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@setfilename gfortran.info
@set copyrights-gfortran 1999-2026

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@settitle The GNU Fortran Compiler

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@copying
Copyright @copyright{} @value{copyrights-gfortran} Free Software Foundation, Inc.

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 the
Invariant Sections being ``Funding Free Software'', the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
``GNU Free Documentation License''.

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.
@end copying

@ifinfo
@dircategory Software development
@direntry
* gfortran: (gfortran).                  The GNU Fortran Compiler.
@end direntry
This file documents the use and the internals of
the GNU Fortran compiler, (@command{gfortran}).

Published by the Free Software Foundation
51 Franklin Street, Fifth Floor
Boston, MA 02110-1301 USA

@insertcopying
@end ifinfo


@setchapternewpage odd
@titlepage
@title Using GNU Fortran
@versionsubtitle
@author The @t{gfortran} team
@page
@vskip 0pt plus 1filll
Published by the Free Software Foundation@*
51 Franklin Street, Fifth Floor@*
Boston, MA 02110-1301, USA@*
@c Last printed ??ber, 19??.@*
@c Printed copies are available for $? each.@*
@c ISBN ???
@sp 1
@insertcopying
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@page

@c ---------------------------------------------------------------------
@c TexInfo table of contents.
@c ---------------------------------------------------------------------

@ifnottex
@node Top
@top Introduction
@cindex Introduction

This manual documents the use of @command{gfortran},
the GNU Fortran compiler.  You can find in this manual how to invoke
@command{gfortran}, as well as its features and incompatibilities.

@ifset DEVELOPMENT
@emph{Warning:} This document, and the compiler it describes, are still
under development.  While efforts are made to keep it up to date, it might
not accurately reflect the status of the most recent GNU Fortran compiler.
@end ifset

@comment
@comment  When you add a new menu item, please keep the right hand
@comment  aligned to the same column.  Do not use tabs.  This provides
@comment  better formatting.
@comment
@menu
* Introduction::

Part I: Invoking GNU Fortran
* Invoking GNU Fortran:: Command options supported by @command{gfortran}.
* Runtime::              Influencing runtime behavior with environment variables.

Part II: Language Reference
* Compiler Characteristics::      User-visible implementation details.
* Extensions::                    Language extensions implemented by GNU Fortran.
* Mixed-Language Programming::    Interoperability with C
* Coarray Programming::
* Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
* Intrinsic Modules::    Intrinsic modules supported by GNU Fortran.

* Contributing::         How you can help.
* Copying::              GNU General Public License says
                         how you can copy and share GNU Fortran.
* GNU Free Documentation License::
                         How you can copy and share this manual.
* Funding::              How to help assure continued work for free software.
* Option Index::         Index of command line options
* Keyword Index::        Index of concepts
@end menu
@end ifnottex

@c ---------------------------------------------------------------------
@c Introduction
@c ---------------------------------------------------------------------

@node Introduction
@chapter Introduction

@c The following duplicates the text on the TexInfo table of contents.
@iftex
This manual documents the use of @command{gfortran}, the GNU Fortran
compiler.  You can find in this manual how to invoke @command{gfortran},
as well as its features and incompatibilities.

@ifset DEVELOPMENT
@emph{Warning:} This document, and the compiler it describes, are still
under development.  While efforts are made to keep it up-to-date, it
might not accurately reflect the status of the most recent GNU Fortran
compiler.
@end ifset
@end iftex

@menu
* About GNU Fortran::    What you should know about the GNU Fortran compiler.
* GNU Fortran and GCC::  You can compile Fortran, C, or other programs.
* Standards::            Standards supported by GNU Fortran.
@end menu


@c ---------------------------------------------------------------------
@c About GNU Fortran
@c ---------------------------------------------------------------------

@node About GNU Fortran
@section About GNU Fortran

The GNU Fortran compiler is the successor to @command{g77}, the
Fortran 77 front end included in GCC prior to version 4 (released in
2005).  While it is backward-compatible with most @command{g77}
extensions and command-line options, @command{gfortran} is a completely new
implementation designed to support more modern dialects of Fortran.
GNU Fortran implements the Fortran 77, 90 and 95 standards
completely, most of the Fortran 2003 and 2008 standards, and some
features from the 2018 standard.  It also implements several extensions
including OpenMP and OpenACC support for parallel programming.

The GNU Fortran compiler passes the
@uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
NIST Fortran 77 Test Suite}, and produces acceptable results on the
@uref{https://www.netlib.org/lapack/faq.html, LAPACK Test Suite}.
It also provides respectable performance on
the @uref{https://polyhedron.com/?page_id=175,
Polyhedron Fortran compiler benchmarks} and the
@uref{https://www.netlib.org/benchmark/livermore,
Livermore Fortran Kernels test}.  It has been used to compile a number of
large real-world programs, including
@uref{http://hirlam.org/, the HARMONIE and HIRLAM weather forecasting code} and
@uref{https://github.com/dylan-jayatilaka/tonto,
the Tonto quantum chemistry package}; see
@url{https://gcc.gnu.org/@/wiki/@/GfortranApps} for an extended list.

GNU Fortran provides the following functionality:

@itemize @bullet
@item
Read a program, stored in a file and containing @dfn{source code}
instructions written in Fortran 77.

@item
Translate the program into instructions a computer
can carry out more quickly than it takes to translate the
original Fortran instructions.
The result after compilation of a program is
@dfn{machine code},
which is efficiently translated and processed
by a machine such as your computer.
Humans usually are not as good writing machine code
as they are at writing Fortran (or C++, Ada, or Java),
because it is easy to make tiny mistakes writing machine code.

@item
Provide information about the reasons why
the compiler may be unable to create a binary from the source code,
for example if the source code is flawed.
The Fortran language standards require that the compiler can point out
mistakes in your code.
An incorrect usage of the language causes an @dfn{error message}.

The compiler also attempts to diagnose cases where your
program contains a correct usage of the language,
but instructs the computer to do something questionable.
This kind of diagnostic message is called a @dfn{warning message}.

@item
Provide optional information about the translation passes
from the source code to machine code.
This can help you to find the cause of
certain bugs which may not be obvious in the source code,
but may be more easily found at a lower level compiler output.
It also helps developers to find bugs in the compiler itself.

@item
Provide information in the generated machine code that can
make it easier to find bugs in the program (using a debugging tool,
called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).

@item
Locate and gather machine code already generated to
perform actions requested by statements in the program.
This machine code is organized into @dfn{modules} and is located
and @dfn{linked} to the user program.
@end itemize

The GNU Fortran compiler consists of several components:

@itemize @bullet
@item
A version of the @command{gcc} command
(which also might be installed as the system's @command{cc} command)
that also understands and accepts Fortran source code.
The @command{gcc} command is the @dfn{driver} program for
all the languages in the GNU Compiler Collection (GCC);
With @command{gcc},
you can compile the source code of any language for
which a front end is available in GCC.

@item
The @command{gfortran} command itself,
which also might be installed as the
system's @command{f95} command.
@command{gfortran} is just another driver program,
but specifically for the Fortran compiler only.
The primary difference between the @command{gcc} and @command{gfortran}
commands is that the latter automatically links the correct libraries
to your program.

@item
A collection of run-time libraries.
These libraries contain the machine code needed to support
capabilities of the Fortran language that are not directly
provided by the machine code generated by the
@command{gfortran} compilation phase,
such as intrinsic functions and subroutines,
and routines for interaction with files and the operating system.
@c and mechanisms to spawn,
@c unleash and pause threads in parallelized code.

@item
The Fortran compiler itself, (@command{f951}).
This is the GNU Fortran parser and code generator,
linked to and interfaced with the GCC backend library.
@command{f951} ``translates'' the source code to
assembler code.  You would typically not use this
program directly;
instead, the @command{gcc} or @command{gfortran} driver
programs call it for you.
@end itemize


@c ---------------------------------------------------------------------
@c GNU Fortran and GCC
@c ---------------------------------------------------------------------

@node GNU Fortran and GCC
@section GNU Fortran and GCC
@cindex GNU Compiler Collection
@cindex GCC

GNU Fortran is a part of GCC, the @dfn{GNU Compiler Collection}.  GCC
consists of a collection of front ends for various languages, which
translate the source code into a language-independent form called
@dfn{GENERIC}.  This is then processed by a common middle end which
provides optimization, and then passed to one of a collection of back
ends which generate code for different computer architectures and
operating systems.

Functionally, this is implemented with a driver program (@command{gcc})
which provides the command-line interface for the compiler.  It calls
the relevant compiler front-end program (e.g., @command{f951} for
Fortran) for each file in the source code, and then calls the assembler
and linker as appropriate to produce the compiled output.  In a copy of
GCC that has been compiled with Fortran language support enabled,
@command{gcc} recognizes files with @file{.f}, @file{.for}, @file{.ftn},
@file{.f90}, @file{.f95}, @file{.f03} and @file{.f08} extensions as
Fortran source code, and compiles it accordingly.  A @command{gfortran}
driver program is also provided, which is identical to @command{gcc}
except that it automatically links the Fortran runtime libraries into the
compiled program.

Source files with @file{.f}, @file{.for}, @file{.fpp}, @file{.ftn}, @file{.F},
@file{.FOR}, @file{.FPP}, and @file{.FTN} extensions are treated as fixed form.
Source files with @file{.f90}, @file{.f95}, @file{.f03}, @file{.f08},
@file{.F90}, @file{.F95}, @file{.F03} and @file{.F08} extensions are
treated as free form.  The capitalized versions of either form are run
through preprocessing.  Source files with the lower case @file{.fpp}
extension are also run through preprocessing.

This manual specifically documents the Fortran front end, which handles
the programming language's syntax and semantics.  The aspects of GCC
that relate to the optimization passes and the back-end code generation
are documented in the GCC manual; see
@ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
The two manuals together provide a complete reference for the GNU
Fortran compiler.

@c ---------------------------------------------------------------------
@c Standards
@c ---------------------------------------------------------------------

@node Standards
@section Standards
@cindex Standards

@menu
* Fortran 95 status::
* Fortran 2003 status::
* Fortran 2008 status::
* Fortran 2018 status::
@end menu

Fortran is developed by the Working Group 5 of Sub-Committee 22 of the
Joint Technical Committee 1 of the International Organization for
Standardization and the International Electrotechnical Commission (IEC).
This group is known as @uref{http://www.nag.co.uk/sc22wg5/, WG5}.
Official Fortran standard documents are available for purchase
from ISO; a collection of free documents (typically final drafts) are
also available on the @uref{https://gcc.gnu.org/wiki/GFortranStandards, wiki}.

The GNU Fortran compiler implements ISO/IEC 1539:1997 (Fortran 95).
As such, it can also compile essentially all standard-compliant
Fortran 90 and Fortran 77 programs.  It also supports the ISO/IEC
TR-15581 enhancements to allocatable arrays.

GNU Fortran also supports almost all of ISO/IEC 1539-1:2004
(Fortran 2003) and ISO/IEC 1539-1:2010 (Fortran 2008).
It has partial support for features introduced in ISO/IEC
1539:2018 (Fortran 2018), the most recent version of the Fortran
language standard, including full support for the Technical Specification
@code{Further Interoperability of Fortran with C} (ISO/IEC TS 29113:2012).
More details on support for these standards can be
found in the following sections of the documentation.

@node Fortran 95 status
@subsection Fortran 95 status
@cindex Varying length strings
@cindex strings, varying length
@cindex conditional compilation

The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000)
varying length character strings.  While GNU Fortran currently does not
support such strings directly, there exist two Fortran implementations
for them, which work with GNU Fortran. One can be found at
@uref{http://user.astro.wisc.edu/~townsend/static.php?ref=iso-varying-string}.

Deferred-length character strings of Fortran 2003 supports part of
the features of @code{ISO_VARYING_STRING} and should be considered as
replacement. (Namely, allocatable or pointers of the type
@code{character(len=:)}.)

Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines
Conditional Compilation, which is not widely used and not directly
supported by the GNU Fortran compiler.  You can use the program coco
to preprocess such files (@uref{http://www.daniellnagle.com/coco.html}).

@node Fortran 2003 status
@subsection Fortran 2003 status

GNU Fortran implements the Fortran 2003 (ISO/IEC 1539-1:2004) standard
except for finalization support, which is incomplete.
See the
@uref{https://gcc.gnu.org/wiki/Fortran2003, wiki page} for a full list
of new features introduced by Fortran 2003 and their implementation status.

@node Fortran 2008 status
@subsection Fortran 2008 status

The GNU Fortran compiler supports almost all features of Fortran 2008;
the @uref{https://gcc.gnu.org/wiki/Fortran2008Status, wiki}
has some information about the current implementation status.
In particular, the following are not yet supported:

@itemize @bullet
@item
@code{DO CONCURRENT} and @code{FORALL} do not recognize a
type-spec in the loop header.

@item
The change to permit any constant expression in subscripts and
nested implied-do limits in a @code{DATA} statement has not been implemented.
@end itemize


@node Fortran 2018 status
@subsection Fortran 2018 status

Fortran 2018 (ISO/IEC 1539:2018) is the most recent version
of the Fortran language standard.  GNU Fortran implements some of the
new features of this standard:

@itemize @bullet
@item
All Fortran 2018 features derived from ISO/IEC TS 29113:2012,
``Further Interoperability of Fortran with C'', are supported by GNU Fortran.
This includes assumed-type and assumed-rank objects and
the @code{SELECT RANK} construct as well as the parts relating to
@code{BIND(C)} functions.
See also @ref{Further Interoperability of Fortran with C}.

@item
GNU Fortran supports a subset of features derived from ISO/IEC TS 18508:2015,
``Additional Parallel Features in Fortran'':

@itemize @bullet
@item
The new atomic ADD, CAS, FETCH and ADD/OR/XOR, OR and XOR intrinsics.

@item
The @code{CO_MIN} and @code{CO_MAX} and @code{SUM} reduction intrinsics,
and the @code{CO_BROADCAST} and @code{CO_REDUCE} intrinsic, except that those
do not support polymorphic types or types with allocatable, pointer or
polymorphic components.

@item
Events (@code{EVENT POST}, @code{EVENT WAIT}, @code{EVENT_QUERY}).

@item
Failed images (@code{FAIL IMAGE}, @code{IMAGE_STATUS},
@code{FAILED_IMAGES}, @code{STOPPED_IMAGES}).

@end itemize

@item
An @code{ERROR STOP} statement is permitted in a @code{PURE}
procedure.

@item
GNU Fortran supports the @code{IMPLICIT NONE} statement with an
@code{implicit-none-spec-list}.

@item
The behavior of the @code{INQUIRE} statement with the @code{RECL=}
specifier now conforms to Fortran 2018.

@end itemize


@c =====================================================================
@c PART I: INVOCATION REFERENCE
@c =====================================================================

@tex
\part{I}{Invoking GNU Fortran}
@end tex

@c ---------------------------------------------------------------------
@c Compiler Options
@c ---------------------------------------------------------------------

@include invoke.texi


@c ---------------------------------------------------------------------
@c Runtime
@c ---------------------------------------------------------------------

@node Runtime
@chapter Runtime:  Influencing runtime behavior with environment variables
@cindex environment variable

The behavior of the @command{gfortran} can be influenced by
environment variables.

Malformed environment variables are silently ignored.

@menu
* TMPDIR:: Directory for scratch files
* GFORTRAN_STDIN_UNIT:: Unit number for standard input
* GFORTRAN_STDOUT_UNIT:: Unit number for standard output
* GFORTRAN_STDERR_UNIT:: Unit number for standard error
* GFORTRAN_UNBUFFERED_ALL:: Do not buffer I/O for all units
* GFORTRAN_UNBUFFERED_PRECONNECTED:: Do not buffer I/O for preconnected units.
* GFORTRAN_SHOW_LOCUS::  Show location for runtime errors
* GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
* GFORTRAN_LIST_SEPARATOR::  Separator for list output
* GFORTRAN_CONVERT_UNIT::  Set conversion for unformatted I/O
* GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
* GFORTRAN_FORMATTED_BUFFER_SIZE:: Buffer size for formatted files
* GFORTRAN_UNFORMATTED_BUFFER_SIZE:: Buffer size for unformatted files
@end menu

@node TMPDIR
@section @env{TMPDIR}---Directory for scratch files

When opening a file with @code{STATUS='SCRATCH'}, GNU Fortran tries to
create the file in one of the potential directories by testing each
directory in the order below.

@enumerate
@item
The environment variable @env{TMPDIR}, if it exists.

@item
On the MinGW target, the directory returned by the @code{GetTempPath}
function. Alternatively, on the Cygwin target, the @env{TMP} and
@env{TEMP} environment variables, if they exist, in that order.

@item
The @code{P_tmpdir} macro if it is defined, otherwise the directory
@file{/tmp}.
@end enumerate

@node GFORTRAN_STDIN_UNIT
@section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input

This environment variable can be used to select the unit number
preconnected to standard input.  This must be a positive integer.
The default value is 5.

@node GFORTRAN_STDOUT_UNIT
@section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output

This environment variable can be used to select the unit number
preconnected to standard output.  This must be a positive integer.
The default value is 6.

@node GFORTRAN_STDERR_UNIT
@section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error

This environment variable can be used to select the unit number
preconnected to standard error.  This must be a positive integer.
The default value is 0.

@node GFORTRAN_UNBUFFERED_ALL
@section @env{GFORTRAN_UNBUFFERED_ALL}---Do not buffer I/O on all units

This environment variable controls whether all I/O is unbuffered.  If
the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
unbuffered.  This slows down small sequential reads and writes.  If
the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
This is the default.

@node GFORTRAN_UNBUFFERED_PRECONNECTED
@section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Do not buffer I/O on preconnected units

The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered.  If
the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered.  This
slows down small sequential reads and writes.  If the first letter
is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.  This is the default.

@node GFORTRAN_SHOW_LOCUS
@section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors

If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
line numbers for runtime errors are printed.  If the first letter is
@samp{n}, @samp{N} or @samp{0}, do not print filename and line numbers
for runtime errors.  The default is to print the location.

@node GFORTRAN_OPTIONAL_PLUS
@section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted

If the first letter is @samp{y}, @samp{Y} or @samp{1},
a plus sign is printed
where permitted by the Fortran standard.  If the first letter
is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
in most cases.  Default is not to print plus signs.

@node GFORTRAN_LIST_SEPARATOR
@section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output

This environment variable specifies the separator when writing
list-directed output.  It may contain any number of spaces and
at most one comma.  If you specify this on the command line,
be sure to quote spaces, as in
@smallexample
$ GFORTRAN_LIST_SEPARATOR='  ,  ' ./a.out
@end smallexample
@noindent
when @command{a.out} is the compiled Fortran program that you want to run.
Default is a single space.

@node GFORTRAN_CONVERT_UNIT
@section @env{GFORTRAN_CONVERT_UNIT}---Set conversion for unformatted I/O

By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
to change the representation of data for unformatted files.
The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable for
most systems is:
@smallexample
GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
exception: mode ':' unit_list | unit_list ;
unit_list: unit_spec | unit_list unit_spec ;
unit_spec: INTEGER | INTEGER '-' INTEGER ;
@end smallexample
The variable consists of an optional default mode, followed by
a list of optional exceptions, which are separated by semicolons
from the preceding default and each other.  Each exception consists
of a format and a comma-separated list of units.  Valid values for
the modes are the same as for the @code{CONVERT} specifier:

@itemize @w{}
@item @code{NATIVE} Use the native format.  This is the default.
@item @code{SWAP} Swap between little- and big-endian.
@item @code{LITTLE_ENDIAN} Use the little-endian format
for unformatted files.
@item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
@end itemize
For POWER systems that support @option{-mabi=ieeelongdouble},
there are additional options, which can be combined with the
others with commas. Those are
@itemize @w{}
@item @code{R16_IEEE} Use IEEE 128-bit format for @code{REAL(KIND=16)}.
@item @code{R16_IBM} Use IBM @code{long double} format for
@code{REAL(KIND=16)}.
@end itemize
A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
@itemize @w{}
@item @code{'big_endian'}  Do all unformatted I/O in big_endian mode.
@item @code{'little_endian;native:10-20,25'}  Do all unformatted I/O
in little_endian mode, except for units 10 to 20 and 25, which are in
native format.
@item @code{'10-20'}  Units 10 to 20 are big-endian, the rest is native.
@item @code{'big_endian,r16_ibm'} Do all unformatted I/O in big-endian
mode and use IBM long double for output of @code{REAL(KIND=16)} values.
@end itemize

Setting the environment variables should be done on the command
line or via the @command{export}
command for @command{sh}-compatible shells and via @command{setenv}
for @command{csh}-compatible shells.

Example for @command{sh}:
@smallexample
$ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
@end smallexample

Example code for @command{csh}:
@smallexample
% gfortran foo.f90
% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
% ./a.out
@end smallexample

Using anything but the native representation for unformatted data
carries a significant speed overhead.  If speed in this area matters
to you, it is best if you use this only for data that needs to be
portable.

@xref{CONVERT specifier}, for an alternative way to specify the
data representation for unformatted files.  @xref{Runtime Options}, for
setting a default data representation for the whole program.  The
@code{CONVERT} specifier overrides the @option{-fconvert} compile options.

Note that the values specified via the @env{GFORTRAN_CONVERT_UNIT}
environment variable override the @code{CONVERT} specifier in the
@code{OPEN} statement.  This is to give control over data formats to
users who do not have the source code of their program available.

@node GFORTRAN_ERROR_BACKTRACE
@section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors

If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to @samp{y},
@samp{Y} or @samp{1} (only the first letter is relevant) then a
backtrace is printed when a serious run-time error occurs.  To disable
the backtracing, set the variable to @samp{n}, @samp{N}, @samp{0}.
Default is to print a backtrace unless the @option{-fno-backtrace}
compile option was used.

@node GFORTRAN_FORMATTED_BUFFER_SIZE
@section @env{GFORTRAN_FORMATTED_BUFFER_SIZE}---Set buffer size for formatted I/O

The @env{GFORTRAN_FORMATTED_BUFFER_SIZE} environment variable
specifies buffer size in bytes to be used for formatted output.
The default value is 8192.

@node GFORTRAN_UNFORMATTED_BUFFER_SIZE
@section @env{GFORTRAN_UNFORMATTED_BUFFER_SIZE}---Set buffer size for unformatted I/O

The @env{GFORTRAN_UNFORMATTED_BUFFER_SIZE} environment variable
specifies buffer size in bytes to be used for unformatted output.
The default value is 131072.

@c =====================================================================
@c PART II: LANGUAGE REFERENCE
@c =====================================================================

@tex
\part{II}{Language Reference}
@end tex



@c ---------------------------------------------------------------------
@c Compiler Characteristics
@c ---------------------------------------------------------------------

@node Compiler Characteristics
@chapter Compiler Characteristics

This chapter describes certain characteristics of the GNU Fortran
compiler that are not specified by the Fortran standard, but which
might in some way or another become visible to the programmer.

@menu
* KIND Type Parameters::
* Internal representation of LOGICAL variables::
* Evaluation of logical expressions::
* MAX and MIN intrinsics with REAL NaN arguments::
* Thread-safety of the runtime library::
* Data consistency and durability::
* Files opened without an explicit ACTION= specifier::
* File operations on symbolic links::
* File format of unformatted sequential files::
* Asynchronous I/O::
* Behavior on integer overflow::
@end menu


@node KIND Type Parameters
@section @code{KIND} Type Parameters
@cindex kind

The @code{KIND} type parameters supported by GNU Fortran for the primitive
data types are:

@table @code

@item INTEGER
1, 2, 4, 8*, 16*, default: 4**

@item LOGICAL
1, 2, 4, 8*, 16*, default: 4**

@item REAL
4, 8, 10*, 16*, default: 4***

@item COMPLEX
4, 8, 10*, 16*, default: 4***

@item DOUBLE PRECISION
4, 8, 10*, 16*, default: 8***

@item CHARACTER
1, 4, default: 1

@end table

@noindent
* not available on all systems @*
** unless @option{-fdefault-integer-8} is used @*
*** unless @option{-fdefault-real-8} is used (see @ref{Fortran Dialect Options})

@noindent
The @code{KIND} value matches the storage size in bytes, except for
@code{COMPLEX} where the storage size is twice as much (or both real and
imaginary part are a real value of the given size).  It is recommended to use
the @ref{SELECTED_CHAR_KIND}, @ref{SELECTED_INT_KIND}, @ref{SELECTED_LOGICAL_KIND}
and @ref{SELECTED_REAL_KIND} intrinsics or the @code{INT8}, @code{INT16},
@code{INT32}, @code{INT64}, @code{REAL32}, @code{REAL64}, and @code{REAL128}
parameters of the @code{ISO_FORTRAN_ENV} module instead of the concrete values.
The available kind parameters can be found in the constant arrays
@code{CHARACTER_KINDS}, @code{INTEGER_KINDS}, @code{LOGICAL_KINDS} and
@code{REAL_KINDS} in the @ref{ISO_FORTRAN_ENV} module.  For C interoperability,
the kind parameters of the @ref{ISO_C_BINDING} module should be used.


@node Internal representation of LOGICAL variables
@section Internal representation of @code{LOGICAL} variables
@cindex logical, variable representation

The Fortran standard does not specify how variables of @code{LOGICAL}
type are represented, beyond requiring that @code{LOGICAL} variables
of default kind have the same storage size as default @code{INTEGER}
and @code{REAL} variables.  The GNU Fortran internal representation is
as follows.

A @code{LOGICAL(KIND=N)} variable is represented as an
@code{INTEGER(KIND=N)} variable, however, with only two permissible
values: @code{1} for @code{.TRUE.} and @code{0} for
@code{.FALSE.}.  Any other integer value results in undefined behavior.

See also @ref{Argument passing conventions} and @ref{Interoperability with C}.


@node Evaluation of logical expressions
@section Evaluation of logical expressions

The Fortran standard does not require the compiler to evaluate all parts of an
expression, if they do not contribute to the final result.  For logical
expressions with @code{.AND.} or @code{.OR.} operators, in particular, GNU
Fortran optimizes out function calls (even to impure functions) if the
result of the expression can be established without them.  However, since not
all compilers do that, and such an optimization can potentially modify the
program flow and subsequent results, GNU Fortran throws warnings for such
situations with the @option{-Wfunction-elimination} flag.


@node MAX and MIN intrinsics with REAL NaN arguments
@section @code{MAX} and @code{MIN} intrinsics with @code{REAL} NaN arguments
@cindex MAX, MIN, NaN

The Fortran standard does not specify what the result of the
@code{MAX} and @code{MIN} intrinsics are if one of the arguments is a
@code{NaN}.  Accordingly, the GNU Fortran compiler does not specify
that either, as this allows for faster and more compact code to be
generated.  If the programmer wishes to take some specific action in
case one of the arguments is a @code{NaN}, it is necessary to
explicitly test the arguments before calling @code{MAX} or @code{MIN},
e.g. with the @code{IEEE_IS_NAN} function from the intrinsic module
@code{IEEE_ARITHMETIC}.


@node Thread-safety of the runtime library
@section Thread-safety of the runtime library
@cindex thread-safety, threads

GNU Fortran can be used in programs with multiple threads, e.g.@: by
using OpenMP, by calling OS thread handling functions via the
@code{ISO_C_BINDING} facility, or by GNU Fortran compiled library code
being called from a multi-threaded program.

The GNU Fortran runtime library, (@code{libgfortran}), supports being
called concurrently from multiple threads with the following
exceptions.

During library initialization, the C @code{getenv} function is used,
which need not be thread-safe.  Similarly, the @code{getenv}
function is used to implement the @code{GET_ENVIRONMENT_VARIABLE} and
@code{GETENV} intrinsics.  It is the responsibility of the user to
ensure that the environment is not being updated concurrently when any
of these actions are taking place.

The @code{EXECUTE_COMMAND_LINE} and @code{SYSTEM} intrinsics are
implemented with the @code{system} function, which need not be
thread-safe.  It is the responsibility of the user to ensure that
@code{system} is not called concurrently.

For platforms not supporting thread-safe POSIX functions, further
functionality might not be thread-safe.  For details, please consult
the documentation for your operating system.

The GNU Fortran runtime library uses various C library functions that
depend on the locale, such as @code{strtod} and @code{snprintf}.  In
order to work correctly in locale-aware programs that set the locale
using @code{setlocale}, the locale is reset to the default ``C''
locale while executing a formatted @code{READ} or @code{WRITE}
statement.  On targets supporting the POSIX 2008 per-thread locale
functions (e.g. @code{newlocale}, @code{uselocale},
@code{freelocale}), these are used and thus the global locale set
using @code{setlocale} or the per-thread locales in other threads are
not affected.  However, on targets lacking this functionality, the
global LC_NUMERIC locale is set to ``C'' during the formatted I/O.
Thus, on such targets it's not safe to call @code{setlocale}
concurrently from another thread while a Fortran formatted I/O
operation is in progress.  Also, other threads doing something
dependent on the LC_NUMERIC locale might not work correctly if a
formatted I/O operation is in progress in another thread.

@node Data consistency and durability
@section Data consistency and durability
@cindex consistency, durability

This section contains a brief overview of data and metadata
consistency and durability issues when doing I/O.

With respect to durability, GNU Fortran makes no effort to ensure that
data is committed to stable storage. If this is required, the GNU
Fortran programmer can use the intrinsic @code{FNUM} to retrieve the
low level file descriptor corresponding to an open Fortran unit. Then,
using e.g. the @code{ISO_C_BINDING} feature, one can call the
underlying system call to flush dirty data to stable storage, such as
@code{fsync} on POSIX, @code{_commit} on MinGW, or @code{fcntl(fd,
F_FULLSYNC, 0)} on macOS. The following example shows how to call
fsync:

@smallexample
  ! Declare the interface for POSIX fsync function
  interface
    function fsync (fd) bind(c,name="fsync")
    use iso_c_binding, only: c_int
      integer(c_int), value :: fd
      integer(c_int) :: fsync
    end function fsync
  end interface

  ! Variable declaration
  integer :: ret

  ! Opening unit 10
  open (10,file="foo")

  ! ...
  ! Perform I/O on unit 10
  ! ...

  ! Flush and sync
  flush(10)
  ret = fsync(fnum(10))

  ! Handle possible error
  if (ret /= 0) stop "Error calling FSYNC"
@end smallexample

With respect to consistency, for regular files GNU Fortran uses
buffered I/O in order to improve performance. This buffer is flushed
automatically when full and in some other situations, e.g. when
closing a unit. It can also be explicitly flushed with the
@code{FLUSH} statement. Also, the buffering can be turned off with the
@code{GFORTRAN_UNBUFFERED_ALL} and
@code{GFORTRAN_UNBUFFERED_PRECONNECTED} environment variables. Special
files, such as terminals and pipes, are always unbuffered. Sometimes,
however, further things may need to be done in order to allow other
processes to see data that GNU Fortran has written, as follows.

The Windows platform supports a relaxed metadata consistency model,
where file metadata is written to the directory lazily. This means
that, for instance, the @code{dir} command can show a stale size for a
file. One can force a directory metadata update by closing the unit,
or by calling @code{_commit} on the file descriptor. Note, though,
that @code{_commit} forces all dirty data to stable storage, which
is often a very slow operation.

The Network File System (NFS) implements a relaxed consistency model
called open-to-close consistency. Closing a file forces dirty data and
metadata to be flushed to the server, and opening a file forces the
client to contact the server in order to revalidate cached
data. @code{fsync} also forces a flush of dirty data and metadata
to the server. Similar to @code{open} and @code{close}, acquiring and
releasing @code{fcntl} file locks, if the server supports them,
also forces cache validation and flushing dirty data and metadata.


@node Files opened without an explicit ACTION= specifier
@section Files opened without an explicit @code{ACTION=} specifier
@cindex open, action

The Fortran standard says that if an @code{OPEN} statement is executed
without an explicit @code{ACTION=} specifier, the default value is
processor dependent.  GNU Fortran behaves as follows:

@enumerate
@item Attempt to open the file with @code{ACTION='READWRITE'}
@item If that fails, try to open with @code{ACTION='READ'}
@item If that fails, try to open with @code{ACTION='WRITE'}
@item If that fails, generate an error
@end enumerate


@node File operations on symbolic links
@section File operations on symbolic links
@cindex file, symbolic link

This section documents the behavior of GNU Fortran for file operations on
symbolic links, on systems that support them.

@itemize

@item Results of @code{INQUIRE} statements of the ``inquire by file'' form
relate to the target of the symbolic link. For example,
@code{INQUIRE(FILE="foo",EXIST=ex)} sets @var{ex} to @var{.true.} if
@var{foo} is a symbolic link pointing to an existing file, and @var{.false.}
if @var{foo} points to an non-existing file (``dangling'' symbolic link).

@item Using the @code{OPEN} statement with a @code{STATUS="NEW"} specifier
on a symbolic link results in an error condition, whether the symbolic
link points to an existing target or is dangling.

@item If a symbolic link was connected, using the @code{CLOSE} statement
with a @code{STATUS="DELETE"} specifier causes the symbolic link itself
to be deleted, not its target.

@end itemize

@node File format of unformatted sequential files
@section File format of unformatted sequential files
@cindex file, unformatted sequential
@cindex unformatted sequential
@cindex sequential, unformatted
@cindex record marker
@cindex subrecord

Unformatted sequential files are stored as logical records using
record markers.  Each logical record consists of one of more
subrecords.

Each subrecord consists of a leading record marker, the data written
by the user program, and a trailing record marker.  The record markers
are four-byte integers by default, and eight-byte integers if the
@option{-fmax-subrecord-length=8} option (which exists for backwards
compatibility only) is in effect.

The representation of the record markers is that of unformatted files
given with the @option{-fconvert} option, the @ref{CONVERT specifier}
in an @code{OPEN} statement or the @ref{GFORTRAN_CONVERT_UNIT} environment
variable.

The maximum number of bytes of user data in a subrecord is 2147483639
(2 GiB - 9) for a four-byte record marker.  This limit can be lowered
with the @option{-fmax-subrecord-length} option, although this is
rarely useful. If the length of a logical record exceeds this limit,
the data is distributed among several subrecords.

The absolute of the number stored in the record markers is the number
of bytes of user data in the corresponding subrecord.  If the leading
record marker of a subrecord contains a negative number, another
subrecord follows the current one.  If the trailing record marker
contains a negative number, then there is a preceding subrecord.

In the most simple case, with only one subrecord per logical record,
both record markers contain the number of bytes of user data in the
record.

The format for unformatted sequential data can be duplicated using
unformatted stream, as shown in the example program for an unformatted
record containing a single subrecord:

@smallexample
program main
  use iso_fortran_env, only: int32
  implicit none
  integer(int32) :: i
  real, dimension(10) :: a, b
  call random_number(a)
  open (10,file='test.dat',form='unformatted',access='stream')
  inquire (iolength=i) a
  write (10) i, a, i
  close (10)
  open (10,file='test.dat',form='unformatted')
  read (10) b
  if (all (a == b)) print *,'success!'
end program main
@end smallexample

@node Asynchronous I/O
@section Asynchronous I/O
@cindex input/output, asynchronous
@cindex asynchronous I/O

Asynchronous I/O is supported if the program is linked against the
POSIX thread library. If that is not the case, all I/O is performed
as synchronous. On systems that do not support pthread condition
variables, such as AIX, I/O is also performed as synchronous.

On some systems, such as Darwin or Solaris, the POSIX thread library
is always linked in, so asynchronous I/O is always performed. On other
systems, such as Linux, it is necessary to specify @option{-pthread},
@option{-lpthread} or @option{-fopenmp} during the linking step.

@c ---------------------------------------------------------------------
@c Extensions
@c ---------------------------------------------------------------------

@c Maybe this chapter should be merged with the 'Standards' section,
@c whenever that is written :-)

@node Behavior on integer overflow
@section Behavior on integer overflow
@cindex integer overflow
@cindex overflow handling

Integer overflow is prohibited by the Fortran standard.  The behavior
of gfortran on integer overflow is undefined by default.  Traditional
code, like linear congruential pseudo-random number generators in old
programs that rely on specific, nonstandard behavior may generate
unexpected results.  The @option{-fsanitize=undefined} option can be
used to detect such code at runtime.

It is recommended to use the intrinsic subroutine @code{RANDOM_NUMBER}
for random number generators or, if the old behavior is desired, to
use the @option{-fwrapv} option.  Note that this option can impact
performance.

@node Extensions
@chapter Extensions
@cindex extensions

The two sections below detail the extensions to standard Fortran that are
implemented in GNU Fortran, as well as some of the popular or
historically important extensions that are not (or not yet) implemented.
For the latter case, we explain the alternatives available to GNU Fortran
users, including replacement by standard-conforming code or GNU
extensions.

@menu
* Extensions implemented in GNU Fortran::
* Extensions not implemented in GNU Fortran::
@end menu


@node Extensions implemented in GNU Fortran
@section Extensions implemented in GNU Fortran
@cindex extensions, implemented

GNU Fortran implements a number of extensions over standard Fortran.
This chapter contains information on their syntax and meaning.  There
are currently two categories of GNU Fortran extensions, those that
provide functionality beyond that provided by any standard, and those
that are supported by GNU Fortran purely for backward compatibility
with legacy compilers.  By default, @option{-std=gnu} allows the
compiler to accept both types of extensions, but to warn about the use
of the latter.  Specifying either @option{-std=f95},
@option{-std=f2003}, @option{-std=f2008}, or @option{-std=f2018}
disables both types of extensions, and @option{-std=legacy} allows
both without warning.  The special compile flag @option{-fdec} enables
additional compatibility extensions along with those enabled by
@option{-std=legacy}.

@menu
* Old-style kind specifications::
* Old-style variable initialization::
* Extensions to namelist::
* X format descriptor without count field::
* Commas in FORMAT specifications::
* Missing period in FORMAT specifications::
* Default widths for F/G/I format descriptors::
* I/O item lists::
* Q exponent-letter::
* BOZ literal constants::
* Real array indices::
* Unary operators::
* Implicitly convert LOGICAL and INTEGER values::
* Hollerith constants support::
* Character conversion::
* Cray pointers::
* CONVERT specifier::
* OpenMP::
* OpenACC::
* Argument list functions::
* Read/Write after EOF marker::
* STRUCTURE and RECORD::
* UNION and MAP::
* Type variants for integer intrinsics::
* AUTOMATIC and STATIC attributes::
* Form feed as whitespace::
* TYPE as an alias for PRINT::
* %LOC as an rvalue::
* .XOR. operator::
* Bitwise logical operators::
* Extended I/O specifiers::
* Legacy PARAMETER statements::
* Default exponents::
* Unsigned integers::
@end menu

@node Old-style kind specifications
@subsection Old-style kind specifications
@cindex kind, old-style

GNU Fortran allows old-style kind specifications in declarations.  These
look like:
@smallexample
      TYPESPEC*@var{size} x,y,z
@end smallexample
@noindent
where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
etc.), and where @var{size} is a byte count corresponding to the
storage size of a valid kind for that type.  (For @code{COMPLEX}
variables, @var{size} is the total size of the real and imaginary
parts.)  The statement then declares @code{x}, @code{y} and @code{z} to
be of type @code{TYPESPEC} with the appropriate kind.  This is
equivalent to the standard-conforming declaration
@smallexample
      TYPESPEC(k) x,y,z
@end smallexample
@noindent
where @code{k} is the kind parameter suitable for the intended precision.  As
kind parameters are implementation-dependent, use the @code{KIND},
@code{SELECTED_INT_KIND}, @code{SELECTED_LOGICAL_KIND} and
@code{SELECTED_REAL_KIND} intrinsics to retrieve the correct value, for
instance @code{REAL*8 x} can be replaced by:
@smallexample
INTEGER, PARAMETER :: dbl = KIND(1.0d0)
REAL(KIND=dbl) :: x
@end smallexample

@node Old-style variable initialization
@subsection Old-style variable initialization

GNU Fortran allows old-style initialization of variables of the
form:
@smallexample
      INTEGER i/1/,j/2/
      REAL x(2,2) /3*0.,1./
@end smallexample
The syntax for the initializers is as for the @code{DATA} statement, but
unlike in a @code{DATA} statement, an initializer only applies to the
variable immediately preceding the initialization.  In other words,
something like @code{INTEGER I,J/2,3/} is not valid.  This style of
initialization is only allowed in declarations without double colons
(@samp{::}); the double colons were introduced in Fortran 90, which also
introduced a standard syntax for initializing variables in type
declarations.

Examples of standard-conforming code equivalent to the above example
are:
@smallexample
! Fortran 90
      INTEGER :: i = 1, j = 2
      REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
! Fortran 77
      INTEGER i, j
      REAL x(2,2)
      DATA i/1/, j/2/, x/3*0.,1./
@end smallexample

Note that variables that are explicitly initialized in declarations
or in @code{DATA} statements automatically acquire the @code{SAVE}
attribute.

@node Extensions to namelist
@subsection Extensions to namelist
@cindex Namelist

GNU Fortran fully supports the Fortran 95 standard for namelist I/O
including array qualifiers, substrings and fully qualified derived types.
The output from a namelist write is compatible with namelist read.  The
output has all names in upper case and indentation to column 1 after the
namelist name.  The following extensions are permitted:

@itemize @bullet

@item
Old-style use of @samp{$} instead of @samp{&}
@smallexample
$MYNML
 X(:)%Y(2) = 1.0 2.0 3.0
 CH(1:4) = "abcd"
$END
@end smallexample

It should be noted that the default terminator is @samp{/} rather than
@samp{&END}.

@item
Querying of the namelist when inputting from stdin.  After at least
one space, entering @samp{?} sends to stdout the namelist name and the names of
the variables in the namelist:
@smallexample
 ?

&mynml
 x
 x%y
 ch
&end
@end smallexample

Entering @samp{=?} outputs the namelist to stdout, as if
@code{WRITE(*,NML = mynml)} had been called:
@smallexample
=?

&MYNML
 X(1)%Y=  0.000000    ,  1.000000    ,  0.000000    ,
 X(2)%Y=  0.000000    ,  2.000000    ,  0.000000    ,
 X(3)%Y=  0.000000    ,  3.000000    ,  0.000000    ,
 CH=abcd,  /
@end smallexample

To aid this dialog, when input is from stdin, errors send their
messages to stderr and execution continues, even if @code{IOSTAT} is set.

@item
@code{PRINT} namelist is permitted.  This causes an error if
@option{-std=f95} is used.
@smallexample
PROGRAM test_print
  REAL, dimension (4)  ::  x = (/1.0, 2.0, 3.0, 4.0/)
  NAMELIST /mynml/ x
  PRINT mynml
END PROGRAM test_print
@end smallexample

@item
Expanded namelist reads are permitted.  This causes an error if
@option{-std=f95} is used.  In the following example, the first element
of the array is given the value 0.00 and the two succeeding
elements are given the values 1.00 and 2.00.
@smallexample
&MYNML
  X(1,1) = 0.00 , 1.00 , 2.00
/
@end smallexample
@end itemize

When writing a namelist, if no @code{DELIM=} is specified, by default a
double quote is used to delimit character strings. With @option{-std=f95}
or later, the delim status is set to @code{'none'}.  Defaulting to
quotes ensures that namelists with character strings can be subsequently
read back in accurately.

@node X format descriptor without count field
@subsection @code{X} format descriptor without count field

To support legacy codes, GNU Fortran permits the count field of the
@code{X} edit descriptor in @code{FORMAT} statements to be omitted.
When omitted, the count is implicitly assumed to be one.

@smallexample
       PRINT 10, 2, 3
10     FORMAT (I1, X, I1)
@end smallexample

@node Commas in FORMAT specifications
@subsection Commas in @code{FORMAT} specifications

To support legacy codes, GNU Fortran allows the comma separator
to be omitted immediately before and after character string edit
descriptors in @code{FORMAT} statements.  A comma with no following format
descriptor is permitted if the @option{-fdec-blank-format-item} is given on
the command line. This is considered non-conforming code and is
discouraged.

@smallexample
       PRINT 10, 2, 3
10     FORMAT ('FOO='I1' BAR='I2)
       print 20, 5, 6
20     FORMAT (I3, I3,)
@end smallexample


@node Missing period in FORMAT specifications
@subsection Missing period in @code{FORMAT} specifications

To support legacy codes, GNU Fortran allows missing periods in format
specifications if and only if @option{-std=legacy} is given on the
command line.  This is considered non-conforming code and is
discouraged.

@smallexample
       REAL :: value
       READ(*,10) value
10     FORMAT ('F4')
@end smallexample

@node Default widths for F/G/I format descriptors
@subsection Default widths for @samp{F}, @samp{G} and @samp{I} format descriptors

To support legacy codes, GNU Fortran allows width to be omitted from format
specifications if and only if @option{-fdec-format-defaults} is given on the
command line.  Default widths are used. This is considered non-conforming
code and is discouraged.

@smallexample
       REAL :: value1
       INTEGER :: value2
       WRITE(*,10) value1, value1, value2
10     FORMAT ('F, G, I')
@end smallexample


@node I/O item lists
@subsection I/O item lists
@cindex I/O item lists

To support legacy codes, GNU Fortran allows the input item list
of the @code{READ} statement, and the output item lists of the
@code{WRITE} and @code{PRINT} statements, to start with a comma.

@node Q exponent-letter
@subsection @samp{Q} exponent-letter
@cindex @samp{Q} exponent-letter

GNU Fortran accepts real literal constants with an exponent-letter
of @samp{Q}, for example, @code{1.23Q45}.  The constant is interpreted
as a @code{REAL(16)} entity on targets that support this type.  If
the target does not support @code{REAL(16)} but has a @code{REAL(10)}
type, then the real-literal-constant is interpreted as a
@code{REAL(10)} entity.  In the absence of @code{REAL(16)} and
@code{REAL(10)}, an error occurs.

@node BOZ literal constants
@subsection BOZ literal constants
@cindex BOZ literal constants

Besides decimal constants, Fortran also supports binary (@samp{b}),
octal (@samp{o}) and hexadecimal (@samp{z}) integer constants.  The
syntax is: @samp{prefix quote digits quote}, where the prefix is
either @samp{b}, @samp{o} or @samp{z}, quote is either @samp{'} or
@samp{"} and the digits are @code{0} or @code{1} for binary,
between @code{0} and @code{7} for octal, and between @code{0} and
@code{F} for hexadecimal.  (Example: @code{b'01011101'}.)

Up to Fortran 95, BOZ literal constants were only allowed to initialize
integer variables in DATA statements.  Since Fortran 2003 BOZ literal
constants are also allowed as actual arguments to the @code{REAL},
@code{DBLE}, @code{INT} and @code{CMPLX} intrinsic functions.
The BOZ literal constant is simply a string of bits, which is padded
or truncated as needed, during conversion to a numeric type.  The 
Fortran standard states that the treatment of the sign bit is processor
dependent.  Gfortran interprets the sign bit as a user would expect.

As a deprecated extension, GNU Fortran allows hexadecimal BOZ literal
constants to be specified using the @samp{X} prefix.  That the BOZ literal
constant can also be specified by adding a suffix to the string, for
example, @code{Z'ABC'} and @code{'ABC'X} are equivalent.  Additionally,
as extension, BOZ literals are permitted in some contexts outside of
@code{DATA} and the intrinsic functions listed in the Fortran standard.
Use @option{-fallow-invalid-boz} to enable the extension.

@node Real array indices
@subsection Real array indices
@cindex array, indices of type real

As an extension, GNU Fortran allows the use of @code{REAL} expressions
or variables as array indices.

@node Unary operators
@subsection Unary operators
@cindex operators, unary

As an extension, GNU Fortran allows unary plus and unary minus operators
to appear as the second operand of binary arithmetic operators without
the need for parenthesis.

@smallexample
       X = Y * -Z
@end smallexample

@node Implicitly convert LOGICAL and INTEGER values
@subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
@cindex conversion, to integer
@cindex conversion, to logical

As an extension for backwards compatibility with other compilers, GNU
Fortran allows the implicit conversion of @code{LOGICAL} values to
@code{INTEGER} values and vice versa.  When converting from a
@code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
zero, and @code{.TRUE.} is interpreted as one.  When converting from
@code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
@code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.

@smallexample
        LOGICAL :: l
        l = 1
@end smallexample
@smallexample
        INTEGER :: i
        i = .TRUE.
@end smallexample

However, there is no implicit conversion of @code{INTEGER} values in
@code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
in I/O operations.

@node Hollerith constants support
@subsection Hollerith constants support
@cindex Hollerith constants

GNU Fortran supports Hollerith constants in assignments, @code{DATA}
statements, function and subroutine arguments. A Hollerith constant is
written as a string of characters preceded by an integer constant 
indicating the character count, and the letter @code{H} or
@code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
@code{REAL}, or @code{COMPLEX}), @code{LOGICAL} or @code{CHARACTER} variable.
The constant is padded with spaces or truncated to fit the size of
the variable in which it is stored.

Examples of valid uses of Hollerith constants:
@smallexample
      complex*16 x(2)
      data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
      x(1) = 16HABCDEFGHIJKLMNOP
      call foo (4h abc)
@end smallexample

Examples of Hollerith constants:
@smallexample
      integer*4 a
      a = 0H         ! Invalid, at least one character is needed.
      a = 4HAB12     ! Valid
      a = 8H12345678 ! Valid, but the Hollerith constant is truncated.
      a = 3Hxyz      ! Valid, but the Hollerith constant is padded.
@end smallexample

In general, Hollerith constants were used to provide a rudimentary
facility for handling character strings in early Fortran compilers,
prior to the introduction of @code{CHARACTER} variables in Fortran 77;
in those cases, the standard-compliant equivalent is to convert the
program to use proper character strings.  On occasion, there may be a
case where the intent is specifically to initialize a numeric variable
with a given byte sequence.  In these cases, the same result can be
obtained by using the @code{TRANSFER} statement, as in this example.
@smallexample
      integer(kind=4) :: a
      a = transfer ("abcd", a)     ! equivalent to: a = 4Habcd
@end smallexample

The use of the @option{-fdec} option extends support of Hollerith constants
to comparisons:
@smallexample
      integer*4 a
      a = 4hABCD
      if (a .ne. 4habcd) then
        write(*,*) "no match"
      end if
@end smallexample

Supported types are numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}),
and @code{CHARACTER}.

@node Character conversion
@subsection Character conversion
@cindex conversion, to character

Allowing character literals to be used in a similar way to Hollerith constants
is a nonstandard extension.  This feature is enabled using
-fdec-char-conversions and only applies to character literals of @code{kind=1}.

Character literals can be used in @code{DATA} statements and assignments with
numeric (@code{INTEGER}, @code{REAL}, or @code{COMPLEX}) or @code{LOGICAL}
variables. Like Hollerith constants they are copied bytewise fashion. The
constant is padded with spaces or truncated to fit the size of the
variable in which it is stored.

Examples:
@smallexample
      integer*4 x
      data x / 'abcd' /

      x = 'A'       ! Is padded.
      x = 'ab1234'  ! Is truncated.
@end smallexample


@node Cray pointers
@subsection Cray pointers
@cindex pointer, Cray

Cray pointers are part of a nonstandard extension that provides a
C-like pointer in Fortran.  This is accomplished through a pair of
variables: an integer ``pointer'' that holds a memory address, and a
``pointee'' that is used to dereference the pointer.

Pointer/pointee pairs are declared in statements of the form:
@smallexample
        pointer ( <pointer> , <pointee> )
@end smallexample
@noindent
or,
@smallexample
        pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
@end smallexample
The pointer is an integer that is intended to hold a memory address.
The pointee may be an array or scalar.
If an assumed-size array is permitted within the scoping unit, a
pointee can be an assumed-size array.
That is, the last dimension may be left unspecified by using a @code{*}
in place of a value. A pointee cannot be an assumed shape array.
No space is allocated for the pointee.

The pointee may have its type declared before or after the pointer
statement, and its array specification (if any) may be declared
before, during, or after the pointer statement.  The pointer may be
declared as an integer prior to the pointer statement.  However, some
machines have default integer sizes that are different than the size
of a pointer, and so the following code is not portable:
@smallexample
        integer ipt
        pointer (ipt, iarr)
@end smallexample
If a pointer is declared with a kind that is too small, the compiler
issues a warning; the resulting binary will probably not work
correctly, because the memory addresses stored in the pointers may be
truncated.  It is safer to omit the first line of the above example;
if explicit declaration of @code{ipt}'s type is omitted, then the compiler
ensures that @code{ipt} is an integer variable large enough to hold a
pointer.

Pointer arithmetic is valid with Cray pointers, but it is not the same
as C pointer arithmetic.  Cray pointers are just ordinary integers, so
the user is responsible for determining how many bytes to add to a
pointer in order to increment it.  Consider the following example:
@smallexample
        real target(10)
        real pointee(10)
        pointer (ipt, pointee)
        ipt = loc (target)
        ipt = ipt + 1
@end smallexample
The last statement does not set @code{ipt} to the address of
@code{target(1)}, as it would in C pointer arithmetic.  Adding @code{1}
to @code{ipt} just adds one byte to the address stored in @code{ipt}.

Any expression involving the pointee is translated to use the
value stored in the pointer as the base address.

To get the address of elements, this extension provides an intrinsic
function @code{LOC()}.  The @code{LOC()} function is equivalent to the
@code{&} operator in C, except the address is cast to an integer type:
@smallexample
        real ar(10)
        pointer(ipt, arpte(10))
        real arpte
        ipt = loc(ar)  ! Makes arpte is an alias for ar
        arpte(1) = 1.0 ! Sets ar(1) to 1.0
@end smallexample
The pointer can also be set by a call to the @code{MALLOC} intrinsic
(see @ref{MALLOC}).

Cray pointees often are used to alias an existing variable.  For
example:
@smallexample
        integer target(10)
        integer iarr(10)
        pointer (ipt, iarr)
        ipt = loc(target)
@end smallexample
As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
@code{target}.  The optimizer, however, does not detect this aliasing, so
it is unsafe to use @code{iarr} and @code{target} simultaneously.  Using
a pointee in any way that violates the Fortran aliasing rules or
assumptions is invalid.  It is the user's responsibility to avoid doing
this; the compiler works under the assumption that no such aliasing
occurs.

Cray pointers work correctly when there is no aliasing (i.e., when
they are used to access a dynamically allocated block of memory), and
also in any routine where a pointee is used, but any variable with which
it shares storage is not used.  Code that violates these rules may not
run as the user intends.  This is not a bug in the optimizer; any code
that violates the aliasing rules is invalid.  (Note that this is not
unique to GNU Fortran; any Fortran compiler that supports Cray pointers
``incorrectly'' optimizes code with invalid aliasing.)

There are a number of restrictions on the attributes that can be applied
to Cray pointers and pointees.  Pointees may not have the
@code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
@code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes.  Pointers
may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
@code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes, nor
may they be function results.  Pointees may not occur in more than one
pointer statement.  A pointee cannot be a pointer.  Pointees cannot occur
in equivalence, common, or data statements.

A Cray pointer may also point to a function or a subroutine.  For
example, the following excerpt is valid:
@smallexample
  implicit none
  external sub
  pointer (subptr,subpte)
  external subpte
  subptr = loc(sub)
  call subpte()
  [...]
  subroutine sub
  [...]
  end subroutine sub
@end smallexample

A pointer may be modified during the course of a program, and this
changes the location to which the pointee refers.  However, when
pointees are passed as arguments, they are treated as ordinary
variables in the invoked function.  Subsequent changes to the pointer
do not change the base address of the array that was passed.

@node CONVERT specifier
@subsection @code{CONVERT} specifier
@cindex @code{CONVERT} specifier

GNU Fortran allows the conversion of unformatted data between little-
and big-endian representation to facilitate moving of data
between different systems.  The conversion can be indicated with
the @code{CONVERT} specifier on the @code{OPEN} statement.
@xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
the data format via an environment variable.

Valid values for @code{CONVERT} on most systems are:
@itemize @w{}
@item @code{CONVERT='NATIVE'} Use the native format.  This is the default.
@item @code{CONVERT='SWAP'} Swap between little- and big-endian.
@item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
for unformatted files.
@item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
unformatted files.
@end itemize
On POWER systems that support @option{-mabi=ieeelongdouble},
there are additional options, which can be combined with the others
with commas. Those are
@itemize @w{}
@item @code{CONVERT='R16_IEEE'} Use IEEE 128-bit format for
@code{REAL(KIND=16)}.
@item @code{CONVERT='R16_IBM'} Use IBM @code{long double} format for
@code{REAL(KIND=16)}.
@end itemize

Using the option could look like this:
@smallexample
  open(file='big.dat',form='unformatted',access='sequential', &
       convert='big_endian')
@end smallexample

The value of the conversion can be queried by using
@code{INQUIRE(CONVERT=ch)}.  The values returned are
@code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.

@code{CONVERT} works between big- and little-endian for
@code{INTEGER} values of all supported kinds and for @code{REAL}
on IEEE systems of kinds 4 and 8.  Conversion between different
``extended double'' types on different architectures such as
m68k and x86_64, which GNU Fortran
supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)},
probably does not work.

Note that the values specified via the @env{GFORTRAN_CONVERT_UNIT}
environment variable overrides the @code{CONVERT} specifier in the
@code{OPEN} statement.  This is to give control over data formats to
users who do not have the source code of their program available.

Using anything but the native representation for unformatted data
carries a significant speed overhead.  If speed in this area matters
to you, it is best if you use this only for data that needs to be
portable.

@node OpenMP
@subsection OpenMP
@cindex OpenMP

OpenMP (Open Multi-Processing) is an application programming
interface (API) that supports multi-platform shared memory
multiprocessing programming in C/C++ and Fortran on many
architectures, including Unix and Microsoft Windows platforms.
It consists of a set of compiler directives, library routines,
and environment variables that influence run-time behavior.

GNU Fortran implements most of the @uref{https://openmp.org/specifications/,
OpenMP Application Program Interface v5.2}, with some omissions and
additional features from later versions.
@xref{OpenMP Implementation Status,,,libgomp,
GNU Offloading and Multi Processing Runtime Library},
for more details about currently supported OpenMP features.

To enable the processing of the OpenMP directive @code{!$omp} in
free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
directives in fixed form; the @code{!$} conditional compilation sentinels
in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
in fixed form, @command{gfortran} needs to be invoked with the
@option{-fopenmp} option.  This option also arranges for automatic linking
of the OpenMP runtime library.
@xref{Top,,,libgomp,GNU Offloading and Multi Processing Runtime Library}.

The OpenMP Fortran runtime library routines are provided both in a
form of a Fortran 90 module named @code{omp_lib} and in a form of
a Fortran @code{include} file named @file{omp_lib.h}.

An example of a parallelized loop taken from Appendix A.1 of
the OpenMP Application Program Interface v2.5:
@smallexample
SUBROUTINE A1(N, A, B)
  INTEGER I, N
  REAL B(N), A(N)
!$OMP PARALLEL DO !I is private by default
  DO I=2,N
    B(I) = (A(I) + A(I-1)) / 2.0
  ENDDO
!$OMP END PARALLEL DO
END SUBROUTINE A1
@end smallexample

@xref{OpenMP and OpenACC Options,,,gcc,
Using the GNU Compiler Collection (GCC)}, for additional options useful with
@option{-fopenmp}.

Please note:
@itemize
@item
@option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
are allocated on the stack.  When porting existing code to OpenMP,
this may lead to surprising results, especially segmentation faults
if the stack size is limited.

@item
On glibc-based systems, OpenMP-enabled applications cannot be statically
linked due to limitations of the underlying pthreads implementation.  It
might be possible to get a working solution if
@command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
to the command line.  However, this is not supported by GCC and
thus not recommended.
@end itemize

@node OpenACC
@subsection OpenACC
@cindex OpenACC

OpenACC is an application programming interface (API) that supports
offloading of code to accelerator devices.  It consists of a set of
compiler directives, library routines, and environment variables that
influence run-time behavior.

GNU Fortran strives to be compatible with the
@uref{https://www.openacc.org/, OpenACC Application Programming
Interface v2.6}.

To enable the processing of the OpenACC directive @code{!$acc} in
free-form source code; the @code{c$acc}, @code{*$acc} and @code{!$acc}
directives in fixed form; the @code{!$} conditional compilation
sentinels in free form; and the @code{c$}, @code{*$} and @code{!$}
sentinels in fixed form, @command{gfortran} needs to be invoked with
the @option{-fopenacc} option.  This option also arranges for automatic
linking of the OpenACC runtime library.
@xref{Top,,,libgomp,GNU Offloading and Multi Processing Runtime Library}.

The OpenACC Fortran runtime library routines are provided both in a
form of a Fortran 90 module named @code{openacc} and in a form of a
Fortran @code{include} file named @file{openacc_lib.h}.

@xref{OpenMP and OpenACC Options,,,gcc,
Using the GNU Compiler Collection (GCC)}, for additional options useful with
@option{-fopenacc}.

@node Argument list functions
@subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
@cindex argument list functions
@cindex @code{%VAL}
@cindex @code{%REF}
@cindex @code{%LOC}

GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
and @code{%LOC} statements, for backward compatibility with g77.
It is recommended that these should be used only for code that is
accessing facilities outside of GNU Fortran, such as operating system
or windowing facilities.  It is best to constrain such uses to isolated
portions of a program--portions that deal specifically and exclusively
with low-level, system-dependent facilities.  Such portions might well
provide a portable interface for use by the program as a whole, but are
themselves not portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.

@code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
reference and @code{%LOC} passes its memory location.  Since gfortran
already passes scalar arguments by reference, @code{%REF} is in effect
a do-nothing.  @code{%LOC} has the same effect as a Fortran pointer.

An example of passing an argument by value to a C subroutine foo.:
@smallexample
C
C prototype      void foo_ (float x);
C
      external foo
      real*4 x
      x = 3.14159
      call foo (%VAL (x))
      end
@end smallexample

For details refer to the g77 manual
@uref{https://gcc.gnu.org/@/onlinedocs/@/gcc-3.4.6/@/g77/@/index.html#Top}.

Also, @code{c_by_val.f} and its partner @code{c_by_val.c} of the
GNU Fortran testsuite are worth a look.

@node Read/Write after EOF marker
@subsection Read/Write after EOF marker
@cindex @code{EOF}
@cindex @code{BACKSPACE}
@cindex @code{REWIND}

Some legacy codes rely on allowing @code{READ} or @code{WRITE} after the
EOF file marker in order to find the end of a file. GNU Fortran normally
rejects these codes with a run-time error message and suggests the user
consider @code{BACKSPACE} or @code{REWIND} to properly position
the file before the EOF marker.  As an extension, the run-time error may
be disabled using -std=legacy.


@node STRUCTURE and RECORD
@subsection @code{STRUCTURE} and @code{RECORD}
@cindex @code{STRUCTURE}
@cindex @code{RECORD}

Record structures are a pre-Fortran-90 vendor extension to create
user-defined aggregate data types.  Support for record structures in GNU
Fortran can be enabled with the @option{-fdec-structure} compile flag.
If you have a choice, you should instead use Fortran 90's ``derived types'',
which have a different syntax.

In many cases, record structures can easily be converted to derived types.
To convert, replace @code{STRUCTURE /}@var{structure-name}@code{/}
by @code{TYPE} @var{type-name}.  Additionally, replace
@code{RECORD /}@var{structure-name}@code{/} by
@code{TYPE(}@var{type-name}@code{)}. Finally, in the component access,
replace the period (@code{.}) by the percent sign (@code{%}).

Here is an example of code using the non portable record structure syntax:

@example
! Declaring a structure named ``item'' and containing three fields:
! an integer ID, an description string and a floating-point price.
STRUCTURE /item/
  INTEGER id
  CHARACTER(LEN=200) description
  REAL price
END STRUCTURE

! Define two variables, an single record of type ``item''
! named ``pear'', and an array of items named ``store_catalog''
RECORD /item/ pear, store_catalog(100)

! We can directly access the fields of both variables
pear.id = 92316
pear.description = "juicy D'Anjou pear"
pear.price = 0.15
store_catalog(7).id = 7831
store_catalog(7).description = "milk bottle"
store_catalog(7).price = 1.2

! We can also manipulate the whole structure
store_catalog(12) = pear
print *, store_catalog(12)
@end example

@noindent
This code can easily be rewritten in the Fortran 90 syntax as following:

@example
! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
! ``TYPE name ... END TYPE''
TYPE item
  INTEGER id
  CHARACTER(LEN=200) description
  REAL price
END TYPE

! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
TYPE(item) pear, store_catalog(100)

! Instead of using a dot (.) to access fields of a record, the
! standard syntax uses a percent sign (%)
pear%id = 92316
pear%description = "juicy D'Anjou pear"
pear%price = 0.15
store_catalog(7)%id = 7831
store_catalog(7)%description = "milk bottle"
store_catalog(7)%price = 1.2

! Assignments of a whole variable do not change
store_catalog(12) = pear
print *, store_catalog(12)
@end example

@noindent
GNU Fortran implements structures like derived types with the following
rules and exceptions:

@itemize @bullet
@item Structures act like derived types with the @code{SEQUENCE} attribute.
Otherwise they may contain no specifiers.

@item Structures may contain a special field with the name @code{%FILL}.
This creates an anonymous component that cannot be accessed but occupies
space just as if a component of the same type was declared in its place, useful
for alignment purposes.  As an example, the following structure consists
of at least sixteen bytes:

@smallexample
structure /padded/
  character(4) start
  character(8) %FILL
  character(4) end
end structure
@end smallexample

@item Structures may share names with other symbols. For example, the following
is invalid for derived types, but valid for structures:

@smallexample
structure /header/
  ! ...
end structure
record /header/ header
@end smallexample

@item Structure types may be declared nested within another parent structure.
The syntax is:
@smallexample
structure /type-name/
    ...
    structure [/<type-name>/] <field-list>
...
@end smallexample

The type name may be omitted, in which case the structure type itself is
anonymous, and other structures of the same type cannot be instantiated. The
following shows some examples:

@example
structure /appointment/
  ! nested structure definition: app_time is an array of two 'time'
  structure /time/ app_time (2)
    integer(1) hour, minute
  end structure
  character(10) memo
end structure

! The 'time' structure is still usable
record /time/ now
now = time(5, 30)

...

structure /appointment/
  ! anonymous nested structure definition
  structure start, end
    integer(1) hour, minute
  end structure
  character(10) memo
end structure
@end example

@item Structures may contain @code{UNION} blocks. For more detail see the
section on @ref{UNION and MAP}.

@item Structures support old-style initialization of components, like
those described in @ref{Old-style variable initialization}. For array
initializers, an initializer may contain a repeat specification of the form
@code{<literal-integer> * <constant-initializer>}. The value of the integer
indicates the number of times to repeat the constant initializer when expanding
the initializer list.
@end itemize

@node UNION and MAP
@subsection @code{UNION} and @code{MAP}
@cindex @code{UNION}
@cindex @code{MAP}

Unions are an old vendor extension which were commonly used with the
nonstandard @ref{STRUCTURE and RECORD} extensions. Use of @code{UNION} and
@code{MAP} is automatically enabled with @option{-fdec-structure}.

A @code{UNION} declaration occurs within a structure; within the definition of
each union is a number of @code{MAP} blocks. Each @code{MAP} shares storage
with its sibling maps (in the same union), and the size of the union is the
size of the largest map within it, just as with unions in C. The major
difference is that component references do not indicate which union or map the
component is in (the compiler gets to figure that out).

Here is a small example:
@smallexample
structure /myunion/
union
  map
    character(2) w0, w1, w2
  end map
  map
    character(6) long
  end map
end union
end structure

record /myunion/ rec
! After this assignment...
rec.long = 'hello!'

! The following is true:
! rec.w0 === 'he'
! rec.w1 === 'll'
! rec.w2 === 'o!'
@end smallexample

The two maps share memory, and the size of the union is ultimately six bytes:

@example
0    1    2    3    4   5   6     Byte offset
-------------------------------
|    |    |    |    |    |    |
-------------------------------

^    W0   ^    W1   ^    W2   ^
 \-------/ \-------/ \-------/

^             LONG            ^
 \---------------------------/
@end example

Following is an example mirroring the layout of an Intel x86_64 register:

@example
structure /reg/
  union ! U0                ! rax
    map
      character(16) rx
    end map
    map
      character(8) rh         ! rah
      union ! U1
        map
          character(8) rl     ! ral
        end map
        map
          character(8) ex     ! eax
        end map
        map
          character(4) eh     ! eah
          union ! U2
            map
              character(4) el ! eal
            end map
            map
              character(4) x  ! ax
            end map
            map
              character(2) h  ! ah
              character(2) l  ! al
            end map
          end union
        end map
      end union
    end map
  end union
end structure
record /reg/ a

! After this assignment...
a.rx     =     'AAAAAAAA.BBB.C.D'

! The following is true:
a.rx === 'AAAAAAAA.BBB.C.D'
a.rh === 'AAAAAAAA'
a.rl ===         '.BBB.C.D'
a.ex ===         '.BBB.C.D'
a.eh ===         '.BBB'
a.el ===             '.C.D'
a.x  ===             '.C.D'
a.h  ===             '.C'
a.l  ===               '.D'
@end example

@node Type variants for integer intrinsics
@subsection Type variants for integer intrinsics
@cindex intrinsics, integer

Similar to the D/C prefixes to real functions to specify the input/output
types, GNU Fortran offers B/I/J/K prefixes to integer functions for
compatibility with DEC programs. The types implied by each are:

@example
@code{B} - @code{INTEGER(kind=1)}
@code{I} - @code{INTEGER(kind=2)}
@code{J} - @code{INTEGER(kind=4)}
@code{K} - @code{INTEGER(kind=8)}
@end example

GNU Fortran supports these with the flag @option{-fdec-intrinsic-ints}.
Intrinsics for which prefixed versions are available and in what form are noted
in @ref{Intrinsic Procedures}. The complete list of supported intrinsics is
here:

@multitable @columnfractions .2 .2 .2 .2 .2

@headitem Intrinsic @tab B @tab I @tab J @tab K

@item @code{@ref{ABS}}
  @tab @code{BABS} @tab @code{IIABS} @tab @code{JIABS} @tab @code{KIABS}
@item @code{@ref{BTEST}}
  @tab @code{BBTEST} @tab @code{BITEST} @tab @code{BJTEST} @tab @code{BKTEST}
@item @code{@ref{IAND}}
  @tab @code{BIAND} @tab @code{IIAND} @tab @code{JIAND} @tab @code{KIAND}
@item @code{@ref{IBCLR}}
  @tab @code{BBCLR} @tab @code{IIBCLR} @tab @code{JIBCLR} @tab @code{KIBCLR}
@item @code{@ref{IBITS}}
  @tab @code{BBITS} @tab @code{IIBITS} @tab @code{JIBITS} @tab @code{KIBITS}
@item @code{@ref{IBSET}}
  @tab @code{BBSET} @tab @code{IIBSET} @tab @code{JIBSET} @tab @code{KIBSET}
@item @code{@ref{IEOR}}
  @tab @code{BIEOR} @tab @code{IIEOR} @tab @code{JIEOR} @tab @code{KIEOR}
@item @code{@ref{IOR}}
  @tab @code{BIOR} @tab @code{IIOR} @tab @code{JIOR} @tab @code{KIOR}
@item @code{@ref{ISHFT}}
  @tab @code{BSHFT} @tab @code{IISHFT} @tab @code{JISHFT} @tab @code{KISHFT}
@item @code{@ref{ISHFTC}}
  @tab @code{BSHFTC} @tab @code{IISHFTC} @tab @code{JISHFTC} @tab @code{KISHFTC}
@item @code{@ref{MOD}}
  @tab @code{BMOD} @tab @code{IMOD} @tab @code{JMOD} @tab @code{KMOD}
@item @code{@ref{NOT}}
  @tab @code{BNOT} @tab @code{INOT} @tab @code{JNOT} @tab @code{KNOT}
@item @code{@ref{REAL}}
  @tab @code{--} @tab @code{FLOATI} @tab @code{FLOATJ} @tab @code{FLOATK}
@end multitable

@node AUTOMATIC and STATIC attributes
@subsection @code{AUTOMATIC} and @code{STATIC} attributes
@cindex variable attributes
@cindex @code{AUTOMATIC}
@cindex @code{STATIC}

With @option{-fdec-static} GNU Fortran supports the DEC extended attributes
@code{STATIC} and @code{AUTOMATIC} to provide explicit specification of entity
storage.  These follow the syntax of the Fortran standard @code{SAVE} attribute.

@code{STATIC} is exactly equivalent to @code{SAVE}, and specifies that
an entity should be allocated in static memory.  As an example, @code{STATIC}
local variables retain their values across multiple calls to a function.

Entities marked @code{AUTOMATIC} are stack automatic whenever possible.
@code{AUTOMATIC} is the default for local variables smaller than
@option{-fmax-stack-var-size}, unless @option{-fno-automatic} is given.  This
attribute overrides @option{-fno-automatic}, @option{-fmax-stack-var-size}, and
blanket @code{SAVE} statements.


Examples:

@example
subroutine f
  integer, automatic :: i  ! automatic variable
  integer x, y             ! static variables
  save
  ...
endsubroutine
@end example
@example
subroutine f
  integer a, b, c, x, y, z
  static :: x
  save y
  automatic z, c
  ! a, b, c, and z are automatic
  ! x and y are static
endsubroutine
@end example
@example
! Compiled with -fno-automatic
subroutine f
  integer a, b, c, d
  automatic :: a
  ! a is automatic; b, c, and d are static
endsubroutine
@end example

@node Form feed as whitespace
@subsection Form feed as whitespace
@cindex form feed whitespace

Historically, legacy compilers allowed insertion of form feed characters ('\f',
ASCII 0xC) at the beginning of lines for formatted output to line printers,
though the Fortran standard does not mention this. GNU Fortran supports the
interpretation of form feed characters in source as whitespace for
compatibility.

@node TYPE as an alias for PRINT
@subsection @code{TYPE} as an alias for @code{PRINT}
@cindex type alias print
For compatibility, GNU Fortran interprets @code{TYPE} statements as
@code{PRINT} statements with the flag @option{-fdec}.  With this flag asserted,
the following two examples are equivalent:

@smallexample
TYPE *, 'hello world'
@end smallexample

@smallexample
PRINT *, 'hello world'
@end smallexample

@node %LOC as an rvalue
@subsection %LOC as an rvalue
@cindex LOC
Normally @code{%LOC} is allowed only in parameter lists.  However the intrinsic
function @code{LOC} does the same thing, and is usable as the right-hand-side of
assignments. For compatibility, GNU Fortran supports the use of @code{%LOC} as
an alias for the builtin @code{LOC} with @option{-std=legacy}.  With this
feature enabled the following two examples are equivalent:

@smallexample
integer :: i, l
l = %loc(i)
call sub(l)
@end smallexample

@smallexample
integer :: i
call sub(%loc(i))
@end smallexample

@node .XOR. operator
@subsection @code{.XOR.} operator
@cindex operators, xor

GNU Fortran supports @code{.XOR.} as a logical operator with @code{-std=legacy}
for compatibility with legacy code. @code{.XOR.} is equivalent to
@code{.NEQV.}. That is, the output is true if and only if the inputs differ.

@node Bitwise logical operators
@subsection Bitwise logical operators
@cindex logical, bitwise

With @option{-fdec}, GNU Fortran relaxes the type constraints on
logical operators to allow integer operands, and performs the corresponding
bitwise operation instead.  This flag is for compatibility only, and should be
avoided in new code.  Consider:

@smallexample
  INTEGER :: i, j
  i = z'33'
  j = z'cc'
  print *, i .AND. j
@end smallexample

In this example, compiled with @option{-fdec}, GNU Fortran
replaces the @code{.AND.} operation with a call to the intrinsic
@code{@ref{IAND}} function, yielding the bitwise-and of @code{i} and @code{j}.

Note that this conversion occurs if at least one operand is of integral
type.  As a result, a logical operand is converted to an integer when the
other operand is an integer in a logical operation.  In this case,
@code{.TRUE.} is converted to @code{1} and @code{.FALSE.} to @code{0}.

Here is the mapping of logical operator to bitwise intrinsic used with
@option{-fdec}:

@multitable @columnfractions .2 .2 .6
@headitem Operator @tab Intrinsic @tab Bitwise operation
@item @code{.NOT.} @tab @code{NOT} @tab complement (@pxref{NOT})
@item @code{.AND.} @tab @code{IAND} @tab intersection (@pxref{IAND})
@item @code{.OR.} @tab @code{IOR} @tab union (@pxref{IOR})
@item @code{.NEQV.} @tab @code{IEOR} @tab exclusive or (@pxref{IEOR})
@item @code{.EQV.} @tab @code{NOT IEOR} @tab complement of exclusive or (@pxref{IEOR})
@end multitable

@node Extended I/O specifiers
@subsection Extended I/O specifiers
@cindex @code{CARRIAGECONTROL}
@cindex @code{READONLY}
@cindex @code{SHARE}
@cindex @code{SHARED}
@cindex @code{NOSHARED}
@cindex I/O specifiers

GNU Fortran supports the additional legacy I/O specifiers
@code{CARRIAGECONTROL}, @code{READONLY}, and @code{SHARE} with the
compile flag @option{-fdec}, for compatibility.

@table @code
@item CARRIAGECONTROL
The @code{CARRIAGECONTROL} specifier allows a user to control line
termination settings between output records for an I/O unit. The specifier has
no meaning for readonly files. When @code{CARRIAGECONTROL} is specified upon
opening a unit for formatted writing, the exact @code{CARRIAGECONTROL} setting
determines what characters to write between output records. The syntax is:

@smallexample
OPEN(..., CARRIAGECONTROL=cc)
@end smallexample

@noindent
where @emph{cc} is a character expression that evaluates to one of the
following values:

@multitable @columnfractions .2 .8
@item @code{'LIST'} @tab One line feed between records (default)
@item @code{'FORTRAN'} @tab Legacy interpretation of the first character (see below)
@item @code{'NONE'} @tab No separator between records
@end multitable

With @code{CARRIAGECONTROL='FORTRAN'}, when a record is written, the first
character of the input record is not written, and instead determines the output
record separator as follows:

@multitable @columnfractions .3 .3 .4
@headitem Leading character @tab Meaning @tab Output separating character(s)
@item @code{'+'} @tab Overprinting @tab Carriage return only
@item @code{'-'} @tab New line @tab Line feed and carriage return
@item @code{'0'} @tab Skip line @tab Two line feeds and carriage return
@item @code{'1'} @tab New page @tab Form feed and carriage return
@item @code{'$'} @tab Prompting @tab Line feed (no carriage return)
@item @code{CHAR(0)} @tab Overprinting (no advance) @tab None
@end multitable

@item READONLY
The @code{READONLY} specifier may be given upon opening a unit, and is
equivalent to specifying @code{ACTION='READ'}, except that the file may not be
deleted on close (i.e. @code{CLOSE} with @code{STATUS="DELETE"}). The syntax
is:

@smallexample
@code{OPEN(..., READONLY)}
@end smallexample

@item SHARE
The @code{SHARE} specifier allows system-level locking on a unit upon opening
it for controlled access from multiple processes/threads. The @code{SHARE}
specifier has several forms:

@smallexample
OPEN(..., SHARE=sh)
OPEN(..., SHARED)
OPEN(..., NOSHARED)
@end smallexample

Where @emph{sh} in the first form is a character expression that evaluates to
a value as seen in the table below. The latter two forms are aliases
for particular values of @emph{sh}:

@multitable @columnfractions .3 .3 .4
@headitem Explicit form @tab Short form @tab Meaning
@item @code{SHARE='DENYRW'} @tab @code{NOSHARED} @tab Exclusive (write) lock
@item @code{SHARE='DENYNONE'} @tab @code{SHARED} @tab Shared (read) lock
@end multitable

In general only one process may hold an exclusive (write) lock for a given file
at a time, whereas many processes may hold shared (read) locks for the same
file.

The behavior of locking may vary with your operating system. On POSIX systems,
locking is implemented with @code{fcntl}. Consult your corresponding operating
system's manual pages for further details. Locking via @code{SHARE=} is not
supported on other systems.

@end table

@node Legacy PARAMETER statements
@subsection Legacy @code{PARAMETER} statements
@cindex PARAMETER

For compatibility, GNU Fortran supports legacy @code{PARAMETER}
statements without
parentheses with @option{-std=legacy}.  A warning is emitted if used with
@option{-std=gnu}, and an error is acknowledged with a real Fortran standard
flag (@option{-std=f95}, etc...).  These statements take the following form:

@smallexample
implicit real (E)
parameter e = 2.718282
real c
parameter c = 3.0e8
@end smallexample

@node Default exponents
@subsection Default exponents
@cindex exponent

For compatibility, GNU Fortran supports a default exponent of zero in real
constants with @option{-fdec}.  For example, @code{9e} would be
interpreted as @code{9e0}, rather than an error.

@node Unsigned integers
@subsection Unsigned integers
@cindex Unsigned integers
If the @option{-funsigned} option is given, GNU Fortran supports
unsigned integers according to
@uref{https://j3-fortran.org/doc/year/24/24-116.txt, J3/24-116}.  The
data type is called @code{UNSIGNED}.  For an unsigned type with @code{n}
bits, it implements integer arithmetic modulo @code{2**n}, comparable
to the @code{unsigned} data type in C.

The data type has @code{KIND} numbers comparable to other Fortran data
types, which can be selected via the @code{SELECTED_UNSIGNED_KIND}
function.

Mixed arithmetic, comparisons and assignment between @code{UNSIGNED}
and other types are only possible via explicit conversion.  Conversion
from @code{UNSIGNED} to other types is done via type conversion
functions like @code{INT} or @code{REAL}. Conversion from other types
to @code{UNSIGNED} is done via @code{UINT}. Unsigned variables cannot be
used as index variables in @code{DO} loops or as array indices.

Unsigned numbers have a trailing @code{u} as suffix, optionally followed
by a @code{KIND} number separated by an underscore.

Input and output can be done using the @samp{I}, @samp{B}, @samp{O}
and @samp{Z} descriptors, plus unformatted I/O.

Unsigned integers as implemented in gfortran are compatible with flang.

Here is a small, somewhat contrived example of their use:
@smallexample
program main
  use iso_fortran_env, only : uint64
  unsigned(kind=uint64) :: v
  v = huge(v) - 32u_uint64
  print *,v
end program main
@end smallexample
@noindent
which outputs the number 18446744073709551583.

Arithmetic operations work on unsigned integers, also for
exponentiation.  As an extension to J3/24-116.txt, unary minus
and exponentiation of unsigned integers are permitted unless
@code{-pedantic} is in force.

In intrinsic procedures, unsigned arguments are typically permitted
for arguments for the data to be processed, analogous to the
use of @code{REAL} arguments. Unsigned values are prohibited
as index variables in @code{DO} loops and as array indices.

Unsigned numbers can be read and written using list-directed,
formatted and unformatted I/O.  For formatted I/O, the @samp{B},
@samp{I}, @samp{O} and @samp{Z} descriptors are valid.  Negative
values and values that would overflow are rejected with
@code{-pedantic}.

@code{SELECT CASE} is supported for unsigned integers.

The following intrinsics take unsigned arguments:
@itemize @bullet
@item @code{BGE}, @pxref{BGE}
@item @code{BGT}, @pxref{BGT}
@item @code{BIT_SIZE}, @pxref{BIT_SIZE}
@item @code{BLE}, @pxref{BLE}
@item @code{BLT}, @pxref{BLT}
@item @code{CMPLX}, @pxref{CMPLX}
@item @code{CSHIFT}, @pxref{CSHIFT}
@item @code{DIGITS}, @pxref{DIGITS}
@item @code{DOT_PRODUCT}, @pxref{DOT_PRODUCT}
@item @code{DSHIFTL}, @pxref{DSHIFTL}
@item @code{DSHIFTR}, @pxref{DSHIFTR}
@item @code{EOSHIFT}, @pxref{EOSHIFT}
@item @code{FINDLOC}, @pxref{FINDLOC}
@item @code{HUGE}, @pxref{HUGE}
@item @code{IALL}, @pxref{IALL}
@item @code{IAND}, @pxref{IAND}
@item @code{IANY}, @pxref{IANY}
@item @code{IBCLR}, @pxref{IBCLR}
@item @code{IBITS}, @pxref{IBITS}
@item @code{IBSET}, @pxref{IBSET}
@item @code{IEOR}, @pxref{IEOR}
@item @code{INT}, @pxref{INT}
@item @code{IOR}, @pxref{IOR}
@item @code{IPARITY}, @pxref{IPARITY}
@item @code{ISHFT}, @pxref{ISHFT}
@item @code{ISHFTC}, @pxref{ISHFTC}
@item @code{MATMUL}, @pxref{MATMUL}
@item @code{MAX}, @pxref{MAX}
@item @code{MAXLOC}, @pxref{MAXLOC}
@item @code{MAXVAL}, @pxref{MAXVAL}
@item @code{MERGE}, @pxref{MERGE}
@item @code{MERGE_BITS}, @pxref{MERGE_BITS}
@item @code{MIN}, @pxref{MIN}
@item @code{MINLOC}, @pxref{MINLOC}
@item @code{MINVAL}, @pxref{MINVAL}
@item @code{MOD}, @pxref{MOD}
@item @code{MODULO}, @pxref{MODULO}
@item @code{MVBITS}, @pxref{MVBITS}
@item @code{NOT}, @pxref{NOT}
@item @code{OUT_OF_RANGE}, @pxref{OUT_OF_RANGE}
@item @code{PRODUCT}, @pxref{PRODUCT}
@item @code{RANDOM_NUMBER}, @pxref{RANDOM_NUMBER}
@item @code{RANGE}, @pxref{RANGE}
@item @code{REAL}, @pxref{REAL}
@item @code{SHIFTA}, @pxref{SHIFTA}
@item @code{SHIFTL}, @pxref{SHIFTL}
@item @code{SHIFTR}, @pxref{SHIFTR}
@item @code{SUM}, @pxref{SUM}
@item @code{TRANSPOSE}, @pxref{TRANSPOSE}
@item @code{TRANSFER}, @pxref{TRANSFER}
@end itemize

The following intrinsics are enabled with @option{-funsigned}:
@itemize @bullet
@item @code{UINT}, @pxref{UINT}
@item @code{UMASKL}, @pxref{UMASKL}
@item @code{UMASKR}, @pxref{UMASKR}
@item @code{SELECTED_UNSIGNED_KIND}, @pxref{SELECTED_UNSIGNED_KIND}
@end itemize

The following constants have been added to the intrinsic
@code{ISO_C_BINDING} module: @code{c_unsigned},
@code{c_unsigned_short}, @code{c_unsigned_char},
@code{c_unsigned_long}, @code{c_unsigned_long_long},
@code{c_uintmax_t}, @code{c_uint8_t}, @code{c_uint16_t},
@code{c_uint32_t}, @code{c_uint64_t}, @code{c_uint128_t},
@code{c_uint_fast8_t}, @code{c_uint_fast16_t}, @code{c_uint_fast32_t},
@code{c_uint_fast64_t}, @code{c_uint_fast128_t},
@code{c_uint_least8_t}, @code{c_uint_least16_t}, @code{c_uint_least32_t},
@code{c_uint_least64_t} and @code{c_uint_least128_t}.

The following constants have been added to the intrinsic
@code{ISO_FORTRAN_ENV} module: @code{uint8}, @code{uint16},
@code{uint32} and @code{uint64}.

@node Extensions not implemented in GNU Fortran
@section Extensions not implemented in GNU Fortran
@cindex extensions, not implemented

The long history of the Fortran language, its wide use and broad
userbase, the large number of different compiler vendors and the lack of
some features crucial to users in the first standards have lead to the
existence of a number of important extensions to the language.  While
some of the most useful or popular extensions are supported by the GNU
Fortran compiler, not all existing extensions are supported.  This section
aims at listing these extensions and offering advice on how best make
code that uses them running with the GNU Fortran compiler.

@c More can be found here:
@c   -- https://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
@c   -- the list of Fortran and libgfortran bugs closed as WONTFIX:
@c      http://tinyurl.com/2u4h5y

@menu
* ENCODE and DECODE statements::
* Variable FORMAT expressions::
@c * TYPE and ACCEPT I/O Statements::
@c * DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
@c * Omitted arguments in procedure call::
* Alternate complex function syntax::
* Volatile COMMON blocks::
* OPEN( ... NAME=)::
* Q edit descriptor::
@end menu

@node ENCODE and DECODE statements
@subsection @code{ENCODE} and @code{DECODE} statements
@cindex @code{ENCODE}
@cindex @code{DECODE}

GNU Fortran does not support the @code{ENCODE} and @code{DECODE}
statements.  These statements are best replaced by @code{READ} and
@code{WRITE} statements involving internal files (@code{CHARACTER}
variables and arrays), which have been part of the Fortran standard since
Fortran 77.  For example, replace a code fragment like

@smallexample
      INTEGER*1 LINE(80)
      REAL A, B, C
c     ... Code that sets LINE
      DECODE (80, 9000, LINE) A, B, C
 9000 FORMAT (1X, 3(F10.5))
@end smallexample

@noindent
with the following:

@smallexample
      CHARACTER(LEN=80) LINE
      REAL A, B, C
c     ... Code that sets LINE
      READ (UNIT=LINE, FMT=9000) A, B, C
 9000 FORMAT (1X, 3(F10.5))
@end smallexample

Similarly, replace a code fragment like

@smallexample
      INTEGER*1 LINE(80)
      REAL A, B, C
c     ... Code that sets A, B and C
      ENCODE (80, 9000, LINE) A, B, C
 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
@end smallexample

@noindent
with the following:

@smallexample
      CHARACTER(LEN=80) LINE
      REAL A, B, C
c     ... Code that sets A, B and C
      WRITE (UNIT=LINE, FMT=9000) A, B, C
 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
@end smallexample


@node Variable FORMAT expressions
@subsection Variable @code{FORMAT} expressions
@cindex @code{FORMAT}

A variable @code{FORMAT} expression is format statement that includes
angle brackets enclosing a Fortran expression: @code{FORMAT(I<N>)}.  GNU
Fortran does not support this legacy extension.  The effect of variable
format expressions can be reproduced by using the more powerful (and
standard) combination of internal output and string formats.  For example,
replace a code fragment like this:

@smallexample
      WRITE(6,20) INT1
 20   FORMAT(I<N+1>)
@end smallexample

@noindent
with the following:

@smallexample
c     Variable declaration
      CHARACTER(LEN=20) FMT
c
c     Other code here...
c
      WRITE(FMT,'("(I", I0, ")")') N+1
      WRITE(6,FMT) INT1
@end smallexample

@noindent
or with:

@smallexample
c     Variable declaration
      CHARACTER(LEN=20) FMT
c
c     Other code here...
c
      WRITE(FMT,*) N+1
      WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1
@end smallexample


@node Alternate complex function syntax
@subsection Alternate complex function syntax
@cindex Complex function

Some Fortran compilers, including @command{g77}, let the user declare
complex functions with the syntax @code{COMPLEX FUNCTION name*16()}, as
well as @code{COMPLEX*16 FUNCTION name()}.  Both are nonstandard legacy
extensions.  @command{gfortran} accepts the latter form, which is more
common, but not the former.


@node Volatile COMMON blocks
@subsection Volatile @code{COMMON} blocks
@cindex @code{VOLATILE}
@cindex @code{COMMON}

Some Fortran compilers, including @command{g77}, let the user declare
@code{COMMON} with the @code{VOLATILE} attribute. This is
invalid standard Fortran syntax and is not supported by
@command{gfortran}.  Note that @command{gfortran} accepts
@code{VOLATILE} variables in @code{COMMON} blocks since revision 4.3.


@node OPEN( ... NAME=)
@subsection @code{OPEN( ... NAME=)}
@cindex @code{NAME}

Some Fortran compilers, including @command{g77}, let the user declare
@code{OPEN( ... NAME=)}. This is
invalid standard Fortran syntax and is not supported by
@command{gfortran}.  @code{OPEN( ... NAME=)} should be replaced
with @code{OPEN( ... FILE=)}.

@node Q edit descriptor
@subsection @code{Q} edit descriptor
@cindex @code{Q} edit descriptor

Some Fortran compilers provide the @code{Q} edit descriptor, which
transfers the number of characters left within an input record into an
integer variable.

A direct replacement of the @code{Q} edit descriptor is not available
in @command{gfortran}.  How to replicate its functionality using
standard-conforming code depends on what the intent of the original
code is.

Options to replace @code{Q} may be to read the whole line into a
character variable and then counting the number of non-blank
characters left using @code{LEN_TRIM}.  Another method may be to use
formatted stream, read the data up to the position where the @code{Q}
descriptor occurred, use @code{INQUIRE} to get the file position,
count the characters up to the next @code{NEW_LINE} and then start
reading from the position marked previously.


@c ---------------------------------------------------------------------
@c ---------------------------------------------------------------------
@c Mixed-Language Programming
@c ---------------------------------------------------------------------

@node Mixed-Language Programming
@chapter Mixed-Language Programming
@cindex Interoperability
@cindex Mixed-language programming

@menu
* Interoperability with C::
* GNU Fortran Compiler Directives::
* Non-Fortran Main Program::
* Naming and argument-passing conventions::
@end menu

This chapter is about mixed-language interoperability, but also
applies if you link Fortran code compiled by different compilers.  In
most cases, use of the C Binding features of the Fortran 2003 and
later standards is sufficient.

For example, it is possible to mix Fortran code with C++ code as well
as C, if you declare the interface functions as @code{extern "C"} on
the C++ side and @code{BIND(C)} on the Fortran side, and follow the
rules for interoperability with C.  Note that you cannot manipulate
C++ class objects in Fortran or vice versa except as opaque pointers.

You can use the @command{gfortran} command to link both Fortran and
non-Fortran code into the same program, or you can use @command{gcc}
or @command{g++} if you also add an explicit @option{-lgfortran} option
to link with the Fortran library.  If your main program is written in
C or some other language instead of Fortran, see
@ref{Non-Fortran Main Program}, below.

@node Interoperability with C
@section Interoperability with C
@cindex interoperability with C
@cindex C interoperability

@menu
* Intrinsic Types::
* Derived Types and struct::
* Interoperable Global Variables::
* Interoperable Subroutines and Functions::
* Working with C Pointers::
* Further Interoperability of Fortran with C::
* Generating C prototypes from Fortran::
@end menu

Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a
standardized way to generate procedure and derived-type
declarations and global variables that are interoperable with C
(ISO/IEC 9899:1999).  The @code{BIND(C)} attribute has been added
to inform the compiler that a symbol shall be interoperable with C;
also, some constraints are added.  Note, however, that not
all C features have a Fortran equivalent or vice versa.  For instance,
neither C's unsigned integers nor C's functions with variable number
of arguments have an equivalent in Fortran.

Note that array dimensions are reversely ordered in C and that arrays in
C always start with index 0 while in Fortran they start by default with
1.  Thus, an array declaration @code{A(n,m)} in Fortran matches
@code{A[m][n]} in C and accessing the element @code{A(i,j)} matches
@code{A[j-1][i-1]}.  The element following @code{A(i,j)} (C: @code{A[j-1][i-1]};
assuming @math{i < n}) in memory is @code{A(i+1,j)} (C: @code{A[j-1][i]}).

@node Intrinsic Types
@subsection Intrinsic Types
@cindex C intrinsic type interoperability
@cindex intrinsic type interoperability with C
@cindex interoperability, intrinsic type

In order to ensure that exactly the same variable type and kind is used
in C and Fortran, you should use the named constants for kind parameters
that are defined in the @code{ISO_C_BINDING} intrinsic module.
That module contains named constants of character type representing
the escaped special characters in C, such as newline.
For a list of the constants, see @ref{ISO_C_BINDING}.

For logical types, please note that the Fortran standard only guarantees
interoperability between C99's @code{_Bool} and Fortran's @code{C_Bool}-kind
logicals and C99 defines that @code{true} has the value 1 and @code{false}
the value 0.  Using any other integer value with GNU Fortran's @code{LOGICAL}
(with any kind parameter) gives an undefined result.  (Passing other integer
values than 0 and 1 to GCC's @code{_Bool} is also undefined, unless the
integer is explicitly or implicitly casted to @code{_Bool}.)

@node Derived Types and struct
@subsection Derived Types and struct
@cindex C derived type and struct interoperability
@cindex derived type interoperability with C
@cindex interoperability, derived type and struct

For compatibility of derived types with @code{struct}, use
the @code{BIND(C)} attribute in the type declaration.  For instance, the
following type declaration

@smallexample
 USE ISO_C_BINDING
 TYPE, BIND(C) :: myType
   INTEGER(C_INT) :: i1, i2
   INTEGER(C_SIGNED_CHAR) :: i3
   REAL(C_DOUBLE) :: d1
   COMPLEX(C_FLOAT_COMPLEX) :: c1
   CHARACTER(KIND=C_CHAR) :: str(5)
 END TYPE
@end smallexample

@noindent
matches the following @code{struct} declaration in C

@smallexample
 struct @{
   int i1, i2;
   /* Note: "char" might be signed or unsigned.  */
   signed char i3;
   double d1;
   float _Complex c1;
   char str[5];
 @} myType;
@end smallexample

Derived types with the C binding attribute shall not have the @code{sequence}
attribute, type parameters, the @code{extends} attribute, nor type-bound
procedures.  Every component must be of interoperable type and kind and may not
have the @code{pointer} or @code{allocatable} attribute.  The names of the
components are irrelevant for interoperability.

As there exist no direct Fortran equivalents, neither unions nor structs
with bit field or variable-length array members are interoperable.

@node Interoperable Global Variables
@subsection Interoperable Global Variables
@cindex C variable interoperability
@cindex variable interoperability with C
@cindex interoperability, variable

Variables can be made accessible from C using the C binding attribute,
optionally together with specifying a binding name.  Those variables
have to be declared in the declaration part of a @code{MODULE},
be of interoperable type, and have neither the @code{pointer} nor
the @code{allocatable} attribute.

@smallexample
  MODULE m
    USE myType_module
    USE ISO_C_BINDING
    integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
    type(myType), bind(C) :: tp
  END MODULE
@end smallexample

Here, @code{_MyProject_flags} is the case-sensitive name of the variable
as seen from C programs while @code{global_flag} is the case-insensitive
name as seen from Fortran.  If no binding name is specified, as for
@var{tp}, the C binding name is the (lowercase) Fortran binding name.
If a binding name is specified, only a single variable may be after the
double colon.  Note of warning: You cannot use a global variable to
access @var{errno} of the C library as the C standard allows it to be
a macro.  Use the @code{IERRNO} intrinsic (GNU extension) instead.

@node Interoperable Subroutines and Functions
@subsection Interoperable Subroutines and Functions
@cindex C procedure interoperability
@cindex procedure interoperability with C
@cindex function interoperability with C
@cindex subroutine interoperability with C
@cindex interoperability, subroutine and function

Subroutines and functions have to have the @code{BIND(C)} attribute to
be compatible with C.  The dummy argument declaration is relatively
straightforward.  However, one needs to be careful because C uses
call-by-value by default while Fortran behaves usually similar to
call-by-reference.  Furthermore, strings and pointers are handled
differently.

To pass a variable by value, use the @code{VALUE} attribute.
Thus, the following C prototype

@smallexample
@code{int func(int i, int *j)}
@end smallexample

@noindent
matches the Fortran declaration

@smallexample
  integer(c_int) function func(i,j)
    use iso_c_binding, only: c_int
    integer(c_int), VALUE :: i
    integer(c_int) :: j
@end smallexample

Note that pointer arguments also frequently need the @code{VALUE} attribute,
see @ref{Working with C Pointers}.

Strings are handled quite differently in C and Fortran.  In C a string
is a @code{NUL}-terminated array of characters while in Fortran each string
has a length associated with it and is thus not terminated (by e.g.
@code{NUL}).  For example, if you want to use the following C function,

@smallexample
  #include <stdio.h>
  void print_C(char *string) /* equivalent: char string[]  */
  @{
     printf("%s\n", string);
  @}
@end smallexample

@noindent
to print ``Hello World'' from Fortran, you can call it using

@smallexample
  use iso_c_binding, only: C_CHAR, C_NULL_CHAR
  interface
    subroutine print_c(string) bind(C, name="print_C")
      use iso_c_binding, only: c_char
      character(kind=c_char) :: string(*)
    end subroutine print_c
  end interface
  call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
@end smallexample

As the example shows, you need to ensure that the
string is @code{NUL} terminated.  Additionally, the dummy argument
@var{string} of @code{print_C} is a length-one assumed-size
array; using @code{character(len=*)} is not allowed.  The example
above uses @code{c_char_"Hello World"} to ensure the string
literal has the right type; typically the default character
kind and @code{c_char} are the same and thus @code{"Hello World"}
is equivalent.  However, the standard does not guarantee this.

The use of strings is now further illustrated using the C library
function @code{strncpy}, whose prototype is

@smallexample
  char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
@end smallexample

@noindent
The function @code{strncpy} copies at most @var{n} characters from
string @var{s2} to @var{s1} and returns @var{s1}.  In the following
example, we ignore the return value:

@smallexample
  use iso_c_binding
  implicit none
  character(len=30) :: str,str2
  interface
    ! Ignore the return value of strncpy -> subroutine
    ! "restrict" is always assumed if we do not pass a pointer
    subroutine strncpy(dest, src, n) bind(C)
      import
      character(kind=c_char),  intent(out) :: dest(*)
      character(kind=c_char),  intent(in)  :: src(*)
      integer(c_size_t), value, intent(in) :: n
    end subroutine strncpy
  end interface
  str = repeat('X',30) ! Initialize whole string with 'X'
  call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
               len(c_char_"Hello World",kind=c_size_t))
  print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
  end
@end smallexample

Fortran 2023 added two new intrinsic functions for converting between
C and Fortran string representations: @code{f_c_string} transforms a
Fortran string into a C string by appending a null character, and
@code{c_f_strpointer} allows access to a null-terminated C string or
simply contiguous array of @code{c_char} as a Fortran deferred-length
character pointer.

The intrinsic procedures are described in @ref{Intrinsic Procedures}.

@node Working with C Pointers
@subsection Working with C Pointers
@cindex C pointers
@cindex pointers, C

C pointers are represented in Fortran via the special opaque derived
type @code{type(c_ptr)} (with private components).  C pointers are distinct
from Fortran objects with the @code{POINTER} attribute.  Thus one needs to
use intrinsic conversion procedures to convert from or to C pointers.
For some applications, using an assumed type (@code{TYPE(*)}) can be
an alternative to a C pointer, and you can also use library routines
to access Fortran pointers from C.  See @ref{Further Interoperability
of Fortran with C}.

Here is an example of using C pointers in Fortran:

@smallexample
  use iso_c_binding
  type(c_ptr) :: cptr1, cptr2
  integer, target :: array(7), scalar
  integer, pointer :: pa(:), ps
  cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
                          ! array is contiguous if required by the C
                          ! procedure
  cptr2 = c_loc(scalar)
  call c_f_pointer(cptr2, ps)
  call c_f_pointer(cptr2, pa, shape=[7])
@end smallexample

When converting C to Fortran arrays, the one-dimensional @code{SHAPE} argument
has to be passed.

If a pointer is a dummy argument of an interoperable procedure, it usually
has to be declared using the @code{VALUE} attribute.  @code{void*}
matches @code{TYPE(C_PTR), VALUE}, while @code{TYPE(C_PTR)} alone
matches @code{void**}.

Procedure pointers are handled analogously to pointers; the C type is
@code{TYPE(C_FUNPTR)} and the intrinsic conversion procedures are
@code{C_F_PROCPOINTER} and @code{C_FUNLOC}.

Let us consider two examples of actually passing a procedure pointer from
C to Fortran and vice versa.  Note that these examples are also very
similar to passing ordinary pointers between both languages. First,
consider this code in C:

@smallexample
/* Procedure implemented in Fortran.  */
void get_values (void (*)(double));

/* Call-back routine we want called from Fortran.  */
void
print_it (double x)
@{
  printf ("Number is %f.\n", x);
@}

/* Call Fortran routine and pass call-back to it.  */
void
foobar ()
@{
  get_values (&print_it);
@}
@end smallexample

A matching implementation for @code{get_values} in Fortran that correctly
receives the procedure pointer from C and is able to call it, is given
in the following @code{MODULE}:

@smallexample
MODULE m
  IMPLICIT NONE

  ! Define interface of call-back routine.
  ABSTRACT INTERFACE
    SUBROUTINE callback (x)
      USE, INTRINSIC :: ISO_C_BINDING
      REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
    END SUBROUTINE callback
  END INTERFACE

CONTAINS

  ! Define C-bound procedure.
  SUBROUTINE get_values (cproc) BIND(C)
    USE, INTRINSIC :: ISO_C_BINDING
    TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc

    PROCEDURE(callback), POINTER :: proc

    ! Convert C to Fortran procedure pointer.
    CALL C_F_PROCPOINTER (cproc, proc)

    ! Call it.
    CALL proc (1.0_C_DOUBLE)
    CALL proc (-42.0_C_DOUBLE)
    CALL proc (18.12_C_DOUBLE)
  END SUBROUTINE get_values

END MODULE m
@end smallexample

Next, we want to call a C routine that expects a procedure pointer argument
and pass it a Fortran procedure (that clearly must be interoperable!).
Again, the C function may be:

@smallexample
int
call_it (int (*func)(int), int arg)
@{
  return func (arg);
@}
@end smallexample

It can be used as in the following Fortran code:

@smallexample
MODULE m
  USE, INTRINSIC :: ISO_C_BINDING
  IMPLICIT NONE

  ! Define interface of C function.
  INTERFACE
    INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
      USE, INTRINSIC :: ISO_C_BINDING
      TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
      INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
    END FUNCTION call_it
  END INTERFACE

CONTAINS

  ! Define procedure passed to C function.
  ! It must be interoperable!
  INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
    INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
    double_it = arg + arg
  END FUNCTION double_it

  ! Call C function.
  SUBROUTINE foobar ()
    TYPE(C_FUNPTR) :: cproc
    INTEGER(KIND=C_INT) :: i

    ! Get C procedure pointer.
    cproc = C_FUNLOC (double_it)

    ! Use it.
    DO i = 1_C_INT, 10_C_INT
      PRINT *, call_it (cproc, i)
    END DO
  END SUBROUTINE foobar

END MODULE m
@end smallexample

@node Further Interoperability of Fortran with C
@subsection Further Interoperability of Fortran with C
@cindex Further Interoperability of Fortran with C
@cindex TS 29113
@cindex array descriptor
@cindex dope vector
@cindex assumed-type
@cindex assumed-rank

GNU Fortran implements the Technical Specification ISO/IEC TS
29113:2012, which extends the interoperability support of Fortran 2003
and Fortran 2008 and is now part of the 2018 Fortran standard.
Besides removing some restrictions and constraints, the Technical
Specification adds assumed-type (@code{TYPE(*)}) and assumed-rank
(@code{DIMENSION(..)}) variables and allows for interoperability of
assumed-shape, assumed-rank, and deferred-shape arrays, as well as
allocatables and pointers.  Objects of these types are passed to
@code{BIND(C)} functions as descriptors with a standard interface,
declared in the header file @code{<ISO_Fortran_binding.h>}.

Note: Currently, GNU Fortran does not use internally the array descriptor
(dope vector) as specified in the Technical Specification, but uses
an array descriptor with different fields in functions without the
@code{BIND(C)} attribute.  Arguments to functions marked @code{BIND(C)}
are converted to the specified form.  If you need to access GNU Fortran's
internal array descriptor, you can use the Chasm Language Interoperability
Tools, @url{http://chasm-interop.sourceforge.net/}.

@node Generating C prototypes from Fortran
@subsection Generating C prototypes from Fortran
@cindex prototypes
@cindex C prototypes
The options @option{-fc-prototypes} can be used to write out C
declarations and function prototypes for @code{BIND(C)} entities.
The same can be done for writing out prototypes for external
procedures using @option{-fc-prototypes-external},
@pxref{Interoperability Options}.

Standard Fortran does not specify an interoperable type for C's
@code{unsigned} integer types.  For interoperability with unsigned
types, GNU Fortran provides unsigned integers, @pxref{Unsigned integers}.

@node GNU Fortran Compiler Directives
@section GNU Fortran Compiler Directives
@cindex GNU Fortran Compiler Directives
@cindex Compiler Directives
@cindex Directives

@menu
* ATTRIBUTES directive::
* UNROLL directive::
* BUILTIN directive::
* IVDEP directive::
* VECTOR directive::
* NOVECTOR directive::
@end menu

@node ATTRIBUTES directive
@subsection @code{ATTRIBUTES} directive
@cindex @code{ATTRIBUTES} directive

The Fortran standard describes how a conforming program shall
behave; however, the exact implementation is not standardized.  In order
to allow the user to choose specific implementation details, compiler
directives can be used to set attributes of variables and procedures
that are not part of the standard.  Whether a given attribute is
supported and its exact effects depend on both the operating system and
on the processor; see
@ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)}
for details.

For procedures and procedure pointers, the following attributes can
be used to change the calling convention:

@itemize
@item @code{CDECL} -- standard C calling convention
@item @code{STDCALL} -- convention where the called procedure pops the stack
@item @code{FASTCALL} -- part of the arguments are passed via registers
instead using the stack
@end itemize

Besides changing the calling convention, the attributes also influence
the decoration of the symbol name, e.g., by a leading underscore or by
a trailing at-sign followed by the number of bytes on the stack.  When
assigning a procedure to a procedure pointer, both should use the same
calling convention.

On some systems, procedures and global variables (module variables and
@code{COMMON} blocks) need special handling to be accessible when they
are in a shared library.  The following attributes are available:

@itemize
@item @code{DLLEXPORT} -- provide a global pointer to a pointer in the DLL
@item @code{DLLIMPORT} -- reference the function or variable using a
global pointer
@end itemize

For dummy arguments, the @code{NO_ARG_CHECK} attribute can be used; in
other compilers, it is also known as @code{IGNORE_TKR}.  For dummy arguments
with this attribute actual arguments of any type and kind (similar to
@code{TYPE(*)}), scalars and arrays of any rank (no equivalent
in Fortran standard) are accepted.  As with @code{TYPE(*)}, the argument
is unlimited polymorphic and no type information is available.
Additionally, the argument may only be passed to dummy arguments
with the @code{NO_ARG_CHECK} attribute and as argument to the
@code{PRESENT} intrinsic function and to @code{C_LOC} of the
@code{ISO_C_BINDING} module.

Variables with @code{NO_ARG_CHECK} attribute shall be of assumed-type
(@code{TYPE(*)}; recommended) or of type @code{INTEGER}, @code{LOGICAL},
@code{REAL} or @code{COMPLEX}. They shall not have the @code{ALLOCATE},
@code{CODIMENSION}, @code{INTENT(OUT)}, @code{POINTER} or @code{VALUE}
attribute; furthermore, they shall be either scalar or of assumed-size
(@code{dimension(*)}). As @code{TYPE(*)}, the @code{NO_ARG_CHECK} attribute
requires an explicit interface.

@itemize
@item @code{NO_ARG_CHECK} -- disable the type, kind and rank checking
@item @code{DEPRECATED} -- print a warning when using a such-tagged
deprecated procedure, variable or parameter; the warning can be suppressed
with @option{-Wno-deprecated-declarations}.
@item @code{NOINLINE} -- prevent inlining given function.
@item @code{INLINE} -- hint that the given function should be inlined, while
still respecting the inlining size limits, corresponding to the C @code{inline}
keyword.
@item @code{ALWAYS_INLINE} -- force inlining of a given function, ignoring the
inlining size limits.  This is the counterpart of @code{NOINLINE} and
corresponds to the C @code{always_inline} attribute.  @code{INLINE} and
@code{ALWAYS_INLINE} are incompatible with @code{NOINLINE}.  Specifying both
for the same procedure makes the inline attribute ignored with a warning.
@item @code{NORETURN} -- add a hint that a given function cannot return.
@item @code{WEAK} -- emit the declaration of an external symbol as a weak
symbol rather than a global.  This is primarily useful in defining library
functions that can be overridden in user code, though it can also be used with
non-function declarations.  The overriding symbol must have the same type as
the weak symbol.
@end itemize


The attributes are specified using the syntax

@code{!GCC$ ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list}

where in free-form source code only whitespace is allowed before @code{!GCC$}
and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall
start in the first column.

For procedures, the compiler directives shall be placed into the body
of the procedure; for variables and procedure pointers, they shall be in
the same declaration part as the variable or procedure pointer.


@node UNROLL directive
@subsection @code{UNROLL} directive
@cindex @code{UNROLL} directive

The syntax of the directive is

@code{!GCC$ unroll N}

You can use this directive to control how many times a loop should be unrolled.
It must be placed immediately before a @code{DO} loop and applies only to the
loop that follows.  N is an integer constant specifying the unrolling factor.
The values of 0 and 1 block any unrolling of the loop.

For @code{DO CONCURRENT} constructs the unrolling specification applies
only to the first loop control variable.


@node BUILTIN directive
@subsection @code{BUILTIN} directive
@cindex @code{BUILTIN} directive

The syntax of the directive is

@code{!GCC$ BUILTIN (@var{B}) attributes simd @var{FLAGS} IF('target')}

You can use this directive to define which middle-end built-ins provide vector
implementations.  @var{B} is name of the middle-end built-in.  @var{FLAGS}
are optional and must be either @code{(inbranch)} or @code{(notinbranch)}.
@code{IF} statement is optional and is used to filter multilib ABIs
for the built-in that should be vectorized.  Example usage:

@smallexample
!GCC$ builtin (sinf) attributes simd (notinbranch) if('x86_64')
@end smallexample

The special target @code{'fastmath'} is used to specify the vector
implementation is only valid if fast-math is enabled:

@smallexample
!GCC$ builtin (exp) attributes simd (notinbranch) if('fastmath')
@end smallexample

The purpose of the directive is to provide an API among the GCC compiler and
the GNU C Library which would define vector implementations of math routines.


@node IVDEP directive
@subsection @code{IVDEP} directive
@cindex @code{IVDEP} directive

The syntax of the directive is

@code{!GCC$ ivdep}

This directive tells the compiler to ignore vector dependencies in the
following loop.  It must be placed immediately before a @code{DO} loop
and applies only to the loop that follows.

Sometimes the compiler may not have sufficient information to decide
whether a particular loop is vectorizable due to potential
dependencies between iterations.  The purpose of the directive is to
tell the compiler that vectorization is safe.

For @code{DO CONCURRENT} constructs this annotation is implicit to all
loop control variables.

This directive is intended for annotation of existing code.  For new
code it is recommended to consider OpenMP SIMD directives as potential
alternative.


@node VECTOR directive
@subsection @code{VECTOR} directive
@cindex @code{VECTOR} directive

The syntax of the directive is

@code{!GCC$ vector}

This directive tells the compiler to vectorize the following loop.  It
must be placed immediately before a @code{DO} loop and applies only to
the loop that follows.

For @code{DO CONCURRENT} constructs this annotation applies to all loops
specified in the concurrent header.


@node NOVECTOR directive
@subsection @code{NOVECTOR} directive
@cindex @code{NOVECTOR} directive

The syntax of the directive is

@code{!GCC$ novector}

This directive tells the compiler to not vectorize the following loop.
It must be placed immediately before a @code{DO} loop and applies only
to the loop that follows.

For @code{DO CONCURRENT} constructs this annotation applies to all loops
specified in the concurrent header.


@node Non-Fortran Main Program
@section Non-Fortran Main Program
@cindex Non-Fortran Main Program
@cindex Main Program

@menu
* _gfortran_set_args:: Save command-line arguments
* _gfortran_set_options:: Set library option flags
* _gfortran_set_convert:: Set endian conversion
* _gfortran_set_record_marker:: Set length of record markers
* _gfortran_set_fpe:: Set when a Floating Point Exception should be raised
* _gfortran_set_max_subrecord_length:: Set subrecord length
@end menu

Even if you are doing mixed-language programming, it is very
likely that you do not need to know or use the information in this
section.  Since it is about the internal structure of GNU Fortran,
it may also change in GCC minor releases.

When you compile a @code{PROGRAM} with GNU Fortran, a function
with the name @code{main} (in the symbol table of the object file)
is generated, which initializes the libgfortran library and then
calls the actual program that uses the name @code{MAIN__}, for
historic reasons.  If you link GNU Fortran compiled procedures
to, e.g., a C or C++ program or to a Fortran program compiled by
a different compiler, the libgfortran library is not initialized
and thus a few intrinsic procedures do not work properly, e.g.
those for obtaining the command-line arguments.

Therefore, if your @code{PROGRAM} is not compiled with
GNU Fortran and the GNU Fortran compiled procedures require
intrinsics relying on the library initialization, you need to
initialize the library yourself.  Using the default options,
gfortran calls @code{_gfortran_set_args} and
@code{_gfortran_set_options}.  The initialization of the former
is needed if the called procedures access the command line
(and for backtracing); the latter sets some flags based on the
standard chosen or to enable backtracing.  In typical programs,
it is not necessary to call any initialization function.

If your @code{PROGRAM} is compiled with GNU Fortran, you shall
not call any of the following functions.  The libgfortran
initialization functions are shown in C syntax but using C
bindings they are also accessible from Fortran.


@node _gfortran_set_args
@subsection @code{_gfortran_set_args} --- Save command-line arguments
@fnindex _gfortran_set_args
@cindex libgfortran initialization, set_args

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_set_args (int argc, char *argv[])}

@item @emph{Description}:
@code{_gfortran_set_args} saves the command-line arguments; this
initialization is required if any of the command-line intrinsics
is called.  Additionally, it shall be called if backtracing is
enabled (see @code{_gfortran_set_options}).

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{argc} @tab number of command line argument strings
@item @var{argv} @tab the command-line argument strings; argv[0]
is the pathname of the executable itself.
@end multitable

@item @emph{Example}:
@smallexample
int main (int argc, char *argv[])
@{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  return 0;
@}
@end smallexample
@end table


@node _gfortran_set_options
@subsection @code{_gfortran_set_options} --- Set library option flags
@fnindex _gfortran_set_options
@cindex libgfortran initialization, set_options

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_set_options (int num, int options[])}

@item @emph{Description}:
@code{_gfortran_set_options} sets several flags related to the Fortran
standard to be used, whether backtracing should be enabled
and whether range checks should be performed.  The syntax allows for
upward compatibility since the number of passed flags is specified; for
non-passed flags, the default value is used.  See also
@pxref{Code Gen Options}.  Please note that not all flags are actually
used.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{num} @tab number of options passed
@item @var{argv} @tab The list of flag values
@end multitable

@item @emph{option flag list}:
@multitable @columnfractions .15 .70
@item @var{option}[0] @tab Allowed standard; can give run-time errors
if e.g. an input-output edit descriptor is invalid in a given
standard.  Possible values are (bitwise or-ed) @code{GFC_STD_F77} (1),
@code{GFC_STD_F95_OBS} (2), @code{GFC_STD_F95_DEL} (4),
@code{GFC_STD_F95} (8), @code{GFC_STD_F2003} (16), @code{GFC_STD_GNU}
(32), @code{GFC_STD_LEGACY} (64), @code{GFC_STD_F2008} (128),
@code{GFC_STD_F2008_OBS} (256), @code{GFC_STD_F2018} (512),
@code{GFC_STD_F2018_OBS} (1024), @code{GFC_STD_F2018_DEL} (2048),
@code{GFC_STD_F2023} (4096), and @code{GFC_STD_F2023_DEL} (8192).
Default: @code{GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F95 |
GFC_STD_F2003 | GFC_STD_F2008 | GFC_STD_F2008_OBS
| GFC_STD_F77 | GFC_STD_F2018 | GFC_STD_F2018_OBS | GFC_STD_F2018_DEL
| GFC_STD_F2023 | GFC_STD_F2023_DEL | GFC_STD_GNU | GFC_STD_LEGACY}.
@item @var{option}[1] @tab Standard-warning flag; prints a warning to
standard error.  Default: @code{GFC_STD_F95_DEL | GFC_STD_LEGACY}.
@item @var{option}[2] @tab If non zero, enable pedantic checking.
Default: off.
@item @var{option}[3] @tab Unused.
@item @var{option}[4] @tab If non zero, enable backtracing on run-time
errors.  Default: off. (Default in the compiler: on.)
Note: Installs a signal handler and requires command-line
initialization using @code{_gfortran_set_args}.
@item @var{option}[5] @tab If non zero, supports signed zeros.
Default: enabled.
@item @var{option}[6] @tab Enables run-time checking.  Possible values
are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2),
GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (8), GFC_RTCHECK_POINTER (16),
GFC_RTCHECK_MEM (32), GFC_RTCHECK_BITS (64).
Default: disabled.
@item @var{option}[7] @tab Unused.
@item @var{option}[8] @tab Show a warning when invoking @code{STOP} and
@code{ERROR STOP} if a floating-point exception occurred. Possible values
are (bitwise or-ed) @code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2),
@code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8),
@code{GFC_FPE_UNDERFLOW} (16), @code{GFC_FPE_INEXACT} (32). Default: None (0).
(Default in the compiler: @code{GFC_FPE_INVALID | GFC_FPE_DENORMAL |
GFC_FPE_ZERO | GFC_FPE_OVERFLOW | GFC_FPE_UNDERFLOW}.)
@end multitable

@item @emph{Example}:
@smallexample
  /* Use gfortran 4.9 default options.  */
  static int options[] = @{68, 511, 0, 0, 1, 1, 0, 0, 31@};
  _gfortran_set_options (9, &options);
@end smallexample
@end table


@node _gfortran_set_convert
@subsection @code{_gfortran_set_convert} --- Set endian conversion
@fnindex _gfortran_set_convert
@cindex libgfortran initialization, set_convert

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_set_convert (int conv)}

@item @emph{Description}:
@code{_gfortran_set_convert} set the representation of data for
unformatted files.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{conv} @tab Endian conversion, possible values:
GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1),
GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).
@end multitable

@item @emph{Example}:
@smallexample
int main (int argc, char *argv[])
@{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_convert (1);
  return 0;
@}
@end smallexample
@end table


@node _gfortran_set_record_marker
@subsection @code{_gfortran_set_record_marker} --- Set length of record markers
@fnindex _gfortran_set_record_marker
@cindex libgfortran initialization, set_record_marker

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_set_record_marker (int val)}

@item @emph{Description}:
@code{_gfortran_set_record_marker} sets the length of record markers
for unformatted files.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{val} @tab Length of the record marker; valid values
are 4 and 8.  Default is 4.
@end multitable

@item @emph{Example}:
@smallexample
int main (int argc, char *argv[])
@{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_record_marker (8);
  return 0;
@}
@end smallexample
@end table


@node _gfortran_set_fpe
@subsection @code{_gfortran_set_fpe} --- Enable floating point exception traps
@fnindex _gfortran_set_fpe
@cindex libgfortran initialization, set_fpe

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_set_fpe (int val)}

@item @emph{Description}:
@code{_gfortran_set_fpe} enables floating point exception traps for
the specified exceptions.  On most systems, this results in a
@code{SIGFPE} signal being sent and the program being aborted.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{option}[0] @tab IEEE exceptions.  Possible values are
(bitwise or-ed) zero (0, default) no trapping,
@code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2),
@code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8),
@code{GFC_FPE_UNDERFLOW} (16), and @code{GFC_FPE_INEXACT} (32).
@end multitable

@item @emph{Example}:
@smallexample
int main (int argc, char *argv[])
@{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  /* FPE for invalid operations such as SQRT(-1.0).  */
  _gfortran_set_fpe (1);
  return 0;
@}
@end smallexample
@end table


@node _gfortran_set_max_subrecord_length
@subsection @code{_gfortran_set_max_subrecord_length} --- Set subrecord length
@fnindex _gfortran_set_max_subrecord_length
@cindex libgfortran initialization, set_max_subrecord_length

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_set_max_subrecord_length (int val)}

@item @emph{Description}:
@code{_gfortran_set_max_subrecord_length} set the maximum length
for a subrecord.  This option only makes sense for testing and
debugging of unformatted I/O.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{val} @tab the maximum length for a subrecord;
the maximum permitted value is 2147483639, which is also
the default.
@end multitable

@item @emph{Example}:
@smallexample
int main (int argc, char *argv[])
@{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_max_subrecord_length (8);
  return 0;
@}
@end smallexample
@end table


@node Naming and argument-passing conventions
@section Naming and argument-passing conventions

This section gives an overview about the naming convention of procedures
and global variables and about the argument passing conventions used by
GNU Fortran.  If a C binding has been specified, the naming convention
and some of the argument-passing conventions change.  If possible,
mixed-language and mixed-compiler projects should use the better defined
C binding for interoperability.  See @pxref{Interoperability with C}.

@menu
* Naming conventions::
* Argument passing conventions::
@end menu


@node Naming conventions
@subsection Naming conventions
@cindex Naming conventions

According the Fortran standard, valid Fortran names consist of a letter
between @code{A} to @code{Z}, @code{a} to @code{z}, digits @code{0},
@code{1} to @code{9} and underscores (@code{_}) with the restriction
that names may only start with a letter.  As vendor extension, the
dollar sign (@code{$}) is additionally permitted with the option
@option{-fdollar-ok}, but not as first character and only if the
target system supports it.

By default, the procedure name is the lowercased Fortran name with an
appended underscore (@code{_}); using @option{-fno-underscoring} no
underscore is appended while @code{-fsecond-underscore} appends two
underscores.  Depending on the target system and the calling convention,
the procedure might be additionally dressed; for instance, on 32bit
Windows with @code{stdcall}, an at-sign @code{@@} followed by an integer
number is appended.  For the changing the calling convention, see
@pxref{GNU Fortran Compiler Directives}.

For common blocks, the same convention is used, i.e. by default an
underscore is appended to the lowercased Fortran name.  Blank commons
have the name @code{__BLNK__}.

For procedures and variables declared in the specification space of a
module, the name is formed by @code{__}, followed by the lowercased
module name, @code{_MOD_}, and the lowercased Fortran name.  Note that
no underscore is appended.


@node Argument passing conventions
@subsection Argument passing conventions
@cindex Argument passing conventions

Subroutines do not return a value (matching C99's @code{void}) while
functions either return a value as specified in the platform ABI or
the result variable is passed as hidden argument to the function and
no result is returned.  A hidden result variable is used when the
result variable is an array or of type @code{CHARACTER}.

Arguments are passed according to the platform ABI. In particular,
complex arguments might not be compatible to a struct with two real
components for the real and imaginary part. The argument passing
matches the one of C99's @code{_Complex}.  Functions with scalar
complex result variables return their value and do not use a
by-reference argument.  Note that with the @option{-ff2c} option,
the argument passing is modified and no longer completely matches
the platform ABI.  Some other Fortran compilers use @code{f2c}
semantic by default; this might cause problems with
interoperability.

GNU Fortran passes most arguments by reference, i.e. by passing a
pointer to the data.  Note that the compiler might use a temporary
variable into which the actual argument has been copied, if required
semantically (copy-in/copy-out).

For arguments with @code{ALLOCATABLE} and @code{POINTER}
attribute (including procedure pointers), a pointer to the pointer
is passed such that the pointer address can be modified in the
procedure.

For dummy arguments with the @code{VALUE} attribute: Scalar arguments
of the type @code{INTEGER}, @code{LOGICAL}, @code{REAL} and
@code{COMPLEX} are passed by value according to the platform ABI.
(As vendor extension and not recommended, using @code{%VAL()} in the
call to a procedure has the same effect.) For @code{TYPE(C_PTR)} and
procedure pointers, the pointer itself is passed such that it can be
modified without affecting the caller.
@c FIXME: Document how VALUE is handled for CHARACTER, TYPE,
@c CLASS and arrays, i.e. whether the copy-in is done in the caller
@c or in the callee.

For Boolean (@code{LOGICAL}) arguments, please note that GCC expects
only the integer value 0 and 1.  If a GNU Fortran @code{LOGICAL}
variable contains another integer value, the result is undefined.
As some other Fortran compilers use @math{-1} for @code{.TRUE.},
extra care has to be taken -- such as passing the value as
@code{INTEGER}.  (The same value restriction also applies to other
front ends of GCC, e.g. to GCC's C99 compiler for @code{_Bool}
or GCC's Ada compiler for @code{Boolean}.)

For arguments of @code{CHARACTER} type, the character length is passed
as a hidden argument at the end of the argument list, except when the
corresponding dummy argument is declared as @code{TYPE(*)}.  For
deferred-length strings, the value is passed by reference, otherwise
by value.  The character length has the C type @code{size_t} (or
@code{INTEGER(kind=C_SIZE_T)} in Fortran).  Note that this is
different to older versions of the GNU Fortran compiler, where the
type of the hidden character length argument was a C @code{int}.  In
order to retain compatibility with older versions, one can e.g. for
the following Fortran procedure

@smallexample
subroutine fstrlen (s, a)
   character(len=*) :: s
   integer :: a
   print*, len(s)
end subroutine fstrlen
@end smallexample

define the corresponding C prototype as follows:

@smallexample
#if __GNUC__ > 7
typedef size_t fortran_charlen_t;
#else
typedef int fortran_charlen_t;
#endif

void fstrlen_ (char*, int*, fortran_charlen_t);
@end smallexample

In order to avoid such compiler-specific details, for new code it is
instead recommended to use the ISO_C_BINDING feature.

Note with C binding, @code{CHARACTER(len=1)} result variables are
returned according to the platform ABI and no hidden length argument
is used for dummy arguments; with @code{VALUE}, those variables are
passed by value.

For @code{OPTIONAL} dummy arguments, an absent argument is denoted
by a NULL pointer, except for scalar dummy arguments of intrinsic type
or derived type (but not @code{CLASS}) and
that have the @code{VALUE} attribute.  For those, a hidden Boolean
argument (@code{logical(kind=C_bool),value}) is used to indicate
whether the argument is present.

Arguments that are assumed-shape, assumed-rank or deferred-rank
arrays or, with @option{-fcoarray=lib}, allocatable scalar coarrays use
an array descriptor.  All other arrays pass the address of the
first element of the array.  With @option{-fcoarray=lib}, the token
and the offset belonging to nonallocatable coarrays dummy arguments
are passed as hidden argument along the character length hidden
arguments.  The token is an opaque pointer identifying the coarray
and the offset is a passed-by-value integer of kind @code{C_PTRDIFF_T},
denoting the byte offset between the base address of the coarray and
the passed scalar or first element of the passed array.

The arguments are passed in the following order
@itemize @bullet
@item Result variable, when the function result is passed by reference
@item Character length of the function result, if it is a of type
@code{CHARACTER} and no C binding is used
@item The arguments in the order in which they appear in the Fortran
declaration
@item The present status for optional arguments with value attribute,
which are internally passed by value
@item The character length and/or coarray token and offset for the first
argument which is a @code{CHARACTER} or a nonallocatable coarray dummy
argument, followed by the hidden arguments of the next dummy argument
of such a type
@end itemize


@c ---------------------------------------------------------------------
@c Coarray Programming
@c ---------------------------------------------------------------------

@node Coarray Programming
@chapter Coarray Programming
@cindex Coarrays

@menu
* Type and enum ABI Documentation::
* Function ABI Documentation::
@end menu


@node Type and enum ABI Documentation
@section Type and enum ABI Documentation
@cindex Coarray type and enum ABI

@menu
* caf_token_t::
* caf_register_t::
* caf_deregister_t::
* caf_reference_t::
* caf_team_t::
@end menu

@node caf_token_t
@subsection @code{caf_token_t}
@cindex @code{caf_token_t}

Typedef of type @code{void *} on the compiler side. Can be any data
type on the library side.

@node caf_register_t
@subsection @code{caf_register_t}
@cindex @code{caf_register_t}

Indicates which kind of coarray variable should be registered.

@smallexample
typedef enum caf_register_t @{
  CAF_REGTYPE_COARRAY_STATIC,
  CAF_REGTYPE_COARRAY_ALLOC,
  CAF_REGTYPE_LOCK_STATIC,
  CAF_REGTYPE_LOCK_ALLOC,
  CAF_REGTYPE_CRITICAL,
  CAF_REGTYPE_EVENT_STATIC,
  CAF_REGTYPE_EVENT_ALLOC,
  CAF_REGTYPE_COARRAY_ALLOC_REGISTER_ONLY,
  CAF_REGTYPE_COARRAY_ALLOC_ALLOCATE_ONLY
@}
caf_register_t;
@end smallexample

The values @code{CAF_REGTYPE_COARRAY_ALLOC_REGISTER_ONLY} and
@code{CAF_REGTYPE_COARRAY_ALLOC_ALLOCATE_ONLY} are for allocatable components
in derived type coarrays only.  The first one sets up the token without
allocating memory for allocatable component.  The latter one only allocates the
memory for an allocatable component in a derived type coarray.  The token
needs to be set up previously by the REGISTER_ONLY.  This allows having
allocatable components unallocated on some images.  The status of whether an
allocatable component is allocated on a remote image can be queried by
@code{_caf_is_present} which used internally by the @code{ALLOCATED}
intrinsic.

@node caf_deregister_t
@subsection @code{caf_deregister_t}
@cindex @code{caf_deregister_t}

@smallexample
typedef enum caf_deregister_t @{
  CAF_DEREGTYPE_COARRAY_DEREGISTER,
  CAF_DEREGTYPE_COARRAY_DEALLOCATE_ONLY
@}
caf_deregister_t;
@end smallexample

Allows to specify the type of deregistration of a coarray object.  The
@code{CAF_DEREGTYPE_COARRAY_DEALLOCATE_ONLY} flag is only allowed for
allocatable components in derived type coarrays.

@node caf_reference_t
@subsection @code{caf_reference_t}
@cindex @code{caf_reference_t}

The structure used for implementing arbitrary reference chains.
A @code{CAF_REFERENCE_T} allows to specify a component reference or any kind
of array reference of any rank supported by gfortran.  For array references all
kinds as known by the compiler/Fortran standard are supported indicated by
a @code{MODE}.

@smallexample
typedef enum caf_ref_type_t @{
  /* Reference a component of a derived type, either regular one or an
     allocatable or pointer type.  For regular ones idx in caf_reference_t is
     set to -1.  */
  CAF_REF_COMPONENT,
  /* Reference an allocatable array.  */
  CAF_REF_ARRAY,
  /* Reference a non-allocatable/non-pointer array.  I.e., the coarray object
     has no array descriptor associated and the addressing is done
     completely using the ref.  */
  CAF_REF_STATIC_ARRAY
@} caf_ref_type_t;
@end smallexample

@smallexample
typedef enum caf_array_ref_t @{
  /* No array ref.  This terminates the array ref.  */
  CAF_ARR_REF_NONE = 0,
  /* Reference array elements given by a vector.  Only for this mode
     caf_reference_t.u.a.dim[i].v is valid.  */
  CAF_ARR_REF_VECTOR,
  /* A full array ref (:).  */
  CAF_ARR_REF_FULL,
  /* Reference a range on elements given by start, end and stride.  */
  CAF_ARR_REF_RANGE,
  /* Only a single item is referenced given in the start member.  */
  CAF_ARR_REF_SINGLE,
  /* An array ref of the kind (i:), where i is an arbitrary valid index in the
     array.  The index i is given in the start member.  */
  CAF_ARR_REF_OPEN_END,
  /* An array ref of the kind (:i), where the lower bound of the array ref
     is given by the remote side.  The index i is given in the end member.  */
  CAF_ARR_REF_OPEN_START
@} caf_array_ref_t;
@end smallexample

@smallexample
/* References to remote components of a derived type.  */
typedef struct caf_reference_t @{
  /* A pointer to the next ref or NULL.  */
  struct caf_reference_t *next;
  /* The type of the reference.  */
  /* caf_ref_type_t, replaced by int to allow specification in fortran FE.  */
  int type;
  /* The size of an item referenced in bytes.  I.e. in an array ref this is
     the factor to advance the array pointer with to get to the next item.
     For component refs this gives just the size of the element referenced.  */
  size_t item_size;
  union @{
    struct @{
      /* The offset (in bytes) of the component in the derived type.
         Unused for allocatable or pointer components.  */
      ptrdiff_t offset;
      /* The offset (in bytes) to the caf_token associated with this
         component.  NULL, when not allocatable/pointer ref.  */
      ptrdiff_t caf_token_offset;
    @} c;
    struct @{
      /* The mode of the array ref.  See CAF_ARR_REF_*.  */
      /* caf_array_ref_t, replaced by unsigned char to allow specification in
         fortran FE.  */
     unsigned char mode[GFC_MAX_DIMENSIONS];
      /* The type of a static array.  Unset for array's with descriptors.  */
      int static_array_type;
      /* Subscript refs (s) or vector refs (v).  */
      union @{
        struct @{
          /* The start and end boundary of the ref and the stride.  */
          index_type start, end, stride;
        @} s;
        struct @{
          /* nvec entries of kind giving the elements to reference.  */
          void *vector;
          /* The number of entries in vector.  */
          size_t nvec;
          /* The integer kind used for the elements in vector.  */
          int kind;
        @} v;
      @} dim[GFC_MAX_DIMENSIONS];
    @} a;
  @} u;
@} caf_reference_t;
@end smallexample

The references make up a single linked list of reference operations.  The
@code{NEXT} member links to the next reference or NULL to indicate the end of
the chain.  Component and array refs can be arbitrarily mixed as long as they
comply to the Fortran standard.

@emph{Notes}:
The member @code{STATIC_ARRAY_TYPE} is used only when the @code{TYPE} is
@code{CAF_REF_STATIC_ARRAY}.  The member gives the type of the data referenced.
Because no array descriptor is available for a descriptorless array and
type conversion still needs to take place the type is transported here.

At the moment @code{CAF_ARR_REF_VECTOR} is not implemented in the front end for
descriptorless arrays.  The library caf_single has untested support for it.

@node caf_team_t
@subsection @code{caf_team_t}

Opaque pointer to represent a team-handle.  This type is a stand-in for the
future implementation of teams.  It is about to change without further notice.

@node Function ABI Documentation
@section Function ABI Documentation
@cindex Coarray Function ABI

@menu
* _gfortran_caf_init:: Initialization function
* _gfortran_caf_finish:: Finalization function
* _gfortran_caf_this_image:: Querying the image number
* _gfortran_caf_num_images:: Querying the maximal number of images
* _gfortran_caf_image_status :: Query the status of an image
* _gfortran_caf_failed_images :: Get an array of the indexes of the failed images
* _gfortran_caf_stopped_images :: Get an array of the indexes of the stopped images
* _gfortran_caf_register:: Registering coarrays
* _gfortran_caf_deregister:: Deregistering coarrays
* _gfortran_caf_register_accessor:: Register an accessor for remote access
* _gfortran_caf_register_accessors_finish:: Finish registering accessor functions
* _gfortran_caf_get_remote_function_index:: Get the index of an accessor
* _gfortran_caf_get_from_remote:: Getting data from a remote image using a remote side accessor
* _gfortran_caf_is_present_on_remote:: Check that a coarray or a part of it is allocated on the remote image
* _gfortran_caf_send_to_remote:: Send data to a remote image using a remote side accessor to store it
* _gfortran_caf_transfer_between_remotes:: Initiate data transfer between to remote images
* _gfortran_caf_sendget_by_ref:: Sending data between remote images using enhanced references
* _gfortran_caf_lock:: Locking a lock variable
* _gfortran_caf_unlock:: Unlocking a lock variable
* _gfortran_caf_event_post:: Post an event
* _gfortran_caf_event_wait:: Wait that an event occurred
* _gfortran_caf_event_query:: Query event count
* _gfortran_caf_sync_all:: All-image barrier
* _gfortran_caf_sync_images:: Barrier for selected images
* _gfortran_caf_sync_memory:: Wait for completion of segment-memory operations
* _gfortran_caf_error_stop:: Error termination with exit code
* _gfortran_caf_error_stop_str:: Error termination with string
* _gfortran_caf_fail_image :: Mark the image failed and end its execution
* _gfortran_caf_atomic_define:: Atomic variable assignment
* _gfortran_caf_atomic_ref:: Atomic variable reference
* _gfortran_caf_atomic_cas:: Atomic compare and swap
* _gfortran_caf_atomic_op:: Atomic operation
* _gfortran_caf_co_broadcast:: Sending data to all images
* _gfortran_caf_co_max:: Collective maximum reduction
* _gfortran_caf_co_min:: Collective minimum reduction
* _gfortran_caf_co_sum:: Collective summing reduction
* _gfortran_caf_co_reduce:: Generic collective reduction
* _gfortran_caf_form_team:: Team creation function
* _gfortran_caf_change_team:: Team activation function
* _gfortran_caf_end_team:: Team termination function
* _gfortran_caf_sync_team:: Synchronize all images of a given team
* _gfortran_caf_get_team:: Get the opaque handle of the specified team
* _gfortran_caf_team_number:: Get the unique id of the given team
@end menu


@node _gfortran_caf_init
@subsection @code{_gfortran_caf_init} --- Initialization function
@cindex Coarray, _gfortran_caf_init

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_init (int *argc, char ***argv)}

@item @emph{Description}:
This function is called at startup of the program before the Fortran main
program, if the latter has been compiled with @option{-fcoarray=lib}.
It takes as arguments the command-line arguments of the program.  It is
permitted to pass two @code{NULL} pointers as argument; if non-@code{NULL},
the library is permitted to modify the arguments.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{argc} @tab intent(inout) An integer pointer with the number of
arguments passed to the program or @code{NULL}.
@item @var{argv} @tab intent(inout) A pointer to an array of strings with the
command-line arguments or @code{NULL}.
@end multitable

@item @emph{Notes}:
The function is modelled after the initialization function of the Message
Passing Interface (MPI) specification.  Due to the way coarray registration
works, it might not be the first call to the library.  If the main program is
not written in Fortran and only a library uses coarrays, it can happen that
this function is never called.  Therefore, it is recommended that the library
does not rely on the passed arguments and whether the call has been done.
@end table


@node _gfortran_caf_finish
@subsection @code{_gfortran_caf_finish} --- Finalization function
@cindex Coarray, _gfortran_caf_finish

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_finish (void)}

@item @emph{Description}:
This function is called at the end of the Fortran main program, if it has
been compiled with the @option{-fcoarray=lib} option.

@item @emph{Notes}:
For non-Fortran programs, it is recommended to call the function at the end
of the main program.  To ensure that the shutdown is also performed for
programs where this function is not explicitly invoked, for instance
non-Fortran programs or calls to the system's exit() function, the library
can use a destructor function.  Note that programs can also be terminated
using the STOP and ERROR STOP statements; those use different library calls.
@end table


@node _gfortran_caf_this_image
@subsection @code{_gfortran_caf_this_image} --- Querying the image number
@cindex Coarray, _gfortran_caf_this_image

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_this_image (caf_team_t team)}

@item @emph{Description}:
Return the current image number in the @var{team}, or in the current team, if
no @var{team} is given.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab intent(in), optional; The team this image's number is
requested for.  If null, the image number in the current team is returned.
@end multitable

@item @emph{Notes}:
Available since Fortran 2008 without argument; Since Fortran 2018 with optional
team argument.  Fortran 2008 uses 0 as argument for team, which is permissible,
because a team handle is always an opaque pointer, which as a special case can
be null here.
@end table


@node _gfortran_caf_num_images
@subsection @code{_gfortran_caf_num_images} --- Querying the maximal number of images
@cindex Coarray, _gfortran_caf_num_images

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_num_images (caf_team_t team, int32_t *team_number)}

@item @emph{Description}:
This function returns the number of images in the team given by @var{team} or
@var{team_number}, if either one is present.  If both are null, then the number
of images in the current team is returned.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab intent(in), optional; The team the number of images is
requested for.  If null, the number of images in the current team is returned.
@item @var{team_number} @tab intent(in), optional; The team id for which the
number of teams is requested; if unset, then number of images in the current
team is returned.
@end multitable

@item @emph{Notes}:
When both argument are given, then it is caf-library dependent which argument
is examined first.  Current implementations prioritize the @var{team} argument,
because it is easier to retrieve the number of images from it.

Fortran 2008 or later, with no arguments; Fortran 2018 or later with two
arguments.
@end table


@node _gfortran_caf_image_status
@subsection @code{_gfortran_caf_image_status} --- Query the status of an image
@cindex Coarray, _gfortran_caf_image_status

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_image_status (int image, caf_team_t * team)}

@item @emph{Description}:
Get the status of the image given by the id @var{image} of the team given by
@var{team}.  Valid results are zero, for image is ok, @code{STAT_STOPPED_IMAGE}
from the ISO_FORTRAN_ENV module to indicate that the image has been stopped and
@code{STAT_FAILED_IMAGE} also from ISO_FORTRAN_ENV to indicate that the image
has executed a @code{FAIL IMAGE} statement.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{image} @tab the positive scalar id of the image in the current TEAM.
@item @var{team} @tab optional; team on the which the inquiry is to be
performed.
@end multitable

@item @emph{Notes}:
This function follows TS18508.  Because team-functionality is not yet
implemented a null pointer is passed for the @var{team} argument at the moment.
@end table


@node _gfortran_caf_failed_images
@subsection @code{_gfortran_caf_failed_images} --- Get an array of the indexes of the failed images
@cindex Coarray, _gfortran_caf_failed_images

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_failed_images (caf_team_t * team, int * kind)}

@item @emph{Description}:
Get an array of image indexes in the current @var{team} that have failed.  The
array is sorted ascendingly.  When @var{team} is not provided the current team
is to be used.  When @var{kind} is provided then the resulting array is of that
integer kind else it is of default integer kind.  The returns an unallocated
size zero array when no images have failed.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab optional; team on the which the inquiry is to be
performed.
@item @var{image} @tab optional; the kind of the resulting integer array.
@end multitable

@item @emph{Notes}:
This function follows TS18508.  Because team-functionality is not yet
implemented a null pointer is passed for the @var{team} argument at the moment.
@end table


@node _gfortran_caf_stopped_images
@subsection @code{_gfortran_caf_stopped_images} --- Get an array of the indexes of the stopped images
@cindex Coarray, _gfortran_caf_stopped_images

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_stopped_images (caf_team_t * team, int * kind)}

@item @emph{Description}:
Get an array of image indexes in the current @var{team} that have stopped.  The
array is sorted ascendingly.  When @var{team} is not provided the current team
is to be used.  When @var{kind} is provided then the resulting array is of that
integer kind else it is of default integer kind.  The returns an unallocated
size zero array when no images have failed.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab optional; team on the which the inquiry is to be
performed.
@item @var{image} @tab optional; the kind of the resulting integer array.
@end multitable

@item @emph{Notes}:
This function follows TS18508.  Because team-functionality is not yet
implemented a null pointer is passed for the @var{team} argument at the moment.
@end table


@node _gfortran_caf_register
@subsection @code{_gfortran_caf_register} --- Registering coarrays
@cindex Coarray, _gfortran_caf_register

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_register (size_t size, caf_register_t type,
caf_token_t *token, gfc_descriptor_t *desc, int *stat, char *errmsg,
size_t errmsg_len)}

@item @emph{Description}:
Registers memory for a coarray and creates a token to identify the coarray.  The
routine is called for both coarrays with @code{SAVE} attribute and using an
explicit @code{ALLOCATE} statement.  If an error occurs and @var{STAT} is a
@code{NULL} pointer, the function shall abort with printing an error message
and starting the error termination.  If no error occurs and @var{STAT} is
present, it shall be set to zero.  Otherwise, it shall be set to a positive
value and, if not-@code{NULL}, @var{ERRMSG} shall be set to a string describing
the failure.  The routine shall register the memory provided in the
@code{DATA}-component of the array descriptor @var{DESC}, when that component
is non-@code{NULL}, else it shall allocate sufficient memory and provide a
pointer to it in the @code{DATA}-component of @var{DESC}.  The array descriptor
has rank zero, when a scalar object is to be registered and the array
descriptor may be invalid after the call to @code{_gfortran_caf_register}.
When an array is to be allocated the descriptor persists.

For @code{CAF_REGTYPE_COARRAY_STATIC} and @code{CAF_REGTYPE_COARRAY_ALLOC},
the passed size is the byte size requested.  For @code{CAF_REGTYPE_LOCK_STATIC},
@code{CAF_REGTYPE_LOCK_ALLOC} and @code{CAF_REGTYPE_CRITICAL} it is the array
size or one for a scalar.

When @code{CAF_REGTYPE_COARRAY_ALLOC_REGISTER_ONLY} is used, then only a token
for an allocatable or pointer component is created.  The @code{SIZE} parameter
is not used then.  On the contrary when
@code{CAF_REGTYPE_COARRAY_ALLOC_ALLOCATE_ONLY} is specified, then the
@var{token} needs to be registered by a previous call with regtype
@code{CAF_REGTYPE_COARRAY_ALLOC_REGISTER_ONLY} and either the memory specified
in the @var{DESC}'s data-ptr is registered or allocate when the data-ptr is
@code{NULL}.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{size} @tab For normal coarrays, the byte size of the coarray to be
allocated; for lock types and event types, the number of elements.
@item @var{type} @tab one of the caf_register_t types.
@item @var{token} @tab intent(out) An opaque pointer identifying the coarray.
@item @var{desc} @tab intent(inout) The (pseudo) array descriptor.
@item @var{stat} @tab intent(out) For allocatable coarrays, stores the STAT=;
may be @code{NULL}
@item @var{errmsg} @tab intent(out) When an error occurs, this is set to
an error message; may be @code{NULL}
@item @var{errmsg_len} @tab the buffer size of errmsg.
@end multitable

@item @emph{Notes}:
Nonallocatable coarrays have to be registered prior use from remote images.
In order to guarantee this, they have to be registered before the main
program. This can be achieved by creating constructor functions. That is what
GCC does such that also for nonallocatable coarrays the memory is allocated and
no static memory is used.  The token permits to identify the coarray; to the
processor, the token is a nonaliasing pointer. The library can, for instance,
store the base address of the coarray in the token, some handle or a more
complicated struct.  The library may also store the array descriptor
@var{DESC} when its rank is nonzero.

For lock types, the value shall only be used for checking the allocation
status. Note that for critical blocks, the locking is only required on one
image; in the locking statement, the processor shall always pass an
image index of one for critical-block lock variables
(@code{CAF_REGTYPE_CRITICAL}). For lock types and critical-block variables,
the initial value shall be unlocked (or, respectively, not in critical
section) such as the value false; for event types, the initial state should
be no event, e.g. zero.
@end table


@node _gfortran_caf_deregister
@subsection @code{_gfortran_caf_deregister} --- Deregistering coarrays
@cindex Coarray, _gfortran_caf_deregister

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_deregister (caf_token_t *token, caf_deregister_t type,
int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Called to free or deregister the memory of a coarray; the processor calls this
function for automatic and explicit deallocation.  In case of an error, this
function shall fail with an error message, unless the @var{STAT} variable is
not null.  The library is only expected to free memory it allocated itself
during a call to @code{_gfortran_caf_register}.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab the token to free.
@item @var{type} @tab the type of action to take for the coarray.  A
@code{CAF_DEREGTYPE_COARRAY_DEALLOCATE_ONLY} is allowed only for allocatable or
pointer components of derived type coarrays.  The action only deallocates the
local memory without deleting the token.
@item @var{stat} @tab intent(out) Stores the STAT=; may be NULL
@item @var{errmsg} @tab intent(out) When an error occurs, this is set
to an error message; may be NULL
@item @var{errmsg_len} @tab the buffer size of errmsg.
@end multitable

@item @emph{Notes}:
For nonallocatable coarrays this function is never called.  If a cleanup is
required, it has to be handled via the finish, stop and error stop functions,
and via destructors.
@end table


@node _gfortran_caf_register_accessor
@subsection @code{_gfortran_caf_register_accessor} --- Register an accessor for remote access
@cindex Coarray, _gfortran_caf_register_accessor

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_register_accessor (const int hash,
void (*accessor)(void **, int32_t *, void *, void *, size_t *,
size_t *))}

@item @emph{Description}:
Identification of access functions across images is done using a unique hash.
For each given hash an accessor has to be registered.  This routine is expected
to register an accessor function pointer for the given hash in nearly constant
time.  I.e. it is expected to add the hash and accessor to a buffer and return.
Sorting shall be done in @code{_gfortran_caf_register_accessors_finish}.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{hash} @tab intent(in)  The unique hash value this accessor is to be
identified by.
@item @var{accessor} @tab intent(in)  A pointer to the function on this image.
The function has the signature @code{void accessor (void **dst_ptr,
int32_t *free_dst, void *src_ptr, void *get_data, size_t *opt_src_charlen,
size_t *opt_dst_charlen)}.  GFortran ensures that functions provided to
@code{_gfortran_caf_register_accessor} adhere to this interface.
@end multitable

@item @emph{Notes}:
This function is required to have a nearly constant runtime complexity, because
it will be called to register multiple accessor in a sequence.  GFortran ensures
that before the first remote accesses commences
@code{_gfortran_caf_register_accessors_finish} is called at least once.  It is
valid to register further accessors after a call to
@code{_gfortran_caf_register_accessors_finish}.  It is invalid to call
@code{_gfortran_caf_register_accessor} after the first remote access has been
done.  See also @ref{_gfortran_caf_register_accessors_finish} and
@ref{_gfortran_caf_get_remote_function_index}
@end table


@node _gfortran_caf_register_accessors_finish
@subsection @code{_gfortran_caf_register_accessors_finish} --- Finish registering accessor functions
@cindex Coarray, _gfortran_caf_register_accessors_finish

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_register_accessors_finish ()}

@item @emph{Description}:
Called to finalize registering of accessor functions.  This function is expected
to prepare a lookup table that has fast lookup time for the hash supplied to
@code{_gfortran_caf_get_remote_function_index} and constant access time for
indexing operations.

@item @emph{Arguments}:
No arguments.

@item @emph{Notes}:
This function may be called multiple times with and without new hash-accessors-
pairs being added.  The post-condition after each call has to be that hashes
can be looked up quickly and indexing on the lookup table of hash-accessor-pairs
is a constant time operation.
@end table


@node _gfortran_caf_get_remote_function_index
@subsection @code{_gfortran_caf_get_remote_function_index} --- Get the index of an accessor
@cindex Coarray, _gfortran_caf_get_remote_function_index

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_get_remote_function_index (const int hash)}

@item @emph{Description}:
Return the index of the accessor in the lookup table build by
@ref{_gfortran_caf_register_accessor} and
@ref{_gfortran_caf_register_accessors_finish}.  This function is expected to be
fast, because it may be called often.  A log(N) lookup time for a given hash is
preferred.  The reference implementation uses @code{bsearch ()}, for example.
The index returned shall be an array index to be used by
@ref{_gfortran_caf_get_from_remote}, i.e. a constant time operation is mandatory
for quick access.

The GFortran compiler ensures that
@code{_gfortran_caf_get_remote_function_index} is called once only for each
hash and the result be stored in a static variable to prevent future redundant
lookups.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{hash} @tab intent(in)  The hash of the accessor desired.
@end multitable

@item @emph{Result}:
The zero based index to access the accessor function in a lookup table.
On error, @code{-1} can be returned.

@item @emph{Notes}:
The function's complexity is expected to be significantly smaller than N,
where N is the number of all accessors registered.  Although returning @code{-1}
is valid, will this most likely crash the Fortran program when accessing the
-1-th accessor function.  It is therefore advised to terminate with an error
message, when the hash could not be found.
@end table


@node _gfortran_caf_get_from_remote
@subsection @code{_gfortran_caf_get_from_remote} --- Getting data from a remote image using a remote side accessor
@cindex Coarray, _gfortran_caf_get_from_remote

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_get_from_remote (caf_token_t token,
const gfc_descriptor_t *opt_src_desc, const size_t *opt_src_charlen,
const int image_index, const size_t dst_size, void **dst_data,
size_t *opt_dst_charlen, gfc_descriptor_t *opt_dst_desc,
const bool may_realloc_dst, const int getter_index, void *get_data,
const size_t get_data_size, int *stat, caf_team_t *team, int *team_number)}

@item @emph{Description}:
Called to get a scalar, an array section or a whole array from a remote image
identified by the @var{image_index}.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{opt_src_desc} @tab intent(in)  A pointer to the descriptor when the
object identified by @var{token} is an array with a descriptor.  The parameter
needs to be set to @code{NULL}, when @var{token} identifies a scalar.
@item @var{opt_src_charlen} @tab intent(in) When the object to get is a char
array with deferred length, then this parameter needs to be set to point to its
length.  Else the parameter needs to be set to @code{NULL}.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number.  @code{this_image ()} is valid.
@item @var{dst_size} @tab intent(in) The size of data expected to be transferred
from the remote image.  If the data type to get is a string or string array,
then this needs to be set to the byte size of each character, i.e. @code{4} for
a @code{CHARACTER (KIND=4)} string.  The length of the string is then returned
in @code{opt_dst_charlen} (also for string arrays).
@item @var{dst_data} @tab intent(inout) A pointer to the address the data is
stored.  To prevent copying of data into an output buffer the address to the
live data is returned here.  When a descriptor is provided also its data-member
is set to that address.  When @var{may_realloc_dst} is set, then the memory may
be reallocated by the remote function, which needs to be replicated by this
function.
@item @var{opt_dst_charlen} @tab intent(inout)  When a char array is returned,
this parameter is set to the length where applicable.  The value can also be
read to prevent reallocation in the accessor.
@item @var{opt_dst_desc} @tab intent(inout)  When a descriptor array is
returned, it is stored in the memory pointed to by this optional parameter.
When @var{may_realloc_dst} is set, then the descriptor may be changed, i.e.
its bounds, but upto now not its rank.
@item @var{may_realloc_dst} @tab intent(in)  Set when the returned data may
require reallocation of the output buffer in @var{dst_data} or
@var{opt_dst_desc}.
@item @var{getter_index} @tab intent(in)  The index of the accessor to execute
as returned by @code{_gfortran_caf_get_remote_function_index ()}.
@item @var{get_data} @tab intent(inout)  Additional data needed in the accessor.
I.e., when an array reference uses a local variable @var{v}, it is transported
in this structure and all references in the accessor are rewritten to access the
member.  The data in the structure of @var{get_data} may be changed by the
accessor, but these changes are lost to the calling Fortran program.
@item @var{get_data_size} @tab intent(in)  The size of the @var{get_data}
structure.
@item @var{stat} @tab intent(out) When non-@code{NULL} give the result of the
operation, i.e., zero on success and non-zero on error.  When @code{NULL} and an
error occurs, then an error message is printed and the program is terminated.
@item @var{team} @tab intent(in)  The opaque team handle as returned by
@code{FORM TEAM}.
@item @var{team_number} @tab intent(in)  The number of the team this access is
to be part of.
@end multitable

@item @emph{Notes}:
It is permitted to have @code{image_index} equal the current image; the memory
to get and the memory to store the data may (partially) overlap.  The
implementation has to take care that it handles this case, e.g. using
@code{memmove} which handles (partially) overlapping memory.
@end table


@node _gfortran_caf_is_present_on_remote
@subsection @code{_gfortran_caf_is_present_on_remote} --- Check that a coarray or a part of it is allocated on the remote image
@cindex Coarray, _gfortran_caf_is_present_on_remote

@table @asis
@item @emph{Synopsis}:
@code{int32_t _gfortran_caf_is_present_on_remote (caf_token_t token,
const int image_index, const int is_present_index, void *add_data,
const size_t add_data_size)}

@item @emph{Description}:
Check if an allocatable coarray or a component of a derived type coarray is
allocated on the remote image identified by the @var{image_index}.  The check
is done by calling routine on the remote side.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number.  @code{this_image ()} is valid.
@item @var{is_present_index} @tab intent(in)  The index of the accessor to
execute as returned by @code{_gfortran_caf_get_remote_function_index ()}.
@item @var{add_data} @tab intent(inout)  Additional data needed in the accessor.
I.e., when an array reference uses a local variable @var{v}, it is transported
in this structure and all references in the accessor are rewritten to access the
member.  The data in the structure of @var{add_data} may be changed by the
accessor, but these changes are lost to the calling Fortran program.
@item @var{add_data_size} @tab intent(in)  The size of the @var{add_data}
structure.
@end multitable
@end table


@node _gfortran_caf_send_to_remote
@subsection @code{_gfortran_caf_send_to_remote} --- Send data to a remote image using a remote side accessor to store it
@cindex Coarray, _gfortran_caf_send_to_remote

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_send_to_remote (caf_token_t token,
gfc_descriptor_t *opt_dst_desc, const size_t *opt_dst_charlen,
const int image_index, const size_t src_size, const void *src_data,
size_t *opt_src_charlen, const gfc_descriptor_t *opt_src_desc,
const int setter_index, void *add_data, const size_t add_data_size, int *stat,
caf_team_t *team, int *team_number)}

@item @emph{Description}:
Called to send a scalar, an array section or a whole array to a remote image
identified by the @var{image_index}. The call modifies the memory of the remote
image.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{opt_dst_desc} @tab intent(inout)  A pointer to the descriptor when
the object identified by @var{token} is an array with a descriptor.  The
parameter needs to be set to @code{NULL}, when @var{token} identifies a scalar
or is an array without a descriptor.
@item @var{opt_dst_charlen} @tab intent(in) When the object to send is a char
array with deferred length, then this parameter needs to be set to point to its
length.  Else the parameter needs to be set to @code{NULL}.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number.  @code{this_image ()} is valid.
@item @var{src_size} @tab intent(in) The size of data expected to be transferred
to the remote image.  If the data type to get is a string or string array,
then this needs to be set to the byte size of each character, i.e. @code{4} for
a @code{CHARACTER (KIND=4)} string.  The length of the string is then given
in @code{opt_src_charlen} (also for string arrays).
@item @var{src_data} @tab intent(in) A pointer the data to be send to the remote
image.  When a descriptor is provided in @code{opt_src_desc} then this parameter
can be ignored by the library implementing the coarray functionality.
@item @var{opt_src_charlen} @tab intent(in)  When a char array is send, this
parameter is set to its length.
@item @var{opt_src_desc} @tab intent(in)  When a descriptor array is send, then
this parameter gives the handle.
@item @var{setter_index} @tab intent(in)  The index of the accessor to execute
as returned by @code{_gfortran_caf_get_remote_function_index ()}.
@item @var{add_data} @tab intent(inout)  Additional data needed in the accessor.
I.e., when an array reference uses a local variable @var{v}, it is transported
in this structure and all references in the accessor are rewritten to access the
member.  The data in the structure of @var{add_data} may be changed by the
accessor, but these changes are lost to the calling Fortran program.
@item @var{add_data_size} @tab intent(in)  The size of the @var{add_data}
structure.
@item @var{stat} @tab intent(out) When non-@code{NULL} give the result of the
operation, i.e., zero on success and non-zero on error.  When @code{NULL} and an
error occurs, then an error message is printed and the program is terminated.
@item @var{team} @tab intent(in)  The opaque team handle as returned by
@code{FORM TEAM}.
@item @var{team_number} @tab intent(in)  The number of the team this access is
to be part of.
@end multitable

@item @emph{Notes}:
It is permitted to have @code{image_index} equal the current image; the memory
to send the data to and the memory to read for the data may (partially) overlap.
The implementation has to take care that it handles this case, e.g. using
@code{memmove} which handles (partially) overlapping memory.
@end table


@node _gfortran_caf_transfer_between_remotes
@subsection @code{_gfortran_caf_transfer_between_remotes} --- Initiate data transfer between to remote images
@cindex Coarray, _gfortran_caf_transfer_between_remotes

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_transfer_between_remotes (caf_token_t dst_token,
gfc_descriptor_t *opt_dst_desc, size_t *opt_dst_charlen,
const int dst_image_index, const int dst_access_index, void *dst_add_data,
const size_t dst_add_data_size, caf_token_t src_token,
const gfc_descriptor_t *opt_src_desc, const size_t *opt_src_charlen,
const int src_image_index, const int src_access_index, void *src_add_data,
const size_t src_add_data_size, const size_t src_size,
const bool scalar_transfer, int *dst_stat, int *src_stat, caf_team_t *dst_team,
int *dst_team_number, caf_team_t *src_team, int *src_team_number)
}

@item @emph{Description}:
Initiates a transfer of data from one remote image to another remote image.
The call modifies the memory of the receiving remote image given by
@code{dst_image_index}.  The @code{src_image_index}'s memory is not modified.
The call returns when the transfer has commenced.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{dst_token} @tab intent(in)  An opaque pointer identifying the coarray
on the receiving image.
@item @var{opt_dst_desc} @tab intent(inout)  A pointer to the descriptor when
the object identified by @var{dst_token} is an array with a descriptor.  The
parameter needs to be set to @code{NULL}, when @var{dst_token} identifies a
scalar or is an array without a descriptor.
@item @var{opt_dst_charlen} @tab intent(in) When the object to modify on the
receiving image is a char array with deferred length, then this parameter needs
to be set to point to its length.  Else the parameter needs to be set to
@code{NULL}.
@item @var{dst_image_index} @tab intent(in)  The ID of the receiving/destination
remote image; must be a positive number.  @code{this_image ()} is valid.
@item @var{dst_access_index} @tab intent(in)  The index of the accessor to
execute on the receiving image as returned by
@code{_gfortran_caf_get_remote_function_index ()}.
@item @var{dst_add_data} @tab intent(inout)  Additional data needed in the
accessor on the receiving side.  I.e., when an array reference uses a local
variable @var{v}, it is transported in this structure and all references in the
accessor are rewritten to access the member.  The data in the structure of
@var{dst_add_data} may be changed by the accessor, but these changes are lost to
the calling Fortran program.
@item @var{dst_add_data_size} @tab intent(in)  The size of the
@var{dst_add_data} structure.
@item @var{src_token} @tab intent(in)  An opaque pointer identifying the coarray
on the sending image.
@item @var{opt_src_desc} @tab intent(inout)  A pointer to the descriptor when
the object identified by @var{src_token} is an array with a descriptor.  The
parameter needs to be set to @code{NULL}, when @var{src_token} identifies a
scalar or is an array without a descriptor.
@item @var{opt_src_charlen} @tab intent(in) When the object to get from the
source image is a char array with deferred length, then this parameter needs
to be set to point to its length.  Else the parameter needs to be set to
@code{NULL}.
@item @var{src_image_index} @tab intent(in)  The ID of the sending/source
remote image; must be a positive number.  @code{this_image ()} is valid.
@item @var{src_access_index} @tab intent(in)  The index of the accessor to
execute on the sending image as returned by
@code{_gfortran_caf_get_remote_function_index ()}.
@item @var{src_add_data} @tab intent(inout)  Additional data needed in the
accessor on the sending side.  I.e., when an array reference uses a local
variable @var{v}, it is transported in this structure and all references in the
accessor are rewritten to access the member.  The data in the structure of
@var{src_add_data} may be changed by the accessor, but these changes are lost to
the calling Fortran program.
@item @var{src_add_data_size} @tab intent(in)  The size of the
@var{src_add_data} structure.
@item @var{src_size} @tab intent(in) The size of data expected to be transferred
between the images.  If the data type to get is a string or string array,
then this needs to be set to the byte size of each character, i.e. @code{4} for
a @code{CHARACTER (KIND=4)} string.  The length of the string is then given
in @code{opt_src_charlen} and @code{opt_dst_charlen} (also for string arrays).
@item @var{scalar_transfer} @tab intent(in)  Is set to true when the data to be
transferred between the two images is not an array with a descriptor.
@item @var{dst_stat} @tab intent(out) When non-@code{NULL} give the result of
the operation on the receiving side, i.e., zero on success and non-zero on
error.  When @code{NULL} and an error occurs, then an error message is printed
and the program is terminated.
@item @var{src_stat} @tab intent(out) When non-@code{NULL} give the result of
the operation on the sending side, i.e., zero on success and non-zero on error.
When @code{NULL} and an error occurs, then an error message is printed and the
program is terminated.
@item @var{dst_team} @tab intent(in)  The opaque team handle as returned by
@code{FORM TEAM}.
@item @var{dst_team_number} @tab intent(in)  The number of the team this access
is to be part of.
@item @var{src_team} @tab intent(in)  The opaque team handle as returned by
@code{FORM TEAM}.
@item @var{src_team_number} @tab intent(in)  The number of the team this access
is to be part of.
@end multitable

@item @emph{Notes}:
It is permitted to have both @code{dst_image_index} and @code{src_image_index}
equal the current image; the memory to send the data to and the memory to read
for the data may (partially) overlap.  The implementation has to take care that
it handles this case, e.g. using @code{memmove} which handles (partially)
overlapping memory.
@end table


@node _gfortran_caf_sendget_by_ref
@subsection @code{_gfortran_caf_sendget_by_ref} --- Sending data between remote images using enhanced references on both sides
@cindex Coarray, _gfortran_caf_sendget_by_ref

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_sendget_by_ref (caf_token_t dst_token,
int dst_image_index, caf_reference_t *dst_refs,
caf_token_t src_token, int src_image_index, caf_reference_t *src_refs,
int dst_kind, int src_kind, bool may_require_tmp, int *dst_stat,
int *src_stat, int dst_type, int src_type)}

@item @emph{Description}:
Called to send a scalar, an array section or a whole array from a remote image
identified by the @var{src_image_index} to a remote image identified by the
@var{dst_image_index}.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{dst_token} @tab intent(in)  An opaque pointer identifying the
destination coarray.
@item @var{dst_image_index} @tab intent(in)  The ID of the destination remote
image; must be a positive number.
@item @var{dst_refs} @tab intent(in) The references on the remote array to store
the data given by the source.  Guaranteed to have at least one entry.
@item @var{src_token} @tab intent(in)  An opaque pointer identifying the source
coarray.
@item @var{src_image_index} @tab intent(in)  The ID of the source remote image;
must be a positive number.
@item @var{src_refs} @tab intent(in) The references to apply to the remote
structure to get the data.
@item @var{dst_kind} @tab intent(in)  Kind of the destination argument
@item @var{src_kind} @tab intent(in)  Kind of the source argument
@item @var{may_require_tmp} @tab intent(in)  The variable is @code{false} when
it is known at compile time that the @var{dest} and @var{src} either cannot
overlap or overlap (fully or partially) such that walking @var{src} and
@var{dest} in elementwise order (honoring the stride value) does not
lead to wrong results.  Otherwise, the value is @code{true}.
@item @var{dst_stat} @tab intent(out) when non-@code{NULL} give the result of
the send-operation, i.e., zero on success and nonzero on error.  When
@code{NULL} and an error occurs, then an error message is printed and the
program is terminated.
@item @var{src_stat} @tab intent(out) When non-@code{NULL} give the result of
the get-operation, i.e., zero on success and nonzero on error.  When
@code{NULL} and an error occurs, then an error message is printed and the
program is terminated.
@item @var{dst_type} @tab intent(in)  Give the type of the destination.  When
the destination is not an array, than the precise type, e.g. of a component in
a derived type, is not known, but provided here.
@item @var{src_type} @tab intent(in)  Give the type of the source.  When the
source is not an array, than the precise type, e.g. of a component in a
derived type, is not known, but provided here.
@end multitable

@item @emph{Notes}:
It is permitted to have the same image index for both @var{src_image_index} and
@var{dst_image_index}; the memory of the send-to and the send-from might
(partially) overlap in that case.  The implementation has to take care that it
handles this case, e.g. using @code{memmove} which handles (partially)
overlapping memory.  If @var{may_require_tmp} is true, the library
might additionally create a temporary variable, unless additional checks show
that this is not required (e.g. because walking backward is possible or because
both arrays are contiguous and @code{memmove} takes care of overlap issues).

Note that the assignment of a scalar to an array is permitted.  In addition,
the library has to handle numeric-type conversion and for strings, padding and
different character kinds.

Because of the more complicated references possible some operations may be
unsupported by certain libraries.  The library is expected to issue a precise
error message why the operation is not permitted.
@end table


@node _gfortran_caf_lock
@subsection @code{_gfortran_caf_lock} --- Locking a lock variable
@cindex Coarray, _gfortran_caf_lock

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_lock (caf_token_t token, size_t index, int image_index,
int *acquired_lock, int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Acquire a lock on the given image on a scalar locking variable or for the
given array element for an array-valued variable.  If the @var{acquired_lock}
is @code{NULL}, the function returns after having obtained the lock.  If it is
non-@code{NULL}, then @var{acquired_lock} is assigned the value true (one) when
the lock could be obtained and false (zero) otherwise.  Locking a lock variable
that has already been locked by the same image is an error.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{index} @tab intent(in)  Array index; first array index is 0.  For
scalars, it is always 0.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number.
@item @var{acquired_lock} @tab intent(out) If not NULL, it returns whether lock
could be obtained.
@item @var{stat} @tab intent(out) Stores the STAT=; may be NULL.
@item @var{errmsg} @tab intent(out) When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
This function is also called for critical blocks; for those, the array index
is always zero and the image index is one.  Libraries are permitted to use other
images for critical-block locking variables.
@end table

@node _gfortran_caf_unlock
@subsection @code{_gfortran_caf_lock} --- Unlocking a lock variable
@cindex Coarray, _gfortran_caf_unlock

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_unlock (caf_token_t token, size_t index, int image_index,
int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Release a lock on the given image on a scalar locking variable or for the
given array element for an array-valued variable. Unlocking a lock variable
that is unlocked or has been locked by a different image is an error.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{index} @tab intent(in)  Array index; first array index is 0.  For
scalars, it is always 0.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number.
@item @var{stat} @tab intent(out) For allocatable coarrays, stores the STAT=;
may be NULL.
@item @var{errmsg} @tab intent(out) When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
This function is also called for critical block; for those, the array index
is always zero and the image index is one.  Libraries are permitted to use other
images for critical-block locking variables.
@end table

@node _gfortran_caf_event_post
@subsection @code{_gfortran_caf_event_post} --- Post an event
@cindex Coarray, _gfortran_caf_event_post

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_event_post (caf_token_t token, size_t index,
int image_index, int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Increment the event count of the specified event variable.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{index} @tab intent(in)  Array index; first array index is 0.  For
scalars, it is always 0.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number; zero indicates the current image, when accessed noncoindexed.
@item @var{stat} @tab intent(out)  Stores the STAT=; may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
This acts like an atomic add of one to the remote image's event variable.
The statement is an image-control statement but does not imply sync memory.
Still, all preceding push communications of this image to the specified
remote image have to be completed before @code{event_wait} on the remote
image returns.
@end table



@node _gfortran_caf_event_wait
@subsection @code{_gfortran_caf_event_wait} --- Wait that an event occurred
@cindex Coarray, _gfortran_caf_event_wait

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_event_wait (caf_token_t token, size_t index,
int until_count, int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Wait until the event count has reached at least the specified
@var{until_count}; if so, atomically decrement the event variable by this
amount and return.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{index} @tab intent(in)  Array index; first array index is 0.  For
scalars, it is always 0.
@item @var{until_count} @tab intent(in)  The number of events that have to be
available before the function returns.
@item @var{stat} @tab intent(out)  Stores the STAT=; may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
This function only operates on a local coarray. It acts like a loop checking
atomically the value of the event variable, breaking if the value is greater
or equal the requested number of counts. Before the function returns, the
event variable has to be decremented by the requested @var{until_count} value.
A possible implementation would be a busy loop for a certain number of spins
(possibly depending on the number of threads relative to the number of available
cores) followed by another waiting strategy such as a sleeping wait (possibly
with an increasing number of sleep time) or, if possible, a futex wait.

The statement is an image-control statement but does not imply sync memory.
Still, all preceding push communications of this image to the specified
remote image have to be completed before @code{event_wait} on the remote
image returns.
@end table



@node _gfortran_caf_event_query
@subsection @code{_gfortran_caf_event_query} --- Query event count
@cindex Coarray, _gfortran_caf_event_query

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_event_query (caf_token_t token, size_t index,
int image_index, int *count, int *stat)}

@item @emph{Description}:
Return the event count of the specified event variable.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{index} @tab intent(in)  Array index; first array index is 0.  For
scalars, it is always 0.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number; zero indicates the current image when accessed noncoindexed.
@item @var{count} @tab intent(out)  The number of events currently posted to
the event variable.
@item @var{stat} @tab intent(out)  Stores the STAT=; may be NULL.
@end multitable

@item @emph{Notes}:
The typical use is to check the local event variable to only call
@code{event_wait} when the data is available. However, a coindexed variable
is permitted; there is no ordering or synchronization implied.  It acts like
an atomic fetch of the value of the event variable.
@end table



@node _gfortran_caf_sync_all
@subsection @code{_gfortran_caf_sync_all} --- All-image barrier
@cindex Coarray, _gfortran_caf_sync_all

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_sync_all (int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Synchronization of all images in the current team; the program only continues
on a given image after this function has been called on all images of the
current team.  Additionally, it ensures that all pending data transfers of
previous segment have completed.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable
@end table



@node _gfortran_caf_sync_images
@subsection @code{_gfortran_caf_sync_images} --- Barrier for selected images
@cindex Coarray, _gfortran_caf_sync_images

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_sync_images (int count, int images[], int *stat,
char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Synchronization between the specified images; the program only continues on a
given image after this function has been called on all images specified for
that image. Note that one image can wait for all other images in the current
team (e.g. via @code{sync images(*)}) while those only wait for that specific
image.  Additionally, @code{sync images} ensures that all pending data
transfers of previous segments have completed.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{count} @tab intent(in)  The number of images that are provided in
the next argument.  For a zero-sized array, the value is zero.  For
@code{sync images (*)}, the value is @math{-1}.
@item @var{images} @tab intent(in)  An array with the images provided by the
user.  If @var{count} is zero, a NULL pointer is passed.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable
@end table



@node _gfortran_caf_sync_memory
@subsection @code{_gfortran_caf_sync_memory} --- Wait for completion of segment-memory operations
@cindex Coarray, _gfortran_caf_sync_memory

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_sync_memory (int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Acts as optimization barrier between different segments. It also ensures that
all pending memory operations of this image have been completed.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
A simple implementation could be
@code{__asm__ __volatile__ ("":::"memory")} to prevent code movements.
@end table



@node _gfortran_caf_error_stop
@subsection @code{_gfortran_caf_error_stop} --- Error termination with exit code
@cindex Coarray, _gfortran_caf_error_stop

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_error_stop (int error)}

@item @emph{Description}:
Invoked for an @code{ERROR STOP} statement that has an integer argument.  The
function should terminate the program with the specified exit code.


@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{error} @tab intent(in)  The exit status to be used.
@end multitable
@end table



@node _gfortran_caf_error_stop_str
@subsection @code{_gfortran_caf_error_stop_str} --- Error termination with string
@cindex Coarray, _gfortran_caf_error_stop_str

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_error_stop (const char *string, size_t len)}

@item @emph{Description}:
Invoked for an @code{ERROR STOP} statement that has a string as argument.  The
function should terminate the program with a nonzero-exit code.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{string} @tab intent(in)  the error message (not zero terminated)
@item @var{len} @tab intent(in)  the length of the string
@end multitable
@end table



@node _gfortran_caf_fail_image
@subsection @code{_gfortran_caf_fail_image} --- Mark the image failed and end its execution
@cindex Coarray, _gfortran_caf_fail_image

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_fail_image ()}

@item @emph{Description}:
Invoked for an @code{FAIL IMAGE} statement.  The function should terminate the
current image.

@item @emph{Notes}:
This function follows TS18508.
@end table



@node _gfortran_caf_atomic_define
@subsection @code{_gfortran_caf_atomic_define} --- Atomic variable assignment
@cindex Coarray, _gfortran_caf_atomic_define

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_atomic_define (caf_token_t token, size_t offset,
int image_index, void *value, int *stat, int type, int kind)}

@item @emph{Description}:
Assign atomically a value to an integer or logical variable.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{offset} @tab intent(in)  The number of bytes the actual data is
shifted compared to the base address of the coarray.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number; zero indicates the current image when used noncoindexed.
@item @var{value} @tab intent(in)  the value to be assigned, passed by reference
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{type} @tab intent(in)  The data type, i.e. @code{BT_INTEGER} (1) or
@code{BT_LOGICAL} (2).
@item @var{kind} @tab intent(in)  The kind value (only 4; always @code{int})
@end multitable
@end table



@node _gfortran_caf_atomic_ref
@subsection @code{_gfortran_caf_atomic_ref} --- Atomic variable reference
@cindex Coarray, _gfortran_caf_atomic_ref

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_atomic_ref (caf_token_t token, size_t offset,
int image_index, void *value, int *stat, int type, int kind)}

@item @emph{Description}:
Reference atomically a value of a kind-4 integer or logical variable.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{offset} @tab intent(in)  The number of bytes the actual data is
shifted compared to the base address of the coarray.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number; zero indicates the current image when used noncoindexed.
@item @var{value} @tab intent(out)  The variable assigned the atomically
referenced variable.
@item @var{stat} @tab intent(out) Stores the status STAT= and may be NULL.
@item @var{type} @tab the data type, i.e. @code{BT_INTEGER} (1) or
@code{BT_LOGICAL} (2).
@item @var{kind} @tab The kind value (only 4; always @code{int})
@end multitable
@end table



@node _gfortran_caf_atomic_cas
@subsection @code{_gfortran_caf_atomic_cas} --- Atomic compare and swap
@cindex Coarray, _gfortran_caf_atomic_cas

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_atomic_cas (caf_token_t token, size_t offset,
int image_index, void *old, void *compare, void *new_val, int *stat,
int type, int kind)}

@item @emph{Description}:
Atomic compare and swap of a kind-4 integer or logical variable. Assigns
atomically the specified value to the atomic variable, if the latter has
the value specified by the passed condition value.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{offset} @tab intent(in)  The number of bytes the actual data is
shifted compared to the base address of the coarray.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number; zero indicates the current image when used noncoindexed.
@item @var{old} @tab intent(out)  The value the atomic variable had
just before the cas operation.
@item @var{compare} @tab intent(in)  The value used for comparison.
@item @var{new_val} @tab intent(in)  The new value for the atomic variable,
assigned to the atomic variable, if @code{compare} equals the value of the
atomic variable.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{type} @tab intent(in)  the data type, i.e. @code{BT_INTEGER} (1) or
@code{BT_LOGICAL} (2).
@item @var{kind} @tab intent(in)  The kind value (only 4; always @code{int})
@end multitable
@end table



@node _gfortran_caf_atomic_op
@subsection @code{_gfortran_caf_atomic_op} --- Atomic operation
@cindex Coarray, _gfortran_caf_atomic_op

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_atomic_op (int op, caf_token_t token, size_t offset,
int image_index, void *value, void *old, int *stat, int type, int kind)}

@item @emph{Description}:
Apply an operation atomically to an atomic integer or logical variable.
After the operation, @var{old} contains the value just before the operation,
which, respectively, adds (GFC_CAF_ATOMIC_ADD) atomically the @code{value} to
the atomic integer variable or does a bitwise AND, OR or exclusive OR
between the atomic variable and @var{value}; the result is then stored in the
atomic variable.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{op} @tab intent(in)  the operation to be performed; possible values
@code{GFC_CAF_ATOMIC_ADD} (1), @code{GFC_CAF_ATOMIC_AND} (2),
@code{GFC_CAF_ATOMIC_OR} (3), @code{GFC_CAF_ATOMIC_XOR} (4).
@item @var{token} @tab intent(in)  An opaque pointer identifying the coarray.
@item @var{offset} @tab intent(in)  The number of bytes the actual data is
shifted compared to the base address of the coarray.
@item @var{image_index} @tab intent(in)  The ID of the remote image; must be a
positive number; zero indicates the current image when used noncoindexed.
@item @var{old} @tab intent(out)  The value the atomic variable had
just before the atomic operation.
@item @var{val} @tab intent(in)  The new value for the atomic variable,
assigned to the atomic variable, if @code{compare} equals the value of the
atomic variable.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{type} @tab intent(in)  the data type, i.e. @code{BT_INTEGER} (1) or
@code{BT_LOGICAL} (2)
@item @var{kind} @tab intent(in)  the kind value (only 4; always @code{int})
@end multitable
@end table




@node _gfortran_caf_co_broadcast
@subsection @code{_gfortran_caf_co_broadcast} --- Sending data to all images
@cindex Coarray, _gfortran_caf_co_broadcast

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_co_broadcast (gfc_descriptor_t *a,
int source_image, int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Distribute a value from a given image to all other images in the team. Has to
be called collectively.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{a} @tab intent(inout)  An array descriptor with the data to be
broadcasted (on @var{source_image}) or to be received (other images).
@item @var{source_image} @tab intent(in)  The ID of the image from which the
data should be broadcasted.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg.
@end multitable
@end table



@node _gfortran_caf_co_max
@subsection @code{_gfortran_caf_co_max} --- Collective maximum reduction
@cindex Coarray, _gfortran_caf_co_max

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_co_max (gfc_descriptor_t *a, int result_image,
int *stat, char *errmsg, int a_len, size_t errmsg_len)}

@item @emph{Description}:
Calculates for each array element of the variable @var{a} the maximum
value for that element in the current team; if @var{result_image} has the
value 0, the result shall be stored on all images, otherwise, only on the
specified image. This function operates on numeric values and character
strings.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{a} @tab intent(inout)  An array descriptor for the data to be
processed.  On the destination image(s) the result overwrites the old content.
@item @var{result_image} @tab intent(in)  The ID of the image to which the
reduced value should be copied to; if zero, it has to be copied to all images.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{a_len} @tab intent(in)  the string length of argument @var{a}
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
If @var{result_image} is nonzero, the data in the array descriptor @var{a} on
all images except of the specified one become undefined; hence, the library may
make use of this.
@end table



@node _gfortran_caf_co_min
@subsection @code{_gfortran_caf_co_min} --- Collective minimum reduction
@cindex Coarray, _gfortran_caf_co_min

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_co_min (gfc_descriptor_t *a, int result_image,
int *stat, char *errmsg, int a_len, size_t errmsg_len)}

@item @emph{Description}:
Calculates for each array element of the variable @var{a} the minimum
value for that element in the current team; if @var{result_image} has the
value 0, the result shall be stored on all images, otherwise, only on the
specified image. This function operates on numeric values and character
strings.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{a} @tab intent(inout)  An array descriptor for the data to be
processed.  On the destination image(s) the result overwrites the old content.
@item @var{result_image} @tab intent(in)  The ID of the image to which the
reduced value should be copied to; if zero, it has to be copied to all images.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{a_len} @tab intent(in)  the string length of argument @var{a}
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
If @var{result_image} is nonzero, the data in the array descriptor @var{a} on
all images except of the specified one become undefined; hence, the library may
make use of this.
@end table



@node _gfortran_caf_co_sum
@subsection @code{_gfortran_caf_co_sum} --- Collective summing reduction
@cindex Coarray, _gfortran_caf_co_sum

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_co_sum (gfc_descriptor_t *a, int result_image,
int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Calculates for each array element of the variable @var{a} the sum of all
values for that element in the current team; if @var{result_image} has the
value 0, the result shall be stored on all images, otherwise, only on the
specified image.  This function operates on numeric values only.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{a} @tab intent(inout)  An array descriptor with the data to be
processed.  On the destination image(s) the result overwrites the old content.
@item @var{result_image} @tab intent(in)  The ID of the image to which the
reduced value should be copied to; if zero, it has to be copied to all images.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
If @var{result_image} is nonzero, the data in the array descriptor @var{a} on
all images except of the specified one become undefined; hence, the library may
make use of this.
@end table



@node _gfortran_caf_co_reduce
@subsection @code{_gfortran_caf_co_reduce} --- Generic collective reduction
@cindex Coarray, _gfortran_caf_co_reduce

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_co_reduce (gfc_descriptor_t *a,
void * (*opr) (void *, void *), int opr_flags, int result_image,
int *stat, char *errmsg, int a_len, size_t errmsg_len)}

@item @emph{Description}:
Calculates for each array element of the variable @var{a} the reduction
value for that element in the current team; if @var{result_image} has the
value 0, the result shall be stored on all images, otherwise, only on the
specified image.  The @var{opr} is a pure function doing a mathematically
commutative and associative operation.

The @var{opr_flags} denote the following; the values are bitwise ored.
@code{GFC_CAF_BYREF} (1) if the result should be returned
by reference; @code{GFC_CAF_HIDDENLEN} (2) whether the result and argument
string lengths shall be specified as hidden arguments;
@code{GFC_CAF_ARG_VALUE} (4) whether the arguments shall be passed by value,
@code{GFC_CAF_ARG_DESC} (8) whether the arguments shall be passed by descriptor.


@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{a} @tab intent(inout)  An array descriptor with the data to be
processed.  On the destination image(s) the result overwrites the old content.
@item @var{opr} @tab intent(in)  Function pointer to the reduction function
@item @var{opr_flags} @tab intent(in)  Flags regarding the reduction function
@item @var{result_image} @tab intent(in)  The ID of the image to which the
reduced value should be copied to; if zero, it has to be copied to all images.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{a_len} @tab intent(in)  the string length of argument @var{a}
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
If @var{result_image} is nonzero, the data in the array descriptor @var{a} on
all images except of the specified one become undefined; hence, the library may
make use of this.

For character arguments, the result is passed as first argument, followed
by the result string length, next come the two string arguments, followed
by the two hidden string length arguments.  With C binding, there are no hidden
arguments and by-reference passing and either only a single character is passed
or an array descriptor.
@end table



@node _gfortran_caf_form_team
@subsection @code{_gfortran_caf_form_team} --- Team creation function
@cindex Coarray, _gfortran_caf_form_team

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_form_team (int team_id, caf_team_t *team,
int *new_index, int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Create a team.  All images giving the same @var{team_id} in a call to
@code{FORM TEAM} will form a new team addressable by the opaque handle
@var{team} which is of type @code{team_type} from the intrinsic module
@ref{ISO_FORTRAN_ENV}.  In the team the image gets the image index given by
@var{new_index} if present.  If @var{new_index} is absent, then an
implementation specific index is assigned.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team_id} @tab intent(in)  A unique id for each team to form.  Images
giving the same @var{team_id} in a call to @code{FORM TEAM} belong to the same
team.
@item @var{team} @tab intent(out)  The opaque pointer to the newly formed team
@item @var{new_index} @tab intent(in)  If non-null gives the unique index of
this image in the newly formed team.  When no @var{new_index} is given, the
caf-library is free to choose a unique index.
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
The id given in @var{team_id} has to be unique in all subsequent calls to
@code{FORM TEAM} on the same image.  That id is the same used in
@code{TEAM_NUMBER=} of coarray indexes, which motivates the uniqueness.

The index given in @var{new_index} needs to be unique among all members of
team to create.  Failing uniqueness may lead to misbehaviour, which depends
on the caf-library's implementation.  The library is free to implement
checks for this, which imposes overhead and therefore may be avoided.
@end table



@node _gfortran_caf_change_team
@subsection @code{_gfortran_caf_change_team} --- Team activation function
@cindex Coarray, _gfortran_caf_change_team

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_change_team (caf_team_t team, int *stat, char *errmsg,
size_t errmsg_len)}

@item @emph{Description}:
Actives the team given by @var{team}, which must be formed but not active
yet.  This routine starts a new epoch on the coarray memory pool.  All
coarrays registered from now on, will be freed once the team is terminated.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab intent(inout)  The opaque pointer to an already formed
team
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable

@item @emph{Notes}:
When an error occurs and @var{stat} is non-null, it will be set.  Nevertheless
will the Fortran program continue with the first statement in the change team
block.
@end table



@node _gfortran_caf_end_team
@subsection @code{_gfortran_caf_end_team} --- Team termination function
@cindex Coarray, _gfortran_caf_end_team

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_end_team (int *stat, char *errmsg, size_t errmsg_len)}

@item @emph{Description}:
Terminates the last team changed to.  The coarray memory epoch is
terminated and all coarrays allocated since the execution of @code{CHANGE TEAM}
are freed.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable
@end table



@node _gfortran_caf_sync_team
@subsection @code{_gfortran_caf_sync_team} --- Synchronize all images of a given team
@cindex Coarray, _gfortran_caf_sync_team

@table @asis
@item @emph{Synopsis}:
@code{void _gfortran_caf_sync_team (caf_team_t team, int *stat, char *errmsg,
size_t errmsg_len)}

@item @emph{Description}:
Blocks execution of the image calling @code{SYNC TEAM} until all images of the
team given by @var{team} have joined the synchronisation call.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab intent(in)  The opaque pointer to an active team
@item @var{stat} @tab intent(out)  Stores the status STAT= and may be NULL.
@item @var{errmsg} @tab intent(out)  When an error occurs, this is set to
an error message; may be NULL.
@item @var{errmsg_len} @tab intent(in)  the buffer size of errmsg
@end multitable
@end table



@node _gfortran_caf_get_team
@subsection @code{_gfortran_caf_get_team} --- Get the opaque handle of the specified team
@cindex Coarray, _gfortran_caf_get_team

@table @asis
@item @emph{Synopsis}:
@code{caf_team_t _gfortran_caf_get_team (int32_t *level)}

@item @emph{Description}:
Get the current team, when @var{level} is null, or the team specified by
@var{level} set to @code{INITIAL_TEAM}, @code{PARENT_TEAM} or
@code{CURRENT_TEAM} from the @code{ISO_FORTRAN_ENV} intrinsic module.  When
being on the @code{INITIAL_TEAM} and requesting its @code{PARENT_TEAM}, then
the initial team is returned.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{level} @tab intent(in)  If set to one of the levels specified in
the @code{ISO_FORTRAN_ENV} module, the function returns the handle of the given
team.  Values different from the allowed ones lead to a runtime error.
@end multitable
@end table



@node _gfortran_caf_team_number
@subsection @code{_gfortran_caf_team_number} --- Get the unique id of the given team
@cindex Coarray, _gfortran_caf_team_number

@table @asis
@item @emph{Synopsis}:
@code{int _gfortran_caf_team_number (caf_team_t team)}

@item @emph{Description}:
The team id given when forming the team @ref{_gfortran_caf_form_team} of the
team specified by @var{team}, if given, or of the current team, if @var{team}
is absent.  It is a runtime error to specify a non-existing team.
The team has to be formed, i.e., it is not necessary that it is changed
into to get the team number.  The initial team has the team number @code{-1}.

@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{team} @tab intent(in)  The team for which the team id is desired.
@end multitable
@end table


@c Intrinsic Procedures
@c ---------------------------------------------------------------------

@include intrinsic.texi


@tex
\blankpart
@end tex

@c ---------------------------------------------------------------------
@c Contributing
@c ---------------------------------------------------------------------

@node Contributing
@unnumbered Contributing
@cindex Contributing

Free software is only possible if people contribute to efforts
to create it.
We're always in need of more people helping out with ideas
and comments, writing documentation and contributing code.

If you want to contribute to GNU Fortran,
have a look at the long lists of projects you can take on.
Some of these projects are small,
some of them are large;
some are completely orthogonal to the rest of what is
happening on GNU Fortran,
but others are ``mainstream'' projects in need of enthusiastic hackers.
All of these projects are important!
We will eventually get around to the things here,
but they are also things doable by someone who is willing and able.

@menu
* Contributors::
* Projects::
@end menu


@node Contributors
@section Contributors to GNU Fortran
@cindex Contributors
@cindex Credits
@cindex Authors

Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
also the initiator of the whole project.  Thanks Andy!
Most of the interface with GCC was written by @emph{Paul Brook}.

The following individuals have contributed code and/or
ideas and significant help to the GNU Fortran project
(in alphabetical order):

@itemize @minus
@item Janne Blomqvist
@item Steven Bosscher
@item Paul Brook
@item Tobias Burnus
@item Fran@,{c}ois-Xavier Coudert
@item Bud Davis
@item Jerry DeLisle
@item Erik Edelmann
@item Bernhard Fischer
@item Daniel Franke
@item Richard Guenther
@item Richard Henderson
@item Katherine Holcomb
@item Jakub Jelinek
@item Niels Kristian Bech Jensen
@item Steven Johnson
@item Steven G. Kargl
@item Thomas Koenig
@item Asher Langton
@item H. J. Lu
@item Toon Moene
@item Brooks Moses
@item Andrew Pinski
@item Tim Prince
@item Christopher D. Rickett
@item Richard Sandiford
@item Tobias Schl@"uter
@item Roger Sayle
@item Paul Thomas
@item Andy Vaught
@item Feng Wang
@item Janus Weil
@item Daniel Kraft
@end itemize

The following people have contributed bug reports,
smaller or larger patches,
and much needed feedback and encouragement for the
GNU Fortran project:

@itemize @minus
@item Bill Clodius
@item Dominique d'Humi@`eres
@item Kate Hedstrom
@item Erik Schnetter
@item Gerhard Steinmetz
@item Joost VandeVondele
@end itemize

Many other individuals have helped debug,
test and improve the GNU Fortran compiler over the past few years,
and we welcome you to do the same!
If you already have done so,
and you would like to see your name listed in the
list above, please contact us.


@node Projects
@section Projects

@table @emph

@item Help build the test suite
Solicit more code for donation to the test suite: the more extensive the
testsuite, the smaller the risk of breaking things in the future! We can
keep code private on request.

@item Bug hunting/squishing
Find bugs and write more test cases! Test cases are especially very
welcome, because it allows us to concentrate on fixing bugs instead of
isolating them.  Going through the bugzilla database at
@url{https://gcc.gnu.org/@/bugzilla/} to reduce testcases posted there and
add more information (for example, for which version does the testcase
work, for which versions does it fail?) is also very helpful.

@item Missing features
For a larger project, consider working on the missing features required for
Fortran language standards compliance (@pxref{Standards}), or contributing
to the implementation of extensions such as OpenMP (@pxref{OpenMP}) or
OpenACC (@pxref{OpenACC}) that are under active development.  Again,
contributing test cases for these features is useful too!

@end table


@c ---------------------------------------------------------------------
@c GNU General Public License
@c ---------------------------------------------------------------------

@include gpl_v3.texi



@c ---------------------------------------------------------------------
@c GNU Free Documentation License
@c ---------------------------------------------------------------------

@include fdl.texi



@c ---------------------------------------------------------------------
@c Funding Free Software
@c ---------------------------------------------------------------------

@include funding.texi

@c ---------------------------------------------------------------------
@c Indices
@c ---------------------------------------------------------------------

@node Option Index
@unnumbered Option Index
@command{gfortran}'s command line options are indexed here without any
initial @samp{-} or @samp{--}.  Where an option has both positive and
negative forms (such as -foption and -fno-option), relevant entries in
the manual are indexed under the most appropriate form; it may sometimes
be useful to look up both forms.
@printindex op

@node Keyword Index
@unnumbered Keyword Index
@printindex cp

@bye
