Implementation Defined Characteristics (GNAT Reference Manual)

## 8 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.

- “Whether or not each recommendation given in Implementation Advice is followed. See 1.1.2(37).”

See Implementation Advice.

- “Capacity limitations of the implementation. See 1.1.3(3).”

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.

- “Variations from the standard that are impractical to avoid given the implementation’s execution environment. See 1.1.3(6).”

There are no variations from the standard.

- “Which code_statements cause external interactions. See 1.1.3(10).”

Any ‘code_statement’ can potentially cause external interactions.

- “The coded representation for the text of an Ada program. See 2.1(4).”

See separate section on source representation.

- “The semantics of an Ada program whose text is not in Normalization Form C. See 2.1(4).”

See separate section on source representation.

- “The representation for an end of line. See 2.2(2).”

See separate section on source representation.

- “Maximum supported line length and lexical element length. See 2.2(15).”

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.

- “Implementation defined pragmas. See 2.8(14).”

See Implementation Defined Pragmas.

- “Effect of pragma `Optimize`. See 2.8(27).”

Pragma `Optimize`, if given with a `Time` or `Space` parameter, checks that the optimization flag is set, and aborts if it is not.

- “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).”

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.

- “The predefined integer types declared in `Standard`. See 3.5.4(25).”

Type | Representation
--- | ---
‘Short_Short_Integer’ | 8-bit signed
‘Short_Integer’ | 16-bit signed
‘Integer’ | 32-bit signed
‘Long_Integer’ | 64-bit signed (on most 64-bit targets, depending on the C definition of long) 32-bit signed (on all other targets)
‘Long_Long_Integer’ | 64-bit signed
‘Long_Long_Long_Integer’ | 128-bit signed (on 64-bit targets) 64-bit signed (on 32-bit targets)

- “Any nonstandard integer types and the operators defined for them. See 3.5.4(26).”

There are no nonstandard integer types.

- “Any nonstandard real types and the operators defined for them. See 3.5.6(8).”

There are no nonstandard real types.

- “What combinations of requested decimal precision and range are supported for floating point types. See 3.5.7(7).”

The precision and range are defined by the IEEE Standard for Floating-Point Arithmetic (IEEE 754-2019).

- “The predefined floating point types declared in `Standard`. See 3.5.7(16).”

Type | Representation
--- | ---
‘Short_Float’ | IEEE Binary32 (Single)
‘Float’ | IEEE Binary32 (Single)
‘Long_Float’ | IEEE Binary64 (Double)
‘Long_Long_Float’ | IEEE Binary64 (Double) on non-x86 architectures IEEE 80-bit Extended on x86 architecture

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 `T'Digits` for any type, for example 30 digits for Long_Float; if more digits are requested, zeros are printed.

- “The small of an ordinary fixed point type. See 3.5.9(8).”

The small is the largest power of two that does not exceed the delta.

- “What combinations of small, range, and digits are supported for fixed point types. See 3.5.9(10).”

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 `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.

- “The result of `Tags.Expanded_Name` for types declared within an unnamed ‘block_statement’. See 3.9(10).”

Block numbers of the form `Bnnn`, where ‘nnn’ is a decimal integer are allocated.

- “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).”

This is handled in the same way as the implementation-defined behavior referenced in A.4.12(34).

- “Implementation-defined attributes. See 4.1.4(12).”

See Implementation Defined Attributes.

- “The value of the parameter to Empty for some container aggregates. See 4.3.5(40).”

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.

- “The maximum number of chunks for a parallel reduction expression without a chunk_specification. See 4.5.10(21).”

Feature unimplemented.

- “Rounding of real static expressions which are exactly half-way between two machine numbers. See 4.9(38).”

Round to even is used in all such cases.

- “The maximum number of chunks for a parallel generalized iterator without a chunk_specification. See 5.5.2(10).”

Feature unimplemented.

- “The number of chunks for an array component iterator. See 5.5.2(11).”

Feature unimplemented.

- “Any extensions of the Global aspect. See 6.1.2(43).”

Feature unimplemented.

- “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).”

Difficult to characterize.

- “Any extensions of the Default_Initial_Condition aspect. See 7.3.3(11).”

SPARK allows specifying ‘null’ as the Default_Initial_Condition aspect of a type. See the SPARK reference manual for further details.

- “Any implementation-defined time types. See 9.6(6).”

There are no implementation-defined time types.

- “The time base associated with relative delays. See 9.6(20).”

See 9.6(20). The time base used is that provided by the C library function `gettimeofday`.

- “The time base of the type `Calendar.Time`. See 9.6(23).”

The time base used is that provided by the C library function `gettimeofday`.

- “The time zone used for package `Calendar` operations. See 9.6(24).”

The time zone used by package `Calendar` is the current system time zone setting for local time, as accessed by the C library function `localtime`.

- “Any limit on ‘delay_until_statements’ of ‘select_statements’. See 9.6(29).”

There are no such limits.

- “The result of Calendar.Formatting.Image if its argument represents more than 100 hours. See 9.6.1(86).”

Calendar.Time_Error is raised.

- “Implementation-defined conflict check policies. See 9.10.1(5).”

There are no implementation-defined conflict check policies.

- “The representation for a compilation. See 10.1(2).”

A compilation is represented by a sequence of files presented to the compiler in a single invocation of the ‘gcc’ command.

- “Any restrictions on compilations that contain multiple compilation_units. See 10.1(4).”

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.

- “The mechanisms for creating an environment and for adding and replacing compilation units. See 10.1.4(3).”

See separate section on compilation model.

- “The manner of explicitly assigning library units to a partition. See 10.2(2).”

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 GNAT User’s Guide.

- “The implementation-defined means, if any, of specifying which compilation units are needed by a given compilation unit. See 10.2(2).”

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.

- “The manner of designating the main subprogram of a partition. See 10.2(7).”

The main program is designated by providing the name of the corresponding `ALI` file as the input parameter to the binder.

- “The order of elaboration of ‘library_items’. See 10.2(18).”

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.

- “Parameter passing and function return for the main subprogram. See 10.2(21).”

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 `Ada.Command_Line.Set_Exit_Status`).

- “The mechanisms for building and running partitions. See 10.2(24).”

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 PolyORB regarding the lifetime of this product.

- “The details of program execution, including program termination. See 10.2(25).”

See separate section on compilation model.

- “The semantics of any non-active partitions supported by the implementation. See 10.2(28).”

Passive partitions are supported on targets where shared memory is provided by the operating system. ‘NB!’ See the note in PolyORB regarding the lifetime of this product.

- “The information returned by `Exception_Message`. See 11.4.1(10).”

Exception message returns the null string unless a specific message has been passed by the program.

- “The result of `Exceptions.Exception_Name` for types declared within an unnamed ‘block_statement’. See 11.4.1(12).”

Blocks have implementation defined names of the form `Bnnn` where ‘nnn’ is an integer.

- “The information returned by `Exception_Information`. See 11.4.1(13).”

`Exception_Information` returns a string in the following format:

    *Exception_Name:* nnnnn
    *Message:* mmmmm
    *PID:* ppp
    *Load address:* 0xhhhh
    *Call stack traceback locations:*
    0xhhhh 0xhhhh 0xhhhh ... 0xhhh

where

> - `nnnn` is the fully qualified name of the exception in all upper case letters. This line is always present.
> - `mmmm` is the message (this line present only if message is non-null)
> - `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.
> - 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 `LF` character (`16#0A#`).

- “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).”

This is handled in the same way as the implementation-defined behavior referenced in A.4.12(34).

- “The information returned by Exception_Information. See 11.4.1(13).”

The exception name and the source location at which the exception was raised are included.

- “Implementation-defined policy_identifiers and assertion_aspect_marks allowed in a pragma Assertion_Policy. See 11.4.2(9).”

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.

- “The default assertion policy. See 11.4.2(10).”

The default assertion policy is Ignore, although this can be overridden via compiler switches such as “-gnata”.

- “Implementation-defined check names. See 11.5(27).”

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 `Suppress` for details.

- “Existence and meaning of second parameter of pragma Unsuppress. See 11.5(27.1).”

The legality rules for and semantics of the second parameter of pragma Unsuppress match those for the second argument of pragma Suppress.

- “The cases that cause conflicts between the representation of the ancestors of a type_declaration. See 13.1(13.1).”

No such cases exist.

- “The interpretation of each representation aspect. See 13.1(20).”

See separate section on data representations.

- “Any restrictions placed upon the specification of representation aspects. See 13.1(20).”

See separate section on data representations.

- “Implementation-defined aspects, including the syntax for specifying such aspects and the legality rules for such aspects. See 13.1.1(38).”

See Implementation Defined Aspects.

- “The set of machine scalars. See 13.3(8.1).”

See separate section on data representations.

- “The meaning of `Size` for indefinite subtypes. See 13.3(48).”

The Size attribute of an indefinite subtype is not less than the Size attribute of any object of that type.

- “The meaning of Object_Size for indefinite subtypes. See 13.3(58).”

The Object_Size attribute of an indefinite subtype is not less than the Object_Size attribute of any object of that type.

- “The default external representation for a type tag. See 13.3(75).”

The default external representation for a type tag is the fully expanded name of the type in upper case letters.

- “What determines whether a compilation unit is the same in two different partitions. See 13.3(76).”

A compilation unit is the same in two different partitions if and only if it derives from the same source file.

- “Implementation-defined components. See 13.5.1(15).”

The only implementation defined component is the tag for a tagged type, which contains a pointer to the dispatching table.

- “If `Word_Size` = `Storage_Unit`, the default bit ordering. See 13.5.3(5).”

`Word_Size` does not equal `Storage_Unit` in this implementation.

- “The contents of the visible part of package `System`. See 13.7(2).”

See the definition of package System in `system.ads`. Note that two declarations are added to package System.

    Max_Priority           : constant Positive := Priority'Last;
    Max_Interrupt_Priority : constant Positive := Interrupt_Priority'Last;

- “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).”

See the definition of package System.Storage_Elements in `s-stoele.ads`.

- “The contents of the visible part of package `System.Machine_Code`, and the meaning of ‘code_statements’. See 13.8(7).”

See the definition and documentation in file `s-maccod.ads`.

- “The result of unchecked conversion for instances with scalar result types whose result is not defined by the language. See 13.9(11).”

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

- “The result of unchecked conversion for instances with nonscalar result types whose result is not defined by the language. See 13.9(11).”

See preceding definition for the scalar result case.

- “Whether or not the implementation provides user-accessible names for the standard pool type(s). See 13.11(17).”

There are 3 different standard pools used by the compiler when `Storage_Pool` is not specified depending whether the type is local to a subprogram or defined at the library level and whether `Storage_Size``is specified or not. See documentation in the runtime library units ``System.Pool_Global`, `System.Pool_Size` and `System.Pool_Local` in files `s-poosiz.ads`, `s-pooglo.ads` and `s-pooloc.ads` for full details on the default pools used. All these pools are accessible by means of withing these units.

- “The meaning of `Storage_Size` when neither the Storage_Size nor the Storage_Pool is specified for an access type. See 13.11(18).”

`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.

- “The effect of specifying aspect Default_Storage_Pool on an instance of a language-defined generic unit. See 13.11.3(5).”

Instances of language-defined generic units are treated the same as other instances with respect to the Default_Storage_Pool aspect.

- “Implementation-defined restrictions allowed in a pragma `Restrictions`. See 13.12(8.7).”

See Standard and Implementation Defined Restrictions.

- “The consequences of violating limitations on `Restrictions` pragmas. See 13.12(9).”

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.

- “Implementation-defined usage profiles allowed in a pragma Profile. See 13.12(15).”

See Implementation Defined Pragmas.

- “The contents of the stream elements read and written by the Read and Write attributes of elementary types. See 13.13.2(9).”

The representation is the in-memory representation of the base type of the type, using the number of bits corresponding to the `type'Size` value, and the natural ordering of the machine.

- “The names and characteristics of the numeric subtypes declared in the visible part of package `Standard`. See A.1(3).”

See items describing the integer and floating-point types supported.

- “The values returned by Strings.Hash. See A.4.9(3).”

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.

- “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).”

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.

- “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).”

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.

- “The accuracy actually achieved by the elementary functions. See A.5.1(1).”

The elementary functions correspond to the functions available in the C library. Only fast math mode is implemented.

- “The sign of a zero result from some of the operators or functions in `Numerics.Generic_Elementary_Functions`, when `Float_Type'Signed_Zeros` is `True`. See A.5.1(46).”

The sign of zeroes follows the requirements of the IEEE 754 standard on floating-point.

- “The value of `Numerics.Float_Random.Max_Image_Width`. See A.5.2(27).”

Maximum image width is 6864, see library file `s-rannum.ads`.

- “The value of `Numerics.Discrete_Random.Max_Image_Width`. See A.5.2(27).”

Maximum image width is 6864, see library file `s-rannum.ads`.

- “The string representation of a random number generator’s state. See A.5.2(38).”

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.

- “The values of the `Model_Mantissa`, `Model_Emin`, `Model_Epsilon`, `Model`, `Safe_First`, and `Safe_Last` attributes, if the Numerics Annex is not supported. See A.5.3(72).”

Running the compiler with ‘-gnatS’ to produce a listing of package `Standard` displays the values of these attributes.

- “The value of `Buffer_Size` in `Storage_IO`. See A.9(10).”

All type representations are contiguous, and the `Buffer_Size` is the value of `type'Size` rounded up to the next storage unit boundary.

- “External files for standard input, standard output, and standard error See A.10(5).”

These files are mapped onto the files provided by the C streams libraries. See source file `i-cstrea.ads` for further details.

- “The accuracy of the value produced by `Put`. See A.10.9(36).”

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.

- “Current size for a stream file for which positioning is not supported. See A.12.1(1.1).”

Positioning is supported.

- “The meaning of `Argument_Count`, `Argument`, and `Command_Name`. See A.15(1).”

These are mapped onto the `argv` and `argc` parameters of the main program in the natural manner.

- “The interpretation of file names and directory names. See A.16(46).”

These names are interpreted consistently with the underlying file system.

- “The maximum value for a file size in Directories. See A.16(87).”

Directories.File_Size’Last is equal to Long_Long_Integer’Last .

- “The result for Directories.Size for a directory or special file. See A.16(93).”

Name_Error is raised.

- “The result for Directories.Modification_Time for a directory or special file. See A.16(93).”

Name_Error is raised.

- “The interpretation of a nonnull search pattern in Directories. See A.16(104).”

When the `Pattern` parameter is not the null string, it is interpreted according to the syntax of regular expressions as defined in the `GNAT.Regexp` package.

See GNAT.Regexp (g-regexp.ads).

- “The results of a Directories search if the contents of the directory are altered while a search is in progress. See A.16(110).”

The effect of a call to Get_Next_Entry is determined by the current state of the directory.

- “The definition and meaning of an environment variable. See A.17(1).”

This definition is determined by the underlying operating system.

- “The circumstances where an environment variable cannot be defined. See A.17(16).”

There are no such implementation-defined circumstances.

- “Environment names for which Set has the effect of Clear. See A.17(17).”

There are no such names.

- “The value of Containers.Hash_Type’Modulus. The value of Containers.Count_Type’Last. See A.18.1(7).”

Containers.Hash_Type’Modulus is 2**32. Containers.Count_Type’Last is 2**31 - 1.

- “Implementation-defined convention names. See B.1(11).”

The following convention names are supported

Convention Name | Interpretation
--- | ---
‘Ada’ | Ada
‘Ada_Pass_By_Copy’ | 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.
‘Ada_Pass_By_Reference’ | 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.
‘Assembler’ | Assembly language
‘Asm’ | Synonym for Assembler
‘Assembly’ | Synonym for Assembler
‘C’ | C
‘C_Pass_By_Copy’ | Allowed only for record types, like C, but also notes that record is to be passed by copy rather than reference.
‘COBOL’ | COBOL
‘C_Plus_Plus (or CPP)’ | C++
‘Default’ | Treated the same as C
‘External’ | Treated the same as C
‘Fortran’ | Fortran
‘Intrinsic’ | For support of pragma `Import` with convention Intrinsic, see separate section on Intrinsic Subprograms.
‘Stdcall’ | 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.
‘DLL’ | Synonym for Stdcall
‘Win32’ | Synonym for Stdcall
‘Stubbed’ | 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 `Program_Error` exception. If a pragma `Import` specifies convention `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.

- “The meaning of link names. See B.1(36).”

Link names are the actual names used by the linker.

- “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).”

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.

- “The effect of pragma `Linker_Options`. See B.1(37).”

The string passed to `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

    pragma Linker_Options ("-labc" & ASCII.NUL & "-ldef");

causes two separate arguments `-labc` and `-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.

- “The contents of the visible part of package `Interfaces` and its language-defined descendants. See B.2(1).”

See files with prefix `i-` in the distributed library.

- “Implementation-defined children of package `Interfaces`. The contents of the visible part of package `Interfaces`. See B.2(11).”

See files with prefix `i-` in the distributed library.

- “The definitions of certain types and constants in Interfaces.C. See B.3(41).”

See source file `i-c.ads`.

- “The types `Floating`, `Long_Floating`, `Binary`, `Long_Binary`, `Decimal_ Element`, and `COBOL_Character`; and the initialization of the variables `Ada_To_COBOL` and `COBOL_To_Ada`, in `Interfaces.COBOL`. See B.4(50).”

COBOL | Ada
--- | ---
‘Floating’ | Float
‘Long_Floating’ | (Floating) Long_Float
‘Binary’ | Integer
‘Long_Binary’ | Long_Long_Integer
‘Decimal_Element’ | Character
‘COBOL_Character’ | Character

For initialization, see the file `i-cobol.ads` in the distributed library.

- “The types Fortran_Integer, Real, Double_Precision, and Character_Set in Interfaces.Fortran. See B.5(17).”

See source file `i-fortra.ads`. These types are derived, respectively, from Integer, Float, Long_Float, and Character.

- “Implementation-defined intrinsic subprograms. See C.1(1).”

See separate section on Intrinsic Subprograms.

- “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).”

There are no such restrictions.

- “Any other forms of interrupt handler supported by the Attach_Handler and Interrupt_Handler aspects. See C.3.1(19).”

There are no such forms.

- “The semantics of some attributes and functions of an entity for which aspect Discard_Names is True. See C.5(7).”

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‘‘Discard_Names‘‘ and `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.

- “The modulus and size of Test_and_Set_Flag. See C.6.3(8).”

The modulus is 2**8. The size is 8.

- “The value used to represent the set value for Atomic_Test_and_Set. See C.6.3(10).”

The value is 1.

- “The result of the `Task_Identification.Image` attribute. See C.7.1(7).”

The result of this attribute is a string that identifies the object or component that denotes a given task. If a variable `Var` has a task type, the image for this task will have the form `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., `Group(5)_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.

- “The value of `Current_Task` when in a protected entry or interrupt handler. See C.7.1(17).”

Protected entries or interrupt handlers can be executed by any convenient thread, so the value of `Current_Task` is undefined.

- “Granularity of locking for Task_Attributes. See C.7.2(16).”

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.

- “The declarations of `Any_Priority` and `Priority`. See D.1(11).”

See declarations in file `system.ads`.

- “Implementation-defined execution resources. See D.1(15).”

There are no implementation-defined execution resources.

- “Whether, on a multiprocessor, a task that is waiting for access to a protected object keeps its processor busy. See D.2.1(3).”

On a multi-processor, a task that is waiting for access to a protected object does not keep its processor busy.

- “The affect of implementation defined execution resources on task dispatching. See D.2.1(9).”

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.

- “Implementation-defined task dispatching policies. See D.2.2(3).”

There are no implementation-defined task dispatching policies.

- “The value of Default_Quantum in Dispatching.Round_Robin. See D.2.5(4).”

The value is 10 milliseconds.

- “Implementation-defined ‘policy_identifiers’ allowed in a pragma `Locking_Policy`. See D.3(4).”

The two implementation defined policies permitted in GNAT are `Inheritance_Locking` and `Concurrent_Readers_Locking`. On targets that support the `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 `Concurrent_Readers_Locking` policy, locking is implemented with a read/write lock allowing multiple protected object functions to enter concurrently.

- “Default ceiling priorities. See D.3(10).”

The ceiling priority of protected objects of the type `System.Interrupt_Priority'Last` as described in the Ada Reference Manual D.3(10),

- “The ceiling of any protected object used internally by the implementation. See D.3(16).”

The ceiling priority of internal protected objects is `System.Priority'Last`.

- “Implementation-defined queuing policies. See D.4(1).”

There are no implementation-defined queuing policies.

- “Implementation-defined admission policies. See D.4.1(1).”

There are no implementation-defined admission policies.

- “Any operations that implicitly require heap storage allocation. See D.7(8).”

The only operation that implicitly requires heap storage allocation is task creation.

- “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).”

Unknown.

- “When restriction No_Task_Termination applies to a partition, what happens when a task terminates. See D.7(15.1).”

Execution is erroneous in that case.

- “The behavior when restriction Max_Storage_At_Blocking is violated. See D.7(17).”

Execution is erroneous in that case.

- “The behavior when restriction Max_Asynchronous_Select_Nesting is violated. See D.7(18).”

Execution is erroneous in that case.

- “The behavior when restriction Max_Tasks is violated. See D.7(19).”

Execution is erroneous in that case.

- “Whether the use of pragma Restrictions results in a reduction in program code or data size or execution time. See D.7(20).”

Yes it can, but the precise circumstances and properties of such reductions are difficult to characterize.

- “The value of Barrier_Limit’Last in Synchronous_Barriers. See D.10.1(4).”

Synchronous_Barriers.Barrier_Limit’Last is Integer’Last .

- “When an aborted task that is waiting on a Synchronous_Barrier is aborted. See D.10.1(13).”

Difficult to characterize.

- “The value of Min_Handler_Ceiling in Execution_Time.Group_Budgets. See D.14.2(7).”

See source file `a-etgrbu.ads`.

- “The value of CPU_Range’Last in System.Multiprocessors. See D.16(4).”

See source file `s-multip.ads`.

- “The processor on which the environment task executes in the absence of a value for the aspect CPU. See D.16(13).”

Unknown.

- “The means for creating and executing distributed programs. See E(5).”

The PolyORB product provides means creating and executing distributed programs. ‘NB!’ See the note in PolyORB regarding the lifetime of this product.

- “Any events that can result in a partition becoming inaccessible. See E.1(7).”

See the PolyORB user guide for full details on such events. ‘NB!’ Consider the note in PolyORB regarding the lifetime of this product.

- “The scheduling policies, treatment of priorities, and management of shared resources between partitions in certain cases. See E.1(11).”

See the PolyORB user guide for full details on these aspects of multi-partition execution. ‘NB!’ Consider the note in PolyORB regarding the lifetime of this product.

- “Whether the execution of the remote subprogram is immediately aborted as a result of cancellation. See E.4(13).”

See the PolyORB user guide for details on the effect of abort in a distributed application. ‘NB!’ Consider the note in PolyORB regarding the lifetime of this product.

- “The range of type System.RPC.Partition_Id. See E.5(14).”

System.RPC.Partition_ID’Last is Integer’Last. See source file `s-rpc.ads`.

- “Implementation-defined interfaces in the PCS. See E.5(26).”

See the PolyORB user guide for a full description of all implementation defined interfaces. ‘NB!’ See the note in PolyORB regarding the lifetime of this product.

- “The values of named numbers in the package `Decimal`. See F.2(7).”

Named Number | Value
--- | ---
‘Max_Scale’ | +18
‘Min_Scale’ | -18
‘Min_Delta’ | 1.0E-18
‘Max_Delta’ | 1.0E+18
‘Max_Decimal_Digits’ | 18

- “The value of `Max_Picture_Length` in the package `Text_IO.Editing`. See F.3.3(16).”

  64

- “The value of `Max_Picture_Length` in the package `Wide_Text_IO.Editing`. See F.3.4(5).”

  64

- “The accuracy actually achieved by the complex elementary functions and by other complex arithmetic operations. See G.1(1).”

Standard library functions are used for the complex arithmetic operations. Only fast math mode is currently supported.

- “The sign of a zero result (or a component thereof) from any operator or function in `Numerics.Generic_Complex_Types`, when `Real'Signed_Zeros` is True. See G.1.1(53).”

The signs of zero values are as recommended by the relevant implementation advice.

- “The sign of a zero result (or a component thereof) from any operator or function in `Numerics.Generic_Complex_Elementary_Functions`, when `Real'Signed_Zeros` is `True`. See G.1.2(45).”

The signs of zero values are as recommended by the relevant implementation advice.

- “Whether the strict mode or the relaxed mode is the default. See G.2(2).”

The strict mode is the default. There is no separate relaxed mode. GNAT provides a highly efficient implementation of strict mode.

- “The result interval in certain cases of fixed-to-float conversion. See G.2.1(10).”

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.

- “The result of a floating point arithmetic operation in overflow situations, when the `Machine_Overflows` attribute of the result type is `False`. See G.2.1(13).”

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.

- “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).”

Not relevant, division is IEEE exact.

- “The definition of close result set, which determines the accuracy of certain fixed point multiplications and divisions. See G.2.3(5).”

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.

- “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).”

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.

- “The result of a fixed point arithmetic operation in overflow situations, when the `Machine_Overflows` attribute of the result type is `False`. See G.2.3(27).”

Not relevant, `Machine_Overflows` is `True` for fixed-point types.

- “The result of an elementary function reference in overflow situations, when the `Machine_Overflows` attribute of the result type is `False`. See G.2.4(4).”

IEEE infinite and Nan values are produced as appropriate.

- “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).”

Information on this subject is not yet available.

- “The accuracy of certain elementary functions for parameters beyond the angle threshold. See G.2.4(10).”

Information on this subject is not yet available.

- “The result of a complex arithmetic operation or complex elementary function reference in overflow situations, when the `Machine_Overflows` attribute of the corresponding real type is `False`. See G.2.6(5).”

IEEE infinite and Nan values are produced as appropriate.

- “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).”

Information on those subjects is not yet available.

- “The accuracy requirements for the subprograms Solve, Inverse, Determinant, Eigenvalues and Eigensystem for type Real_Matrix. See G.3.1(81).”

Information on those subjects is not yet available.

- “The accuracy requirements for the subprograms Solve, Inverse, Determinant, Eigenvalues and Eigensystem for type Complex_Matrix. See G.3.2(149).”

Information on those subjects is not yet available.

- “The consequences of violating No_Hidden_Indirect_Globals. See H.4(23.9).”

Execution is erroneous in that case.