1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
6 @include gcc-common.texi
8 @settitle The GNU Fortran Compiler
10 @c Create a separate index for command line options
12 @c Merge the standard indexes into a single one.
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60 @c Use with @@smallbook.
62 @c %** start of document
64 @c Cause even numbered pages to be printed on the left hand side of
65 @c the page and odd numbered pages to be printed on the right hand
66 @c side of the page. Using this, you can print on both sides of a
67 @c sheet of paper and have the text on the same part of the sheet.
69 @c The text on right hand pages is pushed towards the right hand
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 1.3 or
84 any later version published by the Free Software Foundation; with the
85 Invariant Sections being ``Funding Free Software'', the Front-Cover
86 Texts being (a) (see below), and with the Back-Cover Texts being (b)
87 (see below). A copy of the license is included in the section entitled
88 ``GNU Free Documentation License''.
90 (a) The FSF's Front-Cover Text is:
94 (b) The FSF's Back-Cover Text is:
96 You have freedom to copy and modify this GNU Manual, like GNU
97 software. Copies published by the Free Software Foundation raise
98 funds for GNU development.
102 @dircategory Software development
104 * gfortran: (gfortran). The GNU Fortran Compiler.
106 This file documents the use and the internals of
107 the GNU Fortran compiler, (@command{gfortran}).
109 Published by the Free Software Foundation
110 51 Franklin Street, Fifth Floor
111 Boston, MA 02110-1301 USA
117 @setchapternewpage odd
119 @title Using GNU Fortran
121 @author The @t{gfortran} team
123 @vskip 0pt plus 1filll
124 Published by the Free Software Foundation@*
125 51 Franklin Street, Fifth Floor@*
126 Boston, MA 02110-1301, USA@*
127 @c Last printed ??ber, 19??.@*
128 @c Printed copies are available for $? each.@*
134 @c TODO: The following "Part" definitions are included here temporarily
135 @c until they are incorporated into the official Texinfo distribution.
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151 @c ---------------------------------------------------------------------
152 @c TexInfo table of contents.
153 @c ---------------------------------------------------------------------
160 This manual documents the use of @command{gfortran},
161 the GNU Fortran compiler. You can find in this manual how to invoke
162 @command{gfortran}, as well as its features and incompatibilities.
165 @emph{Warning:} This document, and the compiler it describes, are still
166 under development. While efforts are made to keep it up-to-date, it might
167 not accurately reflect the status of the most recent GNU Fortran compiler.
171 @comment When you add a new menu item, please keep the right hand
172 @comment aligned to the same column. Do not use tabs. This provides
173 @comment better formatting.
178 Part I: Invoking GNU Fortran
179 * Invoking GNU Fortran:: Command options supported by @command{gfortran}.
180 * Runtime:: Influencing runtime behavior with environment variables.
182 Part II: Language Reference
183 * Fortran 2003 and 2008 status:: Fortran 2003 and 2008 features supported by GNU Fortran.
184 * Compiler Characteristics:: User-visible implementation details.
185 * Mixed-Language Programming:: Interoperability with C
186 * Extensions:: Language extensions implemented by GNU Fortran.
187 * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
188 * Intrinsic Modules:: Intrinsic modules supported by GNU Fortran.
190 * Contributing:: How you can help.
191 * Copying:: GNU General Public License says
192 how you can copy and share GNU Fortran.
193 * GNU Free Documentation License::
194 How you can copy and share this manual.
195 * Funding:: How to help assure continued work for free software.
196 * Option Index:: Index of command line options
197 * Keyword Index:: Index of concepts
201 @c ---------------------------------------------------------------------
203 @c ---------------------------------------------------------------------
206 @chapter Introduction
208 @c The following duplicates the text on the TexInfo table of contents.
210 This manual documents the use of @command{gfortran}, the GNU Fortran
211 compiler. You can find in this manual how to invoke @command{gfortran},
212 as well as its features and incompatibilities.
215 @emph{Warning:} This document, and the compiler it describes, are still
216 under development. While efforts are made to keep it up-to-date, it
217 might not accurately reflect the status of the most recent GNU Fortran
222 The GNU Fortran compiler front end was
223 designed initially as a free replacement for,
224 or alternative to, the unix @command{f95} command;
225 @command{gfortran} is the command you'll use to invoke the compiler.
228 * About GNU Fortran:: What you should know about the GNU Fortran compiler.
229 * GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
230 * Preprocessing and conditional compilation:: The Fortran preprocessor
231 * GNU Fortran and G77:: Why we chose to start from scratch.
232 * Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
233 * Standards:: Standards supported by GNU Fortran.
237 @c ---------------------------------------------------------------------
239 @c ---------------------------------------------------------------------
241 @node About GNU Fortran
242 @section About GNU Fortran
244 The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards
245 completely, parts of the Fortran 2003 and Fortran 2008 standards, and
246 several vendor extensions. The development goal is to provide the
251 Read a user's program,
252 stored in a file and containing instructions written
253 in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008.
254 This file contains @dfn{source code}.
257 Translate the user's program into instructions a computer
258 can carry out more quickly than it takes to translate the
259 instructions in the first
260 place. The result after compilation of a program is
262 code designed to be efficiently translated and processed
263 by a machine such as your computer.
264 Humans usually aren't as good writing machine code
265 as they are at writing Fortran (or C++, Ada, or Java),
266 because it is easy to make tiny mistakes writing machine code.
269 Provide the user with information about the reasons why
270 the compiler is unable to create a binary from the source code.
271 Usually this will be the case if the source code is flawed.
272 The Fortran 90 standard requires that the compiler can point out
273 mistakes to the user.
274 An incorrect usage of the language causes an @dfn{error message}.
276 The compiler will also attempt to diagnose cases where the
277 user's program contains a correct usage of the language,
278 but instructs the computer to do something questionable.
279 This kind of diagnostics message is called a @dfn{warning message}.
282 Provide optional information about the translation passes
283 from the source code to machine code.
284 This can help a user of the compiler to find the cause of
285 certain bugs which may not be obvious in the source code,
286 but may be more easily found at a lower level compiler output.
287 It also helps developers to find bugs in the compiler itself.
290 Provide information in the generated machine code that can
291 make it easier to find bugs in the program (using a debugging tool,
292 called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
295 Locate and gather machine code already generated to
296 perform actions requested by statements in the user's program.
297 This machine code is organized into @dfn{modules} and is located
298 and @dfn{linked} to the user program.
301 The GNU Fortran compiler consists of several components:
305 A version of the @command{gcc} command
306 (which also might be installed as the system's @command{cc} command)
307 that also understands and accepts Fortran source code.
308 The @command{gcc} command is the @dfn{driver} program for
309 all the languages in the GNU Compiler Collection (GCC);
311 you can compile the source code of any language for
312 which a front end is available in GCC.
315 The @command{gfortran} command itself,
316 which also might be installed as the
317 system's @command{f95} command.
318 @command{gfortran} is just another driver program,
319 but specifically for the Fortran compiler only.
320 The difference with @command{gcc} is that @command{gfortran}
321 will automatically link the correct libraries to your program.
324 A collection of run-time libraries.
325 These libraries contain the machine code needed to support
326 capabilities of the Fortran language that are not directly
327 provided by the machine code generated by the
328 @command{gfortran} compilation phase,
329 such as intrinsic functions and subroutines,
330 and routines for interaction with files and the operating system.
331 @c and mechanisms to spawn,
332 @c unleash and pause threads in parallelized code.
335 The Fortran compiler itself, (@command{f951}).
336 This is the GNU Fortran parser and code generator,
337 linked to and interfaced with the GCC backend library.
338 @command{f951} ``translates'' the source code to
339 assembler code. You would typically not use this
341 instead, the @command{gcc} or @command{gfortran} driver
342 programs will call it for you.
346 @c ---------------------------------------------------------------------
347 @c GNU Fortran and GCC
348 @c ---------------------------------------------------------------------
350 @node GNU Fortran and GCC
351 @section GNU Fortran and GCC
352 @cindex GNU Compiler Collection
355 GNU Fortran is a part of GCC, the @dfn{GNU Compiler Collection}. GCC
356 consists of a collection of front ends for various languages, which
357 translate the source code into a language-independent form called
358 @dfn{GENERIC}. This is then processed by a common middle end which
359 provides optimization, and then passed to one of a collection of back
360 ends which generate code for different computer architectures and
363 Functionally, this is implemented with a driver program (@command{gcc})
364 which provides the command-line interface for the compiler. It calls
365 the relevant compiler front-end program (e.g., @command{f951} for
366 Fortran) for each file in the source code, and then calls the assembler
367 and linker as appropriate to produce the compiled output. In a copy of
368 GCC which has been compiled with Fortran language support enabled,
369 @command{gcc} will recognize files with @file{.f}, @file{.for}, @file{.ftn},
370 @file{.f90}, @file{.f95}, @file{.f03} and @file{.f08} extensions as
371 Fortran source code, and compile it accordingly. A @command{gfortran}
372 driver program is also provided, which is identical to @command{gcc}
373 except that it automatically links the Fortran runtime libraries into the
376 Source files with @file{.f}, @file{.for}, @file{.fpp}, @file{.ftn}, @file{.F},
377 @file{.FOR}, @file{.FPP}, and @file{.FTN} extensions are treated as fixed form.
378 Source files with @file{.f90}, @file{.f95}, @file{.f03}, @file{.f08},
379 @file{.F90}, @file{.F95}, @file{.F03} and @file{.F08} extensions are
380 treated as free form. The capitalized versions of either form are run
381 through preprocessing. Source files with the lower case @file{.fpp}
382 extension are also run through preprocessing.
384 This manual specifically documents the Fortran front end, which handles
385 the programming language's syntax and semantics. The aspects of GCC
386 which relate to the optimization passes and the back-end code generation
387 are documented in the GCC manual; see
388 @ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
389 The two manuals together provide a complete reference for the GNU
393 @c ---------------------------------------------------------------------
394 @c Preprocessing and conditional compilation
395 @c ---------------------------------------------------------------------
397 @node Preprocessing and conditional compilation
398 @section Preprocessing and conditional compilation
401 @cindex Conditional compilation
402 @cindex Preprocessing
403 @cindex preprocessor, include file handling
405 Many Fortran compilers including GNU Fortran allow passing the source code
406 through a C preprocessor (CPP; sometimes also called the Fortran preprocessor,
407 FPP) to allow for conditional compilation. In the case of GNU Fortran,
408 this is the GNU C Preprocessor in the traditional mode. On systems with
409 case-preserving file names, the preprocessor is automatically invoked if the
410 filename extension is @file{.F}, @file{.FOR}, @file{.FTN}, @file{.fpp},
411 @file{.FPP}, @file{.F90}, @file{.F95}, @file{.F03} or @file{.F08}. To manually
412 invoke the preprocessor on any file, use @option{-cpp}, to disable
413 preprocessing on files where the preprocessor is run automatically, use
416 If a preprocessed file includes another file with the Fortran @code{INCLUDE}
417 statement, the included file is not preprocessed. To preprocess included
418 files, use the equivalent preprocessor statement @code{#include}.
420 If GNU Fortran invokes the preprocessor, @code{__GFORTRAN__}
421 is defined and @code{__GNUC__}, @code{__GNUC_MINOR__} and
422 @code{__GNUC_PATCHLEVEL__} can be used to determine the version of the
423 compiler. See @ref{Top,,Overview,cpp,The C Preprocessor} for details.
425 While CPP is the de-facto standard for preprocessing Fortran code,
426 Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines
427 Conditional Compilation, which is not widely used and not directly
428 supported by the GNU Fortran compiler. You can use the program coco
429 to preprocess such files (@uref{http://www.daniellnagle.com/coco.html}).
432 @c ---------------------------------------------------------------------
433 @c GNU Fortran and G77
434 @c ---------------------------------------------------------------------
436 @node GNU Fortran and G77
437 @section GNU Fortran and G77
439 @cindex @command{g77}
441 The GNU Fortran compiler is the successor to @command{g77}, the Fortran
442 77 front end included in GCC prior to version 4. It is an entirely new
443 program that has been designed to provide Fortran 95 support and
444 extensibility for future Fortran language standards, as well as providing
445 backwards compatibility for Fortran 77 and nearly all of the GNU language
446 extensions supported by @command{g77}.
449 @c ---------------------------------------------------------------------
451 @c ---------------------------------------------------------------------
454 @section Project Status
457 As soon as @command{gfortran} can parse all of the statements correctly,
458 it will be in the ``larva'' state.
459 When we generate code, the ``puppa'' state.
460 When @command{gfortran} is done,
461 we'll see if it will be a beautiful butterfly,
462 or just a big bug....
464 --Andy Vaught, April 2000
467 The start of the GNU Fortran 95 project was announced on
468 the GCC homepage in March 18, 2000
469 (even though Andy had already been working on it for a while,
472 The GNU Fortran compiler is able to compile nearly all
473 standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs,
474 including a number of standard and non-standard extensions, and can be
475 used on real-world programs. In particular, the supported extensions
476 include OpenMP, Cray-style pointers, and several Fortran 2003 and Fortran
477 2008 features, including TR 15581. However, it is still under
478 development and has a few remaining rough edges.
480 At present, the GNU Fortran compiler passes the
481 @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
482 NIST Fortran 77 Test Suite}, and produces acceptable results on the
483 @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}.
484 It also provides respectable performance on
485 the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran
486 compiler benchmarks} and the
487 @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html,
488 Livermore Fortran Kernels test}. It has been used to compile a number of
489 large real-world programs, including
490 @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM
491 weather-forecasting code} and
492 @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum
493 chemistry package}; see @url{http://gcc.gnu.org/@/wiki/@/GfortranApps} for an
496 Among other things, the GNU Fortran compiler is intended as a replacement
497 for G77. At this point, nearly all programs that could be compiled with
498 G77 can be compiled with GNU Fortran, although there are a few minor known
501 The primary work remaining to be done on GNU Fortran falls into three
502 categories: bug fixing (primarily regarding the treatment of invalid code
503 and providing useful error messages), improving the compiler optimizations
504 and the performance of compiled code, and extending the compiler to support
505 future standards---in particular, Fortran 2003 and Fortran 2008.
508 @c ---------------------------------------------------------------------
510 @c ---------------------------------------------------------------------
517 * Varying Length Character Strings::
520 The GNU Fortran compiler implements
521 ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
522 standard-compliant Fortran 90 and Fortran 77 programs. It also supports
523 the ISO/IEC TR-15581 enhancements to allocatable arrays.
525 In the future, the GNU Fortran compiler will also support ISO/IEC
526 1539-1:2004 (Fortran 2003), ISO/IEC 1539-1:2010 (Fortran 2008) and
527 future Fortran standards. Partial support of the Fortran 2003 and
528 Fortran 2008 standard is already provided; the current status of the
529 support is reported in the @ref{Fortran 2003 status} and
530 @ref{Fortran 2008 status} sections of the documentation.
532 Additionally, the GNU Fortran compilers supports the OpenMP specification
533 (version 3.0, @url{http://openmp.org/@/wp/@/openmp-specifications/}).
535 @node Varying Length Character Strings
536 @subsection Varying Length Character Strings
537 @cindex Varying length character strings
538 @cindex Varying length strings
539 @cindex strings, varying length
541 The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000)
542 varying length character strings. While GNU Fortran currently does not
543 support such strings directly, there exist two Fortran implementations
544 for them, which work with GNU Fortran. They can be found at
545 @uref{http://www.fortran.com/@/iso_varying_string.f95} and at
546 @uref{ftp://ftp.nag.co.uk/@/sc22wg5/@/ISO_VARYING_STRING/}.
550 @c =====================================================================
551 @c PART I: INVOCATION REFERENCE
552 @c =====================================================================
555 \part{I}{Invoking GNU Fortran}
558 @c ---------------------------------------------------------------------
560 @c ---------------------------------------------------------------------
565 @c ---------------------------------------------------------------------
567 @c ---------------------------------------------------------------------
570 @chapter Runtime: Influencing runtime behavior with environment variables
571 @cindex environment variable
573 The behavior of the @command{gfortran} can be influenced by
574 environment variables.
576 Malformed environment variables are silently ignored.
579 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
580 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
581 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
582 * GFORTRAN_USE_STDERR:: Send library output to standard error
583 * GFORTRAN_TMPDIR:: Directory for scratch files
584 * GFORTRAN_UNBUFFERED_ALL:: Don't buffer I/O for all units.
585 * GFORTRAN_UNBUFFERED_PRECONNECTED:: Don't buffer I/O for preconnected units.
586 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
587 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
588 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
589 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
590 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
591 * GFORTRAN_ERROR_DUMPCORE:: Dump core on run-time errors
592 * GFORTRAN_ERROR_BACKTRACE:: Show backtrace on run-time errors
595 @node GFORTRAN_STDIN_UNIT
596 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
598 This environment variable can be used to select the unit number
599 preconnected to standard input. This must be a positive integer.
600 The default value is 5.
602 @node GFORTRAN_STDOUT_UNIT
603 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
605 This environment variable can be used to select the unit number
606 preconnected to standard output. This must be a positive integer.
607 The default value is 6.
609 @node GFORTRAN_STDERR_UNIT
610 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
612 This environment variable can be used to select the unit number
613 preconnected to standard error. This must be a positive integer.
614 The default value is 0.
616 @node GFORTRAN_USE_STDERR
617 @section @env{GFORTRAN_USE_STDERR}---Send library output to standard error
619 This environment variable controls where library output is sent.
620 If the first letter is @samp{y}, @samp{Y} or @samp{1}, standard
621 error is used. If the first letter is @samp{n}, @samp{N} or
622 @samp{0}, standard output is used.
624 @node GFORTRAN_TMPDIR
625 @section @env{GFORTRAN_TMPDIR}---Directory for scratch files
627 This environment variable controls where scratch files are
628 created. If this environment variable is missing,
629 GNU Fortran searches for the environment variable @env{TMP}, then @env{TEMP}.
630 If these are missing, the default is @file{/tmp}.
632 @node GFORTRAN_UNBUFFERED_ALL
633 @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer I/O on all units
635 This environment variable controls whether all I/O is unbuffered. If
636 the first letter is @samp{y}, @samp{Y} or @samp{1}, all I/O is
637 unbuffered. This will slow down small sequential reads and writes. If
638 the first letter is @samp{n}, @samp{N} or @samp{0}, I/O is buffered.
641 @node GFORTRAN_UNBUFFERED_PRECONNECTED
642 @section @env{GFORTRAN_UNBUFFERED_PRECONNECTED}---Don't buffer I/O on preconnected units
644 The environment variable named @env{GFORTRAN_UNBUFFERED_PRECONNECTED} controls
645 whether I/O on a preconnected unit (i.e.@: STDOUT or STDERR) is unbuffered. If
646 the first letter is @samp{y}, @samp{Y} or @samp{1}, I/O is unbuffered. This
647 will slow down small sequential reads and writes. If the first letter
648 is @samp{n}, @samp{N} or @samp{0}, I/O is buffered. This is the default.
650 @node GFORTRAN_SHOW_LOCUS
651 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
653 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
654 line numbers for runtime errors are printed. If the first letter is
655 @samp{n}, @samp{N} or @samp{0}, don't print filename and line numbers
656 for runtime errors. The default is to print the location.
658 @node GFORTRAN_OPTIONAL_PLUS
659 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
661 If the first letter is @samp{y}, @samp{Y} or @samp{1},
662 a plus sign is printed
663 where permitted by the Fortran standard. If the first letter
664 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
665 in most cases. Default is not to print plus signs.
667 @node GFORTRAN_DEFAULT_RECL
668 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
670 This environment variable specifies the default record length, in
671 bytes, for files which are opened without a @code{RECL} tag in the
672 @code{OPEN} statement. This must be a positive integer. The
673 default value is 1073741824 bytes (1 GB).
675 @node GFORTRAN_LIST_SEPARATOR
676 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
678 This environment variable specifies the separator when writing
679 list-directed output. It may contain any number of spaces and
680 at most one comma. If you specify this on the command line,
681 be sure to quote spaces, as in
683 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
685 when @command{a.out} is the compiled Fortran program that you want to run.
686 Default is a single space.
688 @node GFORTRAN_CONVERT_UNIT
689 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
691 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
692 to change the representation of data for unformatted files.
693 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
695 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
696 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
697 exception: mode ':' unit_list | unit_list ;
698 unit_list: unit_spec | unit_list unit_spec ;
699 unit_spec: INTEGER | INTEGER '-' INTEGER ;
701 The variable consists of an optional default mode, followed by
702 a list of optional exceptions, which are separated by semicolons
703 from the preceding default and each other. Each exception consists
704 of a format and a comma-separated list of units. Valid values for
705 the modes are the same as for the @code{CONVERT} specifier:
708 @item @code{NATIVE} Use the native format. This is the default.
709 @item @code{SWAP} Swap between little- and big-endian.
710 @item @code{LITTLE_ENDIAN} Use the little-endian format
711 for unformatted files.
712 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
714 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
715 Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
717 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
718 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
719 in little_endian mode, except for units 10 to 20 and 25, which are in
721 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
724 Setting the environment variables should be done on the command
725 line or via the @command{export}
726 command for @command{sh}-compatible shells and via @command{setenv}
727 for @command{csh}-compatible shells.
729 Example for @command{sh}:
732 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
735 Example code for @command{csh}:
738 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
742 Using anything but the native representation for unformatted data
743 carries a significant speed overhead. If speed in this area matters
744 to you, it is best if you use this only for data that needs to be
747 @xref{CONVERT specifier}, for an alternative way to specify the
748 data representation for unformatted files. @xref{Runtime Options}, for
749 setting a default data representation for the whole program. The
750 @code{CONVERT} specifier overrides the @option{-fconvert} compile options.
752 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
753 environment variable will override the CONVERT specifier in the
754 open statement}. This is to give control over data formats to
755 users who do not have the source code of their program available.
757 @node GFORTRAN_ERROR_DUMPCORE
758 @section @env{GFORTRAN_ERROR_DUMPCORE}---Dump core on run-time errors
760 If the @env{GFORTRAN_ERROR_DUMPCORE} variable is set to
761 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
762 then library run-time errors cause core dumps. To disable the core
763 dumps, set the variable to @samp{n}, @samp{N}, @samp{0}. Default
764 is not to core dump unless the @option{-fdump-core} compile option
767 @node GFORTRAN_ERROR_BACKTRACE
768 @section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors
770 If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to
771 @samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
772 then a backtrace is printed when a run-time error occurs.
773 To disable the backtracing, set the variable to
774 @samp{n}, @samp{N}, @samp{0}. Default is not to print a backtrace
775 unless the @option{-fbacktrace} compile option
778 @c =====================================================================
779 @c PART II: LANGUAGE REFERENCE
780 @c =====================================================================
783 \part{II}{Language Reference}
786 @c ---------------------------------------------------------------------
787 @c Fortran 2003 and 2008 Status
788 @c ---------------------------------------------------------------------
790 @node Fortran 2003 and 2008 status
791 @chapter Fortran 2003 and 2008 Status
794 * Fortran 2003 status::
795 * Fortran 2008 status::
798 @node Fortran 2003 status
799 @section Fortran 2003 status
801 GNU Fortran supports several Fortran 2003 features; an incomplete
802 list can be found below. See also the
803 @uref{http://gcc.gnu.org/wiki/Fortran2003, wiki page} about Fortran 2003.
807 Intrinsics @code{command_argument_count}, @code{get_command},
808 @code{get_command_argument}, @code{get_environment_variable}, and
812 @cindex array, constructors
814 Array constructors using square brackets. That is, @code{[...]} rather
815 than @code{(/.../)}. Type-specification for array constructors like
816 @code{(/ some-type :: ... /)}.
819 @cindex @code{FLUSH} statement
820 @cindex statement, @code{FLUSH}
821 @code{FLUSH} statement.
824 @cindex @code{IOMSG=} specifier
825 @code{IOMSG=} specifier for I/O statements.
828 @cindex @code{ENUM} statement
829 @cindex @code{ENUMERATOR} statement
830 @cindex statement, @code{ENUM}
831 @cindex statement, @code{ENUMERATOR}
832 @opindex @code{fshort-enums}
833 Support for the declaration of enumeration constants via the
834 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
835 @command{gcc} is guaranteed also for the case where the
836 @command{-fshort-enums} command line option is given.
843 @cindex @code{ALLOCATABLE} dummy arguments
844 @code{ALLOCATABLE} dummy arguments.
846 @cindex @code{ALLOCATABLE} function results
847 @code{ALLOCATABLE} function results
849 @cindex @code{ALLOCATABLE} components of derived types
850 @code{ALLOCATABLE} components of derived types
854 @cindex @code{ALLOCATE}
855 The @code{ERRMSG=} tag is now supported in @code{ALLOCATE} and
856 @code{DEALLOCATE} statements. The @code{SOURCE=} tag is supported
857 in an @code{ALLOCATE} statement. An @emph{intrinsic-type-spec}
858 can be used as the @emph{type-spec} in an @code{ALLOCATE} statement;
859 while the use of a @emph{derived-type-name} is currently unsupported.
862 @cindex @code{STREAM} I/O
863 @cindex @code{ACCESS='STREAM'} I/O
864 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
865 allowing I/O without any record structure.
868 Namelist input/output for internal files.
871 @cindex @code{PROTECTED} statement
872 @cindex statement, @code{PROTECTED}
873 The @code{PROTECTED} statement and attribute.
876 @cindex @code{VALUE} statement
877 @cindex statement, @code{VALUE}
878 The @code{VALUE} statement and attribute.
881 @cindex @code{VOLATILE} statement
882 @cindex statement, @code{VOLATILE}
883 The @code{VOLATILE} statement and attribute.
886 @cindex @code{IMPORT} statement
887 @cindex statement, @code{IMPORT}
888 The @code{IMPORT} statement, allowing to import
889 host-associated derived types.
892 @cindex @code{USE, INTRINSIC} statement
893 @cindex statement, @code{USE, INTRINSIC}
894 @cindex @code{ISO_FORTRAN_ENV} statement
895 @cindex statement, @code{ISO_FORTRAN_ENV}
896 @code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
897 attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
898 @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
901 Renaming of operators in the @code{USE} statement.
904 @cindex ISO C Bindings
905 Interoperability with C (ISO C Bindings)
908 BOZ as argument of @code{INT}, @code{REAL}, @code{DBLE} and @code{CMPLX}.
911 @cindex type-bound procedure
912 @cindex type-bound operator
913 Type-bound procedures with @code{PROCEDURE} or @code{GENERIC}, and operators
914 bound to a derived-type.
917 @cindex @code{EXTENDS}
918 @cindex derived-type extension
919 Extension of derived-types (the @code{EXTENDS(...)} syntax).
922 @cindex @code{ABSTRACT} type
923 @cindex @code{DEFERRED} procedure binding
924 @code{ABSTRACT} derived-types and declaring procedure bindings @code{DEFERRED}.
929 @node Fortran 2008 status
930 @section Fortran 2008 status
932 The latest version of the Fortran standard is ISO/IEC 1539-1:2010, informally
933 known as Fortran 2008. The official version is available from International
934 Organization for Standardization (ISO) or its national member organizations.
935 The the final draft (FDIS) can be downloaded free of charge from
936 @url{http://www.nag.co.uk/@/sc22wg5/@/links.html}. Fortran is developed by the
937 Working Group 5 of Sub-Committee 22 of the Joint Technical Committee 1 of the
938 International Organization for Standardization and the International
939 Electrotechnical Commission (IEC). This group is known as
940 @uref{http://www.nag.co.uk/sc22wg5/, WG5}.
942 The GNU Fortran supports several of the new features of Fortran 2008; the
943 @uref{http://gcc.gnu.org/wiki/Fortran2008Status, wiki} has some information
944 about the current Fortran 2008 implementation status. In particular, the
945 following is implemented.
948 @item The @option{-std=f2008} option and support for the file extensions
949 @file{.f08} and @file{.F08}.
951 @item The @code{OPEN} statement now supports the @code{NEWUNIT=} option,
952 which returns a unique file unit, thus preventing inadvertent use of the
953 same unit in different parts of the program.
955 @item The @code{g0} format descriptor and unlimited format items.
957 @item The mathematical intrinsics @code{ASINH}, @code{ACOSH}, @code{ATANH},
958 @code{ERF}, @code{ERFC}, @code{GAMMA}, @code{LOG_GAMMA}, @code{BESSEL_J0},
959 @code{BESSEL_J1}, @code{BESSEL_JN}, @code{BESSEL_Y0}, @code{BESSEL_Y1},
960 @code{BESSEL_YN}, @code{HYPOT}, @code{NORM2}, and @code{ERFC_SCALED}.
962 @item Using complex arguments with @code{TAN}, @code{SINH}, @code{COSH},
963 @code{TANH}, @code{ASIN}, @code{ACOS}, and @code{ATAN} is now possible;
964 @code{ATAN}(@var{Y},@var{X}) is now an alias for @code{ATAN2}(@var{Y},@var{X}).
966 @item Support of the @code{PARITY} intrinsic functions.
968 @item The following bit intrinsics: @code{LEADZ} and @code{TRAILZ} for
969 counting the number of leading and trailing zero bits, @code{POPCNT} and
970 @code{POPPAR} for counting the number of one bits and returning the parity;
971 @code{BGE}, @code{BGT}, @code{BLE}, and @code{BLT} for bitwise comparisons;
972 @code{DSHIFTL} and @code{DSHIFTR} for combined left and right shifts,
973 @code{MASKL} and @code{MASKR} for simple left and right justified masks,
974 @code{MERGE_BITS} for a bitwise merge using a mask, @code{SHIFTA},
975 @code{SHIFTL} and @code{SHIFTR} for shift operations, and the
976 transformational bit intrinsics @code{IALL}, @code{IANY} and @code{IPARITY}.
978 @item Support of the @code{EXECUTE_COMMAND_LINE} intrinsic subroutine.
980 @item Support for the @code{STORAGE_SIZE} intrinsic inquiry function.
982 @item The @code{INT@{8,16,32@}} and @code{REAL@{32,64,128@}} kind type
983 parameters and the array-valued named constants @code{INTEGER_KINDS},
984 @code{LOGICAL_KINDS}, @code{REAL_KINDS} and @code{CHARACTER_KINDS} of
985 the intrinsic module @code{ISO_FORTRAN_ENV}.
987 @item The module procedures @code{C_SIZEOF} of the intrinsic module
988 @code{ISO_C_BINDINGS} and @code{COMPILER_VERSION} and @code{COMPILER_OPTIONS}
989 of @code{ISO_FORTRAN_ENV}.
991 @item Experimental coarray support (for one image only), use the
992 @option{-fcoarray=single} flag to enable it.
994 @item The @code{BLOCK} construct is supported.
996 @item The @code{STOP} and the new @code{ERROR STOP} statements now
997 support all constant expressions.
999 @item Support for the @code{CONTIGUOUS} attribute.
1001 @item Support for @code{ALLOCATE} with @code{MOLD}.
1003 @item Support for the @code{IMPURE} attribute for procedures, which
1004 allows for @code{ELEMENTAL} procedures without the restrictions of
1007 @item Null pointers (including @code{NULL()}) and not-allocated variables
1008 can be used as actual argument to optional non-pointer, non-allocatable
1009 dummy arguments, denoting an absent argument.
1011 @item Non-pointer variables with @code{TARGET} attribute can be used as
1012 actual argument to @code{POINTER} dummies with @code{INTENT(IN)}.
1014 @item Pointers including procedure pointers and those in a derived
1015 type (pointer components) can now be initialized by a target instead
1016 of only by @code{NULL}.
1018 @item The @code{EXIT} statement (with construct-name) can be now be
1019 used to leave not only the @code{DO} but also the @code{ASSOCIATE},
1020 @code{BLOCK}, @code{IF}, @code{SELECT CASE} and @code{SELECT TYPE}
1023 @item Internal procedures can now be used as actual argument.
1025 @item Minor features: obsolesce diagnostics for @code{ENTRY} with
1026 @option{-std=f2008}; a line may start with a semicolon; for internal
1027 and module procedures @code{END} can be used instead of
1028 @code{END SUBROUTINE} and @code{END FUNCTION}; @code{SELECTED_REAL_KIND}
1029 now also takes a @code{RADIX} argument; intrinsic types are supported
1030 for @code{TYPE}(@var{intrinsic-type-spec}); multiple type-bound procedures
1031 can be declared in a single @code{PROCEDURE} statement; implied-shape
1032 arrays are supported for named constants (@code{PARAMETER}).
1037 @c ---------------------------------------------------------------------
1038 @c Compiler Characteristics
1039 @c ---------------------------------------------------------------------
1041 @node Compiler Characteristics
1042 @chapter Compiler Characteristics
1044 This chapter describes certain characteristics of the GNU Fortran
1045 compiler, that are not specified by the Fortran standard, but which
1046 might in some way or another become visible to the programmer.
1049 * KIND Type Parameters::
1050 * Internal representation of LOGICAL variables::
1054 @node KIND Type Parameters
1055 @section KIND Type Parameters
1058 The @code{KIND} type parameters supported by GNU Fortran for the primitive
1064 1, 2, 4, 8*, 16*, default: 4 (1)
1067 1, 2, 4, 8*, 16*, default: 4 (1)
1070 4, 8, 10*, 16*, default: 4 (2)
1073 4, 8, 10*, 16*, default: 4 (2)
1081 * = not available on all systems @*
1082 (1) Unless -fdefault-integer-8 is used @*
1083 (2) Unless -fdefault-real-8 is used
1086 The @code{KIND} value matches the storage size in bytes, except for
1087 @code{COMPLEX} where the storage size is twice as much (or both real and
1088 imaginary part are a real value of the given size). It is recommended to use
1089 the @code{SELECTED_CHAR_KIND}, @code{SELECTED_INT_KIND} and
1090 @code{SELECTED_REAL_KIND} intrinsics or the @code{INT8}, @code{INT16},
1091 @code{INT32}, @code{INT64}, @code{REAL32}, @code{REAL64}, and @code{REAL128}
1092 parameters of the @code{ISO_FORTRAN_ENV} module instead of the concrete values.
1093 The available kind parameters can be found in the constant arrays
1094 @code{CHARACTER_KINDS}, @code{INTEGER_KINDS}, @code{LOGICAL_KINDS} and
1095 @code{REAL_KINDS} in the @code{ISO_FORTRAN_ENV} module
1096 (see @ref{ISO_FORTRAN_ENV}).
1099 @node Internal representation of LOGICAL variables
1100 @section Internal representation of LOGICAL variables
1101 @cindex logical, variable representation
1103 The Fortran standard does not specify how variables of @code{LOGICAL}
1104 type are represented, beyond requiring that @code{LOGICAL} variables
1105 of default kind have the same storage size as default @code{INTEGER}
1106 and @code{REAL} variables. The GNU Fortran internal representation is
1109 A @code{LOGICAL(KIND=N)} variable is represented as an
1110 @code{INTEGER(KIND=N)} variable, however, with only two permissible
1111 values: @code{1} for @code{.TRUE.} and @code{0} for
1112 @code{.FALSE.}. Any other integer value results in undefined behavior.
1114 Note that for mixed-language programming using the
1115 @code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can
1116 be used to create @code{LOGICAL(KIND=C_BOOL)} variables which are
1117 interoperable with the C99 _Bool type. The C99 _Bool type has an
1118 internal representation described in the C99 standard, which is
1119 identical to the above description, i.e. with 1 for true and 0 for
1120 false being the only permissible values. Thus the internal
1121 representation of @code{LOGICAL} variables in GNU Fortran is identical
1122 to C99 _Bool, except for a possible difference in storage size
1123 depending on the kind.
1125 @c ---------------------------------------------------------------------
1127 @c ---------------------------------------------------------------------
1129 @c Maybe this chapter should be merged with the 'Standards' section,
1130 @c whenever that is written :-)
1136 The two sections below detail the extensions to standard Fortran that are
1137 implemented in GNU Fortran, as well as some of the popular or
1138 historically important extensions that are not (or not yet) implemented.
1139 For the latter case, we explain the alternatives available to GNU Fortran
1140 users, including replacement by standard-conforming code or GNU
1144 * Extensions implemented in GNU Fortran::
1145 * Extensions not implemented in GNU Fortran::
1149 @node Extensions implemented in GNU Fortran
1150 @section Extensions implemented in GNU Fortran
1151 @cindex extensions, implemented
1153 GNU Fortran implements a number of extensions over standard
1154 Fortran. This chapter contains information on their syntax and
1155 meaning. There are currently two categories of GNU Fortran
1156 extensions, those that provide functionality beyond that provided
1157 by any standard, and those that are supported by GNU Fortran
1158 purely for backward compatibility with legacy compilers. By default,
1159 @option{-std=gnu} allows the compiler to accept both types of
1160 extensions, but to warn about the use of the latter. Specifying
1161 either @option{-std=f95}, @option{-std=f2003} or @option{-std=f2008}
1162 disables both types of extensions, and @option{-std=legacy} allows both
1166 * Old-style kind specifications::
1167 * Old-style variable initialization::
1168 * Extensions to namelist::
1169 * X format descriptor without count field::
1170 * Commas in FORMAT specifications::
1171 * Missing period in FORMAT specifications::
1173 * BOZ literal constants::
1174 * Real array indices::
1176 * Implicitly convert LOGICAL and INTEGER values::
1177 * Hollerith constants support::
1179 * CONVERT specifier::
1181 * Argument list functions::
1184 @node Old-style kind specifications
1185 @subsection Old-style kind specifications
1186 @cindex kind, old-style
1188 GNU Fortran allows old-style kind specifications in declarations. These
1194 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
1195 etc.), and where @code{size} is a byte count corresponding to the
1196 storage size of a valid kind for that type. (For @code{COMPLEX}
1197 variables, @code{size} is the total size of the real and imaginary
1198 parts.) The statement then declares @code{x}, @code{y} and @code{z} to
1199 be of type @code{TYPESPEC} with the appropriate kind. This is
1200 equivalent to the standard-conforming declaration
1205 where @code{k} is the kind parameter suitable for the intended precision. As
1206 kind parameters are implementation-dependent, use the @code{KIND},
1207 @code{SELECTED_INT_KIND} and @code{SELECTED_REAL_KIND} intrinsics to retrieve
1208 the correct value, for instance @code{REAL*8 x} can be replaced by:
1210 INTEGER, PARAMETER :: dbl = KIND(1.0d0)
1214 @node Old-style variable initialization
1215 @subsection Old-style variable initialization
1217 GNU Fortran allows old-style initialization of variables of the
1221 REAL x(2,2) /3*0.,1./
1223 The syntax for the initializers is as for the @code{DATA} statement, but
1224 unlike in a @code{DATA} statement, an initializer only applies to the
1225 variable immediately preceding the initialization. In other words,
1226 something like @code{INTEGER I,J/2,3/} is not valid. This style of
1227 initialization is only allowed in declarations without double colons
1228 (@code{::}); the double colons were introduced in Fortran 90, which also
1229 introduced a standard syntax for initializing variables in type
1232 Examples of standard-conforming code equivalent to the above example
1236 INTEGER :: i = 1, j = 2
1237 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
1241 DATA i/1/, j/2/, x/3*0.,1./
1244 Note that variables which are explicitly initialized in declarations
1245 or in @code{DATA} statements automatically acquire the @code{SAVE}
1248 @node Extensions to namelist
1249 @subsection Extensions to namelist
1252 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
1253 including array qualifiers, substrings and fully qualified derived types.
1254 The output from a namelist write is compatible with namelist read. The
1255 output has all names in upper case and indentation to column 1 after the
1256 namelist name. Two extensions are permitted:
1258 Old-style use of @samp{$} instead of @samp{&}
1261 X(:)%Y(2) = 1.0 2.0 3.0
1266 It should be noted that the default terminator is @samp{/} rather than
1269 Querying of the namelist when inputting from stdin. After at least
1270 one space, entering @samp{?} sends to stdout the namelist name and the names of
1271 the variables in the namelist:
1282 Entering @samp{=?} outputs the namelist to stdout, as if
1283 @code{WRITE(*,NML = mynml)} had been called:
1288 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
1289 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
1290 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
1294 To aid this dialog, when input is from stdin, errors send their
1295 messages to stderr and execution continues, even if @code{IOSTAT} is set.
1297 @code{PRINT} namelist is permitted. This causes an error if
1298 @option{-std=f95} is used.
1301 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
1304 END PROGRAM test_print
1307 Expanded namelist reads are permitted. This causes an error if
1308 @option{-std=f95} is used. In the following example, the first element
1309 of the array will be given the value 0.00 and the two succeeding
1310 elements will be given the values 1.00 and 2.00.
1313 X(1,1) = 0.00 , 1.00 , 2.00
1317 @node X format descriptor without count field
1318 @subsection @code{X} format descriptor without count field
1320 To support legacy codes, GNU Fortran permits the count field of the
1321 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
1322 When omitted, the count is implicitly assumed to be one.
1326 10 FORMAT (I1, X, I1)
1329 @node Commas in FORMAT specifications
1330 @subsection Commas in @code{FORMAT} specifications
1332 To support legacy codes, GNU Fortran allows the comma separator
1333 to be omitted immediately before and after character string edit
1334 descriptors in @code{FORMAT} statements.
1338 10 FORMAT ('FOO='I1' BAR='I2)
1342 @node Missing period in FORMAT specifications
1343 @subsection Missing period in @code{FORMAT} specifications
1345 To support legacy codes, GNU Fortran allows missing periods in format
1346 specifications if and only if @option{-std=legacy} is given on the
1347 command line. This is considered non-conforming code and is
1356 @node I/O item lists
1357 @subsection I/O item lists
1358 @cindex I/O item lists
1360 To support legacy codes, GNU Fortran allows the input item list
1361 of the @code{READ} statement, and the output item lists of the
1362 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1364 @node BOZ literal constants
1365 @subsection BOZ literal constants
1366 @cindex BOZ literal constants
1368 Besides decimal constants, Fortran also supports binary (@code{b}),
1369 octal (@code{o}) and hexadecimal (@code{z}) integer constants. The
1370 syntax is: @samp{prefix quote digits quote}, were the prefix is
1371 either @code{b}, @code{o} or @code{z}, quote is either @code{'} or
1372 @code{"} and the digits are for binary @code{0} or @code{1}, for
1373 octal between @code{0} and @code{7}, and for hexadecimal between
1374 @code{0} and @code{F}. (Example: @code{b'01011101'}.)
1376 Up to Fortran 95, BOZ literals were only allowed to initialize
1377 integer variables in DATA statements. Since Fortran 2003 BOZ literals
1378 are also allowed as argument of @code{REAL}, @code{DBLE}, @code{INT}
1379 and @code{CMPLX}; the result is the same as if the integer BOZ
1380 literal had been converted by @code{TRANSFER} to, respectively,
1381 @code{real}, @code{double precision}, @code{integer} or @code{complex}.
1382 As GNU Fortran extension the intrinsic procedures @code{FLOAT},
1383 @code{DFLOAT}, @code{COMPLEX} and @code{DCMPLX} are treated alike.
1385 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1386 be specified using the @code{X} prefix, in addition to the standard
1387 @code{Z} prefix. The BOZ literal can also be specified by adding a
1388 suffix to the string, for example, @code{Z'ABC'} and @code{'ABC'Z} are
1391 Furthermore, GNU Fortran allows using BOZ literal constants outside
1392 DATA statements and the four intrinsic functions allowed by Fortran 2003.
1393 In DATA statements, in direct assignments, where the right-hand side
1394 only contains a BOZ literal constant, and for old-style initializers of
1395 the form @code{integer i /o'0173'/}, the constant is transferred
1396 as if @code{TRANSFER} had been used; for @code{COMPLEX} numbers, only
1397 the real part is initialized unless @code{CMPLX} is used. In all other
1398 cases, the BOZ literal constant is converted to an @code{INTEGER} value with
1399 the largest decimal representation. This value is then converted
1400 numerically to the type and kind of the variable in question.
1401 (For instance, @code{real :: r = b'0000001' + 1} initializes @code{r}
1402 with @code{2.0}.) As different compilers implement the extension
1403 differently, one should be careful when doing bitwise initialization
1404 of non-integer variables.
1406 Note that initializing an @code{INTEGER} variable with a statement such
1407 as @code{DATA i/Z'FFFFFFFF'/} will give an integer overflow error rather
1408 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1409 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1410 option can be used as a workaround for legacy code that initializes
1411 integers in this manner.
1413 @node Real array indices
1414 @subsection Real array indices
1415 @cindex array, indices of type real
1417 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1418 or variables as array indices.
1420 @node Unary operators
1421 @subsection Unary operators
1422 @cindex operators, unary
1424 As an extension, GNU Fortran allows unary plus and unary minus operators
1425 to appear as the second operand of binary arithmetic operators without
1426 the need for parenthesis.
1432 @node Implicitly convert LOGICAL and INTEGER values
1433 @subsection Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1434 @cindex conversion, to integer
1435 @cindex conversion, to logical
1437 As an extension for backwards compatibility with other compilers, GNU
1438 Fortran allows the implicit conversion of @code{LOGICAL} values to
1439 @code{INTEGER} values and vice versa. When converting from a
1440 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1441 zero, and @code{.TRUE.} is interpreted as one. When converting from
1442 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1443 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1454 However, there is no implicit conversion of @code{INTEGER} values in
1455 @code{if}-statements, nor of @code{LOGICAL} or @code{INTEGER} values
1458 @node Hollerith constants support
1459 @subsection Hollerith constants support
1460 @cindex Hollerith constants
1462 GNU Fortran supports Hollerith constants in assignments, function
1463 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1464 constant is written as a string of characters preceded by an integer
1465 constant indicating the character count, and the letter @code{H} or
1466 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1467 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1468 constant will be padded or truncated to fit the size of the variable in
1471 Examples of valid uses of Hollerith constants:
1474 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1475 x(1) = 16HABCDEFGHIJKLMNOP
1479 Invalid Hollerith constants examples:
1482 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1483 a = 0H ! At least one character is needed.
1486 In general, Hollerith constants were used to provide a rudimentary
1487 facility for handling character strings in early Fortran compilers,
1488 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1489 in those cases, the standard-compliant equivalent is to convert the
1490 program to use proper character strings. On occasion, there may be a
1491 case where the intent is specifically to initialize a numeric variable
1492 with a given byte sequence. In these cases, the same result can be
1493 obtained by using the @code{TRANSFER} statement, as in this example.
1495 INTEGER(KIND=4) :: a
1496 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1501 @subsection Cray pointers
1502 @cindex pointer, Cray
1504 Cray pointers are part of a non-standard extension that provides a
1505 C-like pointer in Fortran. This is accomplished through a pair of
1506 variables: an integer "pointer" that holds a memory address, and a
1507 "pointee" that is used to dereference the pointer.
1509 Pointer/pointee pairs are declared in statements of the form:
1511 pointer ( <pointer> , <pointee> )
1515 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1517 The pointer is an integer that is intended to hold a memory address.
1518 The pointee may be an array or scalar. A pointee can be an assumed
1519 size array---that is, the last dimension may be left unspecified by
1520 using a @code{*} in place of a value---but a pointee cannot be an
1521 assumed shape array. No space is allocated for the pointee.
1523 The pointee may have its type declared before or after the pointer
1524 statement, and its array specification (if any) may be declared
1525 before, during, or after the pointer statement. The pointer may be
1526 declared as an integer prior to the pointer statement. However, some
1527 machines have default integer sizes that are different than the size
1528 of a pointer, and so the following code is not portable:
1533 If a pointer is declared with a kind that is too small, the compiler
1534 will issue a warning; the resulting binary will probably not work
1535 correctly, because the memory addresses stored in the pointers may be
1536 truncated. It is safer to omit the first line of the above example;
1537 if explicit declaration of ipt's type is omitted, then the compiler
1538 will ensure that ipt is an integer variable large enough to hold a
1541 Pointer arithmetic is valid with Cray pointers, but it is not the same
1542 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1543 the user is responsible for determining how many bytes to add to a
1544 pointer in order to increment it. Consider the following example:
1548 pointer (ipt, pointee)
1552 The last statement does not set @code{ipt} to the address of
1553 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1554 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1556 Any expression involving the pointee will be translated to use the
1557 value stored in the pointer as the base address.
1559 To get the address of elements, this extension provides an intrinsic
1560 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1561 @code{&} operator in C, except the address is cast to an integer type:
1564 pointer(ipt, arpte(10))
1566 ipt = loc(ar) ! Makes arpte is an alias for ar
1567 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1569 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1572 Cray pointees often are used to alias an existing variable. For
1580 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1581 @code{target}. The optimizer, however, will not detect this aliasing, so
1582 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1583 a pointee in any way that violates the Fortran aliasing rules or
1584 assumptions is illegal. It is the user's responsibility to avoid doing
1585 this; the compiler works under the assumption that no such aliasing
1588 Cray pointers will work correctly when there is no aliasing (i.e., when
1589 they are used to access a dynamically allocated block of memory), and
1590 also in any routine where a pointee is used, but any variable with which
1591 it shares storage is not used. Code that violates these rules may not
1592 run as the user intends. This is not a bug in the optimizer; any code
1593 that violates the aliasing rules is illegal. (Note that this is not
1594 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1595 will ``incorrectly'' optimize code with illegal aliasing.)
1597 There are a number of restrictions on the attributes that can be applied
1598 to Cray pointers and pointees. Pointees may not have the
1599 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1600 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1601 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1602 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes, nor
1603 may they be function results. Pointees may not occur in more than one
1604 pointer statement. A pointee cannot be a pointer. Pointees cannot occur
1605 in equivalence, common, or data statements.
1607 A Cray pointer may also point to a function or a subroutine. For
1608 example, the following excerpt is valid:
1612 pointer (subptr,subpte)
1622 A pointer may be modified during the course of a program, and this
1623 will change the location to which the pointee refers. However, when
1624 pointees are passed as arguments, they are treated as ordinary
1625 variables in the invoked function. Subsequent changes to the pointer
1626 will not change the base address of the array that was passed.
1628 @node CONVERT specifier
1629 @subsection @code{CONVERT} specifier
1630 @cindex @code{CONVERT} specifier
1632 GNU Fortran allows the conversion of unformatted data between little-
1633 and big-endian representation to facilitate moving of data
1634 between different systems. The conversion can be indicated with
1635 the @code{CONVERT} specifier on the @code{OPEN} statement.
1636 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1637 the data format via an environment variable.
1639 Valid values for @code{CONVERT} are:
1641 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1642 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1643 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1644 for unformatted files.
1645 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1649 Using the option could look like this:
1651 open(file='big.dat',form='unformatted',access='sequential', &
1652 convert='big_endian')
1655 The value of the conversion can be queried by using
1656 @code{INQUIRE(CONVERT=ch)}. The values returned are
1657 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1659 @code{CONVERT} works between big- and little-endian for
1660 @code{INTEGER} values of all supported kinds and for @code{REAL}
1661 on IEEE systems of kinds 4 and 8. Conversion between different
1662 ``extended double'' types on different architectures such as
1663 m68k and x86_64, which GNU Fortran
1664 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1667 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1668 environment variable will override the CONVERT specifier in the
1669 open statement}. This is to give control over data formats to
1670 users who do not have the source code of their program available.
1672 Using anything but the native representation for unformatted data
1673 carries a significant speed overhead. If speed in this area matters
1674 to you, it is best if you use this only for data that needs to be
1681 OpenMP (Open Multi-Processing) is an application programming
1682 interface (API) that supports multi-platform shared memory
1683 multiprocessing programming in C/C++ and Fortran on many
1684 architectures, including Unix and Microsoft Windows platforms.
1685 It consists of a set of compiler directives, library routines,
1686 and environment variables that influence run-time behavior.
1688 GNU Fortran strives to be compatible to the
1689 @uref{http://www.openmp.org/mp-documents/spec30.pdf,
1690 OpenMP Application Program Interface v3.0}.
1692 To enable the processing of the OpenMP directive @code{!$omp} in
1693 free-form source code; the @code{c$omp}, @code{*$omp} and @code{!$omp}
1694 directives in fixed form; the @code{!$} conditional compilation sentinels
1695 in free form; and the @code{c$}, @code{*$} and @code{!$} sentinels
1696 in fixed form, @command{gfortran} needs to be invoked with the
1697 @option{-fopenmp}. This also arranges for automatic linking of the
1698 GNU OpenMP runtime library @ref{Top,,libgomp,libgomp,GNU OpenMP
1701 The OpenMP Fortran runtime library routines are provided both in a
1702 form of a Fortran 90 module named @code{omp_lib} and in a form of
1703 a Fortran @code{include} file named @file{omp_lib.h}.
1705 An example of a parallelized loop taken from Appendix A.1 of
1706 the OpenMP Application Program Interface v2.5:
1708 SUBROUTINE A1(N, A, B)
1711 !$OMP PARALLEL DO !I is private by default
1713 B(I) = (A(I) + A(I-1)) / 2.0
1715 !$OMP END PARALLEL DO
1722 @option{-fopenmp} implies @option{-frecursive}, i.e., all local arrays
1723 will be allocated on the stack. When porting existing code to OpenMP,
1724 this may lead to surprising results, especially to segmentation faults
1725 if the stacksize is limited.
1728 On glibc-based systems, OpenMP enabled applications cannot be statically
1729 linked due to limitations of the underlying pthreads-implementation. It
1730 might be possible to get a working solution if
1731 @command{-Wl,--whole-archive -lpthread -Wl,--no-whole-archive} is added
1732 to the command line. However, this is not supported by @command{gcc} and
1733 thus not recommended.
1736 @node Argument list functions
1737 @subsection Argument list functions @code{%VAL}, @code{%REF} and @code{%LOC}
1738 @cindex argument list functions
1743 GNU Fortran supports argument list functions @code{%VAL}, @code{%REF}
1744 and @code{%LOC} statements, for backward compatibility with g77.
1745 It is recommended that these should be used only for code that is
1746 accessing facilities outside of GNU Fortran, such as operating system
1747 or windowing facilities. It is best to constrain such uses to isolated
1748 portions of a program--portions that deal specifically and exclusively
1749 with low-level, system-dependent facilities. Such portions might well
1750 provide a portable interface for use by the program as a whole, but are
1751 themselves not portable, and should be thoroughly tested each time they
1752 are rebuilt using a new compiler or version of a compiler.
1754 @code{%VAL} passes a scalar argument by value, @code{%REF} passes it by
1755 reference and @code{%LOC} passes its memory location. Since gfortran
1756 already passes scalar arguments by reference, @code{%REF} is in effect
1757 a do-nothing. @code{%LOC} has the same effect as a Fortran pointer.
1759 An example of passing an argument by value to a C subroutine foo.:
1762 C prototype void foo_ (float x);
1771 For details refer to the g77 manual
1772 @uref{http://gcc.gnu.org/@/onlinedocs/@/gcc-3.4.6/@/g77/@/index.html#Top}.
1774 Also, @code{c_by_val.f} and its partner @code{c_by_val.c} of the
1775 GNU Fortran testsuite are worth a look.
1778 @node Extensions not implemented in GNU Fortran
1779 @section Extensions not implemented in GNU Fortran
1780 @cindex extensions, not implemented
1782 The long history of the Fortran language, its wide use and broad
1783 userbase, the large number of different compiler vendors and the lack of
1784 some features crucial to users in the first standards have lead to the
1785 existence of a number of important extensions to the language. While
1786 some of the most useful or popular extensions are supported by the GNU
1787 Fortran compiler, not all existing extensions are supported. This section
1788 aims at listing these extensions and offering advice on how best make
1789 code that uses them running with the GNU Fortran compiler.
1791 @c More can be found here:
1792 @c -- http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Missing-Features.html
1793 @c -- the list of Fortran and libgfortran bugs closed as WONTFIX:
1794 @c http://tinyurl.com/2u4h5y
1797 * STRUCTURE and RECORD::
1798 @c * UNION and MAP::
1799 * ENCODE and DECODE statements::
1800 * Variable FORMAT expressions::
1801 @c * Q edit descriptor::
1802 @c * AUTOMATIC statement::
1803 @c * TYPE and ACCEPT I/O Statements::
1804 @c * .XOR. operator::
1805 @c * CARRIAGECONTROL, DEFAULTFILE, DISPOSE and RECORDTYPE I/O specifiers::
1806 @c * Omitted arguments in procedure call::
1807 * Alternate complex function syntax::
1811 @node STRUCTURE and RECORD
1812 @subsection @code{STRUCTURE} and @code{RECORD}
1813 @cindex @code{STRUCTURE}
1814 @cindex @code{RECORD}
1816 Structures are user-defined aggregate data types; this functionality was
1817 standardized in Fortran 90 with an different syntax, under the name of
1818 ``derived types''. Here is an example of code using the non portable
1822 ! Declaring a structure named ``item'' and containing three fields:
1823 ! an integer ID, an description string and a floating-point price.
1826 CHARACTER(LEN=200) description
1830 ! Define two variables, an single record of type ``item''
1831 ! named ``pear'', and an array of items named ``store_catalog''
1832 RECORD /item/ pear, store_catalog(100)
1834 ! We can directly access the fields of both variables
1836 pear.description = "juicy D'Anjou pear"
1838 store_catalog(7).id = 7831
1839 store_catalog(7).description = "milk bottle"
1840 store_catalog(7).price = 1.2
1842 ! We can also manipulate the whole structure
1843 store_catalog(12) = pear
1844 print *, store_catalog(12)
1848 This code can easily be rewritten in the Fortran 90 syntax as following:
1851 ! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
1852 ! ``TYPE name ... END TYPE''
1855 CHARACTER(LEN=200) description
1859 ! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
1860 TYPE(item) pear, store_catalog(100)
1862 ! Instead of using a dot (.) to access fields of a record, the
1863 ! standard syntax uses a percent sign (%)
1865 pear%description = "juicy D'Anjou pear"
1867 store_catalog(7)%id = 7831
1868 store_catalog(7)%description = "milk bottle"
1869 store_catalog(7)%price = 1.2
1871 ! Assignments of a whole variable don't change
1872 store_catalog(12) = pear
1873 print *, store_catalog(12)
1877 @c @node UNION and MAP
1878 @c @subsection @code{UNION} and @code{MAP}
1879 @c @cindex @code{UNION}
1880 @c @cindex @code{MAP}
1882 @c For help writing this one, see
1883 @c http://www.eng.umd.edu/~nsw/ench250/fortran1.htm#UNION and
1884 @c http://www.tacc.utexas.edu/services/userguides/pgi/pgiws_ug/pgi32u06.htm
1887 @node ENCODE and DECODE statements
1888 @subsection @code{ENCODE} and @code{DECODE} statements
1889 @cindex @code{ENCODE}
1890 @cindex @code{DECODE}
1892 GNU Fortran doesn't support the @code{ENCODE} and @code{DECODE}
1893 statements. These statements are best replaced by @code{READ} and
1894 @code{WRITE} statements involving internal files (@code{CHARACTER}
1895 variables and arrays), which have been part of the Fortran standard since
1896 Fortran 77. For example, replace a code fragment like
1901 c ... Code that sets LINE
1902 DECODE (80, 9000, LINE) A, B, C
1903 9000 FORMAT (1X, 3(F10.5))
1910 CHARACTER(LEN=80) LINE
1912 c ... Code that sets LINE
1913 READ (UNIT=LINE, FMT=9000) A, B, C
1914 9000 FORMAT (1X, 3(F10.5))
1917 Similarly, replace a code fragment like
1922 c ... Code that sets A, B and C
1923 ENCODE (80, 9000, LINE) A, B, C
1924 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1931 CHARACTER(LEN=80) LINE
1933 c ... Code that sets A, B and C
1934 WRITE (UNIT=LINE, FMT=9000) A, B, C
1935 9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
1939 @node Variable FORMAT expressions
1940 @subsection Variable @code{FORMAT} expressions
1941 @cindex @code{FORMAT}
1943 A variable @code{FORMAT} expression is format statement which includes
1944 angle brackets enclosing a Fortran expression: @code{FORMAT(I<N>)}. GNU
1945 Fortran does not support this legacy extension. The effect of variable
1946 format expressions can be reproduced by using the more powerful (and
1947 standard) combination of internal output and string formats. For example,
1948 replace a code fragment like this:
1959 c Variable declaration
1960 CHARACTER(LEN=20) FMT
1962 c Other code here...
1964 WRITE(FMT,'("(I", I0, ")")') N+1
1972 c Variable declaration
1973 CHARACTER(LEN=20) FMT
1975 c Other code here...
1978 WRITE(6,"(I" // ADJUSTL(FMT) // ")") INT1
1982 @node Alternate complex function syntax
1983 @subsection Alternate complex function syntax
1984 @cindex Complex function
1986 Some Fortran compilers, including @command{g77}, let the user declare
1987 complex functions with the syntax @code{COMPLEX FUNCTION name*16()}, as
1988 well as @code{COMPLEX*16 FUNCTION name()}. Both are non-standard, legacy
1989 extensions. @command{gfortran} accepts the latter form, which is more
1990 common, but not the former.
1994 @c ---------------------------------------------------------------------
1995 @c Mixed-Language Programming
1996 @c ---------------------------------------------------------------------
1998 @node Mixed-Language Programming
1999 @chapter Mixed-Language Programming
2000 @cindex Interoperability
2001 @cindex Mixed-language programming
2004 * Interoperability with C::
2005 * GNU Fortran Compiler Directives::
2006 * Non-Fortran Main Program::
2009 This chapter is about mixed-language interoperability, but also applies
2010 if one links Fortran code compiled by different compilers. In most cases,
2011 use of the C Binding features of the Fortran 2003 standard is sufficient,
2012 and their use is highly recommended.
2015 @node Interoperability with C
2016 @section Interoperability with C
2020 * Derived Types and struct::
2021 * Interoperable Global Variables::
2022 * Interoperable Subroutines and Functions::
2023 * Working with Pointers::
2024 * Further Interoperability of Fortran with C::
2027 Since Fortran 2003 (ISO/IEC 1539-1:2004(E)) there is a
2028 standardized way to generate procedure and derived-type
2029 declarations and global variables which are interoperable with C
2030 (ISO/IEC 9899:1999). The @code{bind(C)} attribute has been added
2031 to inform the compiler that a symbol shall be interoperable with C;
2032 also, some constraints are added. Note, however, that not
2033 all C features have a Fortran equivalent or vice versa. For instance,
2034 neither C's unsigned integers nor C's functions with variable number
2035 of arguments have an equivalent in Fortran.
2037 Note that array dimensions are reversely ordered in C and that arrays in
2038 C always start with index 0 while in Fortran they start by default with
2039 1. Thus, an array declaration @code{A(n,m)} in Fortran matches
2040 @code{A[m][n]} in C and accessing the element @code{A(i,j)} matches
2041 @code{A[j-1][i-1]}. The element following @code{A(i,j)} (C: @code{A[j-1][i-1]};
2042 assuming @math{i < n}) in memory is @code{A(i+1,j)} (C: @code{A[j-1][i]}).
2044 @node Intrinsic Types
2045 @subsection Intrinsic Types
2047 In order to ensure that exactly the same variable type and kind is used
2048 in C and Fortran, the named constants shall be used which are defined in the
2049 @code{ISO_C_BINDING} intrinsic module. That module contains named constants
2050 for kind parameters and character named constants for the escape sequences
2051 in C. For a list of the constants, see @ref{ISO_C_BINDING}.
2053 @node Derived Types and struct
2054 @subsection Derived Types and struct
2056 For compatibility of derived types with @code{struct}, one needs to use
2057 the @code{BIND(C)} attribute in the type declaration. For instance, the
2058 following type declaration
2062 TYPE, BIND(C) :: myType
2063 INTEGER(C_INT) :: i1, i2
2064 INTEGER(C_SIGNED_CHAR) :: i3
2065 REAL(C_DOUBLE) :: d1
2066 COMPLEX(C_FLOAT_COMPLEX) :: c1
2067 CHARACTER(KIND=C_CHAR) :: str(5)
2071 matches the following @code{struct} declaration in C
2076 /* Note: "char" might be signed or unsigned. */
2084 Derived types with the C binding attribute shall not have the @code{sequence}
2085 attribute, type parameters, the @code{extends} attribute, nor type-bound
2086 procedures. Every component must be of interoperable type and kind and may not
2087 have the @code{pointer} or @code{allocatable} attribute. The names of the
2088 variables are irrelevant for interoperability.
2090 As there exist no direct Fortran equivalents, neither unions nor structs
2091 with bit field or variable-length array members are interoperable.
2093 @node Interoperable Global Variables
2094 @subsection Interoperable Global Variables
2096 Variables can be made accessible from C using the C binding attribute,
2097 optionally together with specifying a binding name. Those variables
2098 have to be declared in the declaration part of a @code{MODULE},
2099 be of interoperable type, and have neither the @code{pointer} nor
2100 the @code{allocatable} attribute.
2106 integer(C_INT), bind(C, name="_MyProject_flags") :: global_flag
2107 type(myType), bind(C) :: tp
2111 Here, @code{_MyProject_flags} is the case-sensitive name of the variable
2112 as seen from C programs while @code{global_flag} is the case-insensitive
2113 name as seen from Fortran. If no binding name is specified, as for
2114 @var{tp}, the C binding name is the (lowercase) Fortran binding name.
2115 If a binding name is specified, only a single variable may be after the
2116 double colon. Note of warning: You cannot use a global variable to
2117 access @var{errno} of the C library as the C standard allows it to be
2118 a macro. Use the @code{IERRNO} intrinsic (GNU extension) instead.
2120 @node Interoperable Subroutines and Functions
2121 @subsection Interoperable Subroutines and Functions
2123 Subroutines and functions have to have the @code{BIND(C)} attribute to
2124 be compatible with C. The dummy argument declaration is relatively
2125 straightforward. However, one needs to be careful because C uses
2126 call-by-value by default while Fortran behaves usually similar to
2127 call-by-reference. Furthermore, strings and pointers are handled
2128 differently. Note that only explicit size and assumed-size arrays are
2129 supported but not assumed-shape or allocatable arrays.
2131 To pass a variable by value, use the @code{VALUE} attribute.
2132 Thus the following C prototype
2135 @code{int func(int i, int *j)}
2138 matches the Fortran declaration
2141 integer(c_int) function func(i,j)
2142 use iso_c_binding, only: c_int
2143 integer(c_int), VALUE :: i
2147 Note that pointer arguments also frequently need the @code{VALUE} attribute,
2148 see @ref{Working with Pointers}.
2150 Strings are handled quite differently in C and Fortran. In C a string
2151 is a @code{NUL}-terminated array of characters while in Fortran each string
2152 has a length associated with it and is thus not terminated (by e.g.
2153 @code{NUL}). For example, if one wants to use the following C function,
2157 void print_C(char *string) /* equivalent: char string[] */
2159 printf("%s\n", string);
2163 to print ``Hello World'' from Fortran, one can call it using
2166 use iso_c_binding, only: C_CHAR, C_NULL_CHAR
2168 subroutine print_c(string) bind(C, name="print_C")
2169 use iso_c_binding, only: c_char
2170 character(kind=c_char) :: string(*)
2171 end subroutine print_c
2173 call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
2176 As the example shows, one needs to ensure that the
2177 string is @code{NUL} terminated. Additionally, the dummy argument
2178 @var{string} of @code{print_C} is a length-one assumed-size
2179 array; using @code{character(len=*)} is not allowed. The example
2180 above uses @code{c_char_"Hello World"} to ensure the string
2181 literal has the right type; typically the default character
2182 kind and @code{c_char} are the same and thus @code{"Hello World"}
2183 is equivalent. However, the standard does not guarantee this.
2185 The use of strings is now further illustrated using the C library
2186 function @code{strncpy}, whose prototype is
2189 char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
2192 The function @code{strncpy} copies at most @var{n} characters from
2193 string @var{s2} to @var{s1} and returns @var{s1}. In the following
2194 example, we ignore the return value:
2199 character(len=30) :: str,str2
2201 ! Ignore the return value of strncpy -> subroutine
2202 ! "restrict" is always assumed if we do not pass a pointer
2203 subroutine strncpy(dest, src, n) bind(C)
2205 character(kind=c_char), intent(out) :: dest(*)
2206 character(kind=c_char), intent(in) :: src(*)
2207 integer(c_size_t), value, intent(in) :: n
2208 end subroutine strncpy
2210 str = repeat('X',30) ! Initialize whole string with 'X'
2211 call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
2212 len(c_char_"Hello World",kind=c_size_t))
2213 print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
2217 The intrinsic procedures are described in @ref{Intrinsic Procedures}.
2219 @node Working with Pointers
2220 @subsection Working with Pointers
2222 C pointers are represented in Fortran via the special opaque derived type
2223 @code{type(c_ptr)} (with private components). Thus one needs to
2224 use intrinsic conversion procedures to convert from or to C pointers.
2229 type(c_ptr) :: cptr1, cptr2
2230 integer, target :: array(7), scalar
2231 integer, pointer :: pa(:), ps
2232 cptr1 = c_loc(array(1)) ! The programmer needs to ensure that the
2233 ! array is contiguous if required by the C
2235 cptr2 = c_loc(scalar)
2236 call c_f_pointer(cptr2, ps)
2237 call c_f_pointer(cptr2, pa, shape=[7])
2240 When converting C to Fortran arrays, the one-dimensional @code{SHAPE} argument
2243 If a pointer is a dummy-argument of an interoperable procedure, it usually
2244 has to be declared using the @code{VALUE} attribute. @code{void*}
2245 matches @code{TYPE(C_PTR), VALUE}, while @code{TYPE(C_PTR)} alone
2246 matches @code{void**}.
2248 Procedure pointers are handled analogously to pointers; the C type is
2249 @code{TYPE(C_FUNPTR)} and the intrinsic conversion procedures are
2250 @code{C_F_PROCPOINTER} and @code{C_FUNLOC}.
2252 Let's consider two examples of actually passing a procedure pointer from
2253 C to Fortran and vice versa. Note that these examples are also very
2254 similar to passing ordinary pointers between both languages.
2255 First, consider this code in C:
2258 /* Procedure implemented in Fortran. */
2259 void get_values (void (*)(double));
2261 /* Call-back routine we want called from Fortran. */
2265 printf ("Number is %f.\n", x);
2268 /* Call Fortran routine and pass call-back to it. */
2272 get_values (&print_it);
2276 A matching implementation for @code{get_values} in Fortran, that correctly
2277 receives the procedure pointer from C and is able to call it, is given
2278 in the following @code{MODULE}:
2284 ! Define interface of call-back routine.
2286 SUBROUTINE callback (x)
2287 USE, INTRINSIC :: ISO_C_BINDING
2288 REAL(KIND=C_DOUBLE), INTENT(IN), VALUE :: x
2289 END SUBROUTINE callback
2294 ! Define C-bound procedure.
2295 SUBROUTINE get_values (cproc) BIND(C)
2296 USE, INTRINSIC :: ISO_C_BINDING
2297 TYPE(C_FUNPTR), INTENT(IN), VALUE :: cproc
2299 PROCEDURE(callback), POINTER :: proc
2301 ! Convert C to Fortran procedure pointer.
2302 CALL C_F_PROCPOINTER (cproc, proc)
2305 CALL proc (1.0_C_DOUBLE)
2306 CALL proc (-42.0_C_DOUBLE)
2307 CALL proc (18.12_C_DOUBLE)
2308 END SUBROUTINE get_values
2313 Next, we want to call a C routine that expects a procedure pointer argument
2314 and pass it a Fortran procedure (which clearly must be interoperable!).
2315 Again, the C function may be:
2319 call_it (int (*func)(int), int arg)
2325 It can be used as in the following Fortran code:
2329 USE, INTRINSIC :: ISO_C_BINDING
2332 ! Define interface of C function.
2334 INTEGER(KIND=C_INT) FUNCTION call_it (func, arg) BIND(C)
2335 USE, INTRINSIC :: ISO_C_BINDING
2336 TYPE(C_FUNPTR), INTENT(IN), VALUE :: func
2337 INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
2338 END FUNCTION call_it
2343 ! Define procedure passed to C function.
2344 ! It must be interoperable!
2345 INTEGER(KIND=C_INT) FUNCTION double_it (arg) BIND(C)
2346 INTEGER(KIND=C_INT), INTENT(IN), VALUE :: arg
2347 double_it = arg + arg
2348 END FUNCTION double_it
2351 SUBROUTINE foobar ()
2352 TYPE(C_FUNPTR) :: cproc
2353 INTEGER(KIND=C_INT) :: i
2355 ! Get C procedure pointer.
2356 cproc = C_FUNLOC (double_it)
2359 DO i = 1_C_INT, 10_C_INT
2360 PRINT *, call_it (cproc, i)
2362 END SUBROUTINE foobar
2367 @node Further Interoperability of Fortran with C
2368 @subsection Further Interoperability of Fortran with C
2370 Assumed-shape and allocatable arrays are passed using an array descriptor
2371 (dope vector). The internal structure of the array descriptor used
2372 by GNU Fortran is not yet documented and will change. There will also be
2373 a Technical Report (TR 29113) which standardizes an interoperable
2374 array descriptor. Until then, you can use the Chasm Language
2375 Interoperability Tools, @url{http://chasm-interop.sourceforge.net/},
2376 which provide an interface to GNU Fortran's array descriptor.
2378 The technical report 29113 will presumably also include support for
2379 C-interoperable @code{OPTIONAL} and for assumed-rank and assumed-type
2380 dummy arguments. However, the TR has neither been approved nor implemented
2381 in GNU Fortran; therefore, these features are not yet available.
2385 @node GNU Fortran Compiler Directives
2386 @section GNU Fortran Compiler Directives
2388 The Fortran standard standard describes how a conforming program shall
2389 behave; however, the exact implementation is not standardized. In order
2390 to allow the user to choose specific implementation details, compiler
2391 directives can be used to set attributes of variables and procedures
2392 which are not part of the standard. Whether a given attribute is
2393 supported and its exact effects depend on both the operating system and
2394 on the processor; see
2395 @ref{Top,,C Extensions,gcc,Using the GNU Compiler Collection (GCC)}
2398 For procedures and procedure pointers, the following attributes can
2399 be used to change the calling convention:
2402 @item @code{CDECL} -- standard C calling convention
2403 @item @code{STDCALL} -- convention where the called procedure pops the stack
2404 @item @code{FASTCALL} -- part of the arguments are passed via registers
2405 instead using the stack
2408 Besides changing the calling convention, the attributes also influence
2409 the decoration of the symbol name, e.g., by a leading underscore or by
2410 a trailing at-sign followed by the number of bytes on the stack. When
2411 assigning a procedure to a procedure pointer, both should use the same
2414 On some systems, procedures and global variables (module variables and
2415 @code{COMMON} blocks) need special handling to be accessible when they
2416 are in a shared library. The following attributes are available:
2419 @item @code{DLLEXPORT} -- provide a global pointer to a pointer in the DLL
2420 @item @code{DLLIMPORT} -- reference the function or variable using a global pointer
2423 The attributes are specified using the syntax
2425 @code{!GCC$ ATTRIBUTES} @var{attribute-list} @code{::} @var{variable-list}
2427 where in free-form source code only whitespace is allowed before @code{!GCC$}
2428 and in fixed-form source code @code{!GCC$}, @code{cGCC$} or @code{*GCC$} shall
2429 start in the first column.
2431 For procedures, the compiler directives shall be placed into the body
2432 of the procedure; for variables and procedure pointers, they shall be in
2433 the same declaration part as the variable or procedure pointer.
2437 @node Non-Fortran Main Program
2438 @section Non-Fortran Main Program
2441 * _gfortran_set_args:: Save command-line arguments
2442 * _gfortran_set_options:: Set library option flags
2443 * _gfortran_set_convert:: Set endian conversion
2444 * _gfortran_set_record_marker:: Set length of record markers
2445 * _gfortran_set_max_subrecord_length:: Set subrecord length
2446 * _gfortran_set_fpe:: Set when a Floating Point Exception should be raised
2449 Even if you are doing mixed-language programming, it is very
2450 likely that you do not need to know or use the information in this
2451 section. Since it is about the internal structure of GNU Fortran,
2452 it may also change in GCC minor releases.
2454 When you compile a @code{PROGRAM} with GNU Fortran, a function
2455 with the name @code{main} (in the symbol table of the object file)
2456 is generated, which initializes the libgfortran library and then
2457 calls the actual program which uses the name @code{MAIN__}, for
2458 historic reasons. If you link GNU Fortran compiled procedures
2459 to, e.g., a C or C++ program or to a Fortran program compiled by
2460 a different compiler, the libgfortran library is not initialized
2461 and thus a few intrinsic procedures do not work properly, e.g.
2462 those for obtaining the command-line arguments.
2464 Therefore, if your @code{PROGRAM} is not compiled with
2465 GNU Fortran and the GNU Fortran compiled procedures require
2466 intrinsics relying on the library initialization, you need to
2467 initialize the library yourself. Using the default options,
2468 gfortran calls @code{_gfortran_set_args} and
2469 @code{_gfortran_set_options}. The initialization of the former
2470 is needed if the called procedures access the command line
2471 (and for backtracing); the latter sets some flags based on the
2472 standard chosen or to enable backtracing. In typical programs,
2473 it is not necessary to call any initialization function.
2475 If your @code{PROGRAM} is compiled with GNU Fortran, you shall
2476 not call any of the following functions. The libgfortran
2477 initialization functions are shown in C syntax but using C
2478 bindings they are also accessible from Fortran.
2481 @node _gfortran_set_args
2482 @subsection @code{_gfortran_set_args} --- Save command-line arguments
2483 @fnindex _gfortran_set_args
2484 @cindex libgfortran initialization, set_args
2487 @item @emph{Description}:
2488 @code{_gfortran_set_args} saves the command-line arguments; this
2489 initialization is required if any of the command-line intrinsics
2490 is called. Additionally, it shall be called if backtracing is
2491 enabled (see @code{_gfortran_set_options}).
2493 @item @emph{Syntax}:
2494 @code{void _gfortran_set_args (int argc, char *argv[])}
2496 @item @emph{Arguments}:
2497 @multitable @columnfractions .15 .70
2498 @item @var{argc} @tab number of command line argument strings
2499 @item @var{argv} @tab the command-line argument strings; argv[0]
2500 is the pathname of the executable itself.
2503 @item @emph{Example}:
2505 int main (int argc, char *argv[])
2507 /* Initialize libgfortran. */
2508 _gfortran_set_args (argc, argv);
2515 @node _gfortran_set_options
2516 @subsection @code{_gfortran_set_options} --- Set library option flags
2517 @fnindex _gfortran_set_options
2518 @cindex libgfortran initialization, set_options
2521 @item @emph{Description}:
2522 @code{_gfortran_set_options} sets several flags related to the Fortran
2523 standard to be used, whether backtracing or core dumps should be enabled
2524 and whether range checks should be performed. The syntax allows for
2525 upward compatibility since the number of passed flags is specified; for
2526 non-passed flags, the default value is used. See also
2527 @pxref{Code Gen Options}. Please note that not all flags are actually
2530 @item @emph{Syntax}:
2531 @code{void _gfortran_set_options (int num, int options[])}
2533 @item @emph{Arguments}:
2534 @multitable @columnfractions .15 .70
2535 @item @var{num} @tab number of options passed
2536 @item @var{argv} @tab The list of flag values
2539 @item @emph{option flag list}:
2540 @multitable @columnfractions .15 .70
2541 @item @var{option}[0] @tab Allowed standard; can give run-time errors
2542 if e.g. an input-output edit descriptor is invalid in a given standard.
2543 Possible values are (bitwise or-ed) @code{GFC_STD_F77} (1),
2544 @code{GFC_STD_F95_OBS} (2), @code{GFC_STD_F95_DEL} (4), @code{GFC_STD_F95}
2545 (8), @code{GFC_STD_F2003} (16), @code{GFC_STD_GNU} (32),
2546 @code{GFC_STD_LEGACY} (64), @code{GFC_STD_F2008} (128), and
2547 @code{GFC_STD_F2008_OBS} (256). Default: @code{GFC_STD_F95_OBS
2548 | GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003 | GFC_STD_F2008
2549 | GFC_STD_F2008_OBS | GFC_STD_F77 | GFC_STD_GNU | GFC_STD_LEGACY}.
2550 @item @var{option}[1] @tab Standard-warning flag; prints a warning to
2551 standard error. Default: @code{GFC_STD_F95_DEL | GFC_STD_LEGACY}.
2552 @item @var{option}[2] @tab If non zero, enable pedantic checking.
2554 @item @var{option}[3] @tab If non zero, enable core dumps on run-time
2555 errors. Default: off.
2556 @item @var{option}[4] @tab If non zero, enable backtracing on run-time
2557 errors. Default: off.
2558 Note: Installs a signal handler and requires command-line
2559 initialization using @code{_gfortran_set_args}.
2560 @item @var{option}[5] @tab If non zero, supports signed zeros.
2562 @item @var{option}[6] @tab Enables run-time checking. Possible values
2563 are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2),
2564 GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (16), GFC_RTCHECK_POINTER (32).
2566 @item @var{option}[7] @tab If non zero, range checking is enabled.
2567 Default: enabled. See -frange-check (@pxref{Code Gen Options}).
2570 @item @emph{Example}:
2572 /* Use gfortran 4.5 default options. */
2573 static int options[] = @{68, 255, 0, 0, 0, 1, 0, 1@};
2574 _gfortran_set_options (8, &options);
2579 @node _gfortran_set_convert
2580 @subsection @code{_gfortran_set_convert} --- Set endian conversion
2581 @fnindex _gfortran_set_convert
2582 @cindex libgfortran initialization, set_convert
2585 @item @emph{Description}:
2586 @code{_gfortran_set_convert} set the representation of data for
2589 @item @emph{Syntax}:
2590 @code{void _gfortran_set_convert (int conv)}
2592 @item @emph{Arguments}:
2593 @multitable @columnfractions .15 .70
2594 @item @var{conv} @tab Endian conversion, possible values:
2595 GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1),
2596 GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).
2599 @item @emph{Example}:
2601 int main (int argc, char *argv[])
2603 /* Initialize libgfortran. */
2604 _gfortran_set_args (argc, argv);
2605 _gfortran_set_convert (1);
2612 @node _gfortran_set_record_marker
2613 @subsection @code{_gfortran_set_record_marker} --- Set length of record markers
2614 @fnindex _gfortran_set_record_marker
2615 @cindex libgfortran initialization, set_record_marker
2618 @item @emph{Description}:
2619 @code{_gfortran_set_record_marker} sets the length of record markers
2620 for unformatted files.
2622 @item @emph{Syntax}:
2623 @code{void _gfortran_set_record_marker (int val)}
2625 @item @emph{Arguments}:
2626 @multitable @columnfractions .15 .70
2627 @item @var{val} @tab Length of the record marker; valid values
2628 are 4 and 8. Default is 4.
2631 @item @emph{Example}:
2633 int main (int argc, char *argv[])
2635 /* Initialize libgfortran. */
2636 _gfortran_set_args (argc, argv);
2637 _gfortran_set_record_marker (8);
2644 @node _gfortran_set_fpe
2645 @subsection @code{_gfortran_set_fpe} --- Set when a Floating Point Exception should be raised
2646 @fnindex _gfortran_set_fpe
2647 @cindex libgfortran initialization, set_fpe
2650 @item @emph{Description}:
2651 @code{_gfortran_set_fpe} sets the IEEE exceptions for which a
2652 Floating Point Exception (FPE) should be raised. On most systems,
2653 this will result in a SIGFPE signal being sent and the program
2656 @item @emph{Syntax}:
2657 @code{void _gfortran_set_fpe (int val)}
2659 @item @emph{Arguments}:
2660 @multitable @columnfractions .15 .70
2661 @item @var{option}[0] @tab IEEE exceptions. Possible values are
2662 (bitwise or-ed) zero (0, default) no trapping,
2663 @code{GFC_FPE_INVALID} (1), @code{GFC_FPE_DENORMAL} (2),
2664 @code{GFC_FPE_ZERO} (4), @code{GFC_FPE_OVERFLOW} (8),
2665 @code{GFC_FPE_UNDERFLOW} (16), and @code{GFC_FPE_PRECISION} (32).
2668 @item @emph{Example}:
2670 int main (int argc, char *argv[])
2672 /* Initialize libgfortran. */
2673 _gfortran_set_args (argc, argv);
2674 /* FPE for invalid operations such as SQRT(-1.0). */
2675 _gfortran_set_fpe (1);
2682 @node _gfortran_set_max_subrecord_length
2683 @subsection @code{_gfortran_set_max_subrecord_length} --- Set subrecord length
2684 @fnindex _gfortran_set_max_subrecord_length
2685 @cindex libgfortran initialization, set_max_subrecord_length
2688 @item @emph{Description}:
2689 @code{_gfortran_set_max_subrecord_length} set the maximum length
2690 for a subrecord. This option only makes sense for testing and
2691 debugging of unformatted I/O.
2693 @item @emph{Syntax}:
2694 @code{void _gfortran_set_max_subrecord_length (int val)}
2696 @item @emph{Arguments}:
2697 @multitable @columnfractions .15 .70
2698 @item @var{val} @tab the maximum length for a subrecord;
2699 the maximum permitted value is 2147483639, which is also
2703 @item @emph{Example}:
2705 int main (int argc, char *argv[])
2707 /* Initialize libgfortran. */
2708 _gfortran_set_args (argc, argv);
2709 _gfortran_set_max_subrecord_length (8);
2717 @c Intrinsic Procedures
2718 @c ---------------------------------------------------------------------
2720 @include intrinsic.texi
2727 @c ---------------------------------------------------------------------
2729 @c ---------------------------------------------------------------------
2732 @unnumbered Contributing
2733 @cindex Contributing
2735 Free software is only possible if people contribute to efforts
2737 We're always in need of more people helping out with ideas
2738 and comments, writing documentation and contributing code.
2740 If you want to contribute to GNU Fortran,
2741 have a look at the long lists of projects you can take on.
2742 Some of these projects are small,
2743 some of them are large;
2744 some are completely orthogonal to the rest of what is
2745 happening on GNU Fortran,
2746 but others are ``mainstream'' projects in need of enthusiastic hackers.
2747 All of these projects are important!
2748 We'll eventually get around to the things here,
2749 but they are also things doable by someone who is willing and able.
2754 * Proposed Extensions::
2759 @section Contributors to GNU Fortran
2760 @cindex Contributors
2764 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
2765 also the initiator of the whole project. Thanks Andy!
2766 Most of the interface with GCC was written by @emph{Paul Brook}.
2768 The following individuals have contributed code and/or
2769 ideas and significant help to the GNU Fortran project
2770 (in alphabetical order):
2773 @item Janne Blomqvist
2774 @item Steven Bosscher
2777 @item Fran@,{c}ois-Xavier Coudert
2781 @item Bernhard Fischer
2783 @item Richard Guenther
2784 @item Richard Henderson
2785 @item Katherine Holcomb
2787 @item Niels Kristian Bech Jensen
2788 @item Steven Johnson
2789 @item Steven G. Kargl
2797 @item Christopher D. Rickett
2798 @item Richard Sandiford
2799 @item Tobias Schl@"uter
2808 The following people have contributed bug reports,
2809 smaller or larger patches,
2810 and much needed feedback and encouragement for the
2811 GNU Fortran project:
2815 @item Dominique d'Humi@`eres
2817 @item Erik Schnetter
2818 @item Joost VandeVondele
2821 Many other individuals have helped debug,
2822 test and improve the GNU Fortran compiler over the past few years,
2823 and we welcome you to do the same!
2824 If you already have done so,
2825 and you would like to see your name listed in the
2826 list above, please contact us.
2834 @item Help build the test suite
2835 Solicit more code for donation to the test suite: the more extensive the
2836 testsuite, the smaller the risk of breaking things in the future! We can
2837 keep code private on request.
2839 @item Bug hunting/squishing
2840 Find bugs and write more test cases! Test cases are especially very
2841 welcome, because it allows us to concentrate on fixing bugs instead of
2842 isolating them. Going through the bugzilla database at
2843 @url{http://gcc.gnu.org/@/bugzilla/} to reduce testcases posted there and
2844 add more information (for example, for which version does the testcase
2845 work, for which versions does it fail?) is also very helpful.
2850 @node Proposed Extensions
2851 @section Proposed Extensions
2853 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
2854 order. Most of these are necessary to be fully compatible with
2855 existing Fortran compilers, but they are not part of the official
2856 J3 Fortran 95 standard.
2858 @subsection Compiler extensions:
2861 User-specified alignment rules for structures.
2864 Automatically extend single precision constants to double.
2867 Compile code that conserves memory by dynamically allocating common and
2868 module storage either on stack or heap.
2871 Compile flag to generate code for array conformance checking (suggest -CC).
2874 User control of symbol names (underscores, etc).
2877 Compile setting for maximum size of stack frame size before spilling
2878 parts to static or heap.
2881 Flag to force local variables into static space.
2884 Flag to force local variables onto stack.
2888 @subsection Environment Options
2891 Pluggable library modules for random numbers, linear algebra.
2892 LA should use BLAS calling conventions.
2895 Environment variables controlling actions on arithmetic exceptions like
2896 overflow, underflow, precision loss---Generate NaN, abort, default.
2900 Set precision for fp units that support it (i387).
2903 Variable for setting fp rounding mode.
2906 Variable to fill uninitialized variables with a user-defined bit
2910 Environment variable controlling filename that is opened for that unit
2914 Environment variable to clear/trash memory being freed.
2917 Environment variable to control tracing of allocations and frees.
2920 Environment variable to display allocated memory at normal program end.
2923 Environment variable for filename for * IO-unit.
2926 Environment variable for temporary file directory.
2929 Environment variable forcing standard output to be line buffered (unix).
2934 @c ---------------------------------------------------------------------
2935 @c GNU General Public License
2936 @c ---------------------------------------------------------------------
2938 @include gpl_v3.texi
2942 @c ---------------------------------------------------------------------
2943 @c GNU Free Documentation License
2944 @c ---------------------------------------------------------------------
2950 @c ---------------------------------------------------------------------
2951 @c Funding Free Software
2952 @c ---------------------------------------------------------------------
2954 @include funding.texi
2956 @c ---------------------------------------------------------------------
2958 @c ---------------------------------------------------------------------
2961 @unnumbered Option Index
2962 @command{gfortran}'s command line options are indexed here without any
2963 initial @samp{-} or @samp{--}. Where an option has both positive and
2964 negative forms (such as -foption and -fno-option), relevant entries in
2965 the manual are indexed under the most appropriate form; it may sometimes
2966 be useful to look up both forms.
2970 @unnumbered Keyword Index