1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999-2006
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.
21 @c Use with @@smallbook.
23 @c %** start of document
25 @c Cause even numbered pages to be printed on the left hand side of
26 @c the page and odd numbered pages to be printed on the right hand
27 @c side of the page. Using this, you can print on both sides of a
28 @c sheet of paper and have the text on the same part of the sheet.
30 @c The text on right hand pages is pushed towards the right hand
31 @c margin and the text on left hand pages is pushed toward the left
33 @c (To provide the reverse effect, set bindingoffset to -0.75in.)
36 @c \global\bindingoffset=0.75in
37 @c \global\normaloffset =0.75in
41 Copyright @copyright{} @value{copyrights-gfortran} Free Software Foundation, Inc.
43 Permission is granted to copy, distribute and/or modify this document
44 under the terms of the GNU Free Documentation License, Version 1.1 or
45 any later version published by the Free Software Foundation; with the
46 Invariant Sections being ``GNU General Public License'' and ``Funding
47 Free Software'', the Front-Cover
48 texts being (a) (see below), and with the Back-Cover Texts being (b)
49 (see below). A copy of the license is included in the section entitled
50 ``GNU Free Documentation License''.
52 (a) The FSF's Front-Cover Text is:
56 (b) The FSF's Back-Cover Text is:
58 You have freedom to copy and modify this GNU Manual, like GNU
59 software. Copies published by the Free Software Foundation raise
60 funds for GNU development.
64 @dircategory Software development
66 * gfortran: (gfortran). The GNU Fortran Compiler.
68 This file documents the use and the internals of
69 the GNU Fortran compiler, (@command{gfortran}).
71 Published by the Free Software Foundation
72 51 Franklin Street, Fifth Floor
73 Boston, MA 02110-1301 USA
79 @setchapternewpage odd
81 @title Using GNU Fortran
83 @center The gfortran team
85 @vskip 0pt plus 1filll
86 For the @value{version-GCC} Version*
88 Published by the Free Software Foundation @*
89 51 Franklin Street, Fifth Floor@*
90 Boston, MA 02110-1301, USA@*
91 @c Last printed ??ber, 19??.@*
92 @c Printed copies are available for $? each.@*
105 This manual documents the use of @command{gfortran},
106 the GNU Fortran compiler. You can find in this manual how to invoke
107 @command{gfortran}, as well as its features and incompatibilities.
110 @emph{Warning:} This document, and the compiler it describes, are still
111 under development. While efforts are made to keep it up-to-date, it might
112 not accurately reflect the status of the most recent GNU Fortran compiler.
116 @comment When you add a new menu item, please keep the right hand
117 @comment aligned to the same column. Do not use tabs. This provides
118 @comment better formatting.
121 * Getting Started:: What you should know about GNU Fortran.
122 * GNU Fortran and GCC:: You can compile Fortran, C, or other programs.
123 * GNU Fortran and G77:: Why we chose to start from scratch.
124 * Invoking GNU Fortran:: Command options supported by @command{gfortran}.
125 * Project Status:: Status of GNU Fortran, roadmap, proposed extensions.
126 * Contributing:: How you can help.
127 * Standards:: Standards supported by GNU Fortran.
128 * Runtime:: Influencing runtime behavior with environment variables.
129 * Extensions:: Language extensions implemented by GNU Fortran.
130 * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran.
131 * Copying:: GNU General Public License says
132 how you can copy and share GNU Fortran.
133 * GNU Free Documentation License::
134 How you can copy and share this manual.
135 * Funding:: How to help assure continued work for free software.
136 * Index:: Index of this documentation.
141 @c ---------------------------------------------------------------------
143 @c ---------------------------------------------------------------------
145 @node Getting Started
146 @chapter Getting Started
148 The GNU Fortran compiler front end was
149 designed initially as a free replacement for,
150 or alternative to, the unix @command{f95} command;
151 @command{gfortran} is the command you'll use to invoke the compiler.
153 The GNU Fortran compiler is still in an early state of development.
154 It can generate code for most constructs and expressions,
155 but much work remains to be done.
157 When the GNU Fortran compiler is finished,
158 it will do everything you expect from any decent compiler:
162 Read a user's program,
163 stored in a file and containing instructions written
164 in Fortran 77, Fortran 90 or Fortran 95.
165 This file contains @dfn{source code}.
168 Translate the user's program into instructions a computer
169 can carry out more quickly than it takes to translate the
170 instructions in the first
171 place. The result after compilation of a program is
173 code designed to be efficiently translated and processed
174 by a machine such as your computer.
175 Humans usually aren't as good writing machine code
176 as they are at writing Fortran (or C++, Ada, or Java),
177 because is easy to make tiny mistakes writing machine code.
180 Provide the user with information about the reasons why
181 the compiler is unable to create a binary from the source code.
182 Usually this will be the case if the source code is flawed.
183 When writing Fortran, it is easy to make big mistakes.
184 The Fortran 90 requires that the compiler can point out
185 mistakes to the user.
186 An incorrect usage of the language causes an @dfn{error message}.
188 The compiler will also attempt to diagnose cases where the
189 user's program contains a correct usage of the language,
190 but instructs the computer to do something questionable.
191 This kind of diagnostics message is called a @dfn{warning message}.
194 Provide optional information about the translation passes
195 from the source code to machine code.
196 This can help a user of the compiler to find the cause of
197 certain bugs which may not be obvious in the source code,
198 but may be more easily found at a lower level compiler output.
199 It also helps developers to find bugs in the compiler itself.
202 Provide information in the generated machine code that can
203 make it easier to find bugs in the program (using a debugging tool,
204 called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
207 Locate and gather machine code already generated to
208 perform actions requested by statements in the user's program.
209 This machine code is organized into @dfn{modules} and is located
210 and @dfn{linked} to the user program.
213 The GNU Fortran compiler consists of several components:
217 A version of the @command{gcc} command
218 (which also might be installed as the system's @command{cc} command)
219 that also understands and accepts Fortran source code.
220 The @command{gcc} command is the @dfn{driver} program for
221 all the languages in the GNU Compiler Collection (GCC);
223 you can compile the source code of any language for
224 which a front end is available in GCC.
227 The @command{gfortran} command itself,
228 which also might be installed as the
229 system's @command{f95} command.
230 @command{gfortran} is just another driver program,
231 but specifically for the Fortran compiler only.
232 The difference with @command{gcc} is that @command{gfortran}
233 will automatically link the correct libraries to your program.
236 A collection of run-time libraries.
237 These libraries contain the machine code needed to support
238 capabilities of the Fortran language that are not directly
239 provided by the machine code generated by the
240 @command{gfortran} compilation phase,
241 such as intrinsic functions and subroutines,
242 and routines for interaction with files and the operating system.
243 @c and mechanisms to spawn,
244 @c unleash and pause threads in parallelized code.
247 The Fortran compiler itself, (@command{f951}).
248 This is the GNU Fortran parser and code generator,
249 linked to and interfaced with the GCC backend library.
250 @command{f951} ``translates'' the source code to
251 assembler code. You would typically not use this
253 instead, the @command{gcc} or @command{gfortran} driver
254 programs will call it for you.
259 @c ---------------------------------------------------------------------
260 @c GNU Fortran and GCC
261 @c ---------------------------------------------------------------------
263 @node GNU Fortran and GCC
264 @chapter GNU Fortran and GCC
265 @cindex GNU Compiler Collection
267 GCC used to be the GNU ``C'' Compiler,
268 but is now known as the @dfn{GNU Compiler Collection}.
269 GCC provides the GNU system with a very versatile
270 compiler middle end (shared optimization passes),
271 and back ends (code generators) for many different
272 computer architectures and operating systems.
273 The code of the middle end and back end are shared by all
274 compiler front ends that are in the GNU Compiler Collection.
276 A GCC front end is essentially a source code parser
277 and an intermediate code generator. The code generator translates the
278 semantics of the source code into a language independent form called
281 The parser takes a source file written in a
282 particular computer language, reads and parses it,
283 and tries to make sure that the source code conforms to
285 Once the correctness of a program has been established,
286 the compiler will build a data structure known as the
287 @dfn{Abstract Syntax tree},
288 or just @dfn{AST} or ``tree'' for short.
289 This data structure represents the whole program
290 or a subroutine or a function.
291 The ``tree'' is passed to the GCC middle end,
292 which will perform optimization passes on it. The optimized AST is then
293 handed off too the back end which assembles the program unit.
295 Different phases in this translation process can be,
296 and in fact @emph{are} merged in many compiler front ends.
297 GNU Fortran has a strict separation between the
298 parser and code generator.
300 The goal of the GNU Fortran project is to build a new front end for GCC.
301 Specifically, a Fortran 95 front end.
302 In a non-@command{gfortran} installation,
303 @command{gcc} will not be able to compile Fortran source code
304 (only the ``C'' front end has to be compiled if you want to build GCC,
305 all other languages are optional).
306 If you build GCC with @command{gfortran}, @command{gcc} will recognize
307 @file{.f/.f90/.f95} source files and accepts Fortran specific
308 command line options.
312 @c ---------------------------------------------------------------------
313 @c GNU Fortran and G77
314 @c ---------------------------------------------------------------------
316 @node GNU Fortran and G77
317 @chapter GNU Fortran and G77
321 Why do we write a compiler front end from scratch?
322 There's a fine Fortran 77 compiler in the
323 GNU Compiler Collection that accepts some features
324 of the Fortran 90 standard as extensions.
325 Why not start from there and revamp it?
327 One of the reasons is that Craig Burley, the author of G77,
328 has decided to stop working on the G77 front end.
329 On @uref{http://world.std.com/~burley/g77-why.html,
330 Craig explains the reasons for his decision to stop working on G77}
331 in one of the pages in his homepage.
332 Among the reasons is a lack of interest in improvements to
334 Users appear to be quite satisfied with @command{g77} as it is.
335 While @command{g77} is still being maintained (by Toon Moene),
336 it is unlikely that sufficient people will be willing
337 to completely rewrite the existing code.
339 But there are other reasons to start from scratch.
340 Many people, including Craig Burley,
341 no longer agreed with certain design decisions in the G77 front end.
342 Also, the interface of @command{g77} to the back end is written in
343 a style which is confusing and not up to date on recommended practice.
344 In fact, a full rewrite had already been planned for GCC 3.0.
346 When Craig decided to stop,
347 it just seemed to be a better idea to start a new project from scratch,
348 because it was expected to be easier to maintain code we
349 develop ourselves than to do a major overhaul of @command{g77} first,
350 and then build a Fortran 95 compiler out of it.
354 @c ---------------------------------------------------------------------
356 @c ---------------------------------------------------------------------
359 @chapter Project Status
362 As soon as @command{gfortran} can parse all of the statements correctly,
363 it will be in the ``larva'' state.
364 When we generate code, the ``puppa'' state.
365 When @command{gfortran} is done,
366 we'll see if it will be a beautiful butterfly,
367 or just a big bug....
369 --Andy Vaught, April 2000
372 The start of the GNU Fortran 95 project was announced on
373 the GCC homepage in March 18, 2000
374 (even though Andy had already been working on it for a while,
377 The GNU Fortran compiler is currently reaching the stage where is is able to
379 world programs. However it is still under development and has many rough
385 * Proposed Extensions::
388 @node Compiler Status
389 @section Compiler Status
393 This is the part of the GNU Fortran compiler which parses a source file, verifies that it
394 is valid Fortran, performs compile time replacement of constants
395 (PARAMETER variables) and reads and generate module files. This is
396 almost complete. Every Fortran 95 source should be accepted, and most
397 none-Fortran 95 source should be rejected. If you find a source file where
398 this is not true, please tell us. You can use the -fsyntax-only switch to
399 make @command{gfortran} quit after running the front end, effectively reducing it to
402 @item Middle end interface
403 These are the parts of GNU Fortran that take the parse tree generated by the
404 front end and translate it to the GENERIC form required by the GCC back
405 end. Work is ongoing in these parts of GNU Fortran, but a large part has
406 already been completed.
410 @section Library Status
412 Some intrinsic functions map directly to library functions, and in most
413 cases the name of the library function used depends on the type of the
414 arguments. For some intrinsics we generate inline code, and for others,
415 such as sin, cos and sqrt, we rely on the backend to use special
416 instructions in the floating point unit of the CPU if available, or to
417 fall back to a call to libm if these are not available.
419 Implementation of some non-elemental intrinsic functions (eg. DOT_PRODUCT,
420 AVERAGE) is not yet optimal. This is hard because we have to make decisions
421 whether to use inline code (good for small arrays as no function call
422 overhead occurs) or generate function calls (good for large arrays as it
423 allows use of hand-optimized assembly routines, SIMD instructions, etc.)
425 The IO library is in a mostly usable state. Unformatted I/O for
426 @code{REAL(KIND=10)} variables is currently not recommended.
428 Array intrinsics mostly work.
430 @node Proposed Extensions
431 @section Proposed Extensions
433 Here's a list of proposed extensions for the GNU Fortran compiler, in no particular
434 order. Most of these are necessary to be fully compatible with
435 existing Fortran compilers, but they are not part of the official
436 J3 Fortran 95 standard.
438 @subsection Compiler extensions:
441 Flag for defining the kind number for default logicals.
444 User-specified alignment rules for structures.
446 Flag to generate @code{Makefile} info.
449 Automatically extend single precision constants to double.
452 Compile code that conserves memory by dynamically allocating common and
453 module storage either on stack or heap.
456 Flag to cause the compiler to distinguish between upper and lower case
457 names. The Fortran 95 standard does not distinguish them.
460 Compile flag to generate code for array conformance checking (suggest -CC).
463 User control of symbol names (underscores, etc).
466 Compile setting for maximum size of stack frame size before spilling
467 parts to static or heap.
470 Flag to force local variables into static space.
473 Flag to force local variables onto stack.
476 Flag to compile lines beginning with ``D''.
479 Flag to ignore lines beginning with ``D''.
482 Flag for maximum errors before ending compile.
485 Generate code to check for null pointer dereferences -- prints locus of
486 dereference instead of segfaulting. There was some discussion about this
487 option in the g95 development mailing list.
490 Allow setting the default unit number.
493 Option to initialize otherwise uninitialized integer and floating
497 Support for Fortran 200x. This includes several new features including
498 floating point exceptions, extended use of allocatable arrays, C
499 interoperability, Parameterizer data types and function pointers.
503 @subsection Environment Options
506 Pluggable library modules for random numbers, linear algebra.
507 LA should use BLAS calling conventions.
510 Environment variables controlling actions on arithmetic exceptions like
511 overflow, underflow, precision loss -- Generate NaN, abort, default.
515 Set precision for fp units that support it (i387).
518 Variable for setting fp rounding mode.
521 Variable to fill uninitialized variables with a user-defined bit
525 Environment variable controlling filename that is opened for that unit
529 Environment variable to clear/trash memory being freed.
532 Environment variable to control tracing of allocations and frees.
535 Environment variable to display allocated memory at normal program end.
538 Environment variable for filename for * IO-unit.
541 Environment variable for temporary file directory.
544 Environment variable forcing standard output to be line buffered (unix).
549 @chapter Runtime: Influencing runtime behavior with environment variables
552 The behavior of the @command{gfortran} can be influenced by
553 environment variables.
555 Malformed environment variables are silently ignored.
558 * GFORTRAN_STDIN_UNIT:: Unit number for standard input
559 * GFORTRAN_STDOUT_UNIT:: Unit number for standard output
560 * GFORTRAN_STDERR_UNIT:: Unit number for standard error
561 * GFORTRAN_USE_STDERR:: Send library output to standard error
562 * GFORTRAN_TMPDIR:: Directory for scratch files
563 * GFORTRAN_UNBUFFERED_ALL:: Don't buffer output
564 * GFORTRAN_SHOW_LOCUS:: Show location for runtime errors
565 * GFORTRAN_OPTIONAL_PLUS:: Print leading + where permitted
566 * GFORTRAN_DEFAULT_RECL:: Default record length for new files
567 * GFORTRAN_LIST_SEPARATOR:: Separator for list output
568 * GFORTRAN_CONVERT_UNIT:: Set endianness for unformatted I/O
571 @node GFORTRAN_STDIN_UNIT
572 @section @env{GFORTRAN_STDIN_UNIT}---Unit number for standard input
574 This environment variable can be used to select the unit number
575 preconnected to standard input. This must be a positive integer.
576 The default value is 5.
578 @node GFORTRAN_STDOUT_UNIT
579 @section @env{GFORTRAN_STDOUT_UNIT}---Unit number for standard output
581 This environment variable can be used to select the unit number
582 preconnected to standard output. This must be a positive integer.
583 The default value is 6.
585 @node GFORTRAN_STDERR_UNIT
586 @section @env{GFORTRAN_STDERR_UNIT}---Unit number for standard error
588 This environment variable can be used to select the unit number
589 preconnected to standard error. This must be a positive integer.
590 The default value is 0.
592 @node GFORTRAN_USE_STDERR
593 @section @env{GFORTRAN_USE_STDERR}---Send library output to standard error
595 This environment variable controls where library output is sent.
596 If the first letter is @samp{y}, @samp{Y} or @samp{1}, standard
597 error is used. If the first letter is @samp{n}, @samp{N} or
598 @samp{0}, standard output is used.
600 @node GFORTRAN_TMPDIR
601 @section @env{GFORTRAN_TMPDIR}---Directory for scratch files
603 This environment variable controls where scratch files are
604 created. If this environment variable is missing,
605 GNU Fortran searches for the environment variable @env{TMP}. If
606 this is also missing, the default is @file{/tmp}.
608 @node GFORTRAN_UNBUFFERED_ALL
609 @section @env{GFORTRAN_UNBUFFERED_ALL}---Don't buffer output
611 This environment variable controls whether all output is unbuffered.
612 If the first letter is @samp{y}, @samp{Y} or @samp{1}, all output is
613 unbuffered. This will slow down large writes. If the first letter is
614 @samp{n}, @samp{N} or @samp{0}, output is buffered. This is the
617 @node GFORTRAN_SHOW_LOCUS
618 @section @env{GFORTRAN_SHOW_LOCUS}---Show location for runtime errors
620 If the first letter is @samp{y}, @samp{Y} or @samp{1}, filename and
621 line numbers for runtime errors are printed. If the first letter is
622 @samp{n}, @samp{N} or @samp{0}, don't print filename and line numbers
623 for runtime errors. The default is to print the location.
625 @node GFORTRAN_OPTIONAL_PLUS
626 @section @env{GFORTRAN_OPTIONAL_PLUS}---Print leading + where permitted
628 If the first letter is @samp{y}, @samp{Y} or @samp{1},
629 a plus sign is printed
630 where permitted by the Fortran standard. If the first letter
631 is @samp{n}, @samp{N} or @samp{0}, a plus sign is not printed
632 in most cases. Default is not to print plus signs.
634 @node GFORTRAN_DEFAULT_RECL
635 @section @env{GFORTRAN_DEFAULT_RECL}---Default record length for new files
637 This environment variable specifies the default record length for
638 files which are opened without a @code{RECL} tag in the @code{OPEN}
639 statement. This must be a positive integer. The default value is
642 @node GFORTRAN_LIST_SEPARATOR
643 @section @env{GFORTRAN_LIST_SEPARATOR}---Separator for list output
645 This environment variable specifies the separator when writing
646 list-directed output. It may contain any number of spaces and
647 at most one comma. If you specify this on the command line,
648 be sure to quote spaces, as in
650 $ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
652 when @code{a.out} is the compiled Fortran program that you want to run.
653 Default is a single space.
655 @node GFORTRAN_CONVERT_UNIT
656 @section @env{GFORTRAN_CONVERT_UNIT}---Set endianness for unformatted I/O
658 By setting the @env{GFORTRAN_CONVERT_UNIT} variable, it is possible
659 to change the representation of data for unformatted files.
660 The syntax for the @env{GFORTRAN_CONVERT_UNIT} variable is:
662 GFORTRAN_CONVERT_UNIT: mode | mode ';' exception ;
663 mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
664 exception: mode ':' unit_list | unit_list ;
665 unit_list: unit_spec | unit_list unit_spec ;
666 unit_spec: INTEGER | INTEGER '-' INTEGER ;
668 The variable consists of an optional default mode, followed by
669 a list of optional exceptions, which are separated by semicolons
670 from the preceding default and each other. Each exception consists
671 of a format and a comma-separated list of units. Valid values for
672 the modes are the same as for the @code{CONVERT} specifier:
675 @item @code{NATIVE} Use the native format. This is the default.
676 @item @code{SWAP} Swap between little- and big-endian.
677 @item @code{LITTLE_ENDIAN} Use the little-endian format
678 for unformatted files.
679 @item @code{BIG_ENDIAN} Use the big-endian format for unformatted files.
681 A missing mode for an exception is taken to mean @code{BIG_ENDIAN}.
682 Examples of values for @code{GFORTRAN_CONVERT_UNIT} are:
684 @item @code{'big_endian'} Do all unformatted I/O in big_endian mode.
685 @item @code{'little_endian;native:10-20,25'} Do all unformatted I/O
686 in little_endian mode, except for units 10 to 20 and 25, which are in
688 @item @code{'10-20'} Units 10 to 20 are big-endian, the rest is native.
691 Setting the environment variables should be done on the command
692 line or via the @code{export}
693 command for @code{sh}-compatible shells and via @code{setenv}
694 for @code{csh}-compatible shells.
696 Example for @code{sh}:
699 $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
702 Example code for @code{csh}:
705 % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
709 Using anything but the native representation for unformatted data
710 carries a significant speed overhead. If speed in this area matters
711 to you, it is best if you use this only for data that needs to be
714 @xref{CONVERT specifier}, for an alternative way to specify the
715 data representation for unformatted files. @xref{Runtime Options}, for
716 setting a default data representation for the whole program. The
717 @code{CONVERT} specifier overrides the @code{-fconvert} compile options.
719 @c ---------------------------------------------------------------------
721 @c ---------------------------------------------------------------------
723 @c Maybe this chapter should be merged with the 'Standards' section,
724 @c whenever that is written :-)
730 GNU Fortran implements a number of extensions over standard
731 Fortran. This chapter contains information on their syntax and
732 meaning. There are currently two categories of GNU Fortran
733 extensions, those that provide functionality beyond that provided
734 by any standard, and those that are supported by GNU Fortran
735 purely for backward compatibility with legacy compilers. By default,
736 @option{-std=gnu} allows the compiler to accept both types of
737 extensions, but to warn about the use of the latter. Specifying
738 either @option{-std=f95} or @option{-std=f2003} disables both types
739 of extensions, and @option{-std=legacy} allows both without warning.
742 * Old-style kind specifications::
743 * Old-style variable initialization::
744 * Extensions to namelist::
745 * X format descriptor::
746 * Commas in FORMAT specifications::
747 * Missing period in FORMAT specifications::
749 * Hexadecimal constants::
750 * Real array indices::
752 * Implicitly interconvert LOGICAL and INTEGER::
753 * Hollerith constants support::
755 * CONVERT specifier::
759 @node Old-style kind specifications
760 @section Old-style kind specifications
761 @cindex Kind specifications
763 GNU Fortran allows old-style kind specifications in
764 declarations. These look like:
768 where @code{TYPESPEC} is a basic type, and where @code{k} is a valid kind
769 number for that type. The statement then declares @code{x}, @code{y}
770 and @code{z} to be of type @code{TYPESPEC} with kind @code{k}. In
771 other words, it is equivalent to the standard conforming declaration
776 @node Old-style variable initialization
777 @section Old-style variable initialization
778 @cindex Initialization
780 GNU Fortran allows old-style initialization of variables of the
784 REAL*8 x(2,2) /3*0.,1./
786 These are only allowed in declarations without double colons
787 (@code{::}), as these were introduced in Fortran 90 which also
788 introduced a new syntax for variable initializations. The syntax for
789 the individual initializers is as for the @code{DATA} statement, but
790 unlike in a @code{DATA} statement, an initializer only applies to the
791 variable immediately preceding. In other words, something like
792 @code{INTEGER I,J/2,3/} is not valid.
794 Examples of standard conforming code equivalent to the above example, are:
797 INTEGER(4) :: i = 1, j = 2
798 REAL(8) :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
801 DOUBLE PRECISION x(2,2)
802 DATA i,j,x /1,2,3*0.,1./
805 Note that variables initialized in type declarations
806 automatically acquire the @code{SAVE} attribute.
808 @node Extensions to namelist
809 @section Extensions to namelist
812 GNU Fortran fully supports the Fortran 95 standard for namelist I/O
813 including array qualifiers, substrings and fully qualified derived types.
814 The output from a namelist write is compatible with namelist read. The
815 output has all names in upper case and indentation to column 1 after the
816 namelist name. Two extensions are permitted:
818 Old-style use of $ instead of &
821 X(:)%Y(2) = 1.0 2.0 3.0
826 It should be noticed that the default terminator is / rather than &END.
828 Querying of the namelist when inputting from stdin. After at least
829 one space, entering ? sends to stdout the namelist name and the names of
830 the variables in the namelist:
841 Entering =? outputs the namelist to stdout, as if WRITE (*,NML = mynml)
847 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
848 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
849 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
853 To aid this dialog, when input is from stdin, errors send their
854 messages to stderr and execution continues, even if IOSTAT is set.
856 PRINT namelist is permitted. This causes an error if -std=f95 is used.
859 REAL, dimension (4) :: x = (/1.0, 2.0, 3.0, 4.0/)
862 END PROGRAM test_print
865 Expanded namelist reads are permitted. This causes an error if -std=f95
866 is used. In the following example, the first element of the array will be
867 given the value 0.00 and succeeding elements will be 1.00 and 2.00.
870 X(1,1) = 0.00 , 1.00 , 2.00
874 @node X format descriptor
875 @section X format descriptor
876 @cindex X format descriptor
878 To support legacy codes, GNU Fortran permits the count field
879 of the X edit descriptor in FORMAT statements to be omitted. When
880 omitted, the count is implicitly assumed to be one.
884 10 FORMAT (I1, X, I1)
887 @node Commas in FORMAT specifications
888 @section Commas in FORMAT specifications
889 @cindex Commas in FORMAT specifications
891 To support legacy codes, GNU Fortran allows the comma separator
892 to be omitted immediately before and after character string edit
893 descriptors in FORMAT statements.
897 10 FORMAT ('FOO='I1' BAR='I2)
901 @node Missing period in FORMAT specifications
902 @section Missing period in FORMAT specifications
903 @cindex Missing period in FORMAT specifications
905 To support legacy codes, GNU Fortran allows missing periods in format
906 specifications if and only if -std=legacy is given on the command line. This
907 is considered non-conforming code and is discouraged.
916 @section I/O item lists
917 @cindex I/O item lists
919 To support legacy codes, GNU Fortran allows the input item list
920 of the READ statement, and the output item lists of the WRITE and PRINT
921 statements to start with a comma.
923 @node Hexadecimal constants
924 @section Hexadecimal constants
925 @cindex Hexadecimal constants
927 As an extension, GNU Fortran allows hexadecimal constants to
928 be specified using the X prefix, in addition to the standard Z prefix.
929 BOZ literal constants can also be specified by adding a suffix to the string.
930 For example, @code{Z'ABC'} and @code{'ABC'Z} are the same constant.
932 The Fortran standard restricts the appearance of a BOZ literal constant to
933 the @code{DATA} statement, and it is expected to be assigned to an
934 @code{INTEGER} variable. GNU Fortran permits a BOZ literal to appear
935 in any initialization expression as well as assignment statements.
937 Attempts to use a BOZ literal constant to do a bitwise initialization of a
938 variable can lead to confusion. A BOZ literal constant is converted to an
939 @code{INTEGER} value with the kind type with the largest decimal representation,
940 and this value is then converted numerically to the type and kind of the
941 variable in question. Thus, one should not expect a bitwise copy of the BOZ
942 literal constant to be assigned to a @code{REAL} variable.
944 Similarly, initializing an @code{INTEGER} variable with a statement such as
945 @code{DATA i/Z'FFFFFFFF'/} will produce an integer overflow rather than the
946 desired result of @math{-1} when @code{i} is a 32-bit integer on a system that
947 supports 64-bit integers. The @samp{-fno-range-check} option can be used as
948 a workaround for legacy code that initializes integers in this manner.
951 @node Real array indices
952 @section Real array indices
953 @cindex Real array indices
955 As an extension, GNU Fortran allows arrays to be indexed using
956 real types, whose values are implicitly converted to integers.
958 @node Unary operators
959 @section Unary operators
960 @cindex Unary operators
962 As an extension, GNU Fortran allows unary plus and unary
963 minus operators to appear as the second operand of binary arithmetic
964 operators without the need for parenthesis.
970 @node Implicitly interconvert LOGICAL and INTEGER
971 @section Implicitly interconvert LOGICAL and INTEGER
972 @cindex Implicitly interconvert LOGICAL and INTEGER
974 As an extension for backwards compatibility with other compilers,
975 GNU Fortran allows the implicit conversion of LOGICALs to INTEGERs
976 and vice versa. When converting from a LOGICAL to an INTEGER, the numeric
977 value of @code{.FALSE.} is zero, and that of @code{.TRUE.} is one. When
978 converting from INTEGER to LOGICAL, the value zero is interpreted as
979 @code{.FALSE.} and any nonzero value is interpreted as @code{.TRUE.}.
986 @node Hollerith constants support
987 @section Hollerith constants support
988 @cindex Hollerith constants
990 A Hollerith constant is a string of characters preceded by the letter @samp{H}
991 or @samp{h}, and there must be an literal, unsigned, nonzero default integer
992 constant indicating the number of characters in the string. Hollerith constants
993 are stored as byte strings, one character per byte.
995 GNU Fortran supports Hollerith constants. They can be used as the right
996 hands in the @code{DATA} statement and @code{ASSIGN} statement, also as the
997 arguments. The left hands can be of Integer, Real, Complex and Logical type.
998 The constant will be padded or truncated to fit the size of left hand.
1000 Valid Hollerith constants examples:
1003 data x /16Habcdefghijklmnop, 16Hqrstuvwxyz012345/
1005 x(1) = 16Habcdefghijklmnop
1008 Invalid Hollerith constants examples:
1011 a = 8H12345678 ! The Hollerith constant is too long. It will be truncated.
1012 a = 0H ! At least one character needed.
1016 @section Cray pointers
1017 @cindex Cray pointers
1019 Cray pointers are part of a non-standard extension that provides a
1020 C-like pointer in Fortran. This is accomplished through a pair of
1021 variables: an integer "pointer" that holds a memory address, and a
1022 "pointee" that is used to dereference the pointer.
1024 Pointer/pointee pairs are declared in statements of the form:
1026 pointer ( <pointer> , <pointee> )
1030 pointer ( <pointer1> , <pointee1> ), ( <pointer2> , <pointee2> ), ...
1032 The pointer is an integer that is intended to hold a memory address.
1033 The pointee may be an array or scalar. A pointee can be an assumed
1034 size array -- that is, the last dimension may be left unspecified by
1035 using a '*' in place of a value -- but a pointee cannot be an assumed
1036 shape array. No space is allocated for the pointee.
1038 The pointee may have its type declared before or after the pointer
1039 statement, and its array specification (if any) may be declared
1040 before, during, or after the pointer statement. The pointer may be
1041 declared as an integer prior to the pointer statement. However, some
1042 machines have default integer sizes that are different than the size
1043 of a pointer, and so the following code is not portable:
1048 If a pointer is declared with a kind that is too small, the compiler
1049 will issue a warning; the resulting binary will probably not work
1050 correctly, because the memory addresses stored in the pointers may be
1051 truncated. It is safer to omit the first line of the above example;
1052 if explicit declaration of ipt's type is omitted, then the compiler
1053 will ensure that ipt is an integer variable large enough to hold a
1056 Pointer arithmetic is valid with Cray pointers, but it is not the same
1057 as C pointer arithmetic. Cray pointers are just ordinary integers, so
1058 the user is responsible for determining how many bytes to add to a
1059 pointer in order to increment it. Consider the following example:
1063 pointer (ipt, pointee)
1067 The last statement does not set ipt to the address of
1068 @code{target(1)}, as one familiar with C pointer arithmetic might
1069 expect. Adding 1 to ipt just adds one byte to the address stored in
1072 Any expression involving the pointee will be translated to use the
1073 value stored in the pointer as the base address.
1075 To get the address of elements, this extension provides an intrinsic
1076 function loc(), loc() is essentially the C '&' operator, except the
1077 address is cast to an integer type:
1080 pointer(ipt, arpte(10))
1082 ipt = loc(ar) ! Makes arpte is an alias for ar
1083 arpte(1) = 1.0 ! Sets ar(1) to 1.0
1085 The pointer can also be set by a call to the @code{MALLOC} intrinsic
1088 Cray pointees often are used to alias an existing variable. For
1096 As long as ipt remains unchanged, iarr is now an alias for target.
1097 The optimizer, however, will not detect this aliasing, so it is unsafe
1098 to use iarr and target simultaneously. Using a pointee in any way
1099 that violates the Fortran aliasing rules or assumptions is illegal.
1100 It is the user's responsibility to avoid doing this; the compiler
1101 works under the assumption that no such aliasing occurs.
1103 Cray pointers will work correctly when there is no aliasing (i.e.,
1104 when they're used to access a dynamically allocated block of memory),
1105 and also in any routine where a pointee is used, but any variable with
1106 which it shares storage is not used. Code that violates these rules
1107 may not run as the user intends. This is not a bug in the optimizer;
1108 any code that violates the aliasing rules is illegal. (Note that this
1109 is not unique to GNU Fortran; any Fortran compiler that supports Cray
1110 pointers will ``incorrectly'' optimize code with illegal aliasing.)
1112 There are a number of restrictions on the attributes that can be
1113 applied to Cray pointers and pointees. Pointees may not have the
1114 attributes ALLOCATABLE, INTENT, OPTIONAL, DUMMY, TARGET,
1115 INTRINSIC, or POINTER. Pointers may not have the attributes
1116 DIMENSION, POINTER, TARGET, ALLOCATABLE, EXTERNAL, or INTRINSIC.
1117 Pointees may not occur in more than one pointer statement. A pointee
1118 cannot be a pointer. Pointees cannot occur in equivalence, common, or
1121 A Cray pointer may point to a function or a subroutine. For example,
1122 the following excerpt is valid:
1126 pointer (subptr,subpte)
1136 A pointer may be modified during the course of a program, and this
1137 will change the location to which the pointee refers. However, when
1138 pointees are passed as arguments, they are treated as ordinary
1139 variables in the invoked function. Subsequent changes to the pointer
1140 will not change the base address of the array that was passed.
1142 @node CONVERT specifier
1143 @section CONVERT specifier
1144 @cindex CONVERT specifier
1146 GNU Fortran allows the conversion of unformatted data between little-
1147 and big-endian representation to facilitate moving of data
1148 between different systems. The conversion can be indicated with
1149 the @code{CONVERT} specifier on the @code{OPEN} statement.
1150 @xref{GFORTRAN_CONVERT_UNIT}, for an alternative way of specifying
1151 the data format via an environment variable.
1153 Valid values for @code{CONVERT} are:
1155 @item @code{CONVERT='NATIVE'} Use the native format. This is the default.
1156 @item @code{CONVERT='SWAP'} Swap between little- and big-endian.
1157 @item @code{CONVERT='LITTLE_ENDIAN'} Use the little-endian representation
1158 for unformatted files.
1159 @item @code{CONVERT='BIG_ENDIAN'} Use the big-endian representation for
1163 Using the option could look like this:
1165 open(file='big.dat',form='unformatted',access='sequential', &
1166 convert='big_endian')
1169 The value of the conversion can be queried by using
1170 @code{INQUIRE(CONVERT=ch)}. The values returned are
1171 @code{'BIG_ENDIAN'} and @code{'LITTLE_ENDIAN'}.
1173 @code{CONVERT} works between big- and little-endian for
1174 @code{INTEGER} values of all supported kinds and for @code{REAL}
1175 on IEEE systems of kinds 4 and 8. Conversion between different
1176 ``extended double'' types on different architectures such as
1177 m68k and x86_64, which GNU Fortran
1178 supports as @code{REAL(KIND=10)}, will probably not work.
1180 @emph{Note that the values specified via the GFORTRAN_CONVERT_UNIT
1181 environment variable will override the CONVERT specifier in the
1182 open statement}. This is to give control over data formats to
1183 a user who does not have the source code of his program available.
1185 Using anything but the native representation for unformatted data
1186 carries a significant speed overhead. If speed in this area matters
1187 to you, it is best if you use this only for data that needs to be
1194 GNU Fortran attempts to be OpenMP Application Program Interface v2.5
1195 compatible when invoked with the @code{-fopenmp} option. GNU Fortran
1196 then generates parallelized code according to the OpenMP directives
1197 used in the source. The OpenMP Fortran runtime library
1198 routines are provided both in a form of Fortran 90 module named
1199 @code{omp_lib} and in a form of a Fortran @code{include} file named
1202 For details refer to the actual
1203 @uref{http://www.openmp.org/drupal/mp-documents/spec25.pdf,
1204 OpenMP Application Program Interface v2.5} specification.
1206 @c ---------------------------------------------------------------------
1207 @include intrinsic.texi
1208 @c ---------------------------------------------------------------------
1210 @c ---------------------------------------------------------------------
1212 @c ---------------------------------------------------------------------
1215 @chapter Contributing
1216 @cindex Contributing
1218 Free software is only possible if people contribute to efforts
1220 We're always in need of more people helping out with ideas
1221 and comments, writing documentation and contributing code.
1223 If you want to contribute to GNU Fortran,
1224 have a look at the long lists of projects you can take on.
1225 Some of these projects are small,
1226 some of them are large;
1227 some are completely orthogonal to the rest of what is
1228 happening on GNU Fortran,
1229 but others are ``mainstream'' projects in need of enthusiastic hackers.
1230 All of these projects are important!
1231 We'll eventually get around to the things here,
1232 but they are also things doable by someone who is willing and able.
1241 @section Contributors to GNU Fortran
1242 @cindex Contributors
1246 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
1247 also the initiator of the whole project. Thanks Andy!
1248 Most of the interface with GCC was written by @emph{Paul Brook}.
1250 The following individuals have contributed code and/or
1251 ideas and significant help to the GNU Fortran project
1252 (in no particular order):
1256 @item Katherine Holcomb
1257 @item Tobias Schl@"uter
1258 @item Steven Bosscher
1261 @item Niels Kristian Bech Jensen
1262 @item Steven Johnson
1267 @item Fran@,{c}ois-Xavier Coudert
1268 @item Steven G. Kargl
1270 @item Janne Blomqvist
1277 @item Richard Henderson
1278 @item Richard Sandiford
1279 @item Richard Guenther
1280 @item Bernhard Fischer
1283 The following people have contributed bug reports,
1284 smaller or larger patches,
1285 and much needed feedback and encouragement for the
1286 GNU Fortran project:
1289 @item Erik Schnetter
1294 Many other individuals have helped debug,
1295 test and improve the GNU Fortran compiler over the past few years,
1296 and we welcome you to do the same!
1297 If you already have done so,
1298 and you would like to see your name listed in the
1299 list above, please contact us.
1307 @item Help build the test suite
1308 Solicit more code for donation to the test suite.
1309 We can keep code private on request.
1311 @item Bug hunting/squishing
1312 Find bugs and write more test cases!
1313 Test cases are especially very welcome,
1314 because it allows us to concentrate on fixing bugs
1315 instead of isolating them.
1317 @item Smaller projects (``bug'' fixes):
1319 @item Allow init exprs to be numbers raised to integer powers.
1320 @item Implement correct rounding.
1321 @item Implement F restrictions on Fortran 95 syntax.
1322 @item See about making Emacs-parsable error messages.
1326 If you wish to work on the runtime libraries,
1327 please contact a project maintainer.
1331 @c ---------------------------------------------------------------------
1333 @c ---------------------------------------------------------------------
1339 The GNU Fortran compiler aims to be a conforming implementation of
1340 ISO/IEC 1539:1997 (Fortran 95).
1342 In the future it may also support other variants of and extensions to
1343 the Fortran language. These include ANSI Fortran 77, ISO Fortran 90,
1344 ISO Fortran 2003 and OpenMP.
1347 * Fortran 2003 status::
1350 @node Fortran 2003 status
1351 @section Fortran 2003 status
1353 Although GNU Fortran focuses on implementing the Fortran 95
1354 standard for the time being, a few Fortran 2003 features are currently
1359 Intrinsics @code{command_argument_count}, @code{get_command},
1360 @code{get_command_argument}, @code{get_environment_variable}, and
1364 @cindex Array constructors
1365 @cindex @code{[...]}
1366 Array constructors using square brackets. That is, @code{[...]} rather
1367 than @code{(/.../)}.
1370 @cindex @code{FLUSH} statement
1371 @code{FLUSH} statement.
1374 @cindex @code{IOMSG=} specifier
1375 @code{IOMSG=} specifier for I/O statements.
1378 @cindex @code{ENUM} statement
1379 @cindex @code{ENUMERATOR} statement
1380 @cindex @command{-fshort-enums}
1381 Support for the declaration of enumeration constants via the
1382 @code{ENUM} and @code{ENUMERATOR} statements. Interoperability with
1383 @command{gcc} is guaranteed also for the case where the
1384 @command{-fshort-enums} command line option is given.
1391 @cindex @code{ALLOCATABLE} dummy arguments
1392 @code{ALLOCATABLE} dummy arguments.
1394 @cindex @code{ALLOCATABLE} function results
1395 @code{ALLOCATABLE} function results
1397 @cindex @code{ALLOCATABLE} components of derived types
1398 @code{ALLOCATABLE} components of derived types
1402 @cindex @code{STREAM} I/O
1403 @cindex @code{ACCESS='STREAM'} I/O
1404 The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
1405 allowing I/O without any record structure.
1412 @c ---------------------------------------------------------------------
1413 @c GNU General Public License
1414 @c ---------------------------------------------------------------------
1420 @c ---------------------------------------------------------------------
1421 @c GNU Free Documentation License
1422 @c ---------------------------------------------------------------------
1428 @c ---------------------------------------------------------------------
1429 @c Funding Free Software
1430 @c ---------------------------------------------------------------------
1432 @include funding.texi
1434 @c ---------------------------------------------------------------------
1436 @c ---------------------------------------------------------------------