1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999-2008
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
70 @c margin and the text on left hand pages is pushed toward the left
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 1.2 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 * Extensions:: Language extensions implemented by GNU Fortran.
185 * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
186 * Intrinsic Modules:: Intrinsic modules supported by GNU Fortran.
188 * Contributing:: How you can help.
189 * Copying:: GNU General Public License says
190 how you can copy and share GNU Fortran.
191 * GNU Free Documentation License::
192 How you can copy and share this manual.
193 * Funding:: How to help assure continued work for free software.
194 * Option Index:: Index of command line options
195 * Keyword Index:: Index of concepts
199 @c ---------------------------------------------------------------------
201 @c ---------------------------------------------------------------------
204 @chapter Introduction
206 @c The following duplicates the text on the TexInfo table of contents.
208 This manual documents the use of @command{gfortran}, the GNU Fortran
209 compiler. You can find in this manual how to invoke @command{gfortran},
210 as well as its features and incompatibilities.
213 @emph{Warning:} This document, and the compiler it describes, are still
214 under development. While efforts are made to keep it up-to-date, it
215 might not accurately reflect the status of the most recent GNU Fortran
220 The GNU Fortran compiler front end was
221 designed initially as a free replacement for,
222 or alternative to, the unix @command{f95} command;
223 @command{gfortran} is the command you'll use to invoke the compiler.
226 * About GNU Fortran:: What you should know about the GNU Fortran compiler.
227 * GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
228 * Preprocessing and conditional compilation:: The Fortran preprocessor
229 * GNU Fortran and G77:: Why we chose to start from scratch.
230 * Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
231 * Standards:: Standards supported by GNU Fortran.
235 @c ---------------------------------------------------------------------
237 @c ---------------------------------------------------------------------
239 @node About GNU Fortran
240 @section About GNU Fortran
242 The GNU Fortran compiler is still in an early state of development.
243 It can generate code for most constructs and expressions,
244 but much work remains to be done.
246 When the GNU Fortran compiler is finished,
247 it will do everything you expect from any decent compiler:
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 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 When writing Fortran, it is easy to make big mistakes.
273 The Fortran 90 requires that the compiler can point out
274 mistakes to the user.
275 An incorrect usage of the language causes an @dfn{error message}.
277 The compiler will also attempt to diagnose cases where the
278 user's program contains a correct usage of the language,
279 but instructs the computer to do something questionable.
280 This kind of diagnostics message is called a @dfn{warning message}.
283 Provide optional information about the translation passes
284 from the source code to machine code.
285 This can help a user of the compiler to find the cause of
286 certain bugs which may not be obvious in the source code,
287 but may be more easily found at a lower level compiler output.
288 It also helps developers to find bugs in the compiler itself.
291 Provide information in the generated machine code that can
292 make it easier to find bugs in the program (using a debugging tool,
293 called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
296 Locate and gather machine code already generated to
297 perform actions requested by statements in the user's program.
298 This machine code is organized into @dfn{modules} and is located
299 and @dfn{linked} to the user program.
302 The GNU Fortran compiler consists of several components:
306 A version of the @command{gcc} command
307 (which also might be installed as the system's @command{cc} command)
308 that also understands and accepts Fortran source code.
309 The @command{gcc} command is the @dfn{driver} program for
310 all the languages in the GNU Compiler Collection (GCC);
312 you can compile the source code of any language for
313 which a front end is available in GCC.
316 The @command{gfortran} command itself,
317 which also might be installed as the
318 system's @command{f95} command.
319 @command{gfortran} is just another driver program,
320 but specifically for the Fortran compiler only.
321 The difference with @command{gcc} is that @command{gfortran}
322 will automatically link the correct libraries to your program.
325 A collection of run-time libraries.
326 These libraries contain the machine code needed to support
327 capabilities of the Fortran language that are not directly
328 provided by the machine code generated by the
329 @command{gfortran} compilation phase,
330 such as intrinsic functions and subroutines,
331 and routines for interaction with files and the operating system.
332 @c and mechanisms to spawn,
333 @c unleash and pause threads in parallelized code.
336 The Fortran compiler itself, (@command{f951}).
337 This is the GNU Fortran parser and code generator,
338 linked to and interfaced with the GCC backend library.
339 @command{f951} ``translates'' the source code to
340 assembler code. You would typically not use this
342 instead, the @command{gcc} or @command{gfortran} driver
343 programs will call it for you.
347 @c ---------------------------------------------------------------------
348 @c GNU Fortran and GCC
349 @c ---------------------------------------------------------------------
351 @node GNU Fortran and GCC
352 @section GNU Fortran and GCC
353 @cindex GNU Compiler Collection
356 GNU Fortran is a part of GCC, the @dfn{GNU Compiler Collection}. GCC
357 consists of a collection of front ends for various languages, which
358 translate the source code into a language-independent form called
359 @dfn{GENERIC}. This is then processed by a common middle end which
360 provides optimization, and then passed to one of a collection of back
361 ends which generate code for different computer architectures and
364 Functionally, this is implemented with a driver program (@command{gcc})
365 which provides the command-line interface for the compiler. It calls
366 the relevant compiler front-end program (e.g., @command{f951} for
367 Fortran) for each file in the source code, and then calls the assembler
368 and linker as appropriate to produce the compiled output. In a copy of
369 GCC which has been compiled with Fortran language support enabled,
370 @command{gcc} will recognize files with @file{.f}, @file{.for}, @file{.ftn},
371 @file{.f90}, @file{.f95}, @file{.f03} and @file{.f08} extensions as
372 Fortran source code, and compile it accordingly. A @command{gfortran}
373 driver program is also provided, which is identical to @command{gcc}
374 except that it automatically links the Fortran runtime libraries into the
377 Source files with @file{.f}, @file{.for}, @file{.fpp}, @file{.ftn}, @file{.F},
378 @file{.FOR}, @file{.FPP}, and @file{.FTN} extensions are treated as fixed form.
379 Source files with @file{.f90}, @file{.f95}, @file{.f03}, @file{.f08},
380 @file{.F90}, @file{.F95}, @file{.F03} and @file{.F08} extensions are
381 treated as free form. The capitalized versions of either form are run
382 through preprocessing. Source files with the lower case @file{.fpp}
383 extension are also run through preprocessing.
385 This manual specifically documents the Fortran front end, which handles
386 the programming language's syntax and semantics. The aspects of GCC
387 which relate to the optimization passes and the back-end code generation
388 are documented in the GCC manual; see
389 @ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
390 The two manuals together provide a complete reference for the GNU
394 @c ---------------------------------------------------------------------
395 @c Preprocessing and conditional compilation
396 @c ---------------------------------------------------------------------
398 @node Preprocessing and conditional compilation
399 @section Preprocessing and conditional compilation
402 @cindex Conditional compilation
403 @cindex Preprocessing
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 the GNU Fortran invoked the preprocessor, @code{__GFORTRAN__}
417 is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
418 @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
419 compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details.
421 While CPP is the de-facto standard for preprocessing Fortran code,
422 Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines
423 Conditional Compilation, which is not widely used and not directly
424 supported by the GNU Fortran compiler. You can use the program coco
425 to preprocess such files (@uref{http://users.erols.com/dnagle/coco.html}).
428 @c ---------------------------------------------------------------------
429 @c GNU Fortran and G77
430 @c ---------------------------------------------------------------------
432 @node GNU Fortran and G77
433 @section GNU Fortran and G77
435 @cindex @command{g77}
437 The GNU Fortran compiler is the successor to @command{g77}, the Fortran
438 77 front end included in GCC prior to version 4. It is an entirely new
439 program that has been designed to provide Fortran 95 support and
440 extensibility for future Fortran language standards, as well as providing
441 backwards compatibility for Fortran 77 and nearly all of the GNU language
442 extensions supported by @command{g77}.
445 @c ---------------------------------------------------------------------
447 @c ---------------------------------------------------------------------
450 @section Project Status
453 As soon as @command{gfortran} can parse all of the statements correctly,
454 it will be in the ``larva'' state.
455 When we generate code, the ``puppa'' state.
456 When @command{gfortran} is done,
457 we'll see if it will be a beautiful butterfly,
458 or just a big bug....
460 --Andy Vaught, April 2000
463 The start of the GNU Fortran 95 project was announced on
464 the GCC homepage in March 18, 2000
465 (even though Andy had already been working on it for a while,
468 The GNU Fortran compiler is able to compile nearly all
469 standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs,
470 including a number of standard and non-standard extensions, and can be
471 used on real-world programs. In particular, the supported extensions
472 include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran
473 2008 features such as enumeration, stream I/O, and some of the
474 enhancements to allocatable array support from TR 15581. However, it is
475 still under development and has a few remaining rough edges.
477 At present, the GNU Fortran compiler passes the
478 @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
479 NIST Fortran 77 Test Suite}, and produces acceptable results on the
480 @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}.
481 It also provides respectable performance on
482 the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran
483 compiler benchmarks} and the
484 @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html,
485 Livermore Fortran Kernels test}. It has been used to compile a number of
486 large real-world programs, including
487 @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM
488 weather-forecasting code} and
489 @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum
490 chemistry package}; see @url{http://gcc.gnu.org/wiki/GfortranApps} for an
493 Among other things, the GNU Fortran compiler is intended as a replacement
494 for G77. At this point, nearly all programs that could be compiled with
495 G77 can be compiled with GNU Fortran, although there are a few minor known
498 The primary work remaining to be done on GNU Fortran falls into three
499 categories: bug fixing (primarily regarding the treatment of invalid code
500 and providing useful error messages), improving the compiler optimizations
501 and the performance of compiled code, and extending the compiler to support
502 future standards---in particular, Fortran 2003.
505 @c ---------------------------------------------------------------------
507 @c ---------------------------------------------------------------------
513 The GNU Fortran compiler implements
514 ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
515 standard-compliant Fortran 90 and Fortran 77 programs. It also supports
516 the ISO/IEC TR-15581 enhancements to allocatable arrays, and
517 the @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
518 OpenMP Application Program Interface v2.5} specification.
520 In the future, the GNU Fortran compiler will also support ISO/IEC
521 1539-1:2004 (Fortran 2003) and future Fortran standards. Partial support
522 of that standard is already provided; the current status of Fortran 2003
523 support is reported in the @ref{Fortran 2003 status} section of the
526 The next version of the Fortran standard after Fortran 2003 is currently
527 being developed and the GNU Fortran compiler supports some of its new
528 features. This support is based on the latest draft of the standard
529 (available from @url{http://www.nag.co.uk/sc22wg5/}) and no guarantee of
530 future compatibility is made, as the final standard might differ from the
531 draft. For more information, see the @ref{Fortran 2008 status} section.
534 @c =====================================================================
535 @c PART I: INVOCATION REFERENCE
536 @c =====================================================================
539 \part{I}{Invoking GNU Fortran}
542 @c ---------------------------------------------------------------------
544 @c ---------------------------------------------------------------------
549 @c ---------------------------------------------------------------------
551 @c ---------------------------------------------------------------------
554 @chapter Runtime: Influencing runtime behavior with environment variables
555 @cindex environment variable
557 The behavior of the @command{gfortran} can be influenced by
558 environment variables.
560 Malformed environment variables are silently ignored.
563 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
564 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
565 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
566 * GFORTRAN_USE_STDERR:: Send library output to standard error
567 * GFORTRAN_TMPDIR:: Directory for scratch files
568 * GFORTRAN_UNBUFFERED_ALL:: Don't buffer I/O for all units.
569 * GFORTRAN_UNBUFFERED_PRECONNECTED:: Don't buffer I/O for preconnected units.
570 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
571 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
572 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
573 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
574 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
575 * GFORTRAN_ERROR_DUMPCORE:: Dump core on run-time errors
576 * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
579 @node GFORTRAN_STDIN_UNIT
580 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
582 This environment variable can be used to select the unit number
583 preconnected to standard input. This must be a positive integer.
584 The default value is 5.
586 @node GFORTRAN_STDOUT_UNIT
587 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
589 This environment variable can be used to select the unit number
590 preconnected to standard output. This must be a positive integer.
591 The default value is 6.
593 @node GFORTRAN_STDERR_UNIT
594 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
596 This environment variable can be used to select the unit number
597 preconnected to standard error. This must be a positive integer.
598 The default value is 0.
600 @node GFORTRAN_USE_STDERR
601 @section @env{GFORTRAN_USE_STDERR}---Send library output to standard error
603 This environment variable controls where library output is sent.
604 If the first letter is @samp{y}, @samp{Y} or @samp{1}, standard
605 error is used. If the first letter is @samp{n}, @samp{N} or
606 @samp{0}, standard output is used.
608 @node GFORTRAN_TMPDIR
609 @section @env{GFORTRAN_TMPDIR}---Directory for scratch files
611 This environment variable controls where scratch files are
612 created. If this environment variable is missing,
613 GNU Fortran searches for the environment variable @env{TMP}. If
614 this is also missing, the default is @file{/tmp}.
616 @node GFORTRAN_UNBUFFERED_ALL
617 @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer I/O on all units
619 This environment variable controls whether all I/O is unbuffered. If
620 the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
621 unbuffered. This will slow down small sequential reads and writes. If
622 the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
625 @node GFORTRAN_UNBUFFERED_PRECONNECTED
626 @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Don't buffer I/O on preconnected units
628 The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
629 whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If
630 the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This
631 will slow down small sequential reads and writes. If the first letter
632 is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default.
634 @node GFORTRAN_SHOW_LOCUS
635 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
637 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
638 line numbers for runtime errors are printed. If the first letter is
639 @samp{n}, @samp{N} or @samp{0}, don't print filename and line numbers
640 for runtime errors. The default is to print the location.
642 @node GFORTRAN_OPTIONAL_PLUS
643 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
645 If the first letter is @samp{y}, @samp{Y} or @samp{1},
646 a plus sign is printed
647 where permitted by the Fortran standard. If the first letter
648 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
649 in most cases. Default is not to print plus signs.
651 @node GFORTRAN_DEFAULT_RECL
652 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
654 This environment variable specifies the default record length, in
655 bytes, for files which are opened without a @code{RECL} tag in the
656 @code{OPEN} statement. This must be a positive integer. The
657 default value is 1073741824 bytes (1 GB).
659 @node GFORTRAN_LIST_SEPARATOR
660 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
662 This environment variable specifies the separator when writing
663 list-directed output. It may contain any number of spaces and
664 at most one comma. If you specify this on the command line,
665 be sure to quote spaces, as in
667 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
669 when @command{a.out} is the compiled Fortran program that you want to run.
670 Default is a single space.
672 @node GFORTRAN_CONVERT_UNIT
673 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
675 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
676 to change the representation of data for unformatted files.
677 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
679 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
680 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
681 exception: mode ':' unit_list | unit_list ;
682 unit_list: unit_spec | unit_list unit_spec ;
683 unit_spec: INTEGER | INTEGER '-' INTEGER ;
685 The variable consists of an optional default mode, followed by
686 a list of optional exceptions, which are separated by semicolons
687 from the preceding default and each other. Each exception consists
688 of a format and a comma-separated list of units. Valid values for
689 the modes are the same as for the @code{CONVERT} specifier:
692 @item @code{NATIVE} Use the native format. This is the default.
693 @item @code{SWAP} Swap between little- and big-endian.
694 @item @code{LITTLE_ENDIAN} Use the little-endian format
695 for unformatted files.
696 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
698 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
699 Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
701 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
702 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
703 in little_endian mode, except for units 10 to 20 and 25, which are in
705 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
708 Setting the environment variables should be done on the command
709 line or via the @command{export}
710 command for @command{sh}-compatible shells and via @command{setenv}
711 for @command{csh}-compatible shells.
713 Example for @command{sh}:
716 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
719 Example code for @command{csh}:
722 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
726 Using anything but the native representation for unformatted data
727 carries a significant speed overhead. If speed in this area matters
728 to you, it is best if you use this only for data that needs to be
731 @xref{CONVERT specifier}, for an alternative way to specify the
732 data representation for unformatted files. @xref{Runtime Options}, for
733 setting a default data representation for the whole program. The
734 @code{CONVERT} specifier overrides the @option{-fconvert} compile options.
736 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
737 environment variable will override the CONVERT specifier in the
738 open statement}. This is to give control over data formats to
739 users who do not have the source code of their program available.
741 @node GFORTRAN_ERROR_DUMPCORE
742 @section @env{GFORTRAN_ERROR_DUMPCORE}---Dump core on run-time errors
744 If the @env{GFORTRAN_ERROR_DUMPCORE} variable is set to
745 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
746 then library run-time errors cause core dumps. To disable the core
747 dumps, set the variable to @samp{n}, @samp{N}, @samp{0}. Default
748 is not to core dump unless the @option{-fdump-core} compile option
751 @node GFORTRAN_ERROR_BACKTRACE
752 @section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors
754 If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to
755 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
756 then a backtrace is printed when a run-time error occurs.
757 To disable the backtracing, set the variable to
758 @samp{n}, @samp{N}, @samp{0}. Default is not to print a backtrace
759 unless the @option{-fbacktrace} compile option
762 @c =====================================================================
763 @c PART II: LANGUAGE REFERENCE
764 @c =====================================================================
767 \part{II}{Language Reference}
770 @c ---------------------------------------------------------------------
771 @c Fortran 2003 and 2008 Status
772 @c ---------------------------------------------------------------------
774 @node Fortran 2003 and 2008 status
775 @chapter Fortran 2003 and 2008 Status
778 * Fortran 2003 status::
779 * Fortran 2008 status::
782 @node Fortran 2003 status
783 @section Fortran 2003 status
785 Although GNU Fortran focuses on implementing the Fortran 95
786 standard for the time being, a few Fortran 2003 features are currently
791 Intrinsics @code{command_argument_count}, @code{get_command},
792 @code{get_command_argument}, @code{get_environment_variable}, and
796 @cindex array, constructors
798 Array constructors using square brackets. That is, @code{[...]} rather
802 @cindex @code{FLUSH} statement
803 @cindex statement, @code{FLUSH}
804 @code{FLUSH} statement.
807 @cindex @code{IOMSG=} specifier
808 @code{IOMSG=} specifier for I/O statements.
811 @cindex @code{ENUM} statement
812 @cindex @code{ENUMERATOR} statement
813 @cindex statement, @code{ENUM}
814 @cindex statement, @code{ENUMERATOR}
815 @opindex @code{fshort-enums}
816 Support for the declaration of enumeration constants via the
817 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
818 @command{gcc} is guaranteed also for the case where the
819 @command{-fshort-enums} command line option is given.
826 @cindex @code{ALLOCATABLE} dummy arguments
827 @code{ALLOCATABLE} dummy arguments.
829 @cindex @code{ALLOCATABLE} function results
830 @code{ALLOCATABLE} function results
832 @cindex @code{ALLOCATABLE} components of derived types
833 @code{ALLOCATABLE} components of derived types
837 @cindex @code{STREAM} I/O
838 @cindex @code{ACCESS='STREAM'} I/O
839 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
840 allowing I/O without any record structure.
843 Namelist input/output for internal files.
846 @cindex @code{PROTECTED} statement
847 @cindex statement, @code{PROTECTED}
848 The @code{PROTECTED} statement and attribute.
851 @cindex @code{VALUE} statement
852 @cindex statement, @code{VALUE}
853 The @code{VALUE} statement and attribute.
856 @cindex @code{VOLATILE} statement
857 @cindex statement, @code{VOLATILE}
858 The @code{VOLATILE} statement and attribute.
861 @cindex @code{IMPORT} statement
862 @cindex statement, @code{IMPORT}
863 The @code{IMPORT} statement, allowing to import
864 host-associated derived types.
867 @cindex @code{USE, INTRINSIC} statement
868 @cindex statement, @code{USE, INTRINSIC}
869 @cindex @code{ISO_FORTRAN_ENV} statement
870 @cindex statement, @code{ISO_FORTRAN_ENV}
871 @code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
872 attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
873 @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
876 Renaming of operators in the @code{USE} statement.
879 @cindex ISO C Bindings
880 Interoperability with C (ISO C Bindings)
883 BOZ as argument of INT, REAL, DBLE and CMPLX.
888 @node Fortran 2008 status
889 @section Fortran 2008 status
891 The next version of the Fortran standard after Fortran 2003 is currently
892 being worked on by the Working Group 5 of Sub-Committee 22 of the Joint
893 Technical Committee 1 of the International Organization for
894 Standardization (ISO) and the International Electrotechnical Commission
895 (IEC). This group is known at @uref{http://www.nag.co.uk/sc22wg5/, WG5}.
896 The next revision of the Fortran standard is informally referred to as
897 Fortran 2008, reflecting its planned release year. The GNU Fortran
898 compiler has support for some of the new features in Fortran 2008. This
899 support is based on the latest draft, available from
900 @url{http://www.nag.co.uk/sc22wg5/}. However, as the final standard may
901 differ from the drafts, no guarantee of backward compatibility can be
902 made and you should only use it for experimental purposes.
904 @c ---------------------------------------------------------------------
906 @c ---------------------------------------------------------------------
908 @c Maybe this chapter should be merged with the 'Standards' section,
909 @c whenever that is written :-)
915 The two sections below detail the extensions to standard Fortran that are
916 implemented in GNU Fortran, as well as some of the popular or
917 historically important extensions that are not (or not yet) implemented.
918 For the latter case, we explain the alternatives available to GNU Fortran
919 users, including replacement by standard-conforming code or GNU
923 * Extensions implemented in GNU Fortran::
924 * Extensions not implemented in GNU Fortran::
928 @node Extensions implemented in GNU Fortran
929 @section Extensions implemented in GNU Fortran
930 @cindex extensions, implemented
932 GNU Fortran implements a number of extensions over standard
933 Fortran. This chapter contains information on their syntax and
934 meaning. There are currently two categories of GNU Fortran
935 extensions, those that provide functionality beyond that provided
936 by any standard, and those that are supported by GNU Fortran
937 purely for backward compatibility with legacy compilers. By default,
938 @option{-std=gnu} allows the compiler to accept both types of
939 extensions, but to warn about the use of the latter. Specifying
940 either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
941 disables both types of extensions, and @option{-std=legacy} allows both
945 * Old-style kind specifications::
946 * Old-style variable initialization::
947 * Extensions to namelist::
948 * X format descriptor without count field::
949 * Commas in FORMAT specifications::
950 * Missing period in FORMAT specifications::
952 * BOZ literal constants::
953 * Real array indices::
955 * Implicitly convert LOGICAL and INTEGER values::
956 * Hollerith constants support::
958 * CONVERT specifier::
960 * Argument list functions::
963 @node Old-style kind specifications
964 @subsection Old-style kind specifications
965 @cindex kind, old-style
967 GNU Fortran allows old-style kind specifications in declarations. These
973 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
974 etc.), and where @code{size} is a byte count corresponding to the
975 storage size of a valid kind for that type. (For @code{COMPLEX}
976 variables, @code{size} is the total size of the real and imaginary
977 parts.) The statement then declares @code{x}, @code{y} and @code{z} to
978 be of type @code{TYPESPEC} with the appropriate kind. This is
979 equivalent to the standard-conforming declaration
984 where @code{k} is equal to @code{size} for most types, but is equal to
985 @code{size/2} for the @code{COMPLEX} type.
987 @node Old-style variable initialization
988 @subsection Old-style variable initialization
990 GNU Fortran allows old-style initialization of variables of the
994 REAL x(2,2) /3*0.,1./
996 The syntax for the initializers is as for the @code{DATA} statement, but
997 unlike in a @code{DATA} statement, an initializer only applies to the
998 variable immediately preceding the initialization. In other words,
999 something like @code{INTEGER I,J/2,3/} is not valid. This style of
1000 initialization is only allowed in declarations without double colons
1001 (@code{::}); the double colons were introduced in Fortran 90, which also
1002 introduced a standard syntax for initializing variables in type
1005 Examples of standard-conforming code equivalent to the above example
1009 INTEGER :: i = 1, j = 2
1010 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
1014 DATA i/1/, j/2/, x/3*0.,1./
1017 Note that variables which are explicitly initialized in declarations
1018 or in @code{DATA} statements automatically acquire the @code{SAVE}
1021 @node Extensions to namelist
1022 @subsection Extensions to namelist
1025 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
1026 including array qualifiers, substrings and fully qualified derived types.
1027 The output from a namelist write is compatible with namelist read. The
1028 output has all names in upper case and indentation to column 1 after the
1029 namelist name. Two extensions are permitted:
1031 Old-style use of @samp{$} instead of @samp{&}
1034 X(:)%Y(2) = 1.0 2.0 3.0
1039 It should be noted that the default terminator is @samp{/} rather than
1042 Querying of the namelist when inputting from stdin. After at least
1043 one space, entering @samp{?} sends to stdout the namelist name and the names of
1044 the variables in the namelist:
1055 Entering @samp{=?} outputs the namelist to stdout, as if
1056 @code{WRITE(*,NML = mynml)} had been called:
1061 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
1062 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
1063 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
1067 To aid this dialog, when input is from stdin, errors send their
1068 messages to stderr and execution continues, even if @code{IOSTAT} is set.
1070 @code{PRINT} namelist is permitted. This causes an error if
1071 @option{-std=f95} is used.
1074 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
1077 END PROGRAM test_print
1080 Expanded namelist reads are permitted. This causes an error if
1081 @option{-std=f95} is used. In the following example, the first element
1082 of the array will be given the value 0.00 and the two succeeding
1083 elements will be given the values 1.00 and 2.00.
1086 X(1,1) = 0.00 , 1.00 , 2.00
1090 @node X format descriptor without count field
1091 @subsection @code{X} format descriptor without count field
1093 To support legacy codes, GNU Fortran permits the count field of the
1094 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
1095 When omitted, the count is implicitly assumed to be one.
1099 10 FORMAT (I1, X, I1)
1102 @node Commas in FORMAT specifications
1103 @subsection Commas in @code{FORMAT} specifications
1105 To support legacy codes, GNU Fortran allows the comma separator
1106 to be omitted immediately before and after character string edit
1107 descriptors in @code{FORMAT} statements.
1111 10 FORMAT ('FOO='I1' BAR='I2)
1115 @node Missing period in FORMAT specifications
1116 @subsection Missing period in @code{FORMAT} specifications
1118 To support legacy codes, GNU Fortran allows missing periods in format
1119 specifications if and only if @option{-std=legacy} is given on the
1120 command line. This is considered non-conforming code and is
1129 @node I/O item lists
1130 @subsection I/O item lists
1131 @cindex I/O item lists
1133 To support legacy codes, GNU Fortran allows the input item list
1134 of the @code{READ} statement, and the output item lists of the
1135 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1137 @node BOZ literal constants
1138 @subsection BOZ literal constants
1139 @cindex BOZ literal constants
1141 Besides decimal constants, Fortran also supports binary (@code{b}),
1142 octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
1143 syntax is: @samp{prefix quote digits quote}, were the prefix is
1144 either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
1145 @code{"} and the digits are for binary @code{0} or @code{1}, for
1146 octal between @code{0} and @code{7}, and for hexadecimal between
1147 @code{0} and @code{F}. (Example: @code{b'01011101'}.)
1149 Up to Fortran 95, BOZ literals were only allowed to initialize
1150 integer variables in DATA statements. Since Fortran 2003 BOZ literals
1151 are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
1152 and @code{CMPLX}; the result is the same as if the integer BOZ
1153 literal had been converted by @code{TRANSFER} to, respectively,
1154 @code{real}, @code{double precision}, @code{integer} or @code{complex}.
1155 As GNU Fortran extension the intrinsic procedures @code{FLOAT},
1156 @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
1158 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1159 be specified using the @code{X} prefix, in addition to the standard
1160 @code{Z} prefix. The BOZ literal can also be specified by adding a
1161 suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
1164 Furthermore, GNU Fortran allows using BOZ literal constants outside
1165 DATA statements and the four intrinsic functions allowed by Fortran 2003.
1166 In DATA statements, in direct assignments, where the right-hand side
1167 only contains a BOZ literal constant, and for old-style initializers of
1168 the form @code{integer i /o'0173'/}, the constant is transferred
1169 as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
1170 the real part is initialized unless @code{CMPLX} is used. In all other
1171 cases, the BOZ literal constant is converted to an @code{INTEGER} value with
1172 the largest decimal representation. This value is then converted
1173 numerically to the type and kind of the variable in question.
1174 (For instance @code{real :: r = b'0000001' + 1} initializes @code{r}
1175 with @code{2.0}.) As different compilers implement the extension
1176 differently, one should be careful when doing bitwise initialization
1177 of non-integer variables.
1179 Note that initializing an @code{INTEGER} variable with a statement such
1180 as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
1181 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1182 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1183 option can be used as a workaround for legacy code that initializes
1184 integers in this manner.
1186 @node Real array indices
1187 @subsection Real array indices
1188 @cindex array, indices of type real
1190 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1191 or variables as array indices.
1193 @node Unary operators
1194 @subsection Unary operators
1195 @cindex operators, unary
1197 As an extension, GNU Fortran allows unary plus and unary minus operators
1198 to appear as the second operand of binary arithmetic operators without
1199 the need for parenthesis.
1205 @node Implicitly convert LOGICAL and INTEGER values
1206 @subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1207 @cindex conversion, to integer
1208 @cindex conversion, to logical
1210 As an extension for backwards compatibility with other compilers, GNU
1211 Fortran allows the implicit conversion of @code{LOGICAL} values to
1212 @code{INTEGER} values and vice versa. When converting from a
1213 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1214 zero, and @code{.TRUE.} is interpreted as one. When converting from
1215 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1216 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1227 However, there is no implicit conversion of @code{INTEGER} values in
1228 @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
1231 @node Hollerith constants support
1232 @subsection Hollerith constants support
1233 @cindex Hollerith constants
1235 GNU Fortran supports Hollerith constants in assignments, function
1236 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1237 constant is written as a string of characters preceded by an integer
1238 constant indicating the character count, and the letter @code{H} or
1239 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1240 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1241 constant will be padded or truncated to fit the size of the variable in
1244 Examples of valid uses of Hollerith constants:
1247 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1248 x(1) = 16HABCDEFGHIJKLMNOP
1252 Invalid Hollerith constants examples:
1255 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1256 a = 0H ! At least one character is needed.
1259 In general, Hollerith constants were used to provide a rudimentary
1260 facility for handling character strings in early Fortran compilers,
1261 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1262 in those cases, the standard-compliant equivalent is to convert the
1263 program to use proper character strings. On occasion, there may be a
1264 case where the intent is specifically to initialize a numeric variable
1265 with a given byte sequence. In these cases, the same result can be
1266 obtained by using the @code{TRANSFER} statement, as in this example.
1268 INTEGER(KIND=4) :: a
1269 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1274 @subsection Cray pointers
1275 @cindex pointer, Cray
1277 Cray pointers are part of a non-standard extension that provides a
1278 C-like pointer in Fortran. This is accomplished through a pair of
1279 variables: an integer "pointer" that holds a memory address, and a
1280 "pointee" that is used to dereference the pointer.
1282 Pointer/pointee pairs are declared in statements of the form:
1284 pointer ( <pointer> , <pointee> )
1288 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1290 The pointer is an integer that is intended to hold a memory address.
1291 The pointee may be an array or scalar. A pointee can be an assumed
1292 size array---that is, the last dimension may be left unspecified by
1293 using a @code{*} in place of a value---but a pointee cannot be an
1294 assumed shape array. No space is allocated for the pointee.
1296 The pointee may have its type declared before or after the pointer
1297 statement, and its array specification (if any) may be declared
1298 before, during, or after the pointer statement. The pointer may be
1299 declared as an integer prior to the pointer statement. However, some
1300 machines have default integer sizes that are different than the size
1301 of a pointer, and so the following code is not portable:
1306 If a pointer is declared with a kind that is too small, the compiler
1307 will issue a warning; the resulting binary will probably not work
1308 correctly, because the memory addresses stored in the pointers may be
1309 truncated. It is safer to omit the first line of the above example;
1310 if explicit declaration of ipt's type is omitted, then the compiler
1311 will ensure that ipt is an integer variable large enough to hold a
1314 Pointer arithmetic is valid with Cray pointers, but it is not the same
1315 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1316 the user is responsible for determining how many bytes to add to a
1317 pointer in order to increment it. Consider the following example:
1321 pointer (ipt, pointee)
1325 The last statement does not set @code{ipt} to the address of
1326 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1327 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1329 Any expression involving the pointee will be translated to use the
1330 value stored in the pointer as the base address.
1332 To get the address of elements, this extension provides an intrinsic
1333 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1334 @code{&} operator in C, except the address is cast to an integer type:
1337 pointer(ipt, arpte(10))
1339 ipt = loc(ar) ! Makes arpte is an alias for ar
1340 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1342 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1345 Cray pointees often are used to alias an existing variable. For
1353 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1354 @code{target}. The optimizer, however, will not detect this aliasing, so
1355 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1356 a pointee in any way that violates the Fortran aliasing rules or
1357 assumptions is illegal. It is the user's responsibility to avoid doing
1358 this; the compiler works under the assumption that no such aliasing
1361 Cray pointers will work correctly when there is no aliasing (i.e., when
1362 they are used to access a dynamically allocated block of memory), and
1363 also in any routine where a pointee is used, but any variable with which
1364 it shares storage is not used. Code that violates these rules may not
1365 run as the user intends. This is not a bug in the optimizer; any code
1366 that violates the aliasing rules is illegal. (Note that this is not
1367 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1368 will ``incorrectly'' optimize code with illegal aliasing.)
1370 There are a number of restrictions on the attributes that can be applied
1371 to Cray pointers and pointees. Pointees may not have the
1372 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1373 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1374 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1375 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes.
1376 Pointees may not occur in more than one pointer statement. A pointee
1377 cannot be a pointer. Pointees cannot occur in equivalence, common, or
1380 A Cray pointer may also point to a function or a subroutine. For
1381 example, the following excerpt is valid:
1385 pointer (subptr,subpte)
1395 A pointer may be modified during the course of a program, and this
1396 will change the location to which the pointee refers. However, when
1397 pointees are passed as arguments, they are treated as ordinary
1398 variables in the invoked function. Subsequent changes to the pointer
1399 will not change the base address of the array that was passed.
1401 @node CONVERT specifier
1402 @subsection @code{CONVERT} specifier
1403 @cindex @code{CONVERT} specifier
1405 GNU Fortran allows the conversion of unformatted data between little-
1406 and big-endian representation to facilitate moving of data
1407 between different systems. The conversion can be indicated with
1408 the @code{CONVERT} specifier on the @code{OPEN} statement.
1409 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1410 the data format via an environment variable.
1412 Valid values for @code{CONVERT} are:
1414 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1415 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1416 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1417 for unformatted files.
1418 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1422 Using the option could look like this:
1424 open(file='big.dat',form='unformatted',access='sequential', &
1425 convert='big_endian')
1428 The value of the conversion can be queried by using
1429 @code{INQUIRE(CONVERT=ch)}. The values returned are
1430 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1432 @code{CONVERT} works between big- and little-endian for
1433 @code{INTEGER} values of all supported kinds and for @code{REAL}
1434 on IEEE systems of kinds 4 and 8. Conversion between different
1435 ``extended double'' types on different architectures such as
1436 m68k and x86_64, which GNU Fortran
1437 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1440 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1441 environment variable will override the CONVERT specifier in the
1442 open statement}. This is to give control over data formats to
1443 users who do not have the source code of their program available.
1445 Using anything but the native representation for unformatted data
1446 carries a significant speed overhead. If speed in this area matters
1447 to you, it is best if you use this only for data that needs to be
1454 OpenMP (Open Multi-Processing) is an application programming
1455 interface (API) that supports multi-platform shared memory
1456 multiprocessing programming in C/C++ and Fortran on many
1457 architectures, including Unix and Microsoft Windows platforms.
1458 It consists of a set of compiler directives, library routines,
1459 and environment variables that influence run-time behavior.
1461 GNU Fortran strives to be compatible to the
1462 @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
1463 OpenMP Application Program Interface v2.5}.
1465 To enable the processing of the OpenMP directive @code{!$omp} in
1466 free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
1467 directives in fixed form; the @code{!$} conditional compilation sentinels
1468 in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
1469 in fixed form, @command{gfortran} needs to be invoked with the
1470 @option{-fopenmp}. This also arranges for automatic linking of the
1471 GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
1474 The OpenMP Fortran runtime library routines are provided both in a
1475 form of a Fortran 90 module named @code{omp_lib} and in a form of
1476 a Fortran @code{include} file named @file{omp_lib.h}.
1478 An example of a parallelized loop taken from Appendix A.1 of
1479 the OpenMP Application Program Interface v2.5:
1481 SUBROUTINE A1(N, A, B)
1484 !$OMP PARALLEL DO !I is private by default
1486 B(I) = (A(I) + A(I-1)) / 2.0
1488 !$OMP END PARALLEL DO
1495 @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
1496 will be allocated on the stack. When porting existing code to OpenMP,
1497 this may lead to surprising results, especially to segmentation faults
1498 if the stacksize is limited.
1501 On glibc-based systems, OpenMP enabled applications can not be statically
1502 linked due to limitations of the underlying pthreads-implementation. It
1503 might be possible to get a working solution if
1504 @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
1505 to the command line. However, this is not supported by @command{gcc} and
1506 thus not recommended.
1509 @node Argument list functions
1510 @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
1511 @cindex argument list functions
1516 GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
1517 and @code{%LOC} statements, for backward compatibility with g77.
1518 It is recommended that these should be used only for code that is
1519 accessing facilities outside of GNU Fortran, such as operating system
1520 or windowing facilities. It is best to constrain such uses to isolated
1521 portions of a program--portions that deal specifically and exclusively
1522 with low-level, system-dependent facilities. Such portions might well
1523 provide a portable interface for use by the program as a whole, but are
1524 themselves not portable, and should be thoroughly tested each time they
1525 are rebuilt using a new compiler or version of a compiler.
1527 @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
1528 reference and @code{%LOC} passes its memory location. Since gfortran
1529 already passes scalar arguments by reference, @code{%REF} is in effect
1530 a do-nothing. @code{%LOC} has the same effect as a fortran pointer.
1532 An example of passing an argument by value to a C subroutine foo.:
1535 C prototype void foo_ (float x);
1544 For details refer to the g77 manual
1545 @uref{http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top}.
1547 Also, the gfortran testsuite c_by_val.f and its partner c_by_val.c are
1552 @node Extensions not implemented in GNU Fortran
1553 @section Extensions not implemented in GNU Fortran
1554 @cindex extensions, not implemented
1556 The long history of the Fortran language, its wide use and broad
1557 userbase, the large number of different compiler vendors and the lack of
1558 some features crucial to users in the first standards have lead to the
1559 existence of an important number of extensions to the language. While
1560 some of the most useful or popular extensions are supported by the GNU
1561 Fortran compiler, not all existing extensions are supported. This section
1562 aims at listing these extensions and offering advice on how best make
1563 code that uses them running with the GNU Fortran compiler.
1565 @c More can be found here:
1566 @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
1567 @c -- the list of fortran and libgfortran bugs closed as WONTFIX:
1568 @c http://tinyurl.com/2u4h5y
1571 * STRUCTURE and RECORD::
1572 @c * UNION and MAP::
1573 * ENCODE and DECODE statements::
1574 @c * Expressions in FORMAT statements::
1575 @c * Q edit descriptor::
1576 @c * AUTOMATIC statement::
1577 @c * TYPE and ACCEPT I/O Statements::
1578 @c * .XOR. operator::
1579 @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
1580 @c * Omitted arguments in procedure call:
1584 @node STRUCTURE and RECORD
1585 @subsection @code{STRUCTURE} and @code{RECORD}
1586 @cindex @code{STRUCTURE}
1587 @cindex @code{RECORD}
1589 Structures are user-defined aggregate data types; this functionality was
1590 standardized in Fortran 90 with an different syntax, under the name of
1591 ``derived types''. Here is an example of code using the non portable
1595 ! Declaring a structure named ``item'' and containing three fields:
1596 ! an integer ID, an description string and a floating-point price.
1599 CHARACTER(LEN=200) description
1603 ! Define two variables, an single record of type ``item''
1604 ! named ``pear'', and an array of items named ``store_catalog''
1605 RECORD /item/ pear, store_catalog(100)
1607 ! We can directly access the fields of both variables
1609 pear.description = "juicy D'Anjou pear"
1611 store_catalog(7).id = 7831
1612 store_catalog(7).description = "milk bottle"
1613 store_catalog(7).price = 1.2
1615 ! We can also manipulates the whole structure
1616 store_catalog(12) = pear
1617 print *, store_catalog(12)
1621 This code can easily be rewritten in the Fortran 90 syntax as following:
1624 ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
1625 ! ``TYPE name ... END TYPE''
1628 CHARACTER(LEN=200) description
1632 ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
1633 TYPE(item) pear, store_catalog(100)
1635 ! Instead of using a dot (.) to access fields of a record, the
1636 ! standard syntax uses a percent sign (%)
1638 pear%description = "juicy D'Anjou pear"
1640 store_catalog(7)%id = 7831
1641 store_catalog(7)%description = "milk bottle"
1642 store_catalog(7)%price = 1.2
1644 ! Assignments of a whole variable don't change
1645 store_catalog(12) = pear
1646 print *, store_catalog(12)
1650 @c @node UNION and MAP
1651 @c @subsection @code{UNION} and @code{MAP}
1652 @c @cindex @code{UNION}
1653 @c @cindex @code{MAP}
1655 @c For help writing this one, see
1656 @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and
1657 @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm
1660 @node ENCODE and DECODE statements
1661 @subsection @code{ENCODE} and @code{DECODE} statements
1662 @cindex @code{ENCODE}
1663 @cindex @code{DECODE}
1665 GNU Fortran doesn't support the @code{ENCODE} and @code{DECODE}
1666 statements. These statements are best replaced by @code{READ} and
1667 @code{WRITE} statements involving internal files (@code{CHARACTER}
1668 variables and arrays), which have been part of the Fortran standard since
1669 Fortran 77. For example, replace a code fragment like
1674 c ... Code that sets LINE
1675 DECODE (80, 9000, LINE) A, B, C
1676 9000 FORMAT (1X, 3(F10.5))
1683 CHARACTER(LEN=80) LINE
1685 c ... Code that sets LINE
1686 READ (UNIT=LINE, FMT=9000) A, B, C
1687 9000 FORMAT (1X, 3(F10.5))
1690 Similarly, replace a code fragment like
1695 c ... Code that sets A, B and C
1696 ENCODE (80, 9000, LINE) A, B, C
1697 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1706 c ... Code that sets A, B and C
1707 WRITE (UNIT=LINE, FMT=9000) A, B, C
1708 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1712 @c ---------------------------------------------------------------------
1713 @c Intrinsic Procedures
1714 @c ---------------------------------------------------------------------
1716 @include intrinsic.texi
1723 @c ---------------------------------------------------------------------
1725 @c ---------------------------------------------------------------------
1728 @unnumbered Contributing
1729 @cindex Contributing
1731 Free software is only possible if people contribute to efforts
1733 We're always in need of more people helping out with ideas
1734 and comments, writing documentation and contributing code.
1736 If you want to contribute to GNU Fortran,
1737 have a look at the long lists of projects you can take on.
1738 Some of these projects are small,
1739 some of them are large;
1740 some are completely orthogonal to the rest of what is
1741 happening on GNU Fortran,
1742 but others are ``mainstream'' projects in need of enthusiastic hackers.
1743 All of these projects are important!
1744 We'll eventually get around to the things here,
1745 but they are also things doable by someone who is willing and able.
1750 * Proposed Extensions::
1755 @section Contributors to GNU Fortran
1756 @cindex Contributors
1760 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
1761 also the initiator of the whole project. Thanks Andy!
1762 Most of the interface with GCC was written by @emph{Paul Brook}.
1764 The following individuals have contributed code and/or
1765 ideas and significant help to the GNU Fortran project
1766 (in alphabetical order):
1769 @item Janne Blomqvist
1770 @item Steven Bosscher
1773 @item Fran@,{c}ois-Xavier Coudert
1777 @item Bernhard Fischer
1779 @item Richard Guenther
1780 @item Richard Henderson
1781 @item Katherine Holcomb
1783 @item Niels Kristian Bech Jensen
1784 @item Steven Johnson
1785 @item Steven G. Kargl
1793 @item Christopher D. Rickett
1794 @item Richard Sandiford
1795 @item Tobias Schl@"uter
1803 The following people have contributed bug reports,
1804 smaller or larger patches,
1805 and much needed feedback and encouragement for the
1806 GNU Fortran project:
1810 @item Dominique d'Humi@`eres
1812 @item Erik Schnetter
1815 Many other individuals have helped debug,
1816 test and improve the GNU Fortran compiler over the past few years,
1817 and we welcome you to do the same!
1818 If you already have done so,
1819 and you would like to see your name listed in the
1820 list above, please contact us.
1828 @item Help build the test suite
1829 Solicit more code for donation to the test suite: the more extensive the
1830 testsuite, the smaller the risk of breaking things in the future! We can
1831 keep code private on request.
1833 @item Bug hunting/squishing
1834 Find bugs and write more test cases! Test cases are especially very
1835 welcome, because it allows us to concentrate on fixing bugs instead of
1836 isolating them. Going through the bugzilla database at
1837 @url{http://gcc.gnu.org/bugzilla/} to reduce testcases posted there and
1838 add more information (for example, for which version does the testcase
1839 work, for which versions does it fail?) is also very helpful.
1844 @node Proposed Extensions
1845 @section Proposed Extensions
1847 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
1848 order. Most of these are necessary to be fully compatible with
1849 existing Fortran compilers, but they are not part of the official
1850 J3 Fortran 95 standard.
1852 @subsection Compiler extensions:
1855 User-specified alignment rules for structures.
1858 Flag to generate @code{Makefile} info.
1861 Automatically extend single precision constants to double.
1864 Compile code that conserves memory by dynamically allocating common and
1865 module storage either on stack or heap.
1868 Compile flag to generate code for array conformance checking (suggest -CC).
1871 User control of symbol names (underscores, etc).
1874 Compile setting for maximum size of stack frame size before spilling
1875 parts to static or heap.
1878 Flag to force local variables into static space.
1881 Flag to force local variables onto stack.
1885 @subsection Environment Options
1888 Pluggable library modules for random numbers, linear algebra.
1889 LA should use BLAS calling conventions.
1892 Environment variables controlling actions on arithmetic exceptions like
1893 overflow, underflow, precision loss---Generate NaN, abort, default.
1897 Set precision for fp units that support it (i387).
1900 Variable for setting fp rounding mode.
1903 Variable to fill uninitialized variables with a user-defined bit
1907 Environment variable controlling filename that is opened for that unit
1911 Environment variable to clear/trash memory being freed.
1914 Environment variable to control tracing of allocations and frees.
1917 Environment variable to display allocated memory at normal program end.
1920 Environment variable for filename for * IO-unit.
1923 Environment variable for temporary file directory.
1926 Environment variable forcing standard output to be line buffered (unix).
1931 @c ---------------------------------------------------------------------
1932 @c GNU General Public License
1933 @c ---------------------------------------------------------------------
1935 @include gpl_v3.texi
1939 @c ---------------------------------------------------------------------
1940 @c GNU Free Documentation License
1941 @c ---------------------------------------------------------------------
1947 @c ---------------------------------------------------------------------
1948 @c Funding Free Software
1949 @c ---------------------------------------------------------------------
1951 @include funding.texi
1953 @c ---------------------------------------------------------------------
1955 @c ---------------------------------------------------------------------
1958 @unnumbered Option Index
1959 @command{gfortran}'s command line options are indexed here without any
1960 initial @samp{-} or @samp{--}. Where an option has both positive and
1961 negative forms (such as -foption and -fno-option), relevant entries in
1962 the manual are indexed under the most appropriate form; it may sometimes
1963 be useful to look up both forms.
1967 @unnumbered Keyword Index