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
404 @cindex preprocessor, include file handling
406 Many Fortran compilers including GNU Fortran allow passing the source code
407 through a C preprocessor (CPP; sometimes also called the Fortran preprocessor,
408 FPP) to allow for conditional compilation. In the case of GNU Fortran,
409 this is the GNU C Preprocessor in the traditional mode. On systems with
410 case-preserving file names, the preprocessor is automatically invoked if the
411 filename extension is @code{.F}, @code{.FOR}, @code{.FTN}, @code{.fpp},
412 @code{.FPP}, @code{.F90}, @code{.F95}, @code{.F03} or @code{.F08}. To manually
413 invoke the preprocessor on any file, use @option{-cpp}, to disable
414 preprocessing on files where the preprocessor is run automatically, use
417 If a preprocessed file includes another file with the Fortran @code{INCLUDE}
418 statement, the included file is not preprocessed. To preprocess included
419 files, use the equivalent preprocessor statement @code{#include}.
421 If GNU Fortran invokes the preprocessor, @code{__GFORTRAN__}
422 is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
423 @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
424 compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details.
426 While CPP is the de-facto standard for preprocessing Fortran code,
427 Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines
428 Conditional Compilation, which is not widely used and not directly
429 supported by the GNU Fortran compiler. You can use the program coco
430 to preprocess such files (@uref{http://users.erols.com/dnagle/coco.html}).
433 @c ---------------------------------------------------------------------
434 @c GNU Fortran and G77
435 @c ---------------------------------------------------------------------
437 @node GNU Fortran and G77
438 @section GNU Fortran and G77
440 @cindex @command{g77}
442 The GNU Fortran compiler is the successor to @command{g77}, the Fortran
443 77 front end included in GCC prior to version 4. It is an entirely new
444 program that has been designed to provide Fortran 95 support and
445 extensibility for future Fortran language standards, as well as providing
446 backwards compatibility for Fortran 77 and nearly all of the GNU language
447 extensions supported by @command{g77}.
450 @c ---------------------------------------------------------------------
452 @c ---------------------------------------------------------------------
455 @section Project Status
458 As soon as @command{gfortran} can parse all of the statements correctly,
459 it will be in the ``larva'' state.
460 When we generate code, the ``puppa'' state.
461 When @command{gfortran} is done,
462 we'll see if it will be a beautiful butterfly,
463 or just a big bug....
465 --Andy Vaught, April 2000
468 The start of the GNU Fortran 95 project was announced on
469 the GCC homepage in March 18, 2000
470 (even though Andy had already been working on it for a while,
473 The GNU Fortran compiler is able to compile nearly all
474 standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs,
475 including a number of standard and non-standard extensions, and can be
476 used on real-world programs. In particular, the supported extensions
477 include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran
478 2008 features such as enumeration, stream I/O, and some of the
479 enhancements to allocatable array support from TR 15581. However, it is
480 still under development and has a few remaining rough edges.
482 At present, the GNU Fortran compiler passes the
483 @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
484 NIST Fortran 77 Test Suite}, and produces acceptable results on the
485 @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}.
486 It also provides respectable performance on
487 the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran
488 compiler benchmarks} and the
489 @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html,
490 Livermore Fortran Kernels test}. It has been used to compile a number of
491 large real-world programs, including
492 @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM
493 weather-forecasting code} and
494 @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum
495 chemistry package}; see @url{http://gcc.gnu.org/wiki/GfortranApps} for an
498 Among other things, the GNU Fortran compiler is intended as a replacement
499 for G77. At this point, nearly all programs that could be compiled with
500 G77 can be compiled with GNU Fortran, although there are a few minor known
503 The primary work remaining to be done on GNU Fortran falls into three
504 categories: bug fixing (primarily regarding the treatment of invalid code
505 and providing useful error messages), improving the compiler optimizations
506 and the performance of compiled code, and extending the compiler to support
507 future standards---in particular, Fortran 2003.
510 @c ---------------------------------------------------------------------
512 @c ---------------------------------------------------------------------
518 The GNU Fortran compiler implements
519 ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
520 standard-compliant Fortran 90 and Fortran 77 programs. It also supports
521 the ISO/IEC TR-15581 enhancements to allocatable arrays, and
522 the @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
523 OpenMP Application Program Interface v2.5} specification.
525 In the future, the GNU Fortran compiler will also support ISO/IEC
526 1539-1:2004 (Fortran 2003) and future Fortran standards. Partial support
527 of that standard is already provided; the current status of Fortran 2003
528 support is reported in the @ref{Fortran 2003 status} section of the
531 The next version of the Fortran standard after Fortran 2003 is currently
532 being developed and the GNU Fortran compiler supports some of its new
533 features. This support is based on the latest draft of the standard
534 (available from @url{http://www.nag.co.uk/sc22wg5/}) and no guarantee of
535 future compatibility is made, as the final standard might differ from the
536 draft. For more information, see the @ref{Fortran 2008 status} section.
539 @c =====================================================================
540 @c PART I: INVOCATION REFERENCE
541 @c =====================================================================
544 \part{I}{Invoking GNU Fortran}
547 @c ---------------------------------------------------------------------
549 @c ---------------------------------------------------------------------
554 @c ---------------------------------------------------------------------
556 @c ---------------------------------------------------------------------
559 @chapter Runtime: Influencing runtime behavior with environment variables
560 @cindex environment variable
562 The behavior of the @command{gfortran} can be influenced by
563 environment variables.
565 Malformed environment variables are silently ignored.
568 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
569 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
570 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
571 * GFORTRAN_USE_STDERR:: Send library output to standard error
572 * GFORTRAN_TMPDIR:: Directory for scratch files
573 * GFORTRAN_UNBUFFERED_ALL:: Don't buffer I/O for all units.
574 * GFORTRAN_UNBUFFERED_PRECONNECTED:: Don't buffer I/O for preconnected units.
575 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
576 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
577 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
578 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
579 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
580 * GFORTRAN_ERROR_DUMPCORE:: Dump core on run-time errors
581 * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
584 @node GFORTRAN_STDIN_UNIT
585 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
587 This environment variable can be used to select the unit number
588 preconnected to standard input. This must be a positive integer.
589 The default value is 5.
591 @node GFORTRAN_STDOUT_UNIT
592 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
594 This environment variable can be used to select the unit number
595 preconnected to standard output. This must be a positive integer.
596 The default value is 6.
598 @node GFORTRAN_STDERR_UNIT
599 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
601 This environment variable can be used to select the unit number
602 preconnected to standard error. This must be a positive integer.
603 The default value is 0.
605 @node GFORTRAN_USE_STDERR
606 @section @env{GFORTRAN_USE_STDERR}---Send library output to standard error
608 This environment variable controls where library output is sent.
609 If the first letter is @samp{y}, @samp{Y} or @samp{1}, standard
610 error is used. If the first letter is @samp{n}, @samp{N} or
611 @samp{0}, standard output is used.
613 @node GFORTRAN_TMPDIR
614 @section @env{GFORTRAN_TMPDIR}---Directory for scratch files
616 This environment variable controls where scratch files are
617 created. If this environment variable is missing,
618 GNU Fortran searches for the environment variable @env{TMP}. If
619 this is also missing, the default is @file{/tmp}.
621 @node GFORTRAN_UNBUFFERED_ALL
622 @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer I/O on all units
624 This environment variable controls whether all I/O is unbuffered. If
625 the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
626 unbuffered. This will slow down small sequential reads and writes. If
627 the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
630 @node GFORTRAN_UNBUFFERED_PRECONNECTED
631 @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Don't buffer I/O on preconnected units
633 The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
634 whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If
635 the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This
636 will slow down small sequential reads and writes. If the first letter
637 is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default.
639 @node GFORTRAN_SHOW_LOCUS
640 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
642 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
643 line numbers for runtime errors are printed. If the first letter is
644 @samp{n}, @samp{N} or @samp{0}, don't print filename and line numbers
645 for runtime errors. The default is to print the location.
647 @node GFORTRAN_OPTIONAL_PLUS
648 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
650 If the first letter is @samp{y}, @samp{Y} or @samp{1},
651 a plus sign is printed
652 where permitted by the Fortran standard. If the first letter
653 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
654 in most cases. Default is not to print plus signs.
656 @node GFORTRAN_DEFAULT_RECL
657 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
659 This environment variable specifies the default record length, in
660 bytes, for files which are opened without a @code{RECL} tag in the
661 @code{OPEN} statement. This must be a positive integer. The
662 default value is 1073741824 bytes (1 GB).
664 @node GFORTRAN_LIST_SEPARATOR
665 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
667 This environment variable specifies the separator when writing
668 list-directed output. It may contain any number of spaces and
669 at most one comma. If you specify this on the command line,
670 be sure to quote spaces, as in
672 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
674 when @command{a.out} is the compiled Fortran program that you want to run.
675 Default is a single space.
677 @node GFORTRAN_CONVERT_UNIT
678 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
680 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
681 to change the representation of data for unformatted files.
682 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
684 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
685 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
686 exception: mode ':' unit_list | unit_list ;
687 unit_list: unit_spec | unit_list unit_spec ;
688 unit_spec: INTEGER | INTEGER '-' INTEGER ;
690 The variable consists of an optional default mode, followed by
691 a list of optional exceptions, which are separated by semicolons
692 from the preceding default and each other. Each exception consists
693 of a format and a comma-separated list of units. Valid values for
694 the modes are the same as for the @code{CONVERT} specifier:
697 @item @code{NATIVE} Use the native format. This is the default.
698 @item @code{SWAP} Swap between little- and big-endian.
699 @item @code{LITTLE_ENDIAN} Use the little-endian format
700 for unformatted files.
701 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
703 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
704 Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
706 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
707 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
708 in little_endian mode, except for units 10 to 20 and 25, which are in
710 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
713 Setting the environment variables should be done on the command
714 line or via the @command{export}
715 command for @command{sh}-compatible shells and via @command{setenv}
716 for @command{csh}-compatible shells.
718 Example for @command{sh}:
721 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
724 Example code for @command{csh}:
727 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
731 Using anything but the native representation for unformatted data
732 carries a significant speed overhead. If speed in this area matters
733 to you, it is best if you use this only for data that needs to be
736 @xref{CONVERT specifier}, for an alternative way to specify the
737 data representation for unformatted files. @xref{Runtime Options}, for
738 setting a default data representation for the whole program. The
739 @code{CONVERT} specifier overrides the @option{-fconvert} compile options.
741 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
742 environment variable will override the CONVERT specifier in the
743 open statement}. This is to give control over data formats to
744 users who do not have the source code of their program available.
746 @node GFORTRAN_ERROR_DUMPCORE
747 @section @env{GFORTRAN_ERROR_DUMPCORE}---Dump core on run-time errors
749 If the @env{GFORTRAN_ERROR_DUMPCORE} variable is set to
750 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
751 then library run-time errors cause core dumps. To disable the core
752 dumps, set the variable to @samp{n}, @samp{N}, @samp{0}. Default
753 is not to core dump unless the @option{-fdump-core} compile option
756 @node GFORTRAN_ERROR_BACKTRACE
757 @section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors
759 If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to
760 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
761 then a backtrace is printed when a run-time error occurs.
762 To disable the backtracing, set the variable to
763 @samp{n}, @samp{N}, @samp{0}. Default is not to print a backtrace
764 unless the @option{-fbacktrace} compile option
767 @c =====================================================================
768 @c PART II: LANGUAGE REFERENCE
769 @c =====================================================================
772 \part{II}{Language Reference}
775 @c ---------------------------------------------------------------------
776 @c Fortran 2003 and 2008 Status
777 @c ---------------------------------------------------------------------
779 @node Fortran 2003 and 2008 status
780 @chapter Fortran 2003 and 2008 Status
783 * Fortran 2003 status::
784 * Fortran 2008 status::
787 @node Fortran 2003 status
788 @section Fortran 2003 status
790 Although GNU Fortran focuses on implementing the Fortran 95
791 standard for the time being, a few Fortran 2003 features are currently
796 Intrinsics @code{command_argument_count}, @code{get_command},
797 @code{get_command_argument}, @code{get_environment_variable}, and
801 @cindex array, constructors
803 Array constructors using square brackets. That is, @code{[...]} rather
807 @cindex @code{FLUSH} statement
808 @cindex statement, @code{FLUSH}
809 @code{FLUSH} statement.
812 @cindex @code{IOMSG=} specifier
813 @code{IOMSG=} specifier for I/O statements.
816 @cindex @code{ENUM} statement
817 @cindex @code{ENUMERATOR} statement
818 @cindex statement, @code{ENUM}
819 @cindex statement, @code{ENUMERATOR}
820 @opindex @code{fshort-enums}
821 Support for the declaration of enumeration constants via the
822 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
823 @command{gcc} is guaranteed also for the case where the
824 @command{-fshort-enums} command line option is given.
831 @cindex @code{ALLOCATABLE} dummy arguments
832 @code{ALLOCATABLE} dummy arguments.
834 @cindex @code{ALLOCATABLE} function results
835 @code{ALLOCATABLE} function results
837 @cindex @code{ALLOCATABLE} components of derived types
838 @code{ALLOCATABLE} components of derived types
842 @cindex @code{STREAM} I/O
843 @cindex @code{ACCESS='STREAM'} I/O
844 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
845 allowing I/O without any record structure.
848 Namelist input/output for internal files.
851 @cindex @code{PROTECTED} statement
852 @cindex statement, @code{PROTECTED}
853 The @code{PROTECTED} statement and attribute.
856 @cindex @code{VALUE} statement
857 @cindex statement, @code{VALUE}
858 The @code{VALUE} statement and attribute.
861 @cindex @code{VOLATILE} statement
862 @cindex statement, @code{VOLATILE}
863 The @code{VOLATILE} statement and attribute.
866 @cindex @code{IMPORT} statement
867 @cindex statement, @code{IMPORT}
868 The @code{IMPORT} statement, allowing to import
869 host-associated derived types.
872 @cindex @code{USE, INTRINSIC} statement
873 @cindex statement, @code{USE, INTRINSIC}
874 @cindex @code{ISO_FORTRAN_ENV} statement
875 @cindex statement, @code{ISO_FORTRAN_ENV}
876 @code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
877 attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
878 @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
881 Renaming of operators in the @code{USE} statement.
884 @cindex ISO C Bindings
885 Interoperability with C (ISO C Bindings)
888 BOZ as argument of INT, REAL, DBLE and CMPLX.
893 @node Fortran 2008 status
894 @section Fortran 2008 status
896 The next version of the Fortran standard after Fortran 2003 is currently
897 being worked on by the Working Group 5 of Sub-Committee 22 of the Joint
898 Technical Committee 1 of the International Organization for
899 Standardization (ISO) and the International Electrotechnical Commission
900 (IEC). This group is known at @uref{http://www.nag.co.uk/sc22wg5/, WG5}.
901 The next revision of the Fortran standard is informally referred to as
902 Fortran 2008, reflecting its planned release year. The GNU Fortran
903 compiler has support for some of the new features in Fortran 2008. This
904 support is based on the latest draft, available from
905 @url{http://www.nag.co.uk/sc22wg5/}. However, as the final standard may
906 differ from the drafts, no guarantee of backward compatibility can be
907 made and you should only use it for experimental purposes.
909 @c ---------------------------------------------------------------------
911 @c ---------------------------------------------------------------------
913 @c Maybe this chapter should be merged with the 'Standards' section,
914 @c whenever that is written :-)
920 The two sections below detail the extensions to standard Fortran that are
921 implemented in GNU Fortran, as well as some of the popular or
922 historically important extensions that are not (or not yet) implemented.
923 For the latter case, we explain the alternatives available to GNU Fortran
924 users, including replacement by standard-conforming code or GNU
928 * Extensions implemented in GNU Fortran::
929 * Extensions not implemented in GNU Fortran::
933 @node Extensions implemented in GNU Fortran
934 @section Extensions implemented in GNU Fortran
935 @cindex extensions, implemented
937 GNU Fortran implements a number of extensions over standard
938 Fortran. This chapter contains information on their syntax and
939 meaning. There are currently two categories of GNU Fortran
940 extensions, those that provide functionality beyond that provided
941 by any standard, and those that are supported by GNU Fortran
942 purely for backward compatibility with legacy compilers. By default,
943 @option{-std=gnu} allows the compiler to accept both types of
944 extensions, but to warn about the use of the latter. Specifying
945 either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
946 disables both types of extensions, and @option{-std=legacy} allows both
950 * Old-style kind specifications::
951 * Old-style variable initialization::
952 * Extensions to namelist::
953 * X format descriptor without count field::
954 * Commas in FORMAT specifications::
955 * Missing period in FORMAT specifications::
957 * BOZ literal constants::
958 * Real array indices::
960 * Implicitly convert LOGICAL and INTEGER values::
961 * Hollerith constants support::
963 * CONVERT specifier::
965 * Argument list functions::
968 @node Old-style kind specifications
969 @subsection Old-style kind specifications
970 @cindex kind, old-style
972 GNU Fortran allows old-style kind specifications in declarations. These
978 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
979 etc.), and where @code{size} is a byte count corresponding to the
980 storage size of a valid kind for that type. (For @code{COMPLEX}
981 variables, @code{size} is the total size of the real and imaginary
982 parts.) The statement then declares @code{x}, @code{y} and @code{z} to
983 be of type @code{TYPESPEC} with the appropriate kind. This is
984 equivalent to the standard-conforming declaration
989 where @code{k} is equal to @code{size} for most types, but is equal to
990 @code{size/2} for the @code{COMPLEX} type.
992 @node Old-style variable initialization
993 @subsection Old-style variable initialization
995 GNU Fortran allows old-style initialization of variables of the
999 REAL x(2,2) /3*0.,1./
1001 The syntax for the initializers is as for the @code{DATA} statement, but
1002 unlike in a @code{DATA} statement, an initializer only applies to the
1003 variable immediately preceding the initialization. In other words,
1004 something like @code{INTEGER I,J/2,3/} is not valid. This style of
1005 initialization is only allowed in declarations without double colons
1006 (@code{::}); the double colons were introduced in Fortran 90, which also
1007 introduced a standard syntax for initializing variables in type
1010 Examples of standard-conforming code equivalent to the above example
1014 INTEGER :: i = 1, j = 2
1015 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
1019 DATA i/1/, j/2/, x/3*0.,1./
1022 Note that variables which are explicitly initialized in declarations
1023 or in @code{DATA} statements automatically acquire the @code{SAVE}
1026 @node Extensions to namelist
1027 @subsection Extensions to namelist
1030 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
1031 including array qualifiers, substrings and fully qualified derived types.
1032 The output from a namelist write is compatible with namelist read. The
1033 output has all names in upper case and indentation to column 1 after the
1034 namelist name. Two extensions are permitted:
1036 Old-style use of @samp{$} instead of @samp{&}
1039 X(:)%Y(2) = 1.0 2.0 3.0
1044 It should be noted that the default terminator is @samp{/} rather than
1047 Querying of the namelist when inputting from stdin. After at least
1048 one space, entering @samp{?} sends to stdout the namelist name and the names of
1049 the variables in the namelist:
1060 Entering @samp{=?} outputs the namelist to stdout, as if
1061 @code{WRITE(*,NML = mynml)} had been called:
1066 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
1067 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
1068 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
1072 To aid this dialog, when input is from stdin, errors send their
1073 messages to stderr and execution continues, even if @code{IOSTAT} is set.
1075 @code{PRINT} namelist is permitted. This causes an error if
1076 @option{-std=f95} is used.
1079 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
1082 END PROGRAM test_print
1085 Expanded namelist reads are permitted. This causes an error if
1086 @option{-std=f95} is used. In the following example, the first element
1087 of the array will be given the value 0.00 and the two succeeding
1088 elements will be given the values 1.00 and 2.00.
1091 X(1,1) = 0.00 , 1.00 , 2.00
1095 @node X format descriptor without count field
1096 @subsection @code{X} format descriptor without count field
1098 To support legacy codes, GNU Fortran permits the count field of the
1099 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
1100 When omitted, the count is implicitly assumed to be one.
1104 10 FORMAT (I1, X, I1)
1107 @node Commas in FORMAT specifications
1108 @subsection Commas in @code{FORMAT} specifications
1110 To support legacy codes, GNU Fortran allows the comma separator
1111 to be omitted immediately before and after character string edit
1112 descriptors in @code{FORMAT} statements.
1116 10 FORMAT ('FOO='I1' BAR='I2)
1120 @node Missing period in FORMAT specifications
1121 @subsection Missing period in @code{FORMAT} specifications
1123 To support legacy codes, GNU Fortran allows missing periods in format
1124 specifications if and only if @option{-std=legacy} is given on the
1125 command line. This is considered non-conforming code and is
1134 @node I/O item lists
1135 @subsection I/O item lists
1136 @cindex I/O item lists
1138 To support legacy codes, GNU Fortran allows the input item list
1139 of the @code{READ} statement, and the output item lists of the
1140 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1142 @node BOZ literal constants
1143 @subsection BOZ literal constants
1144 @cindex BOZ literal constants
1146 Besides decimal constants, Fortran also supports binary (@code{b}),
1147 octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
1148 syntax is: @samp{prefix quote digits quote}, were the prefix is
1149 either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
1150 @code{"} and the digits are for binary @code{0} or @code{1}, for
1151 octal between @code{0} and @code{7}, and for hexadecimal between
1152 @code{0} and @code{F}. (Example: @code{b'01011101'}.)
1154 Up to Fortran 95, BOZ literals were only allowed to initialize
1155 integer variables in DATA statements. Since Fortran 2003 BOZ literals
1156 are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
1157 and @code{CMPLX}; the result is the same as if the integer BOZ
1158 literal had been converted by @code{TRANSFER} to, respectively,
1159 @code{real}, @code{double precision}, @code{integer} or @code{complex}.
1160 As GNU Fortran extension the intrinsic procedures @code{FLOAT},
1161 @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
1163 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1164 be specified using the @code{X} prefix, in addition to the standard
1165 @code{Z} prefix. The BOZ literal can also be specified by adding a
1166 suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
1169 Furthermore, GNU Fortran allows using BOZ literal constants outside
1170 DATA statements and the four intrinsic functions allowed by Fortran 2003.
1171 In DATA statements, in direct assignments, where the right-hand side
1172 only contains a BOZ literal constant, and for old-style initializers of
1173 the form @code{integer i /o'0173'/}, the constant is transferred
1174 as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
1175 the real part is initialized unless @code{CMPLX} is used. In all other
1176 cases, the BOZ literal constant is converted to an @code{INTEGER} value with
1177 the largest decimal representation. This value is then converted
1178 numerically to the type and kind of the variable in question.
1179 (For instance @code{real :: r = b'0000001' + 1} initializes @code{r}
1180 with @code{2.0}.) As different compilers implement the extension
1181 differently, one should be careful when doing bitwise initialization
1182 of non-integer variables.
1184 Note that initializing an @code{INTEGER} variable with a statement such
1185 as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
1186 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1187 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1188 option can be used as a workaround for legacy code that initializes
1189 integers in this manner.
1191 @node Real array indices
1192 @subsection Real array indices
1193 @cindex array, indices of type real
1195 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1196 or variables as array indices.
1198 @node Unary operators
1199 @subsection Unary operators
1200 @cindex operators, unary
1202 As an extension, GNU Fortran allows unary plus and unary minus operators
1203 to appear as the second operand of binary arithmetic operators without
1204 the need for parenthesis.
1210 @node Implicitly convert LOGICAL and INTEGER values
1211 @subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1212 @cindex conversion, to integer
1213 @cindex conversion, to logical
1215 As an extension for backwards compatibility with other compilers, GNU
1216 Fortran allows the implicit conversion of @code{LOGICAL} values to
1217 @code{INTEGER} values and vice versa. When converting from a
1218 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1219 zero, and @code{.TRUE.} is interpreted as one. When converting from
1220 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1221 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1232 However, there is no implicit conversion of @code{INTEGER} values in
1233 @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
1236 @node Hollerith constants support
1237 @subsection Hollerith constants support
1238 @cindex Hollerith constants
1240 GNU Fortran supports Hollerith constants in assignments, function
1241 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1242 constant is written as a string of characters preceded by an integer
1243 constant indicating the character count, and the letter @code{H} or
1244 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1245 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1246 constant will be padded or truncated to fit the size of the variable in
1249 Examples of valid uses of Hollerith constants:
1252 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1253 x(1) = 16HABCDEFGHIJKLMNOP
1257 Invalid Hollerith constants examples:
1260 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1261 a = 0H ! At least one character is needed.
1264 In general, Hollerith constants were used to provide a rudimentary
1265 facility for handling character strings in early Fortran compilers,
1266 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1267 in those cases, the standard-compliant equivalent is to convert the
1268 program to use proper character strings. On occasion, there may be a
1269 case where the intent is specifically to initialize a numeric variable
1270 with a given byte sequence. In these cases, the same result can be
1271 obtained by using the @code{TRANSFER} statement, as in this example.
1273 INTEGER(KIND=4) :: a
1274 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1279 @subsection Cray pointers
1280 @cindex pointer, Cray
1282 Cray pointers are part of a non-standard extension that provides a
1283 C-like pointer in Fortran. This is accomplished through a pair of
1284 variables: an integer "pointer" that holds a memory address, and a
1285 "pointee" that is used to dereference the pointer.
1287 Pointer/pointee pairs are declared in statements of the form:
1289 pointer ( <pointer> , <pointee> )
1293 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1295 The pointer is an integer that is intended to hold a memory address.
1296 The pointee may be an array or scalar. A pointee can be an assumed
1297 size array---that is, the last dimension may be left unspecified by
1298 using a @code{*} in place of a value---but a pointee cannot be an
1299 assumed shape array. No space is allocated for the pointee.
1301 The pointee may have its type declared before or after the pointer
1302 statement, and its array specification (if any) may be declared
1303 before, during, or after the pointer statement. The pointer may be
1304 declared as an integer prior to the pointer statement. However, some
1305 machines have default integer sizes that are different than the size
1306 of a pointer, and so the following code is not portable:
1311 If a pointer is declared with a kind that is too small, the compiler
1312 will issue a warning; the resulting binary will probably not work
1313 correctly, because the memory addresses stored in the pointers may be
1314 truncated. It is safer to omit the first line of the above example;
1315 if explicit declaration of ipt's type is omitted, then the compiler
1316 will ensure that ipt is an integer variable large enough to hold a
1319 Pointer arithmetic is valid with Cray pointers, but it is not the same
1320 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1321 the user is responsible for determining how many bytes to add to a
1322 pointer in order to increment it. Consider the following example:
1326 pointer (ipt, pointee)
1330 The last statement does not set @code{ipt} to the address of
1331 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1332 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1334 Any expression involving the pointee will be translated to use the
1335 value stored in the pointer as the base address.
1337 To get the address of elements, this extension provides an intrinsic
1338 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1339 @code{&} operator in C, except the address is cast to an integer type:
1342 pointer(ipt, arpte(10))
1344 ipt = loc(ar) ! Makes arpte is an alias for ar
1345 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1347 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1350 Cray pointees often are used to alias an existing variable. For
1358 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1359 @code{target}. The optimizer, however, will not detect this aliasing, so
1360 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1361 a pointee in any way that violates the Fortran aliasing rules or
1362 assumptions is illegal. It is the user's responsibility to avoid doing
1363 this; the compiler works under the assumption that no such aliasing
1366 Cray pointers will work correctly when there is no aliasing (i.e., when
1367 they are used to access a dynamically allocated block of memory), and
1368 also in any routine where a pointee is used, but any variable with which
1369 it shares storage is not used. Code that violates these rules may not
1370 run as the user intends. This is not a bug in the optimizer; any code
1371 that violates the aliasing rules is illegal. (Note that this is not
1372 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1373 will ``incorrectly'' optimize code with illegal aliasing.)
1375 There are a number of restrictions on the attributes that can be applied
1376 to Cray pointers and pointees. Pointees may not have the
1377 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1378 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1379 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1380 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes.
1381 Pointees may not occur in more than one pointer statement. A pointee
1382 cannot be a pointer. Pointees cannot occur in equivalence, common, or
1385 A Cray pointer may also point to a function or a subroutine. For
1386 example, the following excerpt is valid:
1390 pointer (subptr,subpte)
1400 A pointer may be modified during the course of a program, and this
1401 will change the location to which the pointee refers. However, when
1402 pointees are passed as arguments, they are treated as ordinary
1403 variables in the invoked function. Subsequent changes to the pointer
1404 will not change the base address of the array that was passed.
1406 @node CONVERT specifier
1407 @subsection @code{CONVERT} specifier
1408 @cindex @code{CONVERT} specifier
1410 GNU Fortran allows the conversion of unformatted data between little-
1411 and big-endian representation to facilitate moving of data
1412 between different systems. The conversion can be indicated with
1413 the @code{CONVERT} specifier on the @code{OPEN} statement.
1414 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1415 the data format via an environment variable.
1417 Valid values for @code{CONVERT} are:
1419 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1420 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1421 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1422 for unformatted files.
1423 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1427 Using the option could look like this:
1429 open(file='big.dat',form='unformatted',access='sequential', &
1430 convert='big_endian')
1433 The value of the conversion can be queried by using
1434 @code{INQUIRE(CONVERT=ch)}. The values returned are
1435 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1437 @code{CONVERT} works between big- and little-endian for
1438 @code{INTEGER} values of all supported kinds and for @code{REAL}
1439 on IEEE systems of kinds 4 and 8. Conversion between different
1440 ``extended double'' types on different architectures such as
1441 m68k and x86_64, which GNU Fortran
1442 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1445 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1446 environment variable will override the CONVERT specifier in the
1447 open statement}. This is to give control over data formats to
1448 users who do not have the source code of their program available.
1450 Using anything but the native representation for unformatted data
1451 carries a significant speed overhead. If speed in this area matters
1452 to you, it is best if you use this only for data that needs to be
1459 OpenMP (Open Multi-Processing) is an application programming
1460 interface (API) that supports multi-platform shared memory
1461 multiprocessing programming in C/C++ and Fortran on many
1462 architectures, including Unix and Microsoft Windows platforms.
1463 It consists of a set of compiler directives, library routines,
1464 and environment variables that influence run-time behavior.
1466 GNU Fortran strives to be compatible to the
1467 @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
1468 OpenMP Application Program Interface v2.5}.
1470 To enable the processing of the OpenMP directive @code{!$omp} in
1471 free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
1472 directives in fixed form; the @code{!$} conditional compilation sentinels
1473 in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
1474 in fixed form, @command{gfortran} needs to be invoked with the
1475 @option{-fopenmp}. This also arranges for automatic linking of the
1476 GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
1479 The OpenMP Fortran runtime library routines are provided both in a
1480 form of a Fortran 90 module named @code{omp_lib} and in a form of
1481 a Fortran @code{include} file named @file{omp_lib.h}.
1483 An example of a parallelized loop taken from Appendix A.1 of
1484 the OpenMP Application Program Interface v2.5:
1486 SUBROUTINE A1(N, A, B)
1489 !$OMP PARALLEL DO !I is private by default
1491 B(I) = (A(I) + A(I-1)) / 2.0
1493 !$OMP END PARALLEL DO
1500 @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
1501 will be allocated on the stack. When porting existing code to OpenMP,
1502 this may lead to surprising results, especially to segmentation faults
1503 if the stacksize is limited.
1506 On glibc-based systems, OpenMP enabled applications can not be statically
1507 linked due to limitations of the underlying pthreads-implementation. It
1508 might be possible to get a working solution if
1509 @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
1510 to the command line. However, this is not supported by @command{gcc} and
1511 thus not recommended.
1514 @node Argument list functions
1515 @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
1516 @cindex argument list functions
1521 GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
1522 and @code{%LOC} statements, for backward compatibility with g77.
1523 It is recommended that these should be used only for code that is
1524 accessing facilities outside of GNU Fortran, such as operating system
1525 or windowing facilities. It is best to constrain such uses to isolated
1526 portions of a program--portions that deal specifically and exclusively
1527 with low-level, system-dependent facilities. Such portions might well
1528 provide a portable interface for use by the program as a whole, but are
1529 themselves not portable, and should be thoroughly tested each time they
1530 are rebuilt using a new compiler or version of a compiler.
1532 @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
1533 reference and @code{%LOC} passes its memory location. Since gfortran
1534 already passes scalar arguments by reference, @code{%REF} is in effect
1535 a do-nothing. @code{%LOC} has the same effect as a fortran pointer.
1537 An example of passing an argument by value to a C subroutine foo.:
1540 C prototype void foo_ (float x);
1549 For details refer to the g77 manual
1550 @uref{http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top}.
1552 Also, the gfortran testsuite c_by_val.f and its partner c_by_val.c are
1557 @node Extensions not implemented in GNU Fortran
1558 @section Extensions not implemented in GNU Fortran
1559 @cindex extensions, not implemented
1561 The long history of the Fortran language, its wide use and broad
1562 userbase, the large number of different compiler vendors and the lack of
1563 some features crucial to users in the first standards have lead to the
1564 existence of an important number of extensions to the language. While
1565 some of the most useful or popular extensions are supported by the GNU
1566 Fortran compiler, not all existing extensions are supported. This section
1567 aims at listing these extensions and offering advice on how best make
1568 code that uses them running with the GNU Fortran compiler.
1570 @c More can be found here:
1571 @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
1572 @c -- the list of fortran and libgfortran bugs closed as WONTFIX:
1573 @c http://tinyurl.com/2u4h5y
1576 * STRUCTURE and RECORD::
1577 @c * UNION and MAP::
1578 * ENCODE and DECODE statements::
1579 @c * Expressions in FORMAT statements::
1580 @c * Q edit descriptor::
1581 @c * AUTOMATIC statement::
1582 @c * TYPE and ACCEPT I/O Statements::
1583 @c * .XOR. operator::
1584 @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
1585 @c * Omitted arguments in procedure call:
1589 @node STRUCTURE and RECORD
1590 @subsection @code{STRUCTURE} and @code{RECORD}
1591 @cindex @code{STRUCTURE}
1592 @cindex @code{RECORD}
1594 Structures are user-defined aggregate data types; this functionality was
1595 standardized in Fortran 90 with an different syntax, under the name of
1596 ``derived types''. Here is an example of code using the non portable
1600 ! Declaring a structure named ``item'' and containing three fields:
1601 ! an integer ID, an description string and a floating-point price.
1604 CHARACTER(LEN=200) description
1608 ! Define two variables, an single record of type ``item''
1609 ! named ``pear'', and an array of items named ``store_catalog''
1610 RECORD /item/ pear, store_catalog(100)
1612 ! We can directly access the fields of both variables
1614 pear.description = "juicy D'Anjou pear"
1616 store_catalog(7).id = 7831
1617 store_catalog(7).description = "milk bottle"
1618 store_catalog(7).price = 1.2
1620 ! We can also manipulates the whole structure
1621 store_catalog(12) = pear
1622 print *, store_catalog(12)
1626 This code can easily be rewritten in the Fortran 90 syntax as following:
1629 ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
1630 ! ``TYPE name ... END TYPE''
1633 CHARACTER(LEN=200) description
1637 ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
1638 TYPE(item) pear, store_catalog(100)
1640 ! Instead of using a dot (.) to access fields of a record, the
1641 ! standard syntax uses a percent sign (%)
1643 pear%description = "juicy D'Anjou pear"
1645 store_catalog(7)%id = 7831
1646 store_catalog(7)%description = "milk bottle"
1647 store_catalog(7)%price = 1.2
1649 ! Assignments of a whole variable don't change
1650 store_catalog(12) = pear
1651 print *, store_catalog(12)
1655 @c @node UNION and MAP
1656 @c @subsection @code{UNION} and @code{MAP}
1657 @c @cindex @code{UNION}
1658 @c @cindex @code{MAP}
1660 @c For help writing this one, see
1661 @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and
1662 @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm
1665 @node ENCODE and DECODE statements
1666 @subsection @code{ENCODE} and @code{DECODE} statements
1667 @cindex @code{ENCODE}
1668 @cindex @code{DECODE}
1670 GNU Fortran doesn't support the @code{ENCODE} and @code{DECODE}
1671 statements. These statements are best replaced by @code{READ} and
1672 @code{WRITE} statements involving internal files (@code{CHARACTER}
1673 variables and arrays), which have been part of the Fortran standard since
1674 Fortran 77. For example, replace a code fragment like
1679 c ... Code that sets LINE
1680 DECODE (80, 9000, LINE) A, B, C
1681 9000 FORMAT (1X, 3(F10.5))
1688 CHARACTER(LEN=80) LINE
1690 c ... Code that sets LINE
1691 READ (UNIT=LINE, FMT=9000) A, B, C
1692 9000 FORMAT (1X, 3(F10.5))
1695 Similarly, replace a code fragment like
1700 c ... Code that sets A, B and C
1701 ENCODE (80, 9000, LINE) A, B, C
1702 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1711 c ... Code that sets A, B and C
1712 WRITE (UNIT=LINE, FMT=9000) A, B, C
1713 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1717 @c ---------------------------------------------------------------------
1718 @c Intrinsic Procedures
1719 @c ---------------------------------------------------------------------
1721 @include intrinsic.texi
1728 @c ---------------------------------------------------------------------
1730 @c ---------------------------------------------------------------------
1733 @unnumbered Contributing
1734 @cindex Contributing
1736 Free software is only possible if people contribute to efforts
1738 We're always in need of more people helping out with ideas
1739 and comments, writing documentation and contributing code.
1741 If you want to contribute to GNU Fortran,
1742 have a look at the long lists of projects you can take on.
1743 Some of these projects are small,
1744 some of them are large;
1745 some are completely orthogonal to the rest of what is
1746 happening on GNU Fortran,
1747 but others are ``mainstream'' projects in need of enthusiastic hackers.
1748 All of these projects are important!
1749 We'll eventually get around to the things here,
1750 but they are also things doable by someone who is willing and able.
1755 * Proposed Extensions::
1760 @section Contributors to GNU Fortran
1761 @cindex Contributors
1765 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
1766 also the initiator of the whole project. Thanks Andy!
1767 Most of the interface with GCC was written by @emph{Paul Brook}.
1769 The following individuals have contributed code and/or
1770 ideas and significant help to the GNU Fortran project
1771 (in alphabetical order):
1774 @item Janne Blomqvist
1775 @item Steven Bosscher
1778 @item Fran@,{c}ois-Xavier Coudert
1782 @item Bernhard Fischer
1784 @item Richard Guenther
1785 @item Richard Henderson
1786 @item Katherine Holcomb
1788 @item Niels Kristian Bech Jensen
1789 @item Steven Johnson
1790 @item Steven G. Kargl
1798 @item Christopher D. Rickett
1799 @item Richard Sandiford
1800 @item Tobias Schl@"uter
1808 The following people have contributed bug reports,
1809 smaller or larger patches,
1810 and much needed feedback and encouragement for the
1811 GNU Fortran project:
1815 @item Dominique d'Humi@`eres
1817 @item Erik Schnetter
1820 Many other individuals have helped debug,
1821 test and improve the GNU Fortran compiler over the past few years,
1822 and we welcome you to do the same!
1823 If you already have done so,
1824 and you would like to see your name listed in the
1825 list above, please contact us.
1833 @item Help build the test suite
1834 Solicit more code for donation to the test suite: the more extensive the
1835 testsuite, the smaller the risk of breaking things in the future! We can
1836 keep code private on request.
1838 @item Bug hunting/squishing
1839 Find bugs and write more test cases! Test cases are especially very
1840 welcome, because it allows us to concentrate on fixing bugs instead of
1841 isolating them. Going through the bugzilla database at
1842 @url{http://gcc.gnu.org/bugzilla/} to reduce testcases posted there and
1843 add more information (for example, for which version does the testcase
1844 work, for which versions does it fail?) is also very helpful.
1849 @node Proposed Extensions
1850 @section Proposed Extensions
1852 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
1853 order. Most of these are necessary to be fully compatible with
1854 existing Fortran compilers, but they are not part of the official
1855 J3 Fortran 95 standard.
1857 @subsection Compiler extensions:
1860 User-specified alignment rules for structures.
1863 Flag to generate @code{Makefile} info.
1866 Automatically extend single precision constants to double.
1869 Compile code that conserves memory by dynamically allocating common and
1870 module storage either on stack or heap.
1873 Compile flag to generate code for array conformance checking (suggest -CC).
1876 User control of symbol names (underscores, etc).
1879 Compile setting for maximum size of stack frame size before spilling
1880 parts to static or heap.
1883 Flag to force local variables into static space.
1886 Flag to force local variables onto stack.
1890 @subsection Environment Options
1893 Pluggable library modules for random numbers, linear algebra.
1894 LA should use BLAS calling conventions.
1897 Environment variables controlling actions on arithmetic exceptions like
1898 overflow, underflow, precision loss---Generate NaN, abort, default.
1902 Set precision for fp units that support it (i387).
1905 Variable for setting fp rounding mode.
1908 Variable to fill uninitialized variables with a user-defined bit
1912 Environment variable controlling filename that is opened for that unit
1916 Environment variable to clear/trash memory being freed.
1919 Environment variable to control tracing of allocations and frees.
1922 Environment variable to display allocated memory at normal program end.
1925 Environment variable for filename for * IO-unit.
1928 Environment variable for temporary file directory.
1931 Environment variable forcing standard output to be line buffered (unix).
1936 @c ---------------------------------------------------------------------
1937 @c GNU General Public License
1938 @c ---------------------------------------------------------------------
1940 @include gpl_v3.texi
1944 @c ---------------------------------------------------------------------
1945 @c GNU Free Documentation License
1946 @c ---------------------------------------------------------------------
1952 @c ---------------------------------------------------------------------
1953 @c Funding Free Software
1954 @c ---------------------------------------------------------------------
1956 @include funding.texi
1958 @c ---------------------------------------------------------------------
1960 @c ---------------------------------------------------------------------
1963 @unnumbered Option Index
1964 @command{gfortran}'s command line options are indexed here without any
1965 initial @samp{-} or @samp{--}. Where an option has both positive and
1966 negative forms (such as -foption and -fno-option), relevant entries in
1967 the manual are indexed under the most appropriate form; it may sometimes
1968 be useful to look up both forms.
1972 @unnumbered Keyword Index