1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
6 @include gcc-common.texi
8 @settitle The GNU Fortran Compiler
10 @c Create a separate index for command line options
12 @c Merge the standard indexes into a single one.
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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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72 @c (To provide the reverse effect, set bindingoffset to -0.75in.)
75 @c \global\bindingoffset=0.75in
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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'll 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 aren't 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 @code{.F}, @code{.FOR}, @code{.FTN}, @code{.fpp},
411 @code{.FPP}, @code{.F90}, @code{.F95}, @code{.F03} or @code{.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://users.erols.com/dnagle/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 such as enumeration, stream I/O, and some of the
478 enhancements to allocatable array support from TR 15581. However, it is
479 still under development and has a few remaining rough edges.
481 At present, the GNU Fortran compiler passes the
482 @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
483 NIST Fortran 77 Test Suite}, and produces acceptable results on the
484 @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}.
485 It also provides respectable performance on
486 the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran
487 compiler benchmarks} and the
488 @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html,
489 Livermore Fortran Kernels test}. It has been used to compile a number of
490 large real-world programs, including
491 @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM
492 weather-forecasting code} and
493 @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum
494 chemistry package}; see @url{http://gcc.gnu.org/wiki/GfortranApps} for an
497 Among other things, the GNU Fortran compiler is intended as a replacement
498 for G77. At this point, nearly all programs that could be compiled with
499 G77 can be compiled with GNU Fortran, although there are a few minor known
502 The primary work remaining to be done on GNU Fortran falls into three
503 categories: bug fixing (primarily regarding the treatment of invalid code
504 and providing useful error messages), improving the compiler optimizations
505 and the performance of compiled code, and extending the compiler to support
506 future standards---in particular, Fortran 2003 and Fortran 2008.
509 @c ---------------------------------------------------------------------
511 @c ---------------------------------------------------------------------
518 * Varying Length Character Strings::
521 The GNU Fortran compiler implements
522 ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
523 standard-compliant Fortran 90 and Fortran 77 programs. It also supports
524 the ISO/IEC TR-15581 enhancements to allocatable arrays, and
525 the @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
526 OpenMP Application Program Interface v2.5} specification.
528 In the future, the GNU Fortran compiler will also support ISO/IEC
529 1539-1:2004 (Fortran 2003) and future Fortran standards. Partial support
530 of that standard is already provided; the current status of Fortran 2003
531 support is reported in the @ref{Fortran 2003 status} section of the
534 The next version of the Fortran standard (Fortran 2008) is currently
535 being developed and the GNU Fortran compiler supports some of its new
536 features. This support is based on the latest draft of the standard
537 (available from @url{http://www.nag.co.uk/sc22wg5/}) and no guarantee of
538 future compatibility is made, as the final standard might differ from the
539 draft. For more information, see the @ref{Fortran 2008 status} section.
541 Additionally, the GNU Fortran compilers supports the OpenMP specification
542 (version 3.0, @url{http://openmp.org/wp/openmp-specifications/}).
544 @node Varying Length Character Strings
545 @subsection Varying Length Character Strings
546 @cindex Varying length character strings
547 @cindex Varying length strings
548 @cindex strings, varying length
550 The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000)
551 varying length character strings. While GNU Fortran currently does not
552 support such strings directly, there exist two Fortran implementations
553 for them, which work with GNU Fortran. They can be found at
554 @uref{http://www.fortran.com/@/iso_varying_string.f95} and at
555 @uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/ISO_VARYING_STRING/}.
559 @c =====================================================================
560 @c PART I: INVOCATION REFERENCE
561 @c =====================================================================
564 \part{I}{Invoking GNU Fortran}
567 @c ---------------------------------------------------------------------
569 @c ---------------------------------------------------------------------
574 @c ---------------------------------------------------------------------
576 @c ---------------------------------------------------------------------
579 @chapter Runtime: Influencing runtime behavior with environment variables
580 @cindex environment variable
582 The behavior of the @command{gfortran} can be influenced by
583 environment variables.
585 Malformed environment variables are silently ignored.
588 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
589 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
590 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
591 * GFORTRAN_USE_STDERR:: Send library output to standard error
592 * GFORTRAN_TMPDIR:: Directory for scratch files
593 * GFORTRAN_UNBUFFERED_ALL:: Don't buffer I/O for all units.
594 * GFORTRAN_UNBUFFERED_PRECONNECTED:: Don't buffer I/O for preconnected units.
595 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
596 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
597 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
598 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
599 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
600 * GFORTRAN_ERROR_DUMPCORE:: Dump core on run-time errors
601 * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
604 @node GFORTRAN_STDIN_UNIT
605 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
607 This environment variable can be used to select the unit number
608 preconnected to standard input. This must be a positive integer.
609 The default value is 5.
611 @node GFORTRAN_STDOUT_UNIT
612 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
614 This environment variable can be used to select the unit number
615 preconnected to standard output. This must be a positive integer.
616 The default value is 6.
618 @node GFORTRAN_STDERR_UNIT
619 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
621 This environment variable can be used to select the unit number
622 preconnected to standard error. This must be a positive integer.
623 The default value is 0.
625 @node GFORTRAN_USE_STDERR
626 @section @env{GFORTRAN_USE_STDERR}---Send library output to standard error
628 This environment variable controls where library output is sent.
629 If the first letter is @samp{y}, @samp{Y} or @samp{1}, standard
630 error is used. If the first letter is @samp{n}, @samp{N} or
631 @samp{0}, standard output is used.
633 @node GFORTRAN_TMPDIR
634 @section @env{GFORTRAN_TMPDIR}---Directory for scratch files
636 This environment variable controls where scratch files are
637 created. If this environment variable is missing,
638 GNU Fortran searches for the environment variable @env{TMP}, then @env{TEMP}.
639 If these are missing, the default is @file{/tmp}.
641 @node GFORTRAN_UNBUFFERED_ALL
642 @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer I/O on all units
644 This environment variable controls whether all I/O is unbuffered. If
645 the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
646 unbuffered. This will slow down small sequential reads and writes. If
647 the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
650 @node GFORTRAN_UNBUFFERED_PRECONNECTED
651 @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Don't buffer I/O on preconnected units
653 The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
654 whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If
655 the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This
656 will slow down small sequential reads and writes. If the first letter
657 is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default.
659 @node GFORTRAN_SHOW_LOCUS
660 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
662 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
663 line numbers for runtime errors are printed. If the first letter is
664 @samp{n}, @samp{N} or @samp{0}, don't print filename and line numbers
665 for runtime errors. The default is to print the location.
667 @node GFORTRAN_OPTIONAL_PLUS
668 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
670 If the first letter is @samp{y}, @samp{Y} or @samp{1},
671 a plus sign is printed
672 where permitted by the Fortran standard. If the first letter
673 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
674 in most cases. Default is not to print plus signs.
676 @node GFORTRAN_DEFAULT_RECL
677 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
679 This environment variable specifies the default record length, in
680 bytes, for files which are opened without a @code{RECL} tag in the
681 @code{OPEN} statement. This must be a positive integer. The
682 default value is 1073741824 bytes (1 GB).
684 @node GFORTRAN_LIST_SEPARATOR
685 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
687 This environment variable specifies the separator when writing
688 list-directed output. It may contain any number of spaces and
689 at most one comma. If you specify this on the command line,
690 be sure to quote spaces, as in
692 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
694 when @command{a.out} is the compiled Fortran program that you want to run.
695 Default is a single space.
697 @node GFORTRAN_CONVERT_UNIT
698 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
700 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
701 to change the representation of data for unformatted files.
702 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
704 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
705 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
706 exception: mode ':' unit_list | unit_list ;
707 unit_list: unit_spec | unit_list unit_spec ;
708 unit_spec: INTEGER | INTEGER '-' INTEGER ;
710 The variable consists of an optional default mode, followed by
711 a list of optional exceptions, which are separated by semicolons
712 from the preceding default and each other. Each exception consists
713 of a format and a comma-separated list of units. Valid values for
714 the modes are the same as for the @code{CONVERT} specifier:
717 @item @code{NATIVE} Use the native format. This is the default.
718 @item @code{SWAP} Swap between little- and big-endian.
719 @item @code{LITTLE_ENDIAN} Use the little-endian format
720 for unformatted files.
721 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
723 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
724 Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
726 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
727 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
728 in little_endian mode, except for units 10 to 20 and 25, which are in
730 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
733 Setting the environment variables should be done on the command
734 line or via the @command{export}
735 command for @command{sh}-compatible shells and via @command{setenv}
736 for @command{csh}-compatible shells.
738 Example for @command{sh}:
741 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
744 Example code for @command{csh}:
747 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
751 Using anything but the native representation for unformatted data
752 carries a significant speed overhead. If speed in this area matters
753 to you, it is best if you use this only for data that needs to be
756 @xref{CONVERT specifier}, for an alternative way to specify the
757 data representation for unformatted files. @xref{Runtime Options}, for
758 setting a default data representation for the whole program. The
759 @code{CONVERT} specifier overrides the @option{-fconvert} compile options.
761 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
762 environment variable will override the CONVERT specifier in the
763 open statement}. This is to give control over data formats to
764 users who do not have the source code of their program available.
766 @node GFORTRAN_ERROR_DUMPCORE
767 @section @env{GFORTRAN_ERROR_DUMPCORE}---Dump core on run-time errors
769 If the @env{GFORTRAN_ERROR_DUMPCORE} variable is set to
770 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
771 then library run-time errors cause core dumps. To disable the core
772 dumps, set the variable to @samp{n}, @samp{N}, @samp{0}. Default
773 is not to core dump unless the @option{-fdump-core} compile option
776 @node GFORTRAN_ERROR_BACKTRACE
777 @section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors
779 If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to
780 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
781 then a backtrace is printed when a run-time error occurs.
782 To disable the backtracing, set the variable to
783 @samp{n}, @samp{N}, @samp{0}. Default is not to print a backtrace
784 unless the @option{-fbacktrace} compile option
787 @c =====================================================================
788 @c PART II: LANGUAGE REFERENCE
789 @c =====================================================================
792 \part{II}{Language Reference}
795 @c ---------------------------------------------------------------------
796 @c Fortran 2003 and 2008 Status
797 @c ---------------------------------------------------------------------
799 @node Fortran 2003 and 2008 status
800 @chapter Fortran 2003 and 2008 Status
803 * Fortran 2003 status::
804 * Fortran 2008 status::
807 @node Fortran 2003 status
808 @section Fortran 2003 status
810 GNU Fortran supports several Fortran 2003 features; an incomplete
811 list can be found below. See also the
812 @uref{http://gcc.gnu.org/wiki/Fortran2003, wiki page} about Fortran 2003.
816 Intrinsics @code{command_argument_count}, @code{get_command},
817 @code{get_command_argument}, @code{get_environment_variable}, and
821 @cindex array, constructors
823 Array constructors using square brackets. That is, @code{[...]} rather
824 than @code{(/.../)}. Type-specification for array constructors like
825 @code{(/ some-type :: ... /)}.
828 @cindex @code{FLUSH} statement
829 @cindex statement, @code{FLUSH}
830 @code{FLUSH} statement.
833 @cindex @code{IOMSG=} specifier
834 @code{IOMSG=} specifier for I/O statements.
837 @cindex @code{ENUM} statement
838 @cindex @code{ENUMERATOR} statement
839 @cindex statement, @code{ENUM}
840 @cindex statement, @code{ENUMERATOR}
841 @opindex @code{fshort-enums}
842 Support for the declaration of enumeration constants via the
843 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
844 @command{gcc} is guaranteed also for the case where the
845 @command{-fshort-enums} command line option is given.
852 @cindex @code{ALLOCATABLE} dummy arguments
853 @code{ALLOCATABLE} dummy arguments.
855 @cindex @code{ALLOCATABLE} function results
856 @code{ALLOCATABLE} function results
858 @cindex @code{ALLOCATABLE} components of derived types
859 @code{ALLOCATABLE} components of derived types
863 @cindex @code{ALLOCATE}
864 The @code{ERRMSG=} tag is now supported in @code{ALLOCATE} and
865 @code{DEALLOCATE} statements. The @code{SOURCE=} tag is supported
866 in an @code{ALLOCATE} statement. An @emph{intrinsic-type-spec}
867 can be used as the @emph{type-spec} in an @code{ALLOCATE} statement;
868 while the use of a @emph{derived-type-name} is currently unsupported.
871 @cindex @code{STREAM} I/O
872 @cindex @code{ACCESS='STREAM'} I/O
873 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
874 allowing I/O without any record structure.
877 Namelist input/output for internal files.
880 @cindex @code{PROTECTED} statement
881 @cindex statement, @code{PROTECTED}
882 The @code{PROTECTED} statement and attribute.
885 @cindex @code{VALUE} statement
886 @cindex statement, @code{VALUE}
887 The @code{VALUE} statement and attribute.
890 @cindex @code{VOLATILE} statement
891 @cindex statement, @code{VOLATILE}
892 The @code{VOLATILE} statement and attribute.
895 @cindex @code{IMPORT} statement
896 @cindex statement, @code{IMPORT}
897 The @code{IMPORT} statement, allowing to import
898 host-associated derived types.
901 @cindex @code{USE, INTRINSIC} statement
902 @cindex statement, @code{USE, INTRINSIC}
903 @cindex @code{ISO_FORTRAN_ENV} statement
904 @cindex statement, @code{ISO_FORTRAN_ENV}
905 @code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
906 attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
907 @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
910 Renaming of operators in the @code{USE} statement.
913 @cindex ISO C Bindings
914 Interoperability with C (ISO C Bindings)
917 BOZ as argument of @code{INT}, @code{REAL}, @code{DBLE} and @code{CMPLX}.
920 @cindex type-bound procedure
921 @cindex type-bound operator
922 Type-bound procedures with @code{PROCEDURE} or @code{GENERIC}, and operators
923 bound to a derived-type.
926 @cindex @code{EXTENDS}
927 @cindex derived-type extension
928 Extension of derived-types (the @code{EXTENDS(...)} syntax).
931 @cindex @code{ABSTRACT} type
932 @cindex @code{DEFERRED} procedure binding
933 @code{ABSTRACT} derived-types and declaring procedure bindings @code{DEFERRED}.
938 @node Fortran 2008 status
939 @section Fortran 2008 status
941 The next version of the Fortran standard after Fortran 2003 is currently
942 being worked on by the Working Group 5 of Sub-Committee 22 of the Joint
943 Technical Committee 1 of the International Organization for
944 Standardization (ISO) and the International Electrotechnical Commission
945 (IEC). This group is known as @uref{http://www.nag.co.uk/sc22wg5/, WG5}.
946 The next revision of the Fortran standard is informally referred to as
947 Fortran 2008, reflecting its planned release year. The GNU Fortran
948 compiler has support for some of the new features in Fortran 2008. This
949 support is based on the latest draft, available from
950 @url{http://www.nag.co.uk/sc22wg5/}. However, as the final standard may
951 differ from the drafts, no guarantee of backward compatibility can be
952 made and you should only use it for experimental purposes.
954 The @uref{http://gcc.gnu.org/wiki/Fortran2008Status, wiki} has some information
955 about the current Fortran 2008 implementation status.
958 @c ---------------------------------------------------------------------
959 @c Compiler Characteristics
960 @c ---------------------------------------------------------------------
962 @node Compiler Characteristics
963 @chapter Compiler Characteristics
965 This chapter describes certain characteristics of the GNU Fortran
966 compiler, that are not specified by the Fortran standard, but which
967 might in some way or another become visible to the programmer.
970 * KIND Type Parameters::
971 * Internal representation of LOGICAL variables::
975 @node KIND Type Parameters
976 @section KIND Type Parameters
979 The @code{KIND} type parameters supported by GNU Fortran for the primitive
985 1, 2, 4, 8*, 16*, default: 4 (1)
988 1, 2, 4, 8*, 16*, default: 4 (1)
991 4, 8, 10**, 16**, default: 4 (2)
994 4, 8, 10**, 16**, default: 4 (2)
1002 * = not available on all systems @*
1003 ** = not available on all systems; additionally 10 and 16 are never
1004 available at the same time @*
1005 (1) Unless -fdefault-integer-8 is used @*
1006 (2) Unless -fdefault-real-8 is used
1009 The @code{KIND} value matches the storage size in bytes, except for
1010 @code{COMPLEX} where the storage size is twice as much (or both real and
1011 imaginary part are a real value of the given size). It is recommended to use
1012 the @code{SELECT_*_KIND} intrinsics instead of the concrete values.
1015 @node Internal representation of LOGICAL variables
1016 @section Internal representation of LOGICAL variables
1017 @cindex logical, variable representation
1019 The Fortran standard does not specify how variables of @code{LOGICAL}
1020 type are represented, beyond requiring that @code{LOGICAL} variables
1021 of default kind have the same storage size as default @code{INTEGER}
1022 and @code{REAL} variables. The GNU Fortran internal representation is
1025 A @code{LOGICAL(KIND=N)} variable is represented as an
1026 @code{INTEGER(KIND=N)} variable, however, with only two permissible
1027 values: @code{1} for @code{.TRUE.} and @code{0} for
1028 @code{.FALSE.}. Any other integer value results in undefined behavior.
1030 Note that for mixed-language programming using the
1031 @code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can
1032 be used to create @code{LOGICAL(KIND=C_BOOL)} variables which are
1033 interoperable with the C99 _Bool type. The C99 _Bool type has an
1034 internal representation described in the C99 standard, which is
1035 identical to the above description, i.e. with 1 for true and 0 for
1036 false being the only permissible values. Thus the internal
1037 representation of @code{LOGICAL} variables in GNU Fortran is identical
1038 to C99 _Bool, except for a possible difference in storage size
1039 depending on the kind.
1041 @c ---------------------------------------------------------------------
1043 @c ---------------------------------------------------------------------
1045 @c Maybe this chapter should be merged with the 'Standards' section,
1046 @c whenever that is written :-)
1052 The two sections below detail the extensions to standard Fortran that are
1053 implemented in GNU Fortran, as well as some of the popular or
1054 historically important extensions that are not (or not yet) implemented.
1055 For the latter case, we explain the alternatives available to GNU Fortran
1056 users, including replacement by standard-conforming code or GNU
1060 * Extensions implemented in GNU Fortran::
1061 * Extensions not implemented in GNU Fortran::
1065 @node Extensions implemented in GNU Fortran
1066 @section Extensions implemented in GNU Fortran
1067 @cindex extensions, implemented
1069 GNU Fortran implements a number of extensions over standard
1070 Fortran. This chapter contains information on their syntax and
1071 meaning. There are currently two categories of GNU Fortran
1072 extensions, those that provide functionality beyond that provided
1073 by any standard, and those that are supported by GNU Fortran
1074 purely for backward compatibility with legacy compilers. By default,
1075 @option{-std=gnu} allows the compiler to accept both types of
1076 extensions, but to warn about the use of the latter. Specifying
1077 either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
1078 disables both types of extensions, and @option{-std=legacy} allows both
1082 * Old-style kind specifications::
1083 * Old-style variable initialization::
1084 * Extensions to namelist::
1085 * X format descriptor without count field::
1086 * Commas in FORMAT specifications::
1087 * Missing period in FORMAT specifications::
1089 * BOZ literal constants::
1090 * Real array indices::
1092 * Implicitly convert LOGICAL and INTEGER values::
1093 * Hollerith constants support::
1095 * CONVERT specifier::
1097 * Argument list functions::
1100 @node Old-style kind specifications
1101 @subsection Old-style kind specifications
1102 @cindex kind, old-style
1104 GNU Fortran allows old-style kind specifications in declarations. These
1110 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
1111 etc.), and where @code{size} is a byte count corresponding to the
1112 storage size of a valid kind for that type. (For @code{COMPLEX}
1113 variables, @code{size} is the total size of the real and imaginary
1114 parts.) The statement then declares @code{x}, @code{y} and @code{z} to
1115 be of type @code{TYPESPEC} with the appropriate kind. This is
1116 equivalent to the standard-conforming declaration
1121 where @code{k} is the kind parameter suitable for the intended precision. As
1122 kind parameters are implementation-dependent, use the @code{KIND},
1123 @code{SELECTED_INT_KIND} and @code{SELECTED_REAL_KIND} intrinsics to retrieve
1124 the correct value, for instance @code{REAL*8 x} can be replaced by:
1126 INTEGER, PARAMETER :: dbl = KIND(1.0d0)
1130 @node Old-style variable initialization
1131 @subsection Old-style variable initialization
1133 GNU Fortran allows old-style initialization of variables of the
1137 REAL x(2,2) /3*0.,1./
1139 The syntax for the initializers is as for the @code{DATA} statement, but
1140 unlike in a @code{DATA} statement, an initializer only applies to the
1141 variable immediately preceding the initialization. In other words,
1142 something like @code{INTEGER I,J/2,3/} is not valid. This style of
1143 initialization is only allowed in declarations without double colons
1144 (@code{::}); the double colons were introduced in Fortran 90, which also
1145 introduced a standard syntax for initializing variables in type
1148 Examples of standard-conforming code equivalent to the above example
1152 INTEGER :: i = 1, j = 2
1153 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
1157 DATA i/1/, j/2/, x/3*0.,1./
1160 Note that variables which are explicitly initialized in declarations
1161 or in @code{DATA} statements automatically acquire the @code{SAVE}
1164 @node Extensions to namelist
1165 @subsection Extensions to namelist
1168 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
1169 including array qualifiers, substrings and fully qualified derived types.
1170 The output from a namelist write is compatible with namelist read. The
1171 output has all names in upper case and indentation to column 1 after the
1172 namelist name. Two extensions are permitted:
1174 Old-style use of @samp{$} instead of @samp{&}
1177 X(:)%Y(2) = 1.0 2.0 3.0
1182 It should be noted that the default terminator is @samp{/} rather than
1185 Querying of the namelist when inputting from stdin. After at least
1186 one space, entering @samp{?} sends to stdout the namelist name and the names of
1187 the variables in the namelist:
1198 Entering @samp{=?} outputs the namelist to stdout, as if
1199 @code{WRITE(*,NML = mynml)} had been called:
1204 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
1205 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
1206 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
1210 To aid this dialog, when input is from stdin, errors send their
1211 messages to stderr and execution continues, even if @code{IOSTAT} is set.
1213 @code{PRINT} namelist is permitted. This causes an error if
1214 @option{-std=f95} is used.
1217 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
1220 END PROGRAM test_print
1223 Expanded namelist reads are permitted. This causes an error if
1224 @option{-std=f95} is used. In the following example, the first element
1225 of the array will be given the value 0.00 and the two succeeding
1226 elements will be given the values 1.00 and 2.00.
1229 X(1,1) = 0.00 , 1.00 , 2.00
1233 @node X format descriptor without count field
1234 @subsection @code{X} format descriptor without count field
1236 To support legacy codes, GNU Fortran permits the count field of the
1237 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
1238 When omitted, the count is implicitly assumed to be one.
1242 10 FORMAT (I1, X, I1)
1245 @node Commas in FORMAT specifications
1246 @subsection Commas in @code{FORMAT} specifications
1248 To support legacy codes, GNU Fortran allows the comma separator
1249 to be omitted immediately before and after character string edit
1250 descriptors in @code{FORMAT} statements.
1254 10 FORMAT ('FOO='I1' BAR='I2)
1258 @node Missing period in FORMAT specifications
1259 @subsection Missing period in @code{FORMAT} specifications
1261 To support legacy codes, GNU Fortran allows missing periods in format
1262 specifications if and only if @option{-std=legacy} is given on the
1263 command line. This is considered non-conforming code and is
1272 @node I/O item lists
1273 @subsection I/O item lists
1274 @cindex I/O item lists
1276 To support legacy codes, GNU Fortran allows the input item list
1277 of the @code{READ} statement, and the output item lists of the
1278 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1280 @node BOZ literal constants
1281 @subsection BOZ literal constants
1282 @cindex BOZ literal constants
1284 Besides decimal constants, Fortran also supports binary (@code{b}),
1285 octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
1286 syntax is: @samp{prefix quote digits quote}, were the prefix is
1287 either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
1288 @code{"} and the digits are for binary @code{0} or @code{1}, for
1289 octal between @code{0} and @code{7}, and for hexadecimal between
1290 @code{0} and @code{F}. (Example: @code{b'01011101'}.)
1292 Up to Fortran 95, BOZ literals were only allowed to initialize
1293 integer variables in DATA statements. Since Fortran 2003 BOZ literals
1294 are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
1295 and @code{CMPLX}; the result is the same as if the integer BOZ
1296 literal had been converted by @code{TRANSFER} to, respectively,
1297 @code{real}, @code{double precision}, @code{integer} or @code{complex}.
1298 As GNU Fortran extension the intrinsic procedures @code{FLOAT},
1299 @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
1301 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1302 be specified using the @code{X} prefix, in addition to the standard
1303 @code{Z} prefix. The BOZ literal can also be specified by adding a
1304 suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
1307 Furthermore, GNU Fortran allows using BOZ literal constants outside
1308 DATA statements and the four intrinsic functions allowed by Fortran 2003.
1309 In DATA statements, in direct assignments, where the right-hand side
1310 only contains a BOZ literal constant, and for old-style initializers of
1311 the form @code{integer i /o'0173'/}, the constant is transferred
1312 as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
1313 the real part is initialized unless @code{CMPLX} is used. In all other
1314 cases, the BOZ literal constant is converted to an @code{INTEGER} value with
1315 the largest decimal representation. This value is then converted
1316 numerically to the type and kind of the variable in question.
1317 (For instance, @code{real :: r = b'0000001' + 1} initializes @code{r}
1318 with @code{2.0}.) As different compilers implement the extension
1319 differently, one should be careful when doing bitwise initialization
1320 of non-integer variables.
1322 Note that initializing an @code{INTEGER} variable with a statement such
1323 as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
1324 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1325 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1326 option can be used as a workaround for legacy code that initializes
1327 integers in this manner.
1329 @node Real array indices
1330 @subsection Real array indices
1331 @cindex array, indices of type real
1333 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1334 or variables as array indices.
1336 @node Unary operators
1337 @subsection Unary operators
1338 @cindex operators, unary
1340 As an extension, GNU Fortran allows unary plus and unary minus operators
1341 to appear as the second operand of binary arithmetic operators without
1342 the need for parenthesis.
1348 @node Implicitly convert LOGICAL and INTEGER values
1349 @subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1350 @cindex conversion, to integer
1351 @cindex conversion, to logical
1353 As an extension for backwards compatibility with other compilers, GNU
1354 Fortran allows the implicit conversion of @code{LOGICAL} values to
1355 @code{INTEGER} values and vice versa. When converting from a
1356 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1357 zero, and @code{.TRUE.} is interpreted as one. When converting from
1358 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1359 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1370 However, there is no implicit conversion of @code{INTEGER} values in
1371 @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
1374 @node Hollerith constants support
1375 @subsection Hollerith constants support
1376 @cindex Hollerith constants
1378 GNU Fortran supports Hollerith constants in assignments, function
1379 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1380 constant is written as a string of characters preceded by an integer
1381 constant indicating the character count, and the letter @code{H} or
1382 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1383 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1384 constant will be padded or truncated to fit the size of the variable in
1387 Examples of valid uses of Hollerith constants:
1390 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1391 x(1) = 16HABCDEFGHIJKLMNOP
1395 Invalid Hollerith constants examples:
1398 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1399 a = 0H ! At least one character is needed.
1402 In general, Hollerith constants were used to provide a rudimentary
1403 facility for handling character strings in early Fortran compilers,
1404 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1405 in those cases, the standard-compliant equivalent is to convert the
1406 program to use proper character strings. On occasion, there may be a
1407 case where the intent is specifically to initialize a numeric variable
1408 with a given byte sequence. In these cases, the same result can be
1409 obtained by using the @code{TRANSFER} statement, as in this example.
1411 INTEGER(KIND=4) :: a
1412 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1417 @subsection Cray pointers
1418 @cindex pointer, Cray
1420 Cray pointers are part of a non-standard extension that provides a
1421 C-like pointer in Fortran. This is accomplished through a pair of
1422 variables: an integer "pointer" that holds a memory address, and a
1423 "pointee" that is used to dereference the pointer.
1425 Pointer/pointee pairs are declared in statements of the form:
1427 pointer ( <pointer> , <pointee> )
1431 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1433 The pointer is an integer that is intended to hold a memory address.
1434 The pointee may be an array or scalar. A pointee can be an assumed
1435 size array---that is, the last dimension may be left unspecified by
1436 using a @code{*} in place of a value---but a pointee cannot be an
1437 assumed shape array. No space is allocated for the pointee.
1439 The pointee may have its type declared before or after the pointer
1440 statement, and its array specification (if any) may be declared
1441 before, during, or after the pointer statement. The pointer may be
1442 declared as an integer prior to the pointer statement. However, some
1443 machines have default integer sizes that are different than the size
1444 of a pointer, and so the following code is not portable:
1449 If a pointer is declared with a kind that is too small, the compiler
1450 will issue a warning; the resulting binary will probably not work
1451 correctly, because the memory addresses stored in the pointers may be
1452 truncated. It is safer to omit the first line of the above example;
1453 if explicit declaration of ipt's type is omitted, then the compiler
1454 will ensure that ipt is an integer variable large enough to hold a
1457 Pointer arithmetic is valid with Cray pointers, but it is not the same
1458 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1459 the user is responsible for determining how many bytes to add to a
1460 pointer in order to increment it. Consider the following example:
1464 pointer (ipt, pointee)
1468 The last statement does not set @code{ipt} to the address of
1469 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1470 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1472 Any expression involving the pointee will be translated to use the
1473 value stored in the pointer as the base address.
1475 To get the address of elements, this extension provides an intrinsic
1476 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1477 @code{&} operator in C, except the address is cast to an integer type:
1480 pointer(ipt, arpte(10))
1482 ipt = loc(ar) ! Makes arpte is an alias for ar
1483 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1485 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1488 Cray pointees often are used to alias an existing variable. For
1496 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1497 @code{target}. The optimizer, however, will not detect this aliasing, so
1498 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1499 a pointee in any way that violates the Fortran aliasing rules or
1500 assumptions is illegal. It is the user's responsibility to avoid doing
1501 this; the compiler works under the assumption that no such aliasing
1504 Cray pointers will work correctly when there is no aliasing (i.e., when
1505 they are used to access a dynamically allocated block of memory), and
1506 also in any routine where a pointee is used, but any variable with which
1507 it shares storage is not used. Code that violates these rules may not
1508 run as the user intends. This is not a bug in the optimizer; any code
1509 that violates the aliasing rules is illegal. (Note that this is not
1510 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1511 will ``incorrectly'' optimize code with illegal aliasing.)
1513 There are a number of restrictions on the attributes that can be applied
1514 to Cray pointers and pointees. Pointees may not have the
1515 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1516 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1517 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1518 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes, nor
1519 may they be function results. Pointees may not occur in more than one
1520 pointer statement. A pointee cannot be a pointer. Pointees cannot occur
1521 in equivalence, common, or data statements.
1523 A Cray pointer may also point to a function or a subroutine. For
1524 example, the following excerpt is valid:
1528 pointer (subptr,subpte)
1538 A pointer may be modified during the course of a program, and this
1539 will change the location to which the pointee refers. However, when
1540 pointees are passed as arguments, they are treated as ordinary
1541 variables in the invoked function. Subsequent changes to the pointer
1542 will not change the base address of the array that was passed.
1544 @node CONVERT specifier
1545 @subsection @code{CONVERT} specifier
1546 @cindex @code{CONVERT} specifier
1548 GNU Fortran allows the conversion of unformatted data between little-
1549 and big-endian representation to facilitate moving of data
1550 between different systems. The conversion can be indicated with
1551 the @code{CONVERT} specifier on the @code{OPEN} statement.
1552 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1553 the data format via an environment variable.
1555 Valid values for @code{CONVERT} are:
1557 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1558 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1559 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1560 for unformatted files.
1561 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1565 Using the option could look like this:
1567 open(file='big.dat',form='unformatted',access='sequential', &
1568 convert='big_endian')
1571 The value of the conversion can be queried by using
1572 @code{INQUIRE(CONVERT=ch)}. The values returned are
1573 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1575 @code{CONVERT} works between big- and little-endian for
1576 @code{INTEGER} values of all supported kinds and for @code{REAL}
1577 on IEEE systems of kinds 4 and 8. Conversion between different
1578 ``extended double'' types on different architectures such as
1579 m68k and x86_64, which GNU Fortran
1580 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1583 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1584 environment variable will override the CONVERT specifier in the
1585 open statement}. This is to give control over data formats to
1586 users who do not have the source code of their program available.
1588 Using anything but the native representation for unformatted data
1589 carries a significant speed overhead. If speed in this area matters
1590 to you, it is best if you use this only for data that needs to be
1597 OpenMP (Open Multi-Processing) is an application programming
1598 interface (API) that supports multi-platform shared memory
1599 multiprocessing programming in C/C++ and Fortran on many
1600 architectures, including Unix and Microsoft Windows platforms.
1601 It consists of a set of compiler directives, library routines,
1602 and environment variables that influence run-time behavior.
1604 GNU Fortran strives to be compatible to the
1605 @uref{http://www.openmp.org/mp-documents/spec30.pdf,
1606 OpenMP Application Program Interface v3.0}.
1608 To enable the processing of the OpenMP directive @code{!$omp} in
1609 free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
1610 directives in fixed form; the @code{!$} conditional compilation sentinels
1611 in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
1612 in fixed form, @command{gfortran} needs to be invoked with the
1613 @option{-fopenmp}. This also arranges for automatic linking of the
1614 GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
1617 The OpenMP Fortran runtime library routines are provided both in a
1618 form of a Fortran 90 module named @code{omp_lib} and in a form of
1619 a Fortran @code{include} file named @file{omp_lib.h}.
1621 An example of a parallelized loop taken from Appendix A.1 of
1622 the OpenMP Application Program Interface v2.5:
1624 SUBROUTINE A1(N, A, B)
1627 !$OMP PARALLEL DO !I is private by default
1629 B(I) = (A(I) + A(I-1)) / 2.0
1631 !$OMP END PARALLEL DO
1638 @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
1639 will be allocated on the stack. When porting existing code to OpenMP,
1640 this may lead to surprising results, especially to segmentation faults
1641 if the stacksize is limited.
1644 On glibc-based systems, OpenMP enabled applications cannot be statically
1645 linked due to limitations of the underlying pthreads-implementation. It
1646 might be possible to get a working solution if
1647 @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
1648 to the command line. However, this is not supported by @command{gcc} and
1649 thus not recommended.
1652 @node Argument list functions
1653 @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
1654 @cindex argument list functions
1659 GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
1660 and @code{%LOC} statements, for backward compatibility with g77.
1661 It is recommended that these should be used only for code that is
1662 accessing facilities outside of GNU Fortran, such as operating system
1663 or windowing facilities. It is best to constrain such uses to isolated
1664 portions of a program--portions that deal specifically and exclusively
1665 with low-level, system-dependent facilities. Such portions might well
1666 provide a portable interface for use by the program as a whole, but are
1667 themselves not portable, and should be thoroughly tested each time they
1668 are rebuilt using a new compiler or version of a compiler.
1670 @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
1671 reference and @code{%LOC} passes its memory location. Since gfortran
1672 already passes scalar arguments by reference, @code{%REF} is in effect
1673 a do-nothing. @code{%LOC} has the same effect as a Fortran pointer.
1675 An example of passing an argument by value to a C subroutine foo.:
1678 C prototype void foo_ (float x);
1687 For details refer to the g77 manual
1688 @uref{http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top}.
1690 Also, @code{c_by_val.f} and its partner @code{c_by_val.c} of the
1691 GNU Fortran testsuite are worth a look.
1694 @node Extensions not implemented in GNU Fortran
1695 @section Extensions not implemented in GNU Fortran
1696 @cindex extensions, not implemented
1698 The long history of the Fortran language, its wide use and broad
1699 userbase, the large number of different compiler vendors and the lack of
1700 some features crucial to users in the first standards have lead to the
1701 existence of a number of important extensions to the language. While
1702 some of the most useful or popular extensions are supported by the GNU
1703 Fortran compiler, not all existing extensions are supported. This section
1704 aims at listing these extensions and offering advice on how best make
1705 code that uses them running with the GNU Fortran compiler.
1707 @c More can be found here:
1708 @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
1709 @c -- the list of Fortran and libgfortran bugs closed as WONTFIX:
1710 @c http://tinyurl.com/2u4h5y
1713 * STRUCTURE and RECORD::
1714 @c * UNION and MAP::
1715 * ENCODE and DECODE statements::
1716 * Variable FORMAT expressions::
1717 @c * Q edit descriptor::
1718 @c * AUTOMATIC statement::
1719 @c * TYPE and ACCEPT I/O Statements::
1720 @c * .XOR. operator::
1721 @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
1722 @c * Omitted arguments in procedure call::
1723 * Alternate complex function syntax::
1727 @node STRUCTURE and RECORD
1728 @subsection @code{STRUCTURE} and @code{RECORD}
1729 @cindex @code{STRUCTURE}
1730 @cindex @code{RECORD}
1732 Structures are user-defined aggregate data types; this functionality was
1733 standardized in Fortran 90 with an different syntax, under the name of
1734 ``derived types''. Here is an example of code using the non portable
1738 ! Declaring a structure named ``item'' and containing three fields:
1739 ! an integer ID, an description string and a floating-point price.
1742 CHARACTER(LEN=200) description
1746 ! Define two variables, an single record of type ``item''
1747 ! named ``pear'', and an array of items named ``store_catalog''
1748 RECORD /item/ pear, store_catalog(100)
1750 ! We can directly access the fields of both variables
1752 pear.description = "juicy D'Anjou pear"
1754 store_catalog(7).id = 7831
1755 store_catalog(7).description = "milk bottle"
1756 store_catalog(7).price = 1.2
1758 ! We can also manipulate the whole structure
1759 store_catalog(12) = pear
1760 print *, store_catalog(12)
1764 This code can easily be rewritten in the Fortran 90 syntax as following:
1767 ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
1768 ! ``TYPE name ... END TYPE''
1771 CHARACTER(LEN=200) description
1775 ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
1776 TYPE(item) pear, store_catalog(100)
1778 ! Instead of using a dot (.) to access fields of a record, the
1779 ! standard syntax uses a percent sign (%)
1781 pear%description = "juicy D'Anjou pear"
1783 store_catalog(7)%id = 7831
1784 store_catalog(7)%description = "milk bottle"
1785 store_catalog(7)%price = 1.2
1787 ! Assignments of a whole variable don't change
1788 store_catalog(12) = pear
1789 print *, store_catalog(12)
1793 @c @node UNION and MAP
1794 @c @subsection @code{UNION} and @code{MAP}
1795 @c @cindex @code{UNION}
1796 @c @cindex @code{MAP}
1798 @c For help writing this one, see
1799 @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and
1800 @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm
1803 @node ENCODE and DECODE statements
1804 @subsection @code{ENCODE} and @code{DECODE} statements
1805 @cindex @code{ENCODE}
1806 @cindex @code{DECODE}
1808 GNU Fortran doesn't support the @code{ENCODE} and @code{DECODE}
1809 statements. These statements are best replaced by @code{READ} and
1810 @code{WRITE} statements involving internal files (@code{CHARACTER}
1811 variables and arrays), which have been part of the Fortran standard since
1812 Fortran 77. For example, replace a code fragment like
1817 c ... Code that sets LINE
1818 DECODE (80, 9000, LINE) A, B, C
1819 9000 FORMAT (1X, 3(F10.5))
1826 CHARACTER(LEN=80) LINE
1828 c ... Code that sets LINE
1829 READ (UNIT=LINE, FMT=9000) A, B, C
1830 9000 FORMAT (1X, 3(F10.5))
1833 Similarly, replace a code fragment like
1838 c ... Code that sets A, B and C
1839 ENCODE (80, 9000, LINE) A, B, C
1840 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1847 CHARACTER(LEN=80) LINE
1849 c ... Code that sets A, B and C
1850 WRITE (UNIT=LINE, FMT=9000) A, B, C
1851 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1855 @node Variable FORMAT expressions
1856 @subsection Variable @code{FORMAT} expressions
1857 @cindex @code{FORMAT}
1859 A variable @code{FORMAT} expression is format statement which includes
1860 angle brackets enclosing a Fortran expression: @code{FORMAT(I<N>)}. GNU
1861 Fortran does not support this legacy extension. The effect of variable
1862 format expressions can be reproduced by using the more powerful (and
1863 standard) combination of internal output and string formats. For example,
1864 replace a code fragment like this:
1875 c Variable declaration
1876 CHARACTER(LEN=20) FMT
1878 c Other code here...
1880 WRITE(FMT,'("(I", I0, ")")') N+1
1888 c Variable declaration
1889 CHARACTER(LEN=20) FMT
1891 c Other code here...
1894 WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1
1898 @node Alternate complex function syntax
1899 @subsection Alternate complex function syntax
1900 @cindex Complex function
1902 Some Fortran compilers, including @command{g77}, let the user declare
1903 complex functions with the syntax @code{COMPLEX FUNCTION name*16()}, as
1904 well as @code{COMPLEX*16 FUNCTION name()}. Both are non-standard, legacy
1905 extensions. @command{gfortran} accepts the latter form, which is more
1906 common, but not the former.
1910 @c ---------------------------------------------------------------------
1911 @c Mixed-Language Programming
1912 @c ---------------------------------------------------------------------
1914 @node Mixed-Language Programming
1915 @chapter Mixed-Language Programming
1916 @cindex Interoperability
1917 @cindex Mixed-language programming
1920 * Interoperability with C::
1921 * GNU Fortran Compiler Directives::
1922 * Non-Fortran Main Program::
1925 This chapter is about mixed-language interoperability, but also applies
1926 if one links Fortran code compiled by different compilers. In most cases,
1927 use of the C Binding features of the Fortran 2003 standard is sufficient,
1928 and their use is highly recommended.
1931 @node Interoperability with C
1932 @section Interoperability with C
1936 * Further Interoperability of Fortran with C::
1937 * Derived Types and struct::
1938 * Interoperable Global Variables::
1939 * Interoperable Subroutines and Functions::
1942 Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a
1943 standardized way to generate procedure and derived-type
1944 declarations and global variables which are interoperable with C
1945 (ISO/IEC 9899:1999). The @code{bind(C)} attribute has been added
1946 to inform the compiler that a symbol shall be interoperable with C;
1947 also, some constraints are added. Note, however, that not
1948 all C features have a Fortran equivalent or vice versa. For instance,
1949 neither C's unsigned integers nor C's functions with variable number
1950 of arguments have an equivalent in Fortran.
1952 Note that array dimensions are reversely ordered in C and that arrays in
1953 C always start with index 0 while in Fortran they start by default with
1954 1. Thus, an array declaration @code{A(n,m)} in Fortran matches
1955 @code{A[m][n]} in C and accessing the element @code{A(i,j)} matches
1956 @code{A[j-1][i-1]}. The element following @code{A(i,j)} (C: @code{A[j-1][i-1]};
1957 assuming @math{i < n}) in memory is @code{A(i+1,j)} (C: @code{A[j-1][i]}).
1959 @node Intrinsic Types
1960 @subsection Intrinsic Types
1962 In order to ensure that exactly the same variable type and kind is used
1963 in C and Fortran, the named constants shall be used which are defined in the
1964 @code{ISO_C_BINDING} intrinsic module. That module contains named constants
1965 for kind parameters and character named constants for the escape sequences
1966 in C. For a list of the constants, see @ref{ISO_C_BINDING}.
1968 @node Derived Types and struct
1969 @subsection Derived Types and struct
1971 For compatibility of derived types with @code{struct}, one needs to use
1972 the @code{BIND(C)} attribute in the type declaration. For instance, the
1973 following type declaration
1977 TYPE, BIND(C) :: myType
1978 INTEGER(C_INT) :: i1, i2
1979 INTEGER(C_SIGNED_CHAR) :: i3
1980 REAL(C_DOUBLE) :: d1
1981 COMPLEX(C_FLOAT_COMPLEX) :: c1
1982 CHARACTER(KIND=C_CHAR) :: str(5)
1986 matches the following @code{struct} declaration in C
1991 /* Note: "char" might be signed or unsigned. */
1999 Derived types with the C binding attribute shall not have the @code{sequence}
2000 attribute, type parameters, the @code{extends} attribute, nor type-bound
2001 procedures. Every component must be of interoperable type and kind and may not
2002 have the @code{pointer} or @code{allocatable} attribute. The names of the
2003 variables are irrelevant for interoperability.
2005 As there exist no direct Fortran equivalents, neither unions nor structs
2006 with bit field or variable-length array members are interoperable.
2008 @node Interoperable Global Variables
2009 @subsection Interoperable Global Variables
2011 Variables can be made accessible from C using the C binding attribute,
2012 optionally together with specifying a binding name. Those variables
2013 have to be declared in the declaration part of a @code{MODULE},
2014 be of interoperable type, and have neither the @code{pointer} nor
2015 the @code{allocatable} attribute.
2021 integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
2022 type(myType), bind(C) :: tp
2026 Here, @code{_MyProject_flags} is the case-sensitive name of the variable
2027 as seen from C programs while @code{global_flag} is the case-insensitive
2028 name as seen from Fortran. If no binding name is specified, as for
2029 @var{tp}, the C binding name is the (lowercase) Fortran binding name.
2030 If a binding name is specified, only a single variable may be after the
2031 double colon. Note of warning: You cannot use a global variable to
2032 access @var{errno} of the C library as the C standard allows it to be
2033 a macro. Use the @code{IERRNO} intrinsic (GNU extension) instead.
2035 @node Interoperable Subroutines and Functions
2036 @subsection Interoperable Subroutines and Functions
2038 Subroutines and functions have to have the @code{BIND(C)} attribute to
2039 be compatible with C. The dummy argument declaration is relatively
2040 straightforward. However, one needs to be careful because C uses
2041 call-by-value by default while Fortran behaves usually similar to
2042 call-by-reference. Furthermore, strings and pointers are handled
2043 differently. Note that only explicit size and assumed-size arrays are
2044 supported but not assumed-shape or allocatable arrays.
2046 To pass a variable by value, use the @code{VALUE} attribute.
2047 Thus the following C prototype
2050 @code{int func(int i, int *j)}
2053 matches the Fortran declaration
2056 integer(c_int) function func(i,j)
2057 use iso_c_binding, only: c_int
2058 integer(c_int), VALUE :: i
2062 Note that pointer arguments also frequently need the @code{VALUE} attribute.
2064 Strings are handled quite differently in C and Fortran. In C a string
2065 is a @code{NUL}-terminated array of characters while in Fortran each string
2066 has a length associated with it and is thus not terminated (by e.g.
2067 @code{NUL}). For example, if one wants to use the following C function,
2071 void print_C(char *string) /* equivalent: char string[] */
2073 printf("%s\n", string);
2077 to print ``Hello World'' from Fortran, one can call it using
2080 use iso_c_binding, only: C_CHAR, C_NULL_CHAR
2082 subroutine print_c(string) bind(C, name="print_C")
2083 use iso_c_binding, only: c_char
2084 character(kind=c_char) :: string(*)
2085 end subroutine print_c
2087 call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
2090 As the example shows, one needs to ensure that the
2091 string is @code{NUL} terminated. Additionally, the dummy argument
2092 @var{string} of @code{print_C} is a length-one assumed-size
2093 array; using @code{character(len=*)} is not allowed. The example
2094 above uses @code{c_char_"Hello World"} to ensure the string
2095 literal has the right type; typically the default character
2096 kind and @code{c_char} are the same and thus @code{"Hello World"}
2097 is equivalent. However, the standard does not guarantee this.
2099 The use of pointers is now illustrated using the C library
2100 function @code{strncpy}, whose prototype is
2103 char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
2106 The function @code{strncpy} copies at most @var{n} characters from
2107 string @var{s2} to @var{s1} and returns @var{s1}. In the following
2108 example, we ignore the return value:
2113 character(len=30) :: str,str2
2115 ! Ignore the return value of strncpy -> subroutine
2116 ! "restrict" is always assumed if we do not pass a pointer
2117 subroutine strncpy(dest, src, n) bind(C)
2119 character(kind=c_char), intent(out) :: dest(*)
2120 character(kind=c_char), intent(in) :: src(*)
2121 integer(c_size_t), value, intent(in) :: n
2122 end subroutine strncpy
2124 str = repeat('X',30) ! Initialize whole string with 'X'
2125 call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
2126 len(c_char_"Hello World",kind=c_size_t))
2127 print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
2131 C pointers are represented in Fortran via the special derived type
2132 @code{type(c_ptr)}, with private components. Thus one needs to
2133 use intrinsic conversion procedures to convert from or to C pointers.
2138 type(c_ptr) :: cptr1, cptr2
2139 integer, target :: array(7), scalar
2140 integer, pointer :: pa(:), ps
2141 cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
2142 ! array is contiguous if required by the C
2144 cptr2 = c_loc(scalar)
2145 call c_f_pointer(cptr2, ps)
2146 call c_f_pointer(cptr2, pa, shape=[7])
2149 When converting C to Fortran arrays, the one-dimensional @code{SHAPE} argument
2150 has to be passed. Note: A pointer argument @code{void *} matches
2151 @code{TYPE(C_PTR), VALUE} while @code{TYPE(C_PTR)} matches @code{void **}.
2153 Procedure pointers are handled analogously to pointers; the C type is
2154 @code{TYPE(C_FUNPTR)} and the intrinsic conversion procedures are
2155 @code{C_F_PROC_POINTER} and @code{C_FUNLOC}.
2157 The intrinsic procedures are described in @ref{Intrinsic Procedures}.
2159 @node Further Interoperability of Fortran with C
2160 @subsection Further Interoperability of Fortran with C
2162 Assumed-shape and allocatable arrays are passed using an array descriptor
2163 (dope vector). The internal structure of the array descriptor used
2164 by GNU Fortran is not yet documented and will change. There will also be
2165 a Technical Report (TR 29113) which standardizes an interoperable
2166 array descriptor. Until then, you can use the Chasm Language
2167 Interoperability Tools, @url{http://chasm-interop.sourceforge.net/},
2168 which provide an interface to GNU Fortran's array descriptor.
2170 The technical report 29113 will presumably also include support for
2171 C-interoperable @code{OPTIONAL} and for assumed-rank and assumed-type
2172 dummy arguments. However, the TR has neither been approved nor implemented
2173 in GNU Fortran; therefore, these features are not yet available.
2177 @node GNU Fortran Compiler Directives
2178 @section GNU Fortran Compiler Directives
2180 The Fortran standard standard describes how a conforming program shall
2181 behave; however, the exact implementation is not standardized. In order
2182 to allow the user to choose specific implementation details, compiler
2183 directives can be used to set attributes of variables and procedures
2184 which are not part of the standard. Whether a given attribute is
2185 supported and its exact effects depend on both the operating system and
2186 on the processor; see
2187 @ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)}
2190 For procedures and procedure pointers, the following attributes can
2191 be used to change the calling convention:
2194 @item @code{CDECL} -- standard C calling convention
2195 @item @code{STDCALL} -- convention where the called procedure pops the stack
2196 @item @code{FASTCALL} -- part of the arguments are passed via registers
2197 instead using the stack
2200 Besides changing the calling convention, the attributes also influence
2201 the decoration of the symbol name, e.g., by a leading underscore or by
2202 a trailing at-sign followed by the number of bytes on the stack. When
2203 assigning a procedure to a procedure pointer, both should use the same
2206 On some systems, procedures and global variables (module variables and
2207 @code{COMMON} blocks) need special handling to be accessible when they
2208 are in a shared library. The following attributes are available:
2211 @item @code{DLLEXPORT} -- provide a global pointer to a pointer in the DLL
2212 @item @code{DLLIMPORT} -- reference the function or variable using a global pointer
2215 The attributes are specified using the syntax
2217 @code{!GCC$ ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list}
2219 where in free-form source code only whitespace is allowed before @code{!GCC$}
2220 and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall
2221 start in the first column.
2223 For procedures, the compiler directives shall be placed into the body
2224 of the procedure; for variables and procedure pointers, they shall be in
2225 the same declaration part as the variable or procedure pointer.
2229 @node Non-Fortran Main Program
2230 @section Non-Fortran Main Program
2233 * _gfortran_set_args:: Save command-line arguments
2234 * _gfortran_set_options:: Set library option flags
2235 * _gfortran_set_convert:: Set endian conversion
2236 * _gfortran_set_record_marker:: Set length of record markers
2237 * _gfortran_set_max_subrecord_length:: Set subrecord length
2238 * _gfortran_set_fpe:: Set when a Floating Point Exception should be raised
2241 Even if you are doing mixed-language programming, it is very
2242 likely that you do not need to know or use the information in this
2243 section. Since it is about the internal structure of GNU Fortran,
2244 it may also change in GCC minor releases.
2246 When you compile a @code{PROGRAM} with GNU Fortran, a function
2247 with the name @code{main} (in the symbol table of the object file)
2248 is generated, which initializes the libgfortran library and then
2249 calls the actual program which uses the name @code{MAIN__}, for
2250 historic reasons. If you link GNU Fortran compiled procedures
2251 to, e.g., a C or C++ program or to a Fortran program compiled by
2252 a different compiler, the libgfortran library is not initialized
2253 and thus a few intrinsic procedures do not work properly, e.g.
2254 those for obtaining the command-line arguments.
2256 Therefore, if your @code{PROGRAM} is not compiled with
2257 GNU Fortran and the GNU Fortran compiled procedures require
2258 intrinsics relying on the library initialization, you need to
2259 initialize the library yourself. Using the default options,
2260 gfortran calls @code{_gfortran_set_args} and
2261 @code{_gfortran_set_options}. The initialization of the former
2262 is needed if the called procedures access the command line
2263 (and for backtracing); the latter sets some flags based on the
2264 standard chosen or to enable backtracing. In typical programs,
2265 it is not necessary to call any initialization function.
2267 If your @code{PROGRAM} is compiled with GNU Fortran, you shall
2268 not call any of the following functions. The libgfortran
2269 initialization functions are shown in C syntax but using C
2270 bindings they are also accessible from Fortran.
2273 @node _gfortran_set_args
2274 @subsection @code{_gfortran_set_args} --- Save command-line arguments
2275 @fnindex _gfortran_set_args
2276 @cindex libgfortran initialization, set_args
2279 @item @emph{Description}:
2280 @code{_gfortran_set_args} saves the command-line arguments; this
2281 initialization is required if any of the command-line intrinsics
2282 is called. Additionally, it shall be called if backtracing is
2283 enabled (see @code{_gfortran_set_options}).
2285 @item @emph{Syntax}:
2286 @code{void _gfortran_set_args (int argc, char *argv[])}
2288 @item @emph{Arguments}:
2289 @multitable @columnfractions .15 .70
2290 @item @var{argc} @tab number of command line argument strings
2291 @item @var{argv} @tab the command-line argument strings; argv[0]
2292 is the pathname of the executable itself.
2295 @item @emph{Example}:
2297 int main (int argc, char *argv[])
2299 /* Initialize libgfortran. */
2300 _gfortran_set_args (argc, argv);
2307 @node _gfortran_set_options
2308 @subsection @code{_gfortran_set_options} --- Set library option flags
2309 @fnindex _gfortran_set_options
2310 @cindex libgfortran initialization, set_options
2313 @item @emph{Description}:
2314 @code{_gfortran_set_options} sets several flags related to the Fortran
2315 standard to be used, whether backtracing or core dumps should be enabled
2316 and whether range checks should be performed. The syntax allows for
2317 upward compatibility since the number of passed flags is specified; for
2318 non-passed flags, the default value is used. See also
2319 @pxref{Code Gen Options}. Please note that not all flags are actually
2322 @item @emph{Syntax}:
2323 @code{void _gfortran_set_options (int num, int options[])}
2325 @item @emph{Arguments}:
2326 @multitable @columnfractions .15 .70
2327 @item @var{num} @tab number of options passed
2328 @item @var{argv} @tab The list of flag values
2331 @item @emph{option flag list}:
2332 @multitable @columnfractions .15 .70
2333 @item @var{option}[0] @tab Allowed standard; can give run-time errors
2334 if e.g. an input-output edit descriptor is invalid in a given standard.
2335 Possible values are (bitwise or-ed) @code{GFC_STD_F77} (1),
2336 @code{GFC_STD_F95_OBS} (2), @code{GFC_STD_F95_DEL} (4), @code{GFC_STD_F95}
2337 (8), @code{GFC_STD_F2003} (16), @code{GFC_STD_GNU} (32),
2338 @code{GFC_STD_LEGACY} (64), and @code{GFC_STD_F2008} (128).
2339 Default: @code{GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F2003
2340 | GFC_STD_F2008 | GFC_STD_F95 | GFC_STD_F77 | GFC_STD_GNU | GFC_STD_LEGACY}.
2341 @item @var{option}[1] @tab Standard-warning flag; prints a warning to
2342 standard error. Default: @code{GFC_STD_F95_DEL | GFC_STD_LEGACY}.
2343 @item @var{option}[2] @tab If non zero, enable pedantic checking.
2345 @item @var{option}[3] @tab If non zero, enable core dumps on run-time
2346 errors. Default: off.
2347 @item @var{option}[4] @tab If non zero, enable backtracing on run-time
2348 errors. Default: off.
2349 Note: Installs a signal handler and requires command-line
2350 initialization using @code{_gfortran_set_args}.
2351 @item @var{option}[5] @tab If non zero, supports signed zeros.
2353 @item @var{option}[6] @tab Enables run-time checking. Possible values
2354 are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2),
2355 GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (16), GFC_RTCHECK_POINTER (32).
2357 @item @var{option}[7] @tab If non zero, range checking is enabled.
2358 Default: enabled. See -frange-check (@pxref{Code Gen Options}).
2361 @item @emph{Example}:
2363 /* Use gfortran 4.5 default options. */
2364 static int options[] = @{68, 255, 0, 0, 0, 1, 0, 1@};
2365 _gfortran_set_options (8, &options);
2370 @node _gfortran_set_convert
2371 @subsection @code{_gfortran_set_convert} --- Set endian conversion
2372 @fnindex _gfortran_set_convert
2373 @cindex libgfortran initialization, set_convert
2376 @item @emph{Description}:
2377 @code{_gfortran_set_convert} set the representation of data for
2380 @item @emph{Syntax}:
2381 @code{void _gfortran_set_convert (int conv)}
2383 @item @emph{Arguments}:
2384 @multitable @columnfractions .15 .70
2385 @item @var{conv} @tab Endian conversion, possible values:
2386 GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1),
2387 GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).
2390 @item @emph{Example}:
2392 int main (int argc, char *argv[])
2394 /* Initialize libgfortran. */
2395 _gfortran_set_args (argc, argv);
2396 _gfortran_set_convert (1);
2403 @node _gfortran_set_record_marker
2404 @subsection @code{_gfortran_set_record_marker} --- Set length of record markers
2405 @fnindex _gfortran_set_record_marker
2406 @cindex libgfortran initialization, set_record_marker
2409 @item @emph{Description}:
2410 @code{_gfortran_set_record_marker} sets the length of record markers
2411 for unformatted files.
2413 @item @emph{Syntax}:
2414 @code{void _gfortran_set_record_marker (int val)}
2416 @item @emph{Arguments}:
2417 @multitable @columnfractions .15 .70
2418 @item @var{val} @tab Length of the record marker; valid values
2419 are 4 and 8. Default is 4.
2422 @item @emph{Example}:
2424 int main (int argc, char *argv[])
2426 /* Initialize libgfortran. */
2427 _gfortran_set_args (argc, argv);
2428 _gfortran_set_record_marker (8);
2435 @node _gfortran_set_fpe
2436 @subsection @code{_gfortran_set_fpe} --- Set when a Floating Point Exception should be raised
2437 @fnindex _gfortran_set_fpe
2438 @cindex libgfortran initialization, set_fpe
2441 @item @emph{Description}:
2442 @code{_gfortran_set_fpe} sets the IEEE exceptions for which a
2443 Floating Point Exception (FPE) should be raised. On most systems,
2444 this will result in a SIGFPE signal being sent and the program
2447 @item @emph{Syntax}:
2448 @code{void _gfortran_set_fpe (int val)}
2450 @item @emph{Arguments}:
2451 @multitable @columnfractions .15 .70
2452 @item @var{option}[0] @tab IEEE exceptions. Possible values are
2453 (bitwise or-ed) zero (0, default) no trapping,
2454 @code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2),
2455 @code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8),
2456 @code{GFC_FPE_UNDERFLOW} (16), and @code{GFC_FPE_PRECISION} (32).
2459 @item @emph{Example}:
2461 int main (int argc, char *argv[])
2463 /* Initialize libgfortran. */
2464 _gfortran_set_args (argc, argv);
2465 /* FPE for invalid operations such as SQRT(-1.0). */
2466 _gfortran_set_fpe (1);
2473 @node _gfortran_set_max_subrecord_length
2474 @subsection @code{_gfortran_set_max_subrecord_length} --- Set subrecord length
2475 @fnindex _gfortran_set_max_subrecord_length
2476 @cindex libgfortran initialization, set_max_subrecord_length
2479 @item @emph{Description}:
2480 @code{_gfortran_set_max_subrecord_length} set the maximum length
2481 for a subrecord. This option only makes sense for testing and
2482 debugging of unformatted I/O.
2484 @item @emph{Syntax}:
2485 @code{void _gfortran_set_max_subrecord_length (int val)}
2487 @item @emph{Arguments}:
2488 @multitable @columnfractions .15 .70
2489 @item @var{val} @tab the maximum length for a subrecord;
2490 the maximum permitted value is 2147483639, which is also
2494 @item @emph{Example}:
2496 int main (int argc, char *argv[])
2498 /* Initialize libgfortran. */
2499 _gfortran_set_args (argc, argv);
2500 _gfortran_set_max_subrecord_length (8);
2508 @c Intrinsic Procedures
2509 @c ---------------------------------------------------------------------
2511 @include intrinsic.texi
2518 @c ---------------------------------------------------------------------
2520 @c ---------------------------------------------------------------------
2523 @unnumbered Contributing
2524 @cindex Contributing
2526 Free software is only possible if people contribute to efforts
2528 We're always in need of more people helping out with ideas
2529 and comments, writing documentation and contributing code.
2531 If you want to contribute to GNU Fortran,
2532 have a look at the long lists of projects you can take on.
2533 Some of these projects are small,
2534 some of them are large;
2535 some are completely orthogonal to the rest of what is
2536 happening on GNU Fortran,
2537 but others are ``mainstream'' projects in need of enthusiastic hackers.
2538 All of these projects are important!
2539 We'll eventually get around to the things here,
2540 but they are also things doable by someone who is willing and able.
2545 * Proposed Extensions::
2550 @section Contributors to GNU Fortran
2551 @cindex Contributors
2555 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
2556 also the initiator of the whole project. Thanks Andy!
2557 Most of the interface with GCC was written by @emph{Paul Brook}.
2559 The following individuals have contributed code and/or
2560 ideas and significant help to the GNU Fortran project
2561 (in alphabetical order):
2564 @item Janne Blomqvist
2565 @item Steven Bosscher
2568 @item Fran@,{c}ois-Xavier Coudert
2572 @item Bernhard Fischer
2574 @item Richard Guenther
2575 @item Richard Henderson
2576 @item Katherine Holcomb
2578 @item Niels Kristian Bech Jensen
2579 @item Steven Johnson
2580 @item Steven G. Kargl
2588 @item Christopher D. Rickett
2589 @item Richard Sandiford
2590 @item Tobias Schl@"uter
2599 The following people have contributed bug reports,
2600 smaller or larger patches,
2601 and much needed feedback and encouragement for the
2602 GNU Fortran project:
2606 @item Dominique d'Humi@`eres
2608 @item Erik Schnetter
2609 @item Joost VandeVondele
2612 Many other individuals have helped debug,
2613 test and improve the GNU Fortran compiler over the past few years,
2614 and we welcome you to do the same!
2615 If you already have done so,
2616 and you would like to see your name listed in the
2617 list above, please contact us.
2625 @item Help build the test suite
2626 Solicit more code for donation to the test suite: the more extensive the
2627 testsuite, the smaller the risk of breaking things in the future! We can
2628 keep code private on request.
2630 @item Bug hunting/squishing
2631 Find bugs and write more test cases! Test cases are especially very
2632 welcome, because it allows us to concentrate on fixing bugs instead of
2633 isolating them. Going through the bugzilla database at
2634 @url{http://gcc.gnu.org/bugzilla/} to reduce testcases posted there and
2635 add more information (for example, for which version does the testcase
2636 work, for which versions does it fail?) is also very helpful.
2641 @node Proposed Extensions
2642 @section Proposed Extensions
2644 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
2645 order. Most of these are necessary to be fully compatible with
2646 existing Fortran compilers, but they are not part of the official
2647 J3 Fortran 95 standard.
2649 @subsection Compiler extensions:
2652 User-specified alignment rules for structures.
2655 Flag to generate @code{Makefile} info.
2658 Automatically extend single precision constants to double.
2661 Compile code that conserves memory by dynamically allocating common and
2662 module storage either on stack or heap.
2665 Compile flag to generate code for array conformance checking (suggest -CC).
2668 User control of symbol names (underscores, etc).
2671 Compile setting for maximum size of stack frame size before spilling
2672 parts to static or heap.
2675 Flag to force local variables into static space.
2678 Flag to force local variables onto stack.
2682 @subsection Environment Options
2685 Pluggable library modules for random numbers, linear algebra.
2686 LA should use BLAS calling conventions.
2689 Environment variables controlling actions on arithmetic exceptions like
2690 overflow, underflow, precision loss---Generate NaN, abort, default.
2694 Set precision for fp units that support it (i387).
2697 Variable for setting fp rounding mode.
2700 Variable to fill uninitialized variables with a user-defined bit
2704 Environment variable controlling filename that is opened for that unit
2708 Environment variable to clear/trash memory being freed.
2711 Environment variable to control tracing of allocations and frees.
2714 Environment variable to display allocated memory at normal program end.
2717 Environment variable for filename for * IO-unit.
2720 Environment variable for temporary file directory.
2723 Environment variable forcing standard output to be line buffered (unix).
2728 @c ---------------------------------------------------------------------
2729 @c GNU General Public License
2730 @c ---------------------------------------------------------------------
2732 @include gpl_v3.texi
2736 @c ---------------------------------------------------------------------
2737 @c GNU Free Documentation License
2738 @c ---------------------------------------------------------------------
2744 @c ---------------------------------------------------------------------
2745 @c Funding Free Software
2746 @c ---------------------------------------------------------------------
2748 @include funding.texi
2750 @c ---------------------------------------------------------------------
2752 @c ---------------------------------------------------------------------
2755 @unnumbered Option Index
2756 @command{gfortran}'s command line options are indexed here without any
2757 initial @samp{-} or @samp{--}. Where an option has both positive and
2758 negative forms (such as -foption and -fno-option), relevant entries in
2759 the manual are indexed under the most appropriate form; it may sometimes
2760 be useful to look up both forms.
2764 @unnumbered Keyword Index