1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999-2007
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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20 @c until they are incorporated into the official Texinfo distribution.
21 @c They borrow heavily from Texinfo's \unnchapentry definitions.
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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
76 @c \global\normaloffset =0.75in
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 1.1 or
84 any later version published by the Free Software Foundation; with the
85 Invariant Sections being ``GNU General Public License'' and ``Funding
86 Free Software'', the Front-Cover
87 texts being (a) (see below), and with the Back-Cover Texts being (b)
88 (see below). A copy of the license is included in the section entitled
89 ``GNU Free Documentation License''.
91 (a) The FSF's Front-Cover Text is:
95 (b) The FSF's Back-Cover Text is:
97 You have freedom to copy and modify this GNU Manual, like GNU
98 software. Copies published by the Free Software Foundation raise
99 funds for GNU development.
103 @dircategory Software development
105 * gfortran: (gfortran). The GNU Fortran Compiler.
107 This file documents the use and the internals of
108 the GNU Fortran compiler, (@command{gfortran}).
110 Published by the Free Software Foundation
111 51 Franklin Street, Fifth Floor
112 Boston, MA 02110-1301 USA
118 @setchapternewpage odd
120 @title Using GNU Fortran
122 @author The @t{gfortran} team
124 @vskip 0pt plus 1filll
125 Published by the Free Software Foundation@*
126 51 Franklin Street, Fifth Floor@*
127 Boston, MA 02110-1301, USA@*
128 @c Last printed ??ber, 19??.@*
129 @c Printed copies are available for $? each.@*
135 @c TODO: The following "Part" definitions are included here temporarily
136 @c until they are incorporated into the official Texinfo distribution.
139 \global\let\partentry=\dosmallpartentry
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152 @c ---------------------------------------------------------------------
153 @c TexInfo table of contents.
154 @c ---------------------------------------------------------------------
161 This manual documents the use of @command{gfortran},
162 the GNU Fortran compiler. You can find in this manual how to invoke
163 @command{gfortran}, as well as its features and incompatibilities.
166 @emph{Warning:} This document, and the compiler it describes, are still
167 under development. While efforts are made to keep it up-to-date, it might
168 not accurately reflect the status of the most recent GNU Fortran compiler.
172 @comment When you add a new menu item, please keep the right hand
173 @comment aligned to the same column. Do not use tabs. This provides
174 @comment better formatting.
179 Part I: Invoking GNU Fortran
180 * Invoking GNU Fortran:: Command options supported by @command{gfortran}.
181 * Runtime:: Influencing runtime behavior with environment variables.
183 Part II: Language Reference
184 * Fortran 2003 status:: Fortran 2003 features supported by GNU Fortran.
185 * Extensions:: Language extensions implemented by GNU Fortran.
186 * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
188 * Contributing:: How you can help.
189 * Copying:: GNU General Public License says
190 how you can copy and share GNU Fortran.
191 * GNU Free Documentation License::
192 How you can copy and share this manual.
193 * Funding:: How to help assure continued work for free software.
194 * Index:: Index of this documentation.
198 @c ---------------------------------------------------------------------
200 @c ---------------------------------------------------------------------
203 @chapter Introduction
205 @c The following duplicates the text on the TexInfo table of contents.
207 This manual documents the use of @command{gfortran}, the GNU Fortran
208 compiler. You can find in this manual how to invoke @command{gfortran},
209 as well as its features and incompatibilities.
212 @emph{Warning:} This document, and the compiler it describes, are still
213 under development. While efforts are made to keep it up-to-date, it
214 might not accurately reflect the status of the most recent GNU Fortran
219 The GNU Fortran compiler front end was
220 designed initially as a free replacement for,
221 or alternative to, the unix @command{f95} command;
222 @command{gfortran} is the command you'll use to invoke the compiler.
225 * About GNU Fortran:: What you should know about the GNU Fortran compiler.
226 * GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
227 * GNU Fortran and G77:: Why we chose to start from scratch.
228 * Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
229 * Standards:: Standards supported by GNU Fortran.
233 @c ---------------------------------------------------------------------
235 @c ---------------------------------------------------------------------
237 @node About GNU Fortran
238 @section About GNU Fortran
240 The GNU Fortran compiler is still in an early state of development.
241 It can generate code for most constructs and expressions,
242 but much work remains to be done.
244 When the GNU Fortran compiler is finished,
245 it will do everything you expect from any decent compiler:
249 Read a user's program,
250 stored in a file and containing instructions written
251 in Fortran 77, Fortran 90, Fortran 95 or Fortran 2003.
252 This file contains @dfn{source code}.
255 Translate the user's program into instructions a computer
256 can carry out more quickly than it takes to translate the
257 instructions in the first
258 place. The result after compilation of a program is
260 code designed to be efficiently translated and processed
261 by a machine such as your computer.
262 Humans usually aren't as good writing machine code
263 as they are at writing Fortran (or C++, Ada, or Java),
264 because is easy to make tiny mistakes writing machine code.
267 Provide the user with information about the reasons why
268 the compiler is unable to create a binary from the source code.
269 Usually this will be the case if the source code is flawed.
270 When writing Fortran, it is easy to make big mistakes.
271 The Fortran 90 requires that the compiler can point out
272 mistakes to the user.
273 An incorrect usage of the language causes an @dfn{error message}.
275 The compiler will also attempt to diagnose cases where the
276 user's program contains a correct usage of the language,
277 but instructs the computer to do something questionable.
278 This kind of diagnostics message is called a @dfn{warning message}.
281 Provide optional information about the translation passes
282 from the source code to machine code.
283 This can help a user of the compiler to find the cause of
284 certain bugs which may not be obvious in the source code,
285 but may be more easily found at a lower level compiler output.
286 It also helps developers to find bugs in the compiler itself.
289 Provide information in the generated machine code that can
290 make it easier to find bugs in the program (using a debugging tool,
291 called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
294 Locate and gather machine code already generated to
295 perform actions requested by statements in the user's program.
296 This machine code is organized into @dfn{modules} and is located
297 and @dfn{linked} to the user program.
300 The GNU Fortran compiler consists of several components:
304 A version of the @command{gcc} command
305 (which also might be installed as the system's @command{cc} command)
306 that also understands and accepts Fortran source code.
307 The @command{gcc} command is the @dfn{driver} program for
308 all the languages in the GNU Compiler Collection (GCC);
310 you can compile the source code of any language for
311 which a front end is available in GCC.
314 The @command{gfortran} command itself,
315 which also might be installed as the
316 system's @command{f95} command.
317 @command{gfortran} is just another driver program,
318 but specifically for the Fortran compiler only.
319 The difference with @command{gcc} is that @command{gfortran}
320 will automatically link the correct libraries to your program.
323 A collection of run-time libraries.
324 These libraries contain the machine code needed to support
325 capabilities of the Fortran language that are not directly
326 provided by the machine code generated by the
327 @command{gfortran} compilation phase,
328 such as intrinsic functions and subroutines,
329 and routines for interaction with files and the operating system.
330 @c and mechanisms to spawn,
331 @c unleash and pause threads in parallelized code.
334 The Fortran compiler itself, (@command{f951}).
335 This is the GNU Fortran parser and code generator,
336 linked to and interfaced with the GCC backend library.
337 @command{f951} ``translates'' the source code to
338 assembler code. You would typically not use this
340 instead, the @command{gcc} or @command{gfortran} driver
341 programs will call it for you.
345 @c ---------------------------------------------------------------------
346 @c GNU Fortran and GCC
347 @c ---------------------------------------------------------------------
349 @node GNU Fortran and GCC
350 @section GNU Fortran and GCC
351 @cindex GNU Compiler Collection
354 GNU Fortran is a part of GCC, the @dfn{GNU Compiler Collection}. GCC
355 consists of a collection of front ends for various languages, which
356 translate the source code into a language-independent form called
357 @dfn{GENERIC}. This is then processed by a common middle end which
358 provides optimization, and then passed to one of a collection of back
359 ends which generate code for different computer architectures and
362 Functionally, this is implemented with a driver program (@command{gcc})
363 which provides the command-line interface for the compiler. It calls
364 the relevant compiler front-end program (e.g., @command{f951} for
365 Fortran) for each file in the source code, and then calls the assembler
366 and linker as appropriate to produce the compiled output. In a copy of
367 GCC which has been compiled with Fortran language support enabled,
368 @command{gcc} will recognize files with @file{.f}, @file{.f90}, @file{.f95},
369 and @file{.f03} extensions as Fortran source code, and compile it
370 accordingly. A @command{gfortran} driver program is also provided,
371 which is identical to @command{gcc} except that it automatically links
372 the Fortran runtime libraries into the compiled program.
374 This manual specifically documents the Fortran front end, which handles
375 the programming language's syntax and semantics. The aspects of GCC
376 which relate to the optimization passes and the back-end code generation
377 are documented in the GCC manual; see
378 @ref{Top,,Introduction,gcc,Using the GNU Compiler Collection (GCC)}.
379 The two manuals together provide a complete reference for the GNU
383 @c ---------------------------------------------------------------------
384 @c GNU Fortran and G77
385 @c ---------------------------------------------------------------------
387 @node GNU Fortran and G77
388 @section GNU Fortran and G77
392 Why do we write a compiler front end from scratch?
393 There's a fine Fortran 77 compiler in the
394 GNU Compiler Collection that accepts some features
395 of the Fortran 90 standard as extensions.
396 Why not start from there and revamp it?
398 One of the reasons is that Craig Burley, the author of G77,
399 has decided to stop working on the G77 front end.
400 On @uref{http://world.std.com/~burley/g77-why.html,
401 Craig explains the reasons for his decision to stop working on G77}
402 in one of the pages in his homepage.
403 Among the reasons is a lack of interest in improvements to
405 Users appear to be quite satisfied with @command{g77} as it is.
406 While @command{g77} is still being maintained (by Toon Moene),
407 it is unlikely that sufficient people will be willing
408 to completely rewrite the existing code.
410 But there are other reasons to start from scratch.
411 Many people, including Craig Burley,
412 no longer agreed with certain design decisions in the G77 front end.
413 Also, the interface of @command{g77} to the back end is written in
414 a style which is confusing and not up to date on recommended practice.
415 In fact, a full rewrite had already been planned for GCC 3.0.
417 When Craig decided to stop,
418 it just seemed to be a better idea to start a new project from scratch,
419 because it was expected to be easier to maintain code we
420 develop ourselves than to do a major overhaul of @command{g77} first,
421 and then build a Fortran 95 compiler out of it.
424 @c ---------------------------------------------------------------------
426 @c ---------------------------------------------------------------------
429 @section Project Status
432 As soon as @command{gfortran} can parse all of the statements correctly,
433 it will be in the ``larva'' state.
434 When we generate code, the ``puppa'' state.
435 When @command{gfortran} is done,
436 we'll see if it will be a beautiful butterfly,
437 or just a big bug....
439 --Andy Vaught, April 2000
442 The start of the GNU Fortran 95 project was announced on
443 the GCC homepage in March 18, 2000
444 (even though Andy had already been working on it for a while,
447 The GNU Fortran compiler is able to compile nearly all
448 standard-compliant Fortran 95, Fortran 90, and Fortran 77 programs,
449 including a number of standard and non-standard extensions, and can be
450 used on real-world programs. In particular, the supported extensions
451 include OpenMP, Cray-style pointers, and several Fortran 2003 features
452 such as enumeration, stream I/O, and some of the enhancements to
453 allocatable array support from TR 15581. However, it is still under
454 development and has a few remaining rough edges.
456 At present, the GNU Fortran compiler passes the
457 @uref{http://www.fortran-2000.com/ArnaudRecipes/fcvs21_f95.html,
458 NIST Fortran 77 Test Suite}, and produces acceptable results on the
459 @uref{http://www.netlib.org/lapack/faq.html#1.21, LAPACK Test Suite}.
460 It also provides respectable performance on
461 the @uref{http://www.polyhedron.com/pb05.html, Polyhedron Fortran
462 compiler benchmarks} and the
463 @uref{http://www.llnl.gov/asci_benchmarks/asci/limited/lfk/README.html,
464 Livermore Fortran Kernels test}. It has been used to compile a number of
465 large real-world programs, including
466 @uref{http://mysite.verizon.net/serveall/moene.pdf, the HIRLAM
467 weather-forecasting code} and
468 @uref{http://www.theochem.uwa.edu.au/tonto/, the Tonto quantum
469 chemistry package}; see @url{http://gcc.gnu.org/wiki/GfortranApps} for an
472 Among other things, the GNU Fortran compiler is intended as a replacement
473 for G77. At this point, nearly all programs that could be compiled with
474 G77 can be compiled with GNU Fortran, although there are a few minor known
477 The primary work remaining to be done on GNU Fortran falls into three
478 categories: bug fixing (primarily regarding the treatment of invalid code
479 and providing useful error messages), improving the compiler optimizations
480 and the performance of compiled code, and extending the compiler to support
481 future standards---in particular, Fortran 2003.
484 @c ---------------------------------------------------------------------
486 @c ---------------------------------------------------------------------
492 The GNU Fortran compiler implements
493 ISO/IEC 1539:1997 (Fortran 95). As such, it can also compile essentially all
494 standard-compliant Fortran 90 and Fortran 77 programs. It also supports
495 the ISO/IEC TR-15581 enhancements to allocatable arrays, and
496 the @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
497 OpenMP Application Program Interface v2.5} specification.
499 In the future, the GNU Fortran compiler may also support other standard
500 variants of and extensions to the Fortran language. These include
501 ISO/IEC 1539-1:2004 (Fortran 2003).
504 @c =====================================================================
505 @c PART I: INVOCATION REFERENCE
506 @c =====================================================================
509 \part{I}{Invoking GNU Fortran}
512 @c ---------------------------------------------------------------------
514 @c ---------------------------------------------------------------------
519 @c ---------------------------------------------------------------------
521 @c ---------------------------------------------------------------------
524 @chapter Runtime: Influencing runtime behavior with environment variables
527 The behavior of the @command{gfortran} can be influenced by
528 environment variables.
530 Malformed environment variables are silently ignored.
533 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
534 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
535 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
536 * GFORTRAN_USE_STDERR:: Send library output to standard error
537 * GFORTRAN_TMPDIR:: Directory for scratch files
538 * GFORTRAN_UNBUFFERED_ALL:: Don't buffer output
539 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
540 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
541 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
542 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
543 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
546 @node GFORTRAN_STDIN_UNIT
547 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
549 This environment variable can be used to select the unit number
550 preconnected to standard input. This must be a positive integer.
551 The default value is 5.
553 @node GFORTRAN_STDOUT_UNIT
554 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
556 This environment variable can be used to select the unit number
557 preconnected to standard output. This must be a positive integer.
558 The default value is 6.
560 @node GFORTRAN_STDERR_UNIT
561 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
563 This environment variable can be used to select the unit number
564 preconnected to standard error. This must be a positive integer.
565 The default value is 0.
567 @node GFORTRAN_USE_STDERR
568 @section @env{GFORTRAN_USE_STDERR}---Send library output to standard error
570 This environment variable controls where library output is sent.
571 If the first letter is @samp{y}, @samp{Y} or @samp{1}, standard
572 error is used. If the first letter is @samp{n}, @samp{N} or
573 @samp{0}, standard output is used.
575 @node GFORTRAN_TMPDIR
576 @section @env{GFORTRAN_TMPDIR}---Directory for scratch files
578 This environment variable controls where scratch files are
579 created. If this environment variable is missing,
580 GNU Fortran searches for the environment variable @env{TMP}. If
581 this is also missing, the default is @file{/tmp}.
583 @node GFORTRAN_UNBUFFERED_ALL
584 @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer output
586 This environment variable controls whether all output is unbuffered.
587 If the first letter is @samp{y}, @samp{Y} or @samp{1}, all output is
588 unbuffered. This will slow down large writes. If the first letter is
589 @samp{n}, @samp{N} or @samp{0}, output is buffered. This is the
592 @node GFORTRAN_SHOW_LOCUS
593 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
595 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
596 line numbers for runtime errors are printed. If the first letter is
597 @samp{n}, @samp{N} or @samp{0}, don't print filename and line numbers
598 for runtime errors. The default is to print the location.
600 @node GFORTRAN_OPTIONAL_PLUS
601 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
603 If the first letter is @samp{y}, @samp{Y} or @samp{1},
604 a plus sign is printed
605 where permitted by the Fortran standard. If the first letter
606 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
607 in most cases. Default is not to print plus signs.
609 @node GFORTRAN_DEFAULT_RECL
610 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
612 This environment variable specifies the default record length, in
613 bytes, for files which are opened without a @code{RECL} tag in the
614 @code{OPEN} statement. This must be a positive integer. The
615 default value is 1073741824 bytes (1 GB).
617 @node GFORTRAN_LIST_SEPARATOR
618 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
620 This environment variable specifies the separator when writing
621 list-directed output. It may contain any number of spaces and
622 at most one comma. If you specify this on the command line,
623 be sure to quote spaces, as in
625 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
627 when @command{a.out} is the compiled Fortran program that you want to run.
628 Default is a single space.
630 @node GFORTRAN_CONVERT_UNIT
631 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
633 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
634 to change the representation of data for unformatted files.
635 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
637 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception ;
638 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
639 exception: mode ':' unit_list | unit_list ;
640 unit_list: unit_spec | unit_list unit_spec ;
641 unit_spec: INTEGER | INTEGER '-' INTEGER ;
643 The variable consists of an optional default mode, followed by
644 a list of optional exceptions, which are separated by semicolons
645 from the preceding default and each other. Each exception consists
646 of a format and a comma-separated list of units. Valid values for
647 the modes are the same as for the @code{CONVERT} specifier:
650 @item @code{NATIVE} Use the native format. This is the default.
651 @item @code{SWAP} Swap between little- and big-endian.
652 @item @code{LITTLE_ENDIAN} Use the little-endian format
653 for unformatted files.
654 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
656 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
657 Examples of values for @env{GFORTRAN_CONVERT_UNIT} are:
659 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
660 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
661 in little_endian mode, except for units 10 to 20 and 25, which are in
663 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
666 Setting the environment variables should be done on the command
667 line or via the @command{export}
668 command for @command{sh}-compatible shells and via @command{setenv}
669 for @command{csh}-compatible shells.
671 Example for @command{sh}:
674 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
677 Example code for @command{csh}:
680 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
684 Using anything but the native representation for unformatted data
685 carries a significant speed overhead. If speed in this area matters
686 to you, it is best if you use this only for data that needs to be
689 @xref{CONVERT specifier}, for an alternative way to specify the
690 data representation for unformatted files. @xref{Runtime Options}, for
691 setting a default data representation for the whole program. The
692 @code{CONVERT} specifier overrides the @option{-fconvert} compile options.
695 @c =====================================================================
696 @c PART II: LANGUAGE REFERENCE
697 @c =====================================================================
700 \part{II}{Language Reference}
703 @c ---------------------------------------------------------------------
704 @c Fortran 2003 Status
705 @c ---------------------------------------------------------------------
707 @node Fortran 2003 status
708 @chapter Fortran 2003 Status
710 Although GNU Fortran focuses on implementing the Fortran 95
711 standard for the time being, a few Fortran 2003 features are currently
716 Intrinsics @code{command_argument_count}, @code{get_command},
717 @code{get_command_argument}, @code{get_environment_variable}, and
721 @cindex Array constructors
723 Array constructors using square brackets. That is, @code{[...]} rather
727 @cindex @code{FLUSH} statement
728 @code{FLUSH} statement.
731 @cindex @code{IOMSG=} specifier
732 @code{IOMSG=} specifier for I/O statements.
735 @cindex @code{ENUM} statement
736 @cindex @code{ENUMERATOR} statement
737 @cindex @code{-fshort-enums} option
738 Support for the declaration of enumeration constants via the
739 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
740 @command{gcc} is guaranteed also for the case where the
741 @command{-fshort-enums} command line option is given.
748 @cindex @code{ALLOCATABLE} dummy arguments
749 @code{ALLOCATABLE} dummy arguments.
751 @cindex @code{ALLOCATABLE} function results
752 @code{ALLOCATABLE} function results
754 @cindex @code{ALLOCATABLE} components of derived types
755 @code{ALLOCATABLE} components of derived types
759 @cindex @code{STREAM} I/O
760 @cindex @code{ACCESS='STREAM'} I/O
761 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
762 allowing I/O without any record structure.
765 Namelist input/output for internal files.
768 @cindex @code{PROTECTED}
769 The @code{PROTECTED} statement and attribute.
773 The @code{VALUE} statement and attribute.
776 @cindex @code{VOLATILE}
777 The @code{VOLATILE} statement and attribute.
780 @cindex @code{IMPORT}
781 The @code{IMPORT} statement, allowing to import
782 host-associated derived types.
785 @cindex @code{USE, INTRINSIC}
786 @cindex @code{ISO_FORTRAN_ENV}
787 @code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
788 attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
789 @code{OMP_LIB} and @code{OMP_LIB_KINDS}.
794 @c ---------------------------------------------------------------------
796 @c ---------------------------------------------------------------------
798 @c Maybe this chapter should be merged with the 'Standards' section,
799 @c whenever that is written :-)
805 GNU Fortran implements a number of extensions over standard
806 Fortran. This chapter contains information on their syntax and
807 meaning. There are currently two categories of GNU Fortran
808 extensions, those that provide functionality beyond that provided
809 by any standard, and those that are supported by GNU Fortran
810 purely for backward compatibility with legacy compilers. By default,
811 @option{-std=gnu} allows the compiler to accept both types of
812 extensions, but to warn about the use of the latter. Specifying
813 either @option{-std=f95} or @option{-std=f2003} disables both types
814 of extensions, and @option{-std=legacy} allows both without warning.
817 * Old-style kind specifications::
818 * Old-style variable initialization::
819 * Extensions to namelist::
820 * X format descriptor without count field::
821 * Commas in FORMAT specifications::
822 * Missing period in FORMAT specifications::
824 * BOZ literal constants::
825 * Real array indices::
827 * Implicitly convert LOGICAL and INTEGER values::
828 * Hollerith constants support::
830 * CONVERT specifier::
834 @node Old-style kind specifications
835 @section Old-style kind specifications
836 @cindex Kind specifications
838 GNU Fortran allows old-style kind specifications in declarations. These
843 where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
844 etc.), and where @code{size} is a byte count corresponding to a valid
845 kind for that type. The statement then declares @code{x}, @code{y} and
846 @code{z} to be of type @code{TYPESPEC} with the appropriate kind. This
847 is equivalent to the standard conforming declaration
851 where @code{k} is equal to @code{size} for most types, but is equal to
852 @code{size/2} for the @code{COMPLEX} type.
854 @node Old-style variable initialization
855 @section Old-style variable initialization
856 @cindex Initialization
858 GNU Fortran allows old-style initialization of variables of the
862 REAL x(2,2) /3*0.,1./
864 The syntax for the initializers is as for the @code{DATA} statement, but
865 unlike in a @code{DATA} statement, an initializer only applies to the
866 variable immediately preceding the initialization. In other words,
867 something like @code{INTEGER I,J/2,3/} is not valid. This style of
868 initialization is only allowed in declarations without double colons
869 (@code{::}); the double colons were introduced in Fortran 90, which also
870 introduced a standard syntax for initializing variables in type
873 Examples of standard-conforming code equivalent to the above example
877 INTEGER :: i = 1, j = 2
878 REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
882 DATA i/1/, j/2/, x/3*0.,1./
885 Note that variables which are explicitly initialized in declarations
886 or in @code{DATA} statements automatically acquire the @code{SAVE}
889 @node Extensions to namelist
890 @section Extensions to namelist
893 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
894 including array qualifiers, substrings and fully qualified derived types.
895 The output from a namelist write is compatible with namelist read. The
896 output has all names in upper case and indentation to column 1 after the
897 namelist name. Two extensions are permitted:
899 Old-style use of @samp{$} instead of @samp{&}
902 X(:)%Y(2) = 1.0 2.0 3.0
907 It should be noted that the default terminator is @samp{/} rather than
910 Querying of the namelist when inputting from stdin. After at least
911 one space, entering @samp{?} sends to stdout the namelist name and the names of
912 the variables in the namelist:
923 Entering @samp{=?} outputs the namelist to stdout, as if
924 @code{WRITE(*,NML = mynml)} had been called:
929 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
930 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
931 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
935 To aid this dialog, when input is from stdin, errors send their
936 messages to stderr and execution continues, even if @code{IOSTAT} is set.
938 @code{PRINT} namelist is permitted. This causes an error if
939 @option{-std=f95} is used.
942 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
945 END PROGRAM test_print
948 Expanded namelist reads are permitted. This causes an error if
949 @option{-std=f95} is used. In the following example, the first element
950 of the array will be given the value 0.00 and the two succeeding
951 elements will be given the values 1.00 and 2.00.
954 X(1,1) = 0.00 , 1.00 , 2.00
958 @node X format descriptor without count field
959 @section @code{X} format descriptor without count field
960 @cindex @code{X} format descriptor without count field
962 To support legacy codes, GNU Fortran permits the count field of the
963 @code{X} edit descriptor in @code{FORMAT} statements to be omitted.
964 When omitted, the count is implicitly assumed to be one.
968 10 FORMAT (I1, X, I1)
971 @node Commas in FORMAT specifications
972 @section Commas in @code{FORMAT} specifications
973 @cindex Commas in @code{FORMAT} specifications
975 To support legacy codes, GNU Fortran allows the comma separator
976 to be omitted immediately before and after character string edit
977 descriptors in @code{FORMAT} statements.
981 10 FORMAT ('FOO='I1' BAR='I2)
985 @node Missing period in FORMAT specifications
986 @section Missing period in @code{FORMAT} specifications
987 @cindex Missing period in @code{FORMAT} specifications
989 To support legacy codes, GNU Fortran allows missing periods in format
990 specifications if and only if @option{-std=legacy} is given on the
991 command line. This is considered non-conforming code and is
1000 @node I/O item lists
1001 @section I/O item lists
1002 @cindex I/O item lists
1004 To support legacy codes, GNU Fortran allows the input item list
1005 of the @code{READ} statement, and the output item lists of the
1006 @code{WRITE} and @code{PRINT} statements, to start with a comma.
1008 @node BOZ literal constants
1009 @section BOZ literal constants
1010 @cindex BOZ literal constants
1012 As an extension, GNU Fortran allows hexadecimal BOZ literal constants to
1013 be specified using the X prefix, in addition to the standard Z prefix.
1014 BOZ literal constants can also be specified by adding a suffix to the
1015 string. For example, @code{Z'ABC'} and @code{'ABC'Z} are equivalent.
1017 The Fortran standard restricts the appearance of a BOZ literal constant
1018 to the @code{DATA} statement, and it is expected to be assigned to an
1019 @code{INTEGER} variable. GNU Fortran permits a BOZ literal to appear in
1020 any initialization expression as well as assignment statements.
1022 Attempts to use a BOZ literal constant to do a bitwise initialization of
1023 a variable can lead to confusion. A BOZ literal constant is converted
1024 to an @code{INTEGER} value with the kind type with the largest decimal
1025 representation, and this value is then converted numerically to the type
1026 and kind of the variable in question. Thus, one should not expect a
1027 bitwise copy of the BOZ literal constant to be assigned to a @code{REAL}
1030 Similarly, initializing an @code{INTEGER} variable with a statement such
1031 as @code{DATA i/Z'FFFFFFFF'/} will produce an integer overflow rather
1032 than the desired result of @math{-1} when @code{i} is a 32-bit integer
1033 on a system that supports 64-bit integers. The @samp{-fno-range-check}
1034 option can be used as a workaround for legacy code that initializes
1035 integers in this manner.
1037 @node Real array indices
1038 @section Real array indices
1039 @cindex Real array indices
1041 As an extension, GNU Fortran allows the use of @code{REAL} expressions
1042 or variables as array indices.
1044 @node Unary operators
1045 @section Unary operators
1046 @cindex Unary operators
1048 As an extension, GNU Fortran allows unary plus and unary minus operators
1049 to appear as the second operand of binary arithmetic operators without
1050 the need for parenthesis.
1056 @node Implicitly convert LOGICAL and INTEGER values
1057 @section Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1058 @cindex Implicitly convert @code{LOGICAL} and @code{INTEGER} values
1060 As an extension for backwards compatibility with other compilers, GNU
1061 Fortran allows the implicit conversion of @code{LOGICAL} values to
1062 @code{INTEGER} values and vice versa. When converting from a
1063 @code{LOGICAL} to an @code{INTEGER}, @code{.FALSE.} is interpreted as
1064 zero, and @code{.TRUE.} is interpreted as one. When converting from
1065 @code{INTEGER} to @code{LOGICAL}, the value zero is interpreted as
1066 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
1070 IF (i) PRINT *, 'True'
1073 @node Hollerith constants support
1074 @section Hollerith constants support
1075 @cindex Hollerith constants
1077 GNU Fortran supports Hollerith constants in assignments, function
1078 arguments, and @code{DATA} and @code{ASSIGN} statements. A Hollerith
1079 constant is written as a string of characters preceded by an integer
1080 constant indicating the character count, and the letter @code{H} or
1081 @code{h}, and stored in bytewise fashion in a numeric (@code{INTEGER},
1082 @code{REAL}, or @code{complex}) or @code{LOGICAL} variable. The
1083 constant will be padded or truncated to fit the size of the variable in
1086 Examples of valid uses of Hollerith constants:
1089 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1090 x(1) = 16HABCDEFGHIJKLMNOP
1094 Invalid Hollerith constants examples:
1097 a = 8H12345678 ! Valid, but the Hollerith constant will be truncated.
1098 a = 0H ! At least one character is needed.
1101 In general, Hollerith constants were used to provide a rudimentary
1102 facility for handling character strings in early Fortran compilers,
1103 prior to the introduction of @code{CHARACTER} variables in Fortran 77;
1104 in those cases, the standard-compliant equivalent is to convert the
1105 program to use proper character strings. On occasion, there may be a
1106 case where the intent is specifically to initialize a numeric variable
1107 with a given byte sequence. In these cases, the same result can be
1108 obtained by using the @code{TRANSFER} statement, as in this example.
1110 INTEGER(KIND=4) :: a
1111 a = TRANSFER ("abcd", a) ! equivalent to: a = 4Habcd
1116 @section Cray pointers
1117 @cindex Cray pointers
1119 Cray pointers are part of a non-standard extension that provides a
1120 C-like pointer in Fortran. This is accomplished through a pair of
1121 variables: an integer "pointer" that holds a memory address, and a
1122 "pointee" that is used to dereference the pointer.
1124 Pointer/pointee pairs are declared in statements of the form:
1126 pointer ( <pointer> , <pointee> )
1130 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1132 The pointer is an integer that is intended to hold a memory address.
1133 The pointee may be an array or scalar. A pointee can be an assumed
1134 size array---that is, the last dimension may be left unspecified by
1135 using a @code{*} in place of a value---but a pointee cannot be an
1136 assumed shape array. No space is allocated for the pointee.
1138 The pointee may have its type declared before or after the pointer
1139 statement, and its array specification (if any) may be declared
1140 before, during, or after the pointer statement. The pointer may be
1141 declared as an integer prior to the pointer statement. However, some
1142 machines have default integer sizes that are different than the size
1143 of a pointer, and so the following code is not portable:
1148 If a pointer is declared with a kind that is too small, the compiler
1149 will issue a warning; the resulting binary will probably not work
1150 correctly, because the memory addresses stored in the pointers may be
1151 truncated. It is safer to omit the first line of the above example;
1152 if explicit declaration of ipt's type is omitted, then the compiler
1153 will ensure that ipt is an integer variable large enough to hold a
1156 Pointer arithmetic is valid with Cray pointers, but it is not the same
1157 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1158 the user is responsible for determining how many bytes to add to a
1159 pointer in order to increment it. Consider the following example:
1163 pointer (ipt, pointee)
1167 The last statement does not set @code{ipt} to the address of
1168 @code{target(1)}, as it would in C pointer arithmetic. Adding @code{1}
1169 to @code{ipt} just adds one byte to the address stored in @code{ipt}.
1171 Any expression involving the pointee will be translated to use the
1172 value stored in the pointer as the base address.
1174 To get the address of elements, this extension provides an intrinsic
1175 function @code{LOC()}. The @code{LOC()} function is equivalent to the
1176 @code{&} operator in C, except the address is cast to an integer type:
1179 pointer(ipt, arpte(10))
1181 ipt = loc(ar) ! Makes arpte is an alias for ar
1182 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1184 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1187 Cray pointees often are used to alias an existing variable. For
1195 As long as @code{ipt} remains unchanged, @code{iarr} is now an alias for
1196 @code{target}. The optimizer, however, will not detect this aliasing, so
1197 it is unsafe to use @code{iarr} and @code{target} simultaneously. Using
1198 a pointee in any way that violates the Fortran aliasing rules or
1199 assumptions is illegal. It is the user's responsibility to avoid doing
1200 this; the compiler works under the assumption that no such aliasing
1203 Cray pointers will work correctly when there is no aliasing (i.e., when
1204 they are used to access a dynamically allocated block of memory), and
1205 also in any routine where a pointee is used, but any variable with which
1206 it shares storage is not used. Code that violates these rules may not
1207 run as the user intends. This is not a bug in the optimizer; any code
1208 that violates the aliasing rules is illegal. (Note that this is not
1209 unique to GNU Fortran; any Fortran compiler that supports Cray pointers
1210 will ``incorrectly'' optimize code with illegal aliasing.)
1212 There are a number of restrictions on the attributes that can be applied
1213 to Cray pointers and pointees. Pointees may not have the
1214 @code{ALLOCATABLE}, @code{INTENT}, @code{OPTIONAL}, @code{DUMMY},
1215 @code{TARGET}, @code{INTRINSIC}, or @code{POINTER} attributes. Pointers
1216 may not have the @code{DIMENSION}, @code{POINTER}, @code{TARGET},
1217 @code{ALLOCATABLE}, @code{EXTERNAL}, or @code{INTRINSIC} attributes.
1218 Pointees may not occur in more than one pointer statement. A pointee
1219 cannot be a pointer. Pointees cannot occur in equivalence, common, or
1222 A Cray pointer may also point to a function or a subroutine. For
1223 example, the following excerpt is valid:
1227 pointer (subptr,subpte)
1237 A pointer may be modified during the course of a program, and this
1238 will change the location to which the pointee refers. However, when
1239 pointees are passed as arguments, they are treated as ordinary
1240 variables in the invoked function. Subsequent changes to the pointer
1241 will not change the base address of the array that was passed.
1243 @node CONVERT specifier
1244 @section CONVERT specifier
1245 @cindex CONVERT specifier
1247 GNU Fortran allows the conversion of unformatted data between little-
1248 and big-endian representation to facilitate moving of data
1249 between different systems. The conversion can be indicated with
1250 the @code{CONVERT} specifier on the @code{OPEN} statement.
1251 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1252 the data format via an environment variable.
1254 Valid values for @code{CONVERT} are:
1256 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1257 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1258 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1259 for unformatted files.
1260 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1264 Using the option could look like this:
1266 open(file='big.dat',form='unformatted',access='sequential', &
1267 convert='big_endian')
1270 The value of the conversion can be queried by using
1271 @code{INQUIRE(CONVERT=ch)}. The values returned are
1272 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1274 @code{CONVERT} works between big- and little-endian for
1275 @code{INTEGER} values of all supported kinds and for @code{REAL}
1276 on IEEE systems of kinds 4 and 8. Conversion between different
1277 ``extended double'' types on different architectures such as
1278 m68k and x86_64, which GNU Fortran
1279 supports as @code{REAL(KIND=10)} and @code{REAL(KIND=16)}, will
1282 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1283 environment variable will override the CONVERT specifier in the
1284 open statement}. This is to give control over data formats to
1285 users who do not have the source code of their program available.
1287 Using anything but the native representation for unformatted data
1288 carries a significant speed overhead. If speed in this area matters
1289 to you, it is best if you use this only for data that needs to be
1296 GNU Fortran attempts to be OpenMP Application Program Interface v2.5
1297 compatible when invoked with the @option{-fopenmp} option. GNU Fortran
1298 then generates parallelized code according to the OpenMP directives
1299 used in the source. The OpenMP Fortran runtime library
1300 routines are provided both in a form of a Fortran 90 module named
1301 @code{omp_lib} and in a form of a Fortran @code{include} file named
1304 For details refer to the actual
1305 @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
1306 OpenMP Application Program Interface v2.5} specification.
1308 @c ---------------------------------------------------------------------
1309 @c Intrinsic Procedures
1310 @c ---------------------------------------------------------------------
1312 @include intrinsic.texi
1319 @c ---------------------------------------------------------------------
1321 @c ---------------------------------------------------------------------
1324 @unnumbered Contributing
1325 @cindex Contributing
1327 Free software is only possible if people contribute to efforts
1329 We're always in need of more people helping out with ideas
1330 and comments, writing documentation and contributing code.
1332 If you want to contribute to GNU Fortran,
1333 have a look at the long lists of projects you can take on.
1334 Some of these projects are small,
1335 some of them are large;
1336 some are completely orthogonal to the rest of what is
1337 happening on GNU Fortran,
1338 but others are ``mainstream'' projects in need of enthusiastic hackers.
1339 All of these projects are important!
1340 We'll eventually get around to the things here,
1341 but they are also things doable by someone who is willing and able.
1346 * Proposed Extensions::
1351 @section Contributors to GNU Fortran
1352 @cindex Contributors
1356 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
1357 also the initiator of the whole project. Thanks Andy!
1358 Most of the interface with GCC was written by @emph{Paul Brook}.
1360 The following individuals have contributed code and/or
1361 ideas and significant help to the GNU Fortran project
1362 (in no particular order):
1366 @item Katherine Holcomb
1367 @item Tobias Schl@"uter
1368 @item Steven Bosscher
1371 @item Niels Kristian Bech Jensen
1372 @item Steven Johnson
1377 @item Fran@,{c}ois-Xavier Coudert
1378 @item Steven G. Kargl
1380 @item Janne Blomqvist
1387 @item Richard Henderson
1388 @item Richard Sandiford
1389 @item Richard Guenther
1390 @item Bernhard Fischer
1393 The following people have contributed bug reports,
1394 smaller or larger patches,
1395 and much needed feedback and encouragement for the
1396 GNU Fortran project:
1399 @item Erik Schnetter
1404 Many other individuals have helped debug,
1405 test and improve the GNU Fortran compiler over the past few years,
1406 and we welcome you to do the same!
1407 If you already have done so,
1408 and you would like to see your name listed in the
1409 list above, please contact us.
1417 @item Help build the test suite
1418 Solicit more code for donation to the test suite.
1419 We can keep code private on request.
1421 @item Bug hunting/squishing
1422 Find bugs and write more test cases!
1423 Test cases are especially very welcome,
1424 because it allows us to concentrate on fixing bugs
1425 instead of isolating them.
1427 @item Smaller projects (``bug'' fixes):
1429 @item Allow init exprs to be numbers raised to integer powers.
1430 @item Implement correct rounding.
1431 @item Implement F restrictions on Fortran 95 syntax.
1432 @item See about making Emacs-parsable error messages.
1436 If you wish to work on the runtime libraries,
1437 please contact a project maintainer.
1441 @node Proposed Extensions
1442 @section Proposed Extensions
1444 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
1445 order. Most of these are necessary to be fully compatible with
1446 existing Fortran compilers, but they are not part of the official
1447 J3 Fortran 95 standard.
1449 @subsection Compiler extensions:
1452 User-specified alignment rules for structures.
1455 Flag to generate @code{Makefile} info.
1458 Automatically extend single precision constants to double.
1461 Compile code that conserves memory by dynamically allocating common and
1462 module storage either on stack or heap.
1465 Compile flag to generate code for array conformance checking (suggest -CC).
1468 User control of symbol names (underscores, etc).
1471 Compile setting for maximum size of stack frame size before spilling
1472 parts to static or heap.
1475 Flag to force local variables into static space.
1478 Flag to force local variables onto stack.
1481 Flag for maximum errors before ending compile.
1484 Option to initialize otherwise uninitialized integer and floating
1489 @subsection Environment Options
1492 Pluggable library modules for random numbers, linear algebra.
1493 LA should use BLAS calling conventions.
1496 Environment variables controlling actions on arithmetic exceptions like
1497 overflow, underflow, precision loss---Generate NaN, abort, default.
1501 Set precision for fp units that support it (i387).
1504 Variable for setting fp rounding mode.
1507 Variable to fill uninitialized variables with a user-defined bit
1511 Environment variable controlling filename that is opened for that unit
1515 Environment variable to clear/trash memory being freed.
1518 Environment variable to control tracing of allocations and frees.
1521 Environment variable to display allocated memory at normal program end.
1524 Environment variable for filename for * IO-unit.
1527 Environment variable for temporary file directory.
1530 Environment variable forcing standard output to be line buffered (unix).
1535 @c ---------------------------------------------------------------------
1536 @c GNU General Public License
1537 @c ---------------------------------------------------------------------
1543 @c ---------------------------------------------------------------------
1544 @c GNU Free Documentation License
1545 @c ---------------------------------------------------------------------
1551 @c ---------------------------------------------------------------------
1552 @c Funding Free Software
1553 @c ---------------------------------------------------------------------
1555 @include funding.texi
1557 @c ---------------------------------------------------------------------
1559 @c ---------------------------------------------------------------------