1 @c Copyright (C) 2004, 2005, 2006, 2007
2 @c Free Software Foundation, Inc.
3 @c This is part of the GNU Fortran manual.
4 @c For copying conditions, see the file gfortran.texi.
8 Copyright @copyright{} 2004, 2005, 2006, 2007
9 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU Fortran compiler.
33 gfortran [@option{-c}|@option{-S}|@option{-E}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}]
39 [@option{-m}@var{machine-option}@dots{}]
40 [@option{-o} @var{outfile}] @var{infile}@dots{}
42 Only the most useful options are listed here; see below for the
46 gpl(7), gfdl(7), fsf-funding(7),
47 cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
48 and the Info entries for @file{gcc}, @file{cpp}, @file{gfortran}, @file{as},
49 @file{ld}, @file{binutils} and @file{gdb}.
52 For instructions on reporting bugs, see
56 See the Info entry for @command{gfortran} for contributors to GCC and
61 @node Invoking GNU Fortran
62 @chapter GNU Fortran Command Options
63 @cindex GNU Fortran command options
64 @cindex command options
65 @cindex options, @command{gfortran} command
67 @c man begin DESCRIPTION
69 The @command{gfortran} command supports all the options supported by the
70 @command{gcc} command. Only options specific to GNU Fortran are documented here.
72 @xref{Invoking GCC,,GCC Command Options,gcc,Using the GNU Compiler
73 Collection (GCC)}, for information
74 on the non-Fortran-specific aspects of the @command{gcc} command (and,
75 therefore, the @command{gfortran} command).
77 @cindex options, negative forms
78 All GCC and GNU Fortran options
79 are accepted both by @command{gfortran} and by @command{gcc}
80 (as well as any other drivers built at the same time,
81 such as @command{g++}),
82 since adding GNU Fortran to the GCC distribution
83 enables acceptance of GNU Fortran options
84 by all of the relevant drivers.
86 In some cases, options have positive and negative forms;
87 the negative form of @option{-ffoo} would be @option{-fno-foo}.
88 This manual documents only one of these two forms, whichever
89 one is not the default.
93 * Option Summary:: Brief list of all @command{gfortran} options,
95 * Fortran Dialect Options:: Controlling the variant of Fortran language
97 * Error and Warning Options:: How picky should the compiler be?
98 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
99 * Directory Options:: Where to find module files
100 * Link Options :: Influencing the linking step
101 * Runtime Options:: Influencing runtime behavior
102 * Code Gen Options:: Specifying conventions for function calls, data layout
104 * Environment Variables:: Environment variables that affect @command{gfortran}.
108 @section Option summary
112 Here is a summary of all the options specific to GNU Fortran, grouped
113 by type. Explanations are in the following sections.
116 @item Fortran Language Options
117 @xref{Fortran Dialect Options,,Options controlling Fortran dialect}.
118 @gccoptlist{-fall-intrinsics -ffree-form -fno-fixed-form @gol
119 -fdollar-ok -fimplicit-none -fmax-identifier-length @gol
120 -std=@var{std} -fd-lines-as-code -fd-lines-as-comments @gol
121 -ffixed-line-length-@var{n} -ffixed-line-length-none @gol
122 -ffree-line-length-@var{n} -ffree-line-length-none @gol
123 -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 @gol
124 -fcray-pointer -fopenmp -fno-range-check -fno-backslash -fmodule-private}
126 @item Error and Warning Options
127 @xref{Error and Warning Options,,Options to request or suppress errors
129 @gccoptlist{-fmax-errors=@var{n} @gol
130 -fsyntax-only -pedantic -pedantic-errors @gol
131 -Wall -Waliasing -Wampersand -Wcharacter-truncation -Wconversion @gol
132 -Wimplicit-interface -Wline-truncation -Wnonstd-intrinsics -Wsurprising @gol
133 -Wno-tabs -Wunderflow -Wunused-parameter}
135 @item Debugging Options
136 @xref{Debugging Options,,Options for debugging your program or GNU Fortran}.
137 @gccoptlist{-fdump-parse-tree -ffpe-trap=@var{list} @gol
138 -fdump-core -fbacktrace}
140 @item Directory Options
141 @xref{Directory Options,,Options for directory search}.
142 @gccoptlist{-I@var{dir} -J@var{dir} -M@var{dir} -fintrinsic-modules-path @var{dir}}
145 @xref{Link Options,,Options for influencing the linking step}.
146 @gccoptlist{-static-libgfortran}
148 @item Runtime Options
149 @xref{Runtime Options,,Options for influencing runtime behavior}.
150 @gccoptlist{-fconvert=@var{conversion} -frecord-marker=@var{length} @gol
151 -fmax-subrecord-length=@var{length} -fsign-zero}
153 @item Code Generation Options
154 @xref{Code Gen Options,,Options for code generation conventions}.
155 @gccoptlist{-fno-automatic -ff2c -fno-underscoring
156 -fsecond-underscore @gol
157 -fbounds-check -fmax-stack-var-size=@var{n} @gol
158 -fpack-derived -frepack-arrays -fshort-enums -fexternal-blas @gol
159 -fblas-matmul-limit=@var{n} -frecursive -finit-local-zero @gol
160 -finit-integer=@var{n} -finit-real=@var{<zero|inf|-inf|nan>} @gol
161 -finit-logical=@var{<true|false>} -finit-character=@var{n}}
165 * Fortran Dialect Options:: Controlling the variant of Fortran language
167 * Error and Warning Options:: How picky should the compiler be?
168 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
169 * Directory Options:: Where to find module files
170 * Link Options :: Influencing the linking step
171 * Runtime Options:: Influencing runtime behavior
172 * Code Gen Options:: Specifying conventions for function calls, data layout
176 @node Fortran Dialect Options
177 @section Options controlling Fortran dialect
178 @cindex dialect options
179 @cindex language, dialect options
180 @cindex options, dialect
182 The following options control the details of the Fortran dialect
183 accepted by the compiler:
188 @opindex @code{ffree-form}
189 @opindex @code{fno-fixed-form}
190 @cindex options, fortran dialect
191 @cindex file format, free
192 @cindex file format, fixed
193 Specify the layout used by the source file. The free form layout
194 was introduced in Fortran 90. Fixed form was traditionally used in
195 older Fortran programs. When neither option is specified, the source
196 form is determined by the file extension.
198 @item -fall-intrinsics
199 @opindex @code{fall-intrinsics}
200 Accept all of the intrinsic procedures provided in libgfortran
201 without regard to the setting of @option{-std}. In particular,
202 this option can be quite useful with @option{-std=f95}. Additionally,
203 @command{gfortran} will ignore @option{-Wnonstd-intrinsics}.
205 @item -fd-lines-as-code
206 @item -fd-lines-as-comments
207 @opindex @code{fd-lines-as-code}
208 @opindex @code{fd-lines-as-comments}
209 Enable special treatment for lines beginning with @code{d} or @code{D}
210 in fixed form sources. If the @option{-fd-lines-as-code} option is
211 given they are treated as if the first column contained a blank. If the
212 @option{-fd-lines-as-comments} option is given, they are treated as
215 @item -fdefault-double-8
216 @opindex @code{fdefault-double-8}
217 Set the @code{DOUBLE PRECISION} type to an 8 byte wide type.
219 @item -fdefault-integer-8
220 @opindex @code{fdefault-integer-8}
221 Set the default integer and logical types to an 8 byte wide type.
222 Do nothing if this is already the default.
224 @item -fdefault-real-8
225 @opindex @code{fdefault-real-8}
226 Set the default real type to an 8 byte wide type.
227 Do nothing if this is already the default.
230 @opindex @code{fdollar-ok}
233 @cindex character set
234 Allow @samp{$} as a valid character in a symbol name.
237 @opindex @code{fno-backslash}
239 @cindex escape characters
240 Change the interpretation of backslashes in string literals from
241 ``C-style'' escape characters to a single backslash character.
243 @item -fmodule-private
244 @opindex @code{fmodule-private}
245 @cindex module entities
247 Set the default accessibility of module entities to @code{PRIVATE}.
248 Use-associated entities will not be accessible unless they are explicitly
249 declared as @code{PUBLIC}.
251 @item -ffixed-line-length-@var{n}
252 @opindex @code{ffixed-line-length-}@var{n}
253 @cindex file format, fixed
254 Set column after which characters are ignored in typical fixed-form
255 lines in the source file, and through which spaces are assumed (as
256 if padded to that length) after the ends of short fixed-form lines.
258 Popular values for @var{n} include 72 (the
259 standard and the default), 80 (card image), and 132 (corresponding
260 to ``extended-source'' options in some popular compilers).
261 @var{n} may also be @samp{none}, meaning that the entire line is meaningful
262 and that continued character constants never have implicit spaces appended
263 to them to fill out the line.
264 @option{-ffixed-line-length-0} means the same thing as
265 @option{-ffixed-line-length-none}.
267 @item -ffree-line-length-@var{n}
268 @opindex @code{ffree-line-length-}@var{n}
269 @cindex file format, free
270 Set column after which characters are ignored in typical free-form
271 lines in the source file. The default value is 132.
272 @var{n} may be @samp{none}, meaning that the entire line is meaningful.
273 @option{-ffree-line-length-0} means the same thing as
274 @option{-ffree-line-length-none}.
276 @item -fmax-identifier-length=@var{n}
277 @opindex @code{fmax-identifier-length=}@var{n}
278 Specify the maximum allowed identifier length. Typical values are
279 31 (Fortran 95) and 63 (Fortran 2003).
281 @item -fimplicit-none
282 @opindex @code{fimplicit-none}
283 Specify that no implicit typing is allowed, unless overridden by explicit
284 @code{IMPLICIT} statements. This is the equivalent of adding
285 @code{implicit none} to the start of every procedure.
288 @opindex @code{fcray-pointer}
289 Enable the Cray pointer extension, which provides C-like pointer
293 @opindex @code{fopenmp}
295 Enable the OpenMP extensions. This includes OpenMP @code{!$omp} directives
297 and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,
298 @code{!$} conditional compilation sentinels in free form
299 and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form,
300 and when linking arranges for the OpenMP runtime library to be linked
301 in. The option @option{-fopenmp} implies @option{-frecursive}.
303 @item -fno-range-check
304 @opindex @code{frange-check}
305 Disable range checking on results of simplification of constant
306 expressions during compilation. For example, by default, GNU Fortran
307 will give an overflow error at compile time when simplifying @code{a =
308 EXP(1000)}. With @option{-fno-range-check}, no error will be given and
309 the variable @code{a} will be assigned the value @code{+Infinity}.
310 Similarly, @code{DATA i/Z'FFFFFFFF'/} will result in an integer overflow
311 on most systems, but with @option{-fno-range-check} the value will
312 ``wrap around'' and @code{i} will be initialized to @math{-1} instead.
315 @opindex @code{std=}@var{std} option
316 Specify the standard to which the program is expected to conform, which
317 may be one of @samp{f95}, @samp{f2003}, @samp{gnu}, or @samp{legacy}.
318 The default value for @var{std} is @samp{gnu}, which specifies a
319 superset of the Fortran 95 standard that includes all of the extensions
320 supported by GNU Fortran, although warnings will be given for obsolete
321 extensions not recommended for use in new code. The @samp{legacy} value
322 is equivalent but without the warnings for obsolete extensions, and may
323 be useful for old non-standard programs. The @samp{f95} and
324 @samp{f2003} values specify strict conformance to the Fortran 95 and
325 Fortran 2003 standards, respectively; errors are given for all
326 extensions beyond the relevant language standard, and warnings are given
327 for the Fortran 77 features that are permitted but obsolescent in later
332 @node Error and Warning Options
333 @section Options to request or suppress errors and warnings
334 @cindex options, warnings
335 @cindex options, errors
336 @cindex warnings, suppressing
337 @cindex messages, error
338 @cindex messages, warning
339 @cindex suppressing warnings
341 Errors are diagnostic messages that report that the GNU Fortran compiler
342 cannot compile the relevant piece of source code. The compiler will
343 continue to process the program in an attempt to report further errors
344 to aid in debugging, but will not produce any compiled output.
346 Warnings are diagnostic messages that report constructions which
347 are not inherently erroneous but which are risky or suggest there is
348 likely to be a bug in the program. Unless @option{-Werror} is specified,
349 they do not prevent compilation of the program.
351 You can request many specific warnings with options beginning @option{-W},
352 for example @option{-Wimplicit} to request warnings on implicit
353 declarations. Each of these specific warning options also has a
354 negative form beginning @option{-Wno-} to turn off warnings;
355 for example, @option{-Wno-implicit}. This manual lists only one of the
356 two forms, whichever is not the default.
358 These options control the amount and kinds of errors and warnings produced
362 @item -fmax-errors-@var{n}
363 @opindex @code{fmax-errors-}@var{n}
364 @cindex errors, limiting
365 Limits the maximum number of error messages to @var{n}, at which point
366 GNU Fortran bails out rather than attempting to continue processing the
367 source code. If @var{n} is 0, there is no limit on the number of error
371 @opindex @code{fsyntax-only}
372 @cindex syntax checking
373 Check the code for syntax errors, but don't actually compile it. This
374 will generate module files for each module present in the code, but no
378 @opindex @code{pedantic}
379 Issue warnings for uses of extensions to Fortran 95.
380 @option{-pedantic} also applies to C-language constructs where they
381 occur in GNU Fortran source files, such as use of @samp{\e} in a
382 character constant within a directive like @code{#include}.
384 Valid Fortran 95 programs should compile properly with or without
386 However, without this option, certain GNU extensions and traditional
387 Fortran features are supported as well.
388 With this option, many of them are rejected.
390 Some users try to use @option{-pedantic} to check programs for conformance.
391 They soon find that it does not do quite what they want---it finds some
392 nonstandard practices, but not all.
393 However, improvements to GNU Fortran in this area are welcome.
395 This should be used in conjunction with @option{-std=f95} or
398 @item -pedantic-errors
399 @opindex @code{pedantic-errors}
400 Like @option{-pedantic}, except that errors are produced rather than
406 @cindex warnings, all
407 Enables commonly used warning options pertaining to usage that
408 we recommend avoiding and that we believe are easy to avoid.
409 This currently includes @option{-Waliasing},
410 @option{-Wampersand}, @option{-Wsurprising}, @option{-Wnonstd-intrinsics},
411 @option{-Wno-tabs}, and @option{-Wline-truncation}.
414 @opindex @code{Waliasing}
416 @cindex warnings, aliasing
417 Warn about possible aliasing of dummy arguments. Specifically, it warns
418 if the same actual argument is associated with a dummy argument with
419 @code{INTENT(IN)} and a dummy argument with @code{INTENT(OUT)} in a call
420 with an explicit interface.
422 The following example will trigger the warning.
426 integer, intent(in) :: a
427 integer, intent(out) :: b
436 @opindex @code{Wampersand}
437 @cindex warnings, ampersand
439 Warn about missing ampersand in continued character constants. The warning is
440 given with @option{-Wampersand}, @option{-pedantic}, @option{-std=f95}, and
441 @option{-std=f2003}. Note: With no ampersand given in a continued character
442 constant, GNU Fortran assumes continuation at the first non-comment,
443 non-whitespace character after the ampersand that initiated the continuation.
445 @item -Wcharacter-truncation
446 @opindex @code{Wcharacter-truncation}
447 @cindex warnings, character truncation
448 Warn when a character assignment will truncate the assigned string.
451 @opindex @code{Wconversion}
452 @cindex warnings, conversion
454 Warn about implicit conversions between different types.
456 @item -Wimplicit-interface
457 @opindex @code{Wimplicit-interface}
458 @cindex warnings, implicit interface
459 Warn if a procedure is called without an explicit interface.
460 Note this only checks that an explicit interface is present. It does not
461 check that the declared interfaces are consistent across program units.
463 @item -Wnonstd-intrinsics
464 @opindex @code{Wnonstd-intrinsics}
465 @cindex warnings, non-standard intrinsics
466 Warn if the user tries to use an intrinsic that does not belong to the
467 standard the user has chosen via the @option{-std} option.
470 @opindex @code{Wsurprising}
471 @cindex warnings, suspicious code
472 Produce a warning when ``suspicious'' code constructs are encountered.
473 While technically legal these usually indicate that an error has been made.
475 This currently produces a warning under the following circumstances:
479 An INTEGER SELECT construct has a CASE that can never be matched as its
480 lower value is greater than its upper value.
483 A LOGICAL SELECT construct has three CASE statements.
487 @opindex @code{Wtabs}
488 @cindex warnings, tabs
490 By default, tabs are accepted as whitespace, but tabs are not members
491 of the Fortran Character Set. @option{-Wno-tabs} will cause a warning
492 to be issued if a tab is encountered. Note, @option{-Wno-tabs} is active
493 for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003}, and
497 @opindex @code{Wunderflow}
498 @cindex warnings, underflow
500 Produce a warning when numerical constant expressions are
501 encountered, which yield an UNDERFLOW during compilation.
503 @item -Wunused-parameter
504 @opindex @code{Wunused-parameter}
505 @cindex warnings, unused parameter
506 @cindex unused parameter
507 Contrary to @command{gcc}'s meaning of @option{-Wunused-parameter},
508 @command{gfortran}'s implementation of this option does not warn
509 about unused dummy arguments, but about unused @code{PARAMETER} values.
510 @option{-Wunused-parameter} is not included in @option{-Wall} but is
511 implied by @option{-Wall -Wextra}.
514 @opindex @code{Werror}
515 @cindex warnings, to errors
516 Turns all warnings into errors.
519 @xref{Error and Warning Options,,Options to Request or Suppress Errors and
520 Warnings, gcc,Using the GNU Compiler Collection (GCC)}, for information on
521 more options offered by the GBE shared by @command{gfortran}, @command{gcc}
522 and other GNU compilers.
524 Some of these have no effect when compiling programs written in Fortran.
526 @node Debugging Options
527 @section Options for debugging your program or GNU Fortran
528 @cindex options, debugging
529 @cindex debugging information options
531 GNU Fortran has various special options that are used for debugging
532 either your program or the GNU Fortran compiler.
535 @item -fdump-parse-tree
536 @opindex @code{fdump-parse-tree}
537 Output the internal parse tree before starting code generation. Only
538 really useful for debugging the GNU Fortran compiler itself.
540 @item -ffpe-trap=@var{list}
541 @opindex @code{ffpe-trap=}@var{list}
542 Specify a list of IEEE exceptions when a Floating Point Exception
543 (FPE) should be raised. On most systems, this will result in a SIGFPE
544 signal being sent and the program being interrupted, producing a core
545 file useful for debugging. @var{list} is a (possibly empty) comma-separated
546 list of the following IEEE exceptions: @samp{invalid} (invalid floating
547 point operation, such as @code{SQRT(-1.0)}), @samp{zero} (division by
548 zero), @samp{overflow} (overflow in a floating point operation),
549 @samp{underflow} (underflow in a floating point operation),
550 @samp{precision} (loss of precision during operation) and @samp{denormal}
551 (operation produced a denormal value).
554 @opindex @code{fbacktrace}
557 Specify that, when a runtime error is encountered or a deadly signal is
558 emitted (segmentation fault, illegal instruction, bus error or
559 floating-point exception), the Fortran runtime
560 library should output a backtrace of the error. This option
561 only has influence for compilation of the Fortran main program.
565 @opindex @code{fdump-core}
566 Request that a core-dump file is written to disk when a runtime error
567 is encountered on systems that support core dumps. This option is
568 only effective for the compilation of the Fortran main program.
571 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
572 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
575 @node Directory Options
576 @section Options for directory search
577 @cindex directory, options
578 @cindex options, directory search
580 @cindex INCLUDE directive
581 @cindex directive, INCLUDE
582 These options affect how GNU Fortran searches
583 for files specified by the @code{INCLUDE} directive and where it searches
584 for previously compiled modules.
586 It also affects the search paths used by @command{cpp} when used to preprocess
591 @opindex @code{I}@var{dir}
592 @cindex directory, search paths for inclusion
593 @cindex inclusion, directory search paths for
594 @cindex search paths, for included files
595 @cindex paths, search
596 @cindex module search path
597 These affect interpretation of the @code{INCLUDE} directive
598 (as well as of the @code{#include} directive of the @command{cpp}
601 Also note that the general behavior of @option{-I} and
602 @code{INCLUDE} is pretty much the same as of @option{-I} with
603 @code{#include} in the @command{cpp} preprocessor, with regard to
604 looking for @file{header.gcc} files and other such things.
606 This path is also used to search for @file{.mod} files when previously
607 compiled modules are required by a @code{USE} statement.
609 @xref{Directory Options,,Options for Directory Search,
610 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
615 @opindex @code{M}@var{dir}
616 @opindex @code{J}@var{dir}
617 @cindex paths, search
618 @cindex module search path
619 This option specifies where to put @file{.mod} files for compiled modules.
620 It is also added to the list of directories to searched by an @code{USE}
623 The default is the current directory.
625 @option{-J} is an alias for @option{-M} to avoid conflicts with existing
628 @item -fintrinsic-modules-path @var{dir}
629 @opindex @code{fintrinsic-modules-path} @var{dir}
630 @cindex paths, search
631 @cindex module search path
632 This option specifies the location of pre-compiled intrinsic modules, if
633 they are not in the default location expected by the compiler.
637 @section Influencing the linking step
638 @cindex options, linking
639 @cindex linking, static
641 These options come into play when the compiler links object files into an
642 executable output file. They are meaningless if the compiler is not doing
646 @item -static-libgfortran
647 @opindex @code{static-libgfortran}
648 On systems that provide @file{libgfortran} as a shared and a static
649 library, this option forces the use of the static version. If no
650 shared version of @file{libgfortran} was built when the compiler was
651 configured, this option has no effect.
655 @node Runtime Options
656 @section Influencing runtime behavior
657 @cindex options, runtime
659 These options affect the runtime behavior of programs compiled with GNU Fortran.
661 @item -fconvert=@var{conversion}
662 @opindex @code{fconvert=}@var{conversion}
663 Specify the representation of data for unformatted files. Valid
664 values for conversion are: @samp{native}, the default; @samp{swap},
665 swap between big- and little-endian; @samp{big-endian}, use big-endian
666 representation for unformatted files; @samp{little-endian}, use little-endian
667 representation for unformatted files.
669 @emph{This option has an effect only when used in the main program.
670 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
671 variable override the default specified by @option{-fconvert}.}
673 @item -frecord-marker=@var{length}
674 @opindex @code{frecord-marker=}@var{length}
675 Specify the length of record markers for unformatted files.
676 Valid values for @var{length} are 4 and 8. Default is 4.
677 @emph{This is different from previous versions of @command{gfortran}},
678 which specified a default record marker length of 8 on most
679 systems. If you want to read or write files compatible
680 with earlier versions of @command{gfortran}, use @option{-frecord-marker=8}.
682 @item -fmax-subrecord-length=@var{length}
683 @opindex @code{fmax-subrecord-length=}@var{length}
684 Specify the maximum length for a subrecord. The maximum permitted
685 value for length is 2147483639, which is also the default. Only
686 really useful for use by the gfortran testsuite.
689 @opindex @code{fsign-zero}
690 When writing zero values, show the negative sign if the sign bit is set.
691 @code{fno-sign-zero} does not print the negative sign of zero values for
692 compatibility with F77. Default behavior is to show the negative sign.
695 @node Code Gen Options
696 @section Options for code generation conventions
697 @cindex code generation, conventions
698 @cindex options, code generation
699 @cindex options, run-time
701 These machine-independent options control the interface conventions
702 used in code generation.
704 Most of them have both positive and negative forms; the negative form
705 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
706 one of the forms is listed---the one which is not the default. You
707 can figure out the other form by either removing @option{no-} or adding
712 @opindex @code{fno-automatic}
713 @cindex @code{SAVE} statement
714 @cindex statement, @code{SAVE}
715 Treat each program unit (except those marked as RECURSIVE) as if the
716 @code{SAVE} statement were specified for every local variable and array
717 referenced in it. Does not affect common blocks. (Some Fortran compilers
718 provide this option under the name @option{-static} or @option{-save}.)
719 The default, which is @option{-fautomatic}, uses the stack for local
720 variables smaller than the value given by @option{-fmax-stack-var-size}.
721 Use the option @option{-frecursive} to use no static memory.
725 @cindex calling convention
726 @cindex @command{f2c} calling convention
727 @cindex @command{g77} calling convention
728 @cindex libf2c calling convention
729 Generate code designed to be compatible with code generated
730 by @command{g77} and @command{f2c}.
732 The calling conventions used by @command{g77} (originally implemented
733 in @command{f2c}) require functions that return type
734 default @code{REAL} to actually return the C type @code{double}, and
735 functions that return type @code{COMPLEX} to return the values via an
736 extra argument in the calling sequence that points to where to
737 store the return value. Under the default GNU calling conventions, such
738 functions simply return their results as they would in GNU
739 C---default @code{REAL} functions return the C type @code{float}, and
740 @code{COMPLEX} functions return the GNU C type @code{complex}.
741 Additionally, this option implies the @option{-fsecond-underscore}
742 option, unless @option{-fno-second-underscore} is explicitly requested.
744 This does not affect the generation of code that interfaces with
745 the @command{libgfortran} library.
747 @emph{Caution:} It is not a good idea to mix Fortran code compiled with
748 @option{-ff2c} with code compiled with the default @option{-fno-f2c}
749 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
750 functions between program parts which were compiled with different
751 calling conventions will break at execution time.
753 @emph{Caution:} This will break code which passes intrinsic functions
754 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
755 the library implementations use the @option{-fno-f2c} calling conventions.
757 @item -fno-underscoring
758 @opindex @code{fno-underscoring}
760 @cindex symbol names, underscores
761 @cindex transforming symbol names
762 @cindex symbol names, transforming
763 Do not transform names of entities specified in the Fortran
764 source file by appending underscores to them.
766 With @option{-funderscoring} in effect, GNU Fortran appends one
767 underscore to external names with no underscores. This is done to ensure
768 compatibility with code produced by many UNIX Fortran compilers.
770 @emph{Caution}: The default behavior of GNU Fortran is
771 incompatible with @command{f2c} and @command{g77}, please use the
772 @option{-ff2c} option if you want object files compiled with
773 GNU Fortran to be compatible with object code created with these
776 Use of @option{-fno-underscoring} is not recommended unless you are
777 experimenting with issues such as integration of GNU Fortran into
778 existing system environments (vis-a-vis existing libraries, tools, and
781 For example, with @option{-funderscoring}, and assuming other defaults like
782 @option{-fcase-lower} and that @code{j()} and @code{max_count()} are
783 external functions while @code{my_var} and @code{lvar} are local variables,
786 I = J() + MAX_COUNT (MY_VAR, LVAR)
789 is implemented as something akin to:
791 i = j_() + max_count__(&my_var__, &lvar);
794 With @option{-fno-underscoring}, the same statement is implemented as:
797 i = j() + max_count(&my_var, &lvar);
800 Use of @option{-fno-underscoring} allows direct specification of
801 user-defined names while debugging and when interfacing GNU Fortran
802 code with other languages.
804 Note that just because the names match does @emph{not} mean that the
805 interface implemented by GNU Fortran for an external name matches the
806 interface implemented by some other language for that same name.
807 That is, getting code produced by GNU Fortran to link to code produced
808 by some other compiler using this or any other method can be only a
809 small part of the overall solution---getting the code generated by
810 both compilers to agree on issues other than naming can require
811 significant effort, and, unlike naming disagreements, linkers normally
812 cannot detect disagreements in these other areas.
814 Also, note that with @option{-fno-underscoring}, the lack of appended
815 underscores introduces the very real possibility that a user-defined
816 external name will conflict with a name in a system library, which
817 could make finding unresolved-reference bugs quite difficult in some
818 cases---they might occur at program run time, and show up only as
819 buggy behavior at run time.
821 In future versions of GNU Fortran we hope to improve naming and linking
822 issues so that debugging always involves using the names as they appear
823 in the source, even if the names as seen by the linker are mangled to
824 prevent accidental linking between procedures with incompatible
827 @item -fsecond-underscore
828 @opindex @code{fsecond-underscore}
830 @cindex symbol names, underscores
831 @cindex transforming symbol names
832 @cindex symbol names, transforming
833 @cindex @command{f2c} calling convention
834 @cindex @command{g77} calling convention
835 @cindex libf2c calling convention
836 By default, GNU Fortran appends an underscore to external
837 names. If this option is used GNU Fortran appends two
838 underscores to names with underscores and one underscore to external names
839 with no underscores. GNU Fortran also appends two underscores to
840 internal names with underscores to avoid naming collisions with external
843 This option has no effect if @option{-fno-underscoring} is
844 in effect. It is implied by the @option{-ff2c} option.
846 Otherwise, with this option, an external name such as @code{MAX_COUNT}
847 is implemented as a reference to the link-time external symbol
848 @code{max_count__}, instead of @code{max_count_}. This is required
849 for compatibility with @command{g77} and @command{f2c}, and is implied
850 by use of the @option{-ff2c} option.
853 @opindex @code{fbounds-check}
854 @cindex array, bounds checking
855 @cindex bounds checking
856 @cindex range checking
857 @cindex subscript checking
858 @cindex checking subscripts
859 Enable generation of run-time checks for array subscripts
860 and against the declared minimum and maximum values. It also
861 checks array indices for assumed and deferred
862 shape arrays against the actual allocated bounds.
864 Some checks require that @option{-fbounds-check} is set for
865 the compilation of the main probram.
867 In the future this may also include other forms of checking, e.g., checking
868 substring references.
870 @item -fmax-stack-var-size=@var{n}
871 @opindex @code{fmax-stack-var-size}
872 This option specifies the size in bytes of the largest array that will be put
873 on the stack; if the size is exceeded static memory is used (except in
874 procedures marked as RECURSIVE). Use the option @option{-frecursive} to
875 allow for recursive procedures which do not have a RECURSIVE attribute or
876 for parallel programs. Use @option{-fno-automatic} to never use the stack.
878 This option currently only affects local arrays declared with constant
879 bounds, and may not apply to all character variables.
880 Future versions of GNU Fortran may improve this behavior.
882 The default value for @var{n} is 32768.
885 @opindex @code{fpack-derived}
886 @cindex structure packing
887 This option tells GNU Fortran to pack derived type members as closely as
888 possible. Code compiled with this option is likely to be incompatible
889 with code compiled without this option, and may execute slower.
891 @item -frepack-arrays
892 @opindex @code{frepack-arrays}
893 @cindex repacking arrays
894 In some circumstances GNU Fortran may pass assumed shape array
895 sections via a descriptor describing a noncontiguous area of memory.
896 This option adds code to the function prologue to repack the data into
897 a contiguous block at runtime.
899 This should result in faster accesses to the array. However it can introduce
900 significant overhead to the function call, especially when the passed data
904 @opindex @code{fshort-enums}
905 This option is provided for interoperability with C code that was
906 compiled with the @option{-fshort-enums} option. It will make
907 GNU Fortran choose the smallest @code{INTEGER} kind a given
908 enumerator set will fit in, and give all its enumerators this kind.
910 @item -fexternal-blas
911 @opindex @code{fexternal-blas}
912 This option will make @command{gfortran} generate calls to BLAS functions
913 for some matrix operations like @code{MATMUL}, instead of using our own
914 algorithms, if the size of the matrices involved is larger than a given
915 limit (see @option{-fblas-matmul-limit}). This may be profitable if an
916 optimized vendor BLAS library is available. The BLAS library will have
917 to be specified at link time.
919 @item -fblas-matmul-limit=@var{n}
920 @opindex @code{fblas-matmul-limit}
921 Only significant when @option{-fexternal-blas} is in effect.
922 Matrix multiplication of matrices with size larger than (or equal to) @var{n}
923 will be performed by calls to BLAS functions, while others will be
924 handled by @command{gfortran} internal algorithms. If the matrices
925 involved are not square, the size comparison is performed using the
926 geometric mean of the dimensions of the argument and result matrices.
928 The default value for @var{n} is 30.
931 @opindex @code{frecursive}
932 Allow indirect recursion by forcing all local arrays to be allocated
933 on the stack. This flag cannot be used together with
934 @option{-fmax-stack-var-size=} or @option{-fno-automatic}.
936 @item -finit-local-zero
937 @item -finit-integer=@var{n}
938 @item -finit-real=@var{<zero|inf|-inf|nan>}
939 @item -finit-logical=@var{<true|false>}
940 @item -finit-character=@var{n}
941 @opindex @code{finit-local-zero}
942 @opindex @code{finit-integer}
943 @opindex @code{finit-real}
944 @opindex @code{finit-logical}
945 @opindex @code{finit-character}
946 The @option{-finit-local-zero} option instructs the compiler to
947 initialize local @code{INTEGER}, @code{REAL}, and @code{COMPLEX}
948 variables to zero, @code{LOGICAL} variables to false, and
949 @code{CHARACTER} variables to a string of null bytes. Finer-grained
950 initialization options are provided by the
951 @option{-finit-integer=@var{n}},
952 @option{-finit-real=@var{<zero|inf|-inf|nan>}} (which also initializes
953 the real and imaginary parts of local @code{COMPLEX} variables),
954 @option{-finit-logical=@var{<true|false>}}, and
955 @option{-finit-character=@var{n}} (where @var{n} is an ASCII character
956 value) options. These options do not initialize components of derived
957 type variables, nor do they initialize variables that appear in an
958 @code{EQUIVALENCE} statement. (This limitation may be removed in
961 Note that the @option{-finit-real=nan} option initializes @code{REAL}
962 and @code{COMPLEX} variables with a quiet NaN.
965 @xref{Code Gen Options,,Options for Code Generation Conventions,
966 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
968 shared by @command{gfortran}, @command{gcc}, and other GNU compilers.
973 @node Environment Variables
974 @section Environment variables affecting @command{gfortran}
975 @cindex environment variable
977 @c man begin ENVIRONMENT
979 The @command{gfortran} compiler currently does not make use of any environment
980 variables to control its operation above and beyond those
981 that affect the operation of @command{gcc}.
983 @xref{Environment Variables,,Environment Variables Affecting GCC,
984 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
987 @xref{Runtime}, for environment variables that affect the
988 run-time behavior of programs compiled with GNU Fortran.