1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999-2013
6 @include gcc-common.texi
8 @settitle The GNU Fortran Compiler
10 @c Create a separate index for command line options
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60 @c Use with @@smallbook.
62 @c %** start of document
64 @c Cause even numbered pages to be printed on the left hand side of
65 @c the page and odd numbered pages to be printed on the right hand
66 @c side of the page. Using this, you can print on both sides of a
67 @c sheet of paper and have the text on the same part of the sheet.
69 @c The text on right hand pages is pushed towards the right hand
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80 Copyright @copyright{} @value{copyrights-gfortran} Free Software Foundation, Inc.
82 Permission is granted to copy, distribute and/or modify this document
83 under the terms of the GNU Free Documentation License, Version 1.3 or
84 any later version published by the Free Software Foundation; with the
85 Invariant Sections being ``Funding Free Software'', the Front-Cover
86 Texts being (a) (see below), and with the Back-Cover Texts being (b)
87 (see below). A copy of the license is included in the section entitled
88 ``GNU Free Documentation License''.
90 (a) The FSF's Front-Cover Text is:
94 (b) The FSF's Back-Cover Text is:
96 You have freedom to copy and modify this GNU Manual, like GNU
97 software. Copies published by the Free Software Foundation raise
98 funds for GNU development.
102 @dircategory Software development
104 * gfortran: (gfortran). The GNU Fortran Compiler.
106 This file documents the use and the internals of
107 the GNU Fortran compiler, (@command{gfortran}).
109 Published by the Free Software Foundation
110 51 Franklin Street, Fifth Floor
111 Boston, MA 02110-1301 USA
117 @setchapternewpage odd
119 @title Using GNU Fortran
121 @author The @t{gfortran} team
123 @vskip 0pt plus 1filll
124 Published by the Free Software Foundation@*
125 51 Franklin Street, Fifth Floor@*
126 Boston, MA 02110-1301, USA@*
127 @c Last printed ??ber, 19??.@*
128 @c Printed copies are available for $? each.@*
134 @c TODO: The following "Part" definitions are included here temporarily
135 @c until they are incorporated into the official Texinfo distribution.
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151 @c ---------------------------------------------------------------------
152 @c TexInfo table of contents.
153 @c ---------------------------------------------------------------------
160 This manual documents the use of @command{gfortran},
161 the GNU Fortran compiler. You can find in this manual how to invoke
162 @command{gfortran}, as well as its features and incompatibilities.
165 @emph{Warning:} This document, and the compiler it describes, are still
166 under development. While efforts are made to keep it up-to-date, it might
167 not accurately reflect the status of the most recent GNU Fortran compiler.
171 @comment When you add a new menu item, please keep the right hand
172 @comment aligned to the same column. Do not use tabs. This provides
173 @comment better formatting.
178 Part I: Invoking GNU Fortran
179 * Invoking GNU Fortran:: Command options supported by @command{gfortran}.
180 * Runtime:: Influencing runtime behavior with environment variables.
182 Part II: Language Reference
183 * Fortran 2003 and 2008 status:: Fortran 2003 and 2008 features supported by GNU Fortran.
184 * Compiler Characteristics:: User-visible implementation details.
185 * Mixed-Language Programming:: Interoperability with C
186 * Extensions:: Language extensions implemented by GNU Fortran.
187 * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
188 * Intrinsic Modules:: Intrinsic modules supported by GNU Fortran.
190 * Contributing:: How you can help.
191 * Copying:: GNU General Public License says
192 how you can copy and share GNU Fortran.
193 * GNU Free Documentation License::
194 How you can copy and share this manual.
195 * Funding:: How to help assure continued work for free software.
196 * Option Index:: Index of command line options
197 * Keyword Index:: Index of concepts
201 @c ---------------------------------------------------------------------
203 @c ---------------------------------------------------------------------
206 @chapter Introduction
208 @c The following duplicates the text on the TexInfo table of contents.
210 This manual documents the use of @command{gfortran}, the GNU Fortran
211 compiler. You can find in this manual how to invoke @command{gfortran},
212 as well as its features and incompatibilities.
215 @emph{Warning:} This document, and the compiler it describes, are still
216 under development. While efforts are made to keep it up-to-date, it
217 might not accurately reflect the status of the most recent GNU Fortran
222 The GNU Fortran compiler front end was
223 designed initially as a free replacement for,
224 or alternative to, the Unix @command{f95} command;
225 @command{gfortran} is the command you will use to invoke the compiler.
228 * About GNU Fortran:: What you should know about the GNU Fortran compiler.
229 * GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
230 * Preprocessing and conditional compilation:: The Fortran preprocessor
231 * GNU Fortran and G77:: Why we chose to start from scratch.
232 * Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
233 * Standards:: Standards supported by GNU Fortran.
237 @c ---------------------------------------------------------------------
239 @c ---------------------------------------------------------------------
241 @node About GNU Fortran
242 @section About GNU Fortran
244 The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards
245 completely, parts of the Fortran 2003 and Fortran 2008 standards, and
246 several vendor extensions. The development goal is to provide the
251 Read a user's program,
252 stored in a file and containing instructions written
253 in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008.
254 This file contains @dfn{source code}.
257 Translate the user's program into instructions a computer
258 can carry out more quickly than it takes to translate the
259 instructions in the first
260 place. The result after compilation of a program is
262 code designed to be efficiently translated and processed
263 by a machine such as your computer.
264 Humans usually are not as good writing machine code
265 as they are at writing Fortran (or C++, Ada, or Java),
266 because it is easy to make tiny mistakes writing machine code.
269 Provide the user with information about the reasons why
270 the compiler is unable to create a binary from the source code.
271 Usually this will be the case if the source code is flawed.
272 The Fortran 90 standard requires that the compiler can point out
273 mistakes to the user.
274 An incorrect usage of the language causes an @dfn{error message}.
276 The compiler will also attempt to diagnose cases where the
277 user's program contains a correct usage of the language,
278 but instructs the computer to do something questionable.
279 This kind of diagnostics message is called a @dfn{warning message}.
282 Provide optional information about the translation passes
283 from the source code to machine code.
284 This can help a user of the compiler to find the cause of
285 certain bugs which may not be obvious in the source code,
286 but may be more easily found at a lower level compiler output.
287 It also helps developers to find bugs in the compiler itself.
290 Provide information in the generated machine code that can
291 make it easier to find bugs in the program (using a debugging tool,
292 called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
295 Locate and gather machine code already generated to
296 perform actions requested by statements in the user's program.
297 This machine code is organized into @dfn{modules} and is located
298 and @dfn{linked} to the user program.
301 The GNU Fortran compiler consists of several components:
305 A version of the @command{gcc} command
306 (which also might be installed as the system's @command{cc} command)
307 that also understands and accepts Fortran source code.
308 The @command{gcc} command is the @dfn{driver} program for
309 all the languages in the GNU Compiler Collection (GCC);
311 you can compile the source code of any language for
312 which a front end is available in GCC.
315 The @command{gfortran} command itself,
316 which also might be installed as the
317 system's @command{f95} command.
318 @command{gfortran} is just another driver program,
319 but specifically for the Fortran compiler only.
320 The difference with @command{gcc} is that @command{gfortran}
321 will automatically link the correct libraries to your program.
324 A collection of run-time libraries.
325 These libraries contain the machine code needed to support
326 capabilities of the Fortran language that are not directly
327 provided by the machine code generated by the
328 @command{gfortran} compilation phase,
329 such as intrinsic functions and subroutines,
330 and routines for interaction with files and the operating system.
331 @c and mechanisms to spawn,
332 @c unleash and pause threads in parallelized code.
335 The Fortran compiler itself, (@command{f951}).
336 This is the GNU Fortran parser and code generator,
337 linked to and interfaced with the GCC backend library.
338 @command{f951} ``translates'' the source code to
339 assembler code. You would typically not use this
341 instead, the @command{gcc} or @command{gfortran} driver
342 programs will call it for you.
346 @c ---------------------------------------------------------------------
347 @c GNU Fortran and GCC
348 @c ---------------------------------------------------------------------
350 @node GNU Fortran and GCC
351 @section GNU Fortran and GCC
352 @cindex GNU Compiler Collection
355 GNU Fortran is a part of GCC, the @dfn{GNU Compiler Collection}. GCC
356 consists of a collection of front ends for various languages, which
357 translate the source code into a language-independent form called
358 @dfn{GENERIC}. This is then processed by a common middle end which
359 provides optimization, and then passed to one of a collection of back
360 ends which generate code for different computer architectures and
363 Functionally, this is implemented with a driver program (@command{gcc})
364 which provides the command-line interface for the compiler. It calls
365 the relevant compiler front-end program (e.g., @command{f951} for
366 Fortran) for each file in the source code, and then calls the assembler
367 and linker as appropriate to produce the compiled output. In a copy of
368 GCC which has been compiled with Fortran language support enabled,
369 @command{gcc} will recognize files with @file{.f}, @file{.for}, @file{.ftn},
370 @file{.f90}, @file{.f95}, @file{.f03} and @file{.f08} extensions as
371 Fortran source code, and compile it accordingly. A @command{gfortran}
372 driver program is also provided, which is identical to @command{gcc}
373 except that it automatically links the Fortran runtime libraries into the
376 Source files with @file{.f}, @file{.for}, @file{.fpp}, @file{.ftn}, @file{.F},
377 @file{.FOR}, @file{.FPP}, and @file{.FTN} extensions are treated as fixed form.
378 Source files with @file{.f90}, @file{.f95}, @file{.f03}, @file{.f08},
379 @file{.F90}, @file{.F95}, @file{.F03} and @file{.F08} extensions are
380 treated as free form. The capitalized versions of either form are run
381 through preprocessing. Source files with the lower case @file{.fpp}
382 extension are also run through preprocessing.
384 This manual specifically documents the Fortran front end, which handles
385 the programming language's syntax and semantics. The aspects of GCC
386 which relate to the optimization passes and the back-end code generation
387 are documented in the GCC manual; see
388 @ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
389 The two manuals together provide a complete reference for the GNU
393 @c ---------------------------------------------------------------------
394 @c Preprocessing and conditional compilation
395 @c ---------------------------------------------------------------------
397 @node Preprocessing and conditional compilation
398 @section Preprocessing and conditional compilation
401 @cindex Conditional compilation
402 @cindex Preprocessing
403 @cindex preprocessor, include file handling
405 Many Fortran compilers including GNU Fortran allow passing the source code
406 through a C preprocessor (CPP; sometimes also called the Fortran preprocessor,
407 FPP) to allow for conditional compilation. In the case of GNU Fortran,
408 this is the GNU C Preprocessor in the traditional mode. On systems with
409 case-preserving file names, the preprocessor is automatically invoked if the
410 filename extension is @file{.F}, @file{.FOR}, @file{.FTN}, @file{.fpp},
411 @file{.FPP}, @file{.F90}, @file{.F95}, @file{.F03} or @file{.F08}. To manually
412 invoke the preprocessor on any file, use @option{-cpp}, to disable
413 preprocessing on files where the preprocessor is run automatically, use
416 If a preprocessed file includes another file with the Fortran @code{INCLUDE}
417 statement, the included file is not preprocessed. To preprocess included
418 files, use the equivalent preprocessor statement @code{#include}.
420 If GNU Fortran invokes the preprocessor, @code{__GFORTRAN__}
421 is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
422 @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
423 compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details.
425 While CPP is the de-facto standard for preprocessing Fortran code,
426 Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines
427 Conditional Compilation, which is not widely used and not directly
428 supported by the GNU Fortran compiler. You can use the program coco
429 to preprocess such files (@uref{http://www.daniellnagle.com/coco.html}).
432 @c ---------------------------------------------------------------------
433 @c GNU Fortran and G77
434 @c ---------------------------------------------------------------------
436 @node GNU Fortran and G77
437 @section GNU Fortran and G77
439 @cindex @command{g77}
441 The GNU Fortran compiler is the successor to @command{g77}, the Fortran
442 77 front end included in GCC prior to version 4. It is an entirely new
443 program that has been designed to provide Fortran 95 support and
444 extensibility for future Fortran language standards, as well as providing
445 backwards compatibility for Fortran 77 and nearly all of the GNU language
446 extensions supported by @command{g77}.
449 @c ---------------------------------------------------------------------
451 @c ---------------------------------------------------------------------
454 @section Project Status
457 As soon as @command{gfortran} can parse all of the statements correctly,
458 it will be in the ``larva'' state.
459 When we generate code, the ``puppa'' state.
460 When @command{gfortran} is done,
461 we'll see if it will be a beautiful butterfly,
462 or just a big bug....
464 --Andy Vaught, April 2000
467 The start of the GNU Fortran 95 project was announced on
468 the GCC homepage in March 18, 2000
469 (even though Andy had already been working on it for a while,
472 The GNU Fortran compiler is able to compile nearly all
473 standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs,
474 including a number of standard and non-standard extensions, and can be
475 used on real-world programs. In particular, the supported extensions
476 include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran
477 2008 features, including TR 15581. However, it is still under
478 development and has a few remaining rough edges.
480 At present, the GNU Fortran compiler passes the
481 @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
482 NIST Fortran 77 Test Suite}, and produces acceptable results on the
483 @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}.
484 It also provides respectable performance on
485 the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran
486 compiler benchmarks} and the
487 @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html,
488 Livermore Fortran Kernels test}. It has been used to compile a number of
489 large real-world programs, including
490 @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM
491 weather-forecasting code} and
492 @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum
493 chemistry package}; see @url{http://gcc.gnu.org/@/wiki/@/GfortranApps} for an
496 Among other things, the GNU Fortran compiler is intended as a replacement
497 for G77. At this point, nearly all programs that could be compiled with
498 G77 can be compiled with GNU Fortran, although there are a few minor known
501 The primary work remaining to be done on GNU Fortran falls into three
502 categories: bug fixing (primarily regarding the treatment of invalid code
503 and providing useful error messages), improving the compiler optimizations
504 and the performance of compiled code, and extending the compiler to support
505 future standards---in particular, Fortran 2003 and Fortran 2008.
508 @c ---------------------------------------------------------------------
510 @c ---------------------------------------------------------------------
517 * Varying Length Character Strings::
520 The GNU Fortran compiler implements
521 ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
522 standard-compliant Fortran 90 and Fortran 77 programs. It also supports
523 the ISO/IEC TR-15581 enhancements to allocatable arrays.
525 GNU Fortran also have a partial support for ISO/IEC 1539-1:2004 (Fortran
526 2003), ISO/IEC 1539-1:2010 (Fortran 2008), the Technical Specification
527 @code{Further Interoperability of Fortran with C} (ISO/IEC TS 29113:2012).
528 Full support of those standards and future Fortran standards is planned.
529 The current status of the support is can be found in the
530 @ref{Fortran 2003 status}, @ref{Fortran 2008 status} and
531 @ref{TS 29113 status} sections of the documentation.
533 Additionally, the GNU Fortran compilers supports the OpenMP specification
534 (version 3.1, @url{http://openmp.org/@/wp/@/openmp-specifications/}).
536 @node Varying Length Character Strings
537 @subsection Varying Length Character Strings
538 @cindex Varying length character strings
539 @cindex Varying length strings
540 @cindex strings, varying length
542 The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000)
543 varying length character strings. While GNU Fortran currently does not
544 support such strings directly, there exist two Fortran implementations
545 for them, which work with GNU Fortran. They can be found at
546 @uref{http://www.fortran.com/@/iso_varying_string.f95} and at
547 @uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/ISO_VARYING_STRING/}.
549 Deferred-length character strings of Fortran 2003 supports part of
550 the features of @code{ISO_VARYING_STRING} and should be considered as
551 replacement. (Namely, allocatable or pointers of the type
552 @code{character(len=:)}.)
555 @c =====================================================================
556 @c PART I: INVOCATION REFERENCE
557 @c =====================================================================
560 \part{I}{Invoking GNU Fortran}
563 @c ---------------------------------------------------------------------
565 @c ---------------------------------------------------------------------
570 @c ---------------------------------------------------------------------
572 @c ---------------------------------------------------------------------
575 @chapter Runtime: Influencing runtime behavior with environment variables
576 @cindex environment variable
578 The behavior of the @command{gfortran} can be influenced by
579 environment variables.
581 Malformed environment variables are silently ignored.
584 * TMPDIR:: Directory for scratch files
585 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
586 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
587 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
588 * GFORTRAN_UNBUFFERED_ALL:: Do not buffer I/O for all units.
589 * GFORTRAN_UNBUFFERED_PRECONNECTED:: Do not buffer I/O for preconnected units.
590 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
591 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
592 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
593 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
594 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
595 * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
599 @section @env{TMPDIR}---Directory for scratch files
601 When opening a file with @code{STATUS='SCRATCH'}, GNU Fortran tries to
602 create the file in one of the potential directories by testing each
603 directory in the order below.
607 The environment variable @env{TMPDIR}, if it exists.
610 On the MinGW target, the directory returned by the @code{GetTempPath}
611 function. Alternatively, on the Cygwin target, the @env{TMP} and
612 @env{TEMP} environment variables, if they exist, in that order.
615 The @code{P_tmpdir} macro if it is defined, otherwise the directory
619 @node GFORTRAN_STDIN_UNIT
620 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
622 This environment variable can be used to select the unit number
623 preconnected to standard input. This must be a positive integer.
624 The default value is 5.
626 @node GFORTRAN_STDOUT_UNIT
627 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
629 This environment variable can be used to select the unit number
630 preconnected to standard output. This must be a positive integer.
631 The default value is 6.
633 @node GFORTRAN_STDERR_UNIT
634 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
636 This environment variable can be used to select the unit number
637 preconnected to standard error. This must be a positive integer.
638 The default value is 0.
640 @node GFORTRAN_UNBUFFERED_ALL
641 @section @env{GFORTRAN_UNBUFFERED_ALL}---Do not buffer I/O on all units
643 This environment variable controls whether all I/O is unbuffered. If
644 the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
645 unbuffered. This will slow down small sequential reads and writes. If
646 the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
649 @node GFORTRAN_UNBUFFERED_PRECONNECTED
650 @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Do not buffer I/O on preconnected units
652 The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
653 whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If
654 the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This
655 will slow down small sequential reads and writes. If the first letter
656 is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default.
658 @node GFORTRAN_SHOW_LOCUS
659 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
661 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
662 line numbers for runtime errors are printed. If the first letter is
663 @samp{n}, @samp{N} or @samp{0}, do not print filename and line numbers
664 for runtime errors. The default is to print the location.
666 @node GFORTRAN_OPTIONAL_PLUS
667 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
669 If the first letter is @samp{y}, @samp{Y} or @samp{1},
670 a plus sign is printed
671 where permitted by the Fortran standard. If the first letter
672 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
673 in most cases. Default is not to print plus signs.
675 @node GFORTRAN_DEFAULT_RECL
676 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
678 This environment variable specifies the default record length, in
679 bytes, for files which are opened without a @code{RECL} tag in the
680 @code{OPEN} statement. This must be a positive integer. The
681 default value is 1073741824 bytes (1 GB).
683 @node GFORTRAN_LIST_SEPARATOR
684 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
686 This environment variable specifies the separator when writing
687 list-directed output. It may contain any number of spaces and
688 at most one comma. If you specify this on the command line,
689 be sure to quote spaces, as in
691 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
693 when @command{a.out} is the compiled Fortran program that you want to run.
694 Default is a single space.
696 @node GFORTRAN_CONVERT_UNIT
697 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
699 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
700 to change the representation of data for unformatted files.
701 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
703 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
704 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
705 exception: mode ':' unit_list | unit_list ;
706 unit_list: unit_spec | unit_list unit_spec ;
707 unit_spec: INTEGER | INTEGER '-' INTEGER ;
709 The variable consists of an optional default mode, followed by
710 a list of optional exceptions, which are separated by semicolons
711 from the preceding default and each other. Each exception consists
712 of a format and a comma-separated list of units. Valid values for
713 the modes are the same as for the @code{CONVERT} specifier:
716 @item @code{NATIVE} Use the native format. This is the default.
717 @item @code{SWAP} Swap between little- and big-endian.
718 @item @code{LITTLE_ENDIAN} Use the little-endian format
719 for unformatted files.
720 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
722 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
723 Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
725 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
726 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
727 in little_endian mode, except for units 10 to 20 and 25, which are in
729 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
732 Setting the environment variables should be done on the command
733 line or via the @command{export}
734 command for @command{sh}-compatible shells and via @command{setenv}
735 for @command{csh}-compatible shells.
737 Example for @command{sh}:
740 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
743 Example code for @command{csh}:
746 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
750 Using anything but the native representation for unformatted data
751 carries a significant speed overhead. If speed in this area matters
752 to you, it is best if you use this only for data that needs to be
755 @xref{CONVERT specifier}, for an alternative way to specify the
756 data representation for unformatted files. @xref{Runtime Options}, for
757 setting a default data representation for the whole program. The
758 @code{CONVERT} specifier overrides the @option{-fconvert} compile options.
760 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
761 environment variable will override the CONVERT specifier in the
762 open statement}. This is to give control over data formats to
763 users who do not have the source code of their program available.
765 @node GFORTRAN_ERROR_BACKTRACE
766 @section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors
768 If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to @samp{y},
769 @samp{Y} or @samp{1} (only the first letter is relevant) then a
770 backtrace is printed when a serious run-time error occurs. To disable
771 the backtracing, set the variable to @samp{n}, @samp{N}, @samp{0}.
772 Default is to print a backtrace unless the @option{-fno-backtrace}
773 compile option was used.
775 @c =====================================================================
776 @c PART II: LANGUAGE REFERENCE
777 @c =====================================================================
780 \part{II}{Language Reference}
783 @c ---------------------------------------------------------------------
784 @c Fortran 2003 and 2008 Status
785 @c ---------------------------------------------------------------------
787 @node Fortran 2003 and 2008 status
788 @chapter Fortran 2003 and 2008 Status
791 * Fortran 2003 status::
792 * Fortran 2008 status::
796 @node Fortran 2003 status
797 @section Fortran 2003 status
799 GNU Fortran supports several Fortran 2003 features; an incomplete
800 list can be found below. See also the
801 @uref{http://gcc.gnu.org/wiki/Fortran2003, wiki page} about Fortran 2003.
804 @item Procedure pointers including procedure-pointer components with
805 @code{PASS} attribute.
807 @item Procedures which are bound to a derived type (type-bound procedures)
808 including @code{PASS}, @code{PROCEDURE} and @code{GENERIC}, and
809 operators bound to a type.
811 @item Abstract interfaces and type extension with the possibility to
812 override type-bound procedures or to have deferred binding.
814 @item Polymorphic entities (``@code{CLASS}'') for derived types -- including
815 @code{SAME_TYPE_AS}, @code{EXTENDS_TYPE_OF} and @code{SELECT TYPE} for
816 scalars and arrays, including unlimited polymorphism.
818 @item Generic interface names, which have the same name as derived types,
819 are now supported. This allows one to write constructor functions. Note
820 that Fortran does not support static constructor functions. For static
821 variables, only default initialization or structure-constructor
822 initialization are available.
824 @item The @code{ASSOCIATE} construct.
826 @item Interoperability with C including enumerations,
828 @item In structure constructors the components with default values may be
831 @item Extensions to the @code{ALLOCATE} statement, allowing for a
832 type-specification with type parameter and for allocation and initialization
833 from a @code{SOURCE=} expression; @code{ALLOCATE} and @code{DEALLOCATE}
834 optionally return an error message string via @code{ERRMSG=}.
836 @item Reallocation on assignment: If an intrinsic assignment is
837 used, an allocatable variable on the left-hand side is automatically allocated
838 (if unallocated) or reallocated (if the shape is different). Currently, scalar
839 deferred character length left-hand sides are correctly handled but arrays
840 are not yet fully implemented.
842 @item Transferring of allocations via @code{MOVE_ALLOC}.
844 @item The @code{PRIVATE} and @code{PUBLIC} attributes may be given individually
845 to derived-type components.
847 @item In pointer assignments, the lower bound may be specified and
848 the remapping of elements is supported.
850 @item For pointers an @code{INTENT} may be specified which affect the
851 association status not the value of the pointer target.
853 @item Intrinsics @code{command_argument_count}, @code{get_command},
854 @code{get_command_argument}, and @code{get_environment_variable}.
856 @item Support for Unicode characters (ISO 10646) and UTF-8, including
857 the @code{SELECTED_CHAR_KIND} and @code{NEW_LINE} intrinsic functions.
859 @item Support for binary, octal and hexadecimal (BOZ) constants in the
860 intrinsic functions @code{INT}, @code{REAL}, @code{CMPLX} and @code{DBLE}.
862 @item Support for namelist variables with allocatable and pointer
863 attribute and nonconstant length type parameter.
866 @cindex array, constructors
868 Array constructors using square brackets. That is, @code{[...]} rather
869 than @code{(/.../)}. Type-specification for array constructors like
870 @code{(/ some-type :: ... /)}.
872 @item Extensions to the specification and initialization expressions,
873 including the support for intrinsics with real and complex arguments.
875 @item Support for the asynchronous input/output syntax; however, the
876 data transfer is currently always synchronously performed.
879 @cindex @code{FLUSH} statement
880 @cindex statement, @code{FLUSH}
881 @code{FLUSH} statement.
884 @cindex @code{IOMSG=} specifier
885 @code{IOMSG=} specifier for I/O statements.
888 @cindex @code{ENUM} statement
889 @cindex @code{ENUMERATOR} statement
890 @cindex statement, @code{ENUM}
891 @cindex statement, @code{ENUMERATOR}
892 @opindex @code{fshort-enums}
893 Support for the declaration of enumeration constants via the
894 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
895 @command{gcc} is guaranteed also for the case where the
896 @command{-fshort-enums} command line option is given.
903 @cindex @code{ALLOCATABLE} dummy arguments
904 @code{ALLOCATABLE} dummy arguments.
906 @cindex @code{ALLOCATABLE} function results
907 @code{ALLOCATABLE} function results
909 @cindex @code{ALLOCATABLE} components of derived types
910 @code{ALLOCATABLE} components of derived types
914 @cindex @code{STREAM} I/O
915 @cindex @code{ACCESS='STREAM'} I/O
916 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
917 allowing I/O without any record structure.
920 Namelist input/output for internal files.
922 @item Further I/O extensions: Rounding during formatted output, using of
923 a decimal comma instead of a decimal point, setting whether a plus sign
924 should appear for positive numbers.
927 @cindex @code{PROTECTED} statement
928 @cindex statement, @code{PROTECTED}
929 The @code{PROTECTED} statement and attribute.
932 @cindex @code{VALUE} statement
933 @cindex statement, @code{VALUE}
934 The @code{VALUE} statement and attribute.
937 @cindex @code{VOLATILE} statement
938 @cindex statement, @code{VOLATILE}
939 The @code{VOLATILE} statement and attribute.
942 @cindex @code{IMPORT} statement
943 @cindex statement, @code{IMPORT}
944 The @code{IMPORT} statement, allowing to import
945 host-associated derived types.
947 @item The intrinsic modules @code{ISO_FORTRAN_ENVIRONMENT} is supported,
948 which contains parameters of the I/O units, storage sizes. Additionally,
949 procedures for C interoperability are available in the @code{ISO_C_BINDING}
953 @cindex @code{USE, INTRINSIC} statement
954 @cindex statement, @code{USE, INTRINSIC}
955 @cindex @code{ISO_FORTRAN_ENV} statement
956 @cindex statement, @code{ISO_FORTRAN_ENV}
957 @code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
958 attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
959 @code{ISO_C_BINDING}, @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
962 Renaming of operators in the @code{USE} statement.
967 @node Fortran 2008 status
968 @section Fortran 2008 status
970 The latest version of the Fortran standard is ISO/IEC 1539-1:2010, informally
971 known as Fortran 2008. The official version is available from International
972 Organization for Standardization (ISO) or its national member organizations.
973 The the final draft (FDIS) can be downloaded free of charge from
974 @url{http://www.nag.co.uk/@/sc22wg5/@/links.html}. Fortran is developed by the
975 Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the
976 International Organization for Standardization and the International
977 Electrotechnical Commission (IEC). This group is known as
978 @uref{http://www.nag.co.uk/sc22wg5/, WG5}.
980 The GNU Fortran compiler supports several of the new features of Fortran 2008;
981 the @uref{http://gcc.gnu.org/wiki/Fortran2008Status, wiki} has some information
982 about the current Fortran 2008 implementation status. In particular, the
983 following is implemented.
986 @item The @option{-std=f2008} option and support for the file extensions
987 @file{.f08} and @file{.F08}.
989 @item The @code{OPEN} statement now supports the @code{NEWUNIT=} option,
990 which returns a unique file unit, thus preventing inadvertent use of the
991 same unit in different parts of the program.
993 @item The @code{g0} format descriptor and unlimited format items.
995 @item The mathematical intrinsics @code{ASINH}, @code{ACOSH}, @code{ATANH},
996 @code{ERF}, @code{ERFC}, @code{GAMMA}, @code{LOG_GAMMA}, @code{BESSEL_J0},
997 @code{BESSEL_J1}, @code{BESSEL_JN}, @code{BESSEL_Y0}, @code{BESSEL_Y1},
998 @code{BESSEL_YN}, @code{HYPOT}, @code{NORM2}, and @code{ERFC_SCALED}.
1000 @item Using complex arguments with @code{TAN}, @code{SINH}, @code{COSH},
1001 @code{TANH}, @code{ASIN}, @code{ACOS}, and @code{ATAN} is now possible;
1002 @code{ATAN}(@var{Y},@var{X}) is now an alias for @code{ATAN2}(@var{Y},@var{X}).
1004 @item Support of the @code{PARITY} intrinsic functions.
1006 @item The following bit intrinsics: @code{LEADZ} and @code{TRAILZ} for
1007 counting the number of leading and trailing zero bits, @code{POPCNT} and
1008 @code{POPPAR} for counting the number of one bits and returning the parity;
1009 @code{BGE}, @code{BGT}, @code{BLE}, and @code{BLT} for bitwise comparisons;
1010 @code{DSHIFTL} and @code{DSHIFTR} for combined left and right shifts,
1011 @code{MASKL} and @code{MASKR} for simple left and right justified masks,
1012 @code{MERGE_BITS} for a bitwise merge using a mask, @code{SHIFTA},
1013 @code{SHIFTL} and @code{SHIFTR} for shift operations, and the
1014 transformational bit intrinsics @code{IALL}, @code{IANY} and @code{IPARITY}.
1016 @item Support of the @code{EXECUTE_COMMAND_LINE} intrinsic subroutine.
1018 @item Support for the @code{STORAGE_SIZE} intrinsic inquiry function.
1020 @item The @code{INT@{8,16,32@}} and @code{REAL@{32,64,128@}} kind type
1021 parameters and the array-valued named constants @code{INTEGER_KINDS},
1022 @code{LOGICAL_KINDS}, @code{REAL_KINDS} and @code{CHARACTER_KINDS} of
1023 the intrinsic module @code{ISO_FORTRAN_ENV}.
1025 @item The module procedures @code{C_SIZEOF} of the intrinsic module
1026 @code{ISO_C_BINDINGS} and @code{COMPILER_VERSION} and @code{COMPILER_OPTIONS}
1027 of @code{ISO_FORTRAN_ENV}.
1029 @item Coarray support for serial programs with @option{-fcoarray=single} flag
1030 and experimental support for multiple images with the @option{-fcoarray=lib}
1033 @item The @code{DO CONCURRENT} construct is supported.
1035 @item The @code{BLOCK} construct is supported.
1037 @item The @code{STOP} and the new @code{ERROR STOP} statements now
1038 support all constant expressions.
1040 @item Support for the @code{CONTIGUOUS} attribute.
1042 @item Support for @code{ALLOCATE} with @code{MOLD}.
1044 @item Support for the @code{IMPURE} attribute for procedures, which
1045 allows for @code{ELEMENTAL} procedures without the restrictions of
1048 @item Null pointers (including @code{NULL()}) and not-allocated variables
1049 can be used as actual argument to optional non-pointer, non-allocatable
1050 dummy arguments, denoting an absent argument.
1052 @item Non-pointer variables with @code{TARGET} attribute can be used as
1053 actual argument to @code{POINTER} dummies with @code{INTENT(IN)}.
1055 @item Pointers including procedure pointers and those in a derived
1056 type (pointer components) can now be initialized by a target instead
1057 of only by @code{NULL}.
1059 @item The @code{EXIT} statement (with construct-name) can be now be
1060 used to leave not only the @code{DO} but also the @code{ASSOCIATE},
1061 @code{BLOCK}, @code{IF}, @code{SELECT CASE} and @code{SELECT TYPE}
1064 @item Internal procedures can now be used as actual argument.
1066 @item Minor features: obsolesce diagnostics for @code{ENTRY} with
1067 @option{-std=f2008}; a line may start with a semicolon; for internal
1068 and module procedures @code{END} can be used instead of
1069 @code{END SUBROUTINE} and @code{END FUNCTION}; @code{SELECTED_REAL_KIND}
1070 now also takes a @code{RADIX} argument; intrinsic types are supported
1071 for @code{TYPE}(@var{intrinsic-type-spec}); multiple type-bound procedures
1072 can be declared in a single @code{PROCEDURE} statement; implied-shape
1073 arrays are supported for named constants (@code{PARAMETER}).
1078 @node TS 29113 status
1079 @section Technical Specification 29113 Status
1081 GNU Fortran supports some of the new features of the Technical
1082 Specification (TS) 29113 on Further Interoperability of Fortran with C.
1083 The @uref{http://gcc.gnu.org/wiki/TS29113Status, wiki} has some information
1084 about the current TS 29113 implementation status. In particular, the
1085 following is implemented.
1087 See also @ref{Further Interoperability of Fortran with C}.
1090 @item The @option{-std=f2008ts} option.
1092 @item The @code{OPTIONAL} attribute is allowed for dummy arguments
1093 of @code{BIND(C) procedures.}
1095 @item The @code{RANK} intrinsic is supported.
1097 @item GNU Fortran's implementation for variables with @code{ASYNCHRONOUS}
1098 attribute is compatible with TS 29113.
1100 @item Assumed types (@code{TYPE(*)}.
1102 @item Assumed-rank (@code{DIMENSION(..)}). However, the array descriptor
1103 of the TS is not yet supported.
1108 @c ---------------------------------------------------------------------
1109 @c Compiler Characteristics
1110 @c ---------------------------------------------------------------------
1112 @node Compiler Characteristics
1113 @chapter Compiler Characteristics
1115 This chapter describes certain characteristics of the GNU Fortran
1116 compiler, that are not specified by the Fortran standard, but which
1117 might in some way or another become visible to the programmer.
1120 * KIND Type Parameters::
1121 * Internal representation of LOGICAL variables::
1122 * Thread-safety of the runtime library::
1123 * Data consistency and durability::
1127 @node KIND Type Parameters
1128 @section KIND Type Parameters
1131 The @code{KIND} type parameters supported by GNU Fortran for the primitive
1137 1, 2, 4, 8*, 16*, default: 4 (1)
1140 1, 2, 4, 8*, 16*, default: 4 (1)
1143 4, 8, 10*, 16*, default: 4 (2)
1146 4, 8, 10*, 16*, default: 4 (2)
1154 * = not available on all systems @*
1155 (1) Unless -fdefault-integer-8 is used @*
1156 (2) Unless -fdefault-real-8 is used
1159 The @code{KIND} value matches the storage size in bytes, except for
1160 @code{COMPLEX} where the storage size is twice as much (or both real and
1161 imaginary part are a real value of the given size). It is recommended to use
1162 the @code{SELECTED_CHAR_KIND}, @code{SELECTED_INT_KIND} and
1163 @code{SELECTED_REAL_KIND} intrinsics or the @code{INT8}, @code{INT16},
1164 @code{INT32}, @code{INT64}, @code{REAL32}, @code{REAL64}, and @code{REAL128}
1165 parameters of the @code{ISO_FORTRAN_ENV} module instead of the concrete values.
1166 The available kind parameters can be found in the constant arrays
1167 @code{CHARACTER_KINDS}, @code{INTEGER_KINDS}, @code{LOGICAL_KINDS} and
1168 @code{REAL_KINDS} in the @code{ISO_FORTRAN_ENV} module
1169 (see @ref{ISO_FORTRAN_ENV}).
1172 @node Internal representation of LOGICAL variables
1173 @section Internal representation of LOGICAL variables
1174 @cindex logical, variable representation
1176 The Fortran standard does not specify how variables of @code{LOGICAL}
1177 type are represented, beyond requiring that @code{LOGICAL} variables
1178 of default kind have the same storage size as default @code{INTEGER}
1179 and @code{REAL} variables. The GNU Fortran internal representation is
1182 A @code{LOGICAL(KIND=N)} variable is represented as an
1183 @code{INTEGER(KIND=N)} variable, however, with only two permissible
1184 values: @code{1} for @code{.TRUE.} and @code{0} for
1185 @code{.FALSE.}. Any other integer value results in undefined behavior.
1187 Note that for mixed-language programming using the
1188 @code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can
1189 be used to create @code{LOGICAL(KIND=C_BOOL)} variables which are
1190 interoperable with the C99 _Bool type. The C99 _Bool type has an
1191 internal representation described in the C99 standard, which is
1192 identical to the above description, i.e. with 1 for true and 0 for
1193 false being the only permissible values. Thus the internal
1194 representation of @code{LOGICAL} variables in GNU Fortran is identical
1195 to C99 _Bool, except for a possible difference in storage size
1196 depending on the kind.
1199 @node Thread-safety of the runtime library
1200 @section Thread-safety of the runtime library
1201 @cindex thread-safety, threads
1203 GNU Fortran can be used in programs with multiple threads, e.g.@: by
1204 using OpenMP, by calling OS thread handling functions via the
1205 @code{ISO_C_BINDING} facility, or by GNU Fortran compiled library code
1206 being called from a multi-threaded program.
1208 The GNU Fortran runtime library, (@code{libgfortran}), supports being
1209 called concurrently from multiple threads with the following
1212 During library initialization, the C @code{getenv} function is used,
1213 which need not be thread-safe. Similarly, the @code{getenv}
1214 function is used to implement the @code{GET_ENVIRONMENT_VARIABLE} and
1215 @code{GETENV} intrinsics. It is the responsibility of the user to
1216 ensure that the environment is not being updated concurrently when any
1217 of these actions are taking place.
1219 The @code{EXECUTE_COMMAND_LINE} and @code{SYSTEM} intrinsics are
1220 implemented with the @code{system} function, which need not be
1221 thread-safe. It is the responsibility of the user to ensure that
1222 @code{system} is not called concurrently.
1224 Finally, for platforms not supporting thread-safe POSIX functions,
1225 further functionality might not be thread-safe. For details, please
1226 consult the documentation for your operating system.
1229 @node Data consistency and durability
1230 @section Data consistency and durability
1231 @cindex consistency, durability
1233 This section contains a brief overview of data and metadata
1234 consistency and durability issues when doing I/O.
1236 With respect to durability, GNU Fortran makes no effort to ensure that
1237 data is committed to stable storage. If this is required, the GNU
1238 Fortran programmer can use the intrinsic @code{FNUM} to retrieve the
1239 low level file descriptor corresponding to an open Fortran unit. Then,
1240 using e.g. the @code{ISO_C_BINDING} feature, one can call the
1241 underlying system call to flush dirty data to stable storage, such as
1242 @code{fsync} on POSIX, @code{_commit} on MingW, or @code{fcntl(fd,
1243 F_FULLSYNC, 0)} on Mac OS X. The following example shows how to call
1247 ! Declare the interface for POSIX fsync function
1249 function fsync (fd) bind(c,name="fsync")
1250 use iso_c_binding, only: c_int
1251 integer(c_int), value :: fd
1252 integer(c_int) :: fsync
1256 ! Variable declaration
1260 open (10,file="foo")
1263 ! Perform I/O on unit 10
1268 ret = fsync(fnum(10))
1270 ! Handle possible error
1271 if (ret /= 0) stop "Error calling FSYNC"
1274 With respect to consistency, for regular files GNU Fortran uses
1275 buffered I/O in order to improve performance. This buffer is flushed
1276 automatically when full and in some other situations, e.g. when
1277 closing a unit. It can also be explicitly flushed with the
1278 @code{FLUSH} statement. Also, the buffering can be turned off with the
1279 @code{GFORTRAN_UNBUFFERED_ALL} and
1280 @code{GFORTRAN_UNBUFFERED_PRECONNECTED} environment variables. Special
1281 files, such as terminals and pipes, are always unbuffered. Sometimes,
1282 however, further things may need to be done in order to allow other
1283 processes to see data that GNU Fortran has written, as follows.
1285 The Windows platform supports a relaxed metadata consistency model,
1286 where file metadata is written to the directory lazily. This means
1287 that, for instance, the @code{dir} command can show a stale size for a
1288 file. One can force a directory metadata update by closing the unit,
1289 or by calling @code{_commit} on the file descriptor. Note, though,
1290 that @code{_commit} will force all dirty data to stable storage, which
1291 is often a very slow operation.
1293 The Network File System (NFS) implements a relaxed consistency model
1294 called open-to-close consistency. Closing a file forces dirty data and
1295 metadata to be flushed to the server, and opening a file forces the
1296 client to contact the server in order to revalidate cached
1297 data. @code{fsync} will also force a flush of dirty data and metadata
1298 to the server. Similar to @code{open} and @code{close}, acquiring and
1299 releasing @code{fcntl} file locks, if the server supports them, will
1300 also force cache validation and flushing dirty data and metadata.
1303 @c ---------------------------------------------------------------------
1305 @c ---------------------------------------------------------------------
1307 @c Maybe this chapter should be merged with the 'Standards' section,
1308 @c whenever that is written :-)
1314 The two sections below detail the extensions to standard Fortran that are
1315 implemented in GNU Fortran, as well as some of the popular or
1316 historically important extensions that are not (or not yet) implemented.
1317 For the latter case, we explain the alternatives available to GNU Fortran
1318 users, including replacement by standard-conforming code or GNU
1322 * Extensions implemented in GNU Fortran::
1323 * Extensions not implemented in GNU Fortran::
1327 @node Extensions implemented in GNU Fortran
1328 @section Extensions implemented in GNU Fortran
1329 @cindex extensions, implemented
1331 GNU Fortran implements a number of extensions over standard
1332 Fortran. This chapter contains information on their syntax and
1333 meaning. There are currently two categories of GNU Fortran
1334 extensions, those that provide functionality beyond that provided
1335 by any standard, and those that are supported by GNU Fortran
1336 purely for backward compatibility with legacy compilers. By default,
1337 @option{-std=gnu} allows the compiler to accept both types of
1338 extensions, but to warn about the use of the latter. Specifying
1339 either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
1340 disables both types of extensions, and @option{-std=legacy} allows both
1344 * Old-style kind specifications::
1345 * Old-style variable initialization::
1346 * Extensions to namelist::
1347 * X format descriptor without count field::
1348 * Commas in FORMAT specifications::
1349 * Missing period in FORMAT specifications::
1351 * BOZ literal constants::
1352 * @code{Q} exponent-letter::
1353 * Real array indices::
1355 * Implicitly convert LOGICAL and INTEGER values::
1356 * Hollerith constants support::
1358 * CONVERT specifier::
1360 * Argument list functions::
1363 @node Old-style kind specifications
1364 @subsection Old-style kind specifications
1365 @cindex kind, old-style
1367 GNU Fortran allows old-style kind specifications in declarations. These
1373 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
1374 etc.), and where @code{size} is a byte count corresponding to the
1375 storage size of a valid kind for that type. (For @code{COMPLEX}
1376 variables, @code{size} is the total size of the real and imaginary
1377 parts.) The statement then declares @code{x}, @code{y} and @code{z} to
1378 be of type @code{TYPESPEC} with the appropriate kind. This is
1379 equivalent to the standard-conforming declaration
1384 where @code{k} is the kind parameter suitable for the intended precision. As
1385 kind parameters are implementation-dependent, use the @code{KIND},
1386 @code{SELECTED_INT_KIND} and @code{SELECTED_REAL_KIND} intrinsics to retrieve
1387 the correct value, for instance @code{REAL*8 x} can be replaced by:
1389 INTEGER, PARAMETER :: dbl = KIND(1.0d0)
1393 @node Old-style variable initialization
1394 @subsection Old-style variable initialization
1396 GNU Fortran allows old-style initialization of variables of the
1400 REAL x(2,2) /3*0.,1./
1402 The syntax for the initializers is as for the @code{DATA} statement, but
1403 unlike in a @code{DATA} statement, an initializer only applies to the
1404 variable immediately preceding the initialization. In other words,
1405 something like @code{INTEGER I,J/2,3/} is not valid. This style of
1406 initialization is only allowed in declarations without double colons
1407 (@code{::}); the double colons were introduced in Fortran 90, which also
1408 introduced a standard syntax for initializing variables in type
1411 Examples of standard-conforming code equivalent to the above example
1415 INTEGER :: i = 1, j = 2
1416 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
1420 DATA i/1/, j/2/, x/3*0.,1./
1423 Note that variables which are explicitly initialized in declarations
1424 or in @code{DATA} statements automatically acquire the @code{SAVE}
1427 @node Extensions to namelist
1428 @subsection Extensions to namelist
1431 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
1432 including array qualifiers, substrings and fully qualified derived types.
1433 The output from a namelist write is compatible with namelist read. The
1434 output has all names in upper case and indentation to column 1 after the
1435 namelist name. Two extensions are permitted:
1437 Old-style use of @samp{$} instead of @samp{&}
1440 X(:)%Y(2) = 1.0 2.0 3.0
1445 It should be noted that the default terminator is @samp{/} rather than
1448 Querying of the namelist when inputting from stdin. After at least
1449 one space, entering @samp{?} sends to stdout the namelist name and the names of
1450 the variables in the namelist:
1461 Entering @samp{=?} outputs the namelist to stdout, as if
1462 @code{WRITE(*,NML = mynml)} had been called:
1467 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
1468 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
1469 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
1473 To aid this dialog, when input is from stdin, errors send their
1474 messages to stderr and execution continues, even if @code{IOSTAT} is set.
1476 @code{PRINT} namelist is permitted. This causes an error if
1477 @option{-std=f95} is used.
1480 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
1483 END PROGRAM test_print
1486 Expanded namelist reads are permitted. This causes an error if
1487 @option{-std=f95} is used. In the following example, the first element
1488 of the array will be given the value 0.00 and the two succeeding
1489 elements will be given the values 1.00 and 2.00.
1492 X(1,1) = 0.00 , 1.00 , 2.00
1496 @node X format descriptor without count field
1497 @subsection @code{X} format descriptor without count field
1499 To support legacy codes, GNU Fortran permits the count field of the
1500 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
1501 When omitted, the count is implicitly assumed to be one.
1505 10 FORMAT (I1, X, I1)
1508 @node Commas in FORMAT specifications
1509 @subsection Commas in @code{FORMAT} specifications
1511 To support legacy codes, GNU Fortran allows the comma separator
1512 to be omitted immediately before and after character string edit
1513 descriptors in @code{FORMAT} statements.
1517 10 FORMAT ('FOO='I1' BAR='I2)
1521 @node Missing period in FORMAT specifications
1522 @subsection Missing period in @code{FORMAT} specifications
1524 To support legacy codes, GNU Fortran allows missing periods in format
1525 specifications if and only if @option{-std=legacy} is given on the
1526 command line. This is considered non-conforming code and is
1535 @node I/O item lists
1536 @subsection I/O item lists
1537 @cindex I/O item lists
1539 To support legacy codes, GNU Fortran allows the input item list
1540 of the @code{READ} statement, and the output item lists of the
1541 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1543 @node @code{Q} exponent-letter
1544 @subsection @code{Q} exponent-letter
1545 @cindex @code{Q} exponent-letter
1547 GNU Fortran accepts real literal constants with an exponent-letter
1548 of @code{Q}, for example, @code{1.23Q45}. The constant is interpreted
1549 as a @code{REAL(16)} entity on targets that support this type. If
1550 the target does not support @code{REAL(16)} but has a @code{REAL(10)}
1551 type, then the real-literal-constant will be interpreted as a
1552 @code{REAL(10)} entity. In the absence of @code{REAL(16)} and
1553 @code{REAL(10)}, an error will occur.
1555 @node BOZ literal constants
1556 @subsection BOZ literal constants
1557 @cindex BOZ literal constants
1559 Besides decimal constants, Fortran also supports binary (@code{b}),
1560 octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
1561 syntax is: @samp{prefix quote digits quote}, were the prefix is
1562 either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
1563 @code{"} and the digits are for binary @code{0} or @code{1}, for
1564 octal between @code{0} and @code{7}, and for hexadecimal between
1565 @code{0} and @code{F}. (Example: @code{b'01011101'}.)
1567 Up to Fortran 95, BOZ literals were only allowed to initialize
1568 integer variables in DATA statements. Since Fortran 2003 BOZ literals
1569 are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
1570 and @code{CMPLX}; the result is the same as if the integer BOZ
1571 literal had been converted by @code{TRANSFER} to, respectively,
1572 @code{real}, @code{double precision}, @code{integer} or @code{complex}.
1573 As GNU Fortran extension the intrinsic procedures @code{FLOAT},
1574 @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
1576 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1577 be specified using the @code{X} prefix, in addition to the standard
1578 @code{Z} prefix. The BOZ literal can also be specified by adding a
1579 suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
1582 Furthermore, GNU Fortran allows using BOZ literal constants outside
1583 DATA statements and the four intrinsic functions allowed by Fortran 2003.
1584 In DATA statements, in direct assignments, where the right-hand side
1585 only contains a BOZ literal constant, and for old-style initializers of
1586 the form @code{integer i /o'0173'/}, the constant is transferred
1587 as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
1588 the real part is initialized unless @code{CMPLX} is used. In all other
1589 cases, the BOZ literal constant is converted to an @code{INTEGER} value with
1590 the largest decimal representation. This value is then converted
1591 numerically to the type and kind of the variable in question.
1592 (For instance, @code{real :: r = b'0000001' + 1} initializes @code{r}
1593 with @code{2.0}.) As different compilers implement the extension
1594 differently, one should be careful when doing bitwise initialization
1595 of non-integer variables.
1597 Note that initializing an @code{INTEGER} variable with a statement such
1598 as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
1599 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1600 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1601 option can be used as a workaround for legacy code that initializes
1602 integers in this manner.
1604 @node Real array indices
1605 @subsection Real array indices
1606 @cindex array, indices of type real
1608 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1609 or variables as array indices.
1611 @node Unary operators
1612 @subsection Unary operators
1613 @cindex operators, unary
1615 As an extension, GNU Fortran allows unary plus and unary minus operators
1616 to appear as the second operand of binary arithmetic operators without
1617 the need for parenthesis.
1623 @node Implicitly convert LOGICAL and INTEGER values
1624 @subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1625 @cindex conversion, to integer
1626 @cindex conversion, to logical
1628 As an extension for backwards compatibility with other compilers, GNU
1629 Fortran allows the implicit conversion of @code{LOGICAL} values to
1630 @code{INTEGER} values and vice versa. When converting from a
1631 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1632 zero, and @code{.TRUE.} is interpreted as one. When converting from
1633 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1634 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1645 However, there is no implicit conversion of @code{INTEGER} values in
1646 @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
1649 @node Hollerith constants support
1650 @subsection Hollerith constants support
1651 @cindex Hollerith constants
1653 GNU Fortran supports Hollerith constants in assignments, function
1654 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1655 constant is written as a string of characters preceded by an integer
1656 constant indicating the character count, and the letter @code{H} or
1657 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1658 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1659 constant will be padded or truncated to fit the size of the variable in
1662 Examples of valid uses of Hollerith constants:
1665 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1666 x(1) = 16HABCDEFGHIJKLMNOP
1670 Invalid Hollerith constants examples:
1673 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1674 a = 0H ! At least one character is needed.
1677 In general, Hollerith constants were used to provide a rudimentary
1678 facility for handling character strings in early Fortran compilers,
1679 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1680 in those cases, the standard-compliant equivalent is to convert the
1681 program to use proper character strings. On occasion, there may be a
1682 case where the intent is specifically to initialize a numeric variable
1683 with a given byte sequence. In these cases, the same result can be
1684 obtained by using the @code{TRANSFER} statement, as in this example.
1686 INTEGER(KIND=4) :: a
1687 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1692 @subsection Cray pointers
1693 @cindex pointer, Cray
1695 Cray pointers are part of a non-standard extension that provides a
1696 C-like pointer in Fortran. This is accomplished through a pair of
1697 variables: an integer "pointer" that holds a memory address, and a
1698 "pointee" that is used to dereference the pointer.
1700 Pointer/pointee pairs are declared in statements of the form:
1702 pointer ( <pointer> , <pointee> )
1706 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1708 The pointer is an integer that is intended to hold a memory address.
1709 The pointee may be an array or scalar. A pointee can be an assumed
1710 size array---that is, the last dimension may be left unspecified by
1711 using a @code{*} in place of a value---but a pointee cannot be an
1712 assumed shape array. No space is allocated for the pointee.
1714 The pointee may have its type declared before or after the pointer
1715 statement, and its array specification (if any) may be declared
1716 before, during, or after the pointer statement. The pointer may be
1717 declared as an integer prior to the pointer statement. However, some
1718 machines have default integer sizes that are different than the size
1719 of a pointer, and so the following code is not portable:
1724 If a pointer is declared with a kind that is too small, the compiler
1725 will issue a warning; the resulting binary will probably not work
1726 correctly, because the memory addresses stored in the pointers may be
1727 truncated. It is safer to omit the first line of the above example;
1728 if explicit declaration of ipt's type is omitted, then the compiler
1729 will ensure that ipt is an integer variable large enough to hold a
1732 Pointer arithmetic is valid with Cray pointers, but it is not the same
1733 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1734 the user is responsible for determining how many bytes to add to a
1735 pointer in order to increment it. Consider the following example:
1739 pointer (ipt, pointee)
1743 The last statement does not set @code{ipt} to the address of
1744 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1745 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1747 Any expression involving the pointee will be translated to use the
1748 value stored in the pointer as the base address.
1750 To get the address of elements, this extension provides an intrinsic
1751 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1752 @code{&} operator in C, except the address is cast to an integer type:
1755 pointer(ipt, arpte(10))
1757 ipt = loc(ar) ! Makes arpte is an alias for ar
1758 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1760 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1763 Cray pointees often are used to alias an existing variable. For
1771 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1772 @code{target}. The optimizer, however, will not detect this aliasing, so
1773 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1774 a pointee in any way that violates the Fortran aliasing rules or
1775 assumptions is illegal. It is the user's responsibility to avoid doing
1776 this; the compiler works under the assumption that no such aliasing
1779 Cray pointers will work correctly when there is no aliasing (i.e., when
1780 they are used to access a dynamically allocated block of memory), and
1781 also in any routine where a pointee is used, but any variable with which
1782 it shares storage is not used. Code that violates these rules may not
1783 run as the user intends. This is not a bug in the optimizer; any code
1784 that violates the aliasing rules is illegal. (Note that this is not
1785 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1786 will ``incorrectly'' optimize code with illegal aliasing.)
1788 There are a number of restrictions on the attributes that can be applied
1789 to Cray pointers and pointees. Pointees may not have the
1790 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1791 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1792 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1793 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes, nor
1794 may they be function results. Pointees may not occur in more than one
1795 pointer statement. A pointee cannot be a pointer. Pointees cannot occur
1796 in equivalence, common, or data statements.
1798 A Cray pointer may also point to a function or a subroutine. For
1799 example, the following excerpt is valid:
1803 pointer (subptr,subpte)
1813 A pointer may be modified during the course of a program, and this
1814 will change the location to which the pointee refers. However, when
1815 pointees are passed as arguments, they are treated as ordinary
1816 variables in the invoked function. Subsequent changes to the pointer
1817 will not change the base address of the array that was passed.
1819 @node CONVERT specifier
1820 @subsection @code{CONVERT} specifier
1821 @cindex @code{CONVERT} specifier
1823 GNU Fortran allows the conversion of unformatted data between little-
1824 and big-endian representation to facilitate moving of data
1825 between different systems. The conversion can be indicated with
1826 the @code{CONVERT} specifier on the @code{OPEN} statement.
1827 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1828 the data format via an environment variable.
1830 Valid values for @code{CONVERT} are:
1832 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1833 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1834 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1835 for unformatted files.
1836 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1840 Using the option could look like this:
1842 open(file='big.dat',form='unformatted',access='sequential', &
1843 convert='big_endian')
1846 The value of the conversion can be queried by using
1847 @code{INQUIRE(CONVERT=ch)}. The values returned are
1848 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1850 @code{CONVERT} works between big- and little-endian for
1851 @code{INTEGER} values of all supported kinds and for @code{REAL}
1852 on IEEE systems of kinds 4 and 8. Conversion between different
1853 ``extended double'' types on different architectures such as
1854 m68k and x86_64, which GNU Fortran
1855 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1858 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1859 environment variable will override the CONVERT specifier in the
1860 open statement}. This is to give control over data formats to
1861 users who do not have the source code of their program available.
1863 Using anything but the native representation for unformatted data
1864 carries a significant speed overhead. If speed in this area matters
1865 to you, it is best if you use this only for data that needs to be
1872 OpenMP (Open Multi-Processing) is an application programming
1873 interface (API) that supports multi-platform shared memory
1874 multiprocessing programming in C/C++ and Fortran on many
1875 architectures, including Unix and Microsoft Windows platforms.
1876 It consists of a set of compiler directives, library routines,
1877 and environment variables that influence run-time behavior.
1879 GNU Fortran strives to be compatible to the
1880 @uref{http://www.openmp.org/mp-documents/spec31.pdf,
1881 OpenMP Application Program Interface v3.1}.
1883 To enable the processing of the OpenMP directive @code{!$omp} in
1884 free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
1885 directives in fixed form; the @code{!$} conditional compilation sentinels
1886 in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
1887 in fixed form, @command{gfortran} needs to be invoked with the
1888 @option{-fopenmp}. This also arranges for automatic linking of the
1889 GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
1892 The OpenMP Fortran runtime library routines are provided both in a
1893 form of a Fortran 90 module named @code{omp_lib} and in a form of
1894 a Fortran @code{include} file named @file{omp_lib.h}.
1896 An example of a parallelized loop taken from Appendix A.1 of
1897 the OpenMP Application Program Interface v2.5:
1899 SUBROUTINE A1(N, A, B)
1902 !$OMP PARALLEL DO !I is private by default
1904 B(I) = (A(I) + A(I-1)) / 2.0
1906 !$OMP END PARALLEL DO
1913 @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
1914 will be allocated on the stack. When porting existing code to OpenMP,
1915 this may lead to surprising results, especially to segmentation faults
1916 if the stacksize is limited.
1919 On glibc-based systems, OpenMP enabled applications cannot be statically
1920 linked due to limitations of the underlying pthreads-implementation. It
1921 might be possible to get a working solution if
1922 @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
1923 to the command line. However, this is not supported by @command{gcc} and
1924 thus not recommended.
1927 @node Argument list functions
1928 @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
1929 @cindex argument list functions
1934 GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
1935 and @code{%LOC} statements, for backward compatibility with g77.
1936 It is recommended that these should be used only for code that is
1937 accessing facilities outside of GNU Fortran, such as operating system
1938 or windowing facilities. It is best to constrain such uses to isolated
1939 portions of a program--portions that deal specifically and exclusively
1940 with low-level, system-dependent facilities. Such portions might well
1941 provide a portable interface for use by the program as a whole, but are
1942 themselves not portable, and should be thoroughly tested each time they
1943 are rebuilt using a new compiler or version of a compiler.
1945 @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
1946 reference and @code{%LOC} passes its memory location. Since gfortran
1947 already passes scalar arguments by reference, @code{%REF} is in effect
1948 a do-nothing. @code{%LOC} has the same effect as a Fortran pointer.
1950 An example of passing an argument by value to a C subroutine foo.:
1953 C prototype void foo_ (float x);
1962 For details refer to the g77 manual
1963 @uref{http://gcc.gnu.org/@/onlinedocs/@/gcc-3.4.6/@/g77/@/index.html#Top}.
1965 Also, @code{c_by_val.f} and its partner @code{c_by_val.c} of the
1966 GNU Fortran testsuite are worth a look.
1969 @node Extensions not implemented in GNU Fortran
1970 @section Extensions not implemented in GNU Fortran
1971 @cindex extensions, not implemented
1973 The long history of the Fortran language, its wide use and broad
1974 userbase, the large number of different compiler vendors and the lack of
1975 some features crucial to users in the first standards have lead to the
1976 existence of a number of important extensions to the language. While
1977 some of the most useful or popular extensions are supported by the GNU
1978 Fortran compiler, not all existing extensions are supported. This section
1979 aims at listing these extensions and offering advice on how best make
1980 code that uses them running with the GNU Fortran compiler.
1982 @c More can be found here:
1983 @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
1984 @c -- the list of Fortran and libgfortran bugs closed as WONTFIX:
1985 @c http://tinyurl.com/2u4h5y
1988 * STRUCTURE and RECORD::
1989 @c * UNION and MAP::
1990 * ENCODE and DECODE statements::
1991 * Variable FORMAT expressions::
1992 @c * Q edit descriptor::
1993 @c * AUTOMATIC statement::
1994 @c * TYPE and ACCEPT I/O Statements::
1995 @c * .XOR. operator::
1996 @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
1997 @c * Omitted arguments in procedure call::
1998 * Alternate complex function syntax::
2002 @node STRUCTURE and RECORD
2003 @subsection @code{STRUCTURE} and @code{RECORD}
2004 @cindex @code{STRUCTURE}
2005 @cindex @code{RECORD}
2007 Structures are user-defined aggregate data types; this functionality was
2008 standardized in Fortran 90 with an different syntax, under the name of
2009 ``derived types''. Here is an example of code using the non portable
2013 ! Declaring a structure named ``item'' and containing three fields:
2014 ! an integer ID, an description string and a floating-point price.
2017 CHARACTER(LEN=200) description
2021 ! Define two variables, an single record of type ``item''
2022 ! named ``pear'', and an array of items named ``store_catalog''
2023 RECORD /item/ pear, store_catalog(100)
2025 ! We can directly access the fields of both variables
2027 pear.description = "juicy D'Anjou pear"
2029 store_catalog(7).id = 7831
2030 store_catalog(7).description = "milk bottle"
2031 store_catalog(7).price = 1.2
2033 ! We can also manipulate the whole structure
2034 store_catalog(12) = pear
2035 print *, store_catalog(12)
2039 This code can easily be rewritten in the Fortran 90 syntax as following:
2042 ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
2043 ! ``TYPE name ... END TYPE''
2046 CHARACTER(LEN=200) description
2050 ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
2051 TYPE(item) pear, store_catalog(100)
2053 ! Instead of using a dot (.) to access fields of a record, the
2054 ! standard syntax uses a percent sign (%)
2056 pear%description = "juicy D'Anjou pear"
2058 store_catalog(7)%id = 7831
2059 store_catalog(7)%description = "milk bottle"
2060 store_catalog(7)%price = 1.2
2062 ! Assignments of a whole variable do not change
2063 store_catalog(12) = pear
2064 print *, store_catalog(12)
2068 @c @node UNION and MAP
2069 @c @subsection @code{UNION} and @code{MAP}
2070 @c @cindex @code{UNION}
2071 @c @cindex @code{MAP}
2073 @c For help writing this one, see
2074 @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and
2075 @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm
2078 @node ENCODE and DECODE statements
2079 @subsection @code{ENCODE} and @code{DECODE} statements
2080 @cindex @code{ENCODE}
2081 @cindex @code{DECODE}
2083 GNU Fortran does not support the @code{ENCODE} and @code{DECODE}
2084 statements. These statements are best replaced by @code{READ} and
2085 @code{WRITE} statements involving internal files (@code{CHARACTER}
2086 variables and arrays), which have been part of the Fortran standard since
2087 Fortran 77. For example, replace a code fragment like
2092 c ... Code that sets LINE
2093 DECODE (80, 9000, LINE) A, B, C
2094 9000 FORMAT (1X, 3(F10.5))
2101 CHARACTER(LEN=80) LINE
2103 c ... Code that sets LINE
2104 READ (UNIT=LINE, FMT=9000) A, B, C
2105 9000 FORMAT (1X, 3(F10.5))
2108 Similarly, replace a code fragment like
2113 c ... Code that sets A, B and C
2114 ENCODE (80, 9000, LINE) A, B, C
2115 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
2122 CHARACTER(LEN=80) LINE
2124 c ... Code that sets A, B and C
2125 WRITE (UNIT=LINE, FMT=9000) A, B, C
2126 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
2130 @node Variable FORMAT expressions
2131 @subsection Variable @code{FORMAT} expressions
2132 @cindex @code{FORMAT}
2134 A variable @code{FORMAT} expression is format statement which includes
2135 angle brackets enclosing a Fortran expression: @code{FORMAT(I<N>)}. GNU
2136 Fortran does not support this legacy extension. The effect of variable
2137 format expressions can be reproduced by using the more powerful (and
2138 standard) combination of internal output and string formats. For example,
2139 replace a code fragment like this:
2150 c Variable declaration
2151 CHARACTER(LEN=20) FMT
2153 c Other code here...
2155 WRITE(FMT,'("(I", I0, ")")') N+1
2163 c Variable declaration
2164 CHARACTER(LEN=20) FMT
2166 c Other code here...
2169 WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1
2173 @node Alternate complex function syntax
2174 @subsection Alternate complex function syntax
2175 @cindex Complex function
2177 Some Fortran compilers, including @command{g77}, let the user declare
2178 complex functions with the syntax @code{COMPLEX FUNCTION name*16()}, as
2179 well as @code{COMPLEX*16 FUNCTION name()}. Both are non-standard, legacy
2180 extensions. @command{gfortran} accepts the latter form, which is more
2181 common, but not the former.
2185 @c ---------------------------------------------------------------------
2186 @c Mixed-Language Programming
2187 @c ---------------------------------------------------------------------
2189 @node Mixed-Language Programming
2190 @chapter Mixed-Language Programming
2191 @cindex Interoperability
2192 @cindex Mixed-language programming
2195 * Interoperability with C::
2196 * GNU Fortran Compiler Directives::
2197 * Non-Fortran Main Program::
2200 This chapter is about mixed-language interoperability, but also applies
2201 if one links Fortran code compiled by different compilers. In most cases,
2202 use of the C Binding features of the Fortran 2003 standard is sufficient,
2203 and their use is highly recommended.
2206 @node Interoperability with C
2207 @section Interoperability with C
2211 * Derived Types and struct::
2212 * Interoperable Global Variables::
2213 * Interoperable Subroutines and Functions::
2214 * Working with Pointers::
2215 * Further Interoperability of Fortran with C::
2218 Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a
2219 standardized way to generate procedure and derived-type
2220 declarations and global variables which are interoperable with C
2221 (ISO/IEC 9899:1999). The @code{bind(C)} attribute has been added
2222 to inform the compiler that a symbol shall be interoperable with C;
2223 also, some constraints are added. Note, however, that not
2224 all C features have a Fortran equivalent or vice versa. For instance,
2225 neither C's unsigned integers nor C's functions with variable number
2226 of arguments have an equivalent in Fortran.
2228 Note that array dimensions are reversely ordered in C and that arrays in
2229 C always start with index 0 while in Fortran they start by default with
2230 1. Thus, an array declaration @code{A(n,m)} in Fortran matches
2231 @code{A[m][n]} in C and accessing the element @code{A(i,j)} matches
2232 @code{A[j-1][i-1]}. The element following @code{A(i,j)} (C: @code{A[j-1][i-1]};
2233 assuming @math{i < n}) in memory is @code{A(i+1,j)} (C: @code{A[j-1][i]}).
2235 @node Intrinsic Types
2236 @subsection Intrinsic Types
2238 In order to ensure that exactly the same variable type and kind is used
2239 in C and Fortran, the named constants shall be used which are defined in the
2240 @code{ISO_C_BINDING} intrinsic module. That module contains named constants
2241 for kind parameters and character named constants for the escape sequences
2242 in C. For a list of the constants, see @ref{ISO_C_BINDING}.
2244 @node Derived Types and struct
2245 @subsection Derived Types and struct
2247 For compatibility of derived types with @code{struct}, one needs to use
2248 the @code{BIND(C)} attribute in the type declaration. For instance, the
2249 following type declaration
2253 TYPE, BIND(C) :: myType
2254 INTEGER(C_INT) :: i1, i2
2255 INTEGER(C_SIGNED_CHAR) :: i3
2256 REAL(C_DOUBLE) :: d1
2257 COMPLEX(C_FLOAT_COMPLEX) :: c1
2258 CHARACTER(KIND=C_CHAR) :: str(5)
2262 matches the following @code{struct} declaration in C
2267 /* Note: "char" might be signed or unsigned. */
2275 Derived types with the C binding attribute shall not have the @code{sequence}
2276 attribute, type parameters, the @code{extends} attribute, nor type-bound
2277 procedures. Every component must be of interoperable type and kind and may not
2278 have the @code{pointer} or @code{allocatable} attribute. The names of the
2279 components are irrelevant for interoperability.
2281 As there exist no direct Fortran equivalents, neither unions nor structs
2282 with bit field or variable-length array members are interoperable.
2284 @node Interoperable Global Variables
2285 @subsection Interoperable Global Variables
2287 Variables can be made accessible from C using the C binding attribute,
2288 optionally together with specifying a binding name. Those variables
2289 have to be declared in the declaration part of a @code{MODULE},
2290 be of interoperable type, and have neither the @code{pointer} nor
2291 the @code{allocatable} attribute.
2297 integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
2298 type(myType), bind(C) :: tp
2302 Here, @code{_MyProject_flags} is the case-sensitive name of the variable
2303 as seen from C programs while @code{global_flag} is the case-insensitive
2304 name as seen from Fortran. If no binding name is specified, as for
2305 @var{tp}, the C binding name is the (lowercase) Fortran binding name.
2306 If a binding name is specified, only a single variable may be after the
2307 double colon. Note of warning: You cannot use a global variable to
2308 access @var{errno} of the C library as the C standard allows it to be
2309 a macro. Use the @code{IERRNO} intrinsic (GNU extension) instead.
2311 @node Interoperable Subroutines and Functions
2312 @subsection Interoperable Subroutines and Functions
2314 Subroutines and functions have to have the @code{BIND(C)} attribute to
2315 be compatible with C. The dummy argument declaration is relatively
2316 straightforward. However, one needs to be careful because C uses
2317 call-by-value by default while Fortran behaves usually similar to
2318 call-by-reference. Furthermore, strings and pointers are handled
2319 differently. Note that in Fortran 2003 and 2008 only explicit size
2320 and assumed-size arrays are supported but not assumed-shape or
2321 deferred-shape (i.e. allocatable or pointer) arrays. However, those
2322 are allowed since the Technical Specification 29113, see
2323 @ref{Further Interoperability of Fortran with C}
2325 To pass a variable by value, use the @code{VALUE} attribute.
2326 Thus, the following C prototype
2329 @code{int func(int i, int *j)}
2332 matches the Fortran declaration
2335 integer(c_int) function func(i,j)
2336 use iso_c_binding, only: c_int
2337 integer(c_int), VALUE :: i
2341 Note that pointer arguments also frequently need the @code{VALUE} attribute,
2342 see @ref{Working with Pointers}.
2344 Strings are handled quite differently in C and Fortran. In C a string
2345 is a @code{NUL}-terminated array of characters while in Fortran each string
2346 has a length associated with it and is thus not terminated (by e.g.
2347 @code{NUL}). For example, if one wants to use the following C function,
2351 void print_C(char *string) /* equivalent: char string[] */
2353 printf("%s\n", string);
2357 to print ``Hello World'' from Fortran, one can call it using
2360 use iso_c_binding, only: C_CHAR, C_NULL_CHAR
2362 subroutine print_c(string) bind(C, name="print_C")
2363 use iso_c_binding, only: c_char
2364 character(kind=c_char) :: string(*)
2365 end subroutine print_c
2367 call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
2370 As the example shows, one needs to ensure that the
2371 string is @code{NUL} terminated. Additionally, the dummy argument
2372 @var{string} of @code{print_C} is a length-one assumed-size
2373 array; using @code{character(len=*)} is not allowed. The example
2374 above uses @code{c_char_"Hello World"} to ensure the string
2375 literal has the right type; typically the default character
2376 kind and @code{c_char} are the same and thus @code{"Hello World"}
2377 is equivalent. However, the standard does not guarantee this.
2379 The use of strings is now further illustrated using the C library
2380 function @code{strncpy}, whose prototype is
2383 char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
2386 The function @code{strncpy} copies at most @var{n} characters from
2387 string @var{s2} to @var{s1} and returns @var{s1}. In the following
2388 example, we ignore the return value:
2393 character(len=30) :: str,str2
2395 ! Ignore the return value of strncpy -> subroutine
2396 ! "restrict" is always assumed if we do not pass a pointer
2397 subroutine strncpy(dest, src, n) bind(C)
2399 character(kind=c_char), intent(out) :: dest(*)
2400 character(kind=c_char), intent(in) :: src(*)
2401 integer(c_size_t), value, intent(in) :: n
2402 end subroutine strncpy
2404 str = repeat('X',30) ! Initialize whole string with 'X'
2405 call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
2406 len(c_char_"Hello World",kind=c_size_t))
2407 print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
2411 The intrinsic procedures are described in @ref{Intrinsic Procedures}.
2413 @node Working with Pointers
2414 @subsection Working with Pointers
2416 C pointers are represented in Fortran via the special opaque derived type
2417 @code{type(c_ptr)} (with private components). Thus one needs to
2418 use intrinsic conversion procedures to convert from or to C pointers.
2420 For some applications, using an assumed type (@code{TYPE(*)}) can be an
2421 alternative to a C pointer; see
2422 @ref{Further Interoperability of Fortran with C}.
2428 type(c_ptr) :: cptr1, cptr2
2429 integer, target :: array(7), scalar
2430 integer, pointer :: pa(:), ps
2431 cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
2432 ! array is contiguous if required by the C
2434 cptr2 = c_loc(scalar)
2435 call c_f_pointer(cptr2, ps)
2436 call c_f_pointer(cptr2, pa, shape=[7])
2439 When converting C to Fortran arrays, the one-dimensional @code{SHAPE} argument
2442 If a pointer is a dummy-argument of an interoperable procedure, it usually
2443 has to be declared using the @code{VALUE} attribute. @code{void*}
2444 matches @code{TYPE(C_PTR), VALUE}, while @code{TYPE(C_PTR)} alone
2445 matches @code{void**}.
2447 Procedure pointers are handled analogously to pointers; the C type is
2448 @code{TYPE(C_FUNPTR)} and the intrinsic conversion procedures are
2449 @code{C_F_PROCPOINTER} and @code{C_FUNLOC}.
2451 Let us consider two examples of actually passing a procedure pointer from
2452 C to Fortran and vice versa. Note that these examples are also very
2453 similar to passing ordinary pointers between both languages. First,
2454 consider this code in C:
2457 /* Procedure implemented in Fortran. */
2458 void get_values (void (*)(double));
2460 /* Call-back routine we want called from Fortran. */
2464 printf ("Number is %f.\n", x);
2467 /* Call Fortran routine and pass call-back to it. */
2471 get_values (&print_it);
2475 A matching implementation for @code{get_values} in Fortran, that correctly
2476 receives the procedure pointer from C and is able to call it, is given
2477 in the following @code{MODULE}:
2483 ! Define interface of call-back routine.
2485 SUBROUTINE callback (x)
2486 USE, INTRINSIC :: ISO_C_BINDING
2487 REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
2488 END SUBROUTINE callback
2493 ! Define C-bound procedure.
2494 SUBROUTINE get_values (cproc) BIND(C)
2495 USE, INTRINSIC :: ISO_C_BINDING
2496 TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc
2498 PROCEDURE(callback), POINTER :: proc
2500 ! Convert C to Fortran procedure pointer.
2501 CALL C_F_PROCPOINTER (cproc, proc)
2504 CALL proc (1.0_C_DOUBLE)
2505 CALL proc (-42.0_C_DOUBLE)
2506 CALL proc (18.12_C_DOUBLE)
2507 END SUBROUTINE get_values
2512 Next, we want to call a C routine that expects a procedure pointer argument
2513 and pass it a Fortran procedure (which clearly must be interoperable!).
2514 Again, the C function may be:
2518 call_it (int (*func)(int), int arg)
2524 It can be used as in the following Fortran code:
2528 USE, INTRINSIC :: ISO_C_BINDING
2531 ! Define interface of C function.
2533 INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
2534 USE, INTRINSIC :: ISO_C_BINDING
2535 TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
2536 INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
2537 END FUNCTION call_it
2542 ! Define procedure passed to C function.
2543 ! It must be interoperable!
2544 INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
2545 INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
2546 double_it = arg + arg
2547 END FUNCTION double_it
2550 SUBROUTINE foobar ()
2551 TYPE(C_FUNPTR) :: cproc
2552 INTEGER(KIND=C_INT) :: i
2554 ! Get C procedure pointer.
2555 cproc = C_FUNLOC (double_it)
2558 DO i = 1_C_INT, 10_C_INT
2559 PRINT *, call_it (cproc, i)
2561 END SUBROUTINE foobar
2566 @node Further Interoperability of Fortran with C
2567 @subsection Further Interoperability of Fortran with C
2569 The Technical Specification ISO/IEC TS 29113:2012 on further
2570 interoperability of Fortran with C extends the interoperability support
2571 of Fortran 2003 and Fortran 2008. Besides removing some restrictions
2572 and constraints, it adds assumed-type (@code{TYPE(*)}) and assumed-rank
2573 (@code{dimension}) variables and allows for interoperability of
2574 assumed-shape, assumed-rank and deferred-shape arrays, including
2575 allocatables and pointers.
2577 Note: Currently, GNU Fortran does not support the array descriptor
2578 (dope vector) as specified in the Technical Specification, but uses
2579 an array descriptor with different fields. The Chasm Language
2580 Interoperability Tools, @url{http://chasm-interop.sourceforge.net/},
2581 provide an interface to GNU Fortran's array descriptor.
2583 The Technical Specification adds the following new features, which
2584 are supported by GNU Fortran:
2588 @item The @code{ASYNCHRONOUS} attribute has been clarified and
2589 extended to allow its use with asynchronous communication in
2590 user-provided libraries such as in implementations of the
2591 Message Passing Interface specification.
2593 @item Many constraints have been relaxed, in particular for
2594 the @code{C_LOC} and @code{C_F_POINTER} intrinsics.
2596 @item The @code{OPTIONAL} attribute is now allowed for dummy
2597 arguments; an absent argument matches a @code{NULL} pointer.
2599 @item Assumed types (@code{TYPE(*)}) have been added, which may
2600 only be used for dummy arguments. They are unlimited polymorphic
2601 but contrary to @code{CLASS(*)} they do not contain any type
2602 information, similar to C's @code{void *} pointers. Expressions
2603 of any type and kind can be passed; thus, it can be used as
2604 replacement for @code{TYPE(C_PTR)}, avoiding the use of
2605 @code{C_LOC} in the caller.
2607 Note, however, that @code{TYPE(*)} only accepts scalar arguments,
2608 unless the @code{DIMENSION} is explicitly specified. As
2609 @code{DIMENSION(*)} only supports array (including array elements) but
2610 no scalars, it is not a full replacement for @code{C_LOC}. On the
2611 other hand, assumed-type assumed-rank dummy arguments
2612 (@code{TYPE(*), DIMENSION(..)}) allow for both scalars and arrays, but
2613 require special code on the callee side to handle the array descriptor.
2615 @item Assumed-shape arrays (@code{DIMENSION(..)}) as dummy argument
2616 allow that scalars and arrays of any rank can be passed as actual
2617 argument. As the Technical Specification does not provide for direct
2618 means to operate with them, they have to be used either from the C side
2619 or be converted using @code{C_LOC} and @code{C_F_POINTER} to scalars
2620 or arrays of a specific rank. The rank can be determined using the
2621 @code{RANK} intrinisic.
2625 Currently unimplemented:
2629 @item GNU Fortran always uses an array descriptor, which does not
2630 match the one of the Technical Specification. The
2631 @code{ISO_Fortran_binding.h} header file and the C functions it
2632 specifies are not available.
2634 @item Using assumed-shape, assumed-rank and deferred-shape arrays in
2635 @code{BIND(C)} procedures is not fully supported. In particular,
2636 C interoperable strings of other length than one are not supported
2637 as this requires the new array descriptor.
2641 @node GNU Fortran Compiler Directives
2642 @section GNU Fortran Compiler Directives
2644 The Fortran standard describes how a conforming program shall
2645 behave; however, the exact implementation is not standardized. In order
2646 to allow the user to choose specific implementation details, compiler
2647 directives can be used to set attributes of variables and procedures
2648 which are not part of the standard. Whether a given attribute is
2649 supported and its exact effects depend on both the operating system and
2650 on the processor; see
2651 @ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)}
2654 For procedures and procedure pointers, the following attributes can
2655 be used to change the calling convention:
2658 @item @code{CDECL} -- standard C calling convention
2659 @item @code{STDCALL} -- convention where the called procedure pops the stack
2660 @item @code{FASTCALL} -- part of the arguments are passed via registers
2661 instead using the stack
2664 Besides changing the calling convention, the attributes also influence
2665 the decoration of the symbol name, e.g., by a leading underscore or by
2666 a trailing at-sign followed by the number of bytes on the stack. When
2667 assigning a procedure to a procedure pointer, both should use the same
2670 On some systems, procedures and global variables (module variables and
2671 @code{COMMON} blocks) need special handling to be accessible when they
2672 are in a shared library. The following attributes are available:
2675 @item @code{DLLEXPORT} -- provide a global pointer to a pointer in the DLL
2676 @item @code{DLLIMPORT} -- reference the function or variable using a global pointer
2679 The attributes are specified using the syntax
2681 @code{!GCC$ ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list}
2683 where in free-form source code only whitespace is allowed before @code{!GCC$}
2684 and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall
2685 start in the first column.
2687 For procedures, the compiler directives shall be placed into the body
2688 of the procedure; for variables and procedure pointers, they shall be in
2689 the same declaration part as the variable or procedure pointer.
2693 @node Non-Fortran Main Program
2694 @section Non-Fortran Main Program
2697 * _gfortran_set_args:: Save command-line arguments
2698 * _gfortran_set_options:: Set library option flags
2699 * _gfortran_set_convert:: Set endian conversion
2700 * _gfortran_set_record_marker:: Set length of record markers
2701 * _gfortran_set_max_subrecord_length:: Set subrecord length
2702 * _gfortran_set_fpe:: Set when a Floating Point Exception should be raised
2705 Even if you are doing mixed-language programming, it is very
2706 likely that you do not need to know or use the information in this
2707 section. Since it is about the internal structure of GNU Fortran,
2708 it may also change in GCC minor releases.
2710 When you compile a @code{PROGRAM} with GNU Fortran, a function
2711 with the name @code{main} (in the symbol table of the object file)
2712 is generated, which initializes the libgfortran library and then
2713 calls the actual program which uses the name @code{MAIN__}, for
2714 historic reasons. If you link GNU Fortran compiled procedures
2715 to, e.g., a C or C++ program or to a Fortran program compiled by
2716 a different compiler, the libgfortran library is not initialized
2717 and thus a few intrinsic procedures do not work properly, e.g.
2718 those for obtaining the command-line arguments.
2720 Therefore, if your @code{PROGRAM} is not compiled with
2721 GNU Fortran and the GNU Fortran compiled procedures require
2722 intrinsics relying on the library initialization, you need to
2723 initialize the library yourself. Using the default options,
2724 gfortran calls @code{_gfortran_set_args} and
2725 @code{_gfortran_set_options}. The initialization of the former
2726 is needed if the called procedures access the command line
2727 (and for backtracing); the latter sets some flags based on the
2728 standard chosen or to enable backtracing. In typical programs,
2729 it is not necessary to call any initialization function.
2731 If your @code{PROGRAM} is compiled with GNU Fortran, you shall
2732 not call any of the following functions. The libgfortran
2733 initialization functions are shown in C syntax but using C
2734 bindings they are also accessible from Fortran.
2737 @node _gfortran_set_args
2738 @subsection @code{_gfortran_set_args} --- Save command-line arguments
2739 @fnindex _gfortran_set_args
2740 @cindex libgfortran initialization, set_args
2743 @item @emph{Description}:
2744 @code{_gfortran_set_args} saves the command-line arguments; this
2745 initialization is required if any of the command-line intrinsics
2746 is called. Additionally, it shall be called if backtracing is
2747 enabled (see @code{_gfortran_set_options}).
2749 @item @emph{Syntax}:
2750 @code{void _gfortran_set_args (int argc, char *argv[])}
2752 @item @emph{Arguments}:
2753 @multitable @columnfractions .15 .70
2754 @item @var{argc} @tab number of command line argument strings
2755 @item @var{argv} @tab the command-line argument strings; argv[0]
2756 is the pathname of the executable itself.
2759 @item @emph{Example}:
2761 int main (int argc, char *argv[])
2763 /* Initialize libgfortran. */
2764 _gfortran_set_args (argc, argv);
2771 @node _gfortran_set_options
2772 @subsection @code{_gfortran_set_options} --- Set library option flags
2773 @fnindex _gfortran_set_options
2774 @cindex libgfortran initialization, set_options
2777 @item @emph{Description}:
2778 @code{_gfortran_set_options} sets several flags related to the Fortran
2779 standard to be used, whether backtracing should be enabled
2780 and whether range checks should be performed. The syntax allows for
2781 upward compatibility since the number of passed flags is specified; for
2782 non-passed flags, the default value is used. See also
2783 @pxref{Code Gen Options}. Please note that not all flags are actually
2786 @item @emph{Syntax}:
2787 @code{void _gfortran_set_options (int num, int options[])}
2789 @item @emph{Arguments}:
2790 @multitable @columnfractions .15 .70
2791 @item @var{num} @tab number of options passed
2792 @item @var{argv} @tab The list of flag values
2795 @item @emph{option flag list}:
2796 @multitable @columnfractions .15 .70
2797 @item @var{option}[0] @tab Allowed standard; can give run-time errors
2798 if e.g. an input-output edit descriptor is invalid in a given standard.
2799 Possible values are (bitwise or-ed) @code{GFC_STD_F77} (1),
2800 @code{GFC_STD_F95_OBS} (2), @code{GFC_STD_F95_DEL} (4), @code{GFC_STD_F95}
2801 (8), @code{GFC_STD_F2003} (16), @code{GFC_STD_GNU} (32),
2802 @code{GFC_STD_LEGACY} (64), @code{GFC_STD_F2008} (128),
2803 @code{GFC_STD_F2008_OBS} (256) and GFC_STD_F2008_TS (512). Default:
2804 @code{GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003
2805 | GFC_STD_F2008 | GFC_STD_F2008_TS | GFC_STD_F2008_OBS | GFC_STD_F77
2806 | GFC_STD_GNU | GFC_STD_LEGACY}.
2807 @item @var{option}[1] @tab Standard-warning flag; prints a warning to
2808 standard error. Default: @code{GFC_STD_F95_DEL | GFC_STD_LEGACY}.
2809 @item @var{option}[2] @tab If non zero, enable pedantic checking.
2811 @item @var{option}[3] @tab Unused.
2812 @item @var{option}[4] @tab If non zero, enable backtracing on run-time
2813 errors. Default: off.
2814 Note: Installs a signal handler and requires command-line
2815 initialization using @code{_gfortran_set_args}.
2816 @item @var{option}[5] @tab If non zero, supports signed zeros.
2818 @item @var{option}[6] @tab Enables run-time checking. Possible values
2819 are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2),
2820 GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (16), GFC_RTCHECK_POINTER (32).
2824 @item @emph{Example}:
2826 /* Use gfortran 4.8 default options. */
2827 static int options[] = @{68, 511, 0, 0, 1, 1, 0@};
2828 _gfortran_set_options (7, &options);
2833 @node _gfortran_set_convert
2834 @subsection @code{_gfortran_set_convert} --- Set endian conversion
2835 @fnindex _gfortran_set_convert
2836 @cindex libgfortran initialization, set_convert
2839 @item @emph{Description}:
2840 @code{_gfortran_set_convert} set the representation of data for
2843 @item @emph{Syntax}:
2844 @code{void _gfortran_set_convert (int conv)}
2846 @item @emph{Arguments}:
2847 @multitable @columnfractions .15 .70
2848 @item @var{conv} @tab Endian conversion, possible values:
2849 GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1),
2850 GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).
2853 @item @emph{Example}:
2855 int main (int argc, char *argv[])
2857 /* Initialize libgfortran. */
2858 _gfortran_set_args (argc, argv);
2859 _gfortran_set_convert (1);
2866 @node _gfortran_set_record_marker
2867 @subsection @code{_gfortran_set_record_marker} --- Set length of record markers
2868 @fnindex _gfortran_set_record_marker
2869 @cindex libgfortran initialization, set_record_marker
2872 @item @emph{Description}:
2873 @code{_gfortran_set_record_marker} sets the length of record markers
2874 for unformatted files.
2876 @item @emph{Syntax}:
2877 @code{void _gfortran_set_record_marker (int val)}
2879 @item @emph{Arguments}:
2880 @multitable @columnfractions .15 .70
2881 @item @var{val} @tab Length of the record marker; valid values
2882 are 4 and 8. Default is 4.
2885 @item @emph{Example}:
2887 int main (int argc, char *argv[])
2889 /* Initialize libgfortran. */
2890 _gfortran_set_args (argc, argv);
2891 _gfortran_set_record_marker (8);
2898 @node _gfortran_set_fpe
2899 @subsection @code{_gfortran_set_fpe} --- Enable floating point exception traps
2900 @fnindex _gfortran_set_fpe
2901 @cindex libgfortran initialization, set_fpe
2904 @item @emph{Description}:
2905 @code{_gfortran_set_fpe} enables floating point exception traps for
2906 the specified exceptions. On most systems, this will result in a
2907 SIGFPE signal being sent and the program being aborted.
2909 @item @emph{Syntax}:
2910 @code{void _gfortran_set_fpe (int val)}
2912 @item @emph{Arguments}:
2913 @multitable @columnfractions .15 .70
2914 @item @var{option}[0] @tab IEEE exceptions. Possible values are
2915 (bitwise or-ed) zero (0, default) no trapping,
2916 @code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2),
2917 @code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8),
2918 @code{GFC_FPE_UNDERFLOW} (16), and @code{GFC_FPE_INEXACT} (32).
2921 @item @emph{Example}:
2923 int main (int argc, char *argv[])
2925 /* Initialize libgfortran. */
2926 _gfortran_set_args (argc, argv);
2927 /* FPE for invalid operations such as SQRT(-1.0). */
2928 _gfortran_set_fpe (1);
2935 @node _gfortran_set_max_subrecord_length
2936 @subsection @code{_gfortran_set_max_subrecord_length} --- Set subrecord length
2937 @fnindex _gfortran_set_max_subrecord_length
2938 @cindex libgfortran initialization, set_max_subrecord_length
2941 @item @emph{Description}:
2942 @code{_gfortran_set_max_subrecord_length} set the maximum length
2943 for a subrecord. This option only makes sense for testing and
2944 debugging of unformatted I/O.
2946 @item @emph{Syntax}:
2947 @code{void _gfortran_set_max_subrecord_length (int val)}
2949 @item @emph{Arguments}:
2950 @multitable @columnfractions .15 .70
2951 @item @var{val} @tab the maximum length for a subrecord;
2952 the maximum permitted value is 2147483639, which is also
2956 @item @emph{Example}:
2958 int main (int argc, char *argv[])
2960 /* Initialize libgfortran. */
2961 _gfortran_set_args (argc, argv);
2962 _gfortran_set_max_subrecord_length (8);
2970 @c Intrinsic Procedures
2971 @c ---------------------------------------------------------------------
2973 @include intrinsic.texi
2980 @c ---------------------------------------------------------------------
2982 @c ---------------------------------------------------------------------
2985 @unnumbered Contributing
2986 @cindex Contributing
2988 Free software is only possible if people contribute to efforts
2990 We're always in need of more people helping out with ideas
2991 and comments, writing documentation and contributing code.
2993 If you want to contribute to GNU Fortran,
2994 have a look at the long lists of projects you can take on.
2995 Some of these projects are small,
2996 some of them are large;
2997 some are completely orthogonal to the rest of what is
2998 happening on GNU Fortran,
2999 but others are ``mainstream'' projects in need of enthusiastic hackers.
3000 All of these projects are important!
3001 We will eventually get around to the things here,
3002 but they are also things doable by someone who is willing and able.
3007 * Proposed Extensions::
3012 @section Contributors to GNU Fortran
3013 @cindex Contributors
3017 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
3018 also the initiator of the whole project. Thanks Andy!
3019 Most of the interface with GCC was written by @emph{Paul Brook}.
3021 The following individuals have contributed code and/or
3022 ideas and significant help to the GNU Fortran project
3023 (in alphabetical order):
3026 @item Janne Blomqvist
3027 @item Steven Bosscher
3030 @item Fran@,{c}ois-Xavier Coudert
3034 @item Bernhard Fischer
3036 @item Richard Guenther
3037 @item Richard Henderson
3038 @item Katherine Holcomb
3040 @item Niels Kristian Bech Jensen
3041 @item Steven Johnson
3042 @item Steven G. Kargl
3050 @item Christopher D. Rickett
3051 @item Richard Sandiford
3052 @item Tobias Schl@"uter
3061 The following people have contributed bug reports,
3062 smaller or larger patches,
3063 and much needed feedback and encouragement for the
3064 GNU Fortran project:
3068 @item Dominique d'Humi@`eres
3070 @item Erik Schnetter
3071 @item Joost VandeVondele
3074 Many other individuals have helped debug,
3075 test and improve the GNU Fortran compiler over the past few years,
3076 and we welcome you to do the same!
3077 If you already have done so,
3078 and you would like to see your name listed in the
3079 list above, please contact us.
3087 @item Help build the test suite
3088 Solicit more code for donation to the test suite: the more extensive the
3089 testsuite, the smaller the risk of breaking things in the future! We can
3090 keep code private on request.
3092 @item Bug hunting/squishing
3093 Find bugs and write more test cases! Test cases are especially very
3094 welcome, because it allows us to concentrate on fixing bugs instead of
3095 isolating them. Going through the bugzilla database at
3096 @url{http://gcc.gnu.org/@/bugzilla/} to reduce testcases posted there and
3097 add more information (for example, for which version does the testcase
3098 work, for which versions does it fail?) is also very helpful.
3103 @node Proposed Extensions
3104 @section Proposed Extensions
3106 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
3107 order. Most of these are necessary to be fully compatible with
3108 existing Fortran compilers, but they are not part of the official
3109 J3 Fortran 95 standard.
3111 @subsection Compiler extensions:
3114 User-specified alignment rules for structures.
3117 Automatically extend single precision constants to double.
3120 Compile code that conserves memory by dynamically allocating common and
3121 module storage either on stack or heap.
3124 Compile flag to generate code for array conformance checking (suggest -CC).
3127 User control of symbol names (underscores, etc).
3130 Compile setting for maximum size of stack frame size before spilling
3131 parts to static or heap.
3134 Flag to force local variables into static space.
3137 Flag to force local variables onto stack.
3141 @subsection Environment Options
3144 Pluggable library modules for random numbers, linear algebra.
3145 LA should use BLAS calling conventions.
3148 Environment variables controlling actions on arithmetic exceptions like
3149 overflow, underflow, precision loss---Generate NaN, abort, default.
3153 Set precision for fp units that support it (i387).
3156 Variable for setting fp rounding mode.
3159 Variable to fill uninitialized variables with a user-defined bit
3163 Environment variable controlling filename that is opened for that unit
3167 Environment variable to clear/trash memory being freed.
3170 Environment variable to control tracing of allocations and frees.
3173 Environment variable to display allocated memory at normal program end.
3176 Environment variable for filename for * IO-unit.
3179 Environment variable for temporary file directory.
3182 Environment variable forcing standard output to be line buffered (Unix).
3187 @c ---------------------------------------------------------------------
3188 @c GNU General Public License
3189 @c ---------------------------------------------------------------------
3191 @include gpl_v3.texi
3195 @c ---------------------------------------------------------------------
3196 @c GNU Free Documentation License
3197 @c ---------------------------------------------------------------------
3203 @c ---------------------------------------------------------------------
3204 @c Funding Free Software
3205 @c ---------------------------------------------------------------------
3207 @include funding.texi
3209 @c ---------------------------------------------------------------------
3211 @c ---------------------------------------------------------------------
3214 @unnumbered Option Index
3215 @command{gfortran}'s command line options are indexed here without any
3216 initial @samp{-} or @samp{--}. Where an option has both positive and
3217 negative forms (such as -foption and -fno-option), relevant entries in
3218 the manual are indexed under the most appropriate form; it may sometimes
3219 be useful to look up both forms.
3223 @unnumbered Keyword Index