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\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename gfortran.info
@set copyrights-gfortran 1999-2022
@include gcc-common.texi
@settitle The GNU Fortran Compiler
@c Create a separate index for command line options
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@c %** start of document
@c Cause even numbered pages to be printed on the left hand side of
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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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@contents
@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
implemention 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#1.21, 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.
Additionally, the GNU Fortran compilers supports the OpenMP specification
(version 4.5 and partial support of the features of the 5.0 version,
@url{https://openmp.org/@/openmp-specifications/}).
There also is support for the OpenACC specification (targeting
version 2.6, @uref{https://www.openacc.org/}). See
@uref{https://gcc.gnu.org/wiki/OpenACC} for more information.
@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. They can be found at
@uref{https://www.fortran.com/@/iso_varying_string.f95} and at
@uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/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 endianness 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 will slow 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
will slow 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
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 endianness 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 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
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.
@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.
@emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
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::
@end menu
@node KIND Type Parameters
@section 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} 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 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 will optimize 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 MAX and MIN intrinsics with 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 Mac OS X. 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} will force 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} will also force 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, will
also force cache validation and flushing dirty data and metadata.
@node Files opened without an explicit ACTION= specifier
@section Files opened without an explicit 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 INQUIRE statements of the ``inquire by file'' form will
relate to the target of the symbolic link. For example,
@code{INQUIRE(FILE="foo",EXIST=ex)} will set @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 will result 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 will cause 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
compability 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 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 which 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
sytems, 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 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@comma{} G and I format descriptors::
* I/O item lists::
* @code{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::
* Extended math intrinsics::
* 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::
@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*size x,y,z
@end smallexample
@noindent
where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
etc.), and where @code{size} is a byte count corresponding to the
storage size of a valid kind for that type. (For @code{COMPLEX}
variables, @code{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} 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
(@code{::}); 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 which 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. Two extensions are permitted:
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}.
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.
@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
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 will be given the value 0.00 and the two succeeding
elements will be given the values 1.00 and 2.00.
@smallexample
&MYNML
X(1,1) = 0.00 , 1.00 , 2.00
/
@end smallexample
When writing a namelist, if no @code{DELIM=} is specified, by default a
double quote is used to delimit character strings. If -std=F95, F2003,
or F2008, etc, the delim status is set to '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
decriptor is permited 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@comma{} G and I format descriptors
@subsection Default widths for @code{F}, @code{G} and @code{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 will be 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 @code{Q} exponent-letter
@subsection @code{Q} exponent-letter
@cindex @code{Q} exponent-letter
GNU Fortran accepts real literal constants with an exponent-letter
of @code{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 will be interpreted as a
@code{REAL(10)} entity. In the absence of @code{REAL(16)} and
@code{REAL(10)}, an error will occur.
@node BOZ literal constants
@subsection BOZ literal constants
@cindex BOZ literal constants
Besides decimal constants, Fortran also supports binary (@code{b}),
octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
syntax is: @samp{prefix quote digits quote}, where the prefix is
either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
@code{"} 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 @code{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 will be 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 will be truncated.
a = 3Hxyz ! Valid, but the Hollerith constant will be 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 non-standard 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 byte-wise fashion. The
constant will be 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' ! Will be padded.
x = 'ab1234' ! Will be truncated.
@end smallexample
@node Cray pointers
@subsection Cray pointers
@cindex pointer, Cray
Cray pointers are part of a non-standard 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
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
will issue 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 ipt's type is omitted, then the compiler
will ensure that 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 will be 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, will 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 illegal. It is the user's responsibility to avoid doing
this; the compiler works under the assumption that no such aliasing
occurs.
Cray pointers will 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 illegal. (Note that this is not
unique to GNU Fortran; any Fortran compiler that supports Cray pointers
will ``incorrectly'' optimize code with illegal 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
will change 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
will 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} 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
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)}, will
probably not work.
@emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
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 strives to be compatible to the
@uref{https://openmp.org/wp/openmp-specifications/,
OpenMP Application Program Interface v4.5}.
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{!$} sentinels
in fixed form, @command{gfortran} needs to be invoked with the
@option{-fopenmp}. This also arranges for automatic linking of the
GNU Offloading and Multi Processing Runtime Library
@ref{Top,,libgomp,libgomp,GNU Offloading and Multi Processing Runtime
Library}.
The OpenMP Fortran runtime library routines are provided both in a
form of a Fortran 90 module named @code{omp_lib} and in a form of
a Fortran @code{include} file named @file{omp_lib.h}.
An example of a parallelized loop taken from Appendix A.1 of
the OpenMP Application Program Interface v2.5:
@smallexample
SUBROUTINE A1(N, A, B)
INTEGER I, N
REAL B(N), A(N)
!$OMP PARALLEL DO !I is private by default
DO I=2,N
B(I) = (A(I) + A(I-1)) / 2.0
ENDDO
!$OMP END PARALLEL DO
END SUBROUTINE A1
@end smallexample
Please note:
@itemize
@item
@option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
will be allocated on the stack. When porting existing code to OpenMP,
this may lead to surprising results, especially to segmentation faults
if the stacksize is limited.
@item
On glibc-based systems, OpenMP enabled applications cannot be statically
linked due to limitations of the underlying pthreads-implementation. It
might be possible to get a working solution if
@command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
to the command line. However, this is not supported by @command{gcc} and
thus not recommended.
@end itemize
@node OpenACC
@subsection OpenACC
@cindex OpenACC
OpenACC is an application programming interface (API) that supports
offloading of code to accelerator devices. It consists of a set of
compiler directives, library routines, and environment variables that
influence run-time behavior.
GNU Fortran strives to be compatible to the
@uref{https://www.openacc.org/, OpenACC Application Programming
Interface v2.6}.
To enable the processing of the OpenACC directive @code{!$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}. This also arranges for automatic linking of
the GNU Offloading and Multi Processing Runtime Library
@ref{Top,,libgomp,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}.
@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 will create an anonymous component which 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 will consist
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 ommitted, 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
non-standard @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 will retain their values across multiple calls to a function.
Entities marked @code{AUTOMATIC} will be 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 Extended math intrinsics
@subsection Extended math intrinsics
@cindex intrinsics, math
@cindex intrinsics, trigonometric functions
GNU Fortran supports an extended list of mathematical intrinsics with the
compile flag @option{-fdec-math} for compatability with legacy code.
These intrinsics are described fully in @ref{Intrinsic Procedures} where it is
noted that they are extensions and should be avoided whenever possible.
Specifically, @option{-fdec-math} enables the @ref{COTAN} intrinsic, and
trigonometric intrinsics which accept or produce values in degrees instead of
radians. Here is a summary of the new intrinsics:
@multitable @columnfractions .5 .5
@headitem Radians @tab Degrees
@item @code{@ref{ACOS}} @tab @code{@ref{ACOSD}}*
@item @code{@ref{ASIN}} @tab @code{@ref{ASIND}}*
@item @code{@ref{ATAN}} @tab @code{@ref{ATAND}}*
@item @code{@ref{ATAN2}} @tab @code{@ref{ATAN2D}}*
@item @code{@ref{COS}} @tab @code{@ref{COSD}}*
@item @code{@ref{COTAN}}* @tab @code{@ref{COTAND}}*
@item @code{@ref{SIN}} @tab @code{@ref{SIND}}*
@item @code{@ref{TAN}} @tab @code{@ref{TAND}}*
@end multitable
* Enabled with @option{-fdec-math}.
For advanced users, it may be important to know the implementation of these
functions. They are simply wrappers around the standard radian functions, which
have more accurate builtin versions. These functions convert their arguments
(or results) to degrees (or radians) by taking the value modulus 360 (or 2*pi)
and then multiplying it by a constant radian-to-degree (or degree-to-radian)
factor, as appropriate. The factor is computed at compile-time as 180/pi (or
pi/180).
@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 TYPE as an alias for PRINT
@cindex type alias print
For compatibility, GNU Fortran will interpret @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 .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 will
replace 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 will occur if at least one operand is of integral
type. As a result, a logical operand will be 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 .25 .25 .5
@headitem Operator @tab Intrinsic @tab Bitwise operation
@item @code{.NOT.} @tab @code{@ref{NOT}} @tab complement
@item @code{.AND.} @tab @code{@ref{IAND}} @tab intersection
@item @code{.OR.} @tab @code{@ref{IOR}} @tab union
@item @code{.NEQV.} @tab @code{@ref{IEOR}} @tab exclusive or
@item @code{.EQV.} @tab @code{@ref{NOT}(@ref{IEOR})} @tab complement of exclusive or
@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{CARRAIGECONTROL} 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
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 PARAMETER statements
@cindex PARAMETER
For compatibility, GNU Fortran supports legacy 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 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 which 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 non-standard, 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::
@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
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 (which 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 GNU Fortran Compiler Directives
@section GNU Fortran Compiler Directives
@menu
* ATTRIBUTES directive::
* UNROLL directive::
* BUILTIN directive::
* IVDEP directive::
* VECTOR directive::
* NOVECTOR directive::
@end menu
@node ATTRIBUTES directive
@subsection 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
which 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}.
@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 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.
@node BUILTIN directive
@subsection BUILTIN directive
The syntax of the directive is
@code{!GCC$ BUILTIN (B) attributes simd FLAGS IF('target')}
You can use this directive to define which middle-end built-ins provide vector
implementations. @code{B} is name of the middle-end built-in. @code{FLAGS}
are optional and must be either "(inbranch)" or "(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 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 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.
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 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.
@node NOVECTOR directive
@subsection 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.
@node Non-Fortran Main Program
@section Non-Fortran 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 which 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{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{Syntax}:
@code{void _gfortran_set_args (int argc, char *argv[])}
@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{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{Syntax}:
@code{void _gfortran_set_options (int num, int options[])}
@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_F2008_TS} (512),
@code{GFC_STD_F2018} (1024), @code{GFC_STD_F2018_OBS} (2048), and
@code{GFC_STD=F2018_DEL} (4096). Default: @code{GFC_STD_F95_OBS |
GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003 | GFC_STD_F2008 |
GFC_STD_F2008_TS | GFC_STD_F2008_OBS | GFC_STD_F77 | GFC_STD_F2018 |
GFC_STD_F2018_OBS | GFC_STD_F2018_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{Description}:
@code{_gfortran_set_convert} set the representation of data for
unformatted files.
@item @emph{Syntax}:
@code{void _gfortran_set_convert (int conv)}
@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{Description}:
@code{_gfortran_set_record_marker} sets the length of record markers
for unformatted files.
@item @emph{Syntax}:
@code{void _gfortran_set_record_marker (int val)}
@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{Description}:
@code{_gfortran_set_fpe} enables floating point exception traps for
the specified exceptions. On most systems, this will result in a
SIGFPE signal being sent and the program being aborted.
@item @emph{Syntax}:
@code{void _gfortran_set_fpe (int val)}
@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{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{Syntax}:
@code{void _gfortran_set_max_subrecord_length (int val)}
@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
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 lower-cased 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 lower-cased 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 lower-cased
module name, @code{_MOD_}, and the lower-cased Fortran name. Note that
no underscore is appended.
@node Argument passing conventions
@subsection 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
interoperablility.
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. 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 type
@code{INTEGER}, @code{LOGICAL}, @code{REAL} and @code{COMPLEX}
which 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 which 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 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
@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}
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}
Indicates which kind of coarray variable should be registered.
@verbatim
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 verbatim
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 setup previously by the REGISTER_ONLY. This allows to have
allocatable components un-allocated on some images. The status 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}
@verbatim
typedef enum caf_deregister_t {
CAF_DEREGTYPE_COARRAY_DEREGISTER,
CAF_DEREGTYPE_COARRAY_DEALLOCATE_ONLY
}
caf_deregister_t;
@end verbatim
Allows to specifiy 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}
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}.
@verbatim
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 verbatim
@verbatim
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 verbatim
@verbatim
/* 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 unsigend 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 verbatim
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 descriptor-less 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
descriptor-less 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
@menu
* _gfortran_caf_init:: Initialiation 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_is_present:: Query whether an allocatable or pointer component in a derived type coarray is allocated
* _gfortran_caf_send:: Sending data from a local image to a remote image
* _gfortran_caf_get:: Getting data from a remote image
* _gfortran_caf_sendget:: Sending data between remote images
* _gfortran_caf_send_by_ref:: Sending data from a local image to a remote image using enhanced references
* _gfortran_caf_get_by_ref:: Getting data from a remote image using enhanced references
* _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
@end menu
@node _gfortran_caf_init
@subsection @code{_gfortran_caf_init} --- Initialiation function
@cindex Coarray, _gfortran_caf_init
@table @asis
@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{Syntax}:
@code{void _gfortran_caf_init (int *argc, char ***argv)}
@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{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{Syntax}:
@code{void _gfortran_caf_finish (void)}
@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{Description}:
This function returns the current image number, which is a positive number.
@item @emph{Syntax}:
@code{int _gfortran_caf_this_image (int distance)}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{distance} @tab As specified for the @code{this_image} intrinsic
in TS18508. Shall be a non-negative number.
@end multitable
@item @emph{NOTES}
If the Fortran intrinsic @code{this_image} is invoked without an argument, which
is the only permitted form in Fortran 2008, GCC passes @code{0} as
first argument.
@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{Description}:
This function returns the number of images in the current team, if
@var{distance} is 0 or the number of images in the parent team at the specified
distance. If failed is -1, the function returns the number of all images at
the specified distance; if it is 0, the function returns the number of
nonfailed images, and if it is 1, it returns the number of failed images.
@item @emph{Syntax}:
@code{int _gfortran_caf_num_images(int distance, int failed)}
@item @emph{Arguments}:
@multitable @columnfractions .15 .70
@item @var{distance} @tab the distance from this image to the ancestor.
Shall be positive.
@item @var{failed} @tab shall be -1, 0, or 1
@end multitable
@item @emph{NOTES}
This function follows TS18508. If the num_image intrinsic has no arguments,
then the compiler passes @code{distance=0} and @code{failed=-1} to the function.
@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{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{Syntax}:
@code{int _gfortran_caf_image_status (int image, caf_team_t * team)}
@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{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{Syntax}:
@code{int _gfortran_caf_failed_images (caf_team_t * team, int * kind)}
@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{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{Syntax}:
@code{int _gfortran_caf_stopped_images (caf_team_t * team, int * kind)}
@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{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{Syntax}:
@code{void 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{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 will be 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,