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 * Compiler Characteristics:: KIND type parameters supported.
185 * Extensions:: Language extensions implemented by GNU Fortran.
186 * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
187 * Intrinsic Modules:: Intrinsic modules supported by GNU Fortran.
189 * Contributing:: How you can help.
190 * Copying:: GNU General Public License says
191 how you can copy and share GNU Fortran.
192 * GNU Free Documentation License::
193 How you can copy and share this manual.
194 * Funding:: How to help assure continued work for free software.
195 * Option Index:: Index of command line options
196 * Keyword Index:: Index of concepts
200 @c ---------------------------------------------------------------------
202 @c ---------------------------------------------------------------------
205 @chapter Introduction
207 @c The following duplicates the text on the TexInfo table of contents.
209 This manual documents the use of @command{gfortran}, the GNU Fortran
210 compiler. You can find in this manual how to invoke @command{gfortran},
211 as well as its features and incompatibilities.
214 @emph{Warning:} This document, and the compiler it describes, are still
215 under development. While efforts are made to keep it up-to-date, it
216 might not accurately reflect the status of the most recent GNU Fortran
221 The GNU Fortran compiler front end was
222 designed initially as a free replacement for,
223 or alternative to, the unix @command{f95} command;
224 @command{gfortran} is the command you'll use to invoke the compiler.
227 * About GNU Fortran:: What you should know about the GNU Fortran compiler.
228 * GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
229 * Preprocessing and conditional compilation:: The Fortran preprocessor
230 * GNU Fortran and G77:: Why we chose to start from scratch.
231 * Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
232 * Standards:: Standards supported by GNU Fortran.
236 @c ---------------------------------------------------------------------
238 @c ---------------------------------------------------------------------
240 @node About GNU Fortran
241 @section About GNU Fortran
243 The GNU Fortran compiler is still in an early state of development.
244 It can generate code for most constructs and expressions,
245 but much work remains to be done.
247 When the GNU Fortran compiler is finished,
248 it will do everything you expect from any decent compiler:
252 Read a user's program,
253 stored in a file and containing instructions written
254 in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008.
255 This file contains @dfn{source code}.
258 Translate the user's program into instructions a computer
259 can carry out more quickly than it takes to translate the
260 instructions in the first
261 place. The result after compilation of a program is
263 code designed to be efficiently translated and processed
264 by a machine such as your computer.
265 Humans usually aren't as good writing machine code
266 as they are at writing Fortran (or C++, Ada, or Java),
267 because it is easy to make tiny mistakes writing machine code.
270 Provide the user with information about the reasons why
271 the compiler is unable to create a binary from the source code.
272 Usually this will be the case if the source code is flawed.
273 The Fortran 90 standard 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.
910 @c ---------------------------------------------------------------------
911 @c Compiler Characteristics
912 @c ---------------------------------------------------------------------
914 @node Compiler Characteristics
915 @chapter Compiler Characteristics
917 @c TODO: Formulate this introduction a little more generally once
918 @c there is more here than KIND type parameters.
920 This chapter describes certain characteristics of the GNU Fortran compiler,
921 namely the KIND type parameter values supported.
924 * KIND Type Parameters::
928 @node KIND Type Parameters
929 @section KIND Type Parameters
932 The @code{KIND} type parameters supported by GNU Fortran for the primitive
938 1, 2, 4, 8*, 16*, default: 4 (1)
941 1, 2, 4, 8*, 16*, default: 4 (1)
944 4, 8, 10**, 16**, default: 4 (2)
947 4, 8, 10**, 16**, default: 4 (2)
955 * = not available on all systems @*
956 ** = not available on all systems; additionally 10 and 16 are never
957 available at the same time @*
958 (1) Unless -fdefault-integer-8 is used @*
959 (2) Unless -fdefault-real-8 is used
962 The @code{KIND} value matches the storage size in bytes, except for
963 @code{COMPLEX} where the storage size is twice as much (or both real and
964 imaginary part are a real value of the given size). It is recommended to use
965 the @code{SELECT_*_KIND} intrinsics instead of the concrete values.
968 @c ---------------------------------------------------------------------
970 @c ---------------------------------------------------------------------
972 @c Maybe this chapter should be merged with the 'Standards' section,
973 @c whenever that is written :-)
979 The two sections below detail the extensions to standard Fortran that are
980 implemented in GNU Fortran, as well as some of the popular or
981 historically important extensions that are not (or not yet) implemented.
982 For the latter case, we explain the alternatives available to GNU Fortran
983 users, including replacement by standard-conforming code or GNU
987 * Extensions implemented in GNU Fortran::
988 * Extensions not implemented in GNU Fortran::
992 @node Extensions implemented in GNU Fortran
993 @section Extensions implemented in GNU Fortran
994 @cindex extensions, implemented
996 GNU Fortran implements a number of extensions over standard
997 Fortran. This chapter contains information on their syntax and
998 meaning. There are currently two categories of GNU Fortran
999 extensions, those that provide functionality beyond that provided
1000 by any standard, and those that are supported by GNU Fortran
1001 purely for backward compatibility with legacy compilers. By default,
1002 @option{-std=gnu} allows the compiler to accept both types of
1003 extensions, but to warn about the use of the latter. Specifying
1004 either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
1005 disables both types of extensions, and @option{-std=legacy} allows both
1009 * Old-style kind specifications::
1010 * Old-style variable initialization::
1011 * Extensions to namelist::
1012 * X format descriptor without count field::
1013 * Commas in FORMAT specifications::
1014 * Missing period in FORMAT specifications::
1016 * BOZ literal constants::
1017 * Real array indices::
1019 * Implicitly convert LOGICAL and INTEGER values::
1020 * Hollerith constants support::
1022 * CONVERT specifier::
1024 * Argument list functions::
1027 @node Old-style kind specifications
1028 @subsection Old-style kind specifications
1029 @cindex kind, old-style
1031 GNU Fortran allows old-style kind specifications in declarations. These
1037 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
1038 etc.), and where @code{size} is a byte count corresponding to the
1039 storage size of a valid kind for that type. (For @code{COMPLEX}
1040 variables, @code{size} is the total size of the real and imaginary
1041 parts.) The statement then declares @code{x}, @code{y} and @code{z} to
1042 be of type @code{TYPESPEC} with the appropriate kind. This is
1043 equivalent to the standard-conforming declaration
1048 where @code{k} is the kind parameter suitable for the intended precision. As
1049 kind parameters are implementation-dependent, use the @code{KIND},
1050 @code{SELECTED_INT_KIND} and @code{SELECTED_REAL_KIND} intrinsics to retrieve
1051 the correct value, for instance @code{REAL*8 x} can be replaced by:
1053 INTEGER, PARAMETER :: dbl = KIND(1.0d0)
1057 @node Old-style variable initialization
1058 @subsection Old-style variable initialization
1060 GNU Fortran allows old-style initialization of variables of the
1064 REAL x(2,2) /3*0.,1./
1066 The syntax for the initializers is as for the @code{DATA} statement, but
1067 unlike in a @code{DATA} statement, an initializer only applies to the
1068 variable immediately preceding the initialization. In other words,
1069 something like @code{INTEGER I,J/2,3/} is not valid. This style of
1070 initialization is only allowed in declarations without double colons
1071 (@code{::}); the double colons were introduced in Fortran 90, which also
1072 introduced a standard syntax for initializing variables in type
1075 Examples of standard-conforming code equivalent to the above example
1079 INTEGER :: i = 1, j = 2
1080 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
1084 DATA i/1/, j/2/, x/3*0.,1./
1087 Note that variables which are explicitly initialized in declarations
1088 or in @code{DATA} statements automatically acquire the @code{SAVE}
1091 @node Extensions to namelist
1092 @subsection Extensions to namelist
1095 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
1096 including array qualifiers, substrings and fully qualified derived types.
1097 The output from a namelist write is compatible with namelist read. The
1098 output has all names in upper case and indentation to column 1 after the
1099 namelist name. Two extensions are permitted:
1101 Old-style use of @samp{$} instead of @samp{&}
1104 X(:)%Y(2) = 1.0 2.0 3.0
1109 It should be noted that the default terminator is @samp{/} rather than
1112 Querying of the namelist when inputting from stdin. After at least
1113 one space, entering @samp{?} sends to stdout the namelist name and the names of
1114 the variables in the namelist:
1125 Entering @samp{=?} outputs the namelist to stdout, as if
1126 @code{WRITE(*,NML = mynml)} had been called:
1131 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
1132 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
1133 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
1137 To aid this dialog, when input is from stdin, errors send their
1138 messages to stderr and execution continues, even if @code{IOSTAT} is set.
1140 @code{PRINT} namelist is permitted. This causes an error if
1141 @option{-std=f95} is used.
1144 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
1147 END PROGRAM test_print
1150 Expanded namelist reads are permitted. This causes an error if
1151 @option{-std=f95} is used. In the following example, the first element
1152 of the array will be given the value 0.00 and the two succeeding
1153 elements will be given the values 1.00 and 2.00.
1156 X(1,1) = 0.00 , 1.00 , 2.00
1160 @node X format descriptor without count field
1161 @subsection @code{X} format descriptor without count field
1163 To support legacy codes, GNU Fortran permits the count field of the
1164 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
1165 When omitted, the count is implicitly assumed to be one.
1169 10 FORMAT (I1, X, I1)
1172 @node Commas in FORMAT specifications
1173 @subsection Commas in @code{FORMAT} specifications
1175 To support legacy codes, GNU Fortran allows the comma separator
1176 to be omitted immediately before and after character string edit
1177 descriptors in @code{FORMAT} statements.
1181 10 FORMAT ('FOO='I1' BAR='I2)
1185 @node Missing period in FORMAT specifications
1186 @subsection Missing period in @code{FORMAT} specifications
1188 To support legacy codes, GNU Fortran allows missing periods in format
1189 specifications if and only if @option{-std=legacy} is given on the
1190 command line. This is considered non-conforming code and is
1199 @node I/O item lists
1200 @subsection I/O item lists
1201 @cindex I/O item lists
1203 To support legacy codes, GNU Fortran allows the input item list
1204 of the @code{READ} statement, and the output item lists of the
1205 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1207 @node BOZ literal constants
1208 @subsection BOZ literal constants
1209 @cindex BOZ literal constants
1211 Besides decimal constants, Fortran also supports binary (@code{b}),
1212 octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
1213 syntax is: @samp{prefix quote digits quote}, were the prefix is
1214 either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
1215 @code{"} and the digits are for binary @code{0} or @code{1}, for
1216 octal between @code{0} and @code{7}, and for hexadecimal between
1217 @code{0} and @code{F}. (Example: @code{b'01011101'}.)
1219 Up to Fortran 95, BOZ literals were only allowed to initialize
1220 integer variables in DATA statements. Since Fortran 2003 BOZ literals
1221 are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
1222 and @code{CMPLX}; the result is the same as if the integer BOZ
1223 literal had been converted by @code{TRANSFER} to, respectively,
1224 @code{real}, @code{double precision}, @code{integer} or @code{complex}.
1225 As GNU Fortran extension the intrinsic procedures @code{FLOAT},
1226 @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
1228 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1229 be specified using the @code{X} prefix, in addition to the standard
1230 @code{Z} prefix. The BOZ literal can also be specified by adding a
1231 suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
1234 Furthermore, GNU Fortran allows using BOZ literal constants outside
1235 DATA statements and the four intrinsic functions allowed by Fortran 2003.
1236 In DATA statements, in direct assignments, where the right-hand side
1237 only contains a BOZ literal constant, and for old-style initializers of
1238 the form @code{integer i /o'0173'/}, the constant is transferred
1239 as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
1240 the real part is initialized unless @code{CMPLX} is used. In all other
1241 cases, the BOZ literal constant is converted to an @code{INTEGER} value with
1242 the largest decimal representation. This value is then converted
1243 numerically to the type and kind of the variable in question.
1244 (For instance @code{real :: r = b'0000001' + 1} initializes @code{r}
1245 with @code{2.0}.) As different compilers implement the extension
1246 differently, one should be careful when doing bitwise initialization
1247 of non-integer variables.
1249 Note that initializing an @code{INTEGER} variable with a statement such
1250 as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
1251 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1252 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1253 option can be used as a workaround for legacy code that initializes
1254 integers in this manner.
1256 @node Real array indices
1257 @subsection Real array indices
1258 @cindex array, indices of type real
1260 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1261 or variables as array indices.
1263 @node Unary operators
1264 @subsection Unary operators
1265 @cindex operators, unary
1267 As an extension, GNU Fortran allows unary plus and unary minus operators
1268 to appear as the second operand of binary arithmetic operators without
1269 the need for parenthesis.
1275 @node Implicitly convert LOGICAL and INTEGER values
1276 @subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1277 @cindex conversion, to integer
1278 @cindex conversion, to logical
1280 As an extension for backwards compatibility with other compilers, GNU
1281 Fortran allows the implicit conversion of @code{LOGICAL} values to
1282 @code{INTEGER} values and vice versa. When converting from a
1283 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1284 zero, and @code{.TRUE.} is interpreted as one. When converting from
1285 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1286 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1297 However, there is no implicit conversion of @code{INTEGER} values in
1298 @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
1301 @node Hollerith constants support
1302 @subsection Hollerith constants support
1303 @cindex Hollerith constants
1305 GNU Fortran supports Hollerith constants in assignments, function
1306 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1307 constant is written as a string of characters preceded by an integer
1308 constant indicating the character count, and the letter @code{H} or
1309 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1310 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1311 constant will be padded or truncated to fit the size of the variable in
1314 Examples of valid uses of Hollerith constants:
1317 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1318 x(1) = 16HABCDEFGHIJKLMNOP
1322 Invalid Hollerith constants examples:
1325 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1326 a = 0H ! At least one character is needed.
1329 In general, Hollerith constants were used to provide a rudimentary
1330 facility for handling character strings in early Fortran compilers,
1331 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1332 in those cases, the standard-compliant equivalent is to convert the
1333 program to use proper character strings. On occasion, there may be a
1334 case where the intent is specifically to initialize a numeric variable
1335 with a given byte sequence. In these cases, the same result can be
1336 obtained by using the @code{TRANSFER} statement, as in this example.
1338 INTEGER(KIND=4) :: a
1339 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1344 @subsection Cray pointers
1345 @cindex pointer, Cray
1347 Cray pointers are part of a non-standard extension that provides a
1348 C-like pointer in Fortran. This is accomplished through a pair of
1349 variables: an integer "pointer" that holds a memory address, and a
1350 "pointee" that is used to dereference the pointer.
1352 Pointer/pointee pairs are declared in statements of the form:
1354 pointer ( <pointer> , <pointee> )
1358 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1360 The pointer is an integer that is intended to hold a memory address.
1361 The pointee may be an array or scalar. A pointee can be an assumed
1362 size array---that is, the last dimension may be left unspecified by
1363 using a @code{*} in place of a value---but a pointee cannot be an
1364 assumed shape array. No space is allocated for the pointee.
1366 The pointee may have its type declared before or after the pointer
1367 statement, and its array specification (if any) may be declared
1368 before, during, or after the pointer statement. The pointer may be
1369 declared as an integer prior to the pointer statement. However, some
1370 machines have default integer sizes that are different than the size
1371 of a pointer, and so the following code is not portable:
1376 If a pointer is declared with a kind that is too small, the compiler
1377 will issue a warning; the resulting binary will probably not work
1378 correctly, because the memory addresses stored in the pointers may be
1379 truncated. It is safer to omit the first line of the above example;
1380 if explicit declaration of ipt's type is omitted, then the compiler
1381 will ensure that ipt is an integer variable large enough to hold a
1384 Pointer arithmetic is valid with Cray pointers, but it is not the same
1385 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1386 the user is responsible for determining how many bytes to add to a
1387 pointer in order to increment it. Consider the following example:
1391 pointer (ipt, pointee)
1395 The last statement does not set @code{ipt} to the address of
1396 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1397 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1399 Any expression involving the pointee will be translated to use the
1400 value stored in the pointer as the base address.
1402 To get the address of elements, this extension provides an intrinsic
1403 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1404 @code{&} operator in C, except the address is cast to an integer type:
1407 pointer(ipt, arpte(10))
1409 ipt = loc(ar) ! Makes arpte is an alias for ar
1410 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1412 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1415 Cray pointees often are used to alias an existing variable. For
1423 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1424 @code{target}. The optimizer, however, will not detect this aliasing, so
1425 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1426 a pointee in any way that violates the Fortran aliasing rules or
1427 assumptions is illegal. It is the user's responsibility to avoid doing
1428 this; the compiler works under the assumption that no such aliasing
1431 Cray pointers will work correctly when there is no aliasing (i.e., when
1432 they are used to access a dynamically allocated block of memory), and
1433 also in any routine where a pointee is used, but any variable with which
1434 it shares storage is not used. Code that violates these rules may not
1435 run as the user intends. This is not a bug in the optimizer; any code
1436 that violates the aliasing rules is illegal. (Note that this is not
1437 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1438 will ``incorrectly'' optimize code with illegal aliasing.)
1440 There are a number of restrictions on the attributes that can be applied
1441 to Cray pointers and pointees. Pointees may not have the
1442 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1443 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1444 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1445 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes.
1446 Pointees may not occur in more than one pointer statement. A pointee
1447 cannot be a pointer. Pointees cannot occur in equivalence, common, or
1450 A Cray pointer may also point to a function or a subroutine. For
1451 example, the following excerpt is valid:
1455 pointer (subptr,subpte)
1465 A pointer may be modified during the course of a program, and this
1466 will change the location to which the pointee refers. However, when
1467 pointees are passed as arguments, they are treated as ordinary
1468 variables in the invoked function. Subsequent changes to the pointer
1469 will not change the base address of the array that was passed.
1471 @node CONVERT specifier
1472 @subsection @code{CONVERT} specifier
1473 @cindex @code{CONVERT} specifier
1475 GNU Fortran allows the conversion of unformatted data between little-
1476 and big-endian representation to facilitate moving of data
1477 between different systems. The conversion can be indicated with
1478 the @code{CONVERT} specifier on the @code{OPEN} statement.
1479 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1480 the data format via an environment variable.
1482 Valid values for @code{CONVERT} are:
1484 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1485 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1486 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1487 for unformatted files.
1488 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1492 Using the option could look like this:
1494 open(file='big.dat',form='unformatted',access='sequential', &
1495 convert='big_endian')
1498 The value of the conversion can be queried by using
1499 @code{INQUIRE(CONVERT=ch)}. The values returned are
1500 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1502 @code{CONVERT} works between big- and little-endian for
1503 @code{INTEGER} values of all supported kinds and for @code{REAL}
1504 on IEEE systems of kinds 4 and 8. Conversion between different
1505 ``extended double'' types on different architectures such as
1506 m68k and x86_64, which GNU Fortran
1507 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1510 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1511 environment variable will override the CONVERT specifier in the
1512 open statement}. This is to give control over data formats to
1513 users who do not have the source code of their program available.
1515 Using anything but the native representation for unformatted data
1516 carries a significant speed overhead. If speed in this area matters
1517 to you, it is best if you use this only for data that needs to be
1524 OpenMP (Open Multi-Processing) is an application programming
1525 interface (API) that supports multi-platform shared memory
1526 multiprocessing programming in C/C++ and Fortran on many
1527 architectures, including Unix and Microsoft Windows platforms.
1528 It consists of a set of compiler directives, library routines,
1529 and environment variables that influence run-time behavior.
1531 GNU Fortran strives to be compatible to the
1532 @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
1533 OpenMP Application Program Interface v2.5}.
1535 To enable the processing of the OpenMP directive @code{!$omp} in
1536 free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
1537 directives in fixed form; the @code{!$} conditional compilation sentinels
1538 in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
1539 in fixed form, @command{gfortran} needs to be invoked with the
1540 @option{-fopenmp}. This also arranges for automatic linking of the
1541 GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
1544 The OpenMP Fortran runtime library routines are provided both in a
1545 form of a Fortran 90 module named @code{omp_lib} and in a form of
1546 a Fortran @code{include} file named @file{omp_lib.h}.
1548 An example of a parallelized loop taken from Appendix A.1 of
1549 the OpenMP Application Program Interface v2.5:
1551 SUBROUTINE A1(N, A, B)
1554 !$OMP PARALLEL DO !I is private by default
1556 B(I) = (A(I) + A(I-1)) / 2.0
1558 !$OMP END PARALLEL DO
1565 @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
1566 will be allocated on the stack. When porting existing code to OpenMP,
1567 this may lead to surprising results, especially to segmentation faults
1568 if the stacksize is limited.
1571 On glibc-based systems, OpenMP enabled applications can not be statically
1572 linked due to limitations of the underlying pthreads-implementation. It
1573 might be possible to get a working solution if
1574 @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
1575 to the command line. However, this is not supported by @command{gcc} and
1576 thus not recommended.
1579 @node Argument list functions
1580 @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
1581 @cindex argument list functions
1586 GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
1587 and @code{%LOC} statements, for backward compatibility with g77.
1588 It is recommended that these should be used only for code that is
1589 accessing facilities outside of GNU Fortran, such as operating system
1590 or windowing facilities. It is best to constrain such uses to isolated
1591 portions of a program--portions that deal specifically and exclusively
1592 with low-level, system-dependent facilities. Such portions might well
1593 provide a portable interface for use by the program as a whole, but are
1594 themselves not portable, and should be thoroughly tested each time they
1595 are rebuilt using a new compiler or version of a compiler.
1597 @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
1598 reference and @code{%LOC} passes its memory location. Since gfortran
1599 already passes scalar arguments by reference, @code{%REF} is in effect
1600 a do-nothing. @code{%LOC} has the same effect as a fortran pointer.
1602 An example of passing an argument by value to a C subroutine foo.:
1605 C prototype void foo_ (float x);
1614 For details refer to the g77 manual
1615 @uref{http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top}.
1617 Also, the gfortran testsuite c_by_val.f and its partner c_by_val.c are
1622 @node Extensions not implemented in GNU Fortran
1623 @section Extensions not implemented in GNU Fortran
1624 @cindex extensions, not implemented
1626 The long history of the Fortran language, its wide use and broad
1627 userbase, the large number of different compiler vendors and the lack of
1628 some features crucial to users in the first standards have lead to the
1629 existence of a number of important extensions to the language. While
1630 some of the most useful or popular extensions are supported by the GNU
1631 Fortran compiler, not all existing extensions are supported. This section
1632 aims at listing these extensions and offering advice on how best make
1633 code that uses them running with the GNU Fortran compiler.
1635 @c More can be found here:
1636 @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
1637 @c -- the list of fortran and libgfortran bugs closed as WONTFIX:
1638 @c http://tinyurl.com/2u4h5y
1641 * STRUCTURE and RECORD::
1642 @c * UNION and MAP::
1643 * ENCODE and DECODE statements::
1644 * Variable FORMAT expressions::
1645 @c * Q edit descriptor::
1646 @c * AUTOMATIC statement::
1647 @c * TYPE and ACCEPT I/O Statements::
1648 @c * .XOR. operator::
1649 @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
1650 @c * Omitted arguments in procedure call:
1654 @node STRUCTURE and RECORD
1655 @subsection @code{STRUCTURE} and @code{RECORD}
1656 @cindex @code{STRUCTURE}
1657 @cindex @code{RECORD}
1659 Structures are user-defined aggregate data types; this functionality was
1660 standardized in Fortran 90 with an different syntax, under the name of
1661 ``derived types''. Here is an example of code using the non portable
1665 ! Declaring a structure named ``item'' and containing three fields:
1666 ! an integer ID, an description string and a floating-point price.
1669 CHARACTER(LEN=200) description
1673 ! Define two variables, an single record of type ``item''
1674 ! named ``pear'', and an array of items named ``store_catalog''
1675 RECORD /item/ pear, store_catalog(100)
1677 ! We can directly access the fields of both variables
1679 pear.description = "juicy D'Anjou pear"
1681 store_catalog(7).id = 7831
1682 store_catalog(7).description = "milk bottle"
1683 store_catalog(7).price = 1.2
1685 ! We can also manipulate the whole structure
1686 store_catalog(12) = pear
1687 print *, store_catalog(12)
1691 This code can easily be rewritten in the Fortran 90 syntax as following:
1694 ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
1695 ! ``TYPE name ... END TYPE''
1698 CHARACTER(LEN=200) description
1702 ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
1703 TYPE(item) pear, store_catalog(100)
1705 ! Instead of using a dot (.) to access fields of a record, the
1706 ! standard syntax uses a percent sign (%)
1708 pear%description = "juicy D'Anjou pear"
1710 store_catalog(7)%id = 7831
1711 store_catalog(7)%description = "milk bottle"
1712 store_catalog(7)%price = 1.2
1714 ! Assignments of a whole variable don't change
1715 store_catalog(12) = pear
1716 print *, store_catalog(12)
1720 @c @node UNION and MAP
1721 @c @subsection @code{UNION} and @code{MAP}
1722 @c @cindex @code{UNION}
1723 @c @cindex @code{MAP}
1725 @c For help writing this one, see
1726 @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and
1727 @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm
1730 @node ENCODE and DECODE statements
1731 @subsection @code{ENCODE} and @code{DECODE} statements
1732 @cindex @code{ENCODE}
1733 @cindex @code{DECODE}
1735 GNU Fortran doesn't support the @code{ENCODE} and @code{DECODE}
1736 statements. These statements are best replaced by @code{READ} and
1737 @code{WRITE} statements involving internal files (@code{CHARACTER}
1738 variables and arrays), which have been part of the Fortran standard since
1739 Fortran 77. For example, replace a code fragment like
1744 c ... Code that sets LINE
1745 DECODE (80, 9000, LINE) A, B, C
1746 9000 FORMAT (1X, 3(F10.5))
1753 CHARACTER(LEN=80) LINE
1755 c ... Code that sets LINE
1756 READ (UNIT=LINE, FMT=9000) A, B, C
1757 9000 FORMAT (1X, 3(F10.5))
1760 Similarly, replace a code fragment like
1765 c ... Code that sets A, B and C
1766 ENCODE (80, 9000, LINE) A, B, C
1767 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1776 c ... Code that sets A, B and C
1777 WRITE (UNIT=LINE, FMT=9000) A, B, C
1778 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1782 @node Variable FORMAT expressions
1783 @subsection Variable @code{FORMAT} expressions
1784 @cindex @code{FORMAT}
1786 A variable @code{FORMAT} expression is format statement which includes
1787 angle brackets enclosing a Fortran expression: @code{FORMAT(I<N>)}. GNU
1788 Fortran does not support this legacy extension. The effect of variable
1789 format expressions can be reproduced by using the more powerful (and
1790 standard) combination of internal output and string formats. For example,
1791 replace a code fragment like this:
1802 c Variable declaration
1805 c Other code here...
1807 WRITE(FMT,'("(I", I0, ")")') N+1
1815 c Variable declaration
1816 CHARACTER(LEN=20) FMT
1818 c Other code here...
1821 WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1
1827 @c ---------------------------------------------------------------------
1828 @c Intrinsic Procedures
1829 @c ---------------------------------------------------------------------
1831 @include intrinsic.texi
1838 @c ---------------------------------------------------------------------
1840 @c ---------------------------------------------------------------------
1843 @unnumbered Contributing
1844 @cindex Contributing
1846 Free software is only possible if people contribute to efforts
1848 We're always in need of more people helping out with ideas
1849 and comments, writing documentation and contributing code.
1851 If you want to contribute to GNU Fortran,
1852 have a look at the long lists of projects you can take on.
1853 Some of these projects are small,
1854 some of them are large;
1855 some are completely orthogonal to the rest of what is
1856 happening on GNU Fortran,
1857 but others are ``mainstream'' projects in need of enthusiastic hackers.
1858 All of these projects are important!
1859 We'll eventually get around to the things here,
1860 but they are also things doable by someone who is willing and able.
1865 * Proposed Extensions::
1870 @section Contributors to GNU Fortran
1871 @cindex Contributors
1875 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
1876 also the initiator of the whole project. Thanks Andy!
1877 Most of the interface with GCC was written by @emph{Paul Brook}.
1879 The following individuals have contributed code and/or
1880 ideas and significant help to the GNU Fortran project
1881 (in alphabetical order):
1884 @item Janne Blomqvist
1885 @item Steven Bosscher
1888 @item Fran@,{c}ois-Xavier Coudert
1892 @item Bernhard Fischer
1894 @item Richard Guenther
1895 @item Richard Henderson
1896 @item Katherine Holcomb
1898 @item Niels Kristian Bech Jensen
1899 @item Steven Johnson
1900 @item Steven G. Kargl
1908 @item Christopher D. Rickett
1909 @item Richard Sandiford
1910 @item Tobias Schl@"uter
1918 The following people have contributed bug reports,
1919 smaller or larger patches,
1920 and much needed feedback and encouragement for the
1921 GNU Fortran project:
1925 @item Dominique d'Humi@`eres
1927 @item Erik Schnetter
1930 Many other individuals have helped debug,
1931 test and improve the GNU Fortran compiler over the past few years,
1932 and we welcome you to do the same!
1933 If you already have done so,
1934 and you would like to see your name listed in the
1935 list above, please contact us.
1943 @item Help build the test suite
1944 Solicit more code for donation to the test suite: the more extensive the
1945 testsuite, the smaller the risk of breaking things in the future! We can
1946 keep code private on request.
1948 @item Bug hunting/squishing
1949 Find bugs and write more test cases! Test cases are especially very
1950 welcome, because it allows us to concentrate on fixing bugs instead of
1951 isolating them. Going through the bugzilla database at
1952 @url{http://gcc.gnu.org/bugzilla/} to reduce testcases posted there and
1953 add more information (for example, for which version does the testcase
1954 work, for which versions does it fail?) is also very helpful.
1959 @node Proposed Extensions
1960 @section Proposed Extensions
1962 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
1963 order. Most of these are necessary to be fully compatible with
1964 existing Fortran compilers, but they are not part of the official
1965 J3 Fortran 95 standard.
1967 @subsection Compiler extensions:
1970 User-specified alignment rules for structures.
1973 Flag to generate @code{Makefile} info.
1976 Automatically extend single precision constants to double.
1979 Compile code that conserves memory by dynamically allocating common and
1980 module storage either on stack or heap.
1983 Compile flag to generate code for array conformance checking (suggest -CC).
1986 User control of symbol names (underscores, etc).
1989 Compile setting for maximum size of stack frame size before spilling
1990 parts to static or heap.
1993 Flag to force local variables into static space.
1996 Flag to force local variables onto stack.
2000 @subsection Environment Options
2003 Pluggable library modules for random numbers, linear algebra.
2004 LA should use BLAS calling conventions.
2007 Environment variables controlling actions on arithmetic exceptions like
2008 overflow, underflow, precision loss---Generate NaN, abort, default.
2012 Set precision for fp units that support it (i387).
2015 Variable for setting fp rounding mode.
2018 Variable to fill uninitialized variables with a user-defined bit
2022 Environment variable controlling filename that is opened for that unit
2026 Environment variable to clear/trash memory being freed.
2029 Environment variable to control tracing of allocations and frees.
2032 Environment variable to display allocated memory at normal program end.
2035 Environment variable for filename for * IO-unit.
2038 Environment variable for temporary file directory.
2041 Environment variable forcing standard output to be line buffered (unix).
2046 @c ---------------------------------------------------------------------
2047 @c GNU General Public License
2048 @c ---------------------------------------------------------------------
2050 @include gpl_v3.texi
2054 @c ---------------------------------------------------------------------
2055 @c GNU Free Documentation License
2056 @c ---------------------------------------------------------------------
2062 @c ---------------------------------------------------------------------
2063 @c Funding Free Software
2064 @c ---------------------------------------------------------------------
2066 @include funding.texi
2068 @c ---------------------------------------------------------------------
2070 @c ---------------------------------------------------------------------
2073 @unnumbered Option Index
2074 @command{gfortran}'s command line options are indexed here without any
2075 initial @samp{-} or @samp{--}. Where an option has both positive and
2076 negative forms (such as -foption and -fno-option), relevant entries in
2077 the manual are indexed under the most appropriate form; it may sometimes
2078 be useful to look up both forms.
2082 @unnumbered Keyword Index