence of bits that represent a component as an unsigned
integer value).  Low_Order_First (known in the vernacular as
‘little endian’) means the opposite: the first bit is the
least significant.”
@end quotation

Note that the numbering is with respect to the bits of a storage
unit.  In other words, the specification affects only the numbering
of bits within a single storage unit.

We can make the effect clearer by giving an example.

Suppose that we have an external device which presents two bytes, the first
byte presented, which is the first (low addressed byte) of the two byte
record is called Master, and the second byte is called Slave.

The left most (most significant) bit is called Control for each byte, and
the remaining 7 bits are called V1, V2, … V7, where V7 is the rightmost
(least significant) bit.

On a big-endian machine, we can write the following representation clause

@example
type Data is record
   Master_Control : Bit;
   Master_V1      : Bit;
   Master_V2      : Bit;
   Master_V3      : Bit;
   Master_V4      : Bit;
   Master_V5      : Bit;
   Master_V6      : Bit;
   Master_V7      : Bit;
   Slave_Control  : Bit;
   Slave_V1       : Bit;
   Slave_V2       : Bit;
   Slave_V3       : Bit;
   Slave_V4       : Bit;
   Slave_V5       : Bit;
   Slave_V6       : Bit;
   Slave_V7       : Bit;
end record;

for Data use record
   Master_Control at 0 range 0 .. 0;
   Master_V1      at 0 range 1 .. 1;
   Master_V2      at 0 range 2 .. 2;
   Master_V3      at 0 range 3 .. 3;
   Master_V4      at 0 range 4 .. 4;
   Master_V5      at 0 range 5 .. 5;
   Master_V6      at 0 range 6 .. 6;
   Master_V7      at 0 range 7 .. 7;
   Slave_Control  at 1 range 0 .. 0;
   Slave_V1       at 1 range 1 .. 1;
   Slave_V2       at 1 range 2 .. 2;
   Slave_V3       at 1 range 3 .. 3;
   Slave_V4       at 1 range 4 .. 4;
   Slave_V5       at 1 range 5 .. 5;
   Slave_V6       at 1 range 6 .. 6;
   Slave_V7       at 1 range 7 .. 7;
end record;
@end example

Now if we move this to a little endian machine, then the bit ordering within
the byte is backwards, so we have to rewrite the record rep clause as:

@example
for Data use record
   Master_Control at 0 range 7 .. 7;
   Master_V1      at 0 range 6 .. 6;
   Master_V2      at 0 range 5 .. 5;
   Master_V3      at 0 range 4 .. 4;
   Master_V4      at 0 range 3 .. 3;
   Master_V5      at 0 range 2 .. 2;
   Master_V6      at 0 range 1 .. 1;
   Master_V7      at 0 range 0 .. 0;
   Slave_Control  at 1 range 7 .. 7;
   Slave_V1       at 1 range 6 .. 6;
   Slave_V2       at 1 range 5 .. 5;
   Slave_V3       at 1 range 4 .. 4;
   Slave_V4       at 1 range 3 .. 3;
   Slave_V5       at 1 range 2 .. 2;
   Slave_V6       at 1 range 1 .. 1;
   Slave_V7       at 1 range 0 .. 0;
end record;
@end example

It is a nuisance to have to rewrite the clause, especially if
the code has to be maintained on both machines.  However,
this is a case that we can handle with the
@code{Bit_Order} attribute if it is implemented.
Note that the implementation is not required on byte addressed
machines, but it is indeed implemented in GNAT.
This means that we can simply use the
first record clause, together with the declaration

@example
for Data'Bit_Order use High_Order_First;
@end example

and the effect is what is desired, namely the layout is exactly the same,
independent of whether the code is compiled on a big-endian or little-endian
machine.

The important point to understand is that byte ordering is not affected.
A @code{Bit_Order} attribute definition never affects which byte a field
ends up in, only where it ends up in that byte.
To make this clear, let us rewrite the record rep clause of the previous
example as:

@example
for Data'Bit_Order use High_Order_First;
for Data use record
   Master_Control at 0 range  0 .. 0;
   Master_V1      at 0 range  1 .. 1;
   Master_V2      at 0 range  2 .. 2;
   Master_V3      at 0 range  3 .. 3;
   Master_V4      at 0 range  4 .. 4;
   Master_V5      at 0 range  5 .. 5;
   Master_V6      at 0 range  6 .. 6;
   Master_V7      at 0 range  7 .. 7;
   Slave_Control  at 0 range  8 .. 8;
   Slave_V1       at 0 range  9 .. 9;
   Slave_V2       at 0 range 10 .. 10;
   Slave_V3       at 0 range 11 .. 11;
   Slave_V4       at 0 range 12 .. 12;
   Slave_V5       at 0 range 13 .. 13;
   Slave_V6       at 0 range 14 .. 14;
   Slave_V7       at 0 range 15 .. 15;
end record;
@end example

This is exactly equivalent to saying (a repeat of the first example):

@example
for Data'Bit_Order use High_Order_First;
for Data use record
   Master_Control at 0 range 0 .. 0;
   Master_V1      at 0 range 1 .. 1;
   Master_V2      at 0 range 2 .. 2;
   Master_V3      at 0 range 3 .. 3;
   Master_V4      at 0 range 4 .. 4;
   Master_V5      at 0 range 5 .. 5;
   Master_V6      at 0 range 6 .. 6;
   Master_V7      at 0 range 7 .. 7;
   Slave_Control  at 1 range 0 .. 0;
   Slave_V1       at 1 range 1 .. 1;
   Slave_V2       at 1 range 2 .. 2;
   Slave_V3       at 1 range 3 .. 3;
   Slave_V4       at 1 range 4 .. 4;
   Slave_V5       at 1 range 5 .. 5;
   Slave_V6       at 1 range 6 .. 6;
   Slave_V7       at 1 range 7 .. 7;
end record;
@end example

Why are they equivalent? Well take a specific field, the @code{Slave_V2}
field.  The storage place attributes are obtained by normalizing the
values given so that the @code{First_Bit} value is less than 8.  After
normalizing the values (0,10,10) we get (1,2,2) which is exactly what
we specified in the other case.

Now one might expect that the @code{Bit_Order} attribute might affect
bit numbering within the entire record component (two bytes in this
case, thus affecting which byte fields end up in), but that is not
the way this feature is defined, it only affects numbering of bits,
not which byte they end up in.

Consequently it never makes sense to specify a starting bit number
greater than 7 (for a byte addressable field) if an attribute
definition for @code{Bit_Order} has been given, and indeed it
may be actively confusing to specify such a value, so the compiler
generates a warning for such usage.

If you do need to control byte ordering then appropriate conditional
values must be used.  If in our example, the slave byte came first on
some machines we might write:

@example
Master_Byte_First constant Boolean := ...;

Master_Byte : constant Natural :=
                1 - Boolean'Pos (Master_Byte_First);
Slave_Byte  : constant Natural :=
                Boolean'Pos (Master_Byte_First);

for Data'Bit_Order use High_Order_First;
for Data use record
   Master_Control at Master_Byte range 0 .. 0;
   Master_V1      at Master_Byte range 1 .. 1;
   Master_V2      at Master_Byte range 2 .. 2;
   Master_V3      at Master_Byte range 3 .. 3;
   Master_V4      at Master_Byte range 4 .. 4;
   Master_V5      at Master_Byte range 5 .. 5;
   Master_V6      at Master_Byte range 6 .. 6;
   Master_V7      at Master_Byte range 7 .. 7;
   Slave_Control  at Slave_Byte  range 0 .. 0;
   Slave_V1       at Slave_Byte  range 1 .. 1;
   Slave_V2       at Slave_Byte  range 2 .. 2;
   Slave_V3       at Slave_Byte  range 3 .. 3;
   Slave_V4       at Slave_Byte  range 4 .. 4;
   Slave_V5       at Slave_Byte  range 5 .. 5;
   Slave_V6       at Slave_Byte  range 6 .. 6;
   Slave_V7       at Slave_Byte  range 7 .. 7;
end record;
@end example

Now to switch between machines, all that is necessary is
to set the boolean constant @code{Master_Byte_First} in
an appropriate manner.

@node Pragma Pack for Arrays,Pragma Pack for Records,Effect of Bit_Order on Byte Ordering,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id11}@anchor{2b6}@anchor{gnat_rm/representation_clauses_and_pragmas pragma-pack-for-arrays}@anchor{2b7}
@section Pragma Pack for Arrays


@geindex Pragma Pack (for arrays)

Pragma @code{Pack} applied to an array has an effect that depends upon whether the
component type is `packable'.  For a component type to be `packable', it must
be one of the following cases:


@itemize *

@item 
Any elementary type.

@item 
Any small packed array type with a static size.

@item 
Any small simple record type with a static size.
@end itemize

For all these cases, if the component subtype size is in the range
1 through 63 on 32-bit targets, and 1 through 127 on 64-bit targets,
then the effect of the pragma @code{Pack} is exactly as though a
component size were specified giving the component subtype size.

All other types are non-packable, they occupy an integral number of storage
units and the only effect of pragma Pack is to remove alignment gaps.

For example if we have:

@example
type r is range 0 .. 17;

type ar is array (1 .. 8) of r;
pragma Pack (ar);
@end example

Then the component size of @code{ar} will be set to 5 (i.e., to @code{r'size},
and the size of the array @code{ar} will be exactly 40 bits).

Note that in some cases this rather fierce approach to packing can produce
unexpected effects.  For example, in Ada 95 and Ada 2005,
subtype @code{Natural} typically has a size of 31, meaning that if you
pack an array of @code{Natural}, you get 31-bit
close packing, which saves a few bits, but results in far less efficient
access.  Since many other Ada compilers will ignore such a packing request,
GNAT will generate a warning on some uses of pragma @code{Pack} that it guesses
might not be what is intended.  You can easily remove this warning by
using an explicit @code{Component_Size} setting instead, which never generates
a warning, since the intention of the programmer is clear in this case.

GNAT treats packed arrays in one of two ways.  If the size of the array is
known at compile time and is at most 64 bits on 32-bit targets, and at most
128 bits on 64-bit targets, then internally the array is represented as a
single modular type, of exactly the appropriate number of bits.  If the
length is greater than 64 bits on 32-bit targets, and greater than 128
bits on 64-bit targets, or is not known at compile time, then the packed
array is represented as an array of bytes, and its length is always a
multiple of 8 bits.

Note that to represent a packed array as a modular type, the alignment must
be suitable for the modular type involved. For example, on typical machines
a 32-bit packed array will be represented by a 32-bit modular integer with
an alignment of four bytes. If you explicitly override the default alignment
with an alignment clause that is too small, the modular representation
cannot be used. For example, consider the following set of declarations:

@example
type R is range 1 .. 3;
type S is array (1 .. 31) of R;
for S'Component_Size use 2;
for S'Size use 62;
for S'Alignment use 1;
@end example

If the alignment clause were not present, then a 62-bit modular
representation would be chosen (typically with an alignment of 4 or 8
bytes depending on the target). But the default alignment is overridden
with the explicit alignment clause. This means that the modular
representation cannot be used, and instead the array of bytes
representation must be used, meaning that the length must be a multiple
of 8. Thus the above set of declarations will result in a diagnostic
rejecting the size clause and noting that the minimum size allowed is 64.

@geindex Pragma Pack (for type Natural)

@geindex Pragma Pack warning

One special case that is worth noting occurs when the base type of the
component size is 8/16/32 and the subtype is one bit less. Notably this
occurs with subtype @code{Natural}. Consider:

@example
type Arr is array (1 .. 32) of Natural;
pragma Pack (Arr);
@end example

In all commonly used Ada 83 compilers, this pragma Pack would be ignored,
since typically @code{Natural'Size} is 32 in Ada 83, and in any case most
Ada 83 compilers did not attempt 31 bit packing.

In Ada 95 and Ada 2005, @code{Natural'Size} is required to be 31. Furthermore,
GNAT really does pack 31-bit subtype to 31 bits. This may result in a
substantial unintended performance penalty when porting legacy Ada 83 code.
To help prevent this, GNAT generates a warning in such cases. If you really
want 31 bit packing in a case like this, you can set the component size
explicitly:

@example
type Arr is array (1 .. 32) of Natural;
for Arr'Component_Size use 31;
@end example

Here 31-bit packing is achieved as required, and no warning is generated,
since in this case the programmer intention is clear.

@node Pragma Pack for Records,Record Representation Clauses,Pragma Pack for Arrays,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id12}@anchor{2b8}@anchor{gnat_rm/representation_clauses_and_pragmas pragma-pack-for-records}@anchor{2b9}
@section Pragma Pack for Records


@geindex Pragma Pack (for records)

Pragma @code{Pack} applied to a record will pack the components to reduce
wasted space from alignment gaps and by reducing the amount of space
taken by components.  We distinguish between `packable' components and
`non-packable' components.
Components of the following types are considered packable:


@itemize *

@item 
Components of an elementary type are packable unless they are aliased,
independent or atomic.

@item 
Small packed arrays, where the size is statically known, are represented
internally as modular integers, and so they are also packable.

@item 
Small simple records, where the size is statically known, are also packable.
@end itemize

For all these cases, if the @code{'Size} value is in the range 1 through 64 on
32-bit targets, and 1 through 128 on 64-bit targets, the components occupy
the exact number of bits corresponding to this value and are packed with no
padding bits, i.e. they can start on an arbitrary bit boundary.

All other types are non-packable, they occupy an integral number of storage
units and the only effect of pragma @code{Pack} is to remove alignment gaps.

For example, consider the record

@example
type Rb1 is array (1 .. 13) of Boolean;
pragma Pack (Rb1);

type Rb2 is array (1 .. 65) of Boolean;
pragma Pack (Rb2);

type AF is new Float with Atomic;

type X2 is record
   L1 : Boolean;
   L2 : Duration;
   L3 : AF;
   L4 : Boolean;
   L5 : Rb1;
   L6 : Rb2;
end record;
pragma Pack (X2);
@end example

The representation for the record @code{X2} is as follows on 32-bit targets:

@example
for X2'Size use 224;
for X2 use record
   L1 at  0 range  0 .. 0;
   L2 at  0 range  1 .. 64;
   L3 at 12 range  0 .. 31;
   L4 at 16 range  0 .. 0;
   L5 at 16 range  1 .. 13;
   L6 at 18 range  0 .. 71;
end record;
@end example

Studying this example, we see that the packable fields @code{L1}
and @code{L2} are of length equal to their sizes, and placed at
specific bit boundaries (and not byte boundaries) to eliminate
padding.  But @code{L3} is of a non-packable float type (because
it is aliased), so it is on the next appropriate alignment boundary.

The next two fields are fully packable, so @code{L4} and @code{L5} are
minimally packed with no gaps.  However, type @code{Rb2} is a packed
array that is longer than 64 bits, so it is itself non-packable on
32-bit targets.  Thus the @code{L6} field is aligned to the next byte
boundary, and takes an integral number of bytes, i.e., 72 bits.

@node Record Representation Clauses,Handling of Records with Holes,Pragma Pack for Records,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id13}@anchor{2ba}@anchor{gnat_rm/representation_clauses_and_pragmas record-representation-clauses}@anchor{2bb}
@section Record Representation Clauses


@geindex Record Representation Clause

Record representation clauses may be given for all record types, including
types obtained by record extension.  Component clauses are allowed for any
static component.  The restrictions on component clauses depend on the type
of the component.

@geindex Component Clause

For all components of an elementary type, the only restriction on component
clauses is that the size must be at least the @code{'Size} value of the type
(actually the Value_Size).  There are no restrictions due to alignment,
and such components may freely cross storage boundaries.

Packed arrays with a size up to and including 64 bits on 32-bit targets,
and up to and including 128 bits on 64-bit targets, are represented
internally using a modular type with the appropriate number of bits, and
thus the same lack of restriction applies.  For example, if you declare:

@example
type R is array (1 .. 49) of Boolean;
pragma Pack (R);
for R'Size use 49;
@end example

then a component clause for a component of type @code{R} may start on any
specified bit boundary, and may specify a value of 49 bits or greater.

For packed bit arrays that are longer than 64 bits on 32-bit targets,
and longer than 128 bits on 64-bit targets, there are two cases. If the
component size is a power of 2 (1,2,4,8,16,32,64 bits), including the
important case of single bits or boolean values, then there are no
limitations on placement of such components, and they may start and
end at arbitrary bit boundaries.

If the component size is not a power of 2 (e.g., 3 or 5), then an array
of this type must always be placed on on a storage unit (byte) boundary
and occupy an integral number of storage units (bytes). Any component
clause that does not meet this requirement will be rejected.

Any aliased component, or component of an aliased type, must have its
normal alignment and size. A component clause that does not meet this
requirement will be rejected.

The tag field of a tagged type always occupies an address sized field at
the start of the record.  No component clause may attempt to overlay this
tag. When a tagged type appears as a component, the tag field must have
proper alignment

In the case of a record extension @code{T1}, of a type @code{T}, no component
clause applied to the type @code{T1} can specify a storage location that
would overlap the first @code{T'Object_Size} bits of the record.

For all other component types, including non-bit-packed arrays,
the component can be placed at an arbitrary bit boundary,
so for example, the following is permitted:

@example
type R is array (1 .. 10) of Boolean;
for R'Size use 80;

type Q is record
   G, H : Boolean;
   L, M : R;
end record;

for Q use record
   G at 0 range  0 ..   0;
   H at 0 range  1 ..   1;
   L at 0 range  2 ..  81;
   R at 0 range 82 .. 161;
end record;
@end example

@node Handling of Records with Holes,Enumeration Clauses,Record Representation Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas handling-of-records-with-holes}@anchor{2bc}@anchor{gnat_rm/representation_clauses_and_pragmas id14}@anchor{2bd}
@section Handling of Records with Holes


@geindex Handling of Records with Holes

As a result of alignment considerations, records may contain “holes”
or gaps which do not correspond to the data bits of any of the components.
Record representation clauses can also result in holes in records.

GNAT does not attempt to clear these holes, so in record objects,
they should be considered to hold undefined rubbish. The generated
equality routine just tests components so does not access these
undefined bits, and assignment and copy operations may or may not
preserve the contents of these holes (for assignments, the holes
in the target will in practice contain either the bits that are
present in the holes in the source, or the bits that were present
in the target before the assignment).

If it is necessary to ensure that holes in records have all zero
bits, then record objects for which this initialization is desired
should be explicitly set to all zero values using Unchecked_Conversion
or address overlays. For example

@example
type HRec is record
   C : Character;
   I : Integer;
end record;
@end example

On typical machines, integers need to be aligned on a four-byte
boundary, resulting in three bytes of undefined rubbish following
the 8-bit field for C. To ensure that the hole in a variable of
type HRec is set to all zero bits,
you could for example do:

@example
type Base is record
   Dummy1, Dummy2 : Integer := 0;
end record;

BaseVar : Base;
RealVar : Hrec;
for RealVar'Address use BaseVar'Address;
@end example

Now the 8-bytes of the value of RealVar start out containing all zero
bits. A safer approach is to just define dummy fields, avoiding the
holes, as in:

@example
type HRec is record
   C      : Character;
   Dummy1 : Short_Short_Integer := 0;
   Dummy2 : Short_Short_Integer := 0;
   Dummy3 : Short_Short_Integer := 0;
   I      : Integer;
end record;
@end example

And to make absolutely sure that the intent of this is followed, you
can use representation clauses:

@example
for Hrec use record
   C      at 0 range 0 .. 7;
   Dummy1 at 1 range 0 .. 7;
   Dummy2 at 2 range 0 .. 7;
   Dummy3 at 3 range 0 .. 7;
   I      at 4 range 0 .. 31;
end record;
for Hrec'Size use 64;
@end example

@node Enumeration Clauses,Address Clauses,Handling of Records with Holes,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas enumeration-clauses}@anchor{2be}@anchor{gnat_rm/representation_clauses_and_pragmas id15}@anchor{2bf}
@section Enumeration Clauses


The only restriction on enumeration clauses is that the range of values
must be representable.  For the signed case, if one or more of the
representation values are negative, all values must be in the range:

@example
System.Min_Int .. System.Max_Int
@end example

For the unsigned case, where all values are nonnegative, the values must
be in the range:

@example
0 .. System.Max_Binary_Modulus;
@end example

A `confirming' representation clause is one in which the values range
from 0 in sequence, i.e., a clause that confirms the default representation
for an enumeration type.
Such a confirming representation
is permitted by these rules, and is specially recognized by the compiler so
that no extra overhead results from the use of such a clause.

If an array has an index type which is an enumeration type to which an
enumeration clause has been applied, then the array is stored in a compact
manner.  Consider the declarations:

@example
type r is (A, B, C);
for r use (A => 1, B => 5, C => 10);
type t is array (r) of Character;
@end example

The array type t corresponds to a vector with exactly three elements and
has a default size equal to @code{3*Character'Size}.  This ensures efficient
use of space, but means that accesses to elements of the array will incur
the overhead of converting representation values to the corresponding
positional values, (i.e., the value delivered by the @code{Pos} attribute).

@node Address Clauses,Use of Address Clauses for Memory-Mapped I/O,Enumeration Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas address-clauses}@anchor{2c0}@anchor{gnat_rm/representation_clauses_and_pragmas id16}@anchor{2c1}
@section Address Clauses


@geindex Address Clause

The reference manual allows a general restriction on representation clauses,
as found in RM 13.1(22):

@quotation

“An implementation need not support representation
items containing nonstatic expressions, except that
an implementation should support a representation item
for a given entity if each nonstatic expression in the
representation item is a name that statically denotes
a constant declared before the entity.”
@end quotation

In practice this is applicable only to address clauses, since this is the
only case in which a nonstatic expression is permitted by the syntax.  As
the AARM notes in sections 13.1 (22.a-22.h):

@quotation

22.a   Reason: This is to avoid the following sort of thing:

22.b        X : Integer := F(…);
Y : Address := G(…);
for X’Address use Y;

22.c   In the above, we have to evaluate the
initialization expression for X before we
know where to put the result.  This seems
like an unreasonable implementation burden.

22.d   The above code should instead be written
like this:

22.e        Y : constant Address := G(…);
X : Integer := F(…);
for X’Address use Y;

22.f   This allows the expression ‘Y’ to be safely
evaluated before X is created.

22.g   The constant could be a formal parameter of mode in.

22.h   An implementation can support other nonstatic
expressions if it wants to.  Expressions of type
Address are hardly ever static, but their value
might be known at compile time anyway in many
cases.
@end quotation

GNAT does indeed permit many additional cases of nonstatic expressions.  In
particular, if the type involved is elementary there are no restrictions
(since in this case, holding a temporary copy of the initialization value,
if one is present, is inexpensive).  In addition, if there is no implicit or
explicit initialization, then there are no restrictions.  GNAT will reject
only the case where all three of these conditions hold:


@itemize *

@item 
The type of the item is non-elementary (e.g., a record or array).

@item 
There is explicit or implicit initialization required for the object.
Note that access values are always implicitly initialized.

@item 
The address value is nonstatic.  Here GNAT is more permissive than the
RM, and allows the address value to be the address of a previously declared
stand-alone variable, as long as it does not itself have an address clause.

@example
Anchor  : Some_Initialized_Type;
Overlay : Some_Initialized_Type;
for Overlay'Address use Anchor'Address;
@end example

However, the prefix of the address clause cannot be an array component, or
a component of a discriminated record.
@end itemize

As noted above in section 22.h, address values are typically nonstatic.  In
particular the To_Address function, even if applied to a literal value, is
a nonstatic function call.  To avoid this minor annoyance, GNAT provides
the implementation defined attribute ‘To_Address.  The following two
expressions have identical values:

@geindex Attribute

@geindex To_Address

@example
To_Address (16#1234_0000#)
System'To_Address (16#1234_0000#);
@end example

except that the second form is considered to be a static expression, and
thus when used as an address clause value is always permitted.

Additionally, GNAT treats as static an address clause that is an
unchecked_conversion of a static integer value.  This simplifies the porting
of legacy code, and provides a portable equivalent to the GNAT attribute
@code{To_Address}.

Another issue with address clauses is the interaction with alignment
requirements.  When an address clause is given for an object, the address
value must be consistent with the alignment of the object (which is usually
the same as the alignment of the type of the object).  If an address clause
is given that specifies an inappropriately aligned address value, then the
program execution is erroneous.

Since this source of erroneous behavior can have unfortunate effects on
machines with strict alignment requirements, GNAT
checks (at compile time if possible, generating a warning, or at execution
time with a run-time check) that the alignment is appropriate.  If the
run-time check fails, then @code{Program_Error} is raised.  This run-time
check is suppressed if the GNAT check Alignment_Check is suppressed, or if
@code{pragma Restrictions (No_Elaboration_Code)} is in effect. It is also
suppressed by default on non-strict alignment machines (such as the x86).

In some cases, GNAT does not support an address specification (using either
form of aspect specification syntax) for the declaration of an object that has
an indefinite nominal subtype. An object declaration has an indefinite
nominal subtype if it takes its bounds (for an array type), discriminant
values (for a discriminated type whose discriminants lack defaults), or tag
(for a class-wide type) from its initial value, as in

@example
X : String := Some_Function_Call;
-- String has no constraint, so bounds for X come from function call
@end example

This restriction does not apply if the size of the object’s initial value is
known at compile time and the type of the object is not class-wide.

@geindex Export

An address clause cannot be given for an exported object.  More
understandably the real restriction is that objects with an address
clause cannot be exported.  This is because such variables are not
defined by the Ada program, so there is no external object to export.

@geindex Import

It is permissible to give an address clause and a pragma Import for the
same object.  In this case, the variable is not really defined by the
Ada program, so there is no external symbol to be linked.  The link name
and the external name are ignored in this case.  The reason that we allow this
combination is that it provides a useful idiom to avoid unwanted
initializations on objects with address clauses.

When an address clause is given for an object that has implicit or
explicit initialization, then by default initialization takes place.  This
means that the effect of the object declaration is to overwrite the
memory at the specified address.  This is almost always not what the
programmer wants, so GNAT will output a warning:

@example
with System;
package G is
   type R is record
      M : Integer := 0;
   end record;

   Ext : R;
   for Ext'Address use System'To_Address (16#1234_1234#);
       |
>>> warning: implicit initialization of "Ext" may
    modify overlaid storage
>>> warning: use pragma Import for "Ext" to suppress
    initialization (RM B(24))

end G;
@end example

As indicated by the warning message, the solution is to use a (dummy) pragma
Import to suppress this initialization.  The pragma tell the compiler that the
object is declared and initialized elsewhere.  The following package compiles
without warnings (and the initialization is suppressed):

@example
with System;
package G is
   type R is record
      M : Integer := 0;
   end record;

   Ext : R;
   for Ext'Address use System'To_Address (16#1234_1234#);
   pragma Import (Ada, Ext);
end G;
@end example

A final issue with address clauses involves their use for overlaying
variables, as in the following example:

@geindex Overlaying of objects

@example
A : Integer;
B : Integer;
for B'Address use A'Address;
@end example

or alternatively, using the form recommended by the RM:

@example
A    : Integer;
Addr : constant Address := A'Address;
B    : Integer;
for B'Address use Addr;
@end example

In both of these cases, @code{A} and @code{B} become aliased to one another
via the address clause. This use of address clauses to overlay
variables, achieving an effect similar to unchecked conversion
was erroneous in Ada 83, but in Ada 95 and Ada 2005
the effect is implementation defined. Furthermore, the
Ada RM specifically recommends that in a situation
like this, @code{B} should be subject to the following
implementation advice (RM 13.3(19)):

@quotation

“19  If the Address of an object is specified, or it is imported
or exported, then the implementation should not perform
optimizations based on assumptions of no aliases.”
@end quotation

GNAT follows this recommendation, and goes further by also applying
this recommendation to the overlaid variable (@code{A} in the above example)
in this case. This means that the overlay works “as expected”, in that
a modification to one of the variables will affect the value of the other.

More generally, GNAT interprets this recommendation conservatively for
address clauses: in the cases other than overlays, it considers that the
object is effectively subject to pragma @code{Volatile} and implements the
associated semantics.

Note that when address clause overlays are used in this way, there is an
issue of unintentional initialization, as shown by this example:

@example
package Overwrite_Record is
   type R is record
      A : Character := 'C';
      B : Character := 'A';
   end record;
   X : Short_Integer := 3;
   Y : R;
   for Y'Address use X'Address;
       |
>>> warning: default initialization of "Y" may
    modify "X", use pragma Import for "Y" to
    suppress initialization (RM B.1(24))

end Overwrite_Record;
@end example

Here the default initialization of @code{Y} will clobber the value
of @code{X}, which justifies the warning. The warning notes that
this effect can be eliminated by adding a @code{pragma Import}
which suppresses the initialization:

@example
package Overwrite_Record is
   type R is record
      A : Character := 'C';
      B : Character := 'A';
   end record;
   X : Short_Integer := 3;
   Y : R;
   for Y'Address use X'Address;
   pragma Import (Ada, Y);
end Overwrite_Record;
@end example

Note that the use of @code{pragma Initialize_Scalars} may cause variables to
be initialized when they would not otherwise have been in the absence
of the use of this pragma. This may cause an overlay to have this
unintended clobbering effect. The compiler avoids this for scalar
types, but not for composite objects (where in general the effect
of @code{Initialize_Scalars} is part of the initialization routine
for the composite object):

@example
pragma Initialize_Scalars;
with Ada.Text_IO;  use Ada.Text_IO;
procedure Overwrite_Array is
   type Arr is array (1 .. 5) of Integer;
   X : Arr := (others => 1);
   A : Arr;
   for A'Address use X'Address;
       |
>>> warning: default initialization of "A" may
    modify "X", use pragma Import for "A" to
    suppress initialization (RM B.1(24))

begin
   if X /= Arr'(others => 1) then
      Put_Line ("X was clobbered");
   else
      Put_Line ("X was not clobbered");
   end if;
end Overwrite_Array;
@end example

The above program generates the warning as shown, and at execution
time, prints @code{X was clobbered}. If the @code{pragma Import} is
added as suggested:

@example
pragma Initialize_Scalars;
with Ada.Text_IO;  use Ada.Text_IO;
procedure Overwrite_Array is
   type Arr is array (1 .. 5) of Integer;
   X : Arr := (others => 1);
   A : Arr;
   for A'Address use X'Address;
   pragma Import (Ada, A);
begin
   if X /= Arr'(others => 1) then
      Put_Line ("X was clobbered");
   else
      Put_Line ("X was not clobbered");
   end if;
end Overwrite_Array;
@end example

then the program compiles without the warning and when run will generate
the output @code{X was not clobbered}.

@node Use of Address Clauses for Memory-Mapped I/O,Effect of Convention on Representation,Address Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id17}@anchor{2c2}@anchor{gnat_rm/representation_clauses_and_pragmas use-of-address-clauses-for-memory-mapped-i-o}@anchor{2c3}
@section Use of Address Clauses for Memory-Mapped I/O


@geindex Memory-mapped I/O

A common pattern is to use an address clause to map an atomic variable to
a location in memory that corresponds to a memory-mapped I/O operation or
operations, for example:

@example
type Mem_Word is record
   A,B,C,D : Byte;
end record;
pragma Atomic (Mem_Word);
for Mem_Word_Size use 32;

Mem : Mem_Word;
for Mem'Address use some-address;
...
Temp := Mem;
Temp.A := 32;
Mem := Temp;
@end example

For a full access (reference or modification) of the variable (Mem) in this
case, as in the above examples, GNAT guarantees that the entire atomic word
will be accessed, in accordance with the RM C.6(15) clause.

A problem arises with a component access such as:

@example
Mem.A := 32;
@end example

Note that the component A is not declared as atomic. This means that it is
not clear what this assignment means. It could correspond to full word read
and write as given in the first example, or on architectures that supported
such an operation it might be a single byte store instruction. The RM does
not have anything to say in this situation, and GNAT does not make any
guarantee. The code generated may vary from target to target. GNAT will issue
a warning in such a case:

@example
Mem.A := 32;
|
>>> warning: access to non-atomic component of atomic array,
    may cause unexpected accesses to atomic object
@end example

It is best to be explicit in this situation, by either declaring the
components to be atomic if you want the byte store, or explicitly writing
the full word access sequence if that is what the hardware requires.
Alternatively, if the full word access sequence is required, GNAT also
provides the pragma @code{Volatile_Full_Access} which can be used in lieu of
pragma @code{Atomic} and will give the additional guarantee.

@node Effect of Convention on Representation,Conventions and Anonymous Access Types,Use of Address Clauses for Memory-Mapped I/O,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas effect-of-convention-on-representation}@anchor{2c4}@anchor{gnat_rm/representation_clauses_and_pragmas id18}@anchor{2c5}
@section Effect of Convention on Representation


@geindex Convention
@geindex effect on representation

Normally the specification of a foreign language convention for a type or
an object has no effect on the chosen representation.  In particular, the
representation chosen for data in GNAT generally meets the standard system
conventions, and for example records are laid out in a manner that is
consistent with C.  This means that specifying convention C (for example)
has no effect.

There are three exceptions to this general rule:


@itemize *

@item 
`Convention Fortran and array subtypes'.

If pragma Convention Fortran is specified for an array subtype, then in
accordance with the implementation advice in section 3.6.2(11) of the
Ada Reference Manual, the array will be stored in a Fortran-compatible
column-major manner, instead of the normal default row-major order.

@item 
`Convention C and enumeration types'

GNAT normally stores enumeration types in 8, 16, or 32 bits as required
to accommodate all values of the type.  For example, for the enumeration
type declared by:

@example
type Color is (Red, Green, Blue);
@end example

8 bits is sufficient to store all values of the type, so by default, objects
of type @code{Color} will be represented using 8 bits.  However, normal C
convention is to use 32 bits for all enum values in C, since enum values
are essentially of type int.  If pragma @code{Convention C} is specified for an
Ada enumeration type, then the size is modified as necessary (usually to
32 bits) to be consistent with the C convention for enum values.

Note that this treatment applies only to types. If Convention C is given for
an enumeration object, where the enumeration type is not Convention C, then
Object_Size bits are allocated. For example, for a normal enumeration type,
with less than 256 elements, only 8 bits will be allocated for the object.
Since this may be a surprise in terms of what C expects, GNAT will issue a
warning in this situation. The warning can be suppressed by giving an explicit
size clause specifying the desired size.

@item 
`Convention C/Fortran and Boolean types'

In C, the usual convention for boolean values, that is values used for
conditions, is that zero represents false, and nonzero values represent
true.  In Ada, the normal convention is that two specific values, typically
0/1, are used to represent false/true respectively.

Fortran has a similar convention for @code{LOGICAL} values (any nonzero
value represents true).

To accommodate the Fortran and C conventions, if a pragma Convention specifies
C or Fortran convention for a derived Boolean, as in the following example:

@example
type C_Switch is new Boolean;
pragma Convention (C, C_Switch);
@end example

then the GNAT generated code will treat any nonzero value as true.  For truth
values generated by GNAT, the conventional value 1 will be used for True, but
when one of these values is read, any nonzero value is treated as True.
@end itemize

@node Conventions and Anonymous Access Types,Determining the Representations chosen by GNAT,Effect of Convention on Representation,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas conventions-and-anonymous-access-types}@anchor{2c6}@anchor{gnat_rm/representation_clauses_and_pragmas id19}@anchor{2c7}
@section Conventions and Anonymous Access Types


@geindex Anonymous access types

@geindex Convention for anonymous access types

The RM is not entirely clear on convention handling in a number of cases,
and in particular, it is not clear on the convention to be given to
anonymous access types in general, and in particular what is to be
done for the case of anonymous access-to-subprogram.

In GNAT, we decide that if an explicit Convention is applied
to an object or component, and its type is such an anonymous type,
then the convention will apply to this anonymous type as well. This
seems to make sense since it is anomalous in any case to have a
different convention for an object and its type, and there is clearly
no way to explicitly specify a convention for an anonymous type, since
it doesn’t have a name to specify!

Furthermore, we decide that if a convention is applied to a record type,
then this convention is inherited by any of its components that are of an
anonymous access type which do not have an explicitly specified convention.

The following program shows these conventions in action:

@example
package ConvComp is
   type Foo is range 1 .. 10;
   type T1 is record
      A : access function (X : Foo) return Integer;
      B : Integer;
   end record;
   pragma Convention (C, T1);

   type T2 is record
      A : access function (X : Foo) return Integer;
      pragma Convention  (C, A);
      B : Integer;
   end record;
   pragma Convention (COBOL, T2);

   type T3 is record
      A : access function (X : Foo) return Integer;
      pragma Convention  (COBOL, A);
      B : Integer;
   end record;
   pragma Convention (C, T3);

   type T4 is record
      A : access function (X : Foo) return Integer;
      B : Integer;
   end record;
   pragma Convention (COBOL, T4);

   function F (X : Foo) return Integer;
   pragma Convention (C, F);

   function F (X : Foo) return Integer is (13);

   TV1 : T1 := (F'Access, 12);  -- OK
   TV2 : T2 := (F'Access, 13);  -- OK

   TV3 : T3 := (F'Access, 13);  -- ERROR
                |
>>> subprogram "F" has wrong convention
>>> does not match access to subprogram declared at line 17
     38.    TV4 : T4 := (F'Access, 13);  -- ERROR
                |
>>> subprogram "F" has wrong convention
>>> does not match access to subprogram declared at line 24
     39. end ConvComp;
@end example

@node Determining the Representations chosen by GNAT,,Conventions and Anonymous Access Types,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas determining-the-representations-chosen-by-gnat}@anchor{2c8}@anchor{gnat_rm/representation_clauses_and_pragmas id20}@anchor{2c9}
@section Determining the Representations chosen by GNAT


@geindex Representation
@geindex determination of

@geindex -gnatR (gcc)

Although the descriptions in this section are intended to be complete, it is
often easier to simply experiment to see what GNAT accepts and what the
effect is on the layout of types and objects.

As required by the Ada RM, if a representation clause is not accepted, then
it must be rejected as illegal by the compiler.  However, when a
representation clause or pragma is accepted, there can still be questions
of what the compiler actually does.  For example, if a partial record
representation clause specifies the location of some components and not
others, then where are the non-specified components placed? Or if pragma
@code{Pack} is used on a record, then exactly where are the resulting
fields placed? The section on pragma @code{Pack} in this chapter can be
used to answer the second question, but it is often easier to just see
what the compiler does.

For this purpose, GNAT provides the option `-gnatR'.  If you compile
with this option, then the compiler will output information on the actual
representations chosen, in a format similar to source representation
clauses.  For example, if we compile the package:

@example
package q is
   type r (x : boolean) is tagged record
      case x is
         when True => S : String (1 .. 100);
         when False => null;
      end case;
   end record;

   type r2 is new r (false) with record
      y2 : integer;
   end record;

   for r2 use record
      y2 at 16 range 0 .. 31;
   end record;

   type x is record
      y : character;
   end record;

   type x1 is array (1 .. 10) of x;
   for x1'component_size use 11;

   type ia is access integer;

   type Rb1 is array (1 .. 13) of Boolean;
   pragma Pack (rb1);

   type Rb2 is array (1 .. 65) of Boolean;
   pragma Pack (rb2);

   type x2 is record
      l1 : Boolean;
      l2 : Duration;
      l3 : Float;
      l4 : Boolean;
      l5 : Rb1;
      l6 : Rb2;
   end record;
   pragma Pack (x2);
end q;
@end example

using the switch `-gnatR' we obtain the following output:

@example
Representation information for unit q
-------------------------------------

for r'Size use ??;
for r'Alignment use 4;
for r use record
   x    at 4 range  0 .. 7;
   _tag at 0 range  0 .. 31;
   s    at 5 range  0 .. 799;
end record;

for r2'Size use 160;
for r2'Alignment use 4;
for r2 use record
   x       at  4 range  0 .. 7;
   _tag    at  0 range  0 .. 31;
   _parent at  0 range  0 .. 63;
   y2      at 16 range  0 .. 31;
end record;

for x'Size use 8;
for x'Alignment use 1;
for x use record
   y at 0 range  0 .. 7;
end record;

for x1'Size use 112;
for x1'Alignment use 1;
for x1'Component_Size use 11;

for rb1'Size use 13;
for rb1'Alignment use 2;
for rb1'Component_Size use 1;

for rb2'Size use 72;
for rb2'Alignment use 1;
for rb2'Component_Size use 1;

for x2'Size use 224;
for x2'Alignment use 4;
for x2 use record
   l1 at  0 range  0 .. 0;
   l2 at  0 range  1 .. 64;
   l3 at 12 range  0 .. 31;
   l4 at 16 range  0 .. 0;
   l5 at 16 range  1 .. 13;
   l6 at 18 range  0 .. 71;
end record;
@end example

The Size values are actually the Object_Size, i.e., the default size that
will be allocated for objects of the type.
The @code{??} size for type r indicates that we have a variant record, and the
actual size of objects will depend on the discriminant value.

The Alignment values show the actual alignment chosen by the compiler
for each record or array type.

The record representation clause for type r shows where all fields
are placed, including the compiler generated tag field (whose location
cannot be controlled by the programmer).

The record representation clause for the type extension r2 shows all the
fields present, including the parent field, which is a copy of the fields
of the parent type of r2, i.e., r1.

The component size and size clauses for types rb1 and rb2 show
the exact effect of pragma @code{Pack} on these arrays, and the record
representation clause for type x2 shows how pragma @cite{Pack} affects
this record type.

In some cases, it may be useful to cut and paste the representation clauses
generated by the compiler into the original source to fix and guarantee
the actual representation to be used.

@node Standard Library Routines,The Implementation of Standard I/O,Representation Clauses and Pragmas,Top
@anchor{gnat_rm/standard_library_routines doc}@anchor{2ca}@anchor{gnat_rm/standard_library_routines id1}@anchor{2cb}@anchor{gnat_rm/standard_library_routines standard-library-routines}@anchor{f}
@chapter Standard Library Routines


The Ada Reference Manual contains in Annex A a full description of an
extensive set of standard library routines that can be used in any Ada
program, and which must be provided by all Ada compilers.  They are
analogous to the standard C library used by C programs.

GNAT implements all of the facilities described in annex A, and for most
purposes the description in the Ada Reference Manual, or appropriate Ada
text book, will be sufficient for making use of these facilities.

In the case of the input-output facilities,
@ref{10,,The Implementation of Standard I/O},
gives details on exactly how GNAT interfaces to the
file system.  For the remaining packages, the Ada Reference Manual
should be sufficient.  The following is a list of the packages included,
together with a brief description of the functionality that is provided.

For completeness, references are included to other predefined library
routines defined in other sections of the Ada Reference Manual (these are
cross-indexed from Annex A). For further details see the relevant
package declarations in the run-time library. In particular, a few units
are not implemented, as marked by the presence of pragma Unimplemented_Unit,
and in this case the package declaration contains comments explaining why
the unit is not implemented.


@table @asis

@item @code{Ada} `(A.2)'

This is a parent package for all the standard library packages.  It is
usually included implicitly in your program, and itself contains no
useful data or routines.

@item @code{Ada.Assertions} `(11.4.2)'

@code{Assertions} provides the @code{Assert} subprograms, and also
the declaration of the @code{Assertion_Error} exception.

@item @code{Ada.Asynchronous_Task_Control} `(D.11)'

@code{Asynchronous_Task_Control} provides low level facilities for task
synchronization. It is typically not implemented. See package spec for details.

@item @code{Ada.Calendar} `(9.6)'

@code{Calendar} provides time of day access, and routines for
manipulating times and durations.

@item @code{Ada.Calendar.Arithmetic} `(9.6.1)'

This package provides additional arithmetic
operations for @code{Calendar}.

@item @code{Ada.Calendar.Formatting} `(9.6.1)'

This package provides formatting operations for @code{Calendar}.

@item @code{Ada.Calendar.Time_Zones} `(9.6.1)'

This package provides additional @code{Calendar} facilities
for handling time zones.

@item @code{Ada.Characters} `(A.3.1)'

This is a dummy parent package that contains no useful entities

@item @code{Ada.Characters.Conversions} `(A.3.2)'

This package provides character conversion functions.

@item @code{Ada.Characters.Handling} `(A.3.2)'

This package provides some basic character handling capabilities,
including classification functions for classes of characters (e.g., test
for letters, or digits).

@item @code{Ada.Characters.Latin_1} `(A.3.3)'

This package includes a complete set of definitions of the characters
that appear in type CHARACTER.  It is useful for writing programs that
will run in international environments.  For example, if you want an
upper case E with an acute accent in a string, it is often better to use
the definition of @code{UC_E_Acute} in this package.  Then your program
will print in an understandable manner even if your environment does not
support these extended characters.

@item @code{Ada.Command_Line} `(A.15)'

This package provides access to the command line parameters and the name
of the current program (analogous to the use of @code{argc} and @code{argv}
in C), and also allows the exit status for the program to be set in a
system-independent manner.

@item @code{Ada.Complex_Text_IO} `(G.1.3)'

This package provides text input and output of complex numbers.

@item @code{Ada.Containers} `(A.18.1)'

A top level package providing a few basic definitions used by all the
following specific child packages that provide specific kinds of
containers.
@end table

@code{Ada.Containers.Bounded_Priority_Queues} `(A.18.31)'

@code{Ada.Containers.Bounded_Synchronized_Queues} `(A.18.29)'

@code{Ada.Containers.Doubly_Linked_Lists} `(A.18.3)'

@code{Ada.Containers.Generic_Array_Sort} `(A.18.26)'

@code{Ada.Containers.Generic_Constrained_Array_Sort} `(A.18.26)'

@code{Ada.Containers.Generic_Sort} `(A.18.26)'

@code{Ada.Containers.Hashed_Maps} `(A.18.5)'

@code{Ada.Containers.Hashed_Sets} `(A.18.8)'

@code{Ada.Containers.Indefinite_Doubly_Linked_Lists} `(A.18.12)'

@code{Ada.Containers.Indefinite_Hashed_Maps} `(A.18.13)'

@code{Ada.Containers.Indefinite_Hashed_Sets} `(A.18.15)'

@code{Ada.Containers.Indefinite_Holders} `(A.18.18)'

@code{Ada.Containers.Indefinite_Multiway_Trees} `(A.18.17)'

@code{Ada.Containers.Indefinite_Ordered_Maps} `(A.18.14)'

@code{Ada.Containers.Indefinite_Ordered_Sets} `(A.18.16)'

@code{Ada.Containers.Indefinite_Vectors} `(A.18.11)'

@code{Ada.Containers.Multiway_Trees} `(A.18.10)'

@code{Ada.Containers.Ordered_Maps} `(A.18.6)'

@code{Ada.Containers.Ordered_Sets} `(A.18.9)'

@code{Ada.Containers.Synchronized_Queue_Interfaces} `(A.18.27)'

@code{Ada.Containers.Unbounded_Priority_Queues} `(A.18.30)'

@code{Ada.Containers.Unbounded_Synchronized_Queues} `(A.18.28)'

@code{Ada.Containers.Vectors} `(A.18.2)'


@table @asis

@item @code{Ada.Directories} `(A.16)'

This package provides operations on directories.

@item @code{Ada.Directories.Hierarchical_File_Names} `(A.16.1)'

This package provides additional directory operations handling
hierarchical file names.

@item @code{Ada.Directories.Information} `(A.16)'

This is an implementation defined package for additional directory
operations, which is not implemented in GNAT.

@item @code{Ada.Decimal} `(F.2)'

This package provides constants describing the range of decimal numbers
implemented, and also a decimal divide routine (analogous to the COBOL
verb DIVIDE … GIVING … REMAINDER …)

@item @code{Ada.Direct_IO} `(A.8.4)'

This package provides input-output using a model of a set of records of
fixed-length, containing an arbitrary definite Ada type, indexed by an
integer record number.

@item @code{Ada.Dispatching} `(D.2.1)'

A parent package containing definitions for task dispatching operations.

@item @code{Ada.Dispatching.EDF} `(D.2.6)'

Not implemented in GNAT.

@item @code{Ada.Dispatching.Non_Preemptive} `(D.2.4)'

Not implemented in GNAT.

@item @code{Ada.Dispatching.Round_Robin} `(D.2.5)'

Not implemented in GNAT.

@item @code{Ada.Dynamic_Priorities} `(D.5)'

This package allows the priorities of a task to be adjusted dynamically
as the task is running.

@item @code{Ada.Environment_Variables} `(A.17)'

This package provides facilities for accessing environment variables.

@item @code{Ada.Exceptions} `(11.4.1)'

This package provides additional information on exceptions, and also
contains facilities for treating exceptions as data objects, and raising
exceptions with associated messages.

@item @code{Ada.Execution_Time} `(D.14)'

This package provides CPU clock functionalities. It is not implemented on
all targets (see package spec for details).

@item @code{Ada.Execution_Time.Group_Budgets} `(D.14.2)'

Not implemented in GNAT.

@item @code{Ada.Execution_Time.Timers} `(D.14.1)’'

Not implemented in GNAT.

@item @code{Ada.Finalization} `(7.6)'

This package contains the declarations and subprograms to support the
use of controlled types, providing for automatic initialization and
finalization (analogous to the constructors and destructors of C++).

@item @code{Ada.Float_Text_IO} `(A.10.9)'

A library level instantiation of Text_IO.Float_IO for type Float.

@item @code{Ada.Float_Wide_Text_IO} `(A.10.9)'

A library level instantiation of Wide_Text_IO.Float_IO for type Float.

@item @code{Ada.Float_Wide_Wide_Text_IO} `(A.10.9)'

A library level instantiation of Wide_Wide_Text_IO.Float_IO for type Float.

@item @code{Ada.Integer_Text_IO} `(A.10.9)'

A library level instantiation of Text_IO.Integer_IO for type Integer.

@item @code{Ada.Integer_Wide_Text_IO} `(A.10.9)'

A library level instantiation of Wide_Text_IO.Integer_IO for type Integer.

@item @code{Ada.Integer_Wide_Wide_Text_IO} `(A.10.9)'

A library level instantiation of Wide_Wide_Text_IO.Integer_IO for type Integer.

@item @code{Ada.Interrupts} `(C.3.2)'

This package provides facilities for interfacing to interrupts, which
includes the set of signals or conditions that can be raised and
recognized as interrupts.

@item @code{Ada.Interrupts.Names} `(C.3.2)'

This package provides the set of interrupt names (actually signal
or condition names) that can be handled by GNAT.

@item @code{Ada.IO_Exceptions} `(A.13)'

This package defines the set of exceptions that can be raised by use of
the standard IO packages.

@item @code{Ada.Iterator_Interfaces} `(5.5.1)'

This package provides a generic interface to generalized iterators.

@item @code{Ada.Locales} `(A.19)'

This package provides declarations providing information (Language
and Country) about the current locale.

@item @code{Ada.Numerics}

This package contains some standard constants and exceptions used
throughout the numerics packages.  Note that the constants pi and e are
defined here, and it is better to use these definitions than rolling
your own.

@item @code{Ada.Numerics.Complex_Arrays} `(G.3.2)'

Provides operations on arrays of complex numbers.

@item @code{Ada.Numerics.Complex_Elementary_Functions}

Provides the implementation of standard elementary functions (such as
log and trigonometric functions) operating on complex numbers using the
standard @code{Float} and the @code{Complex} and @code{Imaginary} types
created by the package @code{Numerics.Complex_Types}.

@item @code{Ada.Numerics.Complex_Types}

This is a predefined instantiation of
@code{Numerics.Generic_Complex_Types} using @code{Standard.Float} to
build the type @code{Complex} and @code{Imaginary}.

@item @code{Ada.Numerics.Discrete_Random}

This generic package provides a random number generator suitable for generating
uniformly distributed values of a specified discrete subtype. It should not be
used as a cryptographic pseudo-random source.

@item @code{Ada.Numerics.Float_Random}

This package provides a random number generator suitable for generating
uniformly distributed floating point values in the unit interval. It should not
be used as a cryptographic pseudo-random source.

@item @code{Ada.Numerics.Generic_Complex_Elementary_Functions}

This is a generic version of the package that provides the
implementation of standard elementary functions (such as log and
trigonometric functions) for an arbitrary complex type.

The following predefined instantiations of this package are provided:


@itemize *

@item 
@code{Short_Float}

@code{Ada.Numerics.Short_Complex_Elementary_Functions}

@item 
@code{Float}

@code{Ada.Numerics.Complex_Elementary_Functions}

@item 
@code{Long_Float}

@code{Ada.Numerics.Long_Complex_Elementary_Functions}
@end itemize

@item @code{Ada.Numerics.Generic_Complex_Types}

This is a generic package that allows the creation of complex types,
with associated complex arithmetic operations.

The following predefined instantiations of this package exist


@itemize *

@item 
@code{Short_Float}

@code{Ada.Numerics.Short_Complex_Complex_Types}

@item 
@code{Float}

@code{Ada.Numerics.Complex_Complex_Types}

@item 
@code{Long_Float}

@code{Ada.Numerics.Long_Complex_Complex_Types}
@end itemize

@item @code{Ada.Numerics.Generic_Elementary_Functions}

This is a generic package that provides the implementation of standard
elementary functions (such as log an trigonometric functions) for an
arbitrary float type.

The following predefined instantiations of this package exist


@itemize *

@item 
@code{Short_Float}

@code{Ada.Numerics.Short_Elementary_Functions}

@item 
@code{Float}

@code{Ada.Numerics.Elementary_Functions}

@item 
@code{Long_Float}

@code{Ada.Numerics.Long_Elementary_Functions}
@end itemize

@item @code{Ada.Numerics.Generic_Real_Arrays} `(G.3.1)'

Generic operations on arrays of reals

@item @code{Ada.Numerics.Real_Arrays} `(G.3.1)'

Preinstantiation of Ada.Numerics.Generic_Real_Arrays (Float).

@item @code{Ada.Real_Time} `(D.8)'

This package provides facilities similar to those of @code{Calendar}, but
operating with a finer clock suitable for real time control. Note that
annex D requires that there be no backward clock jumps, and GNAT generally
guarantees this behavior, but of course if the external clock on which
the GNAT runtime depends is deliberately reset by some external event,
then such a backward jump may occur.

@item @code{Ada.Real_Time.Timing_Events} `(D.15)'

This package allows procedures to be executed at a specified time without
the use of a task or a delay statement.

@item @code{Ada.Sequential_IO} `(A.8.1)'

This package provides input-output facilities for sequential files,
which can contain a sequence of values of a single type, which can be
any Ada type, including indefinite (unconstrained) types.

@item @code{Ada.Storage_IO} `(A.9)'

This package provides a facility for mapping arbitrary Ada types to and
from a storage buffer.  It is primarily intended for the creation of new
IO packages.

@item @code{Ada.Streams} `(13.13.1)'

This is a generic package that provides the basic support for the
concept of streams as used by the stream attributes (@code{Input},
@code{Output}, @code{Read} and @code{Write}).

@item @code{Ada.Streams.Stream_IO} `(A.12.1)'

This package is a specialization of the type @code{Streams} defined in
package @code{Streams} together with a set of operations providing
Stream_IO capability.  The Stream_IO model permits both random and
sequential access to a file which can contain an arbitrary set of values
of one or more Ada types.

@item @code{Ada.Strings} `(A.4.1)'

This package provides some basic constants used by the string handling
packages.

@item @code{Ada.Strings.Bounded} `(A.4.4)'

This package provides facilities for handling variable length
strings.  The bounded model requires a maximum length.  It is thus
somewhat more limited than the unbounded model, but avoids the use of
dynamic allocation or finalization.

@item @code{Ada.Strings.Bounded.Equal_Case_Insensitive} `(A.4.10)'

Provides case-insensitive comparisons of bounded strings

@item @code{Ada.Strings.Bounded.Hash} `(A.4.9)'

This package provides a generic hash function for bounded strings

@item @code{Ada.Strings.Bounded.Hash_Case_Insensitive} `(A.4.9)'

This package provides a generic hash function for bounded strings that
converts the string to be hashed to lower case.

@item @code{Ada.Strings.Bounded.Less_Case_Insensitive} `(A.4.10)'

This package provides a comparison function for bounded strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.Fixed} `(A.4.3)'

This package provides facilities for handling fixed length strings.

@item @code{Ada.Strings.Fixed.Equal_Case_Insensitive} `(A.4.10)'

This package provides an equality function for fixed strings that compares
the strings after converting both to lower case.

@item @code{Ada.Strings.Fixed.Hash_Case_Insensitive} `(A.4.9)'

This package provides a case insensitive hash function for fixed strings that
converts the string to lower case before computing the hash.

@item @code{Ada.Strings.Fixed.Less_Case_Insensitive} `(A.4.10)'

This package provides a comparison function for fixed strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.Hash} `(A.4.9)'

This package provides a hash function for strings.

@item @code{Ada.Strings.Hash_Case_Insensitive} `(A.4.9)'

This package provides a hash function for strings that is case insensitive.
The string is converted to lower case before computing the hash.

@item @code{Ada.Strings.Less_Case_Insensitive} `(A.4.10)'

This package provides a comparison function for\strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.Maps} `(A.4.2)'

This package provides facilities for handling character mappings and
arbitrarily defined subsets of characters.  For instance it is useful in
defining specialized translation tables.

@item @code{Ada.Strings.Maps.Constants} `(A.4.6)'

This package provides a standard set of predefined mappings and
predefined character sets.  For example, the standard upper to lower case
conversion table is found in this package.  Note that upper to lower case
conversion is non-trivial if you want to take the entire set of
characters, including extended characters like E with an acute accent,
into account.  You should use the mappings in this package (rather than
adding 32 yourself) to do case mappings.

@item @code{Ada.Strings.Unbounded} `(A.4.5)'

This package provides facilities for handling variable length
strings.  The unbounded model allows arbitrary length strings, but
requires the use of dynamic allocation and finalization.

@item @code{Ada.Strings.Unbounded.Equal_Case_Insensitive} `(A.4.10)'

Provides case-insensitive comparisons of unbounded strings

@item @code{Ada.Strings.Unbounded.Hash} `(A.4.9)'

This package provides a generic hash function for unbounded strings

@item @code{Ada.Strings.Unbounded.Hash_Case_Insensitive} `(A.4.9)'

This package provides a generic hash function for unbounded strings that
converts the string to be hashed to lower case.

@item @code{Ada.Strings.Unbounded.Less_Case_Insensitive} `(A.4.10)'

This package provides a comparison function for unbounded strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.UTF_Encoding} `(A.4.11)'

This package provides basic definitions for dealing with UTF-encoded strings.

@item @code{Ada.Strings.UTF_Encoding.Conversions} `(A.4.11)'

This package provides conversion functions for UTF-encoded strings.
@end table

@code{Ada.Strings.UTF_Encoding.Strings} `(A.4.11)'

@code{Ada.Strings.UTF_Encoding.Wide_Strings} `(A.4.11)'


@table @asis

@item @code{Ada.Strings.UTF_Encoding.Wide_Wide_Strings} `(A.4.11)'

These packages provide facilities for handling UTF encodings for
Strings, Wide_Strings and Wide_Wide_Strings.
@end table

@code{Ada.Strings.Wide_Bounded} `(A.4.7)'

@code{Ada.Strings.Wide_Fixed} `(A.4.7)'

@code{Ada.Strings.Wide_Maps} `(A.4.7)'


@table @asis

@item @code{Ada.Strings.Wide_Unbounded} `(A.4.7)'

These packages provide analogous capabilities to the corresponding
packages without @code{Wide_} in the name, but operate with the types
@code{Wide_String} and @code{Wide_Character} instead of @code{String}
and @code{Character}. Versions of all the child packages are available.
@end table

@code{Ada.Strings.Wide_Wide_Bounded} `(A.4.7)'

@code{Ada.Strings.Wide_Wide_Fixed} `(A.4.7)'

@code{Ada.Strings.Wide_Wide_Maps} `(A.4.7)'


@table @asis

@item @code{Ada.Strings.Wide_Wide_Unbounded} `(A.4.7)'

These packages provide analogous capabilities to the corresponding
packages without @code{Wide_} in the name, but operate with the types
@code{Wide_Wide_String} and @code{Wide_Wide_Character} instead
of @code{String} and @code{Character}.

@item @code{Ada.Synchronous_Barriers} `(D.10.1)'

This package provides facilities for synchronizing tasks at a low level
with barriers.

@item @code{Ada.Synchronous_Task_Control} `(D.10)'

This package provides some standard facilities for controlling task
communication in a synchronous manner.

@item @code{Ada.Synchronous_Task_Control.EDF} `(D.10)'

Not implemented in GNAT.

@item @code{Ada.Tags}

This package contains definitions for manipulation of the tags of tagged
values.

@item @code{Ada.Tags.Generic_Dispatching_Constructor} `(3.9)'

This package provides a way of constructing tagged class-wide values given
only the tag value.

@item @code{Ada.Task_Attributes} `(C.7.2)'

This package provides the capability of associating arbitrary
task-specific data with separate tasks.

@item @code{Ada.Task_Identification} `(C.7.1)'

This package provides capabilities for task identification.

@item @code{Ada.Task_Termination} `(C.7.3)'

This package provides control over task termination.

@item @code{Ada.Text_IO}

This package provides basic text input-output capabilities for
character, string and numeric data.  The subpackages of this
package are listed next. Note that although these are defined
as subpackages in the RM, they are actually transparently
implemented as child packages in GNAT, meaning that they
are only loaded if needed.

@item @code{Ada.Text_IO.Decimal_IO}

Provides input-output facilities for decimal fixed-point types

@item @code{Ada.Text_IO.Enumeration_IO}

Provides input-output facilities for enumeration types.

@item @code{Ada.Text_IO.Fixed_IO}

Provides input-output facilities for ordinary fixed-point types.

@item @code{Ada.Text_IO.Float_IO}

Provides input-output facilities for float types.  The following
predefined instantiations of this generic package are available:


@itemize *

@item 
@code{Short_Float}

@code{Short_Float_Text_IO}

@item 
@code{Float}

@code{Float_Text_IO}

@item 
@code{Long_Float}

@code{Long_Float_Text_IO}
@end itemize

@item @code{Ada.Text_IO.Integer_IO}

Provides input-output facilities for integer types.  The following
predefined instantiations of this generic package are available:


@itemize *

@item 
@code{Short_Short_Integer}

@code{Ada.Short_Short_Integer_Text_IO}

@item 
@code{Short_Integer}

@code{Ada.Short_Integer_Text_IO}

@item 
@code{Integer}

@code{Ada.Integer_Text_IO}

@item 
@code{Long_Integer}

@code{Ada.Long_Integer_Text_IO}

@item 
@code{Long_Long_Integer}

@code{Ada.Long_Long_Integer_Text_IO}
@end itemize

@item @code{Ada.Text_IO.Modular_IO}

Provides input-output facilities for modular (unsigned) types.

@item @code{Ada.Text_IO.Bounded_IO (A.10.11)}

Provides input-output facilities for bounded strings.

@item @code{Ada.Text_IO.Complex_IO (G.1.3)}

This package provides basic text input-output capabilities for complex
data.

@item @code{Ada.Text_IO.Editing (F.3.3)}

This package contains routines for edited output, analogous to the use
of pictures in COBOL.  The picture formats used by this package are a
close copy of the facility in COBOL.

@item @code{Ada.Text_IO.Text_Streams (A.12.2)}

This package provides a facility that allows Text_IO files to be treated
as streams, so that the stream attributes can be used for writing
arbitrary data, including binary data, to Text_IO files.

@item @code{Ada.Text_IO.Unbounded_IO (A.10.12)}

This package provides input-output facilities for unbounded strings.

@item @code{Ada.Unchecked_Conversion (13.9)}

This generic package allows arbitrary conversion from one type to
another of the same size, providing for breaking the type safety in
special circumstances.

If the types have the same Size (more accurately the same Value_Size),
then the effect is simply to transfer the bits from the source to the
target type without any modification.  This usage is well defined, and
for simple types whose representation is typically the same across
all implementations, gives a portable method of performing such
conversions.

If the types do not have the same size, then the result is implementation
defined, and thus may be non-portable.  The following describes how GNAT
handles such unchecked conversion cases.

If the types are of different sizes, and are both discrete types, then
the effect is of a normal type conversion without any constraint checking.
In particular if the result type has a larger size, the result will be
zero or sign extended.  If the result type has a smaller size, the result
will be truncated by ignoring high order bits.

If the types are of different sizes, and are not both discrete types,
then the conversion works as though pointers were created to the source
and target, and the pointer value is converted.  The effect is that bits
are copied from successive low order storage units and bits of the source
up to the length of the target type.

A warning is issued if the lengths differ, since the effect in this
case is implementation dependent, and the above behavior may not match
that of some other compiler.

A pointer to one type may be converted to a pointer to another type using
unchecked conversion.  The only case in which the effect is undefined is
when one or both pointers are pointers to unconstrained array types.  In
this case, the bounds information may get incorrectly transferred, and in
particular, GNAT uses double size pointers for such types, and it is
meaningless to convert between such pointer types.  GNAT will issue a
warning if the alignment of the target designated type is more strict
than the alignment of the source designated type (since the result may
be unaligned in this case).

A pointer other than a pointer to an unconstrained array type may be
converted to and from System.Address.  Such usage is common in Ada 83
programs, but note that Ada.Address_To_Access_Conversions is the
preferred method of performing such conversions in Ada 95 and Ada 2005.
Neither
unchecked conversion nor Ada.Address_To_Access_Conversions should be
used in conjunction with pointers to unconstrained objects, since
the bounds information cannot be handled correctly in this case.

@item @code{Ada.Unchecked_Deallocation} `(13.11.2)'

This generic package allows explicit freeing of storage previously
allocated by use of an allocator.

@item @code{Ada.Wide_Text_IO} `(A.11)'

This package is similar to @code{Ada.Text_IO}, except that the external
file supports wide character representations, and the internal types are
@code{Wide_Character} and @code{Wide_String} instead of @code{Character}
and @code{String}. The corresponding set of nested packages and child
packages are defined.

@item @code{Ada.Wide_Wide_Text_IO} `(A.11)'

This package is similar to @code{Ada.Text_IO}, except that the external
file supports wide character representations, and the internal types are
@code{Wide_Character} and @code{Wide_String} instead of @code{Character}
and @code{String}. The corresponding set of nested packages and child
packages are defined.
@end table

For packages in Interfaces and System, all the RM defined packages are
available in GNAT, see the Ada 2012 RM for full details.

@node The Implementation of Standard I/O,The GNAT Library,Standard Library Routines,Top
@anchor{gnat_rm/the_implementation_of_standard_i_o doc}@anchor{2cc}@anchor{gnat_rm/the_implementation_of_standard_i_o id1}@anchor{2cd}@anchor{gnat_rm/the_implementation_of_standard_i_o the-implementation-of-standard-i-o}@anchor{10}
@chapter The Implementation of Standard I/O


GNAT implements all the required input-output facilities described in
A.6 through A.14.  These sections of the Ada Reference Manual describe the
required behavior of these packages from the Ada point of view, and if
you are writing a portable Ada program that does not need to know the
exact manner in which Ada maps to the outside world when it comes to
reading or writing external files, then you do not need to read this
chapter.  As long as your files are all regular files (not pipes or
devices), and as long as you write and read the files only from Ada, the
description in the Ada Reference Manual is sufficient.

However, if you want to do input-output to pipes or other devices, such
as the keyboard or screen, or if the files you are dealing with are
either generated by some other language, or to be read by some other
language, then you need to know more about the details of how the GNAT
implementation of these input-output facilities behaves.

In this chapter we give a detailed description of exactly how GNAT
interfaces to the file system.  As always, the sources of the system are
available to you for answering questions at an even more detailed level,
but for most purposes the information in this chapter will suffice.

Another reason that you may need to know more about how input-output is
implemented arises when you have a program written in mixed languages
where, for example, files are shared between the C and Ada sections of
the same program.  GNAT provides some additional facilities, in the form
of additional child library packages, that facilitate this sharing, and
these additional facilities are also described in this chapter.

@menu
* Standard I/O Packages:: 
* FORM Strings:: 
* Direct_IO:: 
* Sequential_IO:: 
* Text_IO:: 
* Wide_Text_IO:: 
* Wide_Wide_Text_IO:: 
* Stream_IO:: 
* Text Translation:: 
* Shared Files:: 
* Filenames encoding:: 
* File content encoding:: 
* Open Modes:: 
* Operations on C Streams:: 
* Interfacing to C Streams:: 

@end menu

@node Standard I/O Packages,FORM Strings,,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id2}@anchor{2ce}@anchor{gnat_rm/the_implementation_of_standard_i_o standard-i-o-packages}@anchor{2cf}
@section Standard I/O Packages


The Standard I/O packages described in Annex A for


@itemize *

@item 
Ada.Text_IO

@item 
Ada.Text_IO.Complex_IO

@item 
Ada.Text_IO.Text_Streams

@item 
Ada.Wide_Text_IO

@item 
Ada.Wide_Text_IO.Complex_IO

@item 
Ada.Wide_Text_IO.Text_Streams

@item 
Ada.Wide_Wide_Text_IO

@item 
Ada.Wide_Wide_Text_IO.Complex_IO

@item 
Ada.Wide_Wide_Text_IO.Text_Streams

@item 
Ada.Stream_IO

@item 
Ada.Sequential_IO

@item 
Ada.Direct_IO
@end itemize

are implemented using the C
library streams facility; where


@itemize *

@item 
All files are opened using @code{fopen}.

@item 
All input/output operations use @code{fread}/@code{fwrite}.
@end itemize

There is no internal buffering of any kind at the Ada library level. The only
buffering is that provided at the system level in the implementation of the
library routines that support streams. This facilitates shared use of these
streams by mixed language programs. Note though that system level buffering is
explicitly enabled at elaboration of the standard I/O packages and that can
have an impact on mixed language programs, in particular those using I/O before
calling the Ada elaboration routine (e.g., adainit). It is recommended to call
the Ada elaboration routine before performing any I/O or when impractical,
flush the common I/O streams and in particular Standard_Output before
elaborating the Ada code.

@node FORM Strings,Direct_IO,Standard I/O Packages,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o form-strings}@anchor{2d0}@anchor{gnat_rm/the_implementation_of_standard_i_o id3}@anchor{2d1}
@section FORM Strings


The format of a FORM string in GNAT is:

@example
"keyword=value,keyword=value,...,keyword=value"
@end example

where letters may be in upper or lower case, and there are no spaces
between values.  The order of the entries is not important.  Currently
the following keywords defined.

@example
TEXT_TRANSLATION=[YES|NO|TEXT|BINARY|U8TEXT|WTEXT|U16TEXT]
SHARED=[YES|NO]
WCEM=[n|h|u|s|e|8|b]
ENCODING=[UTF8|8BITS]
@end example

The use of these parameters is described later in this section. If an
unrecognized keyword appears in a form string, it is silently ignored
and not considered invalid.

@node Direct_IO,Sequential_IO,FORM Strings,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o direct-io}@anchor{2d2}@anchor{gnat_rm/the_implementation_of_standard_i_o id4}@anchor{2d3}
@section Direct_IO


Direct_IO can only be instantiated for definite types.  This is a
restriction of the Ada language, which means that the records are fixed
length (the length being determined by @code{type'Size}, rounded
up to the next storage unit boundary if necessary).

The records of a Direct_IO file are simply written to the file in index
sequence, with the first record starting at offset zero, and subsequent
records following.  There is no control information of any kind.  For
example, if 32-bit integers are being written, each record takes
4-bytes, so the record at index @code{K} starts at offset @code{(K-1)*4}.

There is no limit on the size of Direct_IO files, they are expanded as
necessary to accommodate whatever records are written to the file.

@node Sequential_IO,Text_IO,Direct_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id5}@anchor{2d4}@anchor{gnat_rm/the_implementation_of_standard_i_o sequential-io}@anchor{2d5}
@section Sequential_IO


Sequential_IO may be instantiated with either a definite (constrained)
or indefinite (unconstrained) type.

For the definite type case, the elements written to the file are simply
the memory images of the data values with no control information of any
kind.  The resulting file should be read using the same type, no validity
checking is performed on input.

For the indefinite type case, the elements written consist of two
parts.  First is the size of the data item, written as the memory image
of a @code{Interfaces.C.size_t} value, followed by the memory image of
the data value.  The resulting file can only be read using the same
(unconstrained) type.  Normal assignment checks are performed on these
read operations, and if these checks fail, @code{Data_Error} is
raised.  In particular, in the array case, the lengths must match, and in
the variant record case, if the variable for a particular read operation
is constrained, the discriminants must match.

Note that it is not possible to use Sequential_IO to write variable
length array items, and then read the data back into different length
arrays.  For example, the following will raise @code{Data_Error}:

@example
package IO is new Sequential_IO (String);
F : IO.File_Type;
S : String (1..4);
...
IO.Create (F)
IO.Write (F, "hello!")
IO.Reset (F, Mode=>In_File);
IO.Read (F, S);
Put_Line (S);
@end example

On some Ada implementations, this will print @code{hell}, but the program is
clearly incorrect, since there is only one element in the file, and that
element is the string @code{hello!}.

In Ada 95 and Ada 2005, this kind of behavior can be legitimately achieved
using Stream_IO, and this is the preferred mechanism.  In particular, the
above program fragment rewritten to use Stream_IO will work correctly.

@node Text_IO,Wide_Text_IO,Sequential_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id6}@anchor{2d6}@anchor{gnat_rm/the_implementation_of_standard_i_o text-io}@anchor{2d7}
@section Text_IO


Text_IO files consist of a stream of characters containing the following
special control characters:

@example
LF (line feed, 16#0A#) Line Mark
FF (form feed, 16#0C#) Page Mark
@end example

A canonical Text_IO file is defined as one in which the following
conditions are met:


@itemize *

@item 
The character @code{LF} is used only as a line mark, i.e., to mark the end
of the line.

@item 
The character @code{FF} is used only as a page mark, i.e., to mark the
end of a page and consequently can appear only immediately following a
@code{LF} (line mark) character.

@item 
The file ends with either @code{LF} (line mark) or @code{LF}-@cite{FF}
(line mark, page mark).  In the former case, the page mark is implicitly
assumed to be present.
@end itemize

A file written using Text_IO will be in canonical form provided that no
explicit @code{LF} or @code{FF} characters are written using @code{Put}
or @code{Put_Line}.  There will be no @code{FF} character at the end of
the file unless an explicit @code{New_Page} operation was performed
before closing the file.

A canonical Text_IO file that is a regular file (i.e., not a device or a
pipe) can be read using any of the routines in Text_IO.  The
semantics in this case will be exactly as defined in the Ada Reference
Manual, and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO
file has one of the following:


@itemize *

@item 
The file contains @code{FF} characters not immediately following a
@code{LF} character.

@item 
The file contains @code{LF} or @code{FF} characters written by
@code{Put} or @code{Put_Line}, which are not logically considered to be
line marks or page marks.

@item 
The file ends in a character other than @code{LF} or @code{FF},
i.e., there is no explicit line mark or page mark at the end of the file.
@end itemize

Text_IO can be used to read such non-standard text files but subprograms
to do with line or page numbers do not have defined meanings.  In
particular, a @code{FF} character that does not follow a @code{LF}
character may or may not be treated as a page mark from the point of
view of page and line numbering.  Every @code{LF} character is considered
to end a line, and there is an implied @code{LF} character at the end of
the file.

@menu
* Stream Pointer Positioning:: 
* Reading and Writing Non-Regular Files:: 
* Get_Immediate:: 
* Treating Text_IO Files as Streams:: 
* Text_IO Extensions:: 
* Text_IO Facilities for Unbounded Strings:: 

@end menu

@node Stream Pointer Positioning,Reading and Writing Non-Regular Files,,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id7}@anchor{2d8}@anchor{gnat_rm/the_implementation_of_standard_i_o stream-pointer-positioning}@anchor{2d9}
@subsection Stream Pointer Positioning


@code{Ada.Text_IO} has a definition of current position for a file that
is being read.  No internal buffering occurs in Text_IO, and usually the
physical position in the stream used to implement the file corresponds
to this logical position defined by Text_IO.  There are two exceptions:


@itemize *

@item 
After a call to @code{End_Of_Page} that returns @code{True}, the stream
is positioned past the @code{LF} (line mark) that precedes the page
mark.  Text_IO maintains an internal flag so that subsequent read
operations properly handle the logical position which is unchanged by
the @code{End_Of_Page} call.

@item 
After a call to @code{End_Of_File} that returns @code{True}, if the
Text_IO file was positioned before the line mark at the end of file
before the call, then the logical position is unchanged, but the stream
is physically positioned right at the end of file (past the line mark,
and past a possible page mark following the line mark.  Again Text_IO
maintains internal flags so that subsequent read operations properly
handle the logical position.
@end itemize

These discrepancies have no effect on the observable behavior of
Text_IO, but if a single Ada stream is shared between a C program and
Ada program, or shared (using @code{shared=yes} in the form string)
between two Ada files, then the difference may be observable in some
situations.

@node Reading and Writing Non-Regular Files,Get_Immediate,Stream Pointer Positioning,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id8}@anchor{2da}@anchor{gnat_rm/the_implementation_of_standard_i_o reading-and-writing-non-regular-files}@anchor{2db}
@subsection Reading and Writing Non-Regular Files


A non-regular file is a device (such as a keyboard), or a pipe.  Text_IO
can be used for reading and writing.  Writing is not affected and the
sequence of characters output is identical to the normal file case, but
for reading, the behavior of Text_IO is modified to avoid undesirable
look-ahead as follows:

An input file that is not a regular file is considered to have no page
marks.  Any @code{Ascii.FF} characters (the character normally used for a
page mark) appearing in the file are considered to be data
characters.  In particular:


@itemize *

@item 
@code{Get_Line} and @code{Skip_Line} do not test for a page mark
following a line mark.  If a page mark appears, it will be treated as a
data character.

@item 
This avoids the need to wait for an extra character to be typed or
entered from the pipe to complete one of these operations.

@item 
@code{End_Of_Page} always returns @code{False}

@item 
@code{End_Of_File} will return @code{False} if there is a page mark at
the end of the file.
@end itemize

Output to non-regular files is the same as for regular files.  Page marks
may be written to non-regular files using @code{New_Page}, but as noted
above they will not be treated as page marks on input if the output is
piped to another Ada program.

Another important discrepancy when reading non-regular files is that the end
of file indication is not ‘sticky’.  If an end of file is entered, e.g., by
pressing the @code{EOT} key,
then end of file
is signaled once (i.e., the test @code{End_Of_File}
will yield @code{True}, or a read will
raise @code{End_Error}), but then reading can resume
to read data past that end of
file indication, until another end of file indication is entered.

@node Get_Immediate,Treating Text_IO Files as Streams,Reading and Writing Non-Regular Files,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o get-immediate}@anchor{2dc}@anchor{gnat_rm/the_implementation_of_standard_i_o id9}@anchor{2dd}
@subsection Get_Immediate


@geindex Get_Immediate

Get_Immediate returns the next character (including control characters)
from the input file.  In particular, Get_Immediate will return LF or FF
characters used as line marks or page marks.  Such operations leave the
file positioned past the control character, and it is thus not treated
as having its normal function.  This means that page, line and column
counts after this kind of Get_Immediate call are set as though the mark
did not occur.  In the case where a Get_Immediate leaves the file
positioned between the line mark and page mark (which is not normally
possible), it is undefined whether the FF character will be treated as a
page mark.

@node Treating Text_IO Files as Streams,Text_IO Extensions,Get_Immediate,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id10}@anchor{2de}@anchor{gnat_rm/the_implementation_of_standard_i_o treating-text-io-files-as-streams}@anchor{2df}
@subsection Treating Text_IO Files as Streams


@geindex Stream files

The package @code{Text_IO.Streams} allows a @code{Text_IO} file to be treated
as a stream.  Data written to a @code{Text_IO} file in this stream mode is
binary data.  If this binary data contains bytes 16#0A# (@code{LF}) or
16#0C# (@code{FF}), the resulting file may have non-standard
format.  Similarly if read operations are used to read from a Text_IO
file treated as a stream, then @code{LF} and @code{FF} characters may be
skipped and the effect is similar to that described above for
@code{Get_Immediate}.

@node Text_IO Extensions,Text_IO Facilities for Unbounded Strings,Treating Text_IO Files as Streams,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id11}@anchor{2e0}@anchor{gnat_rm/the_implementation_of_standard_i_o text-io-extensions}@anchor{2e1}
@subsection Text_IO Extensions


@geindex Text_IO extensions

A package GNAT.IO_Aux in the GNAT library provides some useful extensions
to the standard @code{Text_IO} package:


@itemize *

@item 
function File_Exists (Name : String) return Boolean;
Determines if a file of the given name exists.

@item 
function Get_Line return String;
Reads a string from the standard input file.  The value returned is exactly
the length of the line that was read.

@item 
function Get_Line (File : Ada.Text_IO.File_Type) return String;
Similar, except that the parameter File specifies the file from which
the string is to be read.
@end itemize

@node Text_IO Facilities for Unbounded Strings,,Text_IO Extensions,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id12}@anchor{2e2}@anchor{gnat_rm/the_implementation_of_standard_i_o text-io-facilities-for-unbounded-strings}@anchor{2e3}
@subsection Text_IO Facilities for Unbounded Strings


@geindex Text_IO for unbounded strings

@geindex Unbounded_String
@geindex Text_IO operations

The package @code{Ada.Strings.Unbounded.Text_IO}
in library files @code{a-suteio.ads/adb} contains some GNAT-specific
subprograms useful for Text_IO operations on unbounded strings:


@itemize *

@item 
function Get_Line (File : File_Type) return Unbounded_String;
Reads a line from the specified file
and returns the result as an unbounded string.

@item 
procedure Put (File : File_Type; U : Unbounded_String);
Writes the value of the given unbounded string to the specified file
Similar to the effect of
@code{Put (To_String (U))} except that an extra copy is avoided.

@item 
procedure Put_Line (File : File_Type; U : Unbounded_String);
Writes the value of the given unbounded string to the specified file,
followed by a @code{New_Line}.
Similar to the effect of @code{Put_Line (To_String (U))} except
that an extra copy is avoided.
@end itemize

In the above procedures, @code{File} is of type @code{Ada.Text_IO.File_Type}
and is optional.  If the parameter is omitted, then the standard input or
output file is referenced as appropriate.

The package @code{Ada.Strings.Wide_Unbounded.Wide_Text_IO} in library
files @code{a-swuwti.ads} and @code{a-swuwti.adb} provides similar extended
@code{Wide_Text_IO} functionality for unbounded wide strings.

The package @code{Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO} in library
files @code{a-szuzti.ads} and @code{a-szuzti.adb} provides similar extended
@code{Wide_Wide_Text_IO} functionality for unbounded wide wide strings.

@node Wide_Text_IO,Wide_Wide_Text_IO,Text_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id13}@anchor{2e4}@anchor{gnat_rm/the_implementation_of_standard_i_o wide-text-io}@anchor{2e5}
@section Wide_Text_IO


@code{Wide_Text_IO} is similar in most respects to Text_IO, except that
both input and output files may contain special sequences that represent
wide character values.  The encoding scheme for a given file may be
specified using a FORM parameter:

@example
WCEM=`x`
@end example

as part of the FORM string (WCEM = wide character encoding method),
where @code{x} is one of the following characters


@multitable {xxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Character

@tab

Encoding

@item

`h'

@tab

Hex ESC encoding

@item

`u'

@tab

Upper half encoding

@item

`s'

@tab

Shift-JIS encoding

@item

`e'

@tab

EUC Encoding

@item

`8'

@tab

UTF-8 encoding

@item

`b'

@tab

Brackets encoding

@end multitable


The encoding methods match those that
can be used in a source
program, but there is no requirement that the encoding method used for
the source program be the same as the encoding method used for files,
and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files
for which no WCEM parameter is given in the FORM string matches the
wide character encoding specified for the main program (the default
being brackets encoding if no coding method was specified with -gnatW).


@table @asis

@item `Hex Coding'

In this encoding, a wide character is represented by a five character
sequence:
@end table

@example
ESC a b c d
@end example


@quotation

where @code{a}, @code{b}, @code{c}, @code{d} are the four hexadecimal
characters (using upper case letters) of the wide character code.  For
example, ESC A345 is used to represent the wide character with code
16#A345#.  This scheme is compatible with use of the full
@code{Wide_Character} set.
@end quotation


@table @asis

@item `Upper Half Coding'

The wide character with encoding 16#abcd#, where the upper bit is on
(i.e., a is in the range 8-F) is represented as two bytes 16#ab# and
16#cd#.  The second byte may never be a format control character, but is
not required to be in the upper half.  This method can be also used for
shift-JIS or EUC where the internal coding matches the external coding.

@item `Shift JIS Coding'

A wide character is represented by a two character sequence 16#ab# and
16#cd#, with the restrictions described for upper half encoding as
described above.  The internal character code is the corresponding JIS
character according to the standard algorithm for Shift-JIS
conversion.  Only characters defined in the JIS code set table can be
used with this encoding method.

@item `EUC Coding'

A wide character is represented by a two character sequence 16#ab# and
16#cd#, with both characters being in the upper half.  The internal
character code is the corresponding JIS character according to the EUC
encoding algorithm.  Only characters defined in the JIS code set table
can be used with this encoding method.

@item `UTF-8 Coding'

A wide character is represented using
UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO
10646-1/Am.2.  Depending on the character value, the representation
is a one, two, or three byte sequence:
@end table

@example
16#0000#-16#007f#: 2#0xxxxxxx#
16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
@end example


@quotation

where the @code{xxx} bits correspond to the left-padded bits of the
16-bit character value.  Note that all lower half ASCII characters
are represented as ASCII bytes and all upper half characters and
other wide characters are represented as sequences of upper-half
(The full UTF-8 scheme allows for encoding 31-bit characters as
6-byte sequences, but in this implementation, all UTF-8 sequences
of four or more bytes length will raise a Constraint_Error, as
will all invalid UTF-8 sequences.)
@end quotation


@table @asis

@item `Brackets Coding'

In this encoding, a wide character is represented by the following eight
character sequence:
@end table

@example
[ " a b c d " ]
@end example


@quotation

where @code{a}, @code{b}, @code{c}, @code{d} are the four hexadecimal
characters (using uppercase letters) of the wide character code.  For
example, @code{["A345"]} is used to represent the wide character with code
@code{16#A345#}.
This scheme is compatible with use of the full Wide_Character set.
On input, brackets coding can also be used for upper half characters,
e.g., @code{["C1"]} for lower case a.  However, on output, brackets notation
is only used for wide characters with a code greater than @code{16#FF#}.

Note that brackets coding is not normally used in the context of
Wide_Text_IO or Wide_Wide_Text_IO, since it is really just designed as
a portable way of encoding source files. In the context of Wide_Text_IO
or Wide_Wide_Text_IO, it can only be used if the file does not contain
any instance of the left bracket character other than to encode wide
character values using the brackets encoding method. In practice it is
expected that some standard wide character encoding method such
as UTF-8 will be used for text input output.

If brackets notation is used, then any occurrence of a left bracket
in the input file which is not the start of a valid wide character
sequence will cause Constraint_Error to be raised. It is possible to
encode a left bracket as [“5B”] and Wide_Text_IO and Wide_Wide_Text_IO
input will interpret this as a left bracket.

However, when a left bracket is output, it will be output as a left bracket
and not as [“5B”]. We make this decision because for normal use of
Wide_Text_IO for outputting messages, it is unpleasant to clobber left
brackets. For example, if we write:

@example
Put_Line ("Start of output [first run]");
@end example

we really do not want to have the left bracket in this message clobbered so
that the output reads:
@end quotation

@example
Start of output ["5B"]first run]
@end example


@quotation

In practice brackets encoding is reasonably useful for normal Put_Line use
since we won’t get confused between left brackets and wide character
sequences in the output. But for input, or when files are written out
and read back in, it really makes better sense to use one of the standard
encoding methods such as UTF-8.
@end quotation

For the coding schemes other than UTF-8, Hex, or Brackets encoding,
not all wide character
values can be represented.  An attempt to output a character that cannot
be represented using the encoding scheme for the file causes
Constraint_Error to be raised.  An invalid wide character sequence on
input also causes Constraint_Error to be raised.

@menu
* Stream Pointer Positioning: Stream Pointer Positioning<2>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<2>. 

@end menu

@node Stream Pointer Positioning<2>,Reading and Writing Non-Regular Files<2>,,Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id14}@anchor{2e6}@anchor{gnat_rm/the_implementation_of_standard_i_o stream-pointer-positioning-1}@anchor{2e7}
@subsection Stream Pointer Positioning


@code{Ada.Wide_Text_IO} is similar to @code{Ada.Text_IO} in its handling
of stream pointer positioning (@ref{2d7,,Text_IO}).  There is one additional
case:

If @code{Ada.Wide_Text_IO.Look_Ahead} reads a character outside the
normal lower ASCII set, i.e. a character in the range:

@example
Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)
@end example

then although the logical position of the file pointer is unchanged by
the @code{Look_Ahead} call, the stream is physically positioned past the
wide character sequence.  Again this is to avoid the need for buffering
or backup, and all @code{Wide_Text_IO} routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using @code{Wide_Text_IO}.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.

@node Reading and Writing Non-Regular Files<2>,,Stream Pointer Positioning<2>,Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id15}@anchor{2e8}@anchor{gnat_rm/the_implementation_of_standard_i_o reading-and-writing-non-regular-files-1}@anchor{2e9}
@subsection Reading and Writing Non-Regular Files


As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and @code{End_Of_Page} always returns
@code{False}.  Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

@node Wide_Wide_Text_IO,Stream_IO,Wide_Text_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id16}@anchor{2ea}@anchor{gnat_rm/the_implementation_of_standard_i_o wide-wide-text-io}@anchor{2eb}
@section Wide_Wide_Text_IO


@code{Wide_Wide_Text_IO} is similar in most respects to Text_IO, except that
both input and output files may contain special sequences that represent
wide wide character values.  The encoding scheme for a given file may be
specified using a FORM parameter:

@example
WCEM=`x`
@end example

as part of the FORM string (WCEM = wide character encoding method),
where @code{x} is one of the following characters


@multitable {xxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Character

@tab

Encoding

@item

`h'

@tab

Hex ESC encoding

@item

`u'

@tab

Upper half encoding

@item

`s'

@tab

Shift-JIS encoding

@item

`e'

@tab

EUC Encoding

@item

`8'

@tab

UTF-8 encoding

@item

`b'

@tab

Brackets encoding

@end multitable


The encoding methods match those that
can be used in a source
program, but there is no requirement that the encoding method used for
the source program be the same as the encoding method used for files,
and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files
for which no WCEM parameter is given in the FORM string matches the
wide character encoding specified for the main program (the default
being brackets encoding if no coding method was specified with -gnatW).


@table @asis

@item `UTF-8 Coding'

A wide character is represented using
UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO
10646-1/Am.2.  Depending on the character value, the representation
is a one, two, three, or four byte sequence:
@end table

@example
16#000000#-16#00007f#: 2#0xxxxxxx#
16#000080#-16#0007ff#: 2#110xxxxx# 2#10xxxxxx#
16#000800#-16#00ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
16#010000#-16#10ffff#: 2#11110xxx# 2#10xxxxxx# 2#10xxxxxx# 2#10xxxxxx#
@end example


@quotation

where the @code{xxx} bits correspond to the left-padded bits of the
21-bit character value.  Note that all lower half ASCII characters
are represented as ASCII bytes and all upper half characters and
other wide characters are represented as sequences of upper-half
characters.
@end quotation


@table @asis

@item `Brackets Coding'

In this encoding, a wide wide character is represented by the following eight
character sequence if is in wide character range
@end table

@example
[ " a b c d " ]
@end example


@quotation

and by the following ten character sequence if not
@end quotation

@example
[ " a b c d e f " ]
@end example


@quotation

where @code{a}, @code{b}, @code{c}, @code{d}, @code{e}, and @code{f}
are the four or six hexadecimal
characters (using uppercase letters) of the wide wide character code.  For
example, @code{["01A345"]} is used to represent the wide wide character
with code @code{16#01A345#}.

This scheme is compatible with use of the full Wide_Wide_Character set.
On input, brackets coding can also be used for upper half characters,
e.g., @code{["C1"]} for lower case a.  However, on output, brackets notation
is only used for wide characters with a code greater than @code{16#FF#}.
@end quotation

If is also possible to use the other Wide_Character encoding methods,
such as Shift-JIS, but the other schemes cannot support the full range
of wide wide characters.
An attempt to output a character that cannot
be represented using the encoding scheme for the file causes
Constraint_Error to be raised.  An invalid wide character sequence on
input also causes Constraint_Error to be raised.

@menu
* Stream Pointer Positioning: Stream Pointer Positioning<3>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<3>. 

@end menu

@node Stream Pointer Positioning<3>,Reading and Writing Non-Regular Files<3>,,Wide_Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id17}@anchor{2ec}@anchor{gnat_rm/the_implementation_of_standard_i_o stream-pointer-positioning-2}@anchor{2ed}
@subsection Stream Pointer Positioning


@code{Ada.Wide_Wide_Text_IO} is similar to @code{Ada.Text_IO} in its handling
of stream pointer positioning (@ref{2d7,,Text_IO}).  There is one additional
case:

If @code{Ada.Wide_Wide_Text_IO.Look_Ahead} reads a character outside the
normal lower ASCII set, i.e. a character in the range:

@example
Wide_Wide_Character'Val (16#0080#) .. Wide_Wide_Character'Val (16#10FFFF#)
@end example

then although the logical position of the file pointer is unchanged by
the @code{Look_Ahead} call, the stream is physically positioned past the
wide character sequence.  Again this is to avoid the need for buffering
or backup, and all @code{Wide_Wide_Text_IO} routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using @code{Wide_Wide_Text_IO}.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.

@node Reading and Writing Non-Regular Files<3>,,Stream Pointer Positioning<3>,Wide_Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id18}@anchor{2ee}@anchor{gnat_rm/the_implementation_of_standard_i_o reading-and-writing-non-regular-files-2}@anchor{2ef}
@subsection Reading and Writing Non-Regular Files


As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and @code{End_Of_Page} always returns
@code{False}.  Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

@node Stream_IO,Text Translation,Wide_Wide_Text_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id19}@anchor{2f0}@anchor{gnat_rm/the_implementation_of_standard_i_o stream-io}@anchor{2f1}
@section Stream_IO


A stream file is a sequence of bytes, where individual elements are
written to the file as described in the Ada Reference Manual.  The type
@code{Stream_Element} is simply a byte.  There are two ways to read or
write a stream file.


@itemize *

@item 
The operations @code{Read} and @code{Write} directly read or write a
sequence of stream elements with no control information.

@item 
The stream attributes applied to a stream file transfer data in the
manner described for stream attributes.
@end itemize

@node Text Translation,Shared Files,Stream_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id20}@anchor{2f2}@anchor{gnat_rm/the_implementation_of_standard_i_o text-translation}@anchor{2f3}
@section Text Translation


@code{Text_Translation=xxx} may be used as the Form parameter
passed to Text_IO.Create and Text_IO.Open. @code{Text_Translation=xxx}
has no effect on Unix systems. Possible values are:


@itemize *

@item 
@code{Yes} or @code{Text} is the default, which means to
translate LF to/from CR/LF on Windows systems.

@code{No} disables this translation; i.e. it
uses binary mode. For output files, @code{Text_Translation=No}
may be used to create Unix-style files on
Windows.

@item 
@code{wtext} translation enabled in Unicode mode.
(corresponds to _O_WTEXT).

@item 
@code{u8text} translation enabled in Unicode UTF-8 mode.
(corresponds to O_U8TEXT).

@item 
@code{u16text} translation enabled in Unicode UTF-16
mode. (corresponds to_O_U16TEXT).
@end itemize

@node Shared Files,Filenames encoding,Text Translation,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id21}@anchor{2f4}@anchor{gnat_rm/the_implementation_of_standard_i_o shared-files}@anchor{2f5}
@section Shared Files


Section A.14 of the Ada Reference Manual allows implementations to
provide a wide variety of behavior if an attempt is made to access the
same external file with two or more internal files.

To provide a full range of functionality, while at the same time
minimizing the problems of portability caused by this implementation
dependence, GNAT handles file sharing as follows:


@itemize *

@item 
In the absence of a @code{shared=xxx} form parameter, an attempt
to open two or more files with the same full name is considered an error
and is not supported.  The exception @code{Use_Error} will be
raised.  Note that a file that is not explicitly closed by the program
remains open until the program terminates.

@item 
If the form parameter @code{shared=no} appears in the form string, the
file can be opened or created with its own separate stream identifier,
regardless of whether other files sharing the same external file are
opened.  The exact effect depends on how the C stream routines handle
multiple accesses to the same external files using separate streams.

@item 
If the form parameter @code{shared=yes} appears in the form string for
each of two or more files opened using the same full name, the same
stream is shared between these files, and the semantics are as described
in Ada Reference Manual, Section A.14.
@end itemize

When a program that opens multiple files with the same name is ported
from another Ada compiler to GNAT, the effect will be that
@code{Use_Error} is raised.

The documentation of the original compiler and the documentation of the
program should then be examined to determine if file sharing was
expected, and @code{shared=xxx} parameters added to @code{Open}
and @code{Create} calls as required.

When a program is ported from GNAT to some other Ada compiler, no
special attention is required unless the @code{shared=xxx} form
parameter is used in the program.  In this case, you must examine the
documentation of the new compiler to see if it supports the required
file sharing semantics, and form strings modified appropriately.  Of
course it may be the case that the program cannot be ported if the
target compiler does not support the required functionality.  The best
approach in writing portable code is to avoid file sharing (and hence
the use of the @code{shared=xxx} parameter in the form string)
completely.

One common use of file sharing in Ada 83 is the use of instantiations of
Sequential_IO on the same file with different types, to achieve
heterogeneous input-output.  Although this approach will work in GNAT if
@code{shared=yes} is specified, it is preferable in Ada to use Stream_IO
for this purpose (using the stream attributes).

@node Filenames encoding,File content encoding,Shared Files,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o filenames-encoding}@anchor{2f6}@anchor{gnat_rm/the_implementation_of_standard_i_o id22}@anchor{2f7}
@section Filenames encoding


An encoding form parameter can be used to specify the filename
encoding @code{encoding=xxx}.


@itemize *

@item 
If the form parameter @code{encoding=utf8} appears in the form string, the
filename must be encoded in UTF-8.

@item 
If the form parameter @code{encoding=8bits} appears in the form
string, the filename must be a standard 8bits string.
@end itemize

In the absence of a @code{encoding=xxx} form parameter, the
encoding is controlled by the @code{GNAT_CODE_PAGE} environment
variable. And if not set @code{utf8} is assumed.


@table @asis

@item `CP_ACP'

The current system Windows ANSI code page.

@item `CP_UTF8'

UTF-8 encoding
@end table

This encoding form parameter is only supported on the Windows
platform. On the other Operating Systems the run-time is supporting
UTF-8 natively.

@node File content encoding,Open Modes,Filenames encoding,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o file-content-encoding}@anchor{2f8}@anchor{gnat_rm/the_implementation_of_standard_i_o id23}@anchor{2f9}
@section File content encoding


For text files it is possible to specify the encoding to use. This is
controlled by the by the @code{GNAT_CCS_ENCODING} environment
variable. And if not set @code{TEXT} is assumed.

The possible values are those supported on Windows:


@table @asis

@item `TEXT'

Translated text mode

@item `WTEXT'

Translated unicode encoding

@item `U16TEXT'

Unicode 16-bit encoding

@item `U8TEXT'

Unicode 8-bit encoding
@end table

This encoding is only supported on the Windows platform.

@node Open Modes,Operations on C Streams,File content encoding,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id24}@anchor{2fa}@anchor{gnat_rm/the_implementation_of_standard_i_o open-modes}@anchor{2fb}
@section Open Modes


@code{Open} and @code{Create} calls result in a call to @code{fopen}
using the mode shown in the following table:


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxx} 
@headitem

@code{Open} and @code{Create} Call Modes

@tab

@tab

@item

@tab

`OPEN'

@tab

`CREATE'

@item

Append_File

@tab

“r+”

@tab

“w+”

@item

In_File

@tab

“r”

@tab

“w+”

@item

Out_File (Direct_IO)

@tab

“r+”

@tab

“w”

@item

Out_File (all other cases)

@tab

“w”

@tab

“w”

@item

Inout_File

@tab

“r+”

@tab

“w+”

@end multitable


If text file translation is required, then either @code{b} or @code{t}
is added to the mode, depending on the setting of Text.  Text file
translation refers to the mapping of CR/LF sequences in an external file
to LF characters internally.  This mapping only occurs in DOS and
DOS-like systems, and is not relevant to other systems.

A special case occurs with Stream_IO.  As shown in the above table, the
file is initially opened in @code{r} or @code{w} mode for the
@code{In_File} and @code{Out_File} cases.  If a @code{Set_Mode} operation
subsequently requires switching from reading to writing or vice-versa,
then the file is reopened in @code{r+} mode to permit the required operation.

@node Operations on C Streams,Interfacing to C Streams,Open Modes,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id25}@anchor{2fc}@anchor{gnat_rm/the_implementation_of_standard_i_o operations-on-c-streams}@anchor{2fd}
@section Operations on C Streams


The package @code{Interfaces.C_Streams} provides an Ada program with direct
access to the C library functions for operations on C streams:

@example
package Interfaces.C_Streams is
  -- Note: the reason we do not use the types that are in
  -- Interfaces.C is that we want to avoid dragging in the
  -- code in this unit if possible.
  subtype chars is System.Address;
  -- Pointer to null-terminated array of characters
  subtype FILEs is System.Address;
  -- Corresponds to the C type FILE*
  subtype voids is System.Address;
  -- Corresponds to the C type void*
  subtype int is Integer;
  subtype long is Long_Integer;
  -- Note: the above types are subtypes deliberately, and it
  -- is part of this spec that the above correspondences are
  -- guaranteed.  This means that it is legitimate to, for
  -- example, use Integer instead of int.  We provide these
  -- synonyms for clarity, but in some cases it may be
  -- convenient to use the underlying types (for example to
  -- avoid an unnecessary dependency of a spec on the spec
  -- of this unit).
  type size_t is mod 2 ** Standard'Address_Size;
  NULL_Stream : constant FILEs;
  -- Value returned (NULL in C) to indicate an
  -- fdopen/fopen/tmpfile error
  ----------------------------------
  -- Constants Defined in stdio.h --
  ----------------------------------
  EOF : constant int;
  -- Used by a number of routines to indicate error or
  -- end of file
  IOFBF : constant int;
  IOLBF : constant int;
  IONBF : constant int;
  -- Used to indicate buffering mode for setvbuf call
  SEEK_CUR : constant int;
  SEEK_END : constant int;
  SEEK_SET : constant int;
  -- Used to indicate origin for fseek call
  function stdin return FILEs;
  function stdout return FILEs;
  function stderr return FILEs;
  -- Streams associated with standard files
  --------------------------
  -- Standard C functions --
  --------------------------
  -- The functions selected below are ones that are
  -- available in UNIX (but not necessarily in ANSI C).
  -- These are very thin interfaces
  -- which copy exactly the C headers.  For more
  -- documentation on these functions, see the Microsoft C
  -- "Run-Time Library Reference" (Microsoft Press, 1990,
  -- ISBN 1-55615-225-6), which includes useful information
  -- on system compatibility.
  procedure clearerr (stream : FILEs);
  function fclose (stream : FILEs) return int;
  function fdopen (handle : int; mode : chars) return FILEs;
  function feof (stream : FILEs) return int;
  function ferror (stream : FILEs) return int;
  function fflush (stream : FILEs) return int;
  function fgetc (stream : FILEs) return int;
  function fgets (strng : chars; n : int; stream : FILEs)
      return chars;
  function fileno (stream : FILEs) return int;
  function fopen (filename : chars; Mode : chars)
      return FILEs;
  -- Note: to maintain target independence, use
  -- text_translation_required, a boolean variable defined in
  -- a-sysdep.c to deal with the target dependent text
  -- translation requirement.  If this variable is set,
  -- then  b/t should be appended to the standard mode
  -- argument to set the text translation mode off or on
  -- as required.
  function fputc (C : int; stream : FILEs) return int;
  function fputs (Strng : chars; Stream : FILEs) return int;
  function fread
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t;
  function freopen
     (filename : chars;
      mode : chars;
      stream : FILEs)
      return FILEs;
  function fseek
     (stream : FILEs;
      offset : long;
      origin : int)
      return int;
  function ftell (stream : FILEs) return long;
  function fwrite
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t;
  function isatty (handle : int) return int;
  procedure mktemp (template : chars);
  -- The return value (which is just a pointer to template)
  -- is discarded
  procedure rewind (stream : FILEs);
  function rmtmp return int;
  function setvbuf
     (stream : FILEs;
      buffer : chars;
      mode : int;
      size : size_t)
      return int;

  function tmpfile return FILEs;
  function ungetc (c : int; stream : FILEs) return int;
  function unlink (filename : chars) return int;
  ---------------------
  -- Extra functions --
  ---------------------
  -- These functions supply slightly thicker bindings than
  -- those above.  They are derived from functions in the
  -- C Run-Time Library, but may do a bit more work than
  -- just directly calling one of the Library functions.
  function is_regular_file (handle : int) return int;
  -- Tests if given handle is for a regular file (result 1)
  -- or for a non-regular file (pipe or device, result 0).
  ---------------------------------
  -- Control of Text/Binary Mode --
  ---------------------------------
  -- If text_translation_required is true, then the following
  -- functions may be used to dynamically switch a file from
  -- binary to text mode or vice versa.  These functions have
  -- no effect if text_translation_required is false (i.e., in
  -- normal UNIX mode).  Use fileno to get a stream handle.
  procedure set_binary_mode (handle : int);
  procedure set_text_mode (handle : int);
  ----------------------------
  -- Full Path Name support --
  ----------------------------
  procedure full_name (nam : chars; buffer : chars);
  -- Given a NUL terminated string representing a file
  -- name, returns in buffer a NUL terminated string
  -- representing the full path name for the file name.
  -- On systems where it is relevant the   drive is also
  -- part of the full path name.  It is the responsibility
  -- of the caller to pass an actual parameter for buffer
  -- that is big enough for any full path name.  Use
  -- max_path_len given below as the size of buffer.
  max_path_len : integer;
  -- Maximum length of an allowable full path name on the
  -- system, including a terminating NUL character.
end Interfaces.C_Streams;
@end example

@node Interfacing to C Streams,,Operations on C Streams,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id26}@anchor{2fe}@anchor{gnat_rm/the_implementation_of_standard_i_o interfacing-to-c-streams}@anchor{2ff}
@section Interfacing to C Streams


The packages in this section permit interfacing Ada files to C Stream
operations.

@example
with Interfaces.C_Streams;
package Ada.Sequential_IO.C_Streams is
   function C_Stream (F : File_Type)
      return Interfaces.C_Streams.FILEs;
   procedure Open
     (File : in out File_Type;
      Mode : in File_Mode;
      C_Stream : in Interfaces.C_Streams.FILEs;
      Form : in String := "");
end Ada.Sequential_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Direct_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Direct_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Text_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Text_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Wide_Text_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
end Ada.Wide_Text_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Wide_Wide_Text_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
end Ada.Wide_Wide_Text_IO.C_Streams;

with Interfaces.C_Streams;
package Ada.Stream_IO.C_Streams is
   function C_Stream (F : File_Type)
      return Interfaces.C_Streams.FILEs;
   procedure Open
     (File : in out File_Type;
      Mode : in File_Mode;
      C_Stream : in Interfaces.C_Streams.FILEs;
      Form : in String := "");
end Ada.Stream_IO.C_Streams;
@end example

In each of these six packages, the @code{C_Stream} function obtains the
@code{FILE} pointer from a currently opened Ada file.  It is then
possible to use the @code{Interfaces.C_Streams} package to operate on
this stream, or the stream can be passed to a C program which can
operate on it directly.  Of course the program is responsible for
ensuring that only appropriate sequences of operations are executed.

One particular use of relevance to an Ada program is that the
@code{setvbuf} function can be used to control the buffering of the
stream used by an Ada file.  In the absence of such a call the standard
default buffering is used.

The @code{Open} procedures in these packages open a file giving an
existing C Stream instead of a file name.  Typically this stream is
imported from a C program, allowing an Ada file to operate on an
existing C file.

@node The GNAT Library,Interfacing to Other Languages,The Implementation of Standard I/O,Top
@anchor{gnat_rm/the_gnat_library doc}@anchor{300}@anchor{gnat_rm/the_gnat_library id1}@anchor{301}@anchor{gnat_rm/the_gnat_library the-gnat-library}@anchor{11}
@chapter The GNAT Library


The GNAT library contains a number of general and special purpose packages.
It represents functionality that the GNAT developers have found useful, and
which is made available to GNAT users.  The packages described here are fully
supported, and upwards compatibility will be maintained in future releases,
so you can use these facilities with the confidence that the same functionality
will be available in future releases.

The chapter here simply gives a brief summary of the facilities available.
The full documentation is found in the spec file for the package.  The full
sources of these library packages, including both spec and body, are provided
with all GNAT releases.  For example, to find out the full specifications of
the SPITBOL pattern matching capability, including a full tutorial and
extensive examples, look in the @code{g-spipat.ads} file in the library.

For each entry here, the package name (as it would appear in a @code{with}
clause) is given, followed by the name of the corresponding spec file in
parentheses.  The packages are children in four hierarchies, @code{Ada},
@code{Interfaces}, @code{System}, and @code{GNAT}, the latter being a
GNAT-specific hierarchy.

Note that an application program should only use packages in one of these
four hierarchies if the package is defined in the Ada Reference Manual,
or is listed in this section of the GNAT Programmers Reference Manual.
All other units should be considered internal implementation units and
should not be directly @code{with}ed by application code.  The use of
a @code{with} clause that references one of these internal implementation
units makes an application potentially dependent on changes in versions
of GNAT, and will generate a warning message.

@menu
* Ada.Characters.Latin_9 (a-chlat9.ads): Ada Characters Latin_9 a-chlat9 ads. 
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads): Ada Characters Wide_Latin_1 a-cwila1 ads. 
* Ada.Characters.Wide_Latin_9 (a-cwila9.ads): Ada Characters Wide_Latin_9 a-cwila9 ads. 
* Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads): Ada Characters Wide_Wide_Latin_1 a-chzla1 ads. 
* Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads): Ada Characters Wide_Wide_Latin_9 a-chzla9 ads. 
* Ada.Containers.Bounded_Holders (a-coboho.ads): Ada Containers Bounded_Holders a-coboho ads. 
* Ada.Command_Line.Environment (a-colien.ads): Ada Command_Line Environment a-colien ads. 
* Ada.Command_Line.Remove (a-colire.ads): Ada Command_Line Remove a-colire ads. 
* Ada.Command_Line.Response_File (a-clrefi.ads): Ada Command_Line Response_File a-clrefi ads. 
* Ada.Direct_IO.C_Streams (a-diocst.ads): Ada Direct_IO C_Streams a-diocst ads. 
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads): Ada Exceptions Is_Null_Occurrence a-einuoc ads. 
* Ada.Exceptions.Last_Chance_Handler (a-elchha.ads): Ada Exceptions Last_Chance_Handler a-elchha ads. 
* Ada.Exceptions.Traceback (a-exctra.ads): Ada Exceptions Traceback a-exctra ads. 
* Ada.Sequential_IO.C_Streams (a-siocst.ads): Ada Sequential_IO C_Streams a-siocst ads. 
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads): Ada Streams Stream_IO C_Streams a-ssicst ads. 
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads): Ada Strings Unbounded Text_IO a-suteio ads. 
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads): Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads. 
* Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads): Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads. 
* Ada.Task_Initialization (a-tasini.ads): Ada Task_Initialization a-tasini ads. 
* Ada.Text_IO.C_Streams (a-tiocst.ads): Ada Text_IO C_Streams a-tiocst ads. 
* Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads): Ada Text_IO Reset_Standard_Files a-tirsfi ads. 
* Ada.Wide_Characters.Unicode (a-wichun.ads): Ada Wide_Characters Unicode a-wichun ads. 
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads): Ada Wide_Text_IO C_Streams a-wtcstr ads. 
* Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads): Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads. 
* Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads): Ada Wide_Wide_Characters Unicode a-zchuni ads. 
* Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads): Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads. 
* Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads): Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads. 
* GNAT.Altivec (g-altive.ads): GNAT Altivec g-altive ads. 
* GNAT.Altivec.Conversions (g-altcon.ads): GNAT Altivec Conversions g-altcon ads. 
* GNAT.Altivec.Vector_Operations (g-alveop.ads): GNAT Altivec Vector_Operations g-alveop ads. 
* GNAT.Altivec.Vector_Types (g-alvety.ads): GNAT Altivec Vector_Types g-alvety ads. 
* GNAT.Altivec.Vector_Views (g-alvevi.ads): GNAT Altivec Vector_Views g-alvevi ads. 
* GNAT.Array_Split (g-arrspl.ads): GNAT Array_Split g-arrspl ads. 
* GNAT.AWK (g-awk.ads): GNAT AWK g-awk ads. 
* GNAT.Binary_Search (g-binsea.ads): GNAT Binary_Search g-binsea ads. 
* GNAT.Bind_Environment (g-binenv.ads): GNAT Bind_Environment g-binenv ads. 
* GNAT.Branch_Prediction (g-brapre.ads): GNAT Branch_Prediction g-brapre ads. 
* GNAT.Bounded_Buffers (g-boubuf.ads): GNAT Bounded_Buffers g-boubuf ads. 
* GNAT.Bounded_Mailboxes (g-boumai.ads): GNAT Bounded_Mailboxes g-boumai ads. 
* GNAT.Bubble_Sort (g-bubsor.ads): GNAT Bubble_Sort g-bubsor ads. 
* GNAT.Bubble_Sort_A (g-busora.ads): GNAT Bubble_Sort_A g-busora ads. 
* GNAT.Bubble_Sort_G (g-busorg.ads): GNAT Bubble_Sort_G g-busorg ads. 
* GNAT.Byte_Order_Mark (g-byorma.ads): GNAT Byte_Order_Mark g-byorma ads. 
* GNAT.Byte_Swapping (g-bytswa.ads): GNAT Byte_Swapping g-bytswa ads. 
* GNAT.C_Time (g-c_time.ads): GNAT C_Time g-c_time ads. 
* GNAT.Calendar (g-calend.ads): GNAT Calendar g-calend ads. 
* GNAT.Calendar.Time_IO (g-catiio.ads): GNAT Calendar Time_IO g-catiio ads. 
* GNAT.CRC32 (g-crc32.ads): GNAT CRC32 g-crc32 ads. 
* GNAT.Case_Util (g-casuti.ads): GNAT Case_Util g-casuti ads. 
* GNAT.CGI (g-cgi.ads): GNAT CGI g-cgi ads. 
* GNAT.CGI.Cookie (g-cgicoo.ads): GNAT CGI Cookie g-cgicoo ads. 
* GNAT.CGI.Debug (g-cgideb.ads): GNAT CGI Debug g-cgideb ads. 
* GNAT.Command_Line (g-comlin.ads): GNAT Command_Line g-comlin ads. 
* GNAT.Compiler_Version (g-comver.ads): GNAT Compiler_Version g-comver ads. 
* GNAT.Ctrl_C (g-ctrl_c.ads): GNAT Ctrl_C g-ctrl_c ads. 
* GNAT.Current_Exception (g-curexc.ads): GNAT Current_Exception g-curexc ads. 
* GNAT.Debug_Pools (g-debpoo.ads): GNAT Debug_Pools g-debpoo ads. 
* GNAT.Debug_Utilities (g-debuti.ads): GNAT Debug_Utilities g-debuti ads. 
* GNAT.Decode_String (g-decstr.ads): GNAT Decode_String g-decstr ads. 
* GNAT.Decode_UTF8_String (g-deutst.ads): GNAT Decode_UTF8_String g-deutst ads. 
* GNAT.Directory_Operations (g-dirope.ads): GNAT Directory_Operations g-dirope ads. 
* GNAT.Directory_Operations.Iteration (g-diopit.ads): GNAT Directory_Operations Iteration g-diopit ads. 
* GNAT.Dynamic_HTables (g-dynhta.ads): GNAT Dynamic_HTables g-dynhta ads. 
* GNAT.Dynamic_Tables (g-dyntab.ads): GNAT Dynamic_Tables g-dyntab ads. 
* GNAT.Encode_String (g-encstr.ads): GNAT Encode_String g-encstr ads. 
* GNAT.Encode_UTF8_String (g-enutst.ads): GNAT Encode_UTF8_String g-enutst ads. 
* GNAT.Exception_Actions (g-excact.ads): GNAT Exception_Actions g-excact ads. 
* GNAT.Exception_Traces (g-exctra.ads): GNAT Exception_Traces g-exctra ads. 
* GNAT.Exceptions (g-except.ads): GNAT Exceptions g-except ads. 
* GNAT.Expect (g-expect.ads): GNAT Expect g-expect ads. 
* GNAT.Expect.TTY (g-exptty.ads): GNAT Expect TTY g-exptty ads. 
* GNAT.Float_Control (g-flocon.ads): GNAT Float_Control g-flocon ads. 
* GNAT.Formatted_String (g-forstr.ads): GNAT Formatted_String g-forstr ads. 
* GNAT.Generic_Fast_Math_Functions (g-gfmafu.ads): GNAT Generic_Fast_Math_Functions g-gfmafu ads. 
* GNAT.Heap_Sort (g-heasor.ads): GNAT Heap_Sort g-heasor ads. 
* GNAT.Heap_Sort_A (g-hesora.ads): GNAT Heap_Sort_A g-hesora ads. 
* GNAT.Heap_Sort_G (g-hesorg.ads): GNAT Heap_Sort_G g-hesorg ads. 
* GNAT.HTable (g-htable.ads): GNAT HTable g-htable ads. 
* GNAT.IO (g-io.ads): GNAT IO g-io ads. 
* GNAT.IO_Aux (g-io_aux.ads): GNAT IO_Aux g-io_aux ads. 
* GNAT.Lock_Files (g-locfil.ads): GNAT Lock_Files g-locfil ads. 
* GNAT.MBBS_Discrete_Random (g-mbdira.ads): GNAT MBBS_Discrete_Random g-mbdira ads. 
* GNAT.MBBS_Float_Random (g-mbflra.ads): GNAT MBBS_Float_Random g-mbflra ads. 
* GNAT.MD5 (g-md5.ads): GNAT MD5 g-md5 ads. 
* GNAT.Memory_Dump (g-memdum.ads): GNAT Memory_Dump g-memdum ads. 
* GNAT.Most_Recent_Exception (g-moreex.ads): GNAT Most_Recent_Exception g-moreex ads. 
* GNAT.OS_Lib (g-os_lib.ads): GNAT OS_Lib g-os_lib ads. 
* GNAT.Perfect_Hash_Generators (g-pehage.ads): GNAT Perfect_Hash_Generators g-pehage ads. 
* GNAT.Random_Numbers (g-rannum.ads): GNAT Random_Numbers g-rannum ads. 
* GNAT.Regexp (g-regexp.ads): GNAT Regexp g-regexp ads. 
* GNAT.Registry (g-regist.ads): GNAT Registry g-regist ads. 
* GNAT.Regpat (g-regpat.ads): GNAT Regpat g-regpat ads. 
* GNAT.Rewrite_Data (g-rewdat.ads): GNAT Rewrite_Data g-rewdat ads. 
* GNAT.Secondary_Stack_Info (g-sestin.ads): GNAT Secondary_Stack_Info g-sestin ads. 
* GNAT.Semaphores (g-semaph.ads): GNAT Semaphores g-semaph ads. 
* GNAT.Serial_Communications (g-sercom.ads): GNAT Serial_Communications g-sercom ads. 
* GNAT.SHA1 (g-sha1.ads): GNAT SHA1 g-sha1 ads. 
* GNAT.SHA224 (g-sha224.ads): GNAT SHA224 g-sha224 ads. 
* GNAT.SHA256 (g-sha256.ads): GNAT SHA256 g-sha256 ads. 
* GNAT.SHA384 (g-sha384.ads): GNAT SHA384 g-sha384 ads. 
* GNAT.SHA512 (g-sha512.ads): GNAT SHA512 g-sha512 ads. 
* GNAT.Signals (g-signal.ads): GNAT Signals g-signal ads. 
* GNAT.Sockets (g-socket.ads): GNAT Sockets g-socket ads. 
* GNAT.Source_Info (g-souinf.ads): GNAT Source_Info g-souinf ads. 
* GNAT.Spelling_Checker (g-speche.ads): GNAT Spelling_Checker g-speche ads. 
* GNAT.Spelling_Checker_Generic (g-spchge.ads): GNAT Spelling_Checker_Generic g-spchge ads. 
* GNAT.Spitbol.Patterns (g-spipat.ads): GNAT Spitbol Patterns g-spipat ads. 
* GNAT.Spitbol (g-spitbo.ads): GNAT Spitbol g-spitbo ads. 
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads): GNAT Spitbol Table_Boolean g-sptabo ads. 
* GNAT.Spitbol.Table_Integer (g-sptain.ads): GNAT Spitbol Table_Integer g-sptain ads. 
* GNAT.Spitbol.Table_VString (g-sptavs.ads): GNAT Spitbol Table_VString g-sptavs ads. 
* GNAT.SSE (g-sse.ads): GNAT SSE g-sse ads. 
* GNAT.SSE.Vector_Types (g-ssvety.ads): GNAT SSE Vector_Types g-ssvety ads. 
* GNAT.String_Hash (g-strhas.ads): GNAT String_Hash g-strhas ads. 
* GNAT.Strings (g-string.ads): GNAT Strings g-string ads. 
* GNAT.String_Split (g-strspl.ads): GNAT String_Split g-strspl ads. 
* GNAT.Table (g-table.ads): GNAT Table g-table ads. 
* GNAT.Task_Lock (g-tasloc.ads): GNAT Task_Lock g-tasloc ads. 
* GNAT.Time_Stamp (g-timsta.ads): GNAT Time_Stamp g-timsta ads. 
* GNAT.Threads (g-thread.ads): GNAT Threads g-thread ads. 
* GNAT.Traceback (g-traceb.ads): GNAT Traceback g-traceb ads. 
* GNAT.Traceback.Symbolic (g-trasym.ads): GNAT Traceback Symbolic g-trasym ads. 
* GNAT.UTF_32 (g-utf_32.ads): GNAT UTF_32 g-utf_32 ads. 
* GNAT.UTF_32_Spelling_Checker (g-u3spch.ads): GNAT UTF_32_Spelling_Checker g-u3spch ads. 
* GNAT.Wide_Spelling_Checker (g-wispch.ads): GNAT Wide_Spelling_Checker g-wispch ads. 
* GNAT.Wide_String_Split (g-wistsp.ads): GNAT Wide_String_Split g-wistsp ads. 
* GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads): GNAT Wide_Wide_Spelling_Checker g-zspche ads. 
* GNAT.Wide_Wide_String_Split (g-zistsp.ads): GNAT Wide_Wide_String_Split g-zistsp ads. 
* Interfaces.C.Extensions (i-cexten.ads): Interfaces C Extensions i-cexten ads. 
* Interfaces.C.Streams (i-cstrea.ads): Interfaces C Streams i-cstrea ads. 
* Interfaces.Packed_Decimal (i-pacdec.ads): Interfaces Packed_Decimal i-pacdec ads. 
* Interfaces.VxWorks (i-vxwork.ads): Interfaces VxWorks i-vxwork ads. 
* Interfaces.VxWorks.IO (i-vxwoio.ads): Interfaces VxWorks IO i-vxwoio ads. 
* System.Address_Image (s-addima.ads): System Address_Image s-addima ads. 
* System.Assertions (s-assert.ads): System Assertions s-assert ads. 
* System.Atomic_Counters (s-atocou.ads): System Atomic_Counters s-atocou ads. 
* System.Memory (s-memory.ads): System Memory s-memory ads. 
* System.Multiprocessors (s-multip.ads): System Multiprocessors s-multip ads. 
* System.Multiprocessors.Dispatching_Domains (s-mudido.ads): System Multiprocessors Dispatching_Domains s-mudido ads. 
* System.Partition_Interface (s-parint.ads): System Partition_Interface s-parint ads. 
* System.Pool_Global (s-pooglo.ads): System Pool_Global s-pooglo ads. 
* System.Pool_Local (s-pooloc.ads): System Pool_Local s-pooloc ads. 
* System.Restrictions (s-restri.ads): System Restrictions s-restri ads. 
* System.Rident (s-rident.ads): System Rident s-rident ads. 
* System.Strings.Stream_Ops (s-ststop.ads): System Strings Stream_Ops s-ststop ads. 
* System.Unsigned_Types (s-unstyp.ads): System Unsigned_Types s-unstyp ads. 
* System.Wch_Cnv (s-wchcnv.ads): System Wch_Cnv s-wchcnv ads. 
* System.Wch_Con (s-wchcon.ads): System Wch_Con s-wchcon ads. 

@end menu

@node Ada Characters Latin_9 a-chlat9 ads,Ada Characters Wide_Latin_1 a-cwila1 ads,,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-latin-9-a-chlat9-ads}@anchor{302}@anchor{gnat_rm/the_gnat_library id2}@anchor{303}
@section @code{Ada.Characters.Latin_9} (@code{a-chlat9.ads})


@geindex Ada.Characters.Latin_9 (a-chlat9.ads)

@geindex Latin_9 constants for Character

This child of @code{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @code{Ada.Characters.Latin_1} but with the
few modifications required for @code{Latin-9}.
The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Latin_1 a-cwila1 ads,Ada Characters Wide_Latin_9 a-cwila9 ads,Ada Characters Latin_9 a-chlat9 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-latin-1-a-cwila1-ads}@anchor{304}@anchor{gnat_rm/the_gnat_library id3}@anchor{305}
@section @code{Ada.Characters.Wide_Latin_1} (@code{a-cwila1.ads})


@geindex Ada.Characters.Wide_Latin_1 (a-cwila1.ads)

@geindex Latin_1 constants for Wide_Character

This child of @code{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @code{Ada.Characters.Latin_1} but with the
types of the constants being @code{Wide_Character}
instead of @code{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Latin_9 a-cwila9 ads,Ada Characters Wide_Wide_Latin_1 a-chzla1 ads,Ada Characters Wide_Latin_1 a-cwila1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-latin-9-a-cwila9-ads}@anchor{306}@anchor{gnat_rm/the_gnat_library id4}@anchor{307}
@section @code{Ada.Characters.Wide_Latin_9} (@code{a-cwila9.ads})


@geindex Ada.Characters.Wide_Latin_9 (a-cwila9.ads)

@geindex Latin_9 constants for Wide_Character

This child of @code{Ada.Characters}
provides a set of definitions corresponding to those in the
GNAT defined package @code{Ada.Characters.Latin_9} but with the
types of the constants being @code{Wide_Character}
instead of @code{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Wide_Latin_1 a-chzla1 ads,Ada Characters Wide_Wide_Latin_9 a-chzla9 ads,Ada Characters Wide_Latin_9 a-cwila9 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-wide-latin-1-a-chzla1-ads}@anchor{308}@anchor{gnat_rm/the_gnat_library id5}@anchor{309}
@section @code{Ada.Characters.Wide_Wide_Latin_1} (@code{a-chzla1.ads})


@geindex Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads)

@geindex Latin_1 constants for Wide_Wide_Character

This child of @code{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @code{Ada.Characters.Latin_1} but with the
types of the constants being @code{Wide_Wide_Character}
instead of @code{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Wide_Latin_9 a-chzla9 ads,Ada Containers Bounded_Holders a-coboho ads,Ada Characters Wide_Wide_Latin_1 a-chzla1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-wide-latin-9-a-chzla9-ads}@anchor{30a}@anchor{gnat_rm/the_gnat_library id6}@anchor{30b}
@section @code{Ada.Characters.Wide_Wide_Latin_9} (@code{a-chzla9.ads})


@geindex Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads)

@geindex Latin_9 constants for Wide_Wide_Character

This child of @code{Ada.Characters}
provides a set of definitions corresponding to those in the
GNAT defined package @code{Ada.Characters.Latin_9} but with the
types of the constants being @code{Wide_Wide_Character}
instead of @code{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Containers Bounded_Holders a-coboho ads,Ada Command_Line Environment a-colien ads,Ada Characters Wide_Wide_Latin_9 a-chzla9 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-containers-bounded-holders-a-coboho-ads}@anchor{30c}@anchor{gnat_rm/the_gnat_library id7}@anchor{30d}
@section @code{Ada.Containers.Bounded_Holders} (@code{a-coboho.ads})


@geindex Ada.Containers.Bounded_Holders (a-coboho.ads)

@geindex Formal container for vectors

This child of @code{Ada.Containers} defines a modified version of
Indefinite_Holders that avoids heap allocation.

@node Ada Command_Line Environment a-colien ads,Ada Command_Line Remove a-colire ads,Ada Containers Bounded_Holders a-coboho ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-command-line-environment-a-colien-ads}@anchor{30e}@anchor{gnat_rm/the_gnat_library id8}@anchor{30f}
@section @code{Ada.Command_Line.Environment} (@code{a-colien.ads})


@geindex Ada.Command_Line.Environment (a-colien.ads)

@geindex Environment entries

This child of @code{Ada.Command_Line}
provides a mechanism for obtaining environment values on systems
where this concept makes sense.

@node Ada Command_Line Remove a-colire ads,Ada Command_Line Response_File a-clrefi ads,Ada Command_Line Environment a-colien ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-command-line-remove-a-colire-ads}@anchor{310}@anchor{gnat_rm/the_gnat_library id9}@anchor{311}
@section @code{Ada.Command_Line.Remove} (@code{a-colire.ads})


@geindex Ada.Command_Line.Remove (a-colire.ads)

@geindex Removing command line arguments

@geindex Command line
@geindex argument removal

This child of @code{Ada.Command_Line}
provides a mechanism for logically removing
arguments from the argument list.  Once removed, an argument is not visible
to further calls to the subprograms in @code{Ada.Command_Line}. These calls
will not see the removed argument.

@node Ada Command_Line Response_File a-clrefi ads,Ada Direct_IO C_Streams a-diocst ads,Ada Command_Line Remove a-colire ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-command-line-response-file-a-clrefi-ads}@anchor{312}@anchor{gnat_rm/the_gnat_library id10}@anchor{313}
@section @code{Ada.Command_Line.Response_File} (@code{a-clrefi.ads})


@geindex Ada.Command_Line.Response_File (a-clrefi.ads)

@geindex Response file for command line

@geindex Command line
@geindex response file

@geindex Command line
@geindex handling long command lines

This child of @code{Ada.Command_Line} provides a mechanism facilities for
getting command line arguments from a text file, called a “response file”.
Using a response file allow passing a set of arguments to an executable longer
than the maximum allowed by the system on the command line.

@node Ada Direct_IO C_Streams a-diocst ads,Ada Exceptions Is_Null_Occurrence a-einuoc ads,Ada Command_Line Response_File a-clrefi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-direct-io-c-streams-a-diocst-ads}@anchor{314}@anchor{gnat_rm/the_gnat_library id11}@anchor{315}
@section @code{Ada.Direct_IO.C_Streams} (@code{a-diocst.ads})


@geindex Ada.Direct_IO.C_Streams (a-diocst.ads)

@geindex C Streams
@geindex Interfacing with Direct_IO

This package provides subprograms that allow interfacing between
C streams and @code{Direct_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Exceptions Is_Null_Occurrence a-einuoc ads,Ada Exceptions Last_Chance_Handler a-elchha ads,Ada Direct_IO C_Streams a-diocst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-exceptions-is-null-occurrence-a-einuoc-ads}@anchor{316}@anchor{gnat_rm/the_gnat_library id12}@anchor{317}
@section @code{Ada.Exceptions.Is_Null_Occurrence} (@code{a-einuoc.ads})


@geindex Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)

@geindex Null_Occurrence
@geindex testing for

This child subprogram provides a way of testing for the null
exception occurrence (@code{Null_Occurrence}) without raising
an exception.

@node Ada Exceptions Last_Chance_Handler a-elchha ads,Ada Exceptions Traceback a-exctra ads,Ada Exceptions Is_Null_Occurrence a-einuoc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-exceptions-last-chance-handler-a-elchha-ads}@anchor{318}@anchor{gnat_rm/the_gnat_library id13}@anchor{319}
@section @code{Ada.Exceptions.Last_Chance_Handler} (@code{a-elchha.ads})


@geindex Ada.Exceptions.Last_Chance_Handler (a-elchha.ads)

@geindex Null_Occurrence
@geindex testing for

This child subprogram is used for handling otherwise unhandled
exceptions (hence the name last chance), and perform clean ups before
terminating the program. Note that this subprogram never returns.

@node Ada Exceptions Traceback a-exctra ads,Ada Sequential_IO C_Streams a-siocst ads,Ada Exceptions Last_Chance_Handler a-elchha ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-exceptions-traceback-a-exctra-ads}@anchor{31a}@anchor{gnat_rm/the_gnat_library id14}@anchor{31b}
@section @code{Ada.Exceptions.Traceback} (@code{a-exctra.ads})


@geindex Ada.Exceptions.Traceback (a-exctra.ads)

@geindex Traceback for Exception Occurrence

This child package provides the subprogram (@code{Tracebacks}) to
give a traceback array of addresses based on an exception
occurrence.

@node Ada Sequential_IO C_Streams a-siocst ads,Ada Streams Stream_IO C_Streams a-ssicst ads,Ada Exceptions Traceback a-exctra ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-sequential-io-c-streams-a-siocst-ads}@anchor{31c}@anchor{gnat_rm/the_gnat_library id15}@anchor{31d}
@section @code{Ada.Sequential_IO.C_Streams} (@code{a-siocst.ads})


@geindex Ada.Sequential_IO.C_Streams (a-siocst.ads)

@geindex C Streams
@geindex Interfacing with Sequential_IO

This package provides subprograms that allow interfacing between
C streams and @code{Sequential_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Streams Stream_IO C_Streams a-ssicst ads,Ada Strings Unbounded Text_IO a-suteio ads,Ada Sequential_IO C_Streams a-siocst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-streams-stream-io-c-streams-a-ssicst-ads}@anchor{31e}@anchor{gnat_rm/the_gnat_library id16}@anchor{31f}
@section @code{Ada.Streams.Stream_IO.C_Streams} (@code{a-ssicst.ads})


@geindex Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)

@geindex C Streams
@geindex Interfacing with Stream_IO

This package provides subprograms that allow interfacing between
C streams and @code{Stream_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Strings Unbounded Text_IO a-suteio ads,Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads,Ada Streams Stream_IO C_Streams a-ssicst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-strings-unbounded-text-io-a-suteio-ads}@anchor{320}@anchor{gnat_rm/the_gnat_library id17}@anchor{321}
@section @code{Ada.Strings.Unbounded.Text_IO} (@code{a-suteio.ads})


@geindex Ada.Strings.Unbounded.Text_IO (a-suteio.ads)

@geindex Unbounded_String
@geindex IO support

@geindex Text_IO
@geindex extensions for unbounded strings

This package provides subprograms for Text_IO for unbounded
strings, avoiding the necessity for an intermediate operation
with ordinary strings.

@node Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads,Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads,Ada Strings Unbounded Text_IO a-suteio ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-strings-wide-unbounded-wide-text-io-a-swuwti-ads}@anchor{322}@anchor{gnat_rm/the_gnat_library id18}@anchor{323}
@section @code{Ada.Strings.Wide_Unbounded.Wide_Text_IO} (@code{a-swuwti.ads})


@geindex Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)

@geindex Unbounded_Wide_String
@geindex IO support

@geindex Text_IO
@geindex extensions for unbounded wide strings

This package provides subprograms for Text_IO for unbounded
wide strings, avoiding the necessity for an intermediate operation
with ordinary wide strings.

@node Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads,Ada Task_Initialization a-tasini ads,Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-strings-wide-wide-unbounded-wide-wide-text-io-a-szuzti-ads}@anchor{324}@anchor{gnat_rm/the_gnat_library id19}@anchor{325}
@section @code{Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO} (@code{a-szuzti.ads})


@geindex Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads)

@geindex Unbounded_Wide_Wide_String
@geindex IO support

@geindex Text_IO
@geindex extensions for unbounded wide wide strings

This package provides subprograms for Text_IO for unbounded
wide wide strings, avoiding the necessity for an intermediate operation
with ordinary wide wide strings.

@node Ada Task_Initialization a-tasini ads,Ada Text_IO C_Streams a-tiocst ads,Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-task-initialization-a-tasini-ads}@anchor{326}@anchor{gnat_rm/the_gnat_library id20}@anchor{327}
@section @code{Ada.Task_Initialization} (@code{a-tasini.ads})


@geindex Ada.Task_Initialization (a-tasini.ads)

This package provides a way to set a global initialization handler that
is automatically invoked whenever a task is activated. Handlers are
parameterless procedures. Note that such a handler is only invoked for
those tasks activated after the handler is set.

@node Ada Text_IO C_Streams a-tiocst ads,Ada Text_IO Reset_Standard_Files a-tirsfi ads,Ada Task_Initialization a-tasini ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-text-io-c-streams-a-tiocst-ads}@anchor{328}@anchor{gnat_rm/the_gnat_library id21}@anchor{329}
@section @code{Ada.Text_IO.C_Streams} (@code{a-tiocst.ads})


@geindex Ada.Text_IO.C_Streams (a-tiocst.ads)

@geindex C Streams
@geindex Interfacing with `@w{`}Text_IO`@w{`}

This package provides subprograms that allow interfacing between
C streams and @code{Text_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Text_IO Reset_Standard_Files a-tirsfi ads,Ada Wide_Characters Unicode a-wichun ads,Ada Text_IO C_Streams a-tiocst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-text-io-reset-standard-files-a-tirsfi-ads}@anchor{32a}@anchor{gnat_rm/the_gnat_library id22}@anchor{32b}
@section @code{Ada.Text_IO.Reset_Standard_Files} (@code{a-tirsfi.ads})


@geindex Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads)

@geindex Text_IO resetting standard files

This procedure is used to reset the status of the standard files used
by Ada.Text_IO.  This is useful in a situation (such as a restart in an
embedded application) where the status of the files may change during
execution (for example a standard input file may be redefined to be
interactive).

@node Ada Wide_Characters Unicode a-wichun ads,Ada Wide_Text_IO C_Streams a-wtcstr ads,Ada Text_IO Reset_Standard_Files a-tirsfi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-characters-unicode-a-wichun-ads}@anchor{32c}@anchor{gnat_rm/the_gnat_library id23}@anchor{32d}
@section @code{Ada.Wide_Characters.Unicode} (@code{a-wichun.ads})


@geindex Ada.Wide_Characters.Unicode (a-wichun.ads)

@geindex Unicode categorization
@geindex Wide_Character

This package provides subprograms that allow categorization of
Wide_Character values according to Unicode categories.

@node Ada Wide_Text_IO C_Streams a-wtcstr ads,Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads,Ada Wide_Characters Unicode a-wichun ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-text-io-c-streams-a-wtcstr-ads}@anchor{32e}@anchor{gnat_rm/the_gnat_library id24}@anchor{32f}
@section @code{Ada.Wide_Text_IO.C_Streams} (@code{a-wtcstr.ads})


@geindex Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)

@geindex C Streams
@geindex Interfacing with `@w{`}Wide_Text_IO`@w{`}

This package provides subprograms that allow interfacing between
C streams and @code{Wide_Text_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads,Ada Wide_Wide_Characters Unicode a-zchuni ads,Ada Wide_Text_IO C_Streams a-wtcstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-text-io-reset-standard-files-a-wrstfi-ads}@anchor{330}@anchor{gnat_rm/the_gnat_library id25}@anchor{331}
@section @code{Ada.Wide_Text_IO.Reset_Standard_Files} (@code{a-wrstfi.ads})


@geindex Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads)

@geindex Wide_Text_IO resetting standard files

This procedure is used to reset the status of the standard files used
by Ada.Wide_Text_IO.  This is useful in a situation (such as a restart in an
embedded application) where the status of the files may change during
execution (for example a standard input file may be redefined to be
interactive).

@node Ada Wide_Wide_Characters Unicode a-zchuni ads,Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads,Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-wide-characters-unicode-a-zchuni-ads}@anchor{332}@anchor{gnat_rm/the_gnat_library id26}@anchor{333}
@section @code{Ada.Wide_Wide_Characters.Unicode} (@code{a-zchuni.ads})


@geindex Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads)

@geindex Unicode categorization
@geindex Wide_Wide_Character

This package provides subprograms that allow categorization of
Wide_Wide_Character values according to Unicode categories.

@node Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads,Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads,Ada Wide_Wide_Characters Unicode a-zchuni ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-wide-text-io-c-streams-a-ztcstr-ads}@anchor{334}@anchor{gnat_rm/the_gnat_library id27}@anchor{335}
@section @code{Ada.Wide_Wide_Text_IO.C_Streams} (@code{a-ztcstr.ads})


@geindex Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads)

@geindex C Streams
@geindex Interfacing with `@w{`}Wide_Wide_Text_IO`@w{`}

This package provides subprograms that allow interfacing between
C streams and @code{Wide_Wide_Text_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads,GNAT Altivec g-altive ads,Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-wide-text-io-reset-standard-files-a-zrstfi-ads}@anchor{336}@anchor{gnat_rm/the_gnat_library id28}@anchor{337}
@section @code{Ada.Wide_Wide_Text_IO.Reset_Standard_Files} (@code{a-zrstfi.ads})


@geindex Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads)

@geindex Wide_Wide_Text_IO resetting standard files

This procedure is used to reset the status of the standard files used
by Ada.Wide_Wide_Text_IO. This is useful in a situation (such as a
restart in an embedded application) where the status of the files may
change during execution (for example a standard input file may be
redefined to be interactive).

@node GNAT Altivec g-altive ads,GNAT Altivec Conversions g-altcon ads,Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-g-altive-ads}@anchor{338}@anchor{gnat_rm/the_gnat_library id29}@anchor{339}
@section @code{GNAT.Altivec} (@code{g-altive.ads})


@geindex GNAT.Altivec (g-altive.ads)

@geindex AltiVec

This is the root package of the GNAT AltiVec binding. It provides
definitions of constants and types common to all the versions of the
binding.

@node GNAT Altivec Conversions g-altcon ads,GNAT Altivec Vector_Operations g-alveop ads,GNAT Altivec g-altive ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-conversions-g-altcon-ads}@anchor{33a}@anchor{gnat_rm/the_gnat_library id30}@anchor{33b}
@section @code{GNAT.Altivec.Conversions} (@code{g-altcon.ads})


@geindex GNAT.Altivec.Conversions (g-altcon.ads)

@geindex AltiVec

This package provides the Vector/View conversion routines.

@node GNAT Altivec Vector_Operations g-alveop ads,GNAT Altivec Vector_Types g-alvety ads,GNAT Altivec Conversions g-altcon ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-vector-operations-g-alveop-ads}@anchor{33c}@anchor{gnat_rm/the_gnat_library id31}@anchor{33d}
@section @code{GNAT.Altivec.Vector_Operations} (@code{g-alveop.ads})


@geindex GNAT.Altivec.Vector_Operations (g-alveop.ads)

@geindex AltiVec

This package exposes the Ada interface to the AltiVec operations on
vector objects. A soft emulation is included by default in the GNAT
library. The hard binding is provided as a separate package. This unit
is common to both bindings.

@node GNAT Altivec Vector_Types g-alvety ads,GNAT Altivec Vector_Views g-alvevi ads,GNAT Altivec Vector_Operations g-alveop ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-vector-types-g-alvety-ads}@anchor{33e}@anchor{gnat_rm/the_gnat_library id32}@anchor{33f}
@section @code{GNAT.Altivec.Vector_Types} (@code{g-alvety.ads})


@geindex GNAT.Altivec.Vector_Types (g-alvety.ads)

@geindex AltiVec

This package exposes the various vector types part of the Ada binding
to AltiVec facilities.

@node GNAT Altivec Vector_Views g-alvevi ads,GNAT Array_Split g-arrspl ads,GNAT Altivec Vector_Types g-alvety ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-vector-views-g-alvevi-ads}@anchor{340}@anchor{gnat_rm/the_gnat_library id33}@anchor{341}
@section @code{GNAT.Altivec.Vector_Views} (@code{g-alvevi.ads})


@geindex GNAT.Altivec.Vector_Views (g-alvevi.ads)

@geindex AltiVec

This package provides public ‘View’ data types from/to which private
vector representations can be converted via
GNAT.Altivec.Conversions. This allows convenient access to individual
vector elements and provides a simple way to initialize vector
objects.

@node GNAT Array_Split g-arrspl ads,GNAT AWK g-awk ads,GNAT Altivec Vector_Views g-alvevi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-array-split-g-arrspl-ads}@anchor{342}@anchor{gnat_rm/the_gnat_library id34}@anchor{343}
@section @code{GNAT.Array_Split} (@code{g-arrspl.ads})


@geindex GNAT.Array_Split (g-arrspl.ads)

@geindex Array splitter

Useful array-manipulation routines: given a set of separators, split
an array wherever the separators appear, and provide direct access
to the resulting slices.

@node GNAT AWK g-awk ads,GNAT Binary_Search g-binsea ads,GNAT Array_Split g-arrspl ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-awk-g-awk-ads}@anchor{344}@anchor{gnat_rm/the_gnat_library id35}@anchor{345}
@section @code{GNAT.AWK} (@code{g-awk.ads})


@geindex GNAT.AWK (g-awk.ads)

@geindex Parsing

@geindex AWK

Provides AWK-like parsing functions, with an easy interface for parsing one
or more files containing formatted data.  The file is viewed as a database
where each record is a line and a field is a data element in this line.

@node GNAT Binary_Search g-binsea ads,GNAT Bind_Environment g-binenv ads,GNAT AWK g-awk ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-binary-search-g-binsea-ads}@anchor{346}@anchor{gnat_rm/the_gnat_library id36}@anchor{347}
@section @code{GNAT.Binary_Search} (@code{g-binsea.ads})


@geindex GNAT.Binary_Search (g-binsea.ads)

@geindex Binary search

Allow binary search of a sorted array (or of an array-like container;
the generic does not reference the array directly).

@node GNAT Bind_Environment g-binenv ads,GNAT Branch_Prediction g-brapre ads,GNAT Binary_Search g-binsea ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bind-environment-g-binenv-ads}@anchor{348}@anchor{gnat_rm/the_gnat_library id37}@anchor{349}
@section @code{GNAT.Bind_Environment} (@code{g-binenv.ads})


@geindex GNAT.Bind_Environment (g-binenv.ads)

@geindex Bind environment

Provides access to key=value associations captured at bind time.
These associations can be specified using the @code{-V} binder command
line switch.

@node GNAT Branch_Prediction g-brapre ads,GNAT Bounded_Buffers g-boubuf ads,GNAT Bind_Environment g-binenv ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-branch-prediction-g-brapre-ads}@anchor{34a}@anchor{gnat_rm/the_gnat_library id38}@anchor{34b}
@section @code{GNAT.Branch_Prediction} (@code{g-brapre.ads})


@geindex GNAT.Branch_Prediction (g-brapre.ads)

@geindex Branch Prediction

Provides routines giving hints to the branch predictor of the code generator.

@node GNAT Bounded_Buffers g-boubuf ads,GNAT Bounded_Mailboxes g-boumai ads,GNAT Branch_Prediction g-brapre ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bounded-buffers-g-boubuf-ads}@anchor{34c}@anchor{gnat_rm/the_gnat_library id39}@anchor{34d}
@section @code{GNAT.Bounded_Buffers} (@code{g-boubuf.ads})


@geindex GNAT.Bounded_Buffers (g-boubuf.ads)

@geindex Parsing

@geindex Bounded Buffers

Provides a concurrent generic bounded buffer abstraction.  Instances are
useful directly or as parts of the implementations of other abstractions,
such as mailboxes.

@node GNAT Bounded_Mailboxes g-boumai ads,GNAT Bubble_Sort g-bubsor ads,GNAT Bounded_Buffers g-boubuf ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bounded-mailboxes-g-boumai-ads}@anchor{34e}@anchor{gnat_rm/the_gnat_library id40}@anchor{34f}
@section @code{GNAT.Bounded_Mailboxes} (@code{g-boumai.ads})


@geindex GNAT.Bounded_Mailboxes (g-boumai.ads)

@geindex Parsing

@geindex Mailboxes

Provides a thread-safe asynchronous intertask mailbox communication facility.

@node GNAT Bubble_Sort g-bubsor ads,GNAT Bubble_Sort_A g-busora ads,GNAT Bounded_Mailboxes g-boumai ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bubble-sort-g-bubsor-ads}@anchor{350}@anchor{gnat_rm/the_gnat_library id41}@anchor{351}
@section @code{GNAT.Bubble_Sort} (@code{g-bubsor.ads})


@geindex GNAT.Bubble_Sort (g-bubsor.ads)

@geindex Sorting

@geindex Bubble sort

Provides a general implementation of bubble sort usable for sorting arbitrary
data items.  Exchange and comparison procedures are provided by passing
access-to-procedure values.

@node GNAT Bubble_Sort_A g-busora ads,GNAT Bubble_Sort_G g-busorg ads,GNAT Bubble_Sort g-bubsor ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bubble-sort-a-g-busora-ads}@anchor{352}@anchor{gnat_rm/the_gnat_library id42}@anchor{353}
@section @code{GNAT.Bubble_Sort_A} (@code{g-busora.ads})


@geindex GNAT.Bubble_Sort_A (g-busora.ads)

@geindex Sorting

@geindex Bubble sort

Provides a general implementation of bubble sort usable for sorting arbitrary
data items.  Move and comparison procedures are provided by passing
access-to-procedure values. This is an older version, retained for
compatibility. Usually @code{GNAT.Bubble_Sort} will be preferable.

@node GNAT Bubble_Sort_G g-busorg ads,GNAT Byte_Order_Mark g-byorma ads,GNAT Bubble_Sort_A g-busora ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bubble-sort-g-g-busorg-ads}@anchor{354}@anchor{gnat_rm/the_gnat_library id43}@anchor{355}
@section @code{GNAT.Bubble_Sort_G} (@code{g-busorg.ads})


@geindex GNAT.Bubble_Sort_G (g-busorg.ads)

@geindex Sorting

@geindex Bubble sort

Similar to @code{Bubble_Sort_A} except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.

@node GNAT Byte_Order_Mark g-byorma ads,GNAT Byte_Swapping g-bytswa ads,GNAT Bubble_Sort_G g-busorg ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-byte-order-mark-g-byorma-ads}@anchor{356}@anchor{gnat_rm/the_gnat_library id44}@anchor{357}
@section @code{GNAT.Byte_Order_Mark} (@code{g-byorma.ads})


@geindex GNAT.Byte_Order_Mark (g-byorma.ads)

@geindex UTF-8 representation

@geindex Wide character representations

Provides a routine which given a string, reads the start of the string to
see whether it is one of the standard byte order marks (BOM’s) which signal
the encoding of the string. The routine includes detection of special XML
sequences for various UCS input formats.

@node GNAT Byte_Swapping g-bytswa ads,GNAT C_Time g-c_time ads,GNAT Byte_Order_Mark g-byorma ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-byte-swapping-g-bytswa-ads}@anchor{358}@anchor{gnat_rm/the_gnat_library id45}@anchor{359}
@section @code{GNAT.Byte_Swapping} (@code{g-bytswa.ads})


@geindex GNAT.Byte_Swapping (g-bytswa.ads)

@geindex Byte swapping

@geindex Endianness

General routines for swapping the bytes in 2-, 4-, and 8-byte quantities.
Machine-specific implementations are available in some cases.

@node GNAT C_Time g-c_time ads,GNAT Calendar g-calend ads,GNAT Byte_Swapping g-bytswa ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-c-time-g-c-time-ads}@anchor{35a}@anchor{gnat_rm/the_gnat_library id46}@anchor{35b}
@section @code{GNAT.C_Time} (@code{g-c_time.ads})


@geindex GNAT.C_Time (g-c_time.ads)

@geindex Time

Provides the time_t, timeval and timespec types corresponding to the C
types defined by the OS, as well as various conversion functions.

@node GNAT Calendar g-calend ads,GNAT Calendar Time_IO g-catiio ads,GNAT C_Time g-c_time ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-calendar-g-calend-ads}@anchor{35c}@anchor{gnat_rm/the_gnat_library id47}@anchor{35d}
@section @code{GNAT.Calendar} (@code{g-calend.ads})


@geindex GNAT.Calendar (g-calend.ads)

@geindex Calendar

Extends the facilities provided by @code{Ada.Calendar} to include handling
of days of the week, an extended @code{Split} and @code{Time_Of} capability.

@node GNAT Calendar Time_IO g-catiio ads,GNAT CRC32 g-crc32 ads,GNAT Calendar g-calend ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-calendar-time-io-g-catiio-ads}@anchor{35e}@anchor{gnat_rm/the_gnat_library id48}@anchor{35f}
@section @code{GNAT.Calendar.Time_IO} (@code{g-catiio.ads})


@geindex Calendar

@geindex Time

@geindex GNAT.Calendar.Time_IO (g-catiio.ads)

@node GNAT CRC32 g-crc32 ads,GNAT Case_Util g-casuti ads,GNAT Calendar Time_IO g-catiio ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-crc32-g-crc32-ads}@anchor{360}@anchor{gnat_rm/the_gnat_library id49}@anchor{361}
@section @code{GNAT.CRC32} (@code{g-crc32.ads})


@geindex GNAT.CRC32 (g-crc32.ads)

@geindex CRC32

@geindex Cyclic Redundancy Check

This package implements the CRC-32 algorithm.  For a full description
of this algorithm see
`Computation of Cyclic Redundancy Checks via Table Look-Up',
@cite{Communications of the ACM}, Vol. 31 No. 8, pp. 1008-1013,
Aug. 1988.  Sarwate, D.V.

@node GNAT Case_Util g-casuti ads,GNAT CGI g-cgi ads,GNAT CRC32 g-crc32 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-case-util-g-casuti-ads}@anchor{362}@anchor{gnat_rm/the_gnat_library id50}@anchor{363}
@section @code{GNAT.Case_Util} (@code{g-casuti.ads})


@geindex GNAT.Case_Util (g-casuti.ads)

@geindex Casing utilities

@geindex Character handling (`@w{`}GNAT.Case_Util`@w{`})

A set of simple routines for handling upper and lower casing of strings
without the overhead of the full casing tables
in @code{Ada.Characters.Handling}.

@node GNAT CGI g-cgi ads,GNAT CGI Cookie g-cgicoo ads,GNAT Case_Util g-casuti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-cgi-g-cgi-ads}@anchor{364}@anchor{gnat_rm/the_gnat_library id51}@anchor{365}
@section @code{GNAT.CGI} (@code{g-cgi.ads})


@geindex GNAT.CGI (g-cgi.ads)

@geindex CGI (Common Gateway Interface)

This is a package for interfacing a GNAT program with a Web server via the
Common Gateway Interface (CGI).  Basically this package parses the CGI
parameters, which are a set of key/value pairs sent by the Web server.  It
builds a table whose index is the key and provides some services to deal
with this table.

@node GNAT CGI Cookie g-cgicoo ads,GNAT CGI Debug g-cgideb ads,GNAT CGI g-cgi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-cgi-cookie-g-cgicoo-ads}@anchor{366}@anchor{gnat_rm/the_gnat_library id52}@anchor{367}
@section @code{GNAT.CGI.Cookie} (@code{g-cgicoo.ads})


@geindex GNAT.CGI.Cookie (g-cgicoo.ads)

@geindex CGI (Common Gateway Interface) cookie support

@geindex Cookie support in CGI

This is a package to interface a GNAT program with a Web server via the
Common Gateway Interface (CGI).  It exports services to deal with Web
cookies (piece of information kept in the Web client software).

@node GNAT CGI Debug g-cgideb ads,GNAT Command_Line g-comlin ads,GNAT CGI Cookie g-cgicoo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-cgi-debug-g-cgideb-ads}@anchor{368}@anchor{gnat_rm/the_gnat_library id53}@anchor{369}
@section @code{GNAT.CGI.Debug} (@code{g-cgideb.ads})


@geindex GNAT.CGI.Debug (g-cgideb.ads)

@geindex CGI (Common Gateway Interface) debugging

This is a package to help debugging CGI (Common Gateway Interface)
programs written in Ada.

@node GNAT Command_Line g-comlin ads,GNAT Compiler_Version g-comver ads,GNAT CGI Debug g-cgideb ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-command-line-g-comlin-ads}@anchor{36a}@anchor{gnat_rm/the_gnat_library id54}@anchor{36b}
@section @code{GNAT.Command_Line} (@code{g-comlin.ads})


@geindex GNAT.Command_Line (g-comlin.ads)

@geindex Command line

Provides a high level interface to @code{Ada.Command_Line} facilities,
including the ability to scan for named switches with optional parameters
and expand file names using wildcard notations.

@node GNAT Compiler_Version g-comver ads,GNAT Ctrl_C g-ctrl_c ads,GNAT Command_Line g-comlin ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-compiler-version-g-comver-ads}@anchor{36c}@anchor{gnat_rm/the_gnat_library id55}@anchor{36d}
@section @code{GNAT.Compiler_Version} (@code{g-comver.ads})


@geindex GNAT.Compiler_Version (g-comver.ads)

@geindex Compiler Version

@geindex Version
@geindex of compiler

Provides a routine for obtaining the version of the compiler used to
compile the program. More accurately this is the version of the binder
used to bind the program (this will normally be the same as the version
of the compiler if a consistent tool set is used to compile all units
of a partition).

@node GNAT Ctrl_C g-ctrl_c ads,GNAT Current_Exception g-curexc ads,GNAT Compiler_Version g-comver ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-ctrl-c-g-ctrl-c-ads}@anchor{36e}@anchor{gnat_rm/the_gnat_library id56}@anchor{36f}
@section @code{GNAT.Ctrl_C} (@code{g-ctrl_c.ads})


@geindex GNAT.Ctrl_C (g-ctrl_c.ads)

@geindex Interrupt

Provides a simple interface to handle Ctrl-C keyboard events.

@node GNAT Current_Exception g-curexc ads,GNAT Debug_Pools g-debpoo ads,GNAT Ctrl_C g-ctrl_c ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-current-exception-g-curexc-ads}@anchor{370}@anchor{gnat_rm/the_gnat_library id57}@anchor{371}
@section @code{GNAT.Current_Exception} (@code{g-curexc.ads})


@geindex GNAT.Current_Exception (g-curexc.ads)

@geindex Current exception

@geindex Exception retrieval

Provides access to information on the current exception that has been raised
without the need for using the Ada 95 / Ada 2005 exception choice parameter
specification syntax.
This is particularly useful in simulating typical facilities for
obtaining information about exceptions provided by Ada 83 compilers.

@node GNAT Debug_Pools g-debpoo ads,GNAT Debug_Utilities g-debuti ads,GNAT Current_Exception g-curexc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-debug-pools-g-debpoo-ads}@anchor{372}@anchor{gnat_rm/the_gnat_library id58}@anchor{373}
@section @code{GNAT.Debug_Pools} (@code{g-debpoo.ads})


@geindex GNAT.Debug_Pools (g-debpoo.ads)

@geindex Debugging

@geindex Debug pools

@geindex Memory corruption debugging

Provides a debugging storage pools that helps tracking memory corruption
problems.
See @code{The GNAT Debug_Pool Facility} section in the @cite{GNAT User’s Guide}.

@node GNAT Debug_Utilities g-debuti ads,GNAT Decode_String g-decstr ads,GNAT Debug_Pools g-debpoo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-debug-utilities-g-debuti-ads}@anchor{374}@anchor{gnat_rm/the_gnat_library id59}@anchor{375}
@section @code{GNAT.Debug_Utilities} (@code{g-debuti.ads})


@geindex GNAT.Debug_Utilities (g-debuti.ads)

@geindex Debugging

Provides a few useful utilities for debugging purposes, including conversion
to and from string images of address values. Supports both C and Ada formats
for hexadecimal literals.

@node GNAT Decode_String g-decstr ads,GNAT Decode_UTF8_String g-deutst ads,GNAT Debug_Utilities g-debuti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-decode-string-g-decstr-ads}@anchor{376}@anchor{gnat_rm/the_gnat_library id60}@anchor{377}
@section @code{GNAT.Decode_String} (@code{g-decstr.ads})


@geindex GNAT.Decode_String (g-decstr.ads)

@geindex Decoding strings

@geindex String decoding

@geindex Wide character encoding

@geindex UTF-8

@geindex Unicode

A generic package providing routines for decoding wide character and wide wide
character strings encoded as sequences of 8-bit characters using a specified
encoding method. Includes validation routines, and also routines for stepping
to next or previous encoded character in an encoded string.
Useful in conjunction with Unicode character coding. Note there is a
preinstantiation for UTF-8. See next entry.

@node GNAT Decode_UTF8_String g-deutst ads,GNAT Directory_Operations g-dirope ads,GNAT Decode_String g-decstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-decode-utf8-string-g-deutst-ads}@anchor{378}@anchor{gnat_rm/the_gnat_library id61}@anchor{379}
@section @code{GNAT.Decode_UTF8_String} (@code{g-deutst.ads})


@geindex GNAT.Decode_UTF8_String (g-deutst.ads)

@geindex Decoding strings

@geindex Decoding UTF-8 strings

@geindex UTF-8 string decoding

@geindex Wide character decoding

@geindex UTF-8

@geindex Unicode

A preinstantiation of GNAT.Decode_Strings for UTF-8 encoding.

@node GNAT Directory_Operations g-dirope ads,GNAT Directory_Operations Iteration g-diopit ads,GNAT Decode_UTF8_String g-deutst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-directory-operations-g-dirope-ads}@anchor{37a}@anchor{gnat_rm/the_gnat_library id62}@anchor{37b}
@section @code{GNAT.Directory_Operations} (@code{g-dirope.ads})


@geindex GNAT.Directory_Operations (g-dirope.ads)

@geindex Directory operations

Provides a set of routines for manipulating directories, including changing
the current directory, making new directories, and scanning the files in a
directory.

@node GNAT Directory_Operations Iteration g-diopit ads,GNAT Dynamic_HTables g-dynhta ads,GNAT Directory_Operations g-dirope ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-directory-operations-iteration-g-diopit-ads}@anchor{37c}@anchor{gnat_rm/the_gnat_library id63}@anchor{37d}
@section @code{GNAT.Directory_Operations.Iteration} (@code{g-diopit.ads})


@geindex GNAT.Directory_Operations.Iteration (g-diopit.ads)

@geindex Directory operations iteration

A child unit of GNAT.Directory_Operations providing additional operations
for iterating through directories.

@node GNAT Dynamic_HTables g-dynhta ads,GNAT Dynamic_Tables g-dyntab ads,GNAT Directory_Operations Iteration g-diopit ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-dynamic-htables-g-dynhta-ads}@anchor{37e}@anchor{gnat_rm/the_gnat_library id64}@anchor{37f}
@section @code{GNAT.Dynamic_HTables} (@code{g-dynhta.ads})


@geindex GNAT.Dynamic_HTables (g-dynhta.ads)

@geindex Hash tables

A generic implementation of hash tables that can be used to hash arbitrary
data.  Provided in two forms, a simple form with built in hash functions,
and a more complex form in which the hash function is supplied.

This package provides a facility similar to that of @code{GNAT.HTable},
except that this package declares a type that can be used to define
dynamic instances of the hash table, while an instantiation of
@code{GNAT.HTable} creates a single instance of the hash table.

@node GNAT Dynamic_Tables g-dyntab ads,GNAT Encode_String g-encstr ads,GNAT Dynamic_HTables g-dynhta ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-dynamic-tables-g-dyntab-ads}@anchor{380}@anchor{gnat_rm/the_gnat_library id65}@anchor{381}
@section @code{GNAT.Dynamic_Tables} (@code{g-dyntab.ads})


@geindex GNAT.Dynamic_Tables (g-dyntab.ads)

@geindex Table implementation

@geindex Arrays
@geindex extendable

A generic package providing a single dimension array abstraction where the
length of the array can be dynamically modified.

This package provides a facility similar to that of @code{GNAT.Table},
except that this package declares a type that can be used to define
dynamic instances of the table, while an instantiation of
@code{GNAT.Table} creates a single instance of the table type.

@node GNAT Encode_String g-encstr ads,GNAT Encode_UTF8_String g-enutst ads,GNAT Dynamic_Tables g-dyntab ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-encode-string-g-encstr-ads}@anchor{382}@anchor{gnat_rm/the_gnat_library id66}@anchor{383}
@section @code{GNAT.Encode_String} (@code{g-encstr.ads})


@geindex GNAT.Encode_String (g-encstr.ads)

@geindex Encoding strings

@geindex String encoding

@geindex Wide character encoding

@geindex UTF-8

@geindex Unicode

A generic package providing routines for encoding wide character and wide
wide character strings as sequences of 8-bit characters using a specified
encoding method. Useful in conjunction with Unicode character coding.
Note there is a preinstantiation for UTF-8. See next entry.

@node GNAT Encode_UTF8_String g-enutst ads,GNAT Exception_Actions g-excact ads,GNAT Encode_String g-encstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-encode-utf8-string-g-enutst-ads}@anchor{384}@anchor{gnat_rm/the_gnat_library id67}@anchor{385}
@section @code{GNAT.Encode_UTF8_String} (@code{g-enutst.ads})


@geindex GNAT.Encode_UTF8_String (g-enutst.ads)

@geindex Encoding strings

@geindex Encoding UTF-8 strings

@geindex UTF-8 string encoding

@geindex Wide character encoding

@geindex UTF-8

@geindex Unicode

A preinstantiation of GNAT.Encode_Strings for UTF-8 encoding.

@node GNAT Exception_Actions g-excact ads,GNAT Exception_Traces g-exctra ads,GNAT Encode_UTF8_String g-enutst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-exception-actions-g-excact-ads}@anchor{386}@anchor{gnat_rm/the_gnat_library id68}@anchor{387}
@section @code{GNAT.Exception_Actions} (@code{g-excact.ads})


@geindex GNAT.Exception_Actions (g-excact.ads)

@geindex Exception actions

Provides callbacks when an exception is raised. Callbacks can be registered
for specific exceptions, or when any exception is raised. This
can be used for instance to force a core dump to ease debugging.

@node GNAT Exception_Traces g-exctra ads,GNAT Exceptions g-except ads,GNAT Exception_Actions g-excact ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-exception-traces-g-exctra-ads}@anchor{388}@anchor{gnat_rm/the_gnat_library id69}@anchor{389}
@section @code{GNAT.Exception_Traces} (@code{g-exctra.ads})


@geindex GNAT.Exception_Traces (g-exctra.ads)

@geindex Exception traces

@geindex Debugging

Provides an interface allowing to control automatic output upon exception
occurrences.

@node GNAT Exceptions g-except ads,GNAT Expect g-expect ads,GNAT Exception_Traces g-exctra ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-exceptions-g-except-ads}@anchor{38a}@anchor{gnat_rm/the_gnat_library id70}@anchor{38b}
@section @code{GNAT.Exceptions} (@code{g-except.ads})


@geindex GNAT.Exceptions (g-except.ads)

@geindex Exceptions
@geindex Pure

@geindex Pure packages
@geindex exceptions

Normally it is not possible to raise an exception with
a message from a subprogram in a pure package, since the
necessary types and subprograms are in @code{Ada.Exceptions}
which is not a pure unit. @code{GNAT.Exceptions} provides a
facility for getting around this limitation for a few
predefined exceptions, and for example allows raising
@code{Constraint_Error} with a message from a pure subprogram.

@node GNAT Expect g-expect ads,GNAT Expect TTY g-exptty ads,GNAT Exceptions g-except ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-expect-g-expect-ads}@anchor{38c}@anchor{gnat_rm/the_gnat_library id71}@anchor{38d}
@section @code{GNAT.Expect} (@code{g-expect.ads})


@geindex GNAT.Expect (g-expect.ads)

Provides a set of subprograms similar to what is available
with the standard Tcl Expect tool.
It allows you to easily spawn and communicate with an external process.
You can send commands or inputs to the process, and compare the output
with some expected regular expression. Currently @code{GNAT.Expect}
is implemented on all native GNAT ports.
It is not implemented for cross ports, and in particular is not
implemented for VxWorks or LynxOS.

@node GNAT Expect TTY g-exptty ads,GNAT Float_Control g-flocon ads,GNAT Expect g-expect ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-expect-tty-g-exptty-ads}@anchor{38e}@anchor{gnat_rm/the_gnat_library id72}@anchor{38f}
@section @code{GNAT.Expect.TTY} (@code{g-exptty.ads})


@geindex GNAT.Expect.TTY (g-exptty.ads)

As GNAT.Expect but using pseudo-terminal.
Currently @code{GNAT.Expect.TTY} is implemented on all native GNAT
ports. It is not implemented for cross ports, and
in particular is not implemented for VxWorks or LynxOS.

@node GNAT Float_Control g-flocon ads,GNAT Formatted_String g-forstr ads,GNAT Expect TTY g-exptty ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-float-control-g-flocon-ads}@anchor{390}@anchor{gnat_rm/the_gnat_library id73}@anchor{391}
@section @code{GNAT.Float_Control} (@code{g-flocon.ads})


@geindex GNAT.Float_Control (g-flocon.ads)

@geindex Floating-Point Processor

Provides an interface for resetting the floating-point processor into the
mode required for correct semantic operation in Ada.  Some third party
library calls may cause this mode to be modified, and the Reset procedure
in this package can be used to reestablish the required mode.

@node GNAT Formatted_String g-forstr ads,GNAT Generic_Fast_Math_Functions g-gfmafu ads,GNAT Float_Control g-flocon ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-formatted-string-g-forstr-ads}@anchor{392}@anchor{gnat_rm/the_gnat_library id74}@anchor{393}
@section @code{GNAT.Formatted_String} (@code{g-forstr.ads})


@geindex GNAT.Formatted_String (g-forstr.ads)

@geindex Formatted String

Provides support for C/C++ printf() formatted strings. The format is
copied from the printf() routine and should therefore give identical
output. Some generic routines are provided to be able to use types
derived from Integer, Float or enumerations as values for the
formatted string.

@node GNAT Generic_Fast_Math_Functions g-gfmafu ads,GNAT Heap_Sort g-heasor ads,GNAT Formatted_String g-forstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-generic-fast-math-functions-g-gfmafu-ads}@anchor{394}@anchor{gnat_rm/the_gnat_library id75}@anchor{395}
@section @code{GNAT.Generic_Fast_Math_Functions} (@code{g-gfmafu.ads})


@geindex GNAT.Generic_Fast_Math_Functions (g-gfmafu.ads)

@geindex Mathematical functions

Provides direct access to the underlying implementation of the common
mathematical functions, generally from the system mathematical library.
This differs from @code{Ada.Numerics.Generic_Elementary_Functions} in that
the implementation may deviate from the semantics specified for these
functions in the Reference Manual, for example @code{Numerics.Argument_Error}
is not raised. On selected platforms, some of these functions may also
have a vector implementation that can be automatically used by the
compiler when auto-vectorization is enabled.

@node GNAT Heap_Sort g-heasor ads,GNAT Heap_Sort_A g-hesora ads,GNAT Generic_Fast_Math_Functions g-gfmafu ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-heap-sort-g-heasor-ads}@anchor{396}@anchor{gnat_rm/the_gnat_library id76}@anchor{397}
@section @code{GNAT.Heap_Sort} (@code{g-heasor.ads})


@geindex GNAT.Heap_Sort (g-heasor.ads)

@geindex Sorting

Provides a general implementation of heap sort usable for sorting arbitrary
data items. Exchange and comparison procedures are provided by passing
access-to-procedure values.  The algorithm used is a modified heap sort
that performs approximately N*log(N) comparisons in the worst case.

@node GNAT Heap_Sort_A g-hesora ads,GNAT Heap_Sort_G g-hesorg ads,GNAT Heap_Sort g-heasor ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-heap-sort-a-g-hesora-ads}@anchor{398}@anchor{gnat_rm/the_gnat_library id77}@anchor{399}
@section @code{GNAT.Heap_Sort_A} (@code{g-hesora.ads})


@geindex GNAT.Heap_Sort_A (g-hesora.ads)

@geindex Sorting

Provides a general implementation of heap sort usable for sorting arbitrary
data items. Move and comparison procedures are provided by passing
access-to-procedure values.  The algorithm used is a modified heap sort
that performs approximately N*log(N) comparisons in the worst case.
This differs from @code{GNAT.Heap_Sort} in having a less convenient
interface, but may be slightly more efficient.

@node GNAT Heap_Sort_G g-hesorg ads,GNAT HTable g-htable ads,GNAT Heap_Sort_A g-hesora ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-heap-sort-g-g-hesorg-ads}@anchor{39a}@anchor{gnat_rm/the_gnat_library id78}@anchor{39b}
@section @code{GNAT.Heap_Sort_G} (@code{g-hesorg.ads})


@geindex GNAT.Heap_Sort_G (g-hesorg.ads)

@geindex Sorting

Similar to @code{Heap_Sort_A} except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.

@node GNAT HTable g-htable ads,GNAT IO g-io ads,GNAT Heap_Sort_G g-hesorg ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-htable-g-htable-ads}@anchor{39c}@anchor{gnat_rm/the_gnat_library id79}@anchor{39d}
@section @code{GNAT.HTable} (@code{g-htable.ads})


@geindex GNAT.HTable (g-htable.ads)

@geindex Hash tables

A generic implementation of hash tables that can be used to hash arbitrary
data.  Provides two approaches, one a simple static approach, and the other
allowing arbitrary dynamic hash tables.

@node GNAT IO g-io ads,GNAT IO_Aux g-io_aux ads,GNAT HTable g-htable ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-io-g-io-ads}@anchor{39e}@anchor{gnat_rm/the_gnat_library id80}@anchor{39f}
@section @code{GNAT.IO} (@code{g-io.ads})


@geindex GNAT.IO (g-io.ads)

@geindex Simple I/O

@geindex Input/Output facilities

A simple preelaborable input-output package that provides a subset of
simple Text_IO functions for reading characters and strings from
Standard_Input, and writing characters, strings and integers to either
Standard_Output or Standard_Error.

@node GNAT IO_Aux g-io_aux ads,GNAT Lock_Files g-locfil ads,GNAT IO g-io ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-io-aux-g-io-aux-ads}@anchor{3a0}@anchor{gnat_rm/the_gnat_library id81}@anchor{3a1}
@section @code{GNAT.IO_Aux} (@code{g-io_aux.ads})


@geindex GNAT.IO_Aux (g-io_aux.ads)

@geindex Text_IO

@geindex Input/Output facilities

Provides some auxiliary functions for use with Text_IO, including a test
for whether a file exists, and functions for reading a line of text.

@node GNAT Lock_Files g-locfil ads,GNAT MBBS_Discrete_Random g-mbdira ads,GNAT IO_Aux g-io_aux ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-lock-files-g-locfil-ads}@anchor{3a2}@anchor{gnat_rm/the_gnat_library id82}@anchor{3a3}
@section @code{GNAT.Lock_Files} (@code{g-locfil.ads})


@geindex GNAT.Lock_Files (g-locfil.ads)

@geindex File locking

@geindex Locking using files

Provides a general interface for using files as locks.  Can be used for
providing program level synchronization.

@node GNAT MBBS_Discrete_Random g-mbdira ads,GNAT MBBS_Float_Random g-mbflra ads,GNAT Lock_Files g-locfil ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-mbbs-discrete-random-g-mbdira-ads}@anchor{3a4}@anchor{gnat_rm/the_gnat_library id83}@anchor{3a5}
@section @code{GNAT.MBBS_Discrete_Random} (@code{g-mbdira.ads})


@geindex GNAT.MBBS_Discrete_Random (g-mbdira.ads)

@geindex Random number generation

The original implementation of @code{Ada.Numerics.Discrete_Random}.  Uses
a modified version of the Blum-Blum-Shub generator.

@node GNAT MBBS_Float_Random g-mbflra ads,GNAT MD5 g-md5 ads,GNAT MBBS_Discrete_Random g-mbdira ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-mbbs-float-random-g-mbflra-ads}@anchor{3a6}@anchor{gnat_rm/the_gnat_library id84}@anchor{3a7}
@section @code{GNAT.MBBS_Float_Random} (@code{g-mbflra.ads})


@geindex GNAT.MBBS_Float_Random (g-mbflra.ads)

@geindex Random number generation

The original implementation of @code{Ada.Numerics.Float_Random}.  Uses
a modified version of the Blum-Blum-Shub generator.

@node GNAT MD5 g-md5 ads,GNAT Memory_Dump g-memdum ads,GNAT MBBS_Float_Random g-mbflra ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-md5-g-md5-ads}@anchor{3a8}@anchor{gnat_rm/the_gnat_library id85}@anchor{3a9}
@section @code{GNAT.MD5} (@code{g-md5.ads})


@geindex GNAT.MD5 (g-md5.ads)

@geindex Message Digest MD5

Implements the MD5 Message-Digest Algorithm as described in RFC 1321, and
the HMAC-MD5 message authentication function as described in RFC 2104 and
FIPS PUB 198.

@node GNAT Memory_Dump g-memdum ads,GNAT Most_Recent_Exception g-moreex ads,GNAT MD5 g-md5 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-memory-dump-g-memdum-ads}@anchor{3aa}@anchor{gnat_rm/the_gnat_library id86}@anchor{3ab}
@section @code{GNAT.Memory_Dump} (@code{g-memdum.ads})


@geindex GNAT.Memory_Dump (g-memdum.ads)

@geindex Dump Memory

Provides a convenient routine for dumping raw memory to either the
standard output or standard error files. Uses GNAT.IO for actual
output.

@node GNAT Most_Recent_Exception g-moreex ads,GNAT OS_Lib g-os_lib ads,GNAT Memory_Dump g-memdum ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-most-recent-exception-g-moreex-ads}@anchor{3ac}@anchor{gnat_rm/the_gnat_library id87}@anchor{3ad}
@section @code{GNAT.Most_Recent_Exception} (@code{g-moreex.ads})


@geindex GNAT.Most_Recent_Exception (g-moreex.ads)

@geindex Exception
@geindex obtaining most recent

Provides access to the most recently raised exception.  Can be used for
various logging purposes, including duplicating functionality of some
Ada 83 implementation dependent extensions.

@node GNAT OS_Lib g-os_lib ads,GNAT Perfect_Hash_Generators g-pehage ads,GNAT Most_Recent_Exception g-moreex ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-os-lib-g-os-lib-ads}@anchor{3ae}@anchor{gnat_rm/the_gnat_library id88}@anchor{3af}
@section @code{GNAT.OS_Lib} (@code{g-os_lib.ads})


@geindex GNAT.OS_Lib (g-os_lib.ads)

@geindex Operating System interface

@geindex Spawn capability

Provides a range of target independent operating system interface functions,
including time/date management, file operations, subprocess management,
including a portable spawn procedure, and access to environment variables
and error return codes.

@node GNAT Perfect_Hash_Generators g-pehage ads,GNAT Random_Numbers g-rannum ads,GNAT OS_Lib g-os_lib ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-perfect-hash-generators-g-pehage-ads}@anchor{3b0}@anchor{gnat_rm/the_gnat_library id89}@anchor{3b1}
@section @code{GNAT.Perfect_Hash_Generators} (@code{g-pehage.ads})


@geindex GNAT.Perfect_Hash_Generators (g-pehage.ads)

@geindex Hash functions

Provides a generator of static minimal perfect hash functions. No
collisions occur and each item can be retrieved from the table in one
probe (perfect property). The hash table size corresponds to the exact
size of the key set and no larger (minimal property). The key set has to
be known in advance (static property). The hash functions are also order
preserving. If w2 is inserted after w1 in the generator, their
hashcode are in the same order. These hashing functions are very
convenient for use with realtime applications.

@node GNAT Random_Numbers g-rannum ads,GNAT Regexp g-regexp ads,GNAT Perfect_Hash_Generators g-pehage ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-random-numbers-g-rannum-ads}@anchor{3b2}@anchor{gnat_rm/the_gnat_library id90}@anchor{3b3}
@section @code{GNAT.Random_Numbers} (@code{g-rannum.ads})


@geindex GNAT.Random_Numbers (g-rannum.ads)

@geindex Random number generation

Provides random number capabilities which extend those available in the
standard Ada library and are more convenient to use. This package is
however NOT suitable for situations requiring cryptographically secure
randomness.

@node GNAT Regexp g-regexp ads,GNAT Registry g-regist ads,GNAT Random_Numbers g-rannum ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-regexp-g-regexp-ads}@anchor{287}@anchor{gnat_rm/the_gnat_library id91}@anchor{3b4}
@section @code{GNAT.Regexp} (@code{g-regexp.ads})


@geindex GNAT.Regexp (g-regexp.ads)

@geindex Regular expressions

@geindex Pattern matching

A simple implementation of regular expressions, using a subset of regular
expression syntax copied from familiar Unix style utilities.  This is the
simplest of the three pattern matching packages provided, and is particularly
suitable for ‘file globbing’ applications.

@node GNAT Registry g-regist ads,GNAT Regpat g-regpat ads,GNAT Regexp g-regexp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-registry-g-regist-ads}@anchor{3b5}@anchor{gnat_rm/the_gnat_library id92}@anchor{3b6}
@section @code{GNAT.Registry} (@code{g-regist.ads})


@geindex GNAT.Registry (g-regist.ads)

@geindex Windows Registry

This is a high level binding to the Windows registry.  It is possible to
do simple things like reading a key value, creating a new key.  For full
registry API, but at a lower level of abstraction, refer to the Win32.Winreg
package provided with the Win32Ada binding

@node GNAT Regpat g-regpat ads,GNAT Rewrite_Data g-rewdat ads,GNAT Registry g-regist ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-regpat-g-regpat-ads}@anchor{3b7}@anchor{gnat_rm/the_gnat_library id93}@anchor{3b8}
@section @code{GNAT.Regpat} (@code{g-regpat.ads})


@geindex GNAT.Regpat (g-regpat.ads)

@geindex Regular expressions

@geindex Pattern matching

A complete implementation of Unix-style regular expression matching, copied
from the original V7 style regular expression library written in C by
Henry Spencer (and binary compatible with this C library).

@node GNAT Rewrite_Data g-rewdat ads,GNAT Secondary_Stack_Info g-sestin ads,GNAT Regpat g-regpat ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-rewrite-data-g-rewdat-ads}@anchor{3b9}@anchor{gnat_rm/the_gnat_library id94}@anchor{3ba}
@section @code{GNAT.Rewrite_Data} (@code{g-rewdat.ads})


@geindex GNAT.Rewrite_Data (g-rewdat.ads)

@geindex Rewrite data

A unit to rewrite on-the-fly string occurrences in a stream of
data. The implementation has a very minimal memory footprint as the
full content to be processed is not loaded into memory all at once. This makes
this interface usable for large files or socket streams.

@node GNAT Secondary_Stack_Info g-sestin ads,GNAT Semaphores g-semaph ads,GNAT Rewrite_Data g-rewdat ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-secondary-stack-info-g-sestin-ads}@anchor{3bb}@anchor{gnat_rm/the_gnat_library id95}@anchor{3bc}
@section @code{GNAT.Secondary_Stack_Info} (@code{g-sestin.ads})


@geindex GNAT.Secondary_Stack_Info (g-sestin.ads)

@geindex Secondary Stack Info

Provides the capability to query the high water mark of the current task’s
secondary stack.

@node GNAT Semaphores g-semaph ads,GNAT Serial_Communications g-sercom ads,GNAT Secondary_Stack_Info g-sestin ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-semaphores-g-semaph-ads}@anchor{3bd}@anchor{gnat_rm/the_gnat_library id96}@anchor{3be}
@section @code{GNAT.Semaphores} (@code{g-semaph.ads})


@geindex GNAT.Semaphores (g-semaph.ads)

@geindex Semaphores

Provides classic counting and binary semaphores using protected types.

@node GNAT Serial_Communications g-sercom ads,GNAT SHA1 g-sha1 ads,GNAT Semaphores g-semaph ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-serial-communications-g-sercom-ads}@anchor{3bf}@anchor{gnat_rm/the_gnat_library id97}@anchor{3c0}
@section @code{GNAT.Serial_Communications} (@code{g-sercom.ads})


@geindex GNAT.Serial_Communications (g-sercom.ads)

@geindex Serial_Communications

Provides a simple interface to send and receive data over a serial
port. This is only supported on GNU/Linux and Windows.

@node GNAT SHA1 g-sha1 ads,GNAT SHA224 g-sha224 ads,GNAT Serial_Communications g-sercom ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sha1-g-sha1-ads}@anchor{3c1}@anchor{gnat_rm/the_gnat_library id98}@anchor{3c2}
@section @code{GNAT.SHA1} (@code{g-sha1.ads})


@geindex GNAT.SHA1 (g-sha1.ads)

@geindex Secure Hash Algorithm SHA-1

Implements the SHA-1 Secure Hash Algorithm as described in FIPS PUB 180-3
and RFC 3174, and the HMAC-SHA1 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA224 g-sha224 ads,GNAT SHA256 g-sha256 ads,GNAT SHA1 g-sha1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sha224-g-sha224-ads}@anchor{3c3}@anchor{gnat_rm/the_gnat_library id99}@anchor{3c4}
@section @code{GNAT.SHA224} (@code{g-sha224.ads})


@geindex GNAT.SHA224 (g-sha224.ads)

@geindex Secure Hash Algorithm SHA-224

Implements the SHA-224 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA224 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA256 g-sha256 ads,GNAT SHA384 g-sha384 ads,GNAT SHA224 g-sha224 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sha256-g-sha256-ads}@anchor{3c5}@anchor{gnat_rm/the_gnat_library id100}@anchor{3c6}
@section @code{GNAT.SHA256} (@code{g-sha256.ads})


@geindex GNAT.SHA256 (g-sha256.ads)

@geindex Secure Hash Algorithm SHA-256

Implements the SHA-256 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA256 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA384 g-sha384 ads,GNAT SHA512 g-sha512 ads,GNAT SHA256 g-sha256 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sha384-g-sha384-ads}@anchor{3c7}@anchor{gnat_rm/the_gnat_library id101}@anchor{3c8}
@section @code{GNAT.SHA384} (@code{g-sha384.ads})


@geindex GNAT.SHA384 (g-sha384.ads)

@geindex Secure Hash Algorithm SHA-384

Implements the SHA-384 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA384 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA512 g-sha512 ads,GNAT Signals g-signal ads,GNAT SHA384 g-sha384 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sha512-g-sha512-ads}@anchor{3c9}@anchor{gnat_rm/the_gnat_library id102}@anchor{3ca}
@section @code{GNAT.SHA512} (@code{g-sha512.ads})


@geindex GNAT.SHA512 (g-sha512.ads)

@geindex Secure Hash Algorithm SHA-512

Implements the SHA-512 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA512 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT Signals g-signal ads,GNAT Sockets g-socket ads,GNAT SHA512 g-sha512 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-signals-g-signal-ads}@anchor{3cb}@anchor{gnat_rm/the_gnat_library id103}@anchor{3cc}
@section @code{GNAT.Signals} (@code{g-signal.ads})


@geindex GNAT.Signals (g-signal.ads)

@geindex Signals

Provides the ability to manipulate the blocked status of signals on supported
targets.

@node GNAT Sockets g-socket ads,GNAT Source_Info g-souinf ads,GNAT Signals g-signal ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sockets-g-socket-ads}@anchor{3cd}@anchor{gnat_rm/the_gnat_library id104}@anchor{3ce}
@section @code{GNAT.Sockets} (@code{g-socket.ads})


@geindex GNAT.Sockets (g-socket.ads)

@geindex Sockets

A high level and portable interface to develop sockets based applications.
This package is based on the sockets thin binding found in
@code{GNAT.Sockets.Thin}. Currently @code{GNAT.Sockets} is implemented
on all native GNAT ports and on VxWorks cross ports.  It is not implemented for
the LynxOS cross port.

@node GNAT Source_Info g-souinf ads,GNAT Spelling_Checker g-speche ads,GNAT Sockets g-socket ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-source-info-g-souinf-ads}@anchor{3cf}@anchor{gnat_rm/the_gnat_library id105}@anchor{3d0}
@section @code{GNAT.Source_Info} (@code{g-souinf.ads})


@geindex GNAT.Source_Info (g-souinf.ads)

@geindex Source Information

Provides subprograms that give access to source code information known at
compile time, such as the current file name and line number. Also provides
subprograms yielding the date and time of the current compilation (like the
C macros @code{__DATE__} and @code{__TIME__})

@node GNAT Spelling_Checker g-speche ads,GNAT Spelling_Checker_Generic g-spchge ads,GNAT Source_Info g-souinf ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spelling-checker-g-speche-ads}@anchor{3d1}@anchor{gnat_rm/the_gnat_library id106}@anchor{3d2}
@section @code{GNAT.Spelling_Checker} (@code{g-speche.ads})


@geindex GNAT.Spelling_Checker (g-speche.ads)

@geindex Spell checking

Provides a function for determining whether one string is a plausible
near misspelling of another string.

@node GNAT Spelling_Checker_Generic g-spchge ads,GNAT Spitbol Patterns g-spipat ads,GNAT Spelling_Checker g-speche ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spelling-checker-generic-g-spchge-ads}@anchor{3d3}@anchor{gnat_rm/the_gnat_library id107}@anchor{3d4}
@section @code{GNAT.Spelling_Checker_Generic} (@code{g-spchge.ads})


@geindex GNAT.Spelling_Checker_Generic (g-spchge.ads)

@geindex Spell checking

Provides a generic function that can be instantiated with a string type for
determining whether one string is a plausible near misspelling of another
string.

@node GNAT Spitbol Patterns g-spipat ads,GNAT Spitbol g-spitbo ads,GNAT Spelling_Checker_Generic g-spchge ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spitbol-patterns-g-spipat-ads}@anchor{3d5}@anchor{gnat_rm/the_gnat_library id108}@anchor{3d6}
@section @code{GNAT.Spitbol.Patterns} (@code{g-spipat.ads})


@geindex GNAT.Spitbol.Patterns (g-spipat.ads)

@geindex SPITBOL pattern matching

@geindex Pattern matching

A complete implementation of SNOBOL4 style pattern matching.  This is the
most elaborate of the pattern matching packages provided.  It fully duplicates
the SNOBOL4 dynamic pattern construction and matching capabilities, using the
efficient algorithm developed by Robert Dewar for the SPITBOL system.

@node GNAT Spitbol g-spitbo ads,GNAT Spitbol Table_Boolean g-sptabo ads,GNAT Spitbol Patterns g-spipat ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spitbol-g-spitbo-ads}@anchor{3d7}@anchor{gnat_rm/the_gnat_library id109}@anchor{3d8}
@section @code{GNAT.Spitbol} (@code{g-spitbo.ads})


@geindex GNAT.Spitbol (g-spitbo.ads)

@geindex SPITBOL interface

The top level package of the collection of SPITBOL-style functionality, this
package provides basic SNOBOL4 string manipulation functions, such as
Pad, Reverse, Trim, Substr capability, as well as a generic table function
useful for constructing arbitrary mappings from strings in the style of
the SNOBOL4 TABLE function.

@node GNAT Spitbol Table_Boolean g-sptabo ads,GNAT Spitbol Table_Integer g-sptain ads,GNAT Spitbol g-spitbo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spitbol-table-boolean-g-sptabo-ads}@anchor{3d9}@anchor{gnat_rm/the_gnat_library id110}@anchor{3da}
@section @code{GNAT.Spitbol.Table_Boolean} (@code{g-sptabo.ads})


@geindex GNAT.Spitbol.Table_Boolean (g-sptabo.ads)

@geindex Sets of strings

@geindex SPITBOL Tables

A library level of instantiation of @code{GNAT.Spitbol.Patterns.Table}
for type @code{Standard.Boolean}, giving an implementation of sets of
string values.

@node GNAT Spitbol Table_Integer g-sptain ads,GNAT Spitbol Table_VString g-sptavs ads,GNAT Spitbol Table_Boolean g-sptabo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spitbol-table-integer-g-sptain-ads}@anchor{3db}@anchor{gnat_rm/the_gnat_library id111}@anchor{3dc}
@section @code{GNAT.Spitbol.Table_Integer} (@code{g-sptain.ads})


@geindex GNAT.Spitbol.Table_Integer (g-sptain.ads)

@geindex Integer maps

@geindex Maps

@geindex SPITBOL Tables

A library level of instantiation of @code{GNAT.Spitbol.Patterns.Table}
for type @code{Standard.Integer}, giving an implementation of maps
from string to integer values.

@node GNAT Spitbol Table_VString g-sptavs ads,GNAT SSE g-sse ads,GNAT Spitbol Table_Integer g-sptain ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spitbol-table-vstring-g-sptavs-ads}@anchor{3dd}@anchor{gnat_rm/the_gnat_library id112}@anchor{3de}
@section @code{GNAT.Spitbol.Table_VString} (@code{g-sptavs.ads})


@geindex GNAT.Spitbol.Table_VString (g-sptavs.ads)

@geindex String maps

@geindex Maps

@geindex SPITBOL Tables

A library level of instantiation of @code{GNAT.Spitbol.Patterns.Table} for
a variable length string type, giving an implementation of general
maps from strings to strings.

@node GNAT SSE g-sse ads,GNAT SSE Vector_Types g-ssvety ads,GNAT Spitbol Table_VString g-sptavs ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sse-g-sse-ads}@anchor{3df}@anchor{gnat_rm/the_gnat_library id113}@anchor{3e0}
@section @code{GNAT.SSE} (@code{g-sse.ads})


@geindex GNAT.SSE (g-sse.ads)

Root of a set of units aimed at offering Ada bindings to a subset of
the Intel(r) Streaming SIMD Extensions with GNAT on the x86 family of
targets.  It exposes vector component types together with a general
introduction to the binding contents and use.

@node GNAT SSE Vector_Types g-ssvety ads,GNAT String_Hash g-strhas ads,GNAT SSE g-sse ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sse-vector-types-g-ssvety-ads}@anchor{3e1}@anchor{gnat_rm/the_gnat_library id114}@anchor{3e2}
@section @code{GNAT.SSE.Vector_Types} (@code{g-ssvety.ads})


@geindex GNAT.SSE.Vector_Types (g-ssvety.ads)

SSE vector types for use with SSE related intrinsics.

@node GNAT String_Hash g-strhas ads,GNAT Strings g-string ads,GNAT SSE Vector_Types g-ssvety ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-string-hash-g-strhas-ads}@anchor{3e3}@anchor{gnat_rm/the_gnat_library id115}@anchor{3e4}
@section @code{GNAT.String_Hash} (@code{g-strhas.ads})


@geindex GNAT.String_Hash (g-strhas.ads)

@geindex Hash functions

Provides a generic hash function working on arrays of scalars. Both the scalar
type and the hash result type are parameters.

@node GNAT Strings g-string ads,GNAT String_Split g-strspl ads,GNAT String_Hash g-strhas ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-strings-g-string-ads}@anchor{3e5}@anchor{gnat_rm/the_gnat_library id116}@anchor{3e6}
@section @code{GNAT.Strings} (@code{g-string.ads})


@geindex GNAT.Strings (g-string.ads)

Common String access types and related subprograms. Basically it
defines a string access and an array of string access types.

@node GNAT String_Split g-strspl ads,GNAT Table g-table ads,GNAT Strings g-string ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-string-split-g-strspl-ads}@anchor{3e7}@anchor{gnat_rm/the_gnat_library id117}@anchor{3e8}
@section @code{GNAT.String_Split} (@code{g-strspl.ads})


@geindex GNAT.String_Split (g-strspl.ads)

@geindex String splitter

Useful string manipulation routines: given a set of separators, split
a string wherever the separators appear, and provide direct access
to the resulting slices. This package is instantiated from
@code{GNAT.Array_Split}.

@node GNAT Table g-table ads,GNAT Task_Lock g-tasloc ads,GNAT String_Split g-strspl ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-table-g-table-ads}@anchor{3e9}@anchor{gnat_rm/the_gnat_library id118}@anchor{3ea}
@section @code{GNAT.Table} (@code{g-table.ads})


@geindex GNAT.Table (g-table.ads)

@geindex Table implementation

@geindex Arrays
@geindex extendable

A generic package providing a single dimension array abstraction where the
length of the array can be dynamically modified.

This package provides a facility similar to that of @code{GNAT.Dynamic_Tables},
except that this package declares a single instance of the table type,
while an instantiation of @code{GNAT.Dynamic_Tables} creates a type that can be
used to define dynamic instances of the table.

@node GNAT Task_Lock g-tasloc ads,GNAT Time_Stamp g-timsta ads,GNAT Table g-table ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-task-lock-g-tasloc-ads}@anchor{3eb}@anchor{gnat_rm/the_gnat_library id119}@anchor{3ec}
@section @code{GNAT.Task_Lock} (@code{g-tasloc.ads})


@geindex GNAT.Task_Lock (g-tasloc.ads)

@geindex Task synchronization

@geindex Task locking

@geindex Locking

A very simple facility for locking and unlocking sections of code using a
single global task lock.  Appropriate for use in situations where contention
between tasks is very rarely expected.

@node GNAT Time_Stamp g-timsta ads,GNAT Threads g-thread ads,GNAT Task_Lock g-tasloc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-time-stamp-g-timsta-ads}@anchor{3ed}@anchor{gnat_rm/the_gnat_library id120}@anchor{3ee}
@section @code{GNAT.Time_Stamp} (@code{g-timsta.ads})


@geindex GNAT.Time_Stamp (g-timsta.ads)

@geindex Time stamp

@geindex Current time

Provides a simple function that returns a string YYYY-MM-DD HH:MM:SS.SS that
represents the current date and time in ISO 8601 format. This is a very simple
routine with minimal code and there are no dependencies on any other unit.

@node GNAT Threads g-thread ads,GNAT Traceback g-traceb ads,GNAT Time_Stamp g-timsta ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-threads-g-thread-ads}@anchor{3ef}@anchor{gnat_rm/the_gnat_library id121}@anchor{3f0}
@section @code{GNAT.Threads} (@code{g-thread.ads})


@geindex GNAT.Threads (g-thread.ads)

@geindex Foreign threads

@geindex Threads
@geindex foreign

Provides facilities for dealing with foreign threads which need to be known
by the GNAT run-time system. Consult the documentation of this package for
further details if your program has threads that are created by a non-Ada
environment which then accesses Ada code.

@node GNAT Traceback g-traceb ads,GNAT Traceback Symbolic g-trasym ads,GNAT Threads g-thread ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-traceback-g-traceb-ads}@anchor{3f1}@anchor{gnat_rm/the_gnat_library id122}@anchor{3f2}
@section @code{GNAT.Traceback} (@code{g-traceb.ads})


@geindex GNAT.Traceback (g-traceb.ads)

@geindex Trace back facilities

Provides a facility for obtaining non-symbolic traceback information, useful
in various debugging situations.

@node GNAT Traceback Symbolic g-trasym ads,GNAT UTF_32 g-utf_32 ads,GNAT Traceback g-traceb ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-traceback-symbolic-g-trasym-ads}@anchor{3f3}@anchor{gnat_rm/the_gnat_library id123}@anchor{3f4}
@section @code{GNAT.Traceback.Symbolic} (@code{g-trasym.ads})


@geindex GNAT.Traceback.Symbolic (g-trasym.ads)

@geindex Trace back facilities

@node GNAT UTF_32 g-utf_32 ads,GNAT UTF_32_Spelling_Checker g-u3spch ads,GNAT Traceback Symbolic g-trasym ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-utf-32-g-utf-32-ads}@anchor{3f5}@anchor{gnat_rm/the_gnat_library id124}@anchor{3f6}
@section @code{GNAT.UTF_32} (@code{g-utf_32.ads})


@geindex GNAT.UTF_32 (g-utf_32.ads)

@geindex Wide character codes

This is a package intended to be used in conjunction with the
@code{Wide_Character} type in Ada 95 and the
@code{Wide_Wide_Character} type in Ada 2005 (available
in @code{GNAT} in Ada 2005 mode). This package contains
Unicode categorization routines, as well as lexical
categorization routines corresponding to the Ada 2005
lexical rules for identifiers and strings, and also a
lower case to upper case fold routine corresponding to
the Ada 2005 rules for identifier equivalence.

@node GNAT UTF_32_Spelling_Checker g-u3spch ads,GNAT Wide_Spelling_Checker g-wispch ads,GNAT UTF_32 g-utf_32 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-utf-32-spelling-checker-g-u3spch-ads}@anchor{3f7}@anchor{gnat_rm/the_gnat_library id125}@anchor{3f8}
@section @code{GNAT.UTF_32_Spelling_Checker} (@code{g-u3spch.ads})


@geindex GNAT.UTF_32_Spelling_Checker (g-u3spch.ads)

@geindex Spell checking

Provides a function for determining whether one wide wide string is a plausible
near misspelling of another wide wide string, where the strings are represented
using the UTF_32_String type defined in System.Wch_Cnv.

@node GNAT Wide_Spelling_Checker g-wispch ads,GNAT Wide_String_Split g-wistsp ads,GNAT UTF_32_Spelling_Checker g-u3spch ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-spelling-checker-g-wispch-ads}@anchor{3f9}@anchor{gnat_rm/the_gnat_library id126}@anchor{3fa}
@section @code{GNAT.Wide_Spelling_Checker} (@code{g-wispch.ads})


@geindex GNAT.Wide_Spelling_Checker (g-wispch.ads)

@geindex Spell checking

Provides a function for determining whether one wide string is a plausible
near misspelling of another wide string.

@node GNAT Wide_String_Split g-wistsp ads,GNAT Wide_Wide_Spelling_Checker g-zspche ads,GNAT Wide_Spelling_Checker g-wispch ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-string-split-g-wistsp-ads}@anchor{3fb}@anchor{gnat_rm/the_gnat_library id127}@anchor{3fc}
@section @code{GNAT.Wide_String_Split} (@code{g-wistsp.ads})


@geindex GNAT.Wide_String_Split (g-wistsp.ads)

@geindex Wide_String splitter

Useful wide string manipulation routines: given a set of separators, split
a wide string wherever the separators appear, and provide direct access
to the resulting slices. This package is instantiated from
@code{GNAT.Array_Split}.

@node GNAT Wide_Wide_Spelling_Checker g-zspche ads,GNAT Wide_Wide_String_Split g-zistsp ads,GNAT Wide_String_Split g-wistsp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-wide-spelling-checker-g-zspche-ads}@anchor{3fd}@anchor{gnat_rm/the_gnat_library id128}@anchor{3fe}
@section @code{GNAT.Wide_Wide_Spelling_Checker} (@code{g-zspche.ads})


@geindex GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads)

@geindex Spell checking

Provides a function for determining whether one wide wide string is a plausible
near misspelling of another wide wide string.

@node GNAT Wide_Wide_String_Split g-zistsp ads,Interfaces C Extensions i-cexten ads,GNAT Wide_Wide_Spelling_Checker g-zspche ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-wide-string-split-g-zistsp-ads}@anchor{3ff}@anchor{gnat_rm/the_gnat_library id129}@anchor{400}
@section @code{GNAT.Wide_Wide_String_Split} (@code{g-zistsp.ads})


@geindex GNAT.Wide_Wide_String_Split (g-zistsp.ads)

@geindex Wide_Wide_String splitter

Useful wide wide string manipulation routines: given a set of separators, split
a wide wide string wherever the separators appear, and provide direct access
to the resulting slices. This package is instantiated from
@code{GNAT.Array_Split}.

@node Interfaces C Extensions i-cexten ads,Interfaces C Streams i-cstrea ads,GNAT Wide_Wide_String_Split g-zistsp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id130}@anchor{401}@anchor{gnat_rm/the_gnat_library interfaces-c-extensions-i-cexten-ads}@anchor{402}
@section @code{Interfaces.C.Extensions} (@code{i-cexten.ads})


@geindex Interfaces.C.Extensions (i-cexten.ads)

This package contains additional C-related definitions, intended
for use with either manually or automatically generated bindings
to C libraries.

@node Interfaces C Streams i-cstrea ads,Interfaces Packed_Decimal i-pacdec ads,Interfaces C Extensions i-cexten ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id131}@anchor{403}@anchor{gnat_rm/the_gnat_library interfaces-c-streams-i-cstrea-ads}@anchor{404}
@section @code{Interfaces.C.Streams} (@code{i-cstrea.ads})


@geindex Interfaces.C.Streams (i-cstrea.ads)

@geindex C streams
@geindex interfacing

This package is a binding for the most commonly used operations
on C streams.

@node Interfaces Packed_Decimal i-pacdec ads,Interfaces VxWorks i-vxwork ads,Interfaces C Streams i-cstrea ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id132}@anchor{405}@anchor{gnat_rm/the_gnat_library interfaces-packed-decimal-i-pacdec-ads}@anchor{406}
@section @code{Interfaces.Packed_Decimal} (@code{i-pacdec.ads})


@geindex Interfaces.Packed_Decimal (i-pacdec.ads)

@geindex IBM Packed Format

@geindex Packed Decimal

This package provides a set of routines for conversions to and
from a packed decimal format compatible with that used on IBM
mainframes.

@node Interfaces VxWorks i-vxwork ads,Interfaces VxWorks IO i-vxwoio ads,Interfaces Packed_Decimal i-pacdec ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id133}@anchor{407}@anchor{gnat_rm/the_gnat_library interfaces-vxworks-i-vxwork-ads}@anchor{408}
@section @code{Interfaces.VxWorks} (@code{i-vxwork.ads})


@geindex Interfaces.VxWorks (i-vxwork.ads)

@geindex Interfacing to VxWorks

@geindex VxWorks
@geindex interfacing

This package provides a limited binding to the VxWorks API.

@node Interfaces VxWorks IO i-vxwoio ads,System Address_Image s-addima ads,Interfaces VxWorks i-vxwork ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id134}@anchor{409}@anchor{gnat_rm/the_gnat_library interfaces-vxworks-io-i-vxwoio-ads}@anchor{40a}
@section @code{Interfaces.VxWorks.IO} (@code{i-vxwoio.ads})


@geindex Interfaces.VxWorks.IO (i-vxwoio.ads)

@geindex Interfacing to VxWorks' I/O

@geindex VxWorks
@geindex I/O interfacing

@geindex VxWorks
@geindex Get_Immediate

@geindex Get_Immediate
@geindex VxWorks

This package provides a binding to the ioctl (IO/Control)
function of VxWorks, defining a set of option values and
function codes. A particular use of this package is
to enable the use of Get_Immediate under VxWorks.

@node System Address_Image s-addima ads,System Assertions s-assert ads,Interfaces VxWorks IO i-vxwoio ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id135}@anchor{40b}@anchor{gnat_rm/the_gnat_library system-address-image-s-addima-ads}@anchor{40c}
@section @code{System.Address_Image} (@code{s-addima.ads})


@geindex System.Address_Image (s-addima.ads)

@geindex Address image

@geindex Image
@geindex of an address

This function provides a useful debugging
function that gives an (implementation dependent)
string which identifies an address.

@node System Assertions s-assert ads,System Atomic_Counters s-atocou ads,System Address_Image s-addima ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id136}@anchor{40d}@anchor{gnat_rm/the_gnat_library system-assertions-s-assert-ads}@anchor{40e}
@section @code{System.Assertions} (@code{s-assert.ads})


@geindex System.Assertions (s-assert.ads)

@geindex Assertions

@geindex Assert_Failure
@geindex exception

This package provides the declaration of the exception raised
by an run-time assertion failure, as well as the routine that
is used internally to raise this assertion.

@node System Atomic_Counters s-atocou ads,System Memory s-memory ads,System Assertions s-assert ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id137}@anchor{40f}@anchor{gnat_rm/the_gnat_library system-atomic-counters-s-atocou-ads}@anchor{410}
@section @code{System.Atomic_Counters} (@code{s-atocou.ads})


@geindex System.Atomic_Counters (s-atocou.ads)

This package provides the declaration of an atomic counter type,
together with efficient routines (using hardware
synchronization primitives) for incrementing, decrementing,
and testing of these counters. This package is implemented
on most targets, including all Alpha, AARCH64, ARM, ia64, PowerPC, SPARC V9,
x86, and x86_64 platforms.

@node System Memory s-memory ads,System Multiprocessors s-multip ads,System Atomic_Counters s-atocou ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id138}@anchor{411}@anchor{gnat_rm/the_gnat_library system-memory-s-memory-ads}@anchor{412}
@section @code{System.Memory} (@code{s-memory.ads})


@geindex System.Memory (s-memory.ads)

@geindex Memory allocation

This package provides the interface to the low level routines used
by the generated code for allocation and freeing storage for the
default storage pool (analogous to the C routines malloc and free).
It also provides a reallocation interface analogous to the C routine
realloc. The body of this unit may be modified to provide alternative
allocation mechanisms for the default pool, and in addition, direct
calls to this unit may be made for low level allocation uses (for
example see the body of @code{GNAT.Tables}).

@node System Multiprocessors s-multip ads,System Multiprocessors Dispatching_Domains s-mudido ads,System Memory s-memory ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id139}@anchor{413}@anchor{gnat_rm/the_gnat_library system-multiprocessors-s-multip-ads}@anchor{414}
@section @code{System.Multiprocessors} (@code{s-multip.ads})


@geindex System.Multiprocessors (s-multip.ads)

@geindex Multiprocessor interface

This is an Ada 2012 unit defined in the Ada 2012 Reference Manual, but
in GNAT we also make it available in Ada 95 and Ada 2005 (where it is
technically an implementation-defined addition).

@node System Multiprocessors Dispatching_Domains s-mudido ads,System Partition_Interface s-parint ads,System Multiprocessors s-multip ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id140}@anchor{415}@anchor{gnat_rm/the_gnat_library system-multiprocessors-dispatching-domains-s-mudido-ads}@anchor{416}
@section @code{System.Multiprocessors.Dispatching_Domains} (@code{s-mudido.ads})


@geindex System.Multiprocessors.Dispatching_Domains (s-mudido.ads)

@geindex Multiprocessor interface

This is an Ada 2012 unit defined in the Ada 2012 Reference Manual, but
in GNAT we also make it available in Ada 95 and Ada 2005 (where it is
technically an implementation-defined addition).

@node System Partition_Interface s-parint ads,System Pool_Global s-pooglo ads,System Multiprocessors Dispatching_Domains s-mudido ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id141}@anchor{417}@anchor{gnat_rm/the_gnat_library system-partition-interface-s-parint-ads}@anchor{418}
@section @code{System.Partition_Interface} (@code{s-parint.ads})


@geindex System.Partition_Interface (s-parint.ads)

@geindex Partition interfacing functions

This package provides facilities for partition interfacing.  It
is used primarily in a distribution context when using Annex E
with @code{PolyORB}.  `NB!' See the note in @ref{273,,PolyORB} regarding
the lifetime of this product.

@node System Pool_Global s-pooglo ads,System Pool_Local s-pooloc ads,System Partition_Interface s-parint ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id142}@anchor{419}@anchor{gnat_rm/the_gnat_library system-pool-global-s-pooglo-ads}@anchor{41a}
@section @code{System.Pool_Global} (@code{s-pooglo.ads})


@geindex System.Pool_Global (s-pooglo.ads)

@geindex Storage pool
@geindex global

@geindex Global storage pool

This package provides a storage pool that is equivalent to the default
storage pool used for access types for which no pool is specifically
declared. It uses malloc/free to allocate/free and does not attempt to
do any automatic reclamation.

@node System Pool_Local s-pooloc ads,System Restrictions s-restri ads,System Pool_Global s-pooglo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id143}@anchor{41b}@anchor{gnat_rm/the_gnat_library system-pool-local-s-pooloc-ads}@anchor{41c}
@section @code{System.Pool_Local} (@code{s-pooloc.ads})


@geindex System.Pool_Local (s-pooloc.ads)

@geindex Storage pool
@geindex local

@geindex Local storage pool

This package provides a storage pool that is intended for use with locally
defined access types. It uses malloc/free for allocate/free, and maintains
a list of allocated blocks, so that all storage allocated for the pool can
be freed automatically when the pool is finalized.

@node System Restrictions s-restri ads,System Rident s-rident ads,System Pool_Local s-pooloc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id144}@anchor{41d}@anchor{gnat_rm/the_gnat_library system-restrictions-s-restri-ads}@anchor{41e}
@section @code{System.Restrictions} (@code{s-restri.ads})


@geindex System.Restrictions (s-restri.ads)

@geindex Run-time restrictions access

This package provides facilities for accessing at run time
the status of restrictions specified at compile time for
the partition. Information is available both with regard
to actual restrictions specified, and with regard to
compiler determined information on which restrictions
are violated by one or more packages in the partition.

@node System Rident s-rident ads,System Strings Stream_Ops s-ststop ads,System Restrictions s-restri ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id145}@anchor{41f}@anchor{gnat_rm/the_gnat_library system-rident-s-rident-ads}@anchor{420}
@section @code{System.Rident} (@code{s-rident.ads})


@geindex System.Rident (s-rident.ads)

@geindex Restrictions definitions

This package provides definitions of the restrictions
identifiers supported by GNAT, and also the format of
the restrictions provided in package System.Restrictions.
It is not normally necessary to @code{with} this generic package
since the necessary instantiation is included in
package System.Restrictions.

@node System Strings Stream_Ops s-ststop ads,System Unsigned_Types s-unstyp ads,System Rident s-rident ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id146}@anchor{421}@anchor{gnat_rm/the_gnat_library system-strings-stream-ops-s-ststop-ads}@anchor{422}
@section @code{System.Strings.Stream_Ops} (@code{s-ststop.ads})


@geindex System.Strings.Stream_Ops (s-ststop.ads)

@geindex Stream operations

@geindex String stream operations

This package provides a set of stream subprograms for standard string types.
It is intended primarily to support implicit use of such subprograms when
stream attributes are applied to string types, but the subprograms in this
package can be used directly by application programs.

@node System Unsigned_Types s-unstyp ads,System Wch_Cnv s-wchcnv ads,System Strings Stream_Ops s-ststop ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id147}@anchor{423}@anchor{gnat_rm/the_gnat_library system-unsigned-types-s-unstyp-ads}@anchor{424}
@section @code{System.Unsigned_Types} (@code{s-unstyp.ads})


@geindex System.Unsigned_Types (s-unstyp.ads)

This package contains definitions of standard unsigned types that
correspond in size to the standard signed types declared in Standard,
and (unlike the types in Interfaces) have corresponding names. It
also contains some related definitions for other specialized types
used by the compiler in connection with packed array types.

@node System Wch_Cnv s-wchcnv ads,System Wch_Con s-wchcon ads,System Unsigned_Types s-unstyp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id148}@anchor{425}@anchor{gnat_rm/the_gnat_library system-wch-cnv-s-wchcnv-ads}@anchor{426}
@section @code{System.Wch_Cnv} (@code{s-wchcnv.ads})


@geindex System.Wch_Cnv (s-wchcnv.ads)

@geindex Wide Character
@geindex Representation

@geindex Wide String
@geindex Conversion

@geindex Representation of wide characters

This package provides routines for converting between
wide and wide wide characters and a representation as a value of type
@code{Standard.String}, using a specified wide character
encoding method.  It uses definitions in
package @code{System.Wch_Con}.

@node System Wch_Con s-wchcon ads,,System Wch_Cnv s-wchcnv ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id149}@anchor{427}@anchor{gnat_rm/the_gnat_library system-wch-con-s-wchcon-ads}@anchor{428}
@section @code{System.Wch_Con} (@code{s-wchcon.ads})


@geindex System.Wch_Con (s-wchcon.ads)

This package provides definitions and descriptions of
the various methods used for encoding wide characters
in ordinary strings.  These definitions are used by
the package @code{System.Wch_Cnv}.

@node Interfacing to Other Languages,Specialized Needs Annexes,The GNAT Library,Top
@anchor{gnat_rm/interfacing_to_other_languages doc}@anchor{429}@anchor{gnat_rm/interfacing_to_other_languages id1}@anchor{42a}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-other-languages}@anchor{12}
@chapter Interfacing to Other Languages


The facilities in Annex B of the Ada Reference Manual are fully
implemented in GNAT, and in addition, a full interface to C++ is
provided.

@menu
* Interfacing to C:: 
* Interfacing to C++:: 
* Interfacing to COBOL:: 
* Interfacing to Fortran:: 
* Interfacing to non-GNAT Ada code:: 

@end menu

@node Interfacing to C,Interfacing to C++,,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id2}@anchor{42b}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-c}@anchor{42c}
@section Interfacing to C


Interfacing to C with GNAT can use one of two approaches:


@itemize *

@item 
The types in the package @code{Interfaces.C} may be used.

@item 
Standard Ada types may be used directly.  This may be less portable to
other compilers, but will work on all GNAT compilers, which guarantee
correspondence between the C and Ada types.
@end itemize

Pragma @code{Convention C} may be applied to Ada types, but mostly has no
effect, since this is the default.  The following table shows the
correspondence between Ada scalar types and the corresponding C types.


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Ada Type

@tab

C Type

@item

@code{Integer}

@tab

@code{int}

@item

@code{Short_Integer}

@tab

@code{short}

@item

@code{Short_Short_Integer}

@tab

@code{signed char}

@item

@code{Long_Integer}

@tab

@code{long}

@item

@code{Long_Long_Integer}

@tab

@code{long long}

@item

@code{Short_Float}

@tab

@code{float}

@item

@code{Float}

@tab

@code{float}

@item

@code{Long_Float}

@tab

@code{double}

@item

@code{Long_Long_Float}

@tab

This is the longest floating-point type supported by the hardware.

@end multitable


Additionally, there are the following general correspondences between Ada
and C types:


@itemize *

@item 
Ada enumeration types map to C enumeration types directly if pragma
@code{Convention C} is specified, which causes them to have a length of
32 bits, except for boolean types which map to C99 @code{bool} and for
which the length is 8 bits.
Without pragma @code{Convention C}, Ada enumeration types map to
8, 16, or 32 bits (i.e., C types @code{signed char}, @code{short},
@code{int}, respectively) depending on the number of values passed.
This is the only case in which pragma @code{Convention C} affects the
representation of an Ada type.

@item 
Ada access types map to C pointers, except for the case of pointers to
unconstrained types in Ada, which have no direct C equivalent.

@item 
Ada arrays map directly to C arrays.

@item 
Ada records map directly to C structures.

@item 
Packed Ada records map to C structures where all members are bit fields
of the length corresponding to the @code{type'Size} value in Ada.
@end itemize

@node Interfacing to C++,Interfacing to COBOL,Interfacing to C,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id3}@anchor{4b}@anchor{gnat_rm/interfacing_to_other_languages id4}@anchor{42d}
@section Interfacing to C++


The interface to C++ makes use of the following pragmas, which are
primarily intended to be constructed automatically using a binding generator
tool, although it is possible to construct them by hand.

Using these pragmas it is possible to achieve complete
inter-operability between Ada tagged types and C++ class definitions.
See @ref{7,,Implementation Defined Pragmas}, for more details.


@table @asis

@item @code{pragma CPP_Class ([Entity =>] @var{LOCAL_NAME})}

The argument denotes an entity in the current declarative region that is
declared as a tagged or untagged record type. It indicates that the type
corresponds to an externally declared C++ class type, and is to be laid
out the same way that C++ would lay out the type.

Note: Pragma @code{CPP_Class} is currently obsolete. It is supported
for backward compatibility but its functionality is available
using pragma @code{Import} with @code{Convention} = @code{CPP}.

@item @code{pragma CPP_Constructor ([Entity =>] @var{LOCAL_NAME})}

This pragma identifies an imported function (imported in the usual way
with pragma @code{Import}) as corresponding to a C++ constructor.
@end table

A few restrictions are placed on the use of the @code{Access} attribute
in conjunction with subprograms subject to convention @code{CPP}: the
attribute may be used neither on primitive operations of a tagged
record type with convention @code{CPP}, imported or not, nor on
subprograms imported with pragma @code{CPP_Constructor}.

In addition, C++ exceptions are propagated and can be handled in an
@code{others} choice of an exception handler. The corresponding Ada
occurrence has no message, and the simple name of the exception identity
contains @code{Foreign_Exception}. Finalization and awaiting dependent
tasks works properly when such foreign exceptions are propagated.

It is also possible to import a C++ exception using the following syntax:

@example
LOCAL_NAME : exception;
pragma Import (Cpp,
  [Entity =>] LOCAL_NAME,
  [External_Name =>] static_string_EXPRESSION);
@end example

The @code{External_Name} is the name of the C++ RTTI symbol. You can then
cover a specific C++ exception in an exception handler. If the string
ends with “‘Class”, as if referencing the Class attribute of the C++
type, that enables “class-wide” type matching, i.e., instances of C++
classes derived from the one denoted by the RTTI symbol, that would be
caught by C++ handlers for that type, will also be caught by Ada
handlers for @code{Entity}. For non-class-wide RTTI symbols imported from
C++, only exact type matches will be handled. C++ rethrown (dependent)
exceptions are not distinguishable from the corresponding primary
exceptions: they are handled exactly as if the primary exception had
been raised.

With imported exceptions, especially with base-type matching, a single
handled_sequence_of_statements may have exception handlers with
choices that cover the same C++ types in ways that GNAT cannot detect.
For example, C++ classes @code{base} and @code{derived} may be imported as
exceptions with base-type matching, but GNAT does not know that they
are related by inheritance, only the runtime will know it. Given:

@example
exception
  when Derived_Exception => null;
  when Base_Exception => null;
  when others => null;
@end example

the earliest handler that matches the type of the raised object will
be selected. If an instance of @code{derived} or a further derived type
is raised, the first handler will be used; if an instance of @code{base}
that is not an instance of @code{derived} is raised, the second handler
will be used; raised objects that are not instances of @code{base} will
be handled by the @code{others} handler. However, if the handlers were
reordered (@code{others} must remain last), the @code{Derived_Exception}
handler would never be used, because @code{Base_Exception} would match
any instances of @code{derived} before @code{Derived_Exception} or
@code{others} handlers were considered. Mixing exact-type and base-type
matching exceptions may also involve overlapping handlers that GNAT
will not reject: an exact-type @code{Base_Only_Exception} handler placed
before @code{Base_Exception} will handle instances of @code{base}, whereas
instances of derived types will be handled by
@code{Base_Exception}. Swapping them will cause @code{Base_Exception} to
handle all instances of @code{base} and derived types, so that a
subsequent handler for @code{Base_Only_Exception} will never be selected.

The C++ object associated with a C++ @code{Exception_Occurrence} may be
obtained by calling the @code{GNAT.CPP_Exceptions.Get_Object_Address}
function. There are convenience generic wrappers named @code{Get_Object},
@code{Get_Access_To_Object}, and @code{Get_Access_To_Tagged_Object},
parameterized on the expected Ada type. Note that, for exceptions
imported from C++, the address of the object is that of the subobject
of the type associated with the exception, which may have a different
address from that of the full object; for C++ exceptions handled by
@code{others} handlers, however, the address of the full object is
returned.

E.g., if the imported exception uses the RTTI symbol for the base
class, followed by “‘Class”, and the C++ code raises (throws) an
instance of a derived class, a handler for that imported exception
will catch this @code{Exception_Occurrence}, and @code{Get_Object_Address}
will return the address of the base subobject of the raised derived
object; @code{Get_Object}, @code{Get_Access_To_Object} and
@code{Get_Access_To_Tagged_Object} only convert that address to the
parameterized type, so the specified type ought to be a type that
imports the C++ type whose RTTI symbol was named in the declared
exception, i.e., base, not derived or any other type. GNAT cannot
detect or report if a type is named that does not match the handler’s
RTTI-specified type.

For @code{others} handlers, and for exact type matches, the full object
is obtained. The @code{Get_Type_Info} function that takes an
@code{Exception_Occurrence} argument can be used to verify the type of
the C++ object raised as an exception. The other @code{Get_Type_Info}
function, that takes an @code{Exception_Id}, obtains the type expected by
the handler, and no such type exists for @code{others} handlers.
@code{GNAT.CPP.Std.Name} can then convert the opaque
@code{GNAT.CPP.Std.Type_Info_Ptr} access to @code{std::type_info} objects,
returned by either @code{Get_Type_Info} function, to a C++ mangled type
name.

If an @code{Exception_Occurrence} was raised from C++, or following C++
conventions, @code{GNAT.Exception_Actions.Exception_Language} will return
@code{EL_Cpp}, whether the exception handler is an imported C++ exception
or @code{others}. @code{GNAT.Exception_Actions.Is_Foreign_Exception} returns
True for all of these, as well as for any case in which
@code{Exception_Language} is not @code{EL_Ada}.

@example
--  Given the following partial package specification:

  Base_Exception : exception;
  pragma Import (Cpp, Base_Exception, "_ZTI4base'Class");
  --  Handle instances of base, and of subclasses.

  type Base is limited tagged record
    [...]
  end record;
  pragma Import (Cpp, Base);

  type Derived is limited tagged record
    [...]
  end record;
  pragma Import (Cpp, Derived);

  type Unrelated is access procedure (B : Boolean);

  function Get_Base_Obj_Acc is
    new Get_Access_To_Tagged_Object (Base);
  function Get_Derived_Obj_Acc is
    new Get_Access_To_Tagged_Object (Derived);
  function Get_Unrelated_Obj_Acc is
    new Get_Access_To_Object (Unrelated);

  procedure Raise_Derived;
  --  Raises an instance of derived (with a base subobject).


--  The comments next to each statement indicate the behavior of
--  the following pseudocode blocks:

begin
  Raise_Derived;
exception
  when BEx : Base_Exception =>
    ?? := Is_Foreign_Exception (BEx);  --  True
    ?? := Exception_Language (BEx);    --  EL_Cpp
    ?? := Name (Get_Type_Info (BEx));  --  "7derived"
    ?? := Name (Get_Type_Info (Exception_Identity (BEx)));  --  "4base"
    ?? := Get_Object_Address (BEx);    --  base subobject in derived object
    ?? := Get_Base_Obj_Acc (BEx):      --  ditto, as access to Base
    ?? := Get_Derived_Obj_Acc (BEx):   --  ditto, NO ERROR DETECTED!
    ?? := Get_Unrelated_Obj_Acc (BEx): --  ditto, NO ERROR DETECTED!
end;


begin
  Raise_Derived;
exception
  when BEx : others =>
    ?? := Is_Foreign_Exception (BEx);  --  True
    ?? := Exception_Language (BEx);    --  EL_Cpp
    ?? := Name (Get_Type_Info (BEx));  --  "7derived"
    ?? := Get_Type_Info (Exception_Identity (BEx));  --  null
    ?? := Get_Object_Address (BEx);    --  full derived object
    ?? := Get_Derived_Obj_Acc (BEx):   --  ditto, as access to Derived
    ?? := Get_Base_Obj_Acc (BEx):      --  ditto, NO ERROR DETECTED!
    ?? := Get_Unrelated_Obj_Acc (BEx): --  ditto, NO ERROR DETECTED!
end;
@end example

The calls marked with @code{NO ERROR DETECTED!} will compile successfully,
even though the types specified in the specializations of the generic
function do not match the type of the exception object that the
function is expected to return. Mismatches between derived and base
types are particularly relevant because they will appear to work as
long as there isn’t any offset between pointers to these types. This
may hold in many cases, but is subject to change with various possible
changes to the derived class.

The @code{GNAT.CPP.Std} package offers interfaces corresponding to the
C++ standard type @code{std::type_info}. Function @code{To_Type_Info_Ptr}
builds an opaque @code{Type_Info_Ptr} to reference a @code{std::type_info}
object at a given @code{System.Address}.

@node Interfacing to COBOL,Interfacing to Fortran,Interfacing to C++,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id5}@anchor{42e}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-cobol}@anchor{42f}
@section Interfacing to COBOL


Interfacing to COBOL is achieved as described in section B.4 of
the Ada Reference Manual.

@node Interfacing to Fortran,Interfacing to non-GNAT Ada code,Interfacing to COBOL,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id6}@anchor{430}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-fortran}@anchor{431}
@section Interfacing to Fortran


Interfacing to Fortran is achieved as described in section B.5 of the
Ada Reference Manual.  The pragma @code{Convention Fortran}, applied to a
multi-dimensional array causes the array to be stored in column-major
order as required for convenient interface to Fortran.

@node Interfacing to non-GNAT Ada code,,Interfacing to Fortran,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id7}@anchor{432}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-non-gnat-ada-code}@anchor{433}
@section Interfacing to non-GNAT Ada code


It is possible to specify the convention @code{Ada} in a pragma
@code{Import} or pragma @code{Export}.  However this refers to
the calling conventions used by GNAT, which may or may not be
similar enough to those used by some other Ada 83 / Ada 95 / Ada 2005
compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally
follows system calling conventions, then it may be possible to integrate
files compiled by other Ada compilers, provided that the elaboration
issues are adequately addressed (for example by eliminating the
need for any load time elaboration).

In particular, GNAT running on VMS is designed to
be highly compatible with the DEC Ada 83 compiler, so this is one
case in which it is possible to import foreign units of this type,
provided that the data items passed are restricted to simple scalar
values or simple record types without variants, or simple array
types with fixed bounds.

@node Specialized Needs Annexes,Implementation of Specific Ada Features,Interfacing to Other Languages,Top
@anchor{gnat_rm/specialized_needs_annexes doc}@anchor{434}@anchor{gnat_rm/specialized_needs_annexes id1}@anchor{435}@anchor{gnat_rm/specialized_needs_annexes specialized-needs-annexes}@anchor{13}
@chapter Specialized Needs Annexes


Ada 95, Ada 2005, Ada 2012, and Ada 2022 define a number of Specialized Needs Annexes, which are not required in all implementations.  However, as described in this chapter, GNAT implements all of these annexes:


@table @asis

@item `Systems Programming (Annex C)'

The Systems Programming Annex is fully implemented.

@item `Real-Time Systems (Annex D)'

The Real-Time Systems Annex is fully implemented.

@item `Distributed Systems (Annex E)'

Stub generation is fully implemented in the GNAT compiler.  In addition,
a complete compatible PCS is available as part of @code{PolyORB},
a separate product. `NB!' See the note in @ref{273,,PolyORB} regarding the lifetime of this product.

@item `Information Systems (Annex F)'

The Information Systems annex is fully implemented.

@item `Numerics (Annex G)'

The Numerics Annex is fully implemented.

@item `Safety and Security / High-Integrity Systems (Annex H)'

The Safety and Security Annex (termed the High-Integrity Systems Annex
in Ada 2005) is fully implemented.
@end table

@node Implementation of Specific Ada Features,Implementation of Ada 2022 Features,Specialized Needs Annexes,Top
@anchor{gnat_rm/implementation_of_specific_ada_features doc}@anchor{436}@anchor{gnat_rm/implementation_of_specific_ada_features id1}@anchor{437}@anchor{gnat_rm/implementation_of_specific_ada_features implementation-of-specific-ada-features}@anchor{14}
@chapter Implementation of Specific Ada Features


This chapter describes the GNAT implementation of several Ada language
facilities.

@menu
* Machine Code Insertions:: 
* GNAT Implementation of Tasking:: 
* GNAT Implementation of Shared Passive Packages:: 
* Code Generation for Array Aggregates:: 
* The Size of Discriminated Records with Default Discriminants:: 
* Image Values For Nonscalar Types:: 
* Strict Conformance to the Ada Reference Manual:: 

@end menu

@node Machine Code Insertions,GNAT Implementation of Tasking,,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id2}@anchor{438}@anchor{gnat_rm/implementation_of_specific_ada_features machine-code-insertions}@anchor{184}
@section Machine Code Insertions


@geindex Machine Code insertions

Package @code{Machine_Code} provides machine code support as described
in the Ada Reference Manual in two separate forms:


@itemize *

@item 
Machine code statements, consisting of qualified expressions that
fit the requirements of RM section 13.8.

@item 
An intrinsic callable procedure, providing an alternative mechanism of
including machine instructions in a subprogram.
@end itemize

The two features are similar, and both are closely related to the mechanism
provided by the asm instruction in the GNU C compiler.  Full understanding
and use of the facilities in this package requires understanding the asm
instruction, see the section on Extended Asm in
@cite{Using_the_GNU_Compiler_Collection_(GCC)}.

Calls to the function @code{Asm} and the procedure @code{Asm} have identical
semantic restrictions and effects as described below.  Both are provided so
that the procedure call can be used as a statement, and the function call
can be used to form a code_statement.

Consider this C @code{asm} instruction:

@example
asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));
@end example

The equivalent can be written for GNAT as:

@example
Asm ("fsinx %1 %0",
     My_Float'Asm_Output ("=f", result),
     My_Float'Asm_Input  ("f",  angle));
@end example

The first argument to @code{Asm} is the assembler template, and is
identical to what is used in GNU C.  This string must be a static
expression.  The second argument is the output operand list.  It is
either a single @code{Asm_Output} attribute reference, or a list of such
references enclosed in parentheses (technically an array aggregate of
such references).

The @code{Asm_Output} attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix.  The first (string)
argument is required to be a static expression and designates the
constraint (see the section on Constraints in
@cite{Using_the_GNU_Compiler_Collection_(GCC)})
for the parameter; e.g., what kind of register is required.  The second
argument is the variable to be written or updated with the
result.  The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end.  If there are no output operands, then this argument may
either be omitted, or explicitly given as @code{No_Output_Operands}.
No support is provided for GNU C’s symbolic names for output parameters.

The second argument of @code{my_float'Asm_Output} functions as
though it were an @code{out} parameter, which is a little curious, but
all names have the form of expressions, so there is no syntactic
irregularity, even though normally functions would not be permitted
@code{out} parameters.  The third argument is the list of input
operands.  It is either a single @code{Asm_Input} attribute reference, or
a list of such references enclosed in parentheses (technically an array
aggregate of such references).

The @code{Asm_Input} attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix.  The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g., what kind of register is required).  The second argument is the
value to be used as the input argument.  The possible values for the
constraint are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
No support is provided for GNU C’s symbolic names for input parameters.

If there are no input operands, this argument may either be omitted, or
explicitly given as @code{No_Input_Operands}.  The fourth argument, not
present in the above example, is a list of register names, called the
`clobber' argument.  This argument, if given, must be a static string
expression, and is a space or comma separated list of names of registers
that must be considered destroyed as a result of the @code{Asm} call.  If
this argument is the null string (the default value), then the code
generator assumes that no additional registers are destroyed.
In addition to registers, the special clobbers @code{memory} and
@code{cc} as described in the GNU C docs are both supported.

The fifth argument, not present in the above example, called the
`volatile' argument, is by default @code{False}.  It can be set to
the literal value @code{True} to indicate to the code generator that all
optimizations with respect to the instruction specified should be
suppressed, and in particular an instruction that has outputs
will still be generated, even if none of the outputs are
used.  See @cite{Using_the_GNU_Compiler_Collection_(GCC)}
for the full description.
Generally it is strongly advisable to use Volatile for any ASM statement
that is missing either input or output operands or to avoid unwanted
optimizations. A warning is generated if this advice is not followed.

No support is provided for GNU C’s @code{asm goto} feature.

The @code{Asm} subprograms may be used in two ways.  First the procedure
forms can be used anywhere a procedure call would be valid, and
correspond to what the RM calls ‘intrinsic’ routines.  Such calls can
be used to intersperse machine instructions with other Ada statements.
Second, the function forms, which return a dummy value of the limited
private type @code{Asm_Insn}, can be used in code statements, and indeed
this is the only context where such calls are allowed.  Code statements
appear as aggregates of the form:

@example
Asm_Insn'(Asm (...));
Asm_Insn'(Asm_Volatile (...));
@end example

In accordance with RM rules, such code statements are allowed only
within subprograms whose entire body consists of such statements.  It is
not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient
and more flexible.  The code statement form is provided to meet the RM
suggestion that such a facility should be made available.  The following
is the exact syntax of the call to @code{Asm}. As usual, if named notation
is used, the arguments may be given in arbitrary order, following the
normal rules for use of positional and named arguments:

@example
ASM_CALL ::= Asm (
                 [Template =>] static_string_EXPRESSION
               [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
               [,[Inputs   =>] INPUT_OPERAND_LIST       ]
               [,[Clobber  =>] static_string_EXPRESSION ]
               [,[Volatile =>] static_boolean_EXPRESSION] )

OUTPUT_OPERAND_LIST ::=
  [PREFIX.]No_Output_Operands
| OUTPUT_OPERAND_ATTRIBUTE
| (OUTPUT_OPERAND_ATTRIBUTE @{,OUTPUT_OPERAND_ATTRIBUTE@})

OUTPUT_OPERAND_ATTRIBUTE ::=
  SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)

INPUT_OPERAND_LIST ::=
  [PREFIX.]No_Input_Operands
| INPUT_OPERAND_ATTRIBUTE
| (INPUT_OPERAND_ATTRIBUTE @{,INPUT_OPERAND_ATTRIBUTE@})

INPUT_OPERAND_ATTRIBUTE ::=
  SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)
@end example

The identifiers @code{No_Input_Operands} and @code{No_Output_Operands}
are declared in the package @code{Machine_Code} and must be referenced
according to normal visibility rules. In particular if there is no
@code{use} clause for this package, then appropriate package name
qualification is required.

@node GNAT Implementation of Tasking,GNAT Implementation of Shared Passive Packages,Machine Code Insertions,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features gnat-implementation-of-tasking}@anchor{439}@anchor{gnat_rm/implementation_of_specific_ada_features id3}@anchor{43a}
@section GNAT Implementation of Tasking


This chapter outlines the basic GNAT approach to tasking (in particular,
a multi-layered library for portability) and discusses issues related
to compliance with the Real-Time Systems Annex.

@menu
* Mapping Ada Tasks onto the Underlying Kernel Threads:: 
* Ensuring Compliance with the Real-Time Annex:: 
* Support for Locking Policies:: 

@end menu

@node Mapping Ada Tasks onto the Underlying Kernel Threads,Ensuring Compliance with the Real-Time Annex,,GNAT Implementation of Tasking
@anchor{gnat_rm/implementation_of_specific_ada_features id4}@anchor{43b}@anchor{gnat_rm/implementation_of_specific_ada_features mapping-ada-tasks-onto-the-underlying-kernel-threads}@anchor{43c}
@subsection Mapping Ada Tasks onto the Underlying Kernel Threads


GNAT’s run-time support comprises two layers:


@itemize *

@item 
GNARL (GNAT Run-time Layer)

@item 
GNULL (GNAT Low-level Library)
@end itemize

In GNAT, Ada’s tasking services rely on a platform and OS independent
layer known as GNARL.  This code is responsible for implementing the
correct semantics of Ada’s task creation, rendezvous, protected
operations etc.

GNARL decomposes Ada’s tasking semantics into simpler lower level
operations such as create a thread, set the priority of a thread,
yield, create a lock, lock/unlock, etc.  The spec for these low-level
operations constitutes GNULLI, the GNULL Interface.  This interface is
directly inspired from the POSIX real-time API.

If the underlying executive or OS implements the POSIX standard
faithfully, the GNULL Interface maps as is to the services offered by
the underlying kernel.  Otherwise, some target dependent glue code maps
the services offered by the underlying kernel to the semantics expected
by GNARL.

Whatever the underlying OS (VxWorks, UNIX, Windows, etc.) the
key point is that each Ada task is mapped on a thread in the underlying
kernel.  For example, in the case of VxWorks, one Ada task = one VxWorks task.

In addition Ada task priorities map onto the underlying thread priorities.
Mapping Ada tasks onto the underlying kernel threads has several advantages:


@itemize *

@item 
The underlying scheduler is used to schedule the Ada tasks.  This
makes Ada tasks as efficient as kernel threads from a scheduling
standpoint.

@item 
Interaction with code written in C containing threads is eased
since at the lowest level Ada tasks and C threads map onto the same
underlying kernel concept.

@item 
When an Ada task is blocked during I/O the remaining Ada tasks are
able to proceed.

@item 
On multiprocessor systems Ada tasks can execute in parallel.
@end itemize

Some threads libraries offer a mechanism to fork a new process, with the
child process duplicating the threads from the parent.
GNAT does not
support this functionality when the parent contains more than one task.

@geindex Forking a new process

@node Ensuring Compliance with the Real-Time Annex,Support for Locking Policies,Mapping Ada Tasks onto the Underlying Kernel Threads,GNAT Implementation of Tasking
@anchor{gnat_rm/implementation_of_specific_ada_features ensuring-compliance-with-the-real-time-annex}@anchor{43d}@anchor{gnat_rm/implementation_of_specific_ada_features id5}@anchor{43e}
@subsection Ensuring Compliance with the Real-Time Annex


@geindex Real-Time Systems Annex compliance

Although mapping Ada tasks onto
the underlying threads has significant advantages, it does create some
complications when it comes to respecting the scheduling semantics
specified in the real-time annex (Annex D).

For instance the Annex D requirement for the @code{FIFO_Within_Priorities}
scheduling policy states:

@quotation

`When the active priority of a ready task that is not running
changes, or the setting of its base priority takes effect, the
task is removed from the ready queue for its old active priority
and is added at the tail of the ready queue for its new active
priority, except in the case where the active priority is lowered
due to the loss of inherited priority, in which case the task is
added at the head of the ready queue for its new active priority.'
@end quotation

While most kernels do put tasks at the end of the priority queue when
a task changes its priority, (which respects the main
FIFO_Within_Priorities requirement), almost none keep a thread at the
beginning of its priority queue when its priority drops from the loss
of inherited priority.

As a result most vendors have provided incomplete Annex D implementations.

The GNAT run-time, has a nice cooperative solution to this problem
which ensures that accurate FIFO_Within_Priorities semantics are
respected.

The principle is as follows.  When an Ada task T is about to start
running, it checks whether some other Ada task R with the same
priority as T has been suspended due to the loss of priority
inheritance.  If this is the case, T yields and is placed at the end of
its priority queue.  When R arrives at the front of the queue it
executes.

Note that this simple scheme preserves the relative order of the tasks
that were ready to execute in the priority queue where R has been
placed at the end.

@c Support_for_Locking_Policies

@node Support for Locking Policies,,Ensuring Compliance with the Real-Time Annex,GNAT Implementation of Tasking
@anchor{gnat_rm/implementation_of_specific_ada_features support-for-locking-policies}@anchor{43f}
@subsection Support for Locking Policies


This section specifies which policies specified by pragma Locking_Policy
are supported on which platforms.

GNAT supports the standard @code{Ceiling_Locking} policy, and the
implementation defined @code{Inheritance_Locking} and
@code{Concurrent_Readers_Locking} policies.

@code{Ceiling_Locking} is supported on all platforms if the operating system
supports it. In particular, @code{Ceiling_Locking} is not supported on
VxWorks.
@code{Inheritance_Locking} is supported on
Linux,
Darwin (Mac OS X),
LynxOS 178,
and VxWorks.
@code{Concurrent_Readers_Locking} is supported on Linux.

Notes about @code{Ceiling_Locking} on Linux:
If the process is running as ‘root’, ceiling locking is used.
If the capabilities facility is installed
(“sudo apt-get –assume-yes install libcap-dev” on Ubuntu,
for example),
and the program is linked against that library
(“-largs -lcap”),
and the executable file has the cap_sys_nice capability
(“sudo /sbin/setcap cap_sys_nice=ep executable_file_name”),
then ceiling locking is used.
Otherwise, the @code{Ceiling_Locking} policy is ignored.

@node GNAT Implementation of Shared Passive Packages,Code Generation for Array Aggregates,GNAT Implementation of Tasking,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features gnat-implementation-of-shared-passive-packages}@anchor{440}@anchor{gnat_rm/implementation_of_specific_ada_features id6}@anchor{441}
@section GNAT Implementation of Shared Passive Packages


@geindex Shared passive packages

GNAT fully implements the 
@geindex pragma Shared_Passive
pragma
@code{Shared_Passive} for
the purpose of designating shared passive packages.
This allows the use of passive partitions in the
context described in the Ada Reference Manual; i.e., for communication
between separate partitions of a distributed application using the
features in Annex E.

@geindex Annex E

@geindex Distribution Systems Annex

However, the implementation approach used by GNAT provides for more
extensive usage as follows:


@table @asis

@item `Communication between separate programs'

This allows separate programs to access the data in passive
partitions, using protected objects for synchronization where
needed. The only requirement is that the two programs have a
common shared file system. It is even possible for programs
running on different machines with different architectures
(e.g., different endianness) to communicate via the data in
a passive partition.

@item `Persistence between program runs'

The data in a passive package can persist from one run of a
program to another, so that a later program sees the final
values stored by a previous run of the same program.
@end table

The implementation approach used is to store the data in files. A
separate stream file is created for each object in the package, and
an access to an object causes the corresponding file to be read or
written.

@geindex SHARED_MEMORY_DIRECTORY environment variable

The environment variable @code{SHARED_MEMORY_DIRECTORY} should be
set to the directory to be used for these files.
The files in this directory
have names that correspond to their fully qualified names. For
example, if we have the package

@example
package X is
  pragma Shared_Passive (X);
  Y : Integer;
  Z : Float;
end X;
@end example

and the environment variable is set to @code{/stemp/}, then the files created
will have the names:

@example
/stemp/x.y
/stemp/x.z
@end example

These files are created when a value is initially written to the object, and
the files are retained until manually deleted. This provides the persistence
semantics. If no file exists, it means that no partition has assigned a value
to the variable; in this case the initial value declared in the package
will be used. This model ensures that there are no issues in synchronizing
the elaboration process, since elaboration of passive packages elaborates the
initial values, but does not create the files.

The files are written using normal @code{Stream_IO} access.
If you want to be able
to communicate between programs or partitions running on different
architectures, then you should use the XDR versions of the stream attribute
routines, since these are architecture independent.

If active synchronization is required for access to the variables in the
shared passive package, then as described in the Ada Reference Manual, the
package may contain protected objects used for this purpose. In this case
a lock file (whose name is @code{___lock}, with three underscores)
is created in the shared memory directory.

@geindex ___lock file (for shared passive packages)

This is used to provide the required locking
semantics for proper protected object synchronization.

@node Code Generation for Array Aggregates,The Size of Discriminated Records with Default Discriminants,GNAT Implementation of Shared Passive Packages,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features code-generation-for-array-aggregates}@anchor{442}@anchor{gnat_rm/implementation_of_specific_ada_features id7}@anchor{443}
@section Code Generation for Array Aggregates


Aggregates have a rich syntax and allow the user to specify the values of
complex data structures by means of a single construct.  As a result, the
code generated for aggregates can be quite complex and involve loops, case
statements and multiple assignments.  In the simplest cases, however, the
compiler will recognize aggregates whose components and constraints are
fully static, and in those cases the compiler will generate little or no
executable code.  The following is an outline of the code that GNAT generates
for various aggregate constructs.  For further details, you will find it
useful to examine the output produced by the -gnatG flag to see the expanded
source that is input to the code generator.  You may also want to examine
the assembly code generated at various levels of optimization.

The code generated for aggregates depends on the context, the component values,
and the type.  In the context of an object declaration the code generated is
generally simpler than in the case of an assignment.  As a general rule, static
component values and static subtypes also lead to simpler code.

@menu
* Static constant aggregates with static bounds:: 
* Constant aggregates with unconstrained nominal types:: 
* Aggregates with static bounds:: 
* Aggregates with nonstatic bounds:: 
* Aggregates in assignment statements:: 

@end menu

@node Static constant aggregates with static bounds,Constant aggregates with unconstrained nominal types,,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features id8}@anchor{444}@anchor{gnat_rm/implementation_of_specific_ada_features static-constant-aggregates-with-static-bounds}@anchor{445}
@subsection Static constant aggregates with static bounds


For the declarations:

@example
type One_Dim is array (1..10) of integer;
ar0 : constant One_Dim := (1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
@end example

GNAT generates no executable code: the constant ar0 is placed in static memory.
The same is true for constant aggregates with named associations:

@example
Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1 => 1, 5 .. 10 => 0);
Cr3 : constant One_Dim := (others => 7777);
@end example

The same is true for multidimensional constant arrays such as:

@example
type two_dim is array (1..3, 1..3) of integer;
Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));
@end example

The same is true for arrays of one-dimensional arrays: the following are
static:

@example
type ar1b  is array (1..3) of boolean;
type ar_ar is array (1..3) of ar1b;
None  : constant ar1b := (others => false);     --  fully static
None2 : constant ar_ar := (1..3 => None);       --  fully static
@end example

However, for multidimensional aggregates with named associations, GNAT will
generate assignments and loops, even if all associations are static.  The
following two declarations generate a loop for the first dimension, and
individual component assignments for the second dimension:

@example
Zero1: constant two_dim := (1..3 => (1..3 => 0));
Zero2: constant two_dim := (others => (others => 0));
@end example

@node Constant aggregates with unconstrained nominal types,Aggregates with static bounds,Static constant aggregates with static bounds,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features constant-aggregates-with-unconstrained-nominal-types}@anchor{446}@anchor{gnat_rm/implementation_of_specific_ada_features id9}@anchor{447}
@subsection Constant aggregates with unconstrained nominal types


In such cases the aggregate itself establishes the subtype, so that
associations with @code{others} cannot be used.  GNAT determines the
bounds for the actual subtype of the aggregate, and allocates the
aggregate statically as well.  No code is generated for the following:

@example
type One_Unc is array (natural range <>) of integer;
Cr_Unc : constant One_Unc := (12,24,36);
@end example

@node Aggregates with static bounds,Aggregates with nonstatic bounds,Constant aggregates with unconstrained nominal types,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features aggregates-with-static-bounds}@anchor{448}@anchor{gnat_rm/implementation_of_specific_ada_features id10}@anchor{449}
@subsection Aggregates with static bounds


In all previous examples the aggregate was the initial (and immutable) value
of a constant.  If the aggregate initializes a variable, then code is generated
for it as a combination of individual assignments and loops over the target
object.  The declarations

@example
Cr_Var1 : One_Dim := (2, 5, 7, 11, 0, 0, 0, 0, 0, 0);
Cr_Var2 : One_Dim := (others > -1);
@end example

generate the equivalent of

@example
Cr_Var1 (1) := 2;
Cr_Var1 (2) := 3;
Cr_Var1 (3) := 5;
Cr_Var1 (4) := 11;

for I in Cr_Var2'range loop
   Cr_Var2 (I) := -1;
end loop;
@end example

@node Aggregates with nonstatic bounds,Aggregates in assignment statements,Aggregates with static bounds,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features aggregates-with-nonstatic-bounds}@anchor{44a}@anchor{gnat_rm/implementation_of_specific_ada_features id11}@anchor{44b}
@subsection Aggregates with nonstatic bounds


If the bounds of the aggregate are not statically compatible with the bounds
of the nominal subtype  of the target, then constraint checks have to be
generated on the bounds.  For a multidimensional array, constraint checks may
have to be applied to sub-arrays individually, if they do not have statically
compatible subtypes.

@node Aggregates in assignment statements,,Aggregates with nonstatic bounds,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features aggregates-in-assignment-statements}@anchor{44c}@anchor{gnat_rm/implementation_of_specific_ada_features id12}@anchor{44d}
@subsection Aggregates in assignment statements


In general, aggregate assignment requires the construction of a temporary,
and a copy from the temporary to the target of the assignment.  This is because
it is not always possible to convert the assignment into a series of individual
component assignments.  For example, consider the simple case:

@example
A := (A(2), A(1));
@end example

This cannot be converted into:

@example
A(1) := A(2);
A(2) := A(1);
@end example

So the aggregate has to be built first in a separate location, and then
copied into the target.  GNAT recognizes simple cases where this intermediate
step is not required, and the assignments can be performed in place, directly
into the target.  The following sufficient criteria are applied:


@itemize *

@item 
The bounds of the aggregate are static, and the associations are static.

@item 
The components of the aggregate are static constants, names of
simple variables that are not renamings, or expressions not involving
indexed components whose operands obey these rules.
@end itemize

If any of these conditions are violated, the aggregate will be built in
a temporary (created either by the front-end or the code generator) and then
that temporary will be copied onto the target.

@node The Size of Discriminated Records with Default Discriminants,Image Values For Nonscalar Types,Code Generation for Array Aggregates,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id13}@anchor{44e}@anchor{gnat_rm/implementation_of_specific_ada_features the-size-of-discriminated-records-with-default-discriminants}@anchor{44f}
@section The Size of Discriminated Records with Default Discriminants


If a discriminated type @code{T} has discriminants with default values, it is
possible to declare an object of this type without providing an explicit
constraint:

@example
type Size is range 1..100;

type Rec (D : Size := 15) is record
   Name : String (1..D);
end T;

Word : Rec;
@end example

Such an object is said to be `unconstrained'.
The discriminant of the object
can be modified by a full assignment to the object, as long as it preserves the
relation between the value of the discriminant, and the value of the components
that depend on it:

@example
Word := (3, "yes");

Word := (5, "maybe");

Word := (5, "no"); -- raises Constraint_Error
@end example

In order to support this behavior efficiently, an unconstrained object is
given the maximum size that any value of the type requires. In the case
above, @code{Word} has storage for the discriminant and for
a @code{String} of length 100.
It is important to note that unconstrained objects do not require dynamic
allocation. It would be an improper implementation to place on the heap those
components whose size depends on discriminants. (This improper implementation
was used by some Ada83 compilers, where the @code{Name} component above
would have
been stored as a pointer to a dynamic string). Following the principle that
dynamic storage management should never be introduced implicitly,
an Ada compiler should reserve the full size for an unconstrained declared
object, and place it on the stack.

This maximum size approach
has been a source of surprise to some users, who expect the default
values of the discriminants to determine the size reserved for an
unconstrained object: “If the default is 15, why should the object occupy
a larger size?”
The answer, of course, is that the discriminant may be later modified,
and its full range of values must be taken into account. This is why the
declaration:

@example
type Rec (D : Positive := 15) is record
   Name : String (1..D);
end record;

Too_Large : Rec;
@end example

is flagged by the compiler with a warning:
an attempt to create @code{Too_Large} will raise @code{Storage_Error},
because the required size includes @code{Positive'Last}
bytes. As the first example indicates, the proper approach is to declare an
index type of ‘reasonable’ range so that unconstrained objects are not too
large.

One final wrinkle: if the object is declared to be @code{aliased}, or if it is
created in the heap by means of an allocator, then it is `not'
unconstrained:
it is constrained by the default values of the discriminants, and those values
cannot be modified by full assignment. This is because in the presence of
aliasing all views of the object (which may be manipulated by different tasks,
say) must be consistent, so it is imperative that the object, once created,
remain invariant.

@node Image Values For Nonscalar Types,Strict Conformance to the Ada Reference Manual,The Size of Discriminated Records with Default Discriminants,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id14}@anchor{450}@anchor{gnat_rm/implementation_of_specific_ada_features image-values-for-nonscalar-types}@anchor{451}
@section Image Values For Nonscalar Types


Ada 2022 defines the Image, Wide_Image, and Wide_Wide image attributes
for nonscalar types; earlier Ada versions defined these attributes only
for scalar types. Ada RM 4.10 provides some general guidance regarding
the default implementation of these attributes and the GNAT compiler
follows that guidance. However, beyond that the precise details of the
image text generated in these cases are deliberately not documented and are
subject to change. In particular, users should not rely on formatting details
(such as spaces or line breaking), record field order, image values for access
types, image values for types that have ancestor or subcomponent types
declared in non-Ada2022 code, image values for predefined types, or the
compiler’s choices regarding the implementation permissions described in
Ada RM 4.10. This list is not intended to be exhaustive. If more precise
control of image text is required for some type T, then T’Put_Image should be
explicitly specified.

@node Strict Conformance to the Ada Reference Manual,,Image Values For Nonscalar Types,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id15}@anchor{452}@anchor{gnat_rm/implementation_of_specific_ada_features strict-conformance-to-the-ada-reference-manual}@anchor{453}
@section Strict Conformance to the Ada Reference Manual


The dynamic semantics defined by the Ada Reference Manual impose a set of
run-time checks to be generated. By default, the GNAT compiler will insert many
run-time checks into the compiled code, including most of those required by the
Ada Reference Manual. However, there are two checks that are not enabled in
the default mode for efficiency reasons: checks for access before elaboration
on subprogram calls, and stack overflow checking (most operating systems do not
perform this check by default).

Strict conformance to the Ada Reference Manual can be achieved by adding two
compiler options for dynamic checks for access-before-elaboration on subprogram
calls and generic instantiations (`-gnatE'), and stack overflow checking
(`-fstack-check').

Note that the result of a floating point arithmetic operation in overflow and
invalid situations, when the @code{Machine_Overflows} attribute of the result
type is @code{False}, is to generate IEEE NaN and infinite values. This is the
case for machines compliant with the IEEE floating-point standard, but on
machines that are not fully compliant with this standard, such as Alpha, the
`-mieee' compiler flag must be used for achieving IEEE confirming
behavior (although at the cost of a significant performance penalty), so
infinite and NaN values are properly generated.

@node Implementation of Ada 2022 Features,GNAT language extensions,Implementation of Specific Ada Features,Top
@anchor{gnat_rm/implementation_of_ada_2022_features doc}@anchor{454}@anchor{gnat_rm/implementation_of_ada_2022_features id1}@anchor{455}@anchor{gnat_rm/implementation_of_ada_2022_features implementation-of-ada-2022-features}@anchor{15}
@chapter Implementation of Ada 2022 Features


@geindex Ada 2022 implementation status

@geindex -gnat22 option (gcc)

@geindex pragma Ada_2022

@geindex configuration pragma Ada_2022

@geindex Ada_2022 configuration pragma

This chapter contains a complete list of Ada 2022 features that have been
implemented. Generally, these features are only available if the `-gnat22' (Ada 2022 features enabled) option is set, or if the configuration pragma @code{Ada_2022} is used.

However, new pragmas, attributes, and restrictions are unconditionally available, since the Ada standard allows the addition of new pragmas, attributes, and restrictions (there are exceptions, which are
documented in the individual descriptions), and also certain packages
were made available in earlier versions of Ada.

An ISO date (YYYY-MM-DD) appears in parentheses on the description line.
This date shows the implementation date of the feature. Any wavefront
subsequent to this date will contain the indicated feature, as will any
subsequent releases. A date of 0000-00-00 means that GNAT has always
implemented the feature, or implemented it as soon as it appeared as a
binding interpretation.

Each feature corresponds to an Ada Issue (‘AI’) approved by the Ada
standardization group (ISO/IEC JTC1/SC22/WG9) for inclusion in Ada 2022.

The section “RM references” lists all modified paragraphs in the Ada 2012 reference manual. The details of each modification as well as a complete description of the AIs may be found in
@indicateurl{http://www.ada-auth.org/AI12-SUMMARY.HTML}.

@geindex AI12-0001 (Ada 2022 feature)


@itemize *

@item 
`AI12-0001 Independence and Representation clauses for atomic objects (2019-11-27)'

The compiler accepts packing clauses in all cases, even if they have effectively no influence on the layout. Types, where packing is essentially infeasible are, for instance atomic, aliased and by-reference types.

RM references: 13.02 (6.1/2) 13.02 (7) 13.02 (8) 13.02 (9/3) C.06 (8.1/3)
C.06 (10) C.06 (11) C.06 (21) C.06 (24)
@end itemize

@geindex AI12-0003 (Ada 2022 feature)


@itemize *

@item 
`AI12-0003 Specifying the standard storage pool (2020-06-25)'

Allows the standard storage pool being specified with a @code{Default_Storage_Pool} pragma or aspect.

RM references: 8.02 (11) 13.11.03 (1/3) 13.11.03 (3.1/3) 13.11.03 (4/3)
13.11.03 (4.1/3) 13.11.03 (5/3) 13.11.03 (6.2/3) 13.11.03
(6.3/3)
@end itemize

@geindex AI12-0004 (Ada 2022 feature)


@itemize *

@item 
`AI12-0004 Normalization and allowed characters for identifiers (2020-06-11)'

This AI clarifies that Ada identifiers containing characters which are not
allowed in Normalization Form KC are illegal.

RM references: 2.01 (4.1/3) 2.03 (4/3) A.03.02 (4/3) A.03.02 (32.5/3)
A.03.05 (18/3) A.03.05 (51/3)
@end itemize

@geindex AI12-0020 (Ada 2022 feature)


@itemize *

@item 
`AI12-0020 ‘Image for all types (2020-03-30)'

Put_Image prints out a human-readable representation of an object. The
functionality in Ada2022 RM is fully implemented except the support for
types in the @code{Remote_Types} packages.

RM references: 4.10 (0) 3.05 (27.1/2) 3.05 (27.2/2) 3.05 (27.3/2) 3.05
(27.4/2) 3.05 (27.5/2) 3.05 (27.6/2) 3.05 (27.7/2) 3.05 (28) 3.05
(29) 3.05 (30/3) 3.05 (31) 3.05 (32) 3.05 (33/3) 3.05 (37.1/2)
3.05 (38) 3.05 (39) 3.05 (43/3) 3.05 (55/3) 3.05 (55.1/5) 3.05
(55.2/4) 3.05 (55.3/4) 3.05 (55.4/4) 3.05 (59) H.04 (23) H.04 (23.8/2)
@end itemize

@geindex AI12-0022 (Ada 2022 feature)


@itemize *

@item 
`AI12-0022 Raise_Expressions (2013-01-27)'

This feature allows you to write “raise NAME [with STRING]” in an
expression to rise given exception. It is particularly useful in the case of
assertions such as preconditions allowing to specify which exception a
precondition raises if it fails.

RM references: 4.04 (3/3) 11.02 (6) 11.03 (2/2) 11.03 (3) 11.03 (3.1/2)
11.03 (4/2) 11.04.01 (10.1/3)
@end itemize

@geindex AI12-0027 (Ada 2022 feature)


@itemize *

@item 
`AI12-0027 Access values should never designate unaliased components (2020-06-15)'

AI12-0027 adds a requirement for a value conversion that converts from an array of unaliased components to an array of aliased components to make a copy. It defines such conversions to have a local accessibility, effectively preventing the possibility of unsafe accesses to unaliased components.

RM references: 4.06 (24.17/3) 4.06 (24.21/2) 4.06 (58) 6.02 (10/3) 3.10.02 (10/3)
@end itemize

@geindex AI12-0028 (Ada 2022 feature)


@itemize *

@item 
`AI12-0028 Import of variadic C functions (2020-03-03)'

Ada programs can now properly call variadic C functions by means of the
conventions C_Variadic_<n>, for small integer values <n>.

RM references: B.03 (1/3) B.03 (60.15/3) B.03 (75)
@end itemize

@geindex AI12-0030 (Ada 2022 feature)


@itemize *

@item 
`AI12-0030 Formal derived types and stream attribute availability (2020-08-21)'

Corner cases involving streaming operations for formal derived limited types
that are now defined to raise Program_Error. Before, behavior in these cases
was undefined. Stream attribute availability is more precisely computed in cases where a derived type declaration occurs ahead of a streaming attribute specification for the parent type.

RM references: 12.05.01 (21/3) 13.13.02 (49/2)
@end itemize

@geindex AI12-0031 (Ada 2022 feature)


@itemize *

@item 
`AI12-0031 All_Calls_Remote and indirect calls (0000-00-00)'

Remote indirect calls (i.e., calls through a remote access-to-subprogram type)
behave the same as remote direct calls.

RM references: E.02.03 (19/3)
@end itemize

@geindex AI12-0032 (Ada 2022 feature)


@itemize *

@item 
`AI12-0032 Questions on ‘Old (2020-04-24)'

AI12-0032 resolves several issues related to the ‘Old attribute. The GNAT
compiler already implemented what the AI requires in most of those cases, but two having to do with static and dynamic checking of the accessibility level of the constant object implicitly declared for an ‘Old attribute reference were not yet implemented. Accessibility checking for these constants is now implemented as defined in the AI.

RM references: 4.01.03 (9/3) 6.01.01 (22/3) 6.01.01 (26/3) 6.01.01 (35/3)
@end itemize

@geindex AI12-0033 (Ada 2022 feature)


@itemize *

@item 
`AI12-0033 Sets of CPUs when defining dispatching domains (0000-00-00)'

The set of CPUs associated with a dispatching domain is no longer required
to be a contiguous range of CPU values.

RM references: D.16.01 (7/3) D.16.01 (9/3) D.16.01 (20/3) D.16.01 (23/3)
D.16.01 (24/3) D.16.01 (26/3)
@end itemize

@geindex AI12-0035 (Ada 2022 feature)


@itemize *

@item 
`AI12-0035 Accessibility checks for indefinite elements of containers (0000-00-00)'

If the element type for an instance of one of the indefinite container generics has an access discriminant, then accessibility checks (at run-time) prevent inserting a value into a container object if the value’s discriminant designates an object that is too short-lived (that is, if the designated object has an accessibility level that is deeper than that of the instance). Without this check, dangling references would be possible. GNAT handled this correctly already before this AI was issued.

RM references: A.18 (5/3) A.18.11 (8/2) A.18.12 (7/2) A.18.13 (8/2)
A.18.14 (8/2) A.18.15 (4/2) A.18.16 (4/2) A.18.17 (7/3) A.18.18
(39/3) A.18.18 (47/3)
@end itemize

@geindex AI12-0036 (Ada 2022 feature)


@itemize *

@item 
`AI12-0036 The actual for an untagged formal derived type cannot be tagged (2019-10-21)'

AI12-0036 is a binding interpretation that adds the following legality rule:
The actual type for a formal derived type shall be tagged if and only if the
formal derived type is a private extension. The check is implemented for all Ada dialects, not just Ada 2022.

RM references: 12.05.01 (5.1/3)
@end itemize

@geindex AI12-0037 (Ada 2022 feature)


@itemize *

@item 
`AI12-0037 New types in Ada.Locales can’t be converted to/from strings (2016-09-10)'

The type definitions for Language_Code and Country_Code are now using dynamic
predicates.

RM references: A.19 (4/3)
@end itemize

@geindex AI12-0039 (Ada 2022 feature)


@itemize *

@item 
`AI12-0039 Ambiguity in syntax for membership expression removed (0000-00-00)'

An ambiguity in the syntax for membership expressions was resolved. For example, “A in B and C” can be parsed in only one way because of this AI.

RM references: 4.04 (3/3) 4.04 (3.2/3) 4.05.02 (3.1/3) 4.05.02 (4) 4.05.02
(4.1/3) 4.05.02 (27/3) 4.05.02 (27.1/3) 4.05.02 (28.1/3) 4.05.02
(28.2/3) 4.05.02 (29/3) 4.05.02 (30/3) 4.05.02 (30.1/3) 4.05.02
(30.2/3) 4.05.02 (30.3/3) 4.09 (11/3) 4.09 (32.6/3) 8.06 (27.1/3)
3.02.04 (17/3)
@end itemize

@geindex AI12-0040 (Ada 2022 feature)


@itemize *

@item 
`AI12-0040 Resolving the selecting_expression of a case_expression (0000-00-00)'

The definition of “complete context” is corrected so that selectors of case expressions
and of case statements are treated uniformly.

RM references: 8.06 (9)
@end itemize

@geindex AI12-0041 (Ada 2022 feature)


@itemize *

@item 
`AI12-0041 Type_Invariant’Class for interface types (2016-12-12)'

Subprogram calls within class-wide type invariant expressions get resolved
as primitive operations instead of being dynamically dispatched.

RM references: 7.03.02 (1/3) 7.03.02 (3/3)
@end itemize

@geindex AI12-0042 (Ada 2022 feature)


@itemize *

@item 
`AI12-0042 Type invariant checking rules (2020-06-05)'

AI12-0042 adds rules for type invariants.
Specifically, when inheriting a private dispatching operation when the ancestor operation is visible at the point of the type extension, the operation must be abstract or else overridden. In addition, for a class-wide view conversion from an object of a specific type T to which a type invariant applies, an invariant check is performed when the conversion is within the immediate scope of T.

RM references: 7.03.02 (6/3) 7.03.02 (17/3) 7.03.02 (18/3) 7.03.02 (19/3)
7.03.02 (20/3)
@end itemize

@geindex AI12-0043 (Ada 2022 feature)


@itemize *

@item 
`AI12-0043 Details of the storage pool used when Storage_Size is specified (0000-00-00)'

Clarify that a Storage_Size specification for an access type specifies both an upper bound and a lower bound (not just a lower bound) of the amount of storage allowed for allocated objects.

RM references: 13.11 (18)
@end itemize

@geindex AI12-0044 (Ada 2022 feature)


@itemize *

@item 
`AI12-0044 Calling visible functions from type invariant expressions (2020-05-11)'

AI05-0289-1 extends invariant checking to @cite{in} parameters. However, this makes
it impossible to call a public function of the type from an invariant
expression, as that public function will attempt to check the invariant,
resulting in an infinite recursion.

This AI specifies, that type-invariant checking is performed on parameters
of mode @cite{in} upon return from procedure calls, but not of @cite{in}-mode
parameters in functions.

RM references: 7.03.02 (19/3)
@end itemize

@geindex AI12-0045 (Ada 2022 feature)


@itemize *

@item 
`AI12-0045 Pre- and Postconditions are allowed on generic subprograms (2015-03-17)'

The SPARK toolset now supports contracts on generic subprograms, packages and
their respective bodies.

RM references: 6.01.01 (1/3)
@end itemize

@geindex AI12-0046 (Ada 2022 feature)


@itemize *

@item 
`AI12-0046 Enforcing legality for anonymous access components in record aggregates (0000-00-00)'

For a record aggregate of the form (X | Y => ….), any relevant legality rules are checked for both for X and Y.

For example,

@example
X : aliased constant String := ... ;
type R is record
  F1 : access constant String;
  F2 : access String;
end record;
Obj : R := (F1 | F2 => X'Access); -- ok for F1, but illegal for F2
@end example

RM references: 4.03.01 (16/3)
@end itemize

@geindex AI12-0047 (Ada 2022 feature)


@itemize *

@item 
`AI12-0047 Generalized iterators and discriminant-dependent components (0000-00-00)'

Iterating over the elements of an array is subject to the same legality checks as renaming the array. For example, if an assignment to an enclosing discriminated object could cause an array object to cease to exist then we don’t allow renaming the array. So it is similarly not allowed to iterate over the elements of such an array.

RM references: 5.05.02 (6/3)
@end itemize

@geindex AI12-0048 (Ada 2022 feature)


@itemize *

@item 
`AI12-0048 Default behavior of tasks on a multiprocessor with a specified dispatching policy (0000-00-00)'

Clarify that if the user does not impose requirements about what CPUs a given task might execute on, then the implementation does not get to impose such requirements. This avoids potential problems with priority inversion.

RM references: D.16.01 (30/3)
@end itemize

@geindex AI12-0049 (Ada 2022 feature)


@itemize *

@item 
`AI12-0049 Invariants need to be checked on the initialization of deferred constants (0000-00-00)'

Invariant checking for deferred constants (and subcomponents thereof) is performed. Corrects a clear oversight in the previous RM wording.

RM references: 7.03.02 (10/3)
@end itemize

@geindex AI12-0050 (Ada 2022 feature)


@itemize *

@item 
`AI12-0050 Conformance of quantified expressions (2016-07-22)'

Compiler rejects a subprogram body when an expression for a boolean formal
parameter includes a quantified expression, and the subprogram declaration
contains a textual copy of the same.

RM references: 6.03.01 (20) 6.03.01 (21)
@end itemize

@geindex AI12-0051 (Ada 2022 feature)


@itemize *

@item 
`AI12-0051 The Priority aspect can be specified when Attach_Handler is specified (0000-00-00)'

Previous RM wording had two contradictory rules for determining (in some cases) the priority of a protected subprogram that is attached to an interrupt. The AI clarifies which one of the rules takes precedence.

RM references: D.03 (10/3)
@end itemize

@geindex AI12-0052 (Ada 2022 feature)


@itemize *

@item 
`AI12-0052 Implicit objects are considered overlapping (0000-00-00)'

Clarify that the rules about unsynchronized concurrent access apply as one would expect in the case of predefined routines that access Text_IO’s default input and default output files. There was no compiler changes needed to implement this.

RM references: A (3/2) A.10.03 (21)
@end itemize

@geindex AI12-0054-2 (Ada 2022 feature)


@itemize *

@item 
`AI12-0054-2 Aspect Predicate_Failure (0000-00-00)'

New aspect Predicate_Failure is defined.  A solution for the problem that a predicate like

@example
subtype Open_File is File with Dynamic_Predicate =\> Is_Open (Open_File) or else (raise File_Not_Open);
@end example

does the wrong thing in the case of a membership test.

RM references: 3.02.04 (14/3) 3.02.04 (31/3) 3.02.04 (35/3)
@end itemize

@geindex AI12-0055 (Ada 2022 feature)


@itemize *

@item 
`AI12-0055 All properties of a usage profile are defined by pragmas (2020-06-09)'

AI12-0055 allows the use of the No_Dynamic_CPU_Assignment restriction in pragmas Restrictions and Restrictions_Warnings.

RM references: D.07 (10/3) D.13 (6/3) D.13 (8/3) D.13 (10/3)
@end itemize

@geindex AI12-0059 (Ada 2022 feature)


@itemize *

@item 
`AI12-0059 Object_Size attribute (2019-12-02)'

AI12-0059 brings GNAT-defined attribute Object_Size to Ada standard
and clarifies its semantics. Given that the attribute already existed in
GNAT compiler, the feature is supported for all language versions.

RM references: 4.09.01 (2/3) 13.01 (14) 13.01 (23) 13.03 (9/3) 13.03
(50/2) 13.03 (51) 13.03 (52) 13.03 (58)
@end itemize

@geindex AI12-0061 (Ada 2022 feature)


@itemize *

@item 
`AI12-0061 Iterated component associations in array aggregates (2016-09-01)'

Ada issue AI12-061 introduces a new construct in array aggregates allowing
component associations to be parameterized by a loop variable, for example:

@example
array (1 .. 10) of Integer :=
  (for I in 1 .. 10 => I ** 2);
type Matrix is
array
  (Positive range <>, Positive range <>) of Float;
G : constant Matrix
:=
  (for I in 1 .. 4 =>
      (for J in 1 .. 4 =>
          (if I=J then
1.0 else 0.0))); -- Identity matrix
@end example

The expression in such an association can also be a function that returns a
limited type, and the range can be specified by the ‘others’ choice.

RM references: 4.03.03 (5/2) 4.03.03 (6) 4.03.03 (17/3) 4.03.03 (20)
4.03.03 (23.1/4) 4.03.03 (32/3) 4.03.03 (43) 3.01 (6/3) 3.03 (6)
3.03 (18.1/3) 3.03.01 (23/3) 5.05 (6) 8.01 (2.1/4)
@end itemize

@geindex AI12-0062 (Ada 2022 feature)


@itemize *

@item 
`AI12-0062 Raise exception with failing string function (0000-00-00)'

Clarify that if raising exception E1 is accompanied with a String-valued
expression whose evaluation raises exception E2, then E2 is what gets propagated.

RM references: 11.03 (4/2)
@end itemize

@geindex AI12-0065 (Ada 2022 feature)


@itemize *

@item 
`AI12-0065 Descendants of incomplete views (0000-00-00)'

This AI is a clarification of potentially confusing wording.  GNAT correctly handles the example given in AARM 7.3.1(5.b-5.d), which illustrates the topic of this AI.

RM references: 7.03.01 (5.2/3)
@end itemize

@geindex AI12-0067 (Ada 2022 feature)


@itemize *

@item 
`AI12-0067 Accessibility level of explicitly aliased parameters of procedures and entries (0000-00-00)'

The AI fixes a case where the intent was fairly obvious but the RM wording failed to mention a case (with the result that the accessibility level of an explicitly aliased parameter of a procedure or entry was undefined even though the intent was clear).

RM references: 3.10.02 (7/3)
@end itemize

@geindex AI12-0068 (Ada 2022 feature)


@itemize *

@item 
`AI12-0068 Predicates and the current instance of a subtype (2020-05-06)'

AI12-0068 is a binding interpretation that defines the current instance name in a type or subtype aspect to be a value rather than an object. This affects
attributes whose prefix is a current instance in predicates, type invariants, and @code{Default_Initial_Condition} aspects. In particular, in the case of the @code{Constrained} attribute the value will always be True, and formerly legal attributes that require an object as their prefix (such as @code{Size}, @code{Access}, @code{Address}, etc.) are illegal when applied to a current instance in type and subtype aspects.

RM references: 8.06 (17/3)
@end itemize

@geindex AI12-0069 (Ada 2022 feature)


@itemize *

@item 
`AI12-0069 Inconsistency in Tree container definition (0000-00-00)'

The description of how iteration over a Tree container’s elements was contradictory in some cases regarding whether a cursor designating the Root node is included in the iteration. This contradiction was resolved. In the “!ACATS Test” section of the AI, it says that if an implementation were to get this wrong then almost any attempt to iterate over any tree would fail at runtime.

RM references: A.18.10 (153/3) A.18.10 (155/3) A.18.10 (157/3) A.18.10 (159/3)
@end itemize

@geindex AI12-0070 (Ada 2022 feature)


@itemize *

@item 
`AI12-0070 9.3(2) does not work for anonymous access types (0000-00-00)'

The RM contained some old wording about the master of an allocated object that only made sense for named access types. The AI clarifies the wording to clearly state the scope of validity and ensures that the paragraph does not contradict 3.10.2’s rules for anonymous access  types.

RM references: 3.10.02 (13.1/3) 9.03 (2)
@end itemize

@geindex AI12-0071 (Ada 2022 feature)


@itemize *

@item 
`AI12-0071 Order of evaluation when multiple predicates apply (2015-08-10)'

AI12-0071 specifies the semantics of multiple/inherited predicates on a
single subtype.

RM references: 3.02.04 (4/3) 3.02.04 (6/3) 3.02.04 (30/3) 3.02.04 (31/3)
3.02.04 (32/3) 3.02.04 (33/3) 3.02.04 (35/3) 3.05.05 (7.1/3)
3.05.05 (7.2/3) 3.05.05 (7.3/3) 3.08.01 (10.1/3) 3.08.01 (15/3)
4.05.02 (29/3) 4.05.02 (30/3) 4.06 (51/3) 4.09.01 (10/3) 5.04
(7/3) 5.05 (9/3) 13.09.02 (3/3) 13.09.02 (12)
@end itemize

@geindex AI12-0072 (Ada 2022 feature)


@itemize *

@item 
`AI12-0072 Missing rules for Discard_Names aspect (0000-00-00)'

Clarify that Discard_Names is an aspect, not just a pragma.

RM references: C.05 (1) C.05 (5) C.05 (7/2) C.05 (8)
@end itemize

@geindex AI12-0073 (Ada 2022 feature)


@itemize *

@item 
`AI12-0073 Synchronous Barriers are not allowed with Ravenscar (2020-02-24)'

Ada 2022 adds (as a binding interpretation) a @code{No_Dependence =>
Ada.Synchronous_Barriers} restriction to the Ravenscar profile.

RM references: D.13 (6/3)
@end itemize

@geindex AI12-0074 (Ada 2022 feature)


@itemize *

@item 
`AI12-0074 View conversions and out parameters passed by copy (2020-03-26)'

This Ada 2022 AI makes illegal some cases of out parameters whose type has a
@code{Default_Value} aspect.

RM references: 4.06 (56) 6.04.01 (6.25/3) 6.04.01 (13.1/3)
@end itemize

@geindex AI12-0075 (Ada 2022 feature)


@itemize *

@item 
`AI12-0075 Static expression functions (2020-04-13)'

Ada 2022 defines a new aspect @code{Static} that can be specified on expression
functions. Such an expression function can be called in contexts requiring static expressions when the actual parameters are all static, allowing for greater abstraction in complex static expressions.

RM references: 4.09 (21) 6.08 (3/4) 6.08 (5/4) 6.08 (6/4) 7.03.02 (8.2/5)
7.03.02 (15/4) 7.03.02 (16/4) 7.03.02 (17/4) 7.03.02 (19/4)
7.03.02 (20/5)
@end itemize

@geindex AI12-0076 (Ada 2022 feature)


@itemize *

@item 
`AI12-0076 Variable state in pure packages (0000-00-00)'

Defines an obscure constant-modifying construct to be erroneous. The issue is that the current instance of a type is a variable object, so the following is legal:

@example
  type T;
  type T_Ref (Access_To_Variable : access T) is null record;
  type T is limited record
     Self : T_Ref (T'Access);
     Int : Integer;
  end record;

  Obj : constant T := (Self => <>, Int => 123);
begin
  Obj.Self.Access_To_Variable.Int := 456; -- modifying a component of a constant
@end example

In cases where constancy is really needed (e.g., for an object declared in a Pure context), such a case needs to be erroneous.

RM references: 10.02.01 (17/3) E.02.02 (17/2)
@end itemize

@geindex AI12-0077 (Ada 2022 feature)


@itemize *

@item 
`AI12-0077 Has_Same_Storage on objects of size zero (2020-03-30)'

This binding interpretation requires the Has_Same_Storage attribute
to return always @cite{false} for objects that have a size of zero.

RM references: 13.03 (73.4/3)
@end itemize

@geindex AI12-0078 (Ada 2022 feature)


@itemize *

@item 
`AI12-0078 Definition of node for tree container is confusing (0000-00-00)'

Clarifies the expected behavior in processing tree containers.

RM references: A.18.10 (2/3) A.18.10 (3/3)
@end itemize

@geindex AI12-0081 (Ada 2022 feature)


@itemize *

@item 
`AI12-0081 Real-time aspects need to specify when they are evaluated (0000-00-00)'

Clarify the point at which Priority and Interrupt_Priority aspect expressions are evaluated.

RM references: D.01 (17/3) D.16 (9/3)
@end itemize

@geindex AI12-0084 (Ada 2022 feature)


@itemize *

@item 
`AI12-0084 Box expressions in array aggregates (2014-12-15)'

This AI addresses an issue where compiler used to fail to initialize
components of a multidimensional aggregates with box initialization when
scalar components have a specified default value. The AI clarifies that
in an array aggregate with box (i.e., @code{<>}) component values, the
@code{Default_Component_Value} of the array type (if any) should not be ignored.

RM references: 4.03.03 (23.1/2)
@end itemize

@geindex AI12-0085 (Ada 2022 feature)


@itemize *

@item 
`AI12-0085 Missing aspect cases for Remote_Types (0000-00-00)'

A distributed systems annex (Annex E) clarification. Aspect specifications
that are forbidden using attribute definition clause syntax are also forbidden
using aspect_specification syntax.

RM references: E.02.02 (17/2)
@end itemize

@geindex AI12-0086 (Ada 2022 feature)


@itemize *

@item 
`AI12-0086 Aggregates and variant parts (2019-08-14)'

In Ada 2012, a discriminant value that governs an active variant part in an
aggregate had to be static. AI12-0086 relaxes this restriction: If the subtype of the discriminant value is a static subtype all of whose values select the same variant, then the expression for the discriminant is allowed to be nonstatic.

RM references: 4.03.01 (17/3) 4.03.01 (19/3)
@end itemize

@geindex AI12-0088 (Ada 2022 feature)


@itemize *

@item 
`AI12-0088 UTF_Encoding.Conversions and overlong characters on input (0000-00-00)'

Clarify that overlong characters are acceptable on input even if we never generate them as output.

RM references: A.04.11 (54/3) A.04.11 (55/3)
@end itemize

@geindex AI12-0089 (Ada 2022 feature)


@itemize *

@item 
`AI12-0089 Accessibility rules need to take into account that a generic function is not a (0000-00-00)'

Fix cases in RM wording where the accessibility rules for a function failed to take into account the fact that a generic function is not a function. For example, a generic function with an explicitly aliased parameter should be able to return references to that parameter in the same ways that a (non-generic) function can. The previous wording did not allow that.

RM references: 3.10.02 (7/3) 3.10.02 (19.2/3) 3.10.02 (19.3/3) 6.05 (4/3)
@end itemize

@geindex AI12-0093 (Ada 2022 feature)


@itemize *

@item 
`AI12-0093 Iterator with indefinite cursor (0000-00-00)'

A clarification that confirms what GNAT is already doing.

RM references: 5.05.02 (8/3) 5.05.02 (10/3)
@end itemize

@geindex AI12-0094 (Ada 2022 feature)


@itemize *

@item 
`AI12-0094 An access_definition should be a declarative region (0000-00-00)'

Fixes wording omission in the RM, confirming that the behaviour of GNAT is
correct.

RM references: 8.03 (2) 8.03 (26/3)
@end itemize

@geindex AI12-0095 (Ada 2022 feature)


@itemize *

@item 
`AI12-0095 Generic formal types and constrained partial views (0000-00-00)'

Deciding whether an actual parameter corresponding to an explicitly aliased formal parameter is legal depends on (among other things) whether the parameter type has a constrained partial view. The AI clarifies how this compile-time checking works in the case of a generic formal type (assume the best in the spec and recheck each instance, assume the worst in a generic body).

RM references: 3.10.02 (27.2/3) 4.06 (24.16/2) 6.04.01 (6.2/3) 12.05.01 (15)
@end itemize

@geindex AI12-0096 (Ada 2022 feature)


@itemize *

@item 
`AI12-0096 The exception raised when a subtype conversion fails a predicate check (0000-00-00)'

Clarify that the Predicate_Failure aspect works the same in a subtype conversion as in any other context.

RM references: 4.06 (57/3)
@end itemize

@geindex AI12-0097 (Ada 2022 feature)


@itemize *

@item 
`AI12-0097 Tag of the return object of a simple return expression (0000-00-00)'

Clarify wording about the tag of a function result in the case of a simple (i.e. not extended) return statement in a function with a class-wide result type.

RM references: 6.05 (8/3)
@end itemize

@geindex AI12-0098 (Ada 2022 feature)


@itemize *

@item 
`AI12-0098 Problematic examples for ATC (0000-00-00)'

The AI clarifies reference manual examples, there is no compiler impact.

RM references: 9.07.04 (13)
@end itemize

@geindex AI12-0099 (Ada 2022 feature)


@itemize *

@item 
`AI12-0099 Wording problems with predicates (2020-05-04)'

When extending a task or protected type from an ancestor interface subtype with  a predicate, a link error can occur due to the compiler failing to generate the predicate-checking function. This AI clarifies the requirement for such predicate inheritance for concurrent types.

RM references: 3.02.04 (4/4) 3.02.04 (12/3) 3.02.04 (20/3)
@end itemize

@geindex AI12-0100 (Ada 2022 feature)


@itemize *

@item 
`AI12-0100 A qualified expression makes a predicate check (2020-02-17)'

The compiler now enforces predicate checks on qualified expressions when the
qualifying subtype imposes a predicate.

RM references: 4.07 (4)
@end itemize

@geindex AI12-0101 (Ada 2022 feature)


@itemize *

@item 
`AI12-0101 Incompatibility of hidden untagged record equality (2019-10-31)'

AI12-0101 is a binding interpretation that removes a legality rule that
prohibited the declaration of a primitive equality function for a private type in the private part of its enclosing package (either before or after the completion of the type) when the type is completed as an untagged record type. Such declarations are now accepted in Ada 2012 and later Ada versions.

As a consequence of this work, some cases where the implementation of AI05-0123 was incomplete were corrected.
More specifically, if a user-defined equality operator is present for an untagged record type in an Ada 2012 program, that user-defined equality operator will be (correctly) executed in some difficult-to-characterize cases where the predefined component-by-component comparison was previously being (incorrectly) executed. This can arise, for example, in the case of the predefined equality operation for an enclosing composite type that has a component of the user-defined primitive equality op’s operand type.
This correction means that the impact of this change is not limited solely to code that was previously rejected at compile time.

RM references: 4.05.02 (9.8/3)
@end itemize

@geindex AI12-0102 (Ada 2022 feature)


@itemize *

@item 
`AI12-0102 Stream_IO.File_Type has Preelaborable_Initialization (0000-00-00)'

Modifies the declaration of one type in a predefined package. GNAT’s version of @code{Ada.Streams.Stream_IO} already had this modification (the @code{Preelaborable__Initialization} pragma).

RM references: A.12.01 (5)
@end itemize

@geindex AI12-0103 (Ada 2022 feature)


@itemize *

@item 
`AI12-0103 Expression functions that are completions in package specifications (0000-00-00)'

Clarifies that expression functions that are completions do not cause “general”  freeze-everybody-in-sight freezing like a subprogram body.

RM references: 13.14 (3/3) 13.14 (5/3)
@end itemize

@geindex AI12-0104 (Ada 2022 feature)


@itemize *

@item 
`AI12-0104 Overriding an aspect is undefined (0000-00-00)'

A clarification of the wording in RM, no compiler impact.

RM references: 4.01.06 (4/3) 4.01.06 (17/3)
@end itemize

@geindex AI12-0105 (Ada 2022 feature)


@itemize *

@item 
`AI12-0105 Pre and Post are not allowed on any subprogram completion (0000-00-00)'

Language-defined aspects (e.g., @code{Post}) cannot be specified as part of the completion of  a subprogram declaration. Fix a hole in the RM wording to clarify that this general rule applies even in the special cases where the completion is either an expression function or a null procedure.

RM references: 13.01.01 (18/3)
@end itemize

@geindex AI12-0106 (Ada 2022 feature)


@itemize *

@item 
`AI12-0106 Write’Class aspect (0000-00-00)'

Clarify that the syntax used in an ACATS test BDD2005 for specifying a class-wide streaming aspect is correct.

RM references: 13.01.01 (28/3) 13.13.02 (38/3)
@end itemize

@geindex AI12-0107 (Ada 2022 feature)


@itemize *

@item 
`AI12-0107 A prefixed view of a By_Protected_Procedure interface has convention protected (2020-06-05)'

A prefixed view of a subprogram with aspect Synchronization set to
By_Protected_Procedure has convention protected.

RM references: 6.03.01 (10.1/2) 6.03.01 (12) 6.03.01 (13)
@end itemize

@geindex AI12-0109 (Ada 2022 feature)


@itemize *

@item 
`AI12-0109 Representation of untagged derived types (2019-11-12)'

Ada disallows a nonconforming specification of a type-related representation
aspect of an untagged by-reference type. The motivation for this rule is to ensure that a parent type and a later type derived from the parent agree with respect to such aspects. AI12-0109 disallows a construct that otherwise could be used to get around this rule: an aspect specification for the parent type that occurs after the declaration of the derived type.

RM references: 13.01 (10/3)
@end itemize

@geindex AI12-0110 (Ada 2022 feature)


@itemize *

@item 
`AI12-0110 Tampering checks are performed first (2020-04-14)'

AI12-0110 requires tampering checks in the containers library to be
performed first, before any other checks.

RM references: A.18.02 (97.1/3) A.18.03 (69.1/3) A.18.04 (15.1/3) A.18.07
(14.1/3) A.18.10 (90/3) A.18.18 (35/3)
@end itemize

@geindex AI12-0112 (Ada 2022 feature)


@itemize *

@item 
`AI12-0112 Contracts for container operations (0000-00-00)'

A representation change replacing english descriptions of contracts for
operations on predefined container types with pre/post-conditions. No compiler
impact.

RM references: A.18.02 (99/3) 11.04.02 (23.1/3) 11.05 (23) 11.05 (26) A
(4) A.18 (10)
@end itemize

@geindex AI12-0114 (Ada 2022 feature)


@itemize *

@item 
`AI12-0114 Overlapping objects designated by access parameters are not thread-safe (0000-00-00)'

There are rules saying that concurrent calls to predefined subprograms don’t interfere with each other unless actual parameters overlap. The AI clarifies that such an interference is also possible if overlapping objects are reachable via access dereferencing from actual parameters of the two calls.

RM references: A (3/2)
@end itemize

@geindex AI12-0116 (Ada 2022 feature)


@itemize *

@item 
`AI12-0116 Private types and predicates (0000-00-00)'

Clarify that the same aspect cannot be specified twice for the same type. @code{Dynamic_Predicate}, for example, can be specified on either the partial view of a type or on the completion in the private part, but not on both.

RM references: 13.01 (9/3) 13.01 (9.1/3)
@end itemize

@geindex AI12-0117 (Ada 2022 feature)


@itemize *

@item 
`AI12-0117 Restriction No_Tasks_Unassigned_To_CPU (2020-06-12)'

This AI adds a restriction No_Tasks_Unassigned_To_CPU to provide safe
use of Ravenscar.

The CPU aspect is specified for the environment task. No CPU aspect is
specified to be statically equal to @code{Not_A_Specific_CPU}. If aspect CPU
is specified (dynamically) to the value @code{Not_A_Specific_CPU}, then
Program_Error is raised. If Set_CPU or @code{Delay_Until_And_Set_CPU} are called
with the CPU parameter equal to @code{Not_A_Specific_CPU}, then @code{Program_Error} is raised.

RM references: D.07 (10.8/3)
@end itemize

@geindex AI12-0120 (Ada 2022 feature)


@itemize *

@item 
`AI12-0120 Legality and exceptions of generalized loop iteration (0000-00-00)'

Clarify that the expansion-based definition of generalized loop iteration
includes legality checking. If the expansion would be illegal (for example,
because of passing a constant actual parameter in a call when the mode of
the corresponding formal parameter is in-out), then the loop is illegal too.

RM references: 5.05.02 (6.1/4) 5.05.02 (10/3) 5.05.02 (13/3)
@end itemize

@geindex AI12-0121 (Ada 2022 feature)


@itemize *

@item 
`AI12-0121 Stream-oriented aspects (0000-00-00)'

Clarify that streaming-oriented aspects (e.g., Read) can be specified using
aspect_specification syntax, not just via an attribute definition clause.

RM references: 13.13.02 (38/3)
@end itemize

@geindex AI12-0124 (Ada 2022 feature)


@itemize *

@item 
`AI12-0124 Add Object’Image (2017-03-24)'

The corrigendum of Ada 2012 extends attribute @code{'Image following} the syntax for the GNAT @code{'Img} attribute. This AI fixes a gap in the earlier implementation, which did not recognize function calls and attributes that are functions as valid object prefixes.

RM references: 3.05 (55/3)
@end itemize

@geindex AI12-0125-3 (Ada 2022 feature)


@itemize *

@item 
`AI12-0125-3 Add @@ as an abbreviation for the LHS of an assignment (2016-11-11)'

This AI introduces the use of the character ‘@@’ as an abbreviation for the left-hand side of an assignment statement, usable anywhere within the expression on the right-hand side. To use this feature the compilation flag -gnat2022 must be specified.

RM references: 5.02.01 (0) 2.02 (9) 3.03 (21.1/3) 4.01 (2/3) 8.06 (9/4)
@end itemize

@geindex AI12-0127 (Ada 2022 feature)


@itemize *

@item 
`AI12-0127 Partial aggregate notation (2016-10-12)'

This AI describes a new constructor for aggregates, in terms of an existing record or array object, and a series of component-wise modifications of its value, given by named associations for the modified components. To use this feature the compilation flag @code{-gnat2022} must be specified.

RM references: 4.03 (2) 4.03 (3/2) 4.03 (4) 4.03.01 (9) 4.03.01 (15/3)
4.03.01 (16/4) 4.03.01 (17/5) 4.03.01 (17.1/2) 4.03.03 (4) 4.03.03
(14) 4.03.03 (17/5) 4.03.04 (0) 7.05 (2.6/2)
@end itemize

@geindex AI12-0128 (Ada 2022 feature)


@itemize *

@item 
`AI12-0128 Exact size access to parts of composite atomic objects (2019-11-24)'

According to this AI, the compiler generates full access to atomic composite objects even if the access is only partial in the source code. To use this feature the compilation flag @code{-gnat2022} must be specified.

RM references: C.06 (13.2/3) C.06 (19) C.06 (20) C.06 (22/2) C.06 (25/4)
@end itemize

@geindex AI12-0129 (Ada 2022 feature)


@itemize *

@item 
`AI12-0129 Make protected objects more protecting (2020-07-01)'

A new aspect Exclusive_Functions has been added to the language to force the
use of read/write locks on protected functions when needed.

RM references: 9.05.01 (2) 9.05.01 (4) 9.05.01 (5) 9.05.01 (7) 9.05.03
(15) 9.05.03 (23)
@end itemize

@geindex AI12-0130 (Ada 2022 feature)


@itemize *

@item 
`AI12-0130 All I/O packages should have Flush (2016-07-03)'

The Flush routine has been added for the @code{Sequential_IO} and @code{Direct_IO} standard packages in the Ada 2012 COR.1:2016. The Flush routine here is equivalent to the one found in @code{Text_IO}. The @code{Flush} procedure synchronizes the external file with the internal file (by flushing any internal buffers) without closing the file.

RM references: A.08.01 (10) A.08.02 (28/3) A.08.04 (10) A.10.03 (21)
A.12.01 (28/2) A.12.01 (28.6/1)
@end itemize

@geindex AI12-0131 (Ada 2022 feature)


@itemize *

@item 
`AI12-0131 Inherited Pre’Class when unspecified on initial subprogram (0000-00-00)'

If T1 is a tagged type with a primitive P that has no class-wide precondition,
and if T2 is an extension of T1 which overrides the inherited primitive P, then that overriding P is not allowed to have a class-wide precondition. Allowing it would be ineffective except in corner cases where it would be confusing.

RM references: 6.01.01 (17/3) 6.01.01 (18/3)
@end itemize

@geindex AI12-0132 (Ada 2022 feature)


@itemize *

@item 
`AI12-0132 Freezing of renames-as-body (2020-06-13)'

This AI clarifies that a renames-as-body freezes the expression of any
expression function that it renames.

RM references: 13.14 (5/3)
@end itemize

@geindex AI12-0133 (Ada 2022 feature)


@itemize *

@item 
`AI12-0133 Type invariants and default initialized objects (0000-00-00)'

Clarify that invariant checking for a default-initialized object is performed regardless of where the object is declared (in particular, even when the full view of the type is visible).

RM references: 7.03.02 (10.3/3)
@end itemize

@geindex AI12-0135 (Ada 2022 feature)


@itemize *

@item 
`AI12-0135 Enumeration types should be eligible for convention C (0000-00-00)'

Ada previously allowed but did not require supporting specifying convention C for an enumeration type. Now it is required that an implementation shall support it.

RM references: B.01 (14/3) B.01 (41/3) B.03 (65)
@end itemize

@geindex AI12-0136 (Ada 2022 feature)


@itemize *

@item 
`AI12-0136 Language-defined packages and aspect Default_Storage_Pool (0000-00-00)'

Clarify that the effect of specifying Default_Storage_Pool for an instance of a predefined generic is implementation-defined. No compiler impact.

RM references: 13.11.03 (5/3)
@end itemize

@geindex AI12-0137 (Ada 2022 feature)


@itemize *

@item 
`AI12-0137 Incomplete views and access to class-wide types (0000-00-00)'

If the designated type of an access type is incomplete when the access type is declared, then we have rules about whether we get a complete view when a value of the access type is dereferenced. Clarify that analogous rules apply if the designated type is class-wide.

RM references: 3.10.01 (2.1/2)
@end itemize

@geindex AI12-0138 (Ada 2022 feature)


@itemize *

@item 
`AI12-0138 Iterators of formal derived types (2021-02-11)'

AI12-0138 specifies the legality rules for confirming specifications of
nonoverridable aspects. This completes the legality checks for aspect @code{Implicit_Dereference} and simplifies the checks for those aspects that are inherited operations.

RM references: 13.01.01 (18/4) 13.01.01 (34/3) 4.01.05 (6/3) 4.01.06 (5/3)
4.01.06 (6/3) 4.01.06 (7/3) 4.01.06 (8/3) 4.01.06 (9/3) 5.05.01 (11/3)
@end itemize

@geindex AI12-0140 (Ada 2022 feature)


@itemize *

@item 
`AI12-0140 Access to unconstrained partial view when full view is constrained (0000-00-00)'

Clarify some confusion about about whether what matters when checking whether designated subtypes statically match is the view of the designated type that is currently available v.s. the view that was available when the access type was declared.

RM references: 3.02 (7/2) 7.03.01 (5/1)
@end itemize

@geindex AI12-0143 (Ada 2022 feature)


@itemize *

@item 
`AI12-0143 Using an entry index of a family in a precondition (2022-04-05)'

Ada 2022 adds the @code{Index} attribute, which allows the use of the entry family index of an entry call within preconditions and post-conditions.

RM references: 6.01.01 (30/3) 9.05.04 (5/3)
@end itemize

@geindex AI12-0144 (Ada 2022 feature)


@itemize *

@item 
`AI12-0144 Make Discrete_Random more flexible (2020-01-31)'

A new function Random with First/Last parameters is provided in the
@code{Ada.Numerics.Discrete_Random} package.

RM references: A.05.02 (20) A.05.02 (32) A.05.02 (41) A.05.02 (42)
@end itemize

@geindex AI12-0145 (Ada 2022 feature)


@itemize *

@item 
`AI12-0145 Pool_of_Subpool returns null when called too early (0000-00-00)'

Clarify that if you ask for the pool of a subpool (by calling @code{Pool_Of_Subpool}) before @code{Set_Pool_of_Subpool} is called, then the result is null.

RM references: 13.11.04 (20/3)
@end itemize

@geindex AI12-0147 (Ada 2022 feature)


@itemize *

@item 
`AI12-0147 Expression functions and null procedures can be declared in a protected_body (2015-03-05)'

AI12-0147 specifies that null procedures and expression functions are now
allowed in protected bodies.

RM references: 9.04 (8/1)
@end itemize

@geindex AI12-0149 (Ada 2022 feature)


@itemize *

@item 
`AI12-0149 Type invariants are checked for functions returning access-to-type (0000-00-00)'

Extend the rule saying that @code{Type_Invariant} checks are performed for access-to-T parameters (where T has a specified @code{Type_Invariant}) so that the rule also applies to function results.

RM references: 7.03.02 (19.3/4)
@end itemize

@geindex AI12-0150 (Ada 2022 feature)


@itemize *

@item 
`AI12-0150 Class-wide type invariants and statically bound calls (0000-00-00)'

The same approach used in AI12-0113 to ensure that contract-related calls associated with a call to a subprogram “match” with respect to dispatching also applies to @code{Type_Invariant} checking.

RM references: 7.03.02 (3/3) 7.03.02 (5/3) 7.03.02 (9/3) 7.03.02 (22/3)
@end itemize

@geindex AI12-0154 (Ada 2022 feature)


@itemize *

@item 
`AI12-0154 Aspects of library units (0000-00-00)'

Clarify that an aspect_specification for a library unit is equivalent to a corresponding aspect-specifying pragma.

RM references: 13.01.01 (32/3)
@end itemize

@geindex AI12-0156 (Ada 2022 feature)


@itemize *

@item 
`AI12-0156 Use subtype_indication in generalized iterators (0000-00-00)'

For iterating over an array, we already allow (but do not require) explicitly providing a subtype indication in an iterator_specification. Tee AI generalizes this to handle the case where the element type of the array is of an anonymous access type. This also allows (but does not require) explicitly naming the cursor subtype in a generalized iterator.
The main motivation for allowing these new cases is improving readability by making it easy to infer the (sub)type of the iteration object just by looking at the loop.

RM references: 5.05.02 (2/3) 5.05.02 (5/4) 5.05.02 (7/3) 3.10.02 (11.1/2)
@end itemize

@geindex AI12-0157 (Ada 2022 feature)


@itemize *

@item 
`AI12-0157 Missing rules for expression functions (0000-00-00)'

Clarify that an expression function behaves like a single-return-statement
function in more cases: it can return an aggregate without extra parens, the expression has an applicable index constraint, and the same accessibility rules apply in both cases.

For instance, the code below is legal:

@example
subtype S is String (1 .. 10);
function f return S is (others => '?');
@end example

RM references: 3.10.02 (19.2/4) 3.10.02 (19.3/4) 4.03.03 (11/2) 6.08 (2/3)
6.08 (3/3) 6.08 (5/3) 6.08 (6/3) 6.08 (7/3) 7.05 (2.9/3) 13.14
(5.1/4) 13.14 (5.2/4) 13.14 (8/3) 13.14 (10.1/3) 13.14 (10.2/3)
13.14 (10.3/3)
@end itemize

@geindex AI12-0160 (Ada 2022 feature)


@itemize *

@item 
`AI12-0160 Adding an indexing aspect to an indexable container type (0000-00-00)'

If the parent type of a derived type has exactly one of the two indexing aspects (that is, constant_indexing and variable_indexing) specified, then the derived type cannot have a specification for the other one.

RM references: 4.01.06 (6/4) 4.01.06 (9/4) 3.06 (22.2/3)
@end itemize

@geindex AI12-0162 (Ada 2022 feature)


@itemize *

@item 
`AI12-0162 Memberships and Unchecked_Unions (0000-00-00)'

Clarify that membership tests for unchecked_union types work consistently when
testing membership in more than one subtype (X in AA | BB | CC)  as when
testing for one.

RM references: B.03.03 (25/2)
@end itemize

@geindex AI12-0164 (Ada 2022 feature)


@itemize *

@item 
`AI12-0164 Max_Entry_Queue_Length aspect for entries (2019-06-11)'

AI12-0164 defines pragma and aspect @code{Max_Entry_Queue_Length} in addition
to the GNAT-specific equivalents @code{Max_Queue_Length} and @code{Max_Entry_Queue_Depth}.

RM references: D.04 (16)
@end itemize

@geindex AI12-0165 (Ada 2022 feature)


@itemize *

@item 
`AI12-0165 Operations of class-wide types and formal abstract subprograms (2021-10-19)'

Ada 2022 specifies that when the controlling type of a formal abstract
subprogram declaration is a formal type, and the actual type is a class-wide type T’Class, the actual subprogram can be an implicitly declared subprogram corresponding to a primitive operation of type T.

RM references: 12.06 (8.5/2)
@end itemize

@geindex AI12-0166 (Ada 2022 feature)


@itemize *

@item 
`AI12-0166 External calls to protected functions that appear to be internal calls (2016-11-15)'

According to this AI, the compiler rejects a call to a protected operation when the call appears within a precondition for another protected operation.

RM references: 6.01.01 (34/3) 9.05 (3/3) 9.05 (7.1/3)
@end itemize

@geindex AI12-0167 (Ada 2022 feature)


@itemize *

@item 
`AI12-0167 Type_Invariants and tagged-type View Conversions (0000-00-00)'

This AI clarifies that no invariant check is performed in a case where an invariant-violating value is assigned to a component. This confirms the current compiler behavior.

RM references: 7.03.02 (9/4)
@end itemize

@geindex AI12-0168 (Ada 2022 feature)


@itemize *

@item 
`AI12-0168 Freezing of generic instantiations of generics with bodies (0000-00-00)'

Adjust freezing rules to be compatible with AI12-0103-1. The change confirms the current compiler behavior.

RM references: 13.14 (3/4)
@end itemize

@geindex AI12-0169 (Ada 2022 feature)


@itemize *

@item 
`AI12-0169 Aspect specifications for entry bodies (0000-00-00)'

Change syntax to allow aspect specifications for implementation-defined aspects on entry bodies. The change doesn’t influence any of the language-defined aspects and is solely required for SPARK.

RM references: 9.05.02 (5)
@end itemize

@geindex AI12-0170 (Ada 2022 feature)


@itemize *

@item 
`AI12-0170 Abstract subprogram calls in class-wide precondition expressions (2020-07-06)'

This AI specifies rules for calls to abstract functions within class-wide preconditions and postconditions.

RM references: 3.09.03 (7) 6.01.01 (7/4) 6.01.01 (18/4) 6.01.01 (18.2/4)
@end itemize

@geindex AI12-0172 (Ada 2022 feature)


@itemize *

@item 
`AI12-0172 Raise expressions in limited contexts (2019-07-29)'

The compiler has been enhanced to support the use of raise expressions in
limited contexts.

RM references: 7.05 (2.1/3)
@end itemize

@geindex AI12-0173 (Ada 2022 feature)


@itemize *

@item 
`AI12-0173 Expression of an extended return statement (0000-00-00)'

Fix the wording related to expression of an extended return statement that was made ambiguous by changes of syntax in other AI’s. No compiler changes involved.

RM references: 6.05 (3/2) 6.05 (5/3)
@end itemize

@geindex AI12-0174 (Ada 2022 feature)


@itemize *

@item 
`AI12-0174 Aggregates of Unchecked_Unions using named notation (0000-00-00)'

In many cases, it is illegal to name a discriminant of an unchecked_union type.  Relax this rule to allow the use of named notation in an aggregate of an unchecked_union type.

RM references: B.03.03 (9/3)
@end itemize

@geindex AI12-0175 (Ada 2022 feature)


@itemize *

@item 
`AI12-0175 Preelaborable packages with address clauses (2020-03-20)'

The compiler nows accepts calls to certain functions that are essentially unchecked conversions in preelaborated library units. To use this feature the compilation flag @code{-gnat2022} must be specified.

RM references: 10.02.01 (7)
@end itemize

@geindex AI12-0179 (Ada 2022 feature)


@itemize *

@item 
`AI12-0179 Failure of postconditions of language-defined units (0000-00-00)'

A clarification that expressing postconditions for predefined units via RM wording or via @code{Post} aspect specifications are equivalent. In particular, the expression in such a @code{Post} aspect specification should not yield False. No implementation changes needed.

RM references: 1.01.03 (17/3) 11.04.02 (23.1/3)
@end itemize

@geindex AI12-0180 (Ada 2022 feature)


@itemize *

@item 
`AI12-0180 Using protected subprograms and entries within an invariant (2020-06-22)'

AI12-0180 makes entries and protected subprograms directly visible within Invariant aspects of a task or protected type.

RM references: 13.01.01 (12/3)
@end itemize

@geindex AI12-0181 (Ada 2022 feature)


@itemize *

@item 
`AI12-0181 Self-referencing representation aspects (0000-00-00)'

Clarify that a name or expression which freezes an entity cannot occur in an aspect specification for that entity.

RM references: 13.01 (9/4) 13.01 (9.1/4) 13.14 (19)
@end itemize

@geindex AI12-0182 (Ada 2022 feature)


@itemize *

@item 
`AI12-0182 Pre’Class and protected operations (0000-00-00)'

Confirm that Pre’Class and Post’Class cannot be specified for a protected operation. No language change.

RM references: 13.01.01 (16/3)
@end itemize

@geindex AI12-0184 (Ada 2022 feature)


@itemize *

@item 
`AI12-0184 Long Long C Data Types (2020-01-30)'

Two new types @code{long_long} and @code{unsigned_long_long} are introduced in the package @code{Interfaces.C}.

RM references: B.03 (71.3/3)
@end itemize

@geindex AI12-0185 (Ada 2022 feature)


@itemize *

@item 
`AI12-0185 Resolution of postcondition-specific attributes (0000-00-00)'

Clarify resolution rules for @code{'Old} and @code{'Result} attribute references to match original intent.

RM references: 6.01.01 (7/4) 6.01.01 (8/3) 6.01.01 (26.10/4) 6.01.01 (29/3)
@end itemize

@geindex AI12-0186 (Ada 2022 feature)


@itemize *

@item 
`AI12-0186 Profile freezing for the Access attribute (0000-00-00)'

Clarify that the use of Some_Subprogram’Access does not freeze the profile of Some_Subprogram.

RM references: 13.14 (15)
@end itemize

@geindex AI12-0187 (Ada 2022 feature)


@itemize *

@item 
`AI12-0187 Stable properties of abstract data types (2020-11-04)'

Ada 2022 defines a new aspect, @code{Stable_Properties}, for use in
generating additional postcondition checks for subprograms.

RM references: 7.03.04 (0) 13.01.01 (4/3)
@end itemize

@geindex AI12-0191 (Ada 2022 feature)


@itemize *

@item 
`AI12-0191 Clarify “part” for type invariants (0000-00-00)'

Clarify that for purposes of determining whether an invariant check is required for a “part” of an object, we do not look at “parts” which do not correspond to “parts” of the nominal type of the object. For example, if we have a parameter Param of a tagged type T1 (or equivalently of type T1’Class), and type T2 is an extension of T1 which declares a component Foo, and T1’Class (Param)’Tag = T2’Tag, then no invariant check is performed for Param’s Foo component (or any subcomponent thereof).

RM references: 3.03 (23/5) 3.09.01 (4.1/2) 6.08 (5.8/5) 7.03.02 (8.3/5)
7.03.02 (8.4/5) 7.03.02 (8.5/5) 7.03.02 (8.6/5) 7.03.02 (8.7/5)
7.03.02 (8.8/5) 7.03.02 (8.9/5) 7.03.02 (8.10/5) 7.03.02 (8.11/5)
7.03.02 (8.12/5) 7.03.02 (10.1/4) 7.03.02 (15/5) 7.03.02 (17/4)
7.03.02 (18/4) 7.03.02 (19/4) 13.13.02 (9/3)
@end itemize

@geindex AI12-0192 (Ada 2022 feature)


@itemize *

@item 
`AI12-0192 “requires late initialization” and protected types (2020-03-11)'

This AI clarifies that components of a protected type require late initialization when their initialization references (implicitly) the current instance of the type.

RM references: 3.03.01 (8.1/2)
@end itemize

@geindex AI12-0194 (Ada 2022 feature)


@itemize *

@item 
`AI12-0194 Language-defined aspects and entry bodies (0000-00-00)'

The AI Includes entry bodies on the list of bodies for which no language-defined aspects can be specified (although specifying an implementation-defined aspect may be allowed).

A wording change, no implementation impact.

RM references: 13.01.01 (17/3)
@end itemize

@geindex AI12-0195 (Ada 2022 feature)


@itemize *

@item 
`AI12-0195 Inheriting body but overriding precondition or postcondition (2021-08-11)'

Ada 2022 specifies that if a primitive with a class-wide precondition or
postcondition is inherited, and some primitive function called in the class-wide precondition or postcondition is overridden, then a dispatching call to the first primitive with a controlling operand that has the tag of the overriding type is required to check both the interpretation using the overriding function and the interpretation using the original overridden function.

RM references: 6.01.01 (38/4)
@end itemize

@geindex AI12-0196 (Ada 2022 feature)


@itemize *

@item 
`AI12-0196 Concurrent access to Ada container libraries (0000-00-00)'

Clarify that parallel execution of operations which use cursors to refer to different elements of the same container does not violate the rules about erroneous concurrent access in some cases. That is, if C1 and C2 are cursors that refer to different elements of some container, then it is ok to concurrently execute an operation that is passed C1 and which accesses one element of the container, with another operation (perhaps the same operation, perhaps not) that is passed C2 and which accesses another element of the container.

RM references: A.18 (2/2) A.18.02 (125/2) A.18.02 (133/3) A.18.02 (135/3)
A.18.03 (81/3) A.18.04 (36/3) A.18.07 (34/2) A.18.10 (116/3)
@end itemize

@geindex AI12-0198 (Ada 2022 feature)


@itemize *

@item 
`AI12-0198 Potentially unevaluated components of array aggregates (2020-05-13)'

Ada 2022 enforces the detection of components that belong to a nonstatic or
null range of index values of an array aggregate.

RM references: 6.01.01 (22.1/4)
@end itemize

@geindex AI12-0199 (Ada 2022 feature)


@itemize *

@item 
`AI12-0199 Abstract subprogram calls in class-wide invariant expressions (0000-00-00)'

Class-wide type invariants do not apply to abstract types, to avoid various
problems. Define the notion of a “corresponding expression” for a class-wide
type invariant, replacing references to components as appropriate, taking into
account rules for corresponding and specified discriminants when applying them
to a nonabstract descendant.

RM references: 7.03.02 (5/4) 7.03.02 (8/3)
@end itemize

@geindex AI12-0201 (Ada 2022 feature)


@itemize *

@item 
`AI12-0201 Missing operations of static string types (2020-02-25)'

Relational operators and type conversions of static string types are now static in Ada 2022.

RM references: 4.09 (9) 4.09 (19) 4.09 (20) 4.09 (24)
@end itemize

@geindex AI12-0203 (Ada 2022 feature)


@itemize *

@item 
`AI12-0203 Overriding a nonoverridable aspect (0000-00-00)'

A corner case wording clarification that has no impact on compilers.

RM references: 4.01.05 (5.1/4) 4.01.05 (7/3)
@end itemize

@geindex AI12-0204 (Ada 2022 feature)


@itemize *

@item 
`AI12-0204 Renaming of a prefixed view (2020-02-24)'

AI12-0204 clarifies that the prefix of a prefixed view that is renamed or
passed as a formal subprogram must be renameable as an object.

RM references: 8.05.04 (5.2/2) 12.06 (8.3/2) 4.01.03 (13.1/2) 4.01.06 (9/5)
@end itemize

@geindex AI12-0205 (Ada 2022 feature)


@itemize *

@item 
`AI12-0205 Defaults for generic formal types (2021-04-01)'

AI12-0205 specifies syntax and semantics that provide defaults for formal types of generic units. The legality rules guarantee that the default subtype_mark that is specified for a formal type would be a legal actual in any instantiation of the generic unit.

RM references: 12.03 (7/3) 12.03 (10) 12.05 (2.1/3) 12.05 (2.2/3) 12.05 (7/2)
@end itemize

@geindex AI12-0206 (Ada 2022 feature)


@itemize *

@item 
`AI12-0206 Nonoverridable should allow arbitrary kinds of aspects (0000-00-00)'

A non-overridable aspect can have a value other than a name; for example, @code{Max_Entry_Queue_Length} is non-overridable and it has a scalar value.
Part of adding support for @code{Max_Entry_Queue_Length} (which is already supported by GNAT).

RM references: 13.01.01 (18.2/4) 13.01.01 (18.3/4) 13.01.01 (18.6/4)
@end itemize

@geindex AI12-0207 (Ada 2022 feature)


@itemize *

@item 
`AI12-0207 Convention of anonymous access types (2020-02-01)'

The convention of anonymous access elements of arrays now have the same convention as the array instead of convention Ada.

RM references: 6.03.01 (13.1/3) B.01 (19) B.01 (21/3)
@end itemize

@geindex AI12-0208 (Ada 2022 feature)


@itemize *

@item 
`AI12-0208 Predefined Big numbers support (0000-00-00)'

Add predefined package @code{Ada.Numerics.Big_Numbers}.

RM references: A.05.05 (0) A.05.06 (0) A.05.07 (0)
@end itemize

@geindex AI12-0211 (Ada 2022 feature)


@itemize *

@item 
`AI12-0211 Interface types and inherited nonoverridable aspects (2020-08-24)'

AI12-0211 introduces two new legality rules for Ada 2022. The first says that
if a nonoverridable aspect is explicitly specified for a type that also inherits that aspect from another type (an ancestor or a progenitor), then the explicit aspect specification shall be confirming. The second says that if a type inherits a nonoverridable aspect from two different sources (this can only occur if at least one of the two is an interface type), then the two sources shall agree with respect to the given aspect. This AI is a binding interpretation, so these checks are performed even for earlier Ada versions. Because of compatibility concerns, an escape mechanism for suppressing these legality checks is provided: these new checks always pass if the @code{-gnatd.M} switch (relaxed RM semantics) is specified.

RM references: 13.01.01 (18.3/5) 13.01.01 (18.4/4)
@end itemize

@geindex AI12-0212 (Ada 2022 feature)


@itemize *

@item 
`AI12-0212 Container aggregates; generalized array aggregates (0000-00-00)'

The AI defines a new feature: generalized array aggregates that already exists in GNAT.

RM references: 4.03.05 (0) 1.01.04 (12) 1.01.04 (13) 2.01 (15) 2.02 (9/5)
3.07.01 (3) 3.08.01 (4) 4.03 (2/5) 4.03 (3/5) 4.03.01 (5) 4.03.03
(3/2) 4.03.03 (4/5) 4.03.03 (5.1/5) 4.03.03 (9) 4.03.03 (17/5)
4.03.03 (21) 4.03.03 (23.2/5) 4.03.03 (26) 4.03.03 (27) 4.03.03
(31) 4.03.04 (4/5) 4.04 (3.1/3) 11.02 (3) 13.01.01 (5/3)
13.01.01 (7/3) A.18.02 (8/3) A.18.02 (14/2) A.18.02 (47/2) A.18.02
(175/2) A.18.03 (6/3) A.18.05 (3/3) A.18.06 (4/3) A.18.08 (3/3)
A.18.09 (4/3)
@end itemize

@geindex AI12-0216 (Ada 2022 feature)


@itemize *

@item 
`AI12-0216 6.4.1(6.16-17/3) should never apply to composite objects (0000-00-00)'

Fix wording so that parameter passing cases where there isn’t really any aliasing problems or evaluation order dependency are classified as acceptable.

No compiler impact.

RM references: 6.04.01 (6.17/3)
@end itemize

@geindex AI12-0217 (Ada 2022 feature)


@itemize *

@item 
`AI12-0217 Rules regarding restrictions on the use of the Old attribute are too strict (2020-03-25)'

AI12-0217 loosens the rules regarding what is allowed as the prefix of a ‘Old
attribute reference. In particular, a prefix is now only required to “statically name” (as opposed to the previous “statically denote”) an object. This means that components of composite objects that previously would have been illegal are now legal prefixes.

RM references: 6.01.01 (24/3) 6.01.01 (27/3)
@end itemize

@geindex AI12-0220 (Ada 2022 feature)


@itemize *

@item 
`AI12-0220 Pre/Post for access-to-subprogram types (2020-04-14)'

Contract aspects can now be specified for access-to-subprogram types, as
defined for Ada 2022 in this AI.

RM references: 6.01.01 (1/4) 6.01.01 (2/3) 6.01.01 (4/3) 6.01.01 (19/3)
6.01.01 (28/3) 6.01.01 (29/3) 6.01.01 (39/3) 13.01.01 (12/5)
@end itemize

@geindex AI12-0222 (Ada 2022 feature)


@itemize *

@item 
`AI12-0222 Representation aspects and private types (0000-00-00)'

Clarify that the rule against specifying a representation aspect for a type before the type is completely defined also applies in the case where aspect_specification syntax is used (not just in the case where a pragma or some other kind of representation item is used).

GNAT already implements this.

RM references: 13.01 (9/5) 13.01 (9.1/4) 13.01 (9.2/5)
@end itemize

@geindex AI12-0225 (Ada 2022 feature)


@itemize *

@item 
`AI12-0225 Prefix of Obj’Image (0000-00-00)'

Clarify some Object vs. Value corner cases to allow names that do not denote objects in more contexts, such as a qualified expression as a prefix of an Image attribute.

RM references: 3.05 (55.1/4)
@end itemize

@geindex AI12-0226 (Ada 2022 feature)


@itemize *

@item 
`AI12-0226 Make objects more consistent (0000-00-00)'

Allow value conversions as objects. For instance this example becomes legal: @code{Long_Integer (Duration'Last)'Image}.

RM references: 3.03 (11.1/3) 3.03 (21.1/3) 3.03 (23.8/5) 4.06 (58.1/4)
4.06 (58.3/4)
@end itemize

@geindex AI12-0227 (Ada 2022 feature)


@itemize *

@item 
`AI12-0227 Evaluation of nonstatic universal expressions when no operators are involved (0000-00-00)'

Nonstatic universal integer expressions are always evaluated at runtime as values of type root_integer; similarly, nonstatic universal real expressions are always evaluated at runtime as values of type root_real.
This AI corrects a wording oversight. Previously, the above was only true if a call to operator was involved. With this change it is true in all cases.

No compiler impact.

RM references: 4.04 (10) 8.06 (29)
@end itemize

@geindex AI12-0228 (Ada 2022 feature)


@itemize *

@item 
`AI12-0228 Properties of qualified expressions used as names (2020-02-19)'

This AI clarifies that properties of a qualified object pass through a
qualified expression used as a name. Specifically, “aliased” and “known to be
constrained” are not changed by a qualified expression.

RM references: 3.03 (23.7/3) 3.10 (9/3)
@end itemize

@geindex AI12-0231 (Ada 2022 feature)


@itemize *

@item 
`AI12-0231 Null_Task_Id and Activation_Is_Complete (0000-00-00)'

Add @code{Activation_Is_Complete} to the list of functions that raise P_E if passed @code{Null_Task_Id}, correcting an oversight.

RM references: C.07.01 (15)
@end itemize

@geindex AI12-0232 (Ada 2022 feature)


@itemize *

@item 
`AI12-0232 Rules for pure generic bodies (0000-00-00)'

Clarify the rules for a generic body nested in a pure library unit.

RM references: 10.02.01 (9/3) 10.02.01 (15.1/3) 10.02.01 (15.5/3)
@end itemize

@geindex AI12-0233 (Ada 2022 feature)


@itemize *

@item 
`AI12-0233 Pre’Class for hidden operations of private types (0000-00-00)'

Clarify how @code{Pre'Class} checking interacts with private-part overriding of inherited subprograms. A class-wide precondition can be checked at runtime even if it is specified in a private part that the caller cannot see into.

RM references: 6.01.01 (38/4)
@end itemize

@geindex AI12-0234 (Ada 2022 feature)


@itemize *

@item 
`AI12-0234 Compare-and-swap for atomic objects (0000-00-00)'

New predefined units for atomic operations (@code{System.Atomic_Operations} and child units thereof).

RM references: C.06.01 (0) C.06.02 (0)
@end itemize

@geindex AI12-0235 (Ada 2022 feature)


@itemize *

@item 
`AI12-0235 System.Storage_Pools should be pure (0000-00-00)'

Change the predefined package System.Storage_Pools from preelaborated to pure.

RM references: 13.11 (5)
@end itemize

@geindex AI12-0236 (Ada 2022 feature)


@itemize *

@item 
`AI12-0236 declare expressions (2020-04-08)'

A @code{declare expression} allows constant objects and renamings to be
declared within an expression.

RM references: 2.08 (6) 3.09.02 (3) 3.10.02 (9.1/3) 3.10.02 (16.1/3)
3.10.02 (32.2/3) 4.03.02 (5.4/3) 4.03.03 (15.1/3) 4.04 (7/3)
4.05.09 (0) 6.02 (10/4) 7.05 (2.1/5) 8.01 (2.1/4)
@end itemize

@geindex AI12-0237 (Ada 2022 feature)


@itemize *

@item 
`AI12-0237 Getting the representation of an enumeration value (2020-01-31)'

The GNAT-specific attributes @code{Enum_Rep} and @code{Enum_Val} have been standardized and are now also supported as Ada 2022 attributes.

RM references: 13.04 (10) 13.04 (11/3)
@end itemize

@geindex AI12-0242 (Ada 2022 feature)


@itemize *

@item 
`AI12-0242 Shorthand Reduction Expressions for Objects (0000-00-00)'

Allow reduction expressions to iterate over an an array or an iterable object without having to explicitly create a value sequence.

This allows, for instance, writing @code{A'Reduce("+", 0)} instead of the equivalent (but more verbose) @code{[for Value of A => Value]'Reduce("+", 0);}.

RM references: 4.05.10 (0) 4.01.04 (6)
@end itemize

@geindex AI12-0247 (Ada 2022 feature)


@itemize *

@item 
`AI12-0247 Potentially Blocking goes too far for Detect_Blocking (0000-00-00)'

During a protected action, a call on a subprogram that contains a potentially blocking operation is considered a bounded error (so raising P_E is optional).
This rule imposed an unreasonable implementation burden.
The new rule introduced by this AI allows ignoring (i.e., not detecting) the problem until execution of a potentially blocking operation is actually attempted.

RM references: 9.05 (55/5) 9.05 (56/5) 9.05.01 (18/5) H.05 (5/2)
@end itemize

@geindex AI12-0249 (Ada 2022 feature)


@itemize *

@item 
`AI12-0249 User-defined numeric literals (2020-04-07)'

Compiler support is added for three new aspects (@code{Integer_Literal}, @code{Real_Literal}, and @code{String_Literal}) as described in AI12-0249 (for @code{Integer_Literal} and @code{Real_Literal}), AI12-0295 (for  @code{String_Literal}), and in two follow-up AIs (AI12-0325 and AI12-0342). For pre-Ada 2022 versions of Ada, these are treated as implementation-defined
aspects. Some implementation work remains, particularly in the interactions between these aspects and tagged types.

RM references: 4.02 (9) 4.02.01 (0) 4.09 (3)
@end itemize

@geindex AI12-0250 (Ada 2022 feature)


@itemize *

@item 
`AI12-0250 Iterator Filters (2020-05-19)'

This AI defines Ada 2022 feature of iterator filters, which can be
applied to loop parameter specifications and iterator specifications.

RM references: 4.03.03 (21) 4.03.03 (26) 4.03.03 (31) 4.03.05 (0) 4.05.10
(0) 5.05 (4) 5.05 (7) 5.05 (9/4) 5.05 (9.1/4) 5.05 (10)
5.05.02 (2/3) 5.05.02 (10/3) 5.05.02 (11/3)
@end itemize

@geindex AI12-0252 (Ada 2022 feature)


@itemize *

@item 
`AI12-0252 Duplicate interrupt handlers under Ravenscar (2018-07-05)'

Ada Issue AI12-0252 requires that the runtime shall terminate with a
Program_Error when more than one interrupt handler is attached to the same interrupt and the restriction No_Dynamic_Attachment is in effect.

RM references: C.03.01 (13)
@end itemize

@geindex AI12-0254 (Ada 2022 feature)


@itemize *

@item 
`AI12-0254 Bounded_Indefinite_Holders (2025-12-15)'

This AI defines a new package @code{Ada.Containers.Indefinite_Holders} which
is fully implemented by GNAT.

RM references: A.18.32 (0)
@end itemize

@geindex AI12-0256 (Ada 2022 feature)


@itemize *

@item 
`AI12-0256 Aspect No_Controlled_Parts (2021-01-26)'

The compiler now supports the Ada 2022 aspect No_Controlled_Parts (see
AI12-0256). When specified for a type, this aspect requires that the type and any of its ancestors must not have any controlled parts.

RM references: H.04.01 (0) 13.01.01 (18.7/5)
@end itemize

@geindex AI12-0258 (Ada 2022 feature)


@itemize *

@item 
`AI12-0258 Containers and controlled element types (0000-00-00)'

Most predefined containers are allowed to defer finalization of container elements until the finalization of the container. This allows implementation flexibility  but  causes problems in some cases. AI12-0258 tightens up the rules for the indefinite containers to say that finalization happens earlier - if a client needs the tighter finalization guarantees, then it can use the indefinite containers (even if the element subtype in question is definite). Other solutions involving the holder generic are also possible.

GNAT implements these tighter element finalization requirements for instances of the indefinite container generics.

RM references: A.18 (10/4)
@end itemize

@geindex AI12-0259 (Ada 2022 feature)


@itemize *

@item 
`AI12-0259 Lower bound of strings returned from Ada.Command_Line (0000-00-00)'

Specify that the low-bound of a couple of predefined String-valued functions will always be one.

RM references: A.15 (14) A.15 (16/3)
@end itemize

@geindex AI12-0260 (Ada 2022 feature)


@itemize *

@item 
`AI12-0260 Functions Is_Basic and To_Basic in Wide_Characters.Handling (2020-04-01)'

AI12-0260 is implemented for Ada 2022, providing the new functions @code{Is_Basic} and @code{To_Basic} in package @code{Ada.Wide_Characters.Handling}.

RM references: 1.02 (8/3) A.03.05 (8/3) A.03.05 (20/3) A.03.05 (21/3)
A.03.05 (33/3) A.03.05 (61/3)
@end itemize

@geindex AI12-0261 (Ada 2022 feature)


@itemize *

@item 
`AI12-0261 Conflict in “private with” rules (0000-00-00)'

If a library unit is only visible at some point because of a “private with”, there are legality rules about a name denoting that entity. The AI cleans up the wording so that it captures the intent in a corner case involving a private-child library-unit subprogram. The previous wording incorrectly caused this case to be illegal.

RM references: 10.01.02 (12/3) 10.01.02 (13/2) 10.01.02 (14/2) 10.01.02
(15/2) 10.01.02 (16/2)
@end itemize

@geindex AI12-0262 (Ada 2022 feature)


@itemize *

@item 
`AI12-0262 Map-Reduce attribute (0000-00-00)'

The AI defines Reduction Expressions to allow the programmer to apply the
Map-Reduce paradigm to map/transform a set of values to a new set of values,
and then summarize/reduce the transformed values into a single result value.

RM references: 4.01.04 (1) 4.01.04 (6) 4.01.04 (11) 4.05.10 (0)
@end itemize

@geindex AI12-0263 (Ada 2022 feature)


@itemize *

@item 
`AI12-0263 Update references to ISO/IEC 10646 (0000-00-00)'

Change RM references to ISO/IEC 10646:2011 to instead refer to ISO/IEC 10646:2017. No compiler impact.

RM references: 1.01.04 (14.2/3) 2.01 (1/3) 2.01 (3.1/3) 2.01 (4/3) 2.01
(4.1/5) 2.01 (5/3) 2.01 (15/3) 2.01 (4.1/5) 2.01 (5/3) 2.03
(4.1/5) 2.03 (5/3) 3.05.02 (2/3) 3.05.02 (3/3) 3.05.02 (4/3) A.01
(36.1/3) A.01 (36.2/3) A.03.02 (32.6/5) A.03.05 (51.2/5) A.03.05
(55/3) A.03.05 (59/3) A.04.10 (3/3) B.05 (21/5)
@end itemize

@geindex AI12-0264 (Ada 2022 feature)


@itemize *

@item 
`AI12-0264 Overshifting and overrotating (0000-00-00)'

Clarify Shift and Rotate op behavior with large shift/rotate amounts.

RM references: B.02 (9)
@end itemize

@geindex AI12-0265 (Ada 2022 feature)


@itemize *

@item 
`AI12-0265 Default_Initial_Condition for types (2020-11-13)'

The aspect @code{Default_Initial_Condition}, originally proposed by SPARK and
supported in GNAT, is now also included in Ada 2022. One change from the
original implementation is that when the aspect is specified on ancestor types of a derived type, the ancestors’ check expressions also apply to the derived type.
@code{Default_Initial_Condition} checks are also now applied in cases of default
initialization of components, allocators, ancestor parts of extension aggregates, and box associations of aggregates.

RM references: 7.03.03 (0) 1.01.03 (17.1/5) 11.04.02 (23.2/5) 11.04.02 (23.3/5)
@end itemize

@geindex AI12-0269 (Ada 2022 feature)


@itemize *

@item 
`AI12-0269 Aspect No_Return for functions reprise (2020-03-19)'

This amendment has been implemented under the @code{-gnat2022} switch, and the
compiler now accepts the aspect/pragma No_Return for functions and generic
functions.

RM references: 6.05.01 (0) 6.05.01 (1/3) 6.05.01 (3.1/3) 6.05.01 (3.4/3)
6.05.01 (5/2) 6.05.01 (6/2) 6.05.01 (7/2) J.15.02 (2/3) J.15.02
(3/3) J.15.02 (4/3)
@end itemize

@geindex AI12-0272 (Ada 2022 feature)


@itemize *

@item 
`AI12-0272 (part 1) Pre/Postconditions for formal subprograms (0000-00-00)'

Pre and Post aspects can be specified for a generic formal subprogram. @code{Default_Initial_Condition} can be specified for a generic formal private type.

GNAT implements this with an exception of the part related to @code{Default_Initial_Condition}.

RM references: 6.01.01 (1/5) 6.01.01 (39/5) 7.03.03 (1/5) 7.03.03 (2/5)
7.03.03 (8/5) 7.03.04 (5/5) F.01 (1)
@end itemize

@geindex AI12-0275 (Ada 2022 feature)


@itemize *

@item 
`AI12-0275 Make subtype_mark optional in object renames (2020-01-28)'

AI12-0275 allows object renamings to be declared without an explicit
subtype_mark or access_definition. This feature can be used by compiling
with the switch @code{-gnat2022}.

RM references: 8.05.01 (2/3) 8.05.01 (3/2)
@end itemize

@geindex AI12-0277 (Ada 2022 feature)


@itemize *

@item 
`AI12-0277 The meaning of “accessibility level of the body of F” (2020-09-09)'

Clarify that the only time that an explicitly aliased formal parameter has different accessibility properties than an aliased part of a “normal” parameter is for the accessibility checking associated with a return statement.

RM references: 3.10.02 (19.2/4)
@end itemize

@geindex AI12-0278 (Ada 2022 feature)


@itemize *

@item 
`AI12-0278 Implicit conversions of anonymous return types (0000-00-00)'

If a call to a function with an anonymous-access-type result is converted to a named access type, it doesn’t matter whether the conversion is implicit or explicit. the AI fixes hole where the previous rules didn’t cover the implicit conversion case.

RM references: 3.10.02 (10.3/3)
@end itemize

@geindex AI12-0279 (Ada 2022 feature)


@itemize *

@item 
`AI12-0279 Nonpreemptive dispatching needs more dispatching points (2020-04-17)'

Ada 2022 defines a new aspect @cite{Yield} that can be specified in the declaration of a noninstance subprogram (including a generic formal subprogram), a generic subprogram, or an entry, to ensure that the associated subprogram has at least one task dispatching point during each invocation.

RM references: D.02.01 (1.5/2) D.02.01 (7/5)
@end itemize

@geindex AI12-0280-2 (Ada 2022 feature)


@itemize *

@item 
`AI12-0280-2 Making ‘Old more flexible (2020-07-24)'

For Ada 2022, AI12-0280-2 relaxes Ada’s restrictions on ‘Old attribute
references whose attribute prefix does not statically name an entity. Previously, it was required that such an attribute reference must be unconditionally evaluated when the postcondition is evaluated; with the new rule, conditional evaluation is permitted if the relevant conditions can be evaluated upon entry to the subprogram with the same results as evaluation at the time of the postcondition’s evaluation. In this case, the ‘Old attribute prefix is evaluated conditionally (more specifically, the prefix is evaluated only if the result of that evaluation is going to be referenced later when the
postcondition is evaluated).

RM references: 6.01.01 (20/3) 6.01.01 (21/3) 6.01.01 (22/3) 6.01.01
(22.1/4) 6.01.01 (22.2/5) 6.01.01 (23/3) 6.01.01 (24/3) 6.01.01
(26/4) 6.01.01 (27/5) 6.01.01 (39/5)
@end itemize

@geindex AI12-0282 (Ada 2022 feature)


@itemize *

@item 
`AI12-0282 Atomic, Volatile, and Independent generic formal types (0000-00-00)'

The AI specifies that the aspects @code{Atomic}, @code{Volatile}, @code{Independent}, @code{Atomic_Components}, @code{Volatile_Components}, and @code{Independent_Components} are specifiable for generic formal types. The actual type must have a matching specification.

RM references: C.06 (6.1/3) C.06 (6.3/3) C.06 (6.5/3) C.06 (6.8/3) C.06
(12/3) C.06 (12.1/3) C.06 (21/4)
@end itemize

@geindex AI12-0285 (Ada 2022 feature)


@itemize *

@item 
`AI12-0285 Syntax for Stable_Properties aspects (0000-00-00)'

The AI establishes the required named notation for a Stable_Properties aspect specification in order to avoid syntactic ambiguities.

With the old syntax, an example like

@example
type Ugh is ...
   with Stable_Properties =\> Foo, Bar, Nonblocking, Pack;
@end example

was problematic; @code{Nonblocking} and @code{Pack} are other aspects, while @code{Foo} and @code{Bar} are @code{Stable_Properties} functions. With the clarified syntax, the example above shall be written as:

@example
type Ugh is ...
  with Stable_Properties => (Foo, Bar), Nonblocking, Pack;
@end example

RM references: 7.03.04 (2/5) 7.03.04 (3/5) 7.03.04 (4/5) 7.03.04 (6/5)
7.03.04 (7/5) 7.03.04 (9/5) 7.03.04 (10/5) 7.03.04 (14/5) 13.01.01 (4/5)
@end itemize

@geindex AI12-0287 (Ada 2022 feature)


@itemize *

@item 
`AI12-0287 Legality Rules for null exclusions in renaming are too fierce (2020-02-17)'

The null exclusion legality rules for generic formal object matching and object renaming now only apply to generic formal objects with mode in out.

RM references: 8.05.01 (4.4/2) 8.05.01 (4.5/2) 8.05.01 (4.6/2) 8.05.04
(4.2/2) 12.04 (8.3/2) 12.04 (8.4/2) 12.04 (8.5/2) 12.04 (8.2/5)
12.06 (8.2/5)
@end itemize

@geindex AI12-0289 (Ada 2022 feature)


@itemize *

@item 
`AI12-0289 Implicitly null excluding anonymous access types and conformance (2020-06-09)'

AI12-0289 is implemented for Ada 2022, allowing safer use of access parameters
when the partial view of the designated type is untagged, but the full view is
tagged.

RM references: 3.10 (26)
@end itemize

@geindex AI12-0290 (Ada 2022 feature)


@itemize *

@item 
`AI12-0290 Restriction Pure_Barriers (2020-02-18)'

The GNAT implementation of the Pure_Barriers restriction has
been updated to match the Ada RM’s definition as specified
in this AI. Some constructs that were accepted by the previous
implementation are now rejected, and vice versa. In
particular, the use of a component of a component of a
protected record in a barrier expression, as in “when
Some_Component.Another_Component =>”, formerly was (at least
in some cases) not considered to be a violation of the
Pure_Barriers restriction; that is no longer the case.

RM references: D.07 (2) D.07 (10.10/4)
@end itemize

@geindex AI12-0291 (Ada 2022 feature)


@itemize *

@item 
`AI12-0291 Jorvik Profile (2020-02-19)'

The Jorvik profile is now implemented, as defined in this AI.
For Ada 2012 and earlier versions of Ada, Jorvik is an implementation-defined
profile whose definition matches its Ada 2022 definition.

RM references: D.13 (0) D.13 (1/3) D.13 (4/3) D.13 (6/4) D.13 (9/3) D.13
(10/3) D.13 (11/4) D.13 (12/4)
@end itemize

@geindex AI12-0293 (Ada 2022 feature)


@itemize *

@item 
`AI12-0293 Add predefined FIFO_Streams packages (0000-00-00)'

The AI adds @code{Ada.Streams.Storage} and its two subunits @code{Bounded} and @code{Unbounded}.

RM references: 13.13.01 (1) 13.13.01 (9) 13.13.01 (9.1/1)
@end itemize

@geindex AI12-0295 (Ada 2022 feature)


@itemize *

@item 
`AI12-0295 User-defined string  (2020-04-07)'

Compiler support is added for three new aspects (@code{Integer_Literal}, @code{Real_Literal}, and @code{String_Literal}) as described in AI12-0249 (for @code{Integer_Literal} and @code{Real_Literal}), AI12-0295 (for @code{String_Literal}), and in two follow-up AIs (AI12-0325 and AI12-0342). For pre-Ada 2022 versions of Ada, these are treated as implementation-defined aspects. Some implementation work remains, particularly in the interactions between these aspects and tagged types.

RM references: 4.02 (6) 4.02 (10) 4.02 (11) 3.06.03 (1) 4.02.01 (0) 4.09 (26/3)
@end itemize

@geindex AI12-0301 (Ada 2022 feature)


@itemize *

@item 
`AI12-0301 Predicates should be checked like constraints for types with Default_Value (2020-02-25)'

This AI clarifies that predicate checks apply for objects that are initialized
by default and that are of a type that has any components whose subtypes specify @code{Default_Value} or @code{Default_Component_Value}.

RM references: 3.02.04 (31/4)
@end itemize

@geindex AI12-0304 (Ada 2022 feature)


@itemize *

@item 
`AI12-0304 Image attributes of language-defined types (2020-07-07)'

According to this AI, @code{Put_Image} (and therefore @code{'Image}) is provided for
the containers and for unbounded strings.

RM references: 4.10 (0)
@end itemize

@geindex AI12-0306 (Ada 2022 feature)


@itemize *

@item 
`AI12-0306 Split null array aggregates from positional array aggregates (0000-00-00)'

The AI clarifies the wording of the references RM paragraphs without introducing any language changes.

RM references: 4.03.03 (2) 4.03.03 (3/2) 4.03.03 (9/5) 4.03.03 (26/5)
4.03.03 (26.1/5) 4.03.03 (33/3) 4.03.03 (38) 4.03.03 (39) 4.03.03 (42)
@end itemize

@geindex AI12-0307 (Ada 2022 feature)


@itemize *

@item 
`AI12-0307 Resolution of aggregates (2020-08-13)'

The proposed new syntax for aggregates in Ada 2022 uses square brackets as
delimiters, and in particular allows @code{[]} as a notation for empty array and container aggregates. This syntax is currently available as an experimental feature under the @code{-gnatX} flag.

RM references: 4.03 (3/5)
@end itemize

@geindex AI12-0309 (Ada 2022 feature)


@itemize *

@item 
`AI12-0309 Missing checks for pragma Suppress (0000-00-00)'

The AI includes some previously overlooked run-time checks in the list of checks that are potentially suppressed via a pragma @code{Suppress}. For example, AI12-0251-1 adds a check that the number of chunks in a chunk_specification is not zero or negative. Clarify that suppressing @code{Program_Error_Check} suppresses that check too.

RM references: 11.05 (10) 11.05 (19) 11.05 (20) 11.05 (22) 11.05 (24)
@end itemize

@geindex AI12-0311 (Ada 2022 feature)


@itemize *

@item 
`AI12-0311 Suppressing client-side assertions for language-defined units (0000-00-00)'

The AI defines some new assertion policies that can be given as arguments in a Suppress pragma (e.g., Calendar_Assertion_Check). GNAT recognizes and ignores those new policies, the checks are not implemented.

RM references: 11.04.02 (23.5/5) 11.05 (23) 11.05 (26)
@end itemize

@geindex AI12-0315 (Ada 2022 feature)


@itemize *

@item 
`AI12-0315 Image Attributes subclause improvements (0000-00-00)'

Clarify that a named number or similar can be the prefix of an Image attribute reference.

RM references: 4.10 (0)
@end itemize

@geindex AI12-0318 (Ada 2022 feature)


@itemize *

@item 
`AI12-0318 No_IO should apply to Ada.Directories (2020-01-31)'

The restriction No_IO now applies to and prevents the use of the
@code{Ada.Directories package}.

RM references: H.04 (20/2) H.04 (24/3)
@end itemize

@geindex AI12-0321 (Ada 2022 feature)


@itemize *

@item 
`AI12-0321 Support for Arithmetic Atomic Operations and Test and Set (0000-00-00)'

The AI adds some predefined atomic operations, e.g. package System.`@w{`}Atomic_Operations.Test_And_Set`@w{`}.

RM references: C.06.03 (0) C.06.04 (0)
@end itemize

@geindex AI12-0325 (Ada 2022 feature)


@itemize *

@item 
`AI12-0325 Various issues with user-defined literals (2020-04-07)'

Compiler support is added for three new aspects (@code{Integer_Literal}, @code{Real_Literal}, and @code{String_Literal}) as described in AI12-0249 (for @code{Integer_Literal} and @code{Real_Literal}), AI12-0295 (for @code{String_Literal}), and in two follow-up AIs (AI12-0325 and AI12-0342). For pre-Ada 2022 versions of Ada, these are treated as implementation-defined aspects. Some implementation work remains, particularly in the interactions between these aspects and tagged types.

RM references: 4.02 (6) 4.02 (10) 4.02 (11) 4.02.01 (0)
@end itemize

@geindex AI12-0329 (Ada 2022 feature)


@itemize *

@item 
`AI12-0329 Naming of FIFO_Streams packages (0000-00-00)'

The AI changes the name of predefined package @code{Ada.Streams.FIFO_Streams} to @code{Ada.Streams.Storage}.

RM references: 13.13.01 (9/5) 13.13.01 (9.1/5)
@end itemize

@geindex AI12-0331 (Ada 2022 feature)


@itemize *

@item 
`AI12-0331 Order of finalization of a subpool (0000-00-00)'

Clarify that when a subpool is being finalized, objects allocated from that subpool are finalized before (not after) they cease to exist (i.e. object’s storage has been reclaimed).

RM references: 13.11.05 (5/3) 13.11.05 (6/3) 13.11.05 (7/3) 13.11.05
(7.1/4) 13.11.05 (8/3) 13.11.05 (9/3)
@end itemize

@geindex AI12-0333 (Ada 2022 feature)


@itemize *

@item 
`AI12-0333 Predicate checks on out parameters (0000-00-00)'

If a view conversion is passed as an actual parameter corresponding to an out-mode formal parameter, and if the subtype of the formal parameter has a predicate, then no predicate check associated with the conversion is performed.

RM references: 3.02.04 (31/5) 4.06 (51/4) 6.04.01 (14)
@end itemize

@geindex AI12-0335 (Ada 2022 feature)


@itemize *

@item 
`AI12-0335 Dynamic accessibility check needed for some requeue targets (0000-00-00)'

Define a new runtime accessibility check for a corner case involving requeue statements.

RM references: 9.05.04 (7/4)
@end itemize

@geindex AI12-0336 (Ada 2022 feature)


@itemize *

@item 
`AI12-0336 Meaning of Time_Offset (0000-00-00)'

The AI introduces changes to the predefined package @code{Ada.Calendar.Time_Zones}.

RM references: 9.06.01 (6/2) 9.06.01 (35/2) 9.06.01 (40/2) 9.06.01 (41/2)
9.06.01 (42/3) 9.06.01 (90/2) 9.06.01 (91/2)
@end itemize

@geindex AI12-0337 (Ada 2022 feature)


@itemize *

@item 
`AI12-0337 Simple_Name(“/”) in Ada.Directories (0000-00-00)'

Clarify behavior of subprograms in the predefined package @code{Ada.Directories}. In particular, Simple_Name (“/”) should return “/” on Unix-like systems.

RM references: A.16 (47/2) A.16 (74/2) A.16 (82/3)
@end itemize

@geindex AI12-0338 (Ada 2022 feature)


@itemize *

@item 
`AI12-0338 Type invariant checking and incomplete types (0000-00-00)'

Clarify that type invariants for type T are not checked for incomplete types whose completion is not available, even if that completion has components of type T.

RM references: 7.03.02 (20/5)
@end itemize

@geindex AI12-0339 (Ada 2022 feature)


@itemize *

@item 
`AI12-0339 Empty function for Container aggregates (2020-08-06)'

To provide uniform support for container aggregates, all standard container
libraries have been enhanced with a function Empty, to be used when initializing an aggregate prior to inserting the specified elements in the object being constructed. All products have been updated to remove the ambiguities that may have arisen from previous uses of entities named Empty in our sources, and the expansion of container aggregates uses Empty wherever needed.

RM references: A.18.02 (8/5) A.18.02 (12.3/5) A.18.02 (78.2/5) A.18.02
(98.6/5) A.18.03 (6/5) A.18.03 (10.2/5) A.18.03 (50.2/5) A.18.05
(3/5) A.18.05 (7.2/5) A.18.05 (37.3/5) A.18.05 (46/2) A.18.06
(4/5) A.18.06 (8.2/5) A.18.06 (51.4/5) A.18.08 (3/5) A.18.08
(8.1/5) A.18.08 (59.2/5) A.18.08 (68/2) A.18.09 (4/5) A.18.09
(9.1/5) A.18.09 (74.2/5) A.18.10 (15.2/5) A.18.18 (8.1/5) A.18.19
(6.1/5) A.18.20 (6/3) A.18.21 (6/3) A.18.22 (6/3) A.18.23 (6/3)
A.18.24 (6/3) A.18.25 (8/3)
@end itemize

@geindex AI12-0340 (Ada 2022 feature)


@itemize *

@item 
`AI12-0340 Put_Image should use a Text_Buffer (0000-00-00)'

Add a new predefined package Ada.Strings.Text_Buffers (along with child units) and change the definition of Put_Image attribute to refer to it.

RM references: A.04.12 (0) 4.10 (3.1/5) 4.10 (3.2/5) 4.10 (6/5) 4.10
(25.2/5) 4.10 (28/5) 4.10 (31/5) 4.10 (41/5) H.04 (23.2/5) H.04 (23.11/5)
@end itemize

@geindex AI12-0342 (Ada 2022 feature)


@itemize *

@item 
`AI12-0342 Various issues with user-defined literals (part 2) (2020-04-07)'

Compiler support is added for three new aspects (@code{Integer_Literal}, @code{Real_Literal},  and @code{String_Literal}) as described in AI12-0249 (for @code{Integer_Literal} and @code{Real_Literal}), AI12-0295 (for @code{String_Literal}), and in two follow-up AIs (AI12-0325 and AI12-0342). For pre-Ada 2022 versions of Ada, these are treated as implementation-defined aspects. Some implementation work remains, particularly in the interactions between these aspects and tagged types.

RM references: 4.02.01 (0) 3.09.02 (1/2) 6.03.01 (22)
@end itemize

@geindex AI12-0343 (Ada 2022 feature)


@itemize *

@item 
`AI12-0343 Return Statement Checks (2020-04-02)'

This binding interpretation has been implemented and the accessibility,
predicate, and tag checks prescribed by RM 6.5 are now performed at the appropriate points, as required by this AI.

RM references: 6.05 (5.12/5) 6.05 (8/4) 6.05 (8.1/3) 6.05 (21/3)
@end itemize

@geindex AI12-0345 (Ada 2022 feature)


@itemize *

@item 
`AI12-0345 Dynamic accessibility of explicitly aliased parameters (2020-09-09)'

Further clarify (after AI12-0277) accessibility rules for explicitly aliased parameters.

RM references: 3.10.02 (5) 3.10.02 (7/4) 3.10.02 (10.5/3) 3.10.02 (13.4/4)
3.10.02 (19.2/5) 3.10.02 (21)
@end itemize

@geindex AI12-0350 (Ada 2022 feature)


@itemize *

@item 
`AI12-0350 Swap for Indefinite_Holders (2025-12-15)'

Package @code{Ada.Containers.Indefinite_Holders} is implemented in GNAT, comprising the support for @code{Swap} as specified by this AI.

RM references: A.18.18 (22/5) A.18.18 (67/5) A.18.18 (73/3) A.18.32 (13/5)
@end itemize

@geindex AI12-0351 (Ada 2022 feature)


@itemize *

@item 
`AI12-0351 Matching for actuals for formal derived types (2020-04-03)'

This binding interpretation requires the compiler to checks
that an actual subtype in a generic parameter association of an instantiation is statically compatible (even when the actual is unconstrained) with the ancestor of an associated nondiscriminated generic formal derived type.

RM references: 12.05.01 (7) 12.05.01 (8)
@end itemize

@geindex AI12-0352 (Ada 2022 feature)


@itemize *

@item 
`AI12-0352 Early derivation and equality of untagged types (2020-07-09)'

AI12-0352 clarifies that declaring a user-defined primitive equality operation for a record type T is illegal if it occurs after a type has been derived from T.

RM references: 4.05.02 (9.8/4)
@end itemize

@geindex AI12-0356 (Ada 2022 feature)


@itemize *

@item 
`AI12-0356 Root_Storage_Pool_With_Subpools should have Preelaborable_Initialization (0000-00-00)'

Add Preelaborable_Initialization pragmas for predefined types @code{Root_Storage_Pool_With_Subpools} and @code{Root_Subpool}.

RM references: 13.11.04 (4/3) 13.11.04 (5/3)
@end itemize

@geindex AI12-0363 (Ada 2022 feature)


@itemize *

@item 
`AI12-0363 Fixes for Atomic and Volatile (2020-09-08)'

This amendment has been implemented under the @code{-gnat2022} switch and the compiler now supports the @code{Full_Access_Only} aspect, which is mostly equivalent to GNAT’s @code{Volatile_Full_Access}.

RM references: 3.10.02 (26/3) 9.10 (1/5) C.06 (6.4/3) C.06 (6.10/3) C.06
(8.1/4) C.06 (12/5) C.06 (12.1/5) C.06 (13.3/5) C.06 (19.1/5)
@end itemize

@geindex AI12-0364 (Ada 2022 feature)


@itemize *

@item 
`AI12-0364 Add a modular atomic arithmetic package (0000-00-00)'

Generalize support for atomic integer operations to extend to modular types. Add new predefined generic package,
@code{System.Atomic_Operations.Modular_Arithmetic}.

RM references: C.06.05 (0) C.06.04 (1/5) C.06.04 (2/5) C.06.04 (3/5)
C.06.04 (9/5)
@end itemize

@geindex AI12-0366 (Ada 2022 feature)


@itemize *

@item 
`AI12-0366 Changes to Big_Integer and Big_Real (0000-00-00)'

Simplify @code{Big_Integer `@w{`}and `@w{`}Big_Real} specs by eliminating explicit support for creating “invalid” values. No more
@code{Optional_Big_[Integer,Real]} types.

RM references: A.05.06 (0) A.05.07 (0)
@end itemize

@geindex AI12-0367 (Ada 2022 feature)


@itemize *

@item 
`AI12-0367 Glitches in aspect specifications (0000-00-00)'

The AI clarifies a few wording omissions. For example, a specified Small value for a fixed point type has to be positive.

RM references: 3.05.09 (8/2) 3.05.10 (2/1) 13.01 (9.1/5) 13.14 (10)
@end itemize

@geindex AI12-0368 (Ada 2022 feature)


@itemize *

@item 
`AI12-0368 Declare expressions can be static (2020-05-30)'

AI12-0368 allows declare expressions to be static in Ada 2022.

RM references: 4.09 (8) 4.09 (12.1/3) 4.09 (17) 6.01.01 (24.2/5) 6.01.01
(24.3/5) 6.01.01 (24.4/5) 6.01.01 (24.5/5) C.04 (9)
@end itemize

@geindex AI12-0369 (Ada 2022 feature)


@itemize *

@item 
`AI12-0369 Relaxing barrier restrictions (2020-03-25)'

The definitions of the @code{Simple_Barriers} and @code{Pure_Barriers} restrictions were modified by this AI, replacing uses of “statically denotes” with “statically names”. This means that in many cases (but not all) a barrier expression that references a subcomponent of a component of the protected type while subject to either of the two restrictions is now allowed; with the previous restriction definitions, such a barrier expression would not have been legal.

RM references: D.07 (1.3/5) D.07 (10.12/5)
@end itemize

@geindex AI12-0372 (Ada 2022 feature)


@itemize *

@item 
`AI12-0372 Static accessibility of “master of the call” (2020-09-09)'

Add an extra compile-time accessibility check for explicitly aliased parameters needed to prevent dangling references.

RM references: 3.10.02 (10.5/5) 3.10.02 (19.3/4) 6.04.01 (6.4/3)
@end itemize

@geindex AI12-0373 (Ada 2022 feature)


@itemize *

@item 
`AI12-0373 Bunch of fixes (0000-00-00)'

Small clarifications to various RM entries with minor impact on compiler implementation.

RM references: 3.01 (1) 4.02 (4) 4.02 (8/2) 4.02.01 (3/5) 4.02.01 (4/5)
4.02.01 (5/5) 4.09 (17.3/5) 6.01.01 (41/5) 8.05.04 (4/3) 13.01.01
(4/3) 13.01.01 (11/3) 13.14 (3/5)
@end itemize

@geindex AI12-0376 (Ada 2022 feature)


@itemize *

@item 
`AI12-0376 Representation changes finally allowed for untagged derived types (0000-00-00)'

A change of representation for a derived type is allowed in some previously-illegal cases where a change of representation is required to implement a call to a derived subprogram.

RM references: 13.01 (10/4)
@end itemize

@geindex AI12-0377 (Ada 2022 feature)


@itemize *

@item 
`AI12-0377 View conversions and out parameters of types with Default_Value revisited (2020-06-17)'

This AI clarifies that an actual of an out parameter that is a view conversion
is illegal if either the target or operand type has Default_Value specified while the other does not.

RM references: 6.04.01 (5.1/4) 6.04.01 (5.2/4) 6.04.01 (5.3/4) 6.04.01
(13.1/4) 6.04.01 (13.2/4) 6.04.01 (13.3/4) 6.04.01 (13.4/4) 6.04.01 (15/3)
@end itemize

@geindex AI12-0381 (Ada 2022 feature)


@itemize *

@item 
`AI12-0381 Tag of a delta aggregate (0000-00-00)'

In the case of a delta aggregate of a specific tagged type, the tag of the aggregate comes from the specific type (as opposed to somehow from the base object).

RM references: 4.03.04 (14/5)
@end itemize

@geindex AI12-0382 (Ada 2022 feature)


@itemize *

@item 
`AI12-0382 Loosen type-invariant overriding requirement of AI12-0042-1 (0000-00-00)'

The AI relaxes some corner-case legality rules about type invariants that were added by AI12-0042-1.

RM references: 7.3.2(6.1/4)
@end itemize

@geindex AI12-0383 (Ada 2022 feature)


@itemize *

@item 
`AI12-0383 Renaming values (2020-06-17)'

This AI allow names that denote values rather than objects to nevertheless be
renamed using an object renaming.

RM references: 8.05.01 (1) 8.05.01 (4) 8.05.01 (4.1/2) 8.05.01 (6/2) 8.05.01 (8)
@end itemize

@geindex AI12-0384-2 (Ada 2022 feature)


@itemize *

@item 
`AI12-0384-2 Fixups for Put_Image and Text_Buffers (2021-04-29)'

In GNAT’s initial implementation of the Ada 2022 @code{Put_Image} aspect and
attribute, buffering was performed using a GNAT-defined package,
@code{Ada.Strings.Text_Output}. Ada 2022 requires a different package, Ada.`@w{`}Strings.Text_Buffers`@w{`}, for this role, and that package is now provided, and the older package is eliminated.

RM references: 4.10 (0) A.04.12 (0)
@end itemize

@geindex AI12-0385 (Ada 2022 feature)


@itemize *

@item 
`AI12-0385 Predefined shifts and rotates should be static (0000-00-00)'

This AI allows Shift and Rotate operations in static expressions. GNAT implements this AI partially.

RM references: 4.09 (20)
@end itemize

@geindex AI12-0389 (Ada 2022 feature)


@itemize *

@item 
`AI12-0389 Ignoring unrecognized aspects (2020-10-08)'

Two new restrictions, @code{No_Unrecognized_Aspects} and @code{No_Unrecognized_Pragmas}, are available to make the compiler emit error messages on unrecognized pragmas and aspects.

RM references: 13.01.01 (38/3) 13.12.01 (6.3/3)
@end itemize

@geindex AI12-0394 (Ada 2022 feature)


@itemize *

@item 
`AI12-0394 Named Numbers and User-Defined Numeric Literals (2020-10-05)'

Ada 2022 allows using integer named numbers with types that have an
@code{Integer_Literal} aspect. Similarly, real named numbers may now be used with types that have a @code{Real_Literal} aspect with an overloading that takes two strings, to be used in particular  with
@code{Ada.Numerics.Big_Numbers.Big_Reals}.

RM references: 3.03.02 (3) 4.02.01 (4/5) 4.02.01 (8/5) 4.02.01 (12/5)
4.02.01 (13/5) 4.09 (5)
@end itemize

@geindex AI12-0395 (Ada 2022 feature)


@itemize *

@item 
`AI12-0395 Allow aspect_specifications on formal parameters (0000-00-00)'

Change syntax rules to allow aspect_specifications on formal parameters, if an implementation if an implementation wants to define one. Currently, GNAT doesn’t define any such aspect_specifications.

RM references: 6.01 (15/3)
@end itemize

@geindex AI12-0397 (Ada 2022 feature)


@itemize *

@item 
`AI12-0397 Default_Initial_Condition applied to derived type (2020-12-09)'

The compiler now implements the rules for resolving @code{Default_Initial_Condition}
expressions that involve references to the current instance of types with the aspect, as specified by this AI. The type of the current instance is defined to be like a formal derived type, so for a derived type that inherits the aspect, a call passing the current instance to a primitive means that the call will resolve to invoke the corresponding primitive of the descendant type. This also now permits calls to abstract primitives to occur within the aspect expression of an abstract type.

RM references: 7.03.03 (3/5) 7.03.03 (6/5) 7.03.03 (8/5)
@end itemize

@geindex AI12-0398 (Ada 2022 feature)


@itemize *

@item 
`AI12-0398 Most declarations should have aspect specifications (2020-11-19)'

It is now possible to specify aspects for discriminant specifications, extended return object declarations, and entry index specifications. This is an extension added for Ada 2022 by this AI.

RM references: 3.07 (5/2) 6.03.01 (25) 6.05 (2.1/3) 9.05.02 (8)
@end itemize

@geindex AI12-0399 (Ada 2022 feature)


@itemize *

@item 
`AI12-0399 Aspect specification for Preelaborable_Initialization (0000-00-00)'

Semantics-preserving presentation change. Replace @code{Preelaborable_Initialization} pragmas with equivalent aspect specs in the listed predefined packages. GNAT follows the guidance of this AI partially.

RM references: 9.05 (53/5) 3.09 (6/5) 7.06 (5/2) 7.06 (7/2) 11.04.01 (2/5)
11.04.01 (3/2) 13.11 (6/2) 13.11.04 (4/5) 13.11.04 (5/5) 13.13.01
(3/2) A.04.02 (4/2) A.04.02 (20/2) A.04.05 (4/2) A.04.07 (4/2)
A.04.07 (20/2) A.04.08 (4/2) A.04.08 (20/2) A.12.01 (5/4) A.18.02
(8/5) A.18.02 (9/2) A.18.02 (79.2/5) A.18.02 (79.3/5) A.18.03
(6/5) A.18.03 (7/2) A.18.03 (50.2/5) A.18.03 (50.3/5) A.18.05
(3/5) A.18.05 (4/2) A.18.05 (37.3/5) A.18.05 (37.4/5) A.18.06
(4/5) A.18.06 (5/2) A.18.06 (51.4/5) A.18.06 (51.5/5) A.18.08
(3/5) A.18.08 (4/2) A.18.08 (58.2/5) A.18.08 (58.3/5) A.18.09
(4/5) A.18.09 (5/2) A.18.09 (74.2/5) A.18.09 (74.3/5) A.18.10
(8/5) A.18.10 (9/3) A.18.10 (70.2/5) A.18.10 (70.3/5) A.18.18
(6/5) B.03.01 (5/2) C.07.01 (2/5) G.01.01 (4/2)
@end itemize

@geindex AI12-0400 (Ada 2022 feature)


@itemize *

@item 
`AI12-0400 Ambiguities associated with Vector Append and container aggregates (0000-00-00)'

Change the names of subprograms in the predefined Vector containers from @code{Append} to @code{Append_Vector} and from @code{Prepend} to @code{Prepend_Vector} in order to resolve some ambiguity problems. GNAT adds the subprograms with new names but also keeps the old ones for backward compatibility.

RM references: A.18.02 (8/5) A.18.02 (36/5) A.18.02 (37/5) A.18.02 (38/5)
A.18.02 (44/5) A.18.02 (46/5) A.18.02 (47/5) A.18.02 (58/5)
A.18.02 (79.2/5) A.18.02 (150/5) A.18.02 (151/5) A.18.02 (152/5)
A.18.02 (153/5) A.18.02 (154/5) A.18.02 (155/5) A.18.02 (156/5)
A.18.02 (168/5) A.18.02 (169/5) A.18.02 (172/5) A.18.02 (173/5)
A.18.02 (174/5) A.18.02 (175.1/5) A.18.03 (23/5) A.18.03 (23.1/5)
A.18.03 (58.2/5) A.18.03 (96/5) A.18.03 (97.1/5)
@end itemize

@geindex AI12-0401 (Ada 2022 feature)


@itemize *

@item 
`AI12-0401 Renaming of qualified expression of variable (2020-10-31)'

Ada 2022 AI12-0401 restricts renaming of a qualified expression to cases where
the operand is a constant, or the target subtype statically matches the nominal subtype of the operand, or is unconstrained with no predicates, to prevent setting variables to values outside their range or constraints.

RM references: 3.03 (23.2/3) 8.05.01 (4.7/5) 8.05.01 (5/3)
@end itemize

@geindex AI12-0402 (Ada 2022 feature)


@itemize *

@item 
`AI12-0402 Master of a function call with elementary result type (2026-01-07)'

This AI restricts the rule that makes the master of a function call, which
is used to directly initialize part of an object, that of the object being
initialized, to the case where the result type of the function is composite.

RM references: 3.10.2 (10.2/5)
@end itemize

@geindex AI12-0409 (Ada 2022 feature)


@itemize *

@item 
`AI12-0409 Preelaborable_Initialization and bounded containers (2021-06-23)'

As defined by this AI, the @code{Preelaborable_Initializatio} aspect now has a
corresponding attribute of the same name. Types declared within a generic package specification are permitted to specify the expression of a @code{Prelaborable_Initialization} aspect by including one or more references to the attribute applied to a formal private or formal derived type conjoined by @code{and} operators. This permits the full type of a private type with such an aspect expression to have components of the named formal types, and such a type will have preelaborable initialization in an instance when the
actual types for all referenced formal types have preelaborable initialization.

RM references: 10.02.01 (4.1/2) 10.02.01 (4.2/2) 10.02.01 (11.1/2)
10.02.01 (11.2/2) 10.02.01 (11.6/2) 10.02.01 (11.7/2) 10.02.01
(11.8/2) 13.01 (11/3) A.18.19 (5/5) A.18.20 (5/5) A.18.21 (5/5)
A.18.22 (5/5) A.18.23 (5/5) A.18.24 (5/5) A.18.25 (5/5) A.18.32
(6/5) J.15.14 (0)
@end itemize

@geindex AI12-0411 (Ada 2022 feature)


@itemize *

@item 
`AI12-0411 Add “bool” to Interfaces.C (0000-00-00)'

RM references: B.03 (13) B.03 (43/2) B.03 (65.1/4)
@end itemize

@geindex AI12-0412 (Ada 2022 feature)


@itemize *

@item 
`AI12-0412 Abstract Pre/Post’Class on primitive of abstract type (2021-05-19)'

In Ada 2022, by AI12-0412, it’s legal to specify Pre’Class and Post’Class
aspects on nonabstract primitive subprograms of an abstract type, but if the
expression of such an aspect is nonstatic, then it’s illegal to make a nondispatching call to such a primitive, to apply @code{'Access} to it, or to pass such a primitive as an actual subprogram for a concrete formal subprogram in a generic instantiation.

RM references: 6.01.01 (18.2/4)
@end itemize

@geindex AI12-0413 (Ada 2022 feature)


@itemize *

@item 
`AI12-0413 Reemergence of “=” when defined to be abstract (0000-00-00)'

The AI clarifies rules about operator reemergence in instances, and nondispatching calls to abstract subprograms.

RM references: 3.09.03 (7) 4.05.02 (14.1/3) 4.05.02 (24.1/3) 12.05 (8/3)
@end itemize

@geindex AI12-0423 (Ada 2022 feature)


@itemize *

@item 
`AI12-0423 Aspect inheritance fixups (0000-00-00)'

Clarify that the No_Return aspect behaves as one would expect for an inherited subprogram and that inheritance works as one would expect for a multi-part aspect whose value is specified via an aggregate (e.g., the Aggregate aspect).

RM references: 6.05.01 (3.3/3) 13.01 (15.7/5) 13.01 (15.8/5)
@end itemize

@geindex AI12-0432 (Ada 2022 feature)


@itemize *

@item 
`AI12-0432 View conversions of assignments and predicate checks (2021-05-05)'

When a predicate applies to a tagged type, a view conversion to that type
normally requires a predicate check. However, as specified by AI12-0432, when the view conversion appears as the target of an assignment, a predicate check is not applied to the object in the conversion.

RM references: 3.02.04 (31/5) 4.06 (51.1/5)
@end itemize

@node GNAT language extensions,Security Hardening Features,Implementation of Ada 2022 Features,Top
@anchor{gnat_rm/gnat_language_extensions doc}@anchor{456}@anchor{gnat_rm/gnat_language_extensions gnat-language-extensions}@anchor{457}@anchor{gnat_rm/gnat_language_extensions id1}@anchor{458}
@chapter GNAT language extensions


The GNAT compiler implements a certain number of language extensions on top of
the latest Ada standard, implementing its own extended superset of Ada.

There are two sets of language extensions:


@itemize *

@item 
The first is the curated set. The features in that set are features that we
consider being worthy additions to the Ada language, and that we want to make
available to users early on.

@item 
The second is the experimental set. It includes the first, but also
experimental features, which are considered experimental because
they’re still in an early prototyping phase.
These features might be removed or heavily modified at any time.
@end itemize

@menu
* How to activate the extended GNAT Ada superset:: 
* Curated Extensions:: 
* Experimental Language Extensions:: 

@end menu

@node How to activate the extended GNAT Ada superset,Curated Extensions,,GNAT language extensions
@anchor{gnat_rm/gnat_language_extensions how-to-activate-the-extended-gnat-ada-superset}@anchor{459}
@section How to activate the extended GNAT Ada superset


There are two ways to activate the extended GNAT Ada superset:


@itemize *

@item 
The @ref{6b,,Pragma Extensions_Allowed}. To activate
the curated set of extensions, you should use
@end itemize

@example
pragma Extensions_Allowed (On)
@end example

As a configuration pragma, you can either put it at the beginning of a source
file, or in a @code{.adc} file corresponding to your project.


@itemize *

@item 
The @code{-gnatX} command-line option will
activate the curated subset of extensions.
@end itemize

@cartouche
@quotation Attention 
You can activate the experimental set of extensions
in addition by using either
the @code{-gnatX0} command-line option, or the pragma @code{Extensions_Allowed} with
@code{All_Extensions} as an argument. However, it is not recommended you use
this subset for serious projects; it is only meant as a technology preview
for use in playground experiments.
@end quotation
@end cartouche

@node Curated Extensions,Experimental Language Extensions,How to activate the extended GNAT Ada superset,GNAT language extensions
@anchor{gnat_rm/gnat_language_extensions curated-extensions}@anchor{45a}@anchor{gnat_rm/gnat_language_extensions curated-language-extensions}@anchor{6c}
@section Curated Extensions


Features activated via @code{-gnatX} or
@code{pragma Extensions_Allowed (On)}.

@menu
* Local Declarations Without Block:: 
* Deep delta Aggregates:: 
* Fixed lower bounds for array types and subtypes:: 
* Prefixed-view notation for calls to primitive subprograms of untagged types:: 
* Expression defaults for generic formal functions:: 
* String interpolation:: 
* Constrained attribute for generic objects:: 
* Static aspect on intrinsic functions:: 
* First Controlling Parameter:: 
* Unsigned_Base_Range aspect:: 
* Generalized Finalization:: 

@end menu

@node Local Declarations Without Block,Deep delta Aggregates,,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions local-declarations-without-block}@anchor{45b}
@subsection Local Declarations Without Block


A @code{basic_declarative_item} may appear at the place of any statement. This
avoids the heavy syntax of block_statements just to declare something locally.

The only valid kinds of declarations for now are @code{object_declaration},
@code{object_renaming_declaration}, @code{use_package_clause}, and
@code{use_type_clause}.

For example:

@example
if X > 5 then
   X := X + 1;

   Squared : constant Integer := X**2;

   X := X + Squared;
end if;
@end example

It is generally a good practice to declare local variables (or constants) with as
short a lifetime as possible. However, introducing a declare block to accomplish
this is a relatively heavy syntactic load along with a traditional extra level
of indentation. The alternative syntax supported here allows declarations
in any statement sequence.
The lifetime of such local declarations is until the end of
the enclosing construct. The same enclosing construct cannot contain several
declarations of the same defining name; however, overriding symbols from higher-level
scopes works similarly to the explicit @code{declare} block.

If the enclosing construct allows an exception handler (such as an accept
statement, begin-except-end block or a subprogram body), declarations that
appear at the place of a statement are `not' visible within the handler. Only
declarations that precede the beginning of the construct with an exception
handler would be visible in this handler.

@cartouche
@quotation Attention 
Here are a couple of examples illustrating the scoping rules described above.

@quotation


@enumerate 

@item 
Those declarations are not visible from the potential exception handler:

@example
begin
   A : Integer
   ...
exception
   when others =>
       Put_Line (A'Image) --  ILLEGAL
end;
@end example

@item 
The following is legal

@example
declare
   A : Integer := 10;
begin
   A : Integer := 12;
end;
@end example

because it is roughly expanded into

@example
  declare
     A : Integer := 10;
  begin
     declare
        A : Integer := 12;
     begin
        ...
     end;
  end;

And as such the second `@w{`}A`@w{`} declaration is hiding the first one.
@end example
@end enumerate
@end quotation
@end quotation
@end cartouche

@node Deep delta Aggregates,Fixed lower bounds for array types and subtypes,Local Declarations Without Block,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions deep-delta-aggregates}@anchor{45c}
@subsection Deep delta Aggregates


Ada 2022’s delta aggregates are extended to allow deep updates.

A delta aggregate may be used to specify new values for subcomponents of the
copied base value, instead of only new values for direct components of the
copied base value. This allows a more compact expression of updated values with
a single delta aggregate, instead of multiple nested delta aggregates.

The syntax of delta aggregates in the extended version is the following:

@menu
* Syntax:: 
* Legality Rules:: 
* Dynamic Semantics:: 
* Examples:: 

@end menu

@node Syntax,Legality Rules,,Deep delta Aggregates
@anchor{gnat_rm/gnat_language_extensions syntax}@anchor{45d}
@subsubsection Syntax


@example
delta_aggregate ::= record_delta_aggregate | array_delta_aggregate

record_delta_aggregate ::=
  ( base_expression with delta record_subcomponent_association_list )

record_subcomponent_association_list ::=
  record_subcomponent_association @{, record_subcomponent_association@}

record_subcomponent_association ::=
  record_subcomponent_choice_list => expression

record_subcomponent_choice_list ::=
  record_subcomponent_choice @{'|' record_subcomponent_choice@}

record_subcomponent_choice ::=
    component_selector_name
  | record_subcomponent_choice (expression)
  | record_subcomponent_choice . component_selector_name

array_delta_aggregate ::=
    ( base_expression with delta array_component_association_list )
  | '[' base_expression with delta array_component_association_list ']'
  | ( base_expression with delta array_subcomponent_association_list )
  | '[' base_expression with delta array_subcomponent_association_list ']'

array_subcomponent_association_list ::=
  array_subcomponent_association @{, array_subcomponent_association@}

array_subcomponent_association ::=
  array_subcomponent_choice_list => expression

array_subcomponent_choice_list ::=
  array_subcomponent_choice @{'|' array_subcomponent_choice@}

array_subcomponent_choice ::=
    ( expression )
  | array_subcomponent_choice (expression)
  | array_subcomponent_choice . component_selector_name
@end example

@node Legality Rules,Dynamic Semantics,Syntax,Deep delta Aggregates
@anchor{gnat_rm/gnat_language_extensions legality-rules}@anchor{45e}
@subsubsection Legality Rules



@enumerate 

@item 
For an @code{array_delta_aggregate}, the discrete_choice shall not be `others'.

@item 
For an @code{array_delta_aggregate}, the dimensionality of the type of the
@code{delta_aggregate} shall be 1.

@item 
For an @code{array_delta_aggregate}, the @code{base_expression} and each
expression in every @code{array_component_association} or
@code{array_subcomponent_association} shall be of a nonlimited type.

@item 
For a @code{record_delta_aggregate}, no @code{record_subcomponent_choices} that
consists of only @code{component_selector_names} shall be the same or a prefix
of another record_subcomponent_choice.

@item 
For an @code{array_subcomponent_choice} or a @code{record_subcomponent_choice} the
@code{component_selector_name} shall not be a subcomponent that depends on
discriminants of an unconstrained record subtype with defaulted
discriminants unless its prefix consists of only
@code{component_selector_names}.

[Rationale: As a result of this rule, accessing the subcomponent can only
lead to a discriminant check failure if the subcomponent was not present in
the object denoted by the base_expression, prior to any update.]
@end enumerate

@node Dynamic Semantics,Examples,Legality Rules,Deep delta Aggregates
@anchor{gnat_rm/gnat_language_extensions dynamic-semantics}@anchor{45f}
@subsubsection Dynamic Semantics


The evaluation of a @code{delta_aggregate} begins with the evaluation of the
@code{base_expression} of the delta_aggregate; then that value is used to create
and initialize the anonymous object of the aggregate. The bounds of the
anonymous object of an @code{array_delta_aggregate} and the discriminants (if any)
of the anonymous object of a @code{record_delta_aggregate} are those of the
@code{base_expression}.

If a @code{record_delta_aggregate} is of a specific tagged type, its tag is that
of the specific type; if it is of a class-wide type, its tag is that of the
base_expression.

For a @code{delta_aggregate}, for each @code{discrete_choice} or each subcomponent
associated with a @code{record_subcomponent_associated},
@code{array_component_association} or @code{array_subcomponent_association} (in the
order given in the enclosing @code{discrete_choice_list} or
@code{subcomponent_association_list}, respectively):


@itemize -

@item 
if the associated subcomponent belongs to a variant, a check is made that the
values of the governing discriminants are such that the anonymous object has
this component. The exception @code{Constraint_Error} is raised if this check fails.

@item 
if the associated subcomponent is a subcomponent of an array, then for each
represented index value (in ascending order, if the @code{discrete_choice}
represents a range):

@quotation


@itemize *

@item 
the index value is converted to the index type of the array type.

@item 
a check is made that the index value belongs to the index range of the
corresponding array part of the anonymous object; @code{Constraint_Error} is
raised if this check fails.

@item 
the expression of the @code{record_subcomponent_association},
@code{array_component_association} or @code{array_subcomponent_association} is
evaluated, converted to the nominal subtype of the associated subcomponent,
and assigned to the corresponding subcomponent of the anonymous object.
@end itemize
@end quotation
@end itemize

@node Examples,,Dynamic Semantics,Deep delta Aggregates
@anchor{gnat_rm/gnat_language_extensions examples}@anchor{460}
@subsubsection Examples


@example
declare
   type Point is record
      X, Y : Integer;
   end record;

   type Segment is array (1 .. 2) of Point;
   type Triangle is array (1 .. 3) of Segment;

   S : Segment := (1 .. 2 => (0, 0));
   T : Triangle := (1 .. 3 => (1 .. 2 => (0, 0)));
begin
   S := (S with delta (1).X => 11, (2).Y => 12, (1).Y => 15);

   pragma Assert (S (1).X = 11);
   pragma Assert (S (2).Y = 12);
   pragma Assert (S (1).Y = 15);

   T := (T with delta (2)(1).Y => 18);
   pragma Assert (T (2)(1).Y = 18);
end;
@end example

@node Fixed lower bounds for array types and subtypes,Prefixed-view notation for calls to primitive subprograms of untagged types,Deep delta Aggregates,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions fixed-lower-bounds-for-array-types-and-subtypes}@anchor{461}
@subsection Fixed lower bounds for array types and subtypes


Unconstrained array types and subtypes can be specified with a lower bound that
is fixed to a certain value, by writing an index range that uses the syntax
@code{<lower-bound-expression> .. <>}. This guarantees that all objects of the
type or subtype will have the specified lower bound.

For example, a matrix type with fixed lower bounds of zero for each dimension
can be declared by the following:

@example
type Matrix is
  array (Natural range 0 .. <>, Natural range 0 .. <>) of Integer;
@end example

Objects of type @code{Matrix} declared with an index constraint must have index
ranges starting at zero:

@example
M1 : Matrix (0 .. 9, 0 .. 19);
M2 : Matrix (2 .. 11, 3 .. 22);  -- Warning about bounds; will raise CE
@end example

Similarly, a subtype of @code{String} can be declared that specifies the lower
bound of objects of that subtype to be @code{1}:

@quotation

@example
subtype String_1 is String (1 .. <>);
@end example
@end quotation

If a string slice is passed to a formal of subtype @code{String_1} in a call to a
subprogram @code{S}, the slice’s bounds will “slide” so that the lower bound is
@code{1}.

Within @code{S}, the lower bound of the formal is known to be @code{1}, so, unlike a
normal unconstrained @code{String} formal, there is no need to worry about
accounting for other possible lower-bound values. Sliding of bounds also occurs
in other contexts, such as for object declarations with an unconstrained
subtype with fixed lower bound, as well as in subtype conversions.

Use of this feature increases safety by simplifying code, and can also improve
the efficiency of indexing operations, since the compiler statically knows the
lower bound of unconstrained array formals when the formal’s subtype has index
ranges with static fixed lower bounds.

@node Prefixed-view notation for calls to primitive subprograms of untagged types,Expression defaults for generic formal functions,Fixed lower bounds for array types and subtypes,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions prefixed-view-notation-for-calls-to-primitive-subprograms-of-untagged-types}@anchor{462}
@subsection Prefixed-view notation for calls to primitive subprograms of untagged types


When operating on an untagged type, if it has any primitive operations, and the
first parameter of an operation is of the type (or is an access parameter with
an anonymous type that designates the type), you may invoke these operations
using an @code{object.op(...)} notation, where the parameter that would normally be
the first parameter is brought out front, and the remaining parameters (if any)
appear within parentheses after the name of the primitive operation.

This same notation is already available for tagged types. This extension allows
for untagged types. It is allowed for all primitive operations of the type
independent of whether they were originally declared in a package spec,
or were inherited and/or overridden as part of a derived type
declaration occurring anywhere, so long as the first parameter is of the type,
or an access parameter designating the type.

For example:

@example
generic
   type Elem_Type is private;
package Vectors is
    type Vector is private;
    procedure Add_Element (V : in out Vector; Elem : Elem_Type);
    function Nth_Element (V : Vector; N : Positive) return Elem_Type;
    function Length (V : Vector) return Natural;
private
    function Capacity (V : Vector) return Natural;
       --  Return number of elements that may be added without causing
       --  any new allocation of space

    type Vector is ...
      with Type_Invariant => Vector.Length <= Vector.Capacity;
    ...
end Vectors;

package Int_Vecs is new Vectors(Integer);

V : Int_Vecs.Vector;
...
V.Add_Element(42);
V.Add_Element(-33);

pragma Assert (V.Length = 2);
pragma Assert (V.Nth_Element(1) = 42);
@end example

@node Expression defaults for generic formal functions,String interpolation,Prefixed-view notation for calls to primitive subprograms of untagged types,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions expression-defaults-for-generic-formal-functions}@anchor{463}
@subsection Expression defaults for generic formal functions


The declaration of a generic formal function is allowed to specify
an expression as a default, using the syntax of an expression function.

Here is an example of this feature:

@example
generic
   type T is private;
   with function Copy (Item : T) return T is (Item); -- Defaults to Item
package Stacks is

   type Stack is limited private;

   procedure Push (S : in out Stack; X : T); -- Calls Copy on X
   function Pop (S : in out Stack) return T; -- Calls Copy to return item

private
   -- ...
end Stacks;
@end example

If Stacks is instantiated with an explicit actual for Copy,
then that will be called when Copy is called in the generic body.
If the default is used (i.e. there is no actual corresponding to Copy),
then calls to Copy in the instance will simply return Item.

@node String interpolation,Constrained attribute for generic objects,Expression defaults for generic formal functions,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions string-interpolation}@anchor{464}
@subsection String interpolation


The syntax for string literals is extended to support string interpolation.
An interpolated string literal starts with @code{f}, immediately before
the first double-quote character.

Within an interpolated string literal, an arbitrary expression, when
enclosed in @code{@{ ... @}}, is expanded at run time into the result of calling
@code{'Image} on the result of evaluating the expression enclosed by the brace
characters, unless it is already a string or a single character.

Here is an example of this feature where the expressions @code{Name} and @code{X + Y}
will be evaluated and included in the string.

@example
procedure Test_Interpolation is
   X    : Integer := 12;
   Y    : Integer := 15;
   Name : String := "Leo";
begin
   Put_Line (f"The name is @{Name@} and the sum is @{X + Y@}.");
end Test_Interpolation;
@end example

This will print:

@example
The name is Leo and the sum is 27.
@end example

In addition, an escape character (@code{\}) is provided for inserting certain
standard control characters (such as @code{\t} for tabulation or @code{\n} for
newline) or to escape characters with special significance to the
interpolated string syntax, namely @code{"}, @code{@{}, @code{@}},and @code{\} itself.


@multitable {xxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxx} 
@item

escaped_character

@tab

meaning

@item

@code{\a}

@tab

ALERT

@item

@code{\b}

@tab

BACKSPACE

@item

@code{\f}

@tab

FORM FEED

@item

@code{\n}

@tab

LINE FEED

@item

@code{\r}

@tab

CARRIAGE RETURN

@item

@code{\t}

@tab

CHARACTER TABULATION

@item

@code{\v}

@tab

LINE TABULATION

@item

@code{\0}

@tab

NUL

@item

@code{\\}

@tab

@code{\}

@item

@code{\"}

@tab

@code{"}

@item

@code{\@{}

@tab

@code{@{}

@item

@code{\@}}

@tab

@code{@}}

@end multitable


Note that, unlike normal string literals, doubled double-quote characters have no
special significance. So to include a double-quote or a brace character
in an interpolated string, they must be preceded by a @code{\}.
Multiple interpolated strings are concatenated.
For example:

@example
Put_Line
  (f"X = @{X@} and Y = @{Y@} and X+Y = @{X+Y@};\n" &
   f" a double quote is \" and" &
   f" an open brace is \@{");
@end example

This will print:

@example
X = 12 and Y = 15 and X+Y = 27
a double quote is " and an open brace is @{
@end example

@node Constrained attribute for generic objects,Static aspect on intrinsic functions,String interpolation,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions constrained-attribute-for-generic-objects}@anchor{465}
@subsection Constrained attribute for generic objects


The @code{Constrained} attribute is permitted for objects of generic types. The
result indicates whether the corresponding actual is constrained.

@node Static aspect on intrinsic functions,First Controlling Parameter,Constrained attribute for generic objects,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions static-aspect-on-intrinsic-functions}@anchor{466}
@subsection @code{Static} aspect on intrinsic functions


The Ada 202x @code{Static} aspect can be specified on Intrinsic imported functions
and the compiler will evaluate some of these intrinsics statically, in
particular the @code{Shift_Left} and @code{Shift_Right} intrinsics.

@node First Controlling Parameter,Unsigned_Base_Range aspect,Static aspect on intrinsic functions,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions first-controlling-parameter}@anchor{467}
@subsection First Controlling Parameter


A new pragma/aspect, @code{First_Controlling_Parameter}, is introduced for tagged
types, altering the semantics of primitive/controlling parameters. When a
tagged type is marked with this aspect, only subprograms where the first
parameter is of that type will be considered dispatching primitives. This
pragma/aspect applies to the entire hierarchy, starting from the specified
type, without affecting inherited primitives.

Here is an example of this feature:

@example
package Example is
   type Root is tagged private;

   procedure P (V : Integer; V2 : Root);
   -- Primitive

   type Child is tagged private
     with First_Controlling_Parameter;

private
   type Root is tagged null record;
   type Child is new Root with null record;

   overriding
   procedure P (V : Integer; V2 : Child);
   -- Primitive

   procedure P2 (V : Integer; V2 : Child);
   -- NOT Primitive

   function F return Child; -- NOT Primitive

   function F2 (V : Child) return Child;
   -- Primitive, but only controlling on the first parameter
end Example;
@end example

Note that @code{function F2 (V : Child) return Child;} differs from @code{F2 (V : Child)
return Child'Class;} in that the return type is a specific, definite type. This
is also distinct from the legacy semantics, where further derivations with
added fields would require overriding the function.

The option @code{-gnatw_j}, that you can pass to the compiler directly, enables
warnings related to this new language feature. For instance, compiling the
example above without this switch produces no warnings, but compiling it with
@code{-gnatw_j} generates the following warning on the declaration of procedure P2:

@example
warning: not a dispatching primitive of tagged type "Child"
warning: disallowed by First_Controlling_Parameter on "Child"
@end example

For generic formal tagged types, you can specify whether the type has the
First_Controlling_Parameter aspect enabled:

@example
generic
   type T is tagged private with First_Controlling_Parameter;
package T is
    type U is new T with null record;
    function Foo return U; -- Not a primitive
end T;
@end example

For tagged partial views, the value of the aspect must be consistent between
the partial and full views:

@example
package R is
   type T is tagged private;
...
private
   type T is tagged null record with First_Controlling_Parameter; -- ILLEGAL
end R;
@end example

Restricting the position of controlling parameter offers several advantages:


@itemize *

@item 
Simplification of the dispatching rules improves readability of Ada programs.
One doesn’t need to analyze all subprogram parameters to understand if the given
subprogram is a primitive of a certain tagged type.

@item 
A programmer is free to use any type, including class-wide types, on other
parameters of a subprogram, without the need to consider possible effects of
overriding a primitive or creating new one.

@item 
The result of a function is never a controlling result.
@end itemize

@node Unsigned_Base_Range aspect,Generalized Finalization,First Controlling Parameter,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions unsigned-base-range-aspect}@anchor{468}
@subsection @code{Unsigned_Base_Range} aspect


A new pragma/aspect, @code{Unsigned_Base_Range}, is introduced to explicitly
enforce the use of an unsigned base type for signed integer types.
RM-3.5.4(9) mandates a symmetric base range for signed integer types. This
requirement often requires the use of larger data types for arithmetic
operations, potentially introducing undesirable run-time overhead and
performance penalties, particularly in embedded systems. For instance,
on a 64-bit architecture, a 64-bit multiplication can be performed with
a single hardware instruction, whereas a 128-bit multiplication requires
multiple instructions and intermediate steps.

Here is an example of this feature:

@example
type Uns_64 is range 0 .. 2 ** 64 - 1
  with Size => 64,
       Unsigned_Base_Range => True;
@end example

It ensures that arithmetic operations of type @code{Uns_64} are carried
out using 64 bits.

@node Generalized Finalization,,Unsigned_Base_Range aspect,Curated Extensions
@anchor{gnat_rm/gnat_language_extensions generalized-finalization}@anchor{469}
@subsection Generalized Finalization


The @code{Finalizable} aspect can be applied to any record type, tagged or not,
to specify that it provides the same level of control on the operations of
initialization, finalization, and assignment of objects as the controlled
types (see RM 7.6(2) for a high-level overview). The only restriction is
that the record type must be a root type, in other words not a derived type.

The aspect additionally makes it possible to specify relaxed semantics for
the finalization operations by means of the @code{Relaxed_Finalization} setting.
Here is the archetypal example:

@example
type T is record
  ...
end record
  with Finalizable => (Initialize           => Initialize,
                       Adjust               => Adjust,
                       Finalize             => Finalize,
                       Relaxed_Finalization => True);

procedure Adjust     (Obj : in out T);
procedure Finalize   (Obj : in out T);
procedure Initialize (Obj : in out T);
@end example

The three procedures must be primitive operations of @code{T} and have a single
@code{in out} parameter. They need not be all specified by the aspect. If they
are specified, they have the same dynamic semantics as for controlled types:

@quotation


@itemize -

@item 
@code{Initialize} is called when an object of type @code{T} is declared without
initialization expression.

@item 
@code{Adjust} is called after an object of type @code{T} is assigned a new value.

@item 
@code{Finalize} is called when an object of type @code{T} goes out of scope (for
stack-allocated objects) or is deallocated (for heap-allocated objects).
It is also called when the value is replaced by an assignment.
@end itemize
@end quotation

However, when @code{Relaxed_Finalization} is either @code{True} or not explicitly
specified, the following differences are implemented relative to the semantics
of controlled types:


@itemize *

@item 
The compiler has permission to perform no automatic finalization of
heap-allocated objects: @code{Finalize} is only called when such an object
is explicitly deallocated, or when the designated object is assigned a new
value. As a consequence, no runtime support is needed for performing
implicit deallocation. In particular, no per-object header data is needed
for heap-allocated objects.

Heap-allocated objects allocated through a nested access type will therefore
`not' be deallocated either. The result is simply that memory will be leaked
in this case.

@item 
The @code{Adjust} and @code{Finalize} procedures are automatically considered as
having the @ref{46a,,No_Raise aspect} specified for them. In particular, the
compiler has permission to enforce none of the guarantees specified by the
RM 7.6.1 (14/1) and subsequent subclauses.
@end itemize

Simple example of ref-counted type:

@example
type T is record
   Value     : Integer;
   Ref_Count : Natural := 0;
end record;

procedure Inc_Ref (X : in out T);
procedure Dec_Ref (X : in out T);

type T_Access is access all T;

type T_Ref is record
   Value : T_Access;
end record
   with Finalizable => (Adjust   => Adjust,
                        Finalize => Finalize);

procedure Adjust (Ref : in out T_Ref) is
begin
   Inc_Ref (Ref.Value);
end Adjust;

procedure Finalize (Ref : in out T_Ref) is
begin
   Def_Ref (Ref.Value);
end Finalize;
@end example

Simple file handle that ensures resources are properly released:

@example
package P is
   type File (<>) is limited private;

   function Open (Path : String) return File;

   procedure Close (F : in out File);

private
   type File is limited record
      Handle : ...;
   end record
      with Finalizable (Finalize => Close);
end P;
@end example

@menu
* Finalizable tagged types:: 
* Composite types:: 
* Interoperability with controlled types:: 

@end menu

@node Finalizable tagged types,Composite types,,Generalized Finalization
@anchor{gnat_rm/gnat_language_extensions finalizable-tagged-types}@anchor{46b}
@subsubsection Finalizable tagged types


The aspect is inherited by derived types and the primitives may be overridden
by the derivation. The compiler-generated calls to these operations are then
dispatching whenever it makes sense, i.e. when the object in question is of a
class-wide type and the class includes at least one finalizable tagged type.

@node Composite types,Interoperability with controlled types,Finalizable tagged types,Generalized Finalization
@anchor{gnat_rm/gnat_language_extensions composite-types}@anchor{46c}
@subsubsection Composite types


When a finalizable type is used as a component of a composite type, the latter
becomes finalizable as well. The three primitives are derived automatically
in order to call the primitives of their components. The dynamic semantics is
the same as for controlled components of composite types.

@node Interoperability with controlled types,,Composite types,Generalized Finalization
@anchor{gnat_rm/gnat_language_extensions interoperability-with-controlled-types}@anchor{46d}
@subsubsection Interoperability with controlled types


Finalizable types are fully interoperable with controlled types, in particular
it is possible for a finalizable type to have a controlled component and vice
versa, but the stricter dynamic semantics, in other words that of controlled
types, is applied in this case.

@node Experimental Language Extensions,,Curated Extensions,GNAT language extensions
@anchor{gnat_rm/gnat_language_extensions experimental-language-extensions}@anchor{6d}@anchor{gnat_rm/gnat_language_extensions id2}@anchor{46e}
@section Experimental Language Extensions


Features activated via @code{-gnatX0} or
@code{pragma Extensions_Allowed (All_Extensions)}.

@menu
* Conditional when constructs:: 
* Implicit With:: 
* Storage Model:: 
* Attribute Super:: 
* Simpler Accessibility Model:: 
* Case pattern matching:: 
* Mutably Tagged Types with Size’Class Aspect:: 
* No_Raise aspect:: 
* Inference of Dependent Types in Generic Instantiations:: 
* External_Initialization Aspect:: 
* Finally construct:: 
* Continue statement:: 
* Destructors:: 
* Structural Generic Instantiation:: 

@end menu

@node Conditional when constructs,Implicit With,,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions conditional-when-constructs}@anchor{46f}
@subsection Conditional when constructs


This feature extends the use of @code{when} as a way to condition a control-flow
related statement, to all control-flow related statements.

To do a conditional return in a procedure the following syntax should be used:

@example
procedure P (Condition : Boolean) is
begin
   return when Condition;
end P;
@end example

This will return from the procedure if @code{Condition} is true.

When being used in a function the conditional part comes after the return value:

@example
function Is_Null (I : Integer) return Boolean is
begin
   return True when I = 0;
   return False;
end;
@end example

In a similar way to the @code{exit when} a @code{goto ... when} can be employed:

@example
procedure Low_Level_Optimized is
   Flags : Bitmapping;
begin
   Do_1 (Flags);
   goto Cleanup when Flags (1);

   Do_2 (Flags);
   goto Cleanup when Flags (32);

   --  ...

<<Cleanup>>
   --  ...
end;
@end example

@c code-block

To use a conditional raise construct:

@example
procedure Foo is
begin
   raise Error when Imported_C_Func /= 0;
end;
@end example

An exception message can also be added:

@example
procedure Foo is
begin
   raise Error with "Unix Error"
     when Imported_C_Func /= 0;
end;
@end example

@node Implicit With,Storage Model,Conditional when constructs,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions implicit-with}@anchor{470}
@subsection Implicit With


This feature allows a standalone @code{use} clause in the context clause of a
compilation unit to imply an implicit @code{with} of the same library unit where
an equivalent @code{with} clause would be allowed.

@example
use Ada.Text_IO;
procedure Main is
begin
   Put_Line ("Hello");
end;
@end example

@node Storage Model,Attribute Super,Implicit With,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions storage-model}@anchor{471}
@subsection Storage Model


This extends Storage Pools into a more efficient model allowing higher performance,
easier integration with low footprint embedded run-times and copying data between
different pools of memory. The latter is especially useful when working with distributed
memory models, in particular to support interactions with GPU.

@menu
* Aspect Storage_Model_Type:: 
* Aspect Designated_Storage_Model:: 
* Legacy Storage Pools:: 

@end menu

@node Aspect Storage_Model_Type,Aspect Designated_Storage_Model,,Storage Model
@anchor{gnat_rm/gnat_language_extensions aspect-storage-model-type}@anchor{472}
@subsubsection Aspect Storage_Model_Type


A Storage model is a type with a specified @code{Storage_Model_Type}
aspect, e.g.:

@example
type A_Model is null record
   with Storage_Model_Type (...);
@end example

Storage_Model_Type itself accepts six parameters:


@itemize -

@item 
Address_Type, the type of the address managed by this model. This has to be
a scalar type or derived from System.Address.

@item 
Allocate, a procedure used for allocating memory in this model

@item 
Deallocate, a procedure used for deallocating memory in this model

@item 
Copy_To, a procedure used to copy memory from native memory to this model

@item 
Copy_From, a procedure used to copy memory from this model to native memory

@item 
Storage_Size, a function returning the amount of memory left

@item 
Null_Address, a value for the null address value
@end itemize

By default, Address_Type is System.Address, and the five subprograms
perform native operations (e.g. the allocator is the native @code{new} allocator).
Users can decide to specify one or more of these. When an Address_Type is
specified to be other than System.Address, all of the subprograms have
to be specified.

The prototypes of these procedures are as follows:

@example
procedure Allocate
  (Model           : in out A_Model;
   Storage_Address : out Address_Type;
   Size            : Storage_Count;
   Alignment       : Storage_Count);

procedure Deallocate
  (Model           : in out A_Model;
   Storage_Address : out Address_Type;
   Size            : Storage_Count;
   Alignment       : Storage_Count);

procedure Copy_To
  (Model   : in out A_Model;
   Target  : Address_Type;
   Source  : System.Address;
   Size    : Storage_Count);

procedure Copy_From
  (Model  : in out A_Model;
   Target : System.Address;
   Source : Address_Type;
   Size   : Storage_Count);

function Storage_Size
  (Pool : A_Model)
   return Storage_Count;
@end example

Here’s an example of how this could be instantiated in the context of CUDA:

@example
package CUDA_Memory is

   type CUDA_Storage_Model is null record
      with Storage_Model_Type => (
         Address_Type => CUDA_Address,
         Allocate     => CUDA_Allocate,
         Deallocate   => CUDA_Deallocate,
         Copy_To      => CUDA_Copy_To,
         Copy_From    => CUDA_Copy_From,
         Storage_Size => CUDA_Storage_Size,
         Null_Address => CUDA_Null_Address
      );

   type CUDA_Address is new System.Address;
   --  We're assuming for now same address size on host and device

   procedure CUDA_Allocate
     (Model           : in out CUDA_Storage_Model;
      Storage_Address : out CUDA_Address;
      Size            : Storage_Count;
      Alignment       : Storage_Count);

   procedure CUDA_Deallocate
     (Model           : in out CUDA_Storage_Model;
      Storage_Address : out CUDA_Address;
      Size            : Storage_Count;
      Alignment       : Storage_Count);

   procedure CUDA_Copy_To
     (Model  : in out CUDA_Storage_Model;
      Target : CUDA_Address;
      Source : System.Address;
      Size   : Storage_Count);

   procedure CUDA_Copy_From
     (Model   : in out CUDA_Storage_Model;
      Target  : System.Address;
      Source  : CUDA_Address;
      Size    : Storage_Count);

   function CUDA_Storage_Size
     (Pool : CUDA_Storage_Model)
      return Storage_Count return Storage_Count'Last;

   CUDA_Null_Address : constant CUDA_Address :=
      CUDA_Address (System.Null_Address);

   CUDA_Memory : CUDA_Storage_Model;

end CUDA_Memory;
@end example

@node Aspect Designated_Storage_Model,Legacy Storage Pools,Aspect Storage_Model_Type,Storage Model
@anchor{gnat_rm/gnat_language_extensions aspect-designated-storage-model}@anchor{473}
@subsubsection Aspect Designated_Storage_Model


A new aspect, Designated_Storage_Model, allows to specify the memory model
for the objects pointed by an access type. Under this aspect, allocations
and deallocations will come from the specified memory model instead
of the standard ones. In addition, if write operations are needed for
initialization, or if there is a copy of the target object from and to a
standard memory area, the Copy_To and Copy_From functions will be called.
It allows to encompass the capabilities of storage pools, e.g.:

@example
procedure Main is
   type Integer_Array is array (Integer range <>) of Integer;

   type Host_Array_Access is access all Integer_Array;
   type Device_Array_Access is access Integer_Array
      with Designated_Storage_Model => CUDA_Memory;

   procedure Free is new Unchecked_Deallocation
      (Host_Array_Type, Host_Array_Access);
   procedure Free is new Unchecked_Deallocation
      (Device_Array_Type, Device_Array_Access);

   Host_Array : Host_Array_Access := new Integer_Array (1 .. 10);

   Device_Array : Device_Array_Access := new Host_Array (1 .. 10);
   --  Calls CUDA_Storage_Model.Allocate to allocate the fat pointers and
   --  the bounds, then CUDA_Storage_Model.Copy_In to copy the values of the
   --  boundaries.
begin
   Host_Array.all := (others => 0);

   Device_Array.all := Host_Array.all;
   --  Calls CUDA_Storage_Model.Copy_To to write to the device array from the
   --  native memory.

   Host_Array.all := Device_Array.all;
   --  Calls CUDA_Storage_Model.Copy_From to read from the device array and
   --  write to native memory.

   Free (Host_Array);

   Free (Device_Array);
   --  Calls CUDA_Storage_Model.Deallocate;
end;
@end example

Taking @code{'Address} of an object with a specific memory model returns an object of
the type of the address for that memory category, which may be different from
System.Address.

When copying is performed between two specific memory models, the native memory
is used as a temporary between the two. E.g.:

@example
type Foo_I is access Integer with Designated_Storage_Model => Foo;
type Bar_I is access Integer with Designated_Storage_Model => Bar;

  X : Foo_I := new Integer;
  Y : Bar_I := new Integer;
begin
  X.all := Y.all;
@end example

conceptually becomes:

@example
  X : Foo_I := new Integer;
  T : Integer;
  Y : Bar_I := new Integer;
begin
  T := Y.all;
  X.all := T;
@end example

@node Legacy Storage Pools,,Aspect Designated_Storage_Model,Storage Model
@anchor{gnat_rm/gnat_language_extensions legacy-storage-pools}@anchor{474}
@subsubsection Legacy Storage Pools


Legacy Storage Pools are now replaced by a Storage_Model_Type.
They are implemented as follows:

@example
type Root_Storage_Pool is abstract
  new Ada.Finalization.Limited_Controlled with private
with Storage_Model_Type => (
   Allocate     => Allocate,
   Deallocate   => Deallocate,
   Storage_Size => Storage_Size
);
pragma Preelaborable_Initialization (Root_Storage_Pool);

procedure Allocate
  (Pool                     : in out Root_Storage_Pool;
   Storage_Address          : out System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count)
is abstract;

procedure Deallocate
  (Pool                     : in out Root_Storage_Pool;
   Storage_Address          : System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count)
is abstract;

function Storage_Size
  (Pool : Root_Storage_Pool)
   return System.Storage_Elements.Storage_Count
is abstract;
@end example

The legacy notation:

@example
type My_Pools is new Root_Storage_Pool with record [...]

My_Pool_Instance : Storage_Model_Pool.Storage_Model :=
   My_Pools'(others => <>);

type Acc is access Integer_Array with Storage_Pool => My_Pool;
@end example

can still be accepted as a shortcut for the new syntax.

@node Attribute Super,Simpler Accessibility Model,Storage Model,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions attribute-super}@anchor{475}
@subsection Attribute Super


@geindex Super

The @code{Super} attribute can be applied to objects of tagged types in order
to obtain a view conversion to the most immediate specific parent type.

It cannot be applied to objects of types without any ancestors.

@example
type T1 is tagged null record;
procedure P (V : T1);

type T2 is new T1 with null record;

type T3 is new T2 with null record;
procedure P (V : T3);

procedure Call (
  V1 : T1'Class;
  V2 : T2'Class;
  V3 : T3'Class) is
begin
  V1'Super.P; --  Illegal call as T1 doesn't have any ancestors
  V2'Super.P; --  Equivalent to "T1 (V).P;", a non-dispatching call
              --  to T1's primitive procedure P.
  V3'Super.P; --  Equivalent to "T2 (V).P;"; Since T2 doesn't
              --  override P, a non-dispatching call to T1.P is
              --  executed.
end;
@end example

@node Simpler Accessibility Model,Case pattern matching,Attribute Super,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions simpler-accessibility-model}@anchor{476}
@subsection Simpler Accessibility Model


The goal of this feature is to simplify the accessibility rules by removing
dynamic accessibility checks that are often difficult to understand and debug.
The new rules eliminate the need for runtime accessibility checks by imposing
more conservative legality rules when enabled via a new restriction (see RM 13.12),
No_Dynamic_Accessibility_Checks, which prevents dangling reference problems
at compile time.

This restriction has no effect on the user-visible behavior of a program when executed;
the only effect of this restriction is to enable additional compile-time checks
(described below) which ensure statically that Ada’s dynamic accessibility checks
will not fail.

The feature can be activated with @code{pragma Restrictions (No_Dynamic_Accessibility_Checks);}.
As a result, additional compile-time checks are performed; these checks pertain to
stand-alone objects, subprogram parameters, and function results as described below.

All of the refined rules are compatible with the [use of anonymous access types in SPARK]
(@indicateurl{http://docs.adacore.com/spark2014-docs/html/lrm/declarations-and-types.html#access-types}).

@menu
* Stand-alone objects:: 
* Subprogram parameters:: 
* Function results:: 

@end menu

@node Stand-alone objects,Subprogram parameters,,Simpler Accessibility Model
@anchor{gnat_rm/gnat_language_extensions stand-alone-objects}@anchor{477}
@subsubsection Stand-alone objects


@example
Var        : access T := ...
Var_To_Cst : access constant T := ...
Cst        : constant access T := ...
Cst_To_Cst : constant access constant T := ...
@end example

In this section, we will refer to a stand-alone object of an anonymous access
type as an SO.

When the restriction is in effect, the “statically deeper” relationship
(see RM 3.10.2(4)) does apply to the type of a SO (contrary to RM 3.10.2(19.2))
and, for the purposes of compile-time checks, the accessibility level of the
type of a SO is the accessibility level of that SO.
This supports many common use-cases without the employment of @code{Unchecked_Access}
while still removing the need for dynamic checks.

This statically disallows cases that would otherwise require a dynamic accessibility
check, such as

@example
type Ref is access all Integer;
Ptr : Ref;
Good : aliased Integer;

procedure Proc is
   Bad : aliased Integer;
   Stand_Alone : access Integer;
begin
   if <some condition> then
      Stand_Alone := Good'Access;
   else
      Stand_Alone := Bad'Access;
   end if;
   Ptr := Ref (Stand_Alone);
end Proc;
@end example

If a No_Dynamic_Accessibility_Checks restriction is in effect, then the otherwise-legal
type conversion (the right-hand side of the assignment to Ptr) becomes a violation
of the RM 4.6 rule “The accessibility level of the operand type shall not be
statically deeper than that of the target type …”.

@node Subprogram parameters,Function results,Stand-alone objects,Simpler Accessibility Model
@anchor{gnat_rm/gnat_language_extensions subprogram-parameters}@anchor{478}
@subsubsection Subprogram parameters


@example
procedure P (V : access T; X : access constant T);
@end example

In most cases (the exceptions are described below), a No_Dynamic_Accessibility_Checks
restriction means that the “statically deeper” relationship does apply to the anonymous
type of an access parameter specifying an access-to-object type (contrary to RM 3.10.2(19.1))
and, for purposes of compile-time “statically deeper” checks, the accessibility level
of the type of such a parameter is the accessibility level of the parameter.

This change (at least as described so far) doesn’t affect the caller’s side, but on the
callee’s side it means that object designated by a non-null parameter of an anonymous
access type is treated as having the same accessibility level as a local object declared
immediately within the called subprogram.

With the restriction in effect, the otherwise-legal type conversion in the following example
becomes illegal:

@example
type Ref is access all Integer;
Ptr : Ref;

procedure Proc (Param : access Integer) is
begin
   Ptr := Ref (Param);
end Proc;
@end example

The aforementioned exceptions have to do with return statements from functions that either
return the given parameter (in the case of a function whose result type is an anonymous
access type) or return the given parameter value as an access discriminant of the function
result (or of some discriminated part thereof). More specifically, the “statically deeper”
changes described above do not apply for purposes of checking the “shall not be statically
deeper” rule for access discriminant parts of function results (RM 6.5(5.9)) or in determining
the legality of an (implicit) type conversion from the anonymous access type of a parameter
of a function to an anonymous access result type of that function. In order to prevent these
rule relaxations from introducing the possibility of dynamic accessibility check failures,
compensating compile-time checks are performed at the call site to prevent cases where
including the value of an access parameter as part of a function result could make such
check failures possible (specifically, the discriminant checks of RM 6.5(21) or, in the
case of an anonymous access result type, the RM 4.6(48) check performed when converting
to that result type). These compile-time checks are described in the next section.

From the callee’s perspective, the level of anonymous access formal parameters would be
between the level of the subprogram and the level of the subprogram’s locals. This has the effect
of formal parameters being treated as local to the callee except in:


@itemize *

@item 
Use as a function result

@item 
Use as a value for an access discriminant in result object

@item 
Use as an assignments between formal parameters
@end itemize

Note that with these more restricted rules we lose track of accessibility levels when assigned to
local objects thus making (in the example below) the assignment to Node2.Link from Temp below
compile-time illegal.

@example
type Node is record
   Data : Integer;
   Link : access Node;
end record;

procedure Swap_Links (Node1, Node2 : in out Node) is
   Temp : constant access Node := Node1.Link; -- We lose the "association" to Node1
begin
   Node1.Link := Node2.Link; -- Allowed
   Node2.Link := Temp;       -- Not allowed
end;

function Identity (N : access Node) return access Node is
   Local : constant access Node := N;
begin
   if True then
      return N;              -- Allowed
   else
      return Local;          -- Not allowed
   end if;
end;
@end example

@node Function results,,Subprogram parameters,Simpler Accessibility Model
@anchor{gnat_rm/gnat_language_extensions function-results}@anchor{479}
@subsubsection Function results


@example
function Get (X : Rec) return access T;
@end example

If the result subtype of a function is either an anonymous access (sub)type, a
class-wide (sub)type, an unconstrained subtype with an access discriminant, or
a type with an unconstrained subcomponent subtype that has at least one access
discriminant (this last case is only possible if the access discriminant has a
default value), then we say that the function result type “might require an
anonymous-access-part accessibility check”. If a function has an access parameter,
or a parameter whose subtype “might require an anonymous-access-part accessibility
check”, then we say that the each such parameter “might be used to pass in an
anonymous-access value”. If the first of these conditions holds for the result
subtype of a function and the second condition holds for at least one parameter
that function, then it is possible that a call to that function could return a
result that contains anonymous-access values that were passed in via the parameter.

Given a function call where the result type “might require an anonymous-access-part
accessibility check” and a formal parameter of that function that “might be used to
pass in an anonymous-access value”, either the type of that formal parameter is an
anonymous access type or it is not. If it is, and if a No_Dynamic_Access_Checks
restriction is in effect, then the accessibility level of the type of the actual
parameter shall be statically known to not be deeper than that of the master of
the call. If it isn’t, then the accessibility level of the actual parameter shall
be statically known to not be deeper than that of the master of the call.

Function result example:

@example
declare
   type T is record
      Comp : aliased Integer;
   end record;

   function Identity (Param : access Integer) return access Integer is
   begin
      return Param;        -- Legal
   end;

   function Identity_2 (Param : aliased Integer) return access Integer is
   begin
      return Param'Access; -- Legal
   end;

   X : access Integer;
begin
   X := Identity (X);      -- Legal
   declare
      Y : access Integer;
      Z : aliased Integer;
   begin
      X := Identity (Y);   -- Illegal since Y is too deep
      X := Identity_2 (Z); -- Illegal since Z is too deep
   end;
end;
@end example

In order to avoid having to expand the definition of “might be used to pass in an
anonymous-access value” to include any parameter of a tagged type, the
No_Dynamic_Access_Checks restriction also imposes a requirement that a type extension
cannot include the explicit definition of an access discriminant.

Here is an example of one such case of an upward conversion which would lead to a memory leak:

@example
declare
   type T is tagged null record;
   type T2 (Disc : access Integer) is new T with null record; -- Must be illegal

   function Identity (Param : aliased T'Class) return access Integer is
   begin
      return T2 (T'Class (Param)).Disc; -- Here P gets effectively returned and set to X
   end;

   X : access Integer;
begin
   declare
      P : aliased Integer;
      Y : T2 (P'Access);
   begin
      X := Identity (T'Class (Y)); -- Pass local variable P (via Y's discriminant),
                                 -- leading to a memory leak.
   end;
end;
`@w{`}`

Thus we need to make the following illegal to avoid such situations:

`@w{`}`ada
package Pkg1 is
   type T1 is tagged null record;
   function Func (X1 : T1) return access Integer is (null);
end;

package Pkg2 is
   type T2 (Ptr1, Ptr2 : access Integer) is new Pkg1.T1 with null record; -- Illegal
   ...
end;
@end example

In order to prevent upward conversions of anonymous function results (like below), we
also would need to assure that the level of such a result (from the callee’s perspective)
is statically deeper:

@example
declare
   type Ref is access all Integer;
   Ptr : Ref;
   function Foo (Param : access Integer) return access Integer is
   begin
      return Result : access Integer := Param; do
         Ptr := Ref (Result); -- Not allowed
      end return;
   end;
begin
   declare
      Local : aliased Integer;
   begin
      Foo (Local'Access).all := 123;
   end;
end;
@end example

@node Case pattern matching,Mutably Tagged Types with Size’Class Aspect,Simpler Accessibility Model,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions case-pattern-matching}@anchor{47a}
@subsection Case pattern matching


The selector for a case statement (but not for a case expression) may
be of a composite type, subject to some restrictions (described below).
Aggregate syntax is used for choices of such a case statement; however,
in cases where a “normal” aggregate would require a discrete value, a
discrete subtype may be used instead; box notation can also be used to
match all values.

Consider this example:

@example
type Rec is record
   F1, F2 : Integer;
end record;

procedure Caser_1 (X : Rec) is
begin
   case X is
      when (F1 => Positive, F2 => Positive) =>
         Do_This;
      when (F1 => Natural, F2 => <>) | (F1 => <>, F2 => Natural) =>
         Do_That;
      when others =>
          Do_The_Other_Thing;
   end case;
end Caser_1;
@end example

If @code{Caser_1} is called and both components of X are positive, then
@code{Do_This} will be called; otherwise, if either component is nonnegative
then @code{Do_That} will be called; otherwise, @code{Do_The_Other_Thing} will be
called.

In addition, pattern bindings are supported. This is a mechanism
for binding a name to a component of a matching value for use within
an alternative of a case statement. For a component association
that occurs within a case choice, the expression may be followed by
@code{is <identifier>}. In the special case of a “box” component association,
the identifier may instead be provided within the box. Either of these
indicates that the given identifier denotes (a constant view of) the matching
subcomponent of the case selector.

@cartouche
@quotation Attention 
Binding is not yet supported for arrays or subcomponents
thereof.
@end quotation
@end cartouche

Consider this example (which uses type @code{Rec} from the previous example):

@example
procedure Caser_2 (X : Rec) is
begin
   case X is
      when (F1 => Positive is Abc, F2 => Positive) =>
         Do_This (Abc)
      when (F1 => Natural is N1, F2 => <N2>) |
           (F1 => <N2>, F2 => Natural is N1) =>
         Do_That (Param_1 => N1, Param_2 => N2);
      when others =>
         Do_The_Other_Thing;
   end case;
end Caser_2;
@end example

This example is the same as the previous one with respect to determining
whether @code{Do_This}, @code{Do_That}, or @code{Do_The_Other_Thing} will be called. But
for this version, @code{Do_This} takes a parameter and @code{Do_That} takes two
parameters. If @code{Do_This} is called, the actual parameter in the call will be
@code{X.F1}.

If @code{Do_That} is called, the situation is more complex because there are two
choices for that alternative. If @code{Do_That} is called because the first choice
matched (i.e., because @code{X.F1} is nonnegative and either @code{X.F1} or @code{X.F2}
is zero or negative), then the actual parameters of the call will be (in order)
@code{X.F1} and @code{X.F2}. If @code{Do_That} is called because the second choice
matched (and the first one did not), then the actual parameters will be
reversed.

Within the choice list for single alternative, each choice must define the same
set of bindings and the component subtypes for for a given identifier must all
statically match. Currently, the case of a binding for a nondiscrete component
is not implemented.

If the set of values that match the choice(s) of an earlier alternative
overlaps the corresponding set of a later alternative, then the first set shall
be a proper subset of the second (and the later alternative will not be
executed if the earlier alternative “matches”). All possible values of the
composite type shall be covered. The composite type of the selector shall be an
array or record type that is neither limited nor class-wide. Currently, a “when
others =>” case choice is required; it is intended that this requirement will
be relaxed at some point.

If a subcomponent’s subtype does not meet certain restrictions, then the only
value that can be specified for that subcomponent in a case choice expression
is a “box” component association (which matches all possible values for the
subcomponent). This restriction applies if:


@itemize -

@item 
the component subtype is not a record, array, or discrete type; or

@item 
the component subtype is subject to a non-static constraint or has a
predicate; or:

@item 
the component type is an enumeration type that is subject to an enumeration
representation clause; or

@item 
the component type is a multidimensional array type or an array type with a
nonstatic index subtype.
@end itemize

Support for casing on arrays (and on records that contain arrays) is
currently subject to some restrictions. Non-positional
array aggregates are not supported as (or within) case choices. Likewise
for array type and subtype names. The current implementation exceeds
compile-time capacity limits in some annoyingly common scenarios; the
message generated in such cases is usually “Capacity exceeded in compiling
case statement with composite selector type”.

@node Mutably Tagged Types with Size’Class Aspect,No_Raise aspect,Case pattern matching,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions mutably-tagged-types-with-size-class-aspect}@anchor{47b}
@subsection Mutably Tagged Types with Size’Class Aspect


For a specific tagged nonformal type T that satisfies some conditions
described later in this section, the universal-integer-valued type-related
representation aspect @code{Size'Class} may be specified; any such specified
aspect value shall be static.

Specifying this aspect imposes an upper bound on the sizes of all specific
descendants of T (including T itself). T’Class (but not T) is then said to be
a “mutably tagged” type - meaning that T’Class is a definite subtype and that
the tag of a variable of type T’Class may be modified by assignment in some
cases described later in this section. An inherited @code{Size'Class} aspect
value may be overridden, but not with a larger value.

If the @code{Size'Class} aspect is specified for a type T, then every specific
descendant of T (including T itself)


@itemize *

@item 
shall have a Size that does not exceed the specified value; and

@item 
shall have a (possibly inherited) @code{Size'Class} aspect that does not exceed
the specified value; and

@item 
shall be undiscriminated; and

@item 
shall have no composite subcomponent whose subtype is subject to a nonstatic
constraint; and

@item 
shall not have a tagged partial view other than a private extension; and

@item 
shall not be a descendant of an interface type; and

@item 
shall not have a statically deeper accessibility level than that of T.
@end itemize

If the @code{Size'Class} aspect is not specified for a type T (either explicitly
or by inheritance), then it shall not be specified for any descendant of T.

Example:

@example
type Root_Type is tagged null record with Size'Class => 16 * 8;

type Derived_Type is new Root_Type with record
   Stuff : Some_Type;
end record;  -- ERROR if Derived_Type exceeds 16 bytes
@end example

Because any subtype of a mutably tagged type is definite, it can be used as a
component subtype for enclosing array or record types, as the subtype of a
default-initialized stand-alone object, or as the subtype of an uninitialized
allocator, as in this example:

@example
Obj : Root_Type'Class;
type Array_of_Roots is array (Positive range <>) of Root_Type'Class;
@end example

Default initialization of an object of such a definite subtype proceeds as
for the corresponding specific type, except that Program_Error is raised if
the specific type is abstract. In particular, the initial tag of the object
is that of the corresponding specific type.

There is a general design principle that if a type has a tagged partial view,
then the type’s @code{Size'Class} aspect (or lack thereof) should be determinable
by looking only at the partial view. That provides the motivation for the
rules of the next two paragraphs.

If a type has a tagged partial view, then a @code{Size'Class} aspect specification
may be provided only at the point of the partial view declaration (in other
words, no such aspect specification may be provided when the full view of
the type is declared). All of the above rules (in particular, the rule that
an overriding @code{Size'Class} aspect value shall not be larger than the
overridden inherited value) are also enforced when the full view (which may
have a different ancestor type than that of the partial view) is declared.
If a partial view for a type inherits a @code{Size'Class} aspect value and does
not override that value with an explicit aspect specification, then the
(static) aspect values inherited by the partial view and by the full view
shall be equal.

An actual parameter of an instantiation whose corresponding formal parameter
is a formal tagged private type shall not be either mutably tagged or the
corresponding specific type of a mutably tagged type.

For the legality rules in this section, the RM 12.3(11) rule about legality
checking in the visible part and formal part of an instance is extended (in
the same way that it is extended in many other places in the RM) to include
the private part of an instance.

An object (or a view thereof) of a tagged type is defined to be
“tag-constrained” if it is


@itemize *

@item 
an object whose type is not mutably tagged; or

@item 
a constant object; or

@item 
a view conversion of a tag-constrained object; or

@item 
a view conversion to a type that is not a descendant of the operand’s
type; or

@item 
a formal in out or out parameter whose corresponding actual parameter is
tag-constrained; or

@item 
a dereference of an access value.
@end itemize

In the case of an assignment to a tagged variable that is not
tag-constrained, no check is performed that the tag of the value
of the expression is the same as that of the target (RM 5.2 notwithstanding).
Instead, the tag of the target object becomes that of the source object of
the assignment. Note that the tag of an object of a mutably tagged type MT
will always be the tag of some specific type that is a descendant of MT.
An assignment to a composite object similarly copies the tags of any
subcomponents of the source object that have a mutably tagged type.

The Constrained attribute is defined for any name denoting an object of a
mutably tagged type (RM 3.7.2 notwithstanding). In this case, the
Constrained attribute yields the value True if the object is
tag-constrained and False otherwise.

Renaming is not allowed (see RM 8.5.1) for a type conversion having an operand
of a mutably tagged type MT and a target type TT such that TT (or its
corresponding specific type if TT is class-wide) is not an ancestor of MT
(this is sometimes called a “downward” conversion), nor for any part of
such an object, nor for any slice of any part of such an object. This
rule also applies in any context where a name is required to be one for
which “renaming is allowed” (for example, see RM 12.4).
[This is analogous to the way that renaming is not allowed for a
discriminant-dependent component of an unconstrained variable.]

A name denoting a view of a variable of a mutably tagged type shall not occur
as an operative constituent of the prefix of a name denoting a prefixed
view of a callable entity, except as the callee name in a call to the
callable entity. This disallows, for example, renaming such a prefixed view,
passing the prefixed view name as a generic actual parameter, or using the
prefixed view name as the prefix of an attribute.

The execution of a construct is erroneous if the construct has a constituent
that is a name denoting a subcomponent of a tagged object and the object’s
tag is changed by this execution between evaluating the name and the last
use (within this execution) of the subcomponent denoted by the name.
This is analogous to the RM 3.7.2(4) rule about discriminant-dependent
subcomponents.

If the type of a formal parameter is a specific tagged type, then the execution
of the call is erroneous if the tag of the actual is changed while the formal
parameter exists (that is, before leaving the corresponding callable construct).
This is analogous to the RM 6.4.1(18) rule about discriminated parameters.

@node No_Raise aspect,Inference of Dependent Types in Generic Instantiations,Mutably Tagged Types with Size’Class Aspect,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions id3}@anchor{47c}@anchor{gnat_rm/gnat_language_extensions no-raise-aspect}@anchor{46a}
@subsection No_Raise aspect


The @code{No_Raise} aspect can be applied to a subprogram to declare that this
subprogram is not expected to raise an exception. Should an exception still
be raised during the execution of the subprogram, it is caught at the end of
this execution and @code{Program_Error} is propagated to the caller.

@node Inference of Dependent Types in Generic Instantiations,External_Initialization Aspect,No_Raise aspect,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions inference-of-dependent-types-in-generic-instantiations}@anchor{47d}
@subsection Inference of Dependent Types in Generic Instantiations


If a generic formal type T2 depends on another formal type T1,
the actual for T1 can be inferred from the actual for T2.
That is, you can give the actual for T2, and leave out the one
for T1.

For example, @code{Ada.Unchecked_Deallocation} has two generic formals:

@example
generic
   type Object (<>) is limited private;
   type Name is access Object;
procedure Ada.Unchecked_Deallocation (X : in out Name);
@end example

where @code{Name} depends on @code{Object}. With this language extension,
you can leave out the actual for @code{Object}, as in:

@example
type Integer_Access is access all Integer;

procedure Free is new Unchecked_Deallocation (Name => Integer_Access);
@end example

The compiler will infer that the actual type for @code{Object} is @code{Integer}.
Note that named notation is always required when using inference.

The following inferences are allowed:


@itemize -

@item 
For a formal access type, the designated type can be inferred.

@item 
For a formal array type, the index type(s) and the component
type can be inferred.

@item 
For a formal type with discriminants, the type(s) of the discriminants
can be inferred.
@end itemize

Example for arrays:

@example
generic
   type Element_Type is private;
   type Index_Type is (<>);
   type Array_Type is array (Index_Type range <>) of Element_Type;
package Array_Operations is
   ...
end Array_Operations;

...

type Int_Array is array (Positive range <>) of Integer;

package Int_Array_Operations is new Array_Operations (Array_Type => Int_Array);
@end example

The index and component types of @code{Array_Type} are inferred from
@code{Int_Array}, so that the above instantiation is equivalent to
the following standard-Ada instantiation:

@example
package Int_Array_Operations is new Array_Operations
   (Element_Type => Integer,
      Index_Type   => Positive,
      Array_Type   => Int_Array);
@end example

@node External_Initialization Aspect,Finally construct,Inference of Dependent Types in Generic Instantiations,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions external-initialization-aspect}@anchor{47e}
@subsection External_Initialization Aspect


The @code{External_Initialization} aspect provides a feature similar to Rust’s @code{include_bytes!}
and to C23’s @code{#embed}. It has the effect of initializing an object with the contents of
a file specified by a file path.

Only string objects and objects of type @code{Ada.Streams.Stream_Element_Array} can be subject
to the @code{External_Initialization} aspect.

Example:

@example
with Ada.Streams;

package P is
   S : constant String with External_Initialization => "foo.txt";

   X : constant Ada.Streams.Stream_Element_Array with External_Initialization => "bar.bin";
end P;
@end example

@code{External_Initialization} aspect accepts the following parameters:


@itemize -

@item 
mandatory @code{Path}: the path the compiler uses to access the binary resource.
@end itemize

If @code{Path} is a relative path, it is interpreted relatively to the directory of the file that contains the aspect specification.

@cartouche
@quotation Attention 
The maximum size of loaded files is limited to 2@w{^31} bytes.
@end quotation
@end cartouche

@node Finally construct,Continue statement,External_Initialization Aspect,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions finally-construct}@anchor{47f}
@subsection Finally construct


The @code{finally} keyword makes it possible to have a sequence of statements be executed when
another sequence of statements is completed, whether normally or abnormally.

This feature is similar to the one with the same name in other languages such as Java.

Note that @code{finally} is a keyword but it is not a reserved word. This is a
configuration that does not exist in standard Ada.

@menu
* Syntax: Syntax<2>. 
* Legality Rules: Legality Rules<2>. 
* Dynamic Semantics: Dynamic Semantics<2>. 

@end menu

@node Syntax<2>,Legality Rules<2>,,Finally construct
@anchor{gnat_rm/gnat_language_extensions id4}@anchor{480}
@subsubsection Syntax


@example
handled_sequence_of_statements ::=
     sequence_of_statements
  [exception
     exception_handler
    @{exception_handler@}]
  [finally
    sequence_of_statements]
@end example

@node Legality Rules<2>,Dynamic Semantics<2>,Syntax<2>,Finally construct
@anchor{gnat_rm/gnat_language_extensions id5}@anchor{481}
@subsubsection Legality Rules


Return statements in the @code{sequence_of_statements} attached to the finally that would cause control
to be transferred outside the finally part are forbidden.

Goto & exit where the target is outside of the finally’s @code{sequence_of_statements} are forbidden

@node Dynamic Semantics<2>,,Legality Rules<2>,Finally construct
@anchor{gnat_rm/gnat_language_extensions id6}@anchor{482}
@subsubsection Dynamic Semantics


Statements in the optional @code{sequence_of_statements} contained in the @code{finally} branch will be
executed unconditionally, after the main @code{sequence_of_statements} is executed, and after any
potential @code{exception_handler} is executed.

If an exception is raised in the finally part, it cannot be caught by the @code{exception_handler}.

Abort/ATC (asynchronous transfer of control) cannot interrupt a finally block, nor prevent its
execution, that is the finally block must be executed in full even if the containing task is
aborted, or if the control is transferred out of the block.

@node Continue statement,Destructors,Finally construct,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions continue-statement}@anchor{483}
@subsection Continue statement


The @code{continue} keyword makes it possible to stop execution of a loop iteration
and continue with the next one. A continue statement has the same syntax
(except “exit” is replaced with “continue”), static semantics, and legality
rules as an exit statement. The difference is in the dynamic semantics: where an
exit statement would cause a transfer of control that completes the (implicitly
or explicitly) specified loop_statement, a continue statement would instead
cause a transfer of control that completes only the current iteration of that
loop_statement, like a goto statement targeting a label following the last
statement in the sequence of statements of the specified loop_statement.

Note that @code{continue} is a keyword but it is not a reserved word. This is a
configuration that does not exist in standard Ada.

@node Destructors,Structural Generic Instantiation,Continue statement,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions destructors}@anchor{484}
@subsection Destructors


The @code{Destructor} extension adds a new finalization mechanism that
significantly differs from standard Ada in how it interacts with type
derivation.

New syntax is introduced to make it possible to define “destructors” for record
types, tagged or untagged. Here’s a simple example:

@example
package P is
   type T is record
      ...
   end record;

   procedure T'Destructor (X : in out T);
end P;
@end example

@example
package body P is
   procedure T'Destructor (X : in out T) is
   begin
      ...
   end T'Destructor;
end P;
@end example

Like @code{Finalize} procedures, destructors are called on objects just before they
are destroyed. But destructors are more flexible in how they can used with derived
types. With standard Ada finalization, when you derive from a finalizable type,
you must either inherit the @code{Finalize} procedure or override it completely.

Destructors work differently. You can define a destructor for a type derived from
a parent type that also has a destructor, and then when objects of the derived type
are finalized, both destructors will be called. For example:

@example
type T1 is record
   ...
end record;

procedure T1'Destructor (X : in out T1);

type T2 is new T1;

procedure T2'Destructor (X : in out T2);
@end example

When an object of type @code{T2} is finalized, there will be first a call to
@code{T2'Destructor}, and then a call to @code{T1'Destructor} on the object.

@node Structural Generic Instantiation,,Destructors,Experimental Language Extensions
@anchor{gnat_rm/gnat_language_extensions structural-generic-instantiation}@anchor{485}
@subsection Structural Generic Instantiation


The compiler implements a second kind of generic instantiation, called
“structural”, alongside the traditional instantiation specified by the
language, which is defined as follows: the structural instantiation of
a generic unit on given actual parameters is the anonymous instantiation
of the generic unit on the actual parameters done in the outermost scope
where it would be legal to do an identical traditional instantiation.

There is at most one structural instantiation of a generic unit on given
actual parameters done in a partition.

Structural generic instances (the product of structural instantiation)
are implicitly created whenever a reference to them is made in a place
where a name is accepted by the language.

@menu
* Syntax: Syntax<3>. 
* Legality Rules: Legality Rules<3>. 
* Static Semantics:: 

@end menu

@node Syntax<3>,Legality Rules<3>,,Structural Generic Instantiation
@anchor{gnat_rm/gnat_language_extensions id7}@anchor{486}
@subsubsection Syntax


@example
name ::= @{ set of productions specified in the RM @}
         | structural_generic_instance_name

structural_generic_instance_name ::= name generic_actual_part
@end example

@node Legality Rules<3>,Static Semantics,Syntax<3>,Structural Generic Instantiation
@anchor{gnat_rm/gnat_language_extensions id8}@anchor{487}
@subsubsection Legality Rules


The @code{name} in a @code{structural_generic_instance_name} shall denote a generic
unit that is preelaborated. Note that, unlike in a traditional instantiation,
there are no square brackets around the @code{generic_actual_part} in the second
production, which means that it is mandatory and, therefore, that the generic
unit shall have at least one generic formal parameter.

The generic unit shall not take a generic formal object of mode @code{in out}.
If the generic unit takes a generic formal object of mode @code{in}, then the
corresponding generic actual parameter shall be a static expression.

A @code{structural_generic_instance_name} for a generic package shall not be
present in a library unit if the structural instance is also a library unit
and has a semantic dependence on the former.

For a generic subprogram, if a local entity of the enclosing library-level
package is used as an actual and the structural instance would have a semantic
dependence on the package, the structural instantiation is automatically
demoted to a local instantiation. In this case, several instances of the
generic subprogram may be present in a single partition, unless
whole-partition optimization is performed (e.g., via LTO).

@node Static Semantics,,Legality Rules<3>,Structural Generic Instantiation
@anchor{gnat_rm/gnat_language_extensions static-semantics}@anchor{488}
@subsubsection Static Semantics


A @code{structural_generic_instance_name} denotes the instance that is the
product of the structural instantiation of a generic unit on the specified
actual parameters. This instance is unique to a partition, except when a
generic subprogram instantiation is automatically demoted to a local
instantiation as described under Legality Rules.

Example:

@example
with Ada.Containers.Vectors;

procedure P is
   V : Ada.Containers.Vectors(Positive,Integer).Vector;

begin
   V.Append (1);
   V.Append (0);
   Ada.Containers.Vectors(Positive,Integer).Generic_Sorting("<").Sort (V);
end;
@end example

This procedure references two structural instantiations of two different generic
units: @code{Ada.Containers.Vectors(Positive,Integer)} is the structural instance
of the generic unit @code{Ada.Containers.Vectors} on @code{Positive} and @code{Integer}
and @code{Ada.Containers.Vectors(Positive,Integer).Generic_Sorting("<")} is the
structural instance of the nested generic unit
@code{Ada.Containers.Vectors(Positive,Integer).Generic_Sorting} on @code{"<"}.

Note that the following example is illegal:

@example
with Ada.Containers.Vectors;

package Q is
   type T is record
      I : Integer;
   end record;

   V : Ada.Containers.Vectors(Positive,T).Vector;
end Q;
@end example

The reason is that @code{Ada.Containers.Vectors}, @code{Positive} and @code{Q.T} being
library-level entities, the structural instance @code{Ada.Containers.Vectors(Positive,T)} is a library unit with a dependence
on @code{Q} and, therefore, cannot be referenced from within @code{Q}. This
restriction applies to structural instantiations of generic packages. The
simple way out is to declare a traditional instantiation in this case:

@example
with Ada.Containers.Vectors;

package Q is
   type T is record
      I : Integer;
   end record;

   package Vectors_Of_T is new Ada.Containers.Vectors(Positive,T);

   V : Vectors_Of_T.Vector;
end Q;
@end example

But the following example is legal:

@example
with Ada.Containers.Vectors;

procedure P is
   type T is record
      I : Integer;
   end record;

   V : Ada.Containers.Vectors(Positive,T).Vector;
end;
@end example

because the structural instance @code{Ada.Containers.Vectors(Positive,T)} is
not a library unit.

For generic subprograms, the restriction does not apply: if a local entity of
a library-level package is used as an actual, the structural instantiation is
automatically demoted to a local instantiation. For example:

@example
with Ada.Unchecked_Deallocation;

package Q is
   type T is record
      I : Integer;
   end record;

   type T_Access is access T;
end Q;

package body Q is
   procedure Free_T (X : in out T_Access) is
   begin
      Ada.Unchecked_Deallocation(T, T_Access)(X);
   end Free_T;
end Q;
@end example

is legal: since @code{T} and @code{T_Access} are local entities of @code{Q}, the
structural instantiation of @code{Ada.Unchecked_Deallocation} is demoted to a
local instantiation rather than producing an error. Note that the uniqueness
guarantee no longer holds in this case and several instances of the generic
subprogram may be present in a single partition.

The first example can be rewritten in a less verbose manner:

@example
with Ada.Containers.Vectors; use Ada.Containers.Vectors(Positive,Integer);

procedure P is
   V : Vector;

begin
   V.Append (1);
   V.Append (0);
   Generic_Sorting("<").Sort (V);
end;
@end example

Another example, which additionally uses the inference of dependent types:

@example
with Ada.Unchecked_Deallocation;

procedure P is

   type Integer_Access is access all Integer;

   A : Integer_Access := new Integer'(1);

begin
   Ada.Unchecked_Deallocation(Name => Integer_Access) (A);
end;
@end example

@node Security Hardening Features,Obsolescent Features,GNAT language extensions,Top
@anchor{gnat_rm/security_hardening_features doc}@anchor{489}@anchor{gnat_rm/security_hardening_features id1}@anchor{48a}@anchor{gnat_rm/security_hardening_features security-hardening-features}@anchor{16}
@chapter Security Hardening Features


This chapter describes Ada extensions aimed at security hardening that
are provided by GNAT.

The features in this chapter are currently experimental and subject to
change.

These features are supported only by the GCC back end, not by LLVM.

@c Register Scrubbing:

@menu
* Register Scrubbing:: 
* Stack Scrubbing:: 
* Hardened Conditionals:: 
* Hardened Booleans:: 
* Control Flow Redundancy:: 

@end menu

@node Register Scrubbing,Stack Scrubbing,,Security Hardening Features
@anchor{gnat_rm/security_hardening_features register-scrubbing}@anchor{48b}
@section Register Scrubbing


GNAT can generate code to zero-out hardware registers before returning
from a subprogram.

It can be enabled with the @code{-fzero-call-used-regs=`choice'}
command-line option, to affect all subprograms in a compilation, and
with a @code{Machine_Attribute} pragma, to affect only specific
subprograms.

@example
procedure Foo;
pragma Machine_Attribute (Foo, "zero_call_used_regs", "used");
--  Before returning, Foo scrubs only call-clobbered registers
--  that it uses itself.

function Bar return Integer;
pragma Machine_Attribute (Bar, "zero_call_used_regs", "all");
--  Before returning, Bar scrubs all call-clobbered registers.

function Baz return Integer;
pragma Machine_Attribute (Bar, "zero_call_used_regs", "leafy");
--  Before returning, Bar scrubs call-clobbered registers, either
--  those it uses itself, if it can be identified as a leaf
--  function, or all of them otherwise.
@end example

For usage and more details on the command-line option, on the
@code{zero_call_used_regs} attribute, and on their use with other
programming languages, see @cite{Using the GNU Compiler Collection (GCC)}.

@c Stack Scrubbing:

@node Stack Scrubbing,Hardened Conditionals,Register Scrubbing,Security Hardening Features
@anchor{gnat_rm/security_hardening_features stack-scrubbing}@anchor{48c}
@section Stack Scrubbing


GNAT can generate code to zero-out stack frames used by subprograms.

It can be activated with the @code{Machine_Attribute} pragma, on
specific subprograms and variables, or their types.  (This attribute
always applies to a type, even when it is associated with a subprogram
or a variable.)

@example
function Foo returns Integer;
pragma Machine_Attribute (Foo, "strub");
--  Foo and its callers are modified so as to scrub the stack
--  space used by Foo after it returns.  Shorthand for:
--  pragma Machine_Attribute (Foo, "strub", "at-calls");

procedure Bar;
pragma Machine_Attribute (Bar, "strub", "internal");
--  Bar is turned into a wrapper for its original body,
--  and they scrub the stack used by the original body.

Var : Integer;
pragma Machine_Attribute (Var, "strub");
--  Reading from Var in a subprogram enables stack scrubbing
--  of the stack space used by the subprogram.  Furthermore, if
--  Var is declared within a subprogram, this also enables
--  scrubbing of the stack space used by that subprogram.
@end example

Given these declarations, Foo has its type and body modified as
follows:

@example
function Foo (<WaterMark> : in out System.Address) returns Integer
is
  --  ...
begin
  <__strub_update> (<WaterMark>);  --  Updates the stack WaterMark.
  --  ...
end;
@end example

whereas its callers are modified from:

@example
X := Foo;
@end example

to:

@example
declare
  <WaterMark> : System.Address;
begin
  <__strub_enter> (<WaterMark>);  -- Initialize <WaterMark>.
  X := Foo (<WaterMark>);
  <__strub_leave> (<WaterMark>);  -- Scrubs stack up to <WaterMark>.
end;
@end example

As for Bar, because it is strubbed in internal mode, its callers are
not modified.  Its definition is modified roughly as follows:

@example
procedure Bar is
  <WaterMark> : System.Address;
  procedure Strubbed_Bar (<WaterMark> : in out System.Address) is
  begin
    <__strub_update> (<WaterMark>);  --  Updates the stack WaterMark.
    -- original Bar body.
  end Strubbed_Bar;
begin
  <__strub_enter> (<WaterMark>);  -- Initialize <WaterMark>.
  Strubbed_Bar (<WaterMark>);
  <__strub_leave> (<WaterMark>);  -- Scrubs stack up to <WaterMark>.
end Bar;
@end example

There are also @code{-fstrub=`choice'} command-line options to
control default settings.  For usage and more details on the
command-line options, on the @code{strub} attribute, and their use with
other programming languages, see @cite{Using the GNU Compiler Collection (GCC)}.

Note that Ada secondary stacks are not scrubbed.  The restriction
@code{No_Secondary_Stack} avoids their use, and thus their accidental
preservation of data that should be scrubbed.

Attributes @code{Access} and @code{Unconstrained_Access} of variables and
constants with @code{strub} enabled require types with @code{strub} enabled;
there is no way to express an access-to-strub type otherwise.
@code{Unchecked_Access} bypasses this constraint, but the resulting
access type designates a non-strub type.

@example
VI : aliased Integer;
pragma Machine_Attribute (VI, "strub");
XsVI : access Integer := VI'Access; -- Error.
UXsVI : access Integer := VI'Unchecked_Access; -- OK,
--  UXsVI does *not* enable strub in subprograms that
--  dereference it to obtain the UXsVI.all value.

type Strub_Int is new Integer;
pragma Machine_Attribute (Strub_Int, "strub");
VSI : aliased Strub_Int;
XsVSI : access Strub_Int := VSI'Access; -- OK,
--  VSI and XsVSI.all both enable strub in subprograms that
--  read their values.
@end example

Every access-to-subprogram type, renaming, and overriding and
overridden dispatching operations that may refer to a subprogram with
an attribute-modified interface must be annotated with the same
interface-modifying attribute.  Access-to-subprogram types can be
explicitly converted to different strub modes, as long as they are
interface-compatible (i.e., adding or removing @code{at-calls} is not
allowed).  For example, a @code{strub}-@code{disabled} subprogram can be
turned @code{callable} through such an explicit conversion:

@example
type TBar is access procedure;

type TBar_Callable is access procedure;
pragma Machine_Attribute (TBar_Callable, "strub", "callable");
--  The attribute modifies the procedure type, rather than the
--  access type, because of the extra argument after "strub",
--  only applicable to subprogram types.

Bar_Callable_Ptr : constant TBar_Callable
           := TBar_Callable (TBar'(Bar'Access));

procedure Bar_Callable renames Bar_Callable_Ptr.all;
pragma Machine_Attribute (Bar_Callable, "strub", "callable");
@end example

Note that the renaming declaration is expanded to a full subprogram
body, it won’t be just an alias.  Only if it is inlined will it be as
efficient as a call by dereferencing the access-to-subprogram constant
Bar_Callable_Ptr.

@c Hardened Conditionals:

@node Hardened Conditionals,Hardened Booleans,Stack Scrubbing,Security Hardening Features
@anchor{gnat_rm/security_hardening_features hardened-conditionals}@anchor{48d}
@section Hardened Conditionals


GNAT can harden conditionals to protect against control-flow attacks.

This is accomplished by two complementary transformations, each
activated by a separate command-line option.

The option @code{-fharden-compares} enables hardening of compares
that compute results stored in variables, adding verification that the
reversed compare yields the opposite result, turning:

@example
B := X = Y;
@end example

into:

@example
B := X = Y;
declare
  NotB : Boolean := X /= Y; -- Computed independently of B.
begin
  if B = NotB then
    <__builtin_trap>;
  end if;
end;
@end example

The option @code{-fharden-conditional-branches} enables hardening
of compares that guard conditional branches, adding verification of
the reversed compare to both execution paths, turning:

@example
if X = Y then
  X := Z + 1;
else
  Y := Z - 1;
end if;
@end example

into:

@example
if X = Y then
  if X /= Y then -- Computed independently of X = Y.
    <__builtin_trap>;
  end if;
  X := Z + 1;
else
  if X /= Y then -- Computed independently of X = Y.
    null;
  else
    <__builtin_trap>;
  end if;
  Y := Z - 1;
end if;
@end example

These transformations are introduced late in the compilation pipeline,
long after boolean expressions are decomposed into separate compares,
each one turned into either a conditional branch or a compare whose
result is stored in a boolean variable or temporary.  Compiler
optimizations, if enabled, may also turn conditional branches into
stored compares, and vice-versa, or into operations with implied
conditionals (e.g. MIN and MAX).  Conditionals may also be optimized
out entirely, if their value can be determined at compile time, and
occasionally multiple compares can be combined into one.

It is thus difficult to predict which of these two options will affect
a specific compare operation expressed in source code.  Using both
options ensures that every compare that is neither optimized out nor
optimized into implied conditionals will be hardened.

The addition of reversed compares can be observed by enabling the dump
files of the corresponding passes, through command-line options
@code{-fdump-tree-hardcmp} and @code{-fdump-tree-hardcbr},
respectively.

They are separate options, however, because of the significantly
different performance impact of the hardening transformations.

For usage and more details on the command-line options, see
@cite{Using the GNU Compiler Collection (GCC)}.  These options can
be used with other programming languages supported by GCC.

@c Hardened Booleans:

@node Hardened Booleans,Control Flow Redundancy,Hardened Conditionals,Security Hardening Features
@anchor{gnat_rm/security_hardening_features hardened-booleans}@anchor{48e}
@section Hardened Booleans


Ada has built-in support for introducing boolean types with
alternative representations, using representation clauses:

@example
type HBool is new Boolean;
for HBool use (16#5a#, 16#a5#);
for HBool'Size use 8;
@end example

When validity checking is enabled, the compiler will check that
variables of such types hold values corresponding to the selected
representations.

There are multiple strategies for where to introduce validity checking
(see @code{-gnatV} options).  Their goal is to guard against
various kinds of programming errors, and GNAT strives to omit checks
when program logic rules out an invalid value, and optimizers may
further remove checks found to be redundant.

For additional hardening, the @code{hardbool} @code{Machine_Attribute}
pragma can be used to annotate boolean types with representation
clauses, so that expressions of such types used as conditions are
checked even when compiling with @code{-gnatVT}:

@example
pragma Machine_Attribute (HBool, "hardbool");

function To_Boolean (X : HBool) returns Boolean is (Boolean (X));
@end example

is compiled roughly like:

@example
function To_Boolean (X : HBool) returns Boolean is
begin
  if X not in True | False then
    raise Constraint_Error;
  elsif X in True then
    return True;
  else
    return False;
  end if;
end To_Boolean;
@end example

Note that @code{-gnatVn} will disable even @code{hardbool} testing.

Analogous behavior is available as a GCC extension to the C and
Objective C programming languages, through the @code{hardbool} attribute,
with the difference that, instead of raising a Constraint_Error
exception, when a hardened boolean variable is found to hold a value
that stands for neither True nor False, the program traps.  For usage
and more details on that attribute, see @cite{Using the GNU Compiler Collection (GCC)}.

@c Control Flow Redundancy:

@node Control Flow Redundancy,,Hardened Booleans,Security Hardening Features
@anchor{gnat_rm/security_hardening_features control-flow-redundancy}@anchor{48f}
@section Control Flow Redundancy


GNAT can guard against unexpected execution flows, such as branching
into the middle of subprograms, as in Return Oriented Programming
exploits.

In units compiled with @code{-fharden-control-flow-redundancy},
subprograms are instrumented so that, every time they are called,
basic blocks take note as control flows through them, and, before
returning, subprograms verify that the taken notes are consistent with
the control-flow graph.

The performance impact of verification on leaf subprograms can be much
higher, while the averted risks are much lower on them.
Instrumentation can be disabled for leaf subprograms with
@code{-fhardcfr-skip-leaf}.

Functions with too many basic blocks, or with multiple return points,
call a run-time function to perform the verification.  Other functions
perform the verification inline before returning.

Optimizing the inlined verification can be quite time consuming, so
the default upper limit for the inline mode is set at 16 blocks.
Command-line option @code{--param hardcfr-max-inline-blocks=} can
override it.

Even though typically sparse control-flow graphs exhibit run-time
verification time nearly proportional to the block count of a
subprogram, it may become very significant for generated subprograms
with thousands of blocks.  Command-line option
@code{--param hardcfr-max-blocks=} can set an upper limit for
instrumentation.

For each block that is marked as visited, the mechanism checks that at
least one of its predecessors, and at least one of its successors, are
also marked as visited.

Verification is performed just before a subprogram returns.  The
following fragment:

@example
if X then
  Y := F (Z);
  return;
end if;
@end example

gets turned into:

@example
type Visited_Bitmap is array (1..N) of Boolean with Pack;
Visited : aliased Visited_Bitmap := (others => False);
--  Bitmap of visited blocks.  N is the basic block count.
[...]
--  Basic block #I
Visited(I) := True;
if X then
  --  Basic block #J
  Visited(J) := True;
  Y := F (Z);
  CFR.Check (N, Visited'Access, CFG'Access);
  --  CFR is a hypothetical package whose Check procedure calls
  --  libgcc's __hardcfr_check, that traps if the Visited bitmap
  --  does not hold a valid path in CFG, the run-time
  --  representation of the control flow graph in the enclosing
  --  subprogram.
  return;
end if;
--  Basic block #K
Visited(K) := True;
@end example

Verification would also be performed before tail calls, if any
front-ends marked them as mandatory or desirable, but none do.
Regular calls are optimized into tail calls too late for this
transformation to act on it.

In order to avoid adding verification after potential tail calls,
which would prevent tail-call optimization, we recognize returning
calls, i.e., calls whose result, if any, is returned by the calling
subprogram to its caller immediately after the call returns.
Verification is performed before such calls, whether or not they are
ultimately optimized to tail calls.  This behavior is enabled by
default whenever sibcall optimization is enabled (see
@code{-foptimize-sibling-calls}); it may be disabled with
@code{-fno-hardcfr-check-returning-calls}, or enabled with
@code{-fhardcfr-check-returning-calls}, regardless of the
optimization, but the lack of other optimizations may prevent calls
from being recognized as returning calls:

@example
--  CFR.Check here, with -fhardcfr-check-returning-calls.
P (X);
--  CFR.Check here, with -fno-hardcfr-check-returning-calls.
return;
@end example

or:

@example
--  CFR.Check here, with -fhardcfr-check-returning-calls.
R := F (X);
--  CFR.Check here, with -fno-hardcfr-check-returning-calls.
return R;
@end example

Any subprogram from which an exception may escape, i.e., that may
raise or propagate an exception that isn’t handled internally, is
conceptually enclosed by a cleanup handler that performs verification,
unless this is disabled with @code{-fno-hardcfr-check-exceptions}.
With this feature enabled, a subprogram body containing:

@example
--  ...
  Y := F (X);  -- May raise exceptions.
--  ...
  raise E;  -- Not handled internally.
--  ...
@end example

gets modified as follows:

@example
begin
  --  ...
    Y := F (X);  -- May raise exceptions.
  --  ...
    raise E;  -- Not handled internally.
  --  ...
exception
  when others =>
    CFR.Check (N, Visited'Access, CFG'Access);
    raise;
end;
@end example

Verification may also be performed before No_Return calls, whether all
of them, with @code{-fhardcfr-check-noreturn-calls=always}; all but
internal subprograms involved in exception-raising or -reraising or
subprograms explicitly marked with both @code{No_Return} and
@code{Machine_Attribute} @code{expected_throw} pragmas, with
@code{-fhardcfr-check-noreturn-calls=no-xthrow} (default); only
nothrow ones, with @code{-fhardcfr-check-noreturn-calls=nothrow};
or none, with @code{-fhardcfr-check-noreturn-calls=never}.

When a No_Return call returns control to its caller through an
exception, verification may have already been performed before the
call, if @code{-fhardcfr-check-noreturn-calls=always} or
@code{-fhardcfr-check-noreturn-calls=no-xthrow} is in effect.  The
compiler arranges for already-checked No_Return calls without a
preexisting handler to bypass the implicitly-added cleanup handler and
thus the redundant check, but a local exception or cleanup handler, if
present, will modify the set of visited blocks, and checking will take
place again when the caller reaches the next verification point,
whether it is a return or reraise statement after the exception is
otherwise handled, or even another No_Return call.

The instrumentation for hardening with control flow redundancy can be
observed in dump files generated by the command-line option
@code{-fdump-tree-hardcfr}.

For more details on the control flow redundancy command-line options,
see @cite{Using the GNU Compiler Collection (GCC)}.  These options
can be used with other programming languages supported by GCC.

@node Obsolescent Features,Compatibility and Porting Guide,Security Hardening Features,Top
@anchor{gnat_rm/obsolescent_features doc}@anchor{490}@anchor{gnat_rm/obsolescent_features id1}@anchor{491}@anchor{gnat_rm/obsolescent_features obsolescent-features}@anchor{17}
@chapter Obsolescent Features


This chapter describes features that are provided by GNAT, but are
considered obsolescent since there are other, more appropriate, ways
of achieving the same effect. These features are provided solely for
historical compatibility purposes.

@menu
* PolyORB:: 
* pragma No_Run_Time:: 
* pragma Ravenscar:: 
* pragma Restricted_Run_Time:: 
* pragma Task_Info:: 
* package System.Task_Info (s-tasinf.ads): package System Task_Info s-tasinf ads. 

@end menu

@node PolyORB,pragma No_Run_Time,,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id2}@anchor{492}@anchor{gnat_rm/obsolescent_features polyorb}@anchor{273}
@section PolyORB


PolyORB is a deprecated product. It will be baselined with the GNAT Pro
release 28. After this release, there will be no new versions of this
product. Contact AdaCore support to get recommendations for replacements.

@node pragma No_Run_Time,pragma Ravenscar,PolyORB,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id3}@anchor{493}@anchor{gnat_rm/obsolescent_features pragma-no-run-time}@anchor{494}
@section pragma No_Run_Time


The pragma @code{No_Run_Time} is used to achieve an affect similar
to the use of the “Zero Foot Print” configurable run time, but without
requiring a specially configured run time. The result of using this
pragma, which must be used for all units in a partition, is to restrict
the use of any language features requiring run-time support code. The
preferred usage is to use an appropriately configured run-time that
includes just those features that are to be made accessible.

@node pragma Ravenscar,pragma Restricted_Run_Time,pragma No_Run_Time,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id4}@anchor{495}@anchor{gnat_rm/obsolescent_features pragma-ravenscar}@anchor{496}
@section pragma Ravenscar


The pragma @code{Ravenscar} has exactly the same effect as pragma
@code{Profile (Ravenscar)}. The latter usage is preferred since it
is part of the new Ada 2005 standard.

@node pragma Restricted_Run_Time,pragma Task_Info,pragma Ravenscar,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id5}@anchor{497}@anchor{gnat_rm/obsolescent_features pragma-restricted-run-time}@anchor{498}
@section pragma Restricted_Run_Time


The pragma @code{Restricted_Run_Time} has exactly the same effect as
pragma @code{Profile (Restricted)}. The latter usage is
preferred since the Ada 2005 pragma @code{Profile} is intended for
this kind of implementation dependent addition.

@node pragma Task_Info,package System Task_Info s-tasinf ads,pragma Restricted_Run_Time,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id6}@anchor{499}@anchor{gnat_rm/obsolescent_features pragma-task-info}@anchor{49a}
@section pragma Task_Info


The functionality provided by pragma @code{Task_Info} is now part of the
Ada language. The @code{CPU} aspect and the package
@code{System.Multiprocessors} offer a less system-dependent way to specify
task affinity or to query the number of processors.

Syntax

@example
pragma Task_Info (EXPRESSION);
@end example

This pragma appears within a task definition (like pragma
@code{Priority}) and applies to the task in which it appears.  The
argument must be of type @code{System.Task_Info.Task_Info_Type}.
The @code{Task_Info} pragma provides system dependent control over
aspects of tasking implementation, for example, the ability to map
tasks to specific processors.  For details on the facilities available
for the version of GNAT that you are using, see the documentation
in the spec of package System.Task_Info in the runtime
library.

@node package System Task_Info s-tasinf ads,,pragma Task_Info,Obsolescent Features
@anchor{gnat_rm/obsolescent_features package-system-task-info}@anchor{49b}@anchor{gnat_rm/obsolescent_features package-system-task-info-s-tasinf-ads}@anchor{49c}
@section package System.Task_Info (@code{s-tasinf.ads})


This package provides target dependent functionality that is used
to support the @code{Task_Info} pragma. The predefined Ada package
@code{System.Multiprocessors} and the @code{CPU} aspect now provide a
standard replacement for GNAT’s @code{Task_Info} functionality.

@node Compatibility and Porting Guide,GNU Free Documentation License,Obsolescent Features,Top
@anchor{gnat_rm/compatibility_and_porting_guide doc}@anchor{49d}@anchor{gnat_rm/compatibility_and_porting_guide compatibility-and-porting-guide}@anchor{18}@anchor{gnat_rm/compatibility_and_porting_guide id1}@anchor{49e}
@chapter Compatibility and Porting Guide


This chapter presents some guidelines for developing portable Ada code,
describes the compatibility issues that may arise between
GNAT and other Ada compilation systems (including those for Ada 83),
and shows how GNAT can expedite porting
applications developed in other Ada environments.

@menu
* Writing Portable Fixed-Point Declarations:: 
* Compatibility with Ada 83:: 
* Compatibility between Ada 95 and Ada 2005:: 
* Implementation-dependent characteristics:: 
* Compatibility with Other Ada Systems:: 
* Representation Clauses:: 
* Compatibility with HP Ada 83:: 

@end menu

@node Writing Portable Fixed-Point Declarations,Compatibility with Ada 83,,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide id2}@anchor{49f}@anchor{gnat_rm/compatibility_and_porting_guide writing-portable-fixed-point-declarations}@anchor{4a0}
@section Writing Portable Fixed-Point Declarations


The Ada Reference Manual gives an implementation freedom to choose bounds
that are narrower by @code{Small} from the given bounds.
For example, if we write

@example
type F1 is delta 1.0 range -128.0 .. +128.0;
@end example

then the implementation is allowed to choose -128.0 .. +127.0 if it
likes, but is not required to do so.

This leads to possible portability problems, so let’s have a closer
look at this, and figure out how to avoid these problems.

First, why does this freedom exist, and why would an implementation
take advantage of it? To answer this, take a closer look at the type
declaration for @code{F1} above. If the compiler uses the given bounds,
it would need 9 bits to hold the largest positive value (and typically
that means 16 bits on all machines). But if the implementation chooses
the +127.0 bound then it can fit values of the type in 8 bits.

Why not make the user write +127.0 if that’s what is wanted?
The rationale is that if you are thinking of fixed point
as a kind of ‘poor man’s floating-point’, then you don’t want
to be thinking about the scaled integers that are used in its
representation. Let’s take another example:

@example
type F2 is delta 2.0**(-15) range -1.0 .. +1.0;
@end example

Looking at this declaration, it seems casually as though
it should fit in 16 bits, but again that extra positive value
+1.0 has the scaled integer equivalent of 2**15 which is one too
big for signed 16 bits. The implementation can treat this as:

@example
type F2 is delta 2.0**(-15) range -1.0 .. +1.0-(2.0**(-15));
@end example

and the Ada language design team felt that this was too annoying
to require. We don’t need to debate this decision at this point,
since it is well established (the rule about narrowing the ranges
dates to Ada 83).

But the important point is that an implementation is not required
to do this narrowing, so we have a potential portability problem.
We could imagine three types of implementation:


@enumerate a

@item 
those that narrow the range automatically if they can figure
out that the narrower range will allow storage in a smaller machine unit,

@item 
those that will narrow only if forced to by a @code{'Size} clause, and

@item 
those that will never narrow.
@end enumerate

Now if we are language theoreticians, we can imagine a fourth
approach: to narrow all the time, e.g. to treat

@example
type F3 is delta 1.0 range -10.0 .. +23.0;
@end example

as though it had been written:

@example
type F3 is delta 1.0 range -9.0 .. +22.0;
@end example

But although technically allowed, such a behavior would be hostile and silly,
and no real compiler would do this. All real compilers will fall into one of
the categories (a), (b) or (c) above.

So, how do you get the compiler to do what you want? The answer is give the
actual bounds you want, and then use a @code{'Small} clause and a
@code{'Size} clause to absolutely pin down what the compiler does.
E.g., for @code{F2} above, we will write:

@example
My_Small : constant := 2.0**(-15);
My_First : constant := -1.0;
My_Last  : constant := +1.0 - My_Small;

type F2 is delta My_Small range My_First .. My_Last;
@end example

and then add

@example
for F2'Small use my_Small;
for F2'Size  use 16;
@end example

In practice all compilers will do the same thing here and will give you
what you want, so the above declarations are fully portable. If you really
want to play language lawyer and guard against ludicrous behavior by the
compiler you could add

@example
Test1 : constant := 1 / Boolean'Pos (F2'First = My_First);
Test2 : constant := 1 / Boolean'Pos (F2'Last  = My_Last);
@end example

One or other or both are allowed to be illegal if the compiler is
behaving in a silly manner, but at least the silly compiler will not
get away with silently messing with your (very clear) intentions.

If you follow this scheme you will be guaranteed that your fixed-point
types will be portable.

@node Compatibility with Ada 83,Compatibility between Ada 95 and Ada 2005,Writing Portable Fixed-Point Declarations,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-with-ada-83}@anchor{4a1}@anchor{gnat_rm/compatibility_and_porting_guide id3}@anchor{4a2}
@section Compatibility with Ada 83


@geindex Compatibility (between Ada 83 and Ada 95 / Ada 2005 / Ada 2012 / Ada 2022)

Ada 95 and the subsequent revisions Ada 2005, Ada 2012, Ada 2022
are highly upwards compatible with Ada 83.  In
particular, the design intention was that the difficulties associated
with moving from Ada 83 to later versions of the standard should be no greater
than those that occur when moving from one Ada 83 system to another.

However, there are a number of points at which there are minor
incompatibilities.  The @cite{Ada 95 Annotated Reference Manual} contains
full details of these issues as they relate to Ada 95,
and should be consulted for a complete treatment.
In practice the
following subsections treat the most likely issues to be encountered.

@menu
* Legal Ada 83 programs that are illegal in Ada 95:: 
* More deterministic semantics:: 
* Changed semantics:: 
* Other language compatibility issues:: 

@end menu

@node Legal Ada 83 programs that are illegal in Ada 95,More deterministic semantics,,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide id4}@anchor{4a3}@anchor{gnat_rm/compatibility_and_porting_guide legal-ada-83-programs-that-are-illegal-in-ada-95}@anchor{4a4}
@subsection Legal Ada 83 programs that are illegal in Ada 95


Some legal Ada 83 programs are illegal (i.e., they will fail to compile) in
Ada 95 and later versions of the standard:


@itemize *

@item 
`Character literals'

Some uses of character literals are ambiguous.  Since Ada 95 has introduced
@code{Wide_Character} as a new predefined character type, some uses of
character literals that were legal in Ada 83 are illegal in Ada 95.
For example:

@example
for Char in 'A' .. 'Z' loop ... end loop;
@end example

The problem is that ‘A’ and ‘Z’ could be from either
@code{Character} or @code{Wide_Character}.  The simplest correction
is to make the type explicit; e.g.:

@example
for Char in Character range 'A' .. 'Z' loop ... end loop;
@end example

@item 
`New reserved words'

The identifiers @code{abstract}, @code{aliased}, @code{protected},
@code{requeue}, @code{tagged}, and @code{until} are reserved in Ada 95.
Existing Ada 83 code using any of these identifiers must be edited to
use some alternative name.

@item 
`Freezing rules'

The rules in Ada 95 are slightly different with regard to the point at
which entities are frozen, and representation pragmas and clauses are
not permitted past the freeze point.  This shows up most typically in
the form of an error message complaining that a representation item
appears too late, and the appropriate corrective action is to move
the item nearer to the declaration of the entity to which it refers.

A particular case is that representation pragmas
cannot be applied to a subprogram body.  If necessary, a separate subprogram
declaration must be introduced to which the pragma can be applied.

@item 
`Optional bodies for library packages'

In Ada 83, a package that did not require a package body was nevertheless
allowed to have one.  This lead to certain surprises in compiling large
systems (situations in which the body could be unexpectedly ignored by the
binder).  In Ada 95, if a package does not require a body then it is not
permitted to have a body.  To fix this problem, simply remove a redundant
body if it is empty, or, if it is non-empty, introduce a dummy declaration
into the spec that makes the body required.  One approach is to add a private
part to the package declaration (if necessary), and define a parameterless
procedure called @code{Requires_Body}, which must then be given a dummy
procedure body in the package body, which then becomes required.
Another approach (assuming that this does not introduce elaboration
circularities) is to add an @code{Elaborate_Body} pragma to the package spec,
since one effect of this pragma is to require the presence of a package body.

@item 
`Numeric_Error is the same exception as Constraint_Error'

In Ada 95, the exception @code{Numeric_Error} is a renaming of @code{Constraint_Error}.
This means that it is illegal to have separate exception handlers for
the two exceptions.  The fix is simply to remove the handler for the
@code{Numeric_Error} case (since even in Ada 83, a compiler was free to raise
@code{Constraint_Error} in place of @code{Numeric_Error} in all cases).

@item 
`Indefinite subtypes in generics'

In Ada 83, it was permissible to pass an indefinite type (e.g, @code{String})
as the actual for a generic formal private type, but then the instantiation
would be illegal if there were any instances of declarations of variables
of this type in the generic body.  In Ada 95, to avoid this clear violation
of the methodological principle known as the ‘contract model’,
the generic declaration explicitly indicates whether
or not such instantiations are permitted.  If a generic formal parameter
has explicit unknown discriminants, indicated by using @code{(<>)} after the
subtype name, then it can be instantiated with indefinite types, but no
stand-alone variables can be declared of this type.  Any attempt to declare
such a variable will result in an illegality at the time the generic is
declared.  If the @code{(<>)} notation is not used, then it is illegal
to instantiate the generic with an indefinite type.
This is the potential incompatibility issue when porting Ada 83 code to Ada 95.
It will show up as a compile time error, and
the fix is usually simply to add the @code{(<>)} to the generic declaration.
@end itemize

@node More deterministic semantics,Changed semantics,Legal Ada 83 programs that are illegal in Ada 95,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide id5}@anchor{4a5}@anchor{gnat_rm/compatibility_and_porting_guide more-deterministic-semantics}@anchor{4a6}
@subsection More deterministic semantics



@itemize *

@item 
`Conversions'

Conversions from real types to integer types round away from 0.  In Ada 83
the conversion Integer(2.5) could deliver either 2 or 3 as its value.  This
implementation freedom was intended to support unbiased rounding in
statistical applications, but in practice it interfered with portability.
In Ada 95 the conversion semantics are unambiguous, and rounding away from 0
is required.  Numeric code may be affected by this change in semantics.
Note, though, that this issue is no worse than already existed in Ada 83
when porting code from one vendor to another.

@item 
`Tasking'

The Real-Time Annex introduces a set of policies that define the behavior of
features that were implementation dependent in Ada 83, such as the order in
which open select branches are executed.
@end itemize

@node Changed semantics,Other language compatibility issues,More deterministic semantics,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide changed-semantics}@anchor{4a7}@anchor{gnat_rm/compatibility_and_porting_guide id6}@anchor{4a8}
@subsection Changed semantics


The worst kind of incompatibility is one where a program that is legal in
Ada 83 is also legal in Ada 95 but can have an effect in Ada 95 that was not
possible in Ada 83.  Fortunately this is extremely rare, but the one
situation that you should be alert to is the change in the predefined type
@code{Character} from 7-bit ASCII to 8-bit Latin-1.

@quotation

@geindex Latin-1
@end quotation


@itemize *

@item 
`Range of type `@w{`}Character`@w{`}'

The range of @code{Standard.Character} is now the full 256 characters
of Latin-1, whereas in most Ada 83 implementations it was restricted
to 128 characters. Although some of the effects of
this change will be manifest in compile-time rejection of legal
Ada 83 programs it is possible for a working Ada 83 program to have
a different effect in Ada 95, one that was not permitted in Ada 83.
As an example, the expression
@code{Character'Pos(Character'Last)} returned @code{127} in Ada 83 and now
delivers @code{255} as its value.
In general, you should look at the logic of any
character-processing Ada 83 program and see whether it needs to be adapted
to work correctly with Latin-1.  Note that the predefined Ada 95 API has a
character handling package that may be relevant if code needs to be adapted
to account for the additional Latin-1 elements.
The desirable fix is to
modify the program to accommodate the full character set, but in some cases
it may be convenient to define a subtype or derived type of Character that
covers only the restricted range.
@end itemize

@node Other language compatibility issues,,Changed semantics,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide id7}@anchor{4a9}@anchor{gnat_rm/compatibility_and_porting_guide other-language-compatibility-issues}@anchor{4aa}
@subsection Other language compatibility issues



@itemize *

@item 
`-gnat83' switch

All implementations of GNAT provide a switch that causes GNAT to operate
in Ada 83 mode.  In this mode, some but not all compatibility problems
of the type described above are handled automatically.  For example, the
new reserved words introduced in Ada 95 and Ada 2005 are treated simply
as identifiers as in Ada 83.  However,
in practice, it is usually advisable to make the necessary modifications
to the program to remove the need for using this switch.
See the @code{Compiling Different Versions of Ada} section in
the @cite{GNAT User’s Guide}.

@item 
Support for removed Ada 83 pragmas and attributes

A number of pragmas and attributes from Ada 83 were removed from Ada 95,
generally because they were replaced by other mechanisms.  Ada 95 and Ada 2005
compilers are allowed, but not required, to implement these missing
elements.  In contrast with some other compilers, GNAT implements all
such pragmas and attributes, eliminating this compatibility concern.  These
include @code{pragma Interface} and the floating point type attributes
(@code{Emax}, @code{Mantissa}, etc.), among other items.
@end itemize

@node Compatibility between Ada 95 and Ada 2005,Implementation-dependent characteristics,Compatibility with Ada 83,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-between-ada-95-and-ada-2005}@anchor{4ab}@anchor{gnat_rm/compatibility_and_porting_guide id8}@anchor{4ac}
@section Compatibility between Ada 95 and Ada 2005


@geindex Compatibility between Ada 95 and Ada 2005

Although Ada 2005 was designed to be upwards compatible with Ada 95, there are
a number of incompatibilities. Several are enumerated below;
for a complete description please see the
@cite{Annotated Ada 2005 Reference Manual}, or section 9.1.1 in
@cite{Rationale for Ada 2005}.


@itemize *

@item 
`New reserved words.'

The words @code{interface}, @code{overriding} and @code{synchronized} are
reserved in Ada 2005.
A pre-Ada 2005 program that uses any of these as an identifier will be
illegal.

@item 
`New declarations in predefined packages.'

A number of packages in the predefined environment contain new declarations:
@code{Ada.Exceptions}, @code{Ada.Real_Time}, @code{Ada.Strings},
@code{Ada.Strings.Fixed}, @code{Ada.Strings.Bounded},
@code{Ada.Strings.Unbounded}, @code{Ada.Strings.Wide_Fixed},
@code{Ada.Strings.Wide_Bounded}, @code{Ada.Strings.Wide_Unbounded},
@code{Ada.Tags}, @code{Ada.Text_IO}, and @code{Interfaces.C}.
If an Ada 95 program does a @code{with} and @code{use} of any of these
packages, the new declarations may cause name clashes.

@item 
`Access parameters.'

A nondispatching subprogram with an access parameter cannot be renamed
as a dispatching operation.  This was permitted in Ada 95.

@item 
`Access types, discriminants, and constraints.'

Rule changes in this area have led to some incompatibilities; for example,
constrained subtypes of some access types are not permitted in Ada 2005.

@item 
`Aggregates for limited types.'

The allowance of aggregates for limited types in Ada 2005 raises the
possibility of ambiguities in legal Ada 95 programs, since additional types
now need to be considered in expression resolution.

@item 
`Fixed-point multiplication and division.'

Certain expressions involving ‘*’ or ‘/’ for a fixed-point type, which
were legal in Ada 95 and invoked the predefined versions of these operations,
are now ambiguous.
The ambiguity may be resolved either by applying a type conversion to the
expression, or by explicitly invoking the operation from package
@code{Standard}.

@item 
`Return-by-reference types.'

The Ada 95 return-by-reference mechanism has been removed.  Instead, the user
can declare a function returning a value from an anonymous access type.
@end itemize

@node Implementation-dependent characteristics,Compatibility with Other Ada Systems,Compatibility between Ada 95 and Ada 2005,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide id9}@anchor{4ad}@anchor{gnat_rm/compatibility_and_porting_guide implementation-dependent-characteristics}@anchor{4ae}
@section Implementation-dependent characteristics


Although the Ada language defines the semantics of each construct as
precisely as practical, in some situations (for example for reasons of
efficiency, or where the effect is heavily dependent on the host or target
platform) the implementation is allowed some freedom.  In porting Ada 83
code to GNAT, you need to be aware of whether / how the existing code
exercised such implementation dependencies.  Such characteristics fall into
several categories, and GNAT offers specific support in assisting the
transition from certain Ada 83 compilers.

@menu
* Implementation-defined pragmas:: 
* Implementation-defined attributes:: 
* Libraries:: 
* Elaboration order:: 
* Target-specific aspects:: 

@end menu

@node Implementation-defined pragmas,Implementation-defined attributes,,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide id10}@anchor{4af}@anchor{gnat_rm/compatibility_and_porting_guide implementation-defined-pragmas}@anchor{4b0}
@subsection Implementation-defined pragmas


Ada compilers are allowed to supplement the language-defined pragmas, and
these are a potential source of non-portability.  All GNAT-defined pragmas
are described in @ref{7,,Implementation Defined Pragmas},
and these include several that are specifically
intended to correspond to other vendors’ Ada 83 pragmas.
For migrating from VADS, the pragma @code{Use_VADS_Size} may be useful.
For compatibility with HP Ada 83, GNAT supplies the pragmas
@code{Extend_System}, @code{Ident}, @code{Inline_Generic},
@code{Interface_Name}, @code{Passive}, @code{Suppress_All},
and @code{Volatile}.
Other relevant pragmas include @code{External} and @code{Link_With}.
Some vendor-specific
Ada 83 pragmas (@code{Share_Generic}, @code{Subtitle}, and @code{Title}) are
recognized, thus
avoiding compiler rejection of units that contain such pragmas; they are not
relevant in a GNAT context and hence are not otherwise implemented.

@node Implementation-defined attributes,Libraries,Implementation-defined pragmas,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide id11}@anchor{4b1}@anchor{gnat_rm/compatibility_and_porting_guide implementation-defined-attributes}@anchor{4b2}
@subsection Implementation-defined attributes


Analogous to pragmas, the set of attributes may be extended by an
implementation.  All GNAT-defined attributes are described in
@ref{8,,Implementation Defined Attributes},
and these include several that are specifically intended
to correspond to other vendors’ Ada 83 attributes.  For migrating from VADS,
the attribute @code{VADS_Size} may be useful.  For compatibility with HP
Ada 83, GNAT supplies the attributes @code{Bit}, @code{Machine_Size} and
@code{Type_Class}.

@node Libraries,Elaboration order,Implementation-defined attributes,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide id12}@anchor{4b3}@anchor{gnat_rm/compatibility_and_porting_guide libraries}@anchor{4b4}
@subsection Libraries


Vendors may supply libraries to supplement the standard Ada API.  If Ada 83
code uses vendor-specific libraries then there are several ways to manage
this in Ada 95 and later versions of the standard:


@itemize *

@item 
If the source code for the libraries (specs and bodies) are
available, then the libraries can be migrated in the same way as the
application.

@item 
If the source code for the specs but not the bodies are
available, then you can reimplement the bodies.

@item 
Some features introduced by Ada 95 obviate the need for library support.  For
example most Ada 83 vendors supplied a package for unsigned integers.  The
Ada 95 modular type feature is the preferred way to handle this need, so
instead of migrating or reimplementing the unsigned integer package it may
be preferable to retrofit the application using modular types.
@end itemize

@node Elaboration order,Target-specific aspects,Libraries,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide elaboration-order}@anchor{4b5}@anchor{gnat_rm/compatibility_and_porting_guide id13}@anchor{4b6}
@subsection Elaboration order


The implementation can choose any elaboration order consistent with the unit
dependency relationship.  This freedom means that some orders can result in
Program_Error being raised due to an ‘Access Before Elaboration’: an attempt
to invoke a subprogram before its body has been elaborated, or to instantiate
a generic before the generic body has been elaborated.  By default GNAT
attempts to choose a safe order (one that will not encounter access before
elaboration problems) by implicitly inserting @code{Elaborate} or
@code{Elaborate_All} pragmas where
needed.  However, this can lead to the creation of elaboration circularities
and a resulting rejection of the program by gnatbind.  This issue is
thoroughly described in the `Elaboration Order Handling in GNAT' appendix
in the @cite{GNAT User’s Guide}.
In brief, there are several
ways to deal with this situation:


@itemize *

@item 
Modify the program to eliminate the circularities, e.g., by moving
elaboration-time code into explicitly-invoked procedures

@item 
Constrain the elaboration order by including explicit @code{Elaborate_Body} or
@code{Elaborate} pragmas, and then inhibit the generation of implicit
@code{Elaborate_All}
pragmas either globally (as an effect of the `-gnatE' switch) or locally
(by selectively suppressing elaboration checks via pragma
@code{Suppress(Elaboration_Check)} when it is safe to do so).
@end itemize

@node Target-specific aspects,,Elaboration order,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide id14}@anchor{4b7}@anchor{gnat_rm/compatibility_and_porting_guide target-specific-aspects}@anchor{4b8}
@subsection Target-specific aspects


Low-level applications need to deal with machine addresses, data
representations, interfacing with assembler code, and similar issues.  If
such an Ada 83 application is being ported to different target hardware (for
example where the byte endianness has changed) then you will need to
carefully examine the program logic; the porting effort will heavily depend
on the robustness of the original design.  Moreover, Ada 95 (and thus
Ada 2005, Ada 2012, and Ada 2022) are sometimes
incompatible with typical Ada 83 compiler practices regarding implicit
packing, the meaning of the Size attribute, and the size of access values.
GNAT’s approach to these issues is described in @ref{4b9,,Representation Clauses}.

@node Compatibility with Other Ada Systems,Representation Clauses,Implementation-dependent characteristics,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-with-other-ada-systems}@anchor{4ba}@anchor{gnat_rm/compatibility_and_porting_guide id15}@anchor{4bb}
@section Compatibility with Other Ada Systems


If programs avoid the use of implementation dependent and
implementation defined features, as documented in the
@cite{Ada Reference Manual}, there should be a high degree of portability between
GNAT and other Ada systems.  The following are specific items which
have proved troublesome in moving Ada 95 programs from GNAT to other Ada 95
compilers, but do not affect porting code to GNAT.
(As of January 2007, GNAT is the only compiler available for Ada 2005;
the following issues may or may not arise for Ada 2005 programs
when other compilers appear.)


@itemize *

@item 
`Ada 83 Pragmas and Attributes'

Ada 95 compilers are allowed, but not required, to implement the missing
Ada 83 pragmas and attributes that are no longer defined in Ada 95.
GNAT implements all such pragmas and attributes, eliminating this as
a compatibility concern, but some other Ada 95 compilers reject these
pragmas and attributes.

@item 
`Specialized Needs Annexes'

GNAT implements the full set of special needs annexes.  At the
current time, it is the only Ada 95 compiler to do so.  This means that
programs making use of these features may not be portable to other Ada
95 compilation systems.

@item 
`Representation Clauses'

Some other Ada 95 compilers implement only the minimal set of
representation clauses required by the Ada 95 reference manual.  GNAT goes
far beyond this minimal set, as described in the next section.
@end itemize

@node Representation Clauses,Compatibility with HP Ada 83,Compatibility with Other Ada Systems,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide id16}@anchor{4bc}@anchor{gnat_rm/compatibility_and_porting_guide representation-clauses}@anchor{4b9}
@section Representation Clauses


The Ada 83 reference manual was quite vague in describing both the minimal
required implementation of representation clauses, and also their precise
effects.  Ada 95 (and thus also Ada 2005) are much more explicit, but the
minimal set of capabilities required is still quite limited.

GNAT implements the full required set of capabilities in
Ada 95 and Ada 2005, but also goes much further, and in particular
an effort has been made to be compatible with existing Ada 83 usage to the
greatest extent possible.

A few cases exist in which Ada 83 compiler behavior is incompatible with
the requirements in Ada 95 (and thus also Ada 2005).  These are instances of
intentional or accidental dependence on specific implementation dependent
characteristics of these Ada 83 compilers.  The following is a list of
the cases most likely to arise in existing Ada 83 code.


@itemize *

@item 
`Implicit Packing'

Some Ada 83 compilers allowed a Size specification to cause implicit
packing of an array or record.  This could cause expensive implicit
conversions for change of representation in the presence of derived
types, and the Ada design intends to avoid this possibility.
Subsequent AI’s were issued to make it clear that such implicit
change of representation in response to a Size clause is inadvisable,
and this recommendation is represented explicitly in the Ada 95 (and Ada 2005)
Reference Manuals as implementation advice that is followed by GNAT.
The problem will show up as an error
message rejecting the size clause.  The fix is simply to provide
the explicit pragma @code{Pack}, or for more fine tuned control, provide
a Component_Size clause.

@item 
`Meaning of Size Attribute'

The Size attribute in Ada 95 (and Ada 2005) for discrete types is defined as
the minimal number of bits required to hold values of the type.  For example,
on a 32-bit machine, the size of @code{Natural} will typically be 31 and not
32 (since no sign bit is required).  Some Ada 83 compilers gave 31, and
some 32 in this situation.  This problem will usually show up as a compile
time error, but not always.  It is a good idea to check all uses of the
‘Size attribute when porting Ada 83 code.  The GNAT specific attribute
Object_Size can provide a useful way of duplicating the behavior of
some Ada 83 compiler systems.

@item 
`Size of Access Types'

A common assumption in Ada 83 code is that an access type is in fact a pointer,
and that therefore it will be the same size as a System.Address value.  This
assumption is true for GNAT in most cases with one exception.  For the case of
a pointer to an unconstrained array type (where the bounds may vary from one
value of the access type to another), the default is to use a ‘fat pointer’,
which is represented as two separate pointers, one to the bounds, and one to
the array.  This representation has a number of advantages, including improved
efficiency.  However, it may cause some difficulties in porting existing Ada 83
code which makes the assumption that, for example, pointers fit in 32 bits on
a machine with 32-bit addressing.

To get around this problem, GNAT also permits the use of ‘thin pointers’ for
access types in this case (where the designated type is an unconstrained array
type).  These thin pointers are indeed the same size as a System.Address value.
To specify a thin pointer, use a size clause for the type, for example:

@example
type X is access all String;
for X'Size use Standard'Address_Size;
@end example

which will cause the type X to be represented using a single pointer.
When using this representation, the bounds are right behind the array.
This representation is slightly less efficient, and does not allow quite
such flexibility in the use of foreign pointers or in using the
Unrestricted_Access attribute to create pointers to non-aliased objects.
But for any standard portable use of the access type it will work in
a functionally correct manner and allow porting of existing code.
Note that another way of forcing a thin pointer representation
is to use a component size clause for the element size in an array,
or a record representation clause for an access field in a record.

See the documentation of Unrestricted_Access in the GNAT RM for a
full discussion of possible problems using this attribute in conjunction
with thin pointers.
@end itemize

@node Compatibility with HP Ada 83,,Representation Clauses,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-with-hp-ada-83}@anchor{4bd}@anchor{gnat_rm/compatibility_and_porting_guide id17}@anchor{4be}
@section Compatibility with HP Ada 83


All the HP Ada 83 pragmas and attributes are recognized, although only a subset
of them can sensibly be implemented.  The description of pragmas in
@ref{7,,Implementation Defined Pragmas} indicates whether or not they are
applicable to GNAT.


@itemize *

@item 
`Default floating-point representation'

In GNAT, the default floating-point format is IEEE, whereas in HP Ada 83,
it is VMS format.

@item 
`System'

the package System in GNAT exactly corresponds to the definition in the
Ada 95 reference manual, which means that it excludes many of the
HP Ada 83 extensions.  However, a separate package Aux_DEC is provided
that contains the additional definitions, and a special pragma,
Extend_System allows this package to be treated transparently as an
extension of package System.
@end itemize

@node GNU Free Documentation License,Index,Compatibility and Porting Guide,Top
@anchor{share/gnu_free_documentation_license doc}@anchor{4bf}@anchor{share/gnu_free_documentation_license gnu-fdl}@anchor{1}@anchor{share/gnu_free_documentation_license gnu-free-documentation-license}@anchor{4c0}
@chapter GNU Free Documentation License


Version 1.3, 3 November 2008

Copyright  2000, 2001, 2002, 2007, 2008  Free Software Foundation, Inc
@indicateurl{https://fsf.org/}

Everyone is permitted to copy and distribute verbatim copies of this
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@enumerate A

@item 
Use in the Title Page (and on the covers, if any) a title distinct
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of the Document).  You may use the same title as a previous version
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@item 
List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

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State on the Title page the name of the publisher of the
Modified Version, as the publisher.

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Preserve all the copyright notices of the Document.

@item 
Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

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Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

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Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document’s license notice.

@item 
Include an unaltered copy of this License.

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Preserve the section Entitled “History”, Preserve its Title, and add
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You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

@item 
For any section Entitled “Acknowledgements” or “Dedications”,
Preserve the Title of the section, and preserve in the section all
the substance and tone of each of the contributor acknowledgements
and/or dedications given therein.

@item 
Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

@item 
Delete any section Entitled “Endorsements”.  Such a section
may not be included in the Modified Version.

@item 
Do not retitle any existing section to be Entitled “Endorsements”
or to conflict in title with any Invariant Section.

@item 
Preserve any Warranty Disclaimers.
@end enumerate

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled “Endorsements”, provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity.  If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

`5. COMBINING DOCUMENTS'

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy.  If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled
“History”; likewise combine any sections Entitled “Acknowledgements”,
and any sections Entitled “Dedications”.  You must delete all sections
Entitled “Endorsements”.

`6. COLLECTIONS OF DOCUMENTS'

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

`7. AGGREGATION WITH INDEPENDENT WORKS'

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation’s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document’s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

`8. TRANSLATION'

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers.  In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements”,
“Dedications”, or “History”, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

`9. TERMINATION'

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense, or distribute it is void, and
will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License.  If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

`10. FUTURE REVISIONS OF THIS LICENSE'

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
@indicateurl{https://www.gnu.org/copyleft/}.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.  If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy’s public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.

`11. RELICENSING'

“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works.  A
public wiki that anybody can edit is an example of such a server.  A
“Massive Multiauthor Collaboration” (or “MMC”) contained in the
site means any set of copyrightable works thus published on the MMC
site.

“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

“Incorporate” means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is “eligible for relicensing” if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.

`ADDENDUM: How to use this License for your documents'

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

@quotation

Copyright © YEAR  YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled “GNU
Free Documentation License”.
@end quotation

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with … Texts.” line with this:

@quotation

with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
@end quotation

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.

@node Index,,GNU Free Documentation License,Top
@unnumbered Index


@printindex ge


@c %**end of body
@bye
