1 \input texinfo @c -*-texinfo-*-
3 @setfilename gfortran.info
4 @set copyrights-gfortran 1999-2005
6 @include gcc-common.texi
8 @settitle The GNU Fortran 95 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 Programming
66 * gfortran: (gfortran). The GNU Fortran 95 Compiler.
68 This file documents the use and the internals of
69 the GNU Fortran 95 compiler, (@command{gfortran}).
71 Published by the Free Software Foundation
72 51 Franklin Street, Fifth Floor
73 Boston, MA 02110-1301 USA
78 Contributed by Steven Bosscher (@email{s.bosscher@@gcc.gnu.org}).
80 @setchapternewpage odd
82 @title Using GNU Fortran 95
84 @center Steven Bosscher
86 @vskip 0pt plus 1filll
87 For the @value{version-GCC} Version*
89 Published by the Free Software Foundation @*
90 51 Franklin Street, Fifth Floor@*
91 Boston, MA 02110-1301, USA@*
92 @c Last printed ??ber, 19??.@*
93 @c Printed copies are available for $? each.@*
102 @node Top, Copying,, (DIR)
106 This manual documents the use of @command{gfortran},
107 the GNU Fortran 95 compiler. You can find in this manual how to invoke
108 @command{gfortran}, as well as its features and incompatibilities.
111 @emph{Warning:} This document, and the compiler it describes, are still
112 under development. While efforts are made to keep it up-to-date, it might
113 not accurately reflect the status of the most recent @command{gfortran}.
117 @comment When you add a new menu item, please keep the right hand
118 @comment aligned to the same column. Do not use tabs. This provides
119 @comment better formatting.
122 * Copying:: GNU General Public License says
123 how you can copy and share GNU Fortran.
124 * GNU Free Documentation License::
125 How you can copy and share this manual.
126 * Funding:: How to help assure continued work for free software.
127 * Getting Started:: What you should know about @command{gfortran}.
128 * GFORTRAN and GCC:: You can compile Fortran, C, or other programs.
129 * GFORTRAN and G77:: Why we chose to start from scratch.
130 * Invoking GFORTRAN:: Command options supported by @command{gfortran}.
131 * Project Status:: Status of @command{gfortran}, roadmap, proposed extensions.
132 * Contributing:: How you can help.
133 * Standards:: Standards supported by @command{gfortran}
134 * Extensions:: Language extensions implemented by @command{gfortran}
135 * Intrinsic Procedures:: Intrinsic procedures supported by @command{gfortran}
136 * Index:: Index of this documentation.
141 @c ---------------------------------------------------------------------
142 @c GNU General Public License
143 @c ---------------------------------------------------------------------
149 @c ---------------------------------------------------------------------
150 @c GNU Free Documentation License
151 @c ---------------------------------------------------------------------
157 @c ---------------------------------------------------------------------
158 @c Funding Free Software
159 @c ---------------------------------------------------------------------
161 @include funding.texi
165 @c ---------------------------------------------------------------------
167 @c ---------------------------------------------------------------------
169 @node Getting Started
170 @chapter Getting Started
172 Gfortran is the GNU Fortran 95 compiler front end,
173 designed initially as a free replacement for,
174 or alternative to, the unix @command{f95} command;
175 @command{gfortran} is command you'll use to invoke the compiler.
177 Gfortran is still in an early state of development.
178 @command{gfortran} can generate code for most constructs and expressions,
179 but much work remains to be done.
181 When @command{gfortran} is finished,
182 it will do everything you expect from any decent compiler:
186 Read a user's program,
187 stored in a file and containing instructions written
188 in Fortran 77, Fortran 90 or Fortran 95.
189 This file contains @dfn{source code}.
192 Translate the user's program into instructions a computer
193 can carry out more quickly than it takes to translate the
194 instructions in the first
195 place. The result after compilation of a program is
197 code designed to be efficiently translated and processed
198 by a machine such as your computer.
199 Humans usually aren't as good writing machine code
200 as they are at writing Fortran (or C++, Ada, or Java),
201 because is easy to make tiny mistakes writing machine code.
204 Provide the user with information about the reasons why
205 the compiler is unable to create a binary from the source code.
206 Usually this will be the case if the source code is flawed.
207 When writing Fortran, it is easy to make big mistakes.
208 The Fortran 90 requires that the compiler can point out
209 mistakes to the user.
210 An incorrect usage of the language causes an @dfn{error message}.
212 The compiler will also attempt to diagnose cases where the
213 user's program contains a correct usage of the language,
214 but instructs the computer to do something questionable.
215 This kind of diagnostics message is called a @dfn{warning message}.
218 Provide optional information about the translation passes
219 from the source code to machine code.
220 This can help a user of the compiler to find the cause of
221 certain bugs which may not be obvious in the source code,
222 but may be more easily found at a lower level compiler output.
223 It also helps developers to find bugs in the compiler itself.
226 Provide information in the generated machine code that can
227 make it easier to find bugs in the program (using a debugging tool,
228 called a @dfn{debugger}, such as the GNU Debugger @command{gdb}).
231 Locate and gather machine code already generated to
232 perform actions requested by statements in the user's program.
233 This machine code is organized into @dfn{modules} and is located
234 and @dfn{linked} to the user program.
237 Gfortran consists of several components:
241 A version of the @command{gcc} command
242 (which also might be installed as the system's @command{cc} command)
243 that also understands and accepts Fortran source code.
244 The @command{gcc} command is the @dfn{driver} program for
245 all the languages in the GNU Compiler Collection (GCC);
247 you can compiler the source code of any language for
248 which a front end is available in GCC.
251 The @command{gfortran} command itself,
252 which also might be installed as the
253 system's @command{f95} command.
254 @command{gfortran} is just another driver program,
255 but specifically for the Fortran 95 compiler only.
256 The difference with @command{gcc} is that @command{gfortran}
257 will automatically link the correct libraries to your program.
260 A collection of run-time libraries.
261 These libraries contains the machine code needed to support
262 capabilities of the Fortran language that are not directly
263 provided by the machine code generated by the
264 @command{gfortran} compilation phase,
265 such as intrinsic functions and subroutines,
266 and routines for interaction with files and the operating system.
267 @c and mechanisms to spawn,
268 @c unleash and pause threads in parallelized code.
271 The Fortran compiler itself, (@command{f951}).
272 This is the gfortran parser and code generator,
273 linked to and interfaced with the GCC backend library.
274 @command{f951} ``translates'' the source code to
275 assembler code. You would typically not use this
277 instead, the @command{gcc} or @command{gfortran} driver
278 programs will call it for you.
283 @c ---------------------------------------------------------------------
285 @c ---------------------------------------------------------------------
287 @node GFORTRAN and GCC
288 @chapter GFORTRAN and GCC
289 @cindex GNU Compiler Collection
291 GCC used to be the GNU ``C'' Compiler,
292 but is now known as the @dfn{GNU Compiler Collection}.
293 GCC provides the GNU system with a very versatile
294 compiler middle end (shared optimization passes),
295 and with back ends (code generators) for many different
296 computer architectures and operating systems.
297 The code of the middle end and back end are shared by all
298 compiler front ends that are in the GNU Compiler Collection.
300 A GCC front end is essentially a source code parser
301 and a pass to generate a representation of the semantics
302 of the program in the source code in the GCC language
303 independent intermediate language,
304 called @dfn{GENERIC}.
306 The parser takes a source file written in a
307 particular computer language, reads and parses it,
308 and tries to make sure that the source code conforms to
310 Once the correctness of a program has been established,
311 the compiler will build a data structure known as the
312 @dfn{Abstract Syntax tree},
313 or just @dfn{AST} or ``tree'' for short.
314 This data structure represents the whole program
315 or a subroutine or a function.
316 The ``tree'' is passed to the GCC middle end,
317 which will perform optimization passes on it,
318 pass the optimized AST and generate assembly
319 for the program unit.
321 Different phases in this translation process can be,
322 and in fact @emph{are} merged in many compiler front ends.
323 GNU Fortran 95 has a strict separation between the
324 parser and code generator.
326 The goal of the gfortran project is to build a new front end for GCC:
327 A Fortran 95 front end.
328 In a non-gfortran installation,
329 @command{gcc} will not be able to compile Fortran 95 source code
330 (only the ``C'' front end has to be compiled if you want to build GCC,
331 all other languages are optional).
332 If you build GCC with gfortran, @command{gcc} will recognize
333 @file{.f/.f90/.f95} source files and accepts Fortran 95 specific
334 command line options.
338 @c ---------------------------------------------------------------------
340 @c ---------------------------------------------------------------------
342 @node GFORTRAN and G77
343 @chapter GFORTRAN and G77
347 Why do we write a compiler front end from scratch?
348 There's a fine Fortran 77 compiler in the
349 GNU Compiler Collection that accepts some features
350 of the Fortran 90 standard as extensions.
351 Why not start from there and revamp it?
353 One of the reasons is that Craig Burley, the author of G77,
354 has decided to stop working on the G77 front end.
355 On @uref{http://world.std.com/~burley/g77-why.html,
356 Craig explains the reasons for his decision to stop working on G77}
357 in one of the pages in his homepage.
358 Among the reasons is a lack of interest in improvements to
360 Users appear to be quite satisfied with @command{g77} as it is.
361 While @command{g77} is still being maintained (by Toon Moene),
362 it is unlikely that sufficient people will be willing
363 to completely rewrite the existing code.
365 But there are other reasons to start from scratch.
366 Many people, including Craig Burley,
367 no longer agreed with certain design decisions in the G77 front end.
368 Also, the interface of @command{g77} to the back end is written in
369 a style which is confusing and not up to date on recommended practice.
370 In fact, a full rewrite had already been planned for GCC 3.0.
372 When Craig decided to stop,
373 it just seemed to be a better idea to start a new project from scratch,
374 because it was expected to be easier to maintain code we
375 develop ourselves than to do a major overhaul of @command{g77} first,
376 and then build a Fortran 95 compiler out of it.
380 @c ---------------------------------------------------------------------
382 @c ---------------------------------------------------------------------
385 @chapter Project Status
388 As soon as gfortran can parse all of the statements correctly,
389 it will be in the ``larva'' state.
390 When we generate code, the ``puppa'' state.
391 When gfortran is done,
392 we'll see if it will be a beautiful butterfly,
393 or just a big bug....
395 --Andy Vaught, April 2000
398 The start of the GNU Fortran 95 project was announced on
399 the GCC homepage in March 18, 2000
400 (even though Andy had already been working on it for a while,
403 Gfortran is currently reaching the stage where is is able to compile real
404 world programs. However it is still under development and has many rough
410 * Proposed Extensions::
413 @node Compiler Status
414 @section Compiler Status
418 This is the part of gfortran which parses a source file, verifies that it
419 is valid Fortran 95, performs compile time replacement of constants
420 (PARAMETER variables) and reads and generate module files. This is
421 almost complete. Every Fortran 95 source should be accepted, and most
422 none-Fortran 95 source should be rejected. If you find a source file where
423 this is not true, please tell us. You can use the -fsyntax-only switch to
424 make gfortran quit after running the front end, effectively reducing it to
427 @item Middle end interface
428 These are the parts of gfortran that take the parse tree generated by the
429 front end and translate it to the GENERIC form required by the GCC back
430 end. Work is ongoing in these parts of gfortran, but a large part has
431 already been completed.
435 @section Library Status
437 Some intrinsic functions map directly to library functions, and in most
438 cases the name of the library function used depends on the type of the
439 arguments. For some intrinsics we generate inline code, and for others,
440 such as sin, cos and sqrt, we rely on the backend to use special
441 instructions in the floating point unit of the CPU if available, or to
442 fall back to a call to libm if these are not available.
444 Implementation of some non-elemental intrinsic functions (eg. DOT_PRODUCT,
445 AVERAGE) is not yet optimal. This is hard because we have to make decisions
446 whether to use inline code (good for small arrays as no function call
447 overhead occurs) or generate function calls (good for large arrays as it
448 allows use of hand-optimized assembly routines, SIMD instructions, etc.)
450 The IO library is still under development. The following features should be
451 usable for real programs:
455 @item Unformatted sequential
461 @item Formatted sequential ('T' edit descriptor, and others)
467 @item Unformatted direct access
468 @item Formatted direct access
471 Many Fortran programs only use a small subset of the available IO
472 capabilities, so your mileage may vary.
474 @node Proposed Extensions
475 @section Proposed Extensions
477 Here's a list of proposed extensions for @command{gfortran}, in no particular
478 order. Most of these are necessary to be fully compatible with
479 existing Fortran compilers, but they are not part of the official
480 J3 Fortran 95 standard.
482 @subsection Compiler extensions:
485 Flag for defining the kind number for default logicals.
488 User-specified alignment rules for structures.
490 Flag to generate a @code{Makefile} info.
493 Automatically extend single precision constants to double.
496 Cray pointers (this was high on the @command{g77} wishlist).
499 Compile code that conserves memory by dynamically allocating common and
500 module storage either on stack or heap.
503 Flag to cause the compiler to distinguish between upper and lower case
504 names. The Fortran 95 standard does not distinguish them.
507 Compile flag to generate code for array conformance checking (suggest -CC).
510 User control of symbol names (underscores, etc).
513 Compile setting for maximum size of stack frame size before spilling
514 parts to static or heap.
517 Flag to force local variables into static space.
520 Flag to force local variables onto stack.
523 Flag to compile lines beginning with ``D''.
526 Flag to ignore lines beginning with ``D''.
529 Flag for maximum errors before ending compile.
532 Generate code to check for null pointer dereferences -- prints locus of
533 dereference instead of segfaulting. There was some discussion about this
534 option in the g95 development mailing list.
537 Allow setting default unit number.
540 Option to initialize of otherwise uninitialized integer and floating
544 Support for OpenMP directives. This also requires support from the runtime
545 library and the rest of the compiler.
548 Support for Fortran 200x. This includes several new features including
549 floating point exceptions, extended use of allocatable arrays, C
550 interoperability, Parameterizer data types and function pointers.
554 @subsection Environment Options
557 Pluggable library modules for random numbers, linear algebra.
558 LA should use BLAS calling conventions.
561 Environment variables controlling actions on arithmetic exceptions like
562 overflow, underflow, precision loss -- Generate NaN, abort, default.
566 Set precision for fp units that support it (i387).
569 Variables for setting fp rounding mode.
572 Variable to fill uninitialized variables with a user-defined bit
576 Environment variable controlling filename that is opened for that unit
580 Environment variable to clear/trash memory being freed.
583 Environment variable to control tracing of allocations and frees.
586 Environment variable to display allocated memory at normal program end.
589 Environment variable for filename for * IO-unit.
592 Environment variable for temporary file directory.
595 Environment variable forcing standard output to be line buffered (unix).
598 Variable for swapping endianness during unformatted read.
601 Variable for swapping Endianness during unformatted write.
604 @c ---------------------------------------------------------------------
606 @c ---------------------------------------------------------------------
608 @c Maybe this chapter should be merged with the 'Standards' section,
609 @c whenever that is written :-)
615 @command{gfortran} implements a number of extensions over standard
616 Fortran. This chapter contains information on their syntax and
617 meaning. There are currently two categories of @command{gfortran}
618 extensions, those that provide functionality beyond that provided
619 by any standard, and those that are supported by @command{gfortran}
620 purely for backward compatibility with legacy compilers. By default,
621 @option{-std=gnu} allows the compiler to accept both types of
622 extensions, but to warn about the use of the latter. Specifying
623 either @option{-std=f95} or @option{-std=f2003} disables both types
624 of extensions, and @option{-std=legacy} allows both without warning.
627 * Old-style kind specifications::
628 * Old-style variable initialization::
629 * Extensions to namelist::
630 * X format descriptor::
631 * Commas in FORMAT specifications::
633 * Hexadecimal constants::
634 * Real array indices::
636 * Implicitly interconvert LOGICAL and INTEGER::
639 @node Old-style kind specifications
640 @section Old-style kind specifications
641 @cindex Kind specifications
643 @command{gfortran} allows old-style kind specifications in
644 declarations. These look like:
648 where @code{TYPESPEC} is a basic type, and where @code{k} is a valid kind
649 number for that type. The statement then declares @code{x}, @code{y}
650 and @code{z} to be of type @code{TYPESPEC} with kind @code{k}. In
651 other words, it is equivalent to the standard conforming declaration
656 @node Old-style variable initialization
657 @section Old-style variable initialization
658 @cindex Initialization
660 @command{gfortran} allows old-style initialization of variables of the
664 REAL*8 x(2,2) /3*0.,1./
666 These are only allowed in declarations without double colons
667 (@code{::}), as these were introduced in Fortran 90 which also
668 introduced a new syntax for variable initializations. The syntax for
669 the individual initializers is as for the @code{DATA} statement, but
670 unlike in a @code{DATA} statement, an initializer only applies to the
671 variable immediately preceding. In other words, something like
672 @code{INTEGER I,J/2,3/} is not valid.
674 Examples of standard conforming code equivalent to the above example, are:
677 INTEGER(4) :: i = 1, j = 2
678 REAL(8) :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
681 DOUBLE PRECISION x(2,2)
682 DATA i,j,x /1,2,3*0.,1./
685 @node Extensions to namelist
686 @section Extensions to namelist
689 @command{gfortran} fully supports the fortran95 standard for namelist io
690 including array qualifiers, substrings and fully qualified derived types.
691 The output from a namelist write is compatible with namelist read. The
692 output has all names in upper case and indentation to column 1 after the
693 namelist name. Two extensions are permitted:
695 Old-style use of $ instead of &
698 X(:)%Y(2) = 1.0 2.0 3.0
703 It should be noticed that the default terminator is / rather than &END.
705 Querying of the namelist when inputting from stdin. After at least
706 one space, entering ? sends to stdout the namelist name and the names of
707 the variables in the namelist:
718 Entering =? outputs the namelist to stdout, as if WRITE (*,NML = mynml)
724 X(1)%Y= 0.000000 , 1.000000 , 0.000000 ,
725 X(2)%Y= 0.000000 , 2.000000 , 0.000000 ,
726 X(3)%Y= 0.000000 , 3.000000 , 0.000000 ,
730 To aid this dialog, when input is from stdin, errors produce send their
731 messages to stderr and execution continues, even if IOSTAT is set.
733 @node X format descriptor
734 @section X format descriptor
735 @cindex X format descriptor
737 To support legacy codes, @command{gfortran} permits the count field
738 of the X edit descriptor in FORMAT statements to be omitted. When
739 omitted, the count is implicitly assumed to be one.
743 10 FORMAT (I1, X, I1)
746 @node Commas in FORMAT specifications
747 @section Commas in FORMAT specifications
748 @cindex Commas in FORMAT specifications
750 To support legacy codes, @command{gfortran} allows the comma separator
751 to be omitted immediately before and after character string edit
752 descriptors in FORMAT statements.
756 10 FORMAT ('FOO='I1' BAR='I2)
760 @section I/O item lists
761 @cindex I/O item lists
763 To support legacy codes, @command{gfortran} allows the input item list
764 of the READ statement, and the output item lists of the WRITE and PRINT
765 statements to start with a comma.
767 @node Hexadecimal constants
768 @section Hexadecimal constants
769 @cindex Hexadecimal constants
771 As a GNU extension, @command{gfortran} allows hexadecimal constants to
772 be specified using the X prefix, in addition to the standard Z prefix.
774 @node Real array indices
775 @section Real array indices
776 @cindex Real array indices
778 As a GNU extension, @command{gfortran} allows arrays to be indexed using
779 real types, whose values are implicitly converted to integers.
781 @node Unary operators
782 @section Unary operators
783 @cindex Unary operators
785 As a GNU extension, @command{gfortran} allows unary plus and unary
786 minus operators to appear as the second operand of binary arithmetic
787 operators without the need for parenthesis.
793 @node Implicitly interconvert LOGICAL and INTEGER
794 @section Implicitly interconvert LOGICAL and INTEGER
795 @cindex Implicitly interconvert LOGICAL and INTEGER
797 As a GNU extension for backwards compatability with other compilers,
798 @command{gfortran} allows the implicit conversion of LOGICALs to INTEGERs
799 and vice versa. When converting from a LOGICAL to an INTEGER, the numeric
800 value of @code{.FALSE.} is zero, and that of @code{.TRUE.} is one. When
801 converting from INTEGER to LOGICAL, the value zero is interpreted as
802 @code{.FALSE.} and any non-zero value is interpreted as @code{.TRUE.}.
809 @include intrinsic.texi
810 @c ---------------------------------------------------------------------
812 @c ---------------------------------------------------------------------
815 @chapter Contributing
818 Free software is only possible if people contribute to efforts
820 We're always in need of more people helping out with ideas
821 and comments, writing documentation and contributing code.
823 If you want to contribute to GNU Fortran 95,
824 have a look at the long lists of projects you can take on.
825 Some of these projects are small,
826 some of them are large;
827 some are completely orthogonal to the rest of what is
828 happening on @command{gfortran},
829 but others are ``mainstream'' projects in need of enthusiastic hackers.
830 All of these projects are important!
831 We'll eventually get around to the things here,
832 but they are also things doable by someone who is willing and able.
841 @section Contributors to GNU Fortran 95
846 Most of the parser was hand-crafted by @emph{Andy Vaught}, who is
847 also the initiator of the whole project. Thanks Andy!
848 Most of the interface with GCC was written by @emph{Paul Brook}.
850 The following individuals have contributed code and/or
851 ideas and significant help to the gfortran project
852 (in no particular order):
856 @item Katherine Holcomb
857 @item Tobias Schlüter
858 @item Steven Bosscher
861 @item Niels Kristian Bech Jensen
868 The following people have contributed bug reports,
869 smaller or larger patches,
870 and much needed feedback and encouragement for the
871 @command{gfortran} project:
879 Many other individuals have helped debug,
880 test and improve @command{gfortran} over the past two years,
881 and we welcome you to do the same!
882 If you already have done so,
883 and you would like to see your name listed in the
884 list above, please contact us.
892 @item Help build the test suite
893 Solicit more code for donation to the test suite.
894 We can keep code private on request.
896 @item Bug hunting/squishing
897 Find bugs and write more test cases!
898 Test cases are especially very welcome,
899 because it allows us to concentrate on fixing bugs
900 instead of isolating them.
902 @item Smaller projects (``bug'' fixes):
904 @item Allow init exprs to be numbers raised to integer powers.
905 @item Implement correct rounding.
906 @item Implement F restrictions on Fortran 95 syntax.
907 @item See about making Emacs-parsable error messages.
911 If you wish to work on the runtime libraries,
912 please contact a project maintainer.
916 @c ---------------------------------------------------------------------
918 @c ---------------------------------------------------------------------
924 The GNU Fortran 95 Compiler aims to be a conforming implementation of
925 ISO/IEC 1539:1997 (Fortran 95).
927 In the future it may also support other variants and extensions to the Fortran
928 language. This includes ANSI Fortran 77, Fortran 90, Fortran 2000 (not yet
929 finalized), and OpenMP.