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 -fbackslash -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{backslash}
239 @cindex escape characters
240 Change the interpretation of backslashes in string literals
241 from a single backslash character to ``C-style'' escape characters.
242 The following combinations are expanded \a, \b, \f, \n, \r, \t,
243 \v, \\, and \0 to the ASCII characters alert, backspace, form feed,
244 newline, carriage return, horizontal tab, vertical tab, backslash,
245 and NUL, respectively. All other combinations of a character preceded
248 @item -fmodule-private
249 @opindex @code{fmodule-private}
250 @cindex module entities
252 Set the default accessibility of module entities to @code{PRIVATE}.
253 Use-associated entities will not be accessible unless they are explicitly
254 declared as @code{PUBLIC}.
256 @item -ffixed-line-length-@var{n}
257 @opindex @code{ffixed-line-length-}@var{n}
258 @cindex file format, fixed
259 Set column after which characters are ignored in typical fixed-form
260 lines in the source file, and through which spaces are assumed (as
261 if padded to that length) after the ends of short fixed-form lines.
263 Popular values for @var{n} include 72 (the
264 standard and the default), 80 (card image), and 132 (corresponding
265 to ``extended-source'' options in some popular compilers).
266 @var{n} may also be @samp{none}, meaning that the entire line is meaningful
267 and that continued character constants never have implicit spaces appended
268 to them to fill out the line.
269 @option{-ffixed-line-length-0} means the same thing as
270 @option{-ffixed-line-length-none}.
272 @item -ffree-line-length-@var{n}
273 @opindex @code{ffree-line-length-}@var{n}
274 @cindex file format, free
275 Set column after which characters are ignored in typical free-form
276 lines in the source file. The default value is 132.
277 @var{n} may be @samp{none}, meaning that the entire line is meaningful.
278 @option{-ffree-line-length-0} means the same thing as
279 @option{-ffree-line-length-none}.
281 @item -fmax-identifier-length=@var{n}
282 @opindex @code{fmax-identifier-length=}@var{n}
283 Specify the maximum allowed identifier length. Typical values are
284 31 (Fortran 95) and 63 (Fortran 2003).
286 @item -fimplicit-none
287 @opindex @code{fimplicit-none}
288 Specify that no implicit typing is allowed, unless overridden by explicit
289 @code{IMPLICIT} statements. This is the equivalent of adding
290 @code{implicit none} to the start of every procedure.
293 @opindex @code{fcray-pointer}
294 Enable the Cray pointer extension, which provides C-like pointer
298 @opindex @code{fopenmp}
300 Enable the OpenMP extensions. This includes OpenMP @code{!$omp} directives
302 and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,
303 @code{!$} conditional compilation sentinels in free form
304 and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form,
305 and when linking arranges for the OpenMP runtime library to be linked
306 in. The option @option{-fopenmp} implies @option{-frecursive}.
308 @item -fno-range-check
309 @opindex @code{frange-check}
310 Disable range checking on results of simplification of constant
311 expressions during compilation. For example, GNU Fortran will give
312 an error at compile time when simplifying @code{a = 1. / 0}.
313 With this option, no error will be given and @code{a} will be assigned
314 the value @code{+Infinity}. If an expression evaluates to a value
315 outside of the relevant range of [@code{-HUGE()}:@code{HUGE()}],
316 then the expression will be replaced by @code{-Inf} or @code{+Inf}
318 Similarly, @code{DATA i/Z'FFFFFFFF'/} will result in an integer overflow
319 on most systems, but with @option{-fno-range-check} the value will
320 ``wrap around'' and @code{i} will be initialized to @math{-1} instead.
323 @opindex @code{std=}@var{std} option
324 Specify the standard to which the program is expected to conform, which
325 may be one of @samp{f95}, @samp{f2003}, @samp{gnu}, or @samp{legacy}.
326 The default value for @var{std} is @samp{gnu}, which specifies a
327 superset of the Fortran 95 standard that includes all of the extensions
328 supported by GNU Fortran, although warnings will be given for obsolete
329 extensions not recommended for use in new code. The @samp{legacy} value
330 is equivalent but without the warnings for obsolete extensions, and may
331 be useful for old non-standard programs. The @samp{f95} and
332 @samp{f2003} values specify strict conformance to the Fortran 95 and
333 Fortran 2003 standards, respectively; errors are given for all
334 extensions beyond the relevant language standard, and warnings are given
335 for the Fortran 77 features that are permitted but obsolescent in later
340 @node Error and Warning Options
341 @section Options to request or suppress errors and warnings
342 @cindex options, warnings
343 @cindex options, errors
344 @cindex warnings, suppressing
345 @cindex messages, error
346 @cindex messages, warning
347 @cindex suppressing warnings
349 Errors are diagnostic messages that report that the GNU Fortran compiler
350 cannot compile the relevant piece of source code. The compiler will
351 continue to process the program in an attempt to report further errors
352 to aid in debugging, but will not produce any compiled output.
354 Warnings are diagnostic messages that report constructions which
355 are not inherently erroneous but which are risky or suggest there is
356 likely to be a bug in the program. Unless @option{-Werror} is specified,
357 they do not prevent compilation of the program.
359 You can request many specific warnings with options beginning @option{-W},
360 for example @option{-Wimplicit} to request warnings on implicit
361 declarations. Each of these specific warning options also has a
362 negative form beginning @option{-Wno-} to turn off warnings;
363 for example, @option{-Wno-implicit}. This manual lists only one of the
364 two forms, whichever is not the default.
366 These options control the amount and kinds of errors and warnings produced
370 @item -fmax-errors=@var{n}
371 @opindex @code{fmax-errors=}@var{n}
372 @cindex errors, limiting
373 Limits the maximum number of error messages to @var{n}, at which point
374 GNU Fortran bails out rather than attempting to continue processing the
375 source code. If @var{n} is 0, there is no limit on the number of error
379 @opindex @code{fsyntax-only}
380 @cindex syntax checking
381 Check the code for syntax errors, but don't actually compile it. This
382 will generate module files for each module present in the code, but no
386 @opindex @code{pedantic}
387 Issue warnings for uses of extensions to Fortran 95.
388 @option{-pedantic} also applies to C-language constructs where they
389 occur in GNU Fortran source files, such as use of @samp{\e} in a
390 character constant within a directive like @code{#include}.
392 Valid Fortran 95 programs should compile properly with or without
394 However, without this option, certain GNU extensions and traditional
395 Fortran features are supported as well.
396 With this option, many of them are rejected.
398 Some users try to use @option{-pedantic} to check programs for conformance.
399 They soon find that it does not do quite what they want---it finds some
400 nonstandard practices, but not all.
401 However, improvements to GNU Fortran in this area are welcome.
403 This should be used in conjunction with @option{-std=f95} or
406 @item -pedantic-errors
407 @opindex @code{pedantic-errors}
408 Like @option{-pedantic}, except that errors are produced rather than
414 @cindex warnings, all
415 Enables commonly used warning options pertaining to usage that
416 we recommend avoiding and that we believe are easy to avoid.
417 This currently includes @option{-Waliasing},
418 @option{-Wampersand}, @option{-Wsurprising}, @option{-Wnonstd-intrinsics},
419 @option{-Wno-tabs}, and @option{-Wline-truncation}.
422 @opindex @code{Waliasing}
424 @cindex warnings, aliasing
425 Warn about possible aliasing of dummy arguments. Specifically, it warns
426 if the same actual argument is associated with a dummy argument with
427 @code{INTENT(IN)} and a dummy argument with @code{INTENT(OUT)} in a call
428 with an explicit interface.
430 The following example will trigger the warning.
434 integer, intent(in) :: a
435 integer, intent(out) :: b
444 @opindex @code{Wampersand}
445 @cindex warnings, ampersand
447 Warn about missing ampersand in continued character constants. The warning is
448 given with @option{-Wampersand}, @option{-pedantic}, @option{-std=f95}, and
449 @option{-std=f2003}. Note: With no ampersand given in a continued character
450 constant, GNU Fortran assumes continuation at the first non-comment,
451 non-whitespace character after the ampersand that initiated the continuation.
453 @item -Wcharacter-truncation
454 @opindex @code{Wcharacter-truncation}
455 @cindex warnings, character truncation
456 Warn when a character assignment will truncate the assigned string.
459 @opindex @code{Wconversion}
460 @cindex warnings, conversion
462 Warn about implicit conversions between different types.
464 @item -Wimplicit-interface
465 @opindex @code{Wimplicit-interface}
466 @cindex warnings, implicit interface
467 Warn if a procedure is called without an explicit interface.
468 Note this only checks that an explicit interface is present. It does not
469 check that the declared interfaces are consistent across program units.
471 @item -Wnonstd-intrinsics
472 @opindex @code{Wnonstd-intrinsics}
473 @cindex warnings, non-standard intrinsics
474 Warn if the user tries to use an intrinsic that does not belong to the
475 standard the user has chosen via the @option{-std} option.
478 @opindex @code{Wsurprising}
479 @cindex warnings, suspicious code
480 Produce a warning when ``suspicious'' code constructs are encountered.
481 While technically legal these usually indicate that an error has been made.
483 This currently produces a warning under the following circumstances:
487 An INTEGER SELECT construct has a CASE that can never be matched as its
488 lower value is greater than its upper value.
491 A LOGICAL SELECT construct has three CASE statements.
494 A TRANSFER specifies a source that is shorter than the destination.
498 @opindex @code{Wtabs}
499 @cindex warnings, tabs
501 By default, tabs are accepted as whitespace, but tabs are not members
502 of the Fortran Character Set. @option{-Wno-tabs} will cause a warning
503 to be issued if a tab is encountered. Note, @option{-Wno-tabs} is active
504 for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003}, and
508 @opindex @code{Wunderflow}
509 @cindex warnings, underflow
511 Produce a warning when numerical constant expressions are
512 encountered, which yield an UNDERFLOW during compilation.
514 @item -Wunused-parameter
515 @opindex @code{Wunused-parameter}
516 @cindex warnings, unused parameter
517 @cindex unused parameter
518 Contrary to @command{gcc}'s meaning of @option{-Wunused-parameter},
519 @command{gfortran}'s implementation of this option does not warn
520 about unused dummy arguments, but about unused @code{PARAMETER} values.
521 @option{-Wunused-parameter} is not included in @option{-Wall} but is
522 implied by @option{-Wall -Wextra}.
525 @opindex @code{Werror}
526 @cindex warnings, to errors
527 Turns all warnings into errors.
530 @xref{Error and Warning Options,,Options to Request or Suppress Errors and
531 Warnings, gcc,Using the GNU Compiler Collection (GCC)}, for information on
532 more options offered by the GBE shared by @command{gfortran}, @command{gcc}
533 and other GNU compilers.
535 Some of these have no effect when compiling programs written in Fortran.
537 @node Debugging Options
538 @section Options for debugging your program or GNU Fortran
539 @cindex options, debugging
540 @cindex debugging information options
542 GNU Fortran has various special options that are used for debugging
543 either your program or the GNU Fortran compiler.
546 @item -fdump-parse-tree
547 @opindex @code{fdump-parse-tree}
548 Output the internal parse tree before starting code generation. Only
549 really useful for debugging the GNU Fortran compiler itself.
551 @item -ffpe-trap=@var{list}
552 @opindex @code{ffpe-trap=}@var{list}
553 Specify a list of IEEE exceptions when a Floating Point Exception
554 (FPE) should be raised. On most systems, this will result in a SIGFPE
555 signal being sent and the program being interrupted, producing a core
556 file useful for debugging. @var{list} is a (possibly empty) comma-separated
557 list of the following IEEE exceptions: @samp{invalid} (invalid floating
558 point operation, such as @code{SQRT(-1.0)}), @samp{zero} (division by
559 zero), @samp{overflow} (overflow in a floating point operation),
560 @samp{underflow} (underflow in a floating point operation),
561 @samp{precision} (loss of precision during operation) and @samp{denormal}
562 (operation produced a denormal value).
564 Some of the routines in the Fortran runtime library, like
565 @samp{CPU_TIME}, are likely to to trigger floating point exceptions when
566 @code{ffpe-trap=precision} is used. For this reason, the use of
567 @code{ffpe-trap=precision} is not recommended.
570 @opindex @code{fbacktrace}
573 Specify that, when a runtime error is encountered or a deadly signal is
574 emitted (segmentation fault, illegal instruction, bus error or
575 floating-point exception), the Fortran runtime
576 library should output a backtrace of the error. This option
577 only has influence for compilation of the Fortran main program.
581 @opindex @code{fdump-core}
582 Request that a core-dump file is written to disk when a runtime error
583 is encountered on systems that support core dumps. This option is
584 only effective for the compilation of the Fortran main program.
587 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
588 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
591 @node Directory Options
592 @section Options for directory search
593 @cindex directory, options
594 @cindex options, directory search
596 @cindex INCLUDE directive
597 @cindex directive, INCLUDE
598 These options affect how GNU Fortran searches
599 for files specified by the @code{INCLUDE} directive and where it searches
600 for previously compiled modules.
602 It also affects the search paths used by @command{cpp} when used to preprocess
607 @opindex @code{I}@var{dir}
608 @cindex directory, search paths for inclusion
609 @cindex inclusion, directory search paths for
610 @cindex search paths, for included files
611 @cindex paths, search
612 @cindex module search path
613 These affect interpretation of the @code{INCLUDE} directive
614 (as well as of the @code{#include} directive of the @command{cpp}
617 Also note that the general behavior of @option{-I} and
618 @code{INCLUDE} is pretty much the same as of @option{-I} with
619 @code{#include} in the @command{cpp} preprocessor, with regard to
620 looking for @file{header.gcc} files and other such things.
622 This path is also used to search for @file{.mod} files when previously
623 compiled modules are required by a @code{USE} statement.
625 @xref{Directory Options,,Options for Directory Search,
626 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
631 @opindex @code{M}@var{dir}
632 @opindex @code{J}@var{dir}
633 @cindex paths, search
634 @cindex module search path
635 This option specifies where to put @file{.mod} files for compiled modules.
636 It is also added to the list of directories to searched by an @code{USE}
639 The default is the current directory.
641 @option{-J} is an alias for @option{-M} to avoid conflicts with existing
644 @item -fintrinsic-modules-path @var{dir}
645 @opindex @code{fintrinsic-modules-path} @var{dir}
646 @cindex paths, search
647 @cindex module search path
648 This option specifies the location of pre-compiled intrinsic modules, if
649 they are not in the default location expected by the compiler.
653 @section Influencing the linking step
654 @cindex options, linking
655 @cindex linking, static
657 These options come into play when the compiler links object files into an
658 executable output file. They are meaningless if the compiler is not doing
662 @item -static-libgfortran
663 @opindex @code{static-libgfortran}
664 On systems that provide @file{libgfortran} as a shared and a static
665 library, this option forces the use of the static version. If no
666 shared version of @file{libgfortran} was built when the compiler was
667 configured, this option has no effect.
671 @node Runtime Options
672 @section Influencing runtime behavior
673 @cindex options, runtime
675 These options affect the runtime behavior of programs compiled with GNU Fortran.
677 @item -fconvert=@var{conversion}
678 @opindex @code{fconvert=}@var{conversion}
679 Specify the representation of data for unformatted files. Valid
680 values for conversion are: @samp{native}, the default; @samp{swap},
681 swap between big- and little-endian; @samp{big-endian}, use big-endian
682 representation for unformatted files; @samp{little-endian}, use little-endian
683 representation for unformatted files.
685 @emph{This option has an effect only when used in the main program.
686 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
687 variable override the default specified by @option{-fconvert}.}
689 @item -frecord-marker=@var{length}
690 @opindex @code{frecord-marker=}@var{length}
691 Specify the length of record markers for unformatted files.
692 Valid values for @var{length} are 4 and 8. Default is 4.
693 @emph{This is different from previous versions of @command{gfortran}},
694 which specified a default record marker length of 8 on most
695 systems. If you want to read or write files compatible
696 with earlier versions of @command{gfortran}, use @option{-frecord-marker=8}.
698 @item -fmax-subrecord-length=@var{length}
699 @opindex @code{fmax-subrecord-length=}@var{length}
700 Specify the maximum length for a subrecord. The maximum permitted
701 value for length is 2147483639, which is also the default. Only
702 really useful for use by the gfortran testsuite.
705 @opindex @code{fsign-zero}
706 When writing zero values, show the negative sign if the sign bit is set.
707 @code{fno-sign-zero} does not print the negative sign of zero values for
708 compatibility with F77. Default behavior is to show the negative sign.
711 @node Code Gen Options
712 @section Options for code generation conventions
713 @cindex code generation, conventions
714 @cindex options, code generation
715 @cindex options, run-time
717 These machine-independent options control the interface conventions
718 used in code generation.
720 Most of them have both positive and negative forms; the negative form
721 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
722 one of the forms is listed---the one which is not the default. You
723 can figure out the other form by either removing @option{no-} or adding
728 @opindex @code{fno-automatic}
729 @cindex @code{SAVE} statement
730 @cindex statement, @code{SAVE}
731 Treat each program unit (except those marked as RECURSIVE) as if the
732 @code{SAVE} statement were specified for every local variable and array
733 referenced in it. Does not affect common blocks. (Some Fortran compilers
734 provide this option under the name @option{-static} or @option{-save}.)
735 The default, which is @option{-fautomatic}, uses the stack for local
736 variables smaller than the value given by @option{-fmax-stack-var-size}.
737 Use the option @option{-frecursive} to use no static memory.
741 @cindex calling convention
742 @cindex @command{f2c} calling convention
743 @cindex @command{g77} calling convention
744 @cindex libf2c calling convention
745 Generate code designed to be compatible with code generated
746 by @command{g77} and @command{f2c}.
748 The calling conventions used by @command{g77} (originally implemented
749 in @command{f2c}) require functions that return type
750 default @code{REAL} to actually return the C type @code{double}, and
751 functions that return type @code{COMPLEX} to return the values via an
752 extra argument in the calling sequence that points to where to
753 store the return value. Under the default GNU calling conventions, such
754 functions simply return their results as they would in GNU
755 C---default @code{REAL} functions return the C type @code{float}, and
756 @code{COMPLEX} functions return the GNU C type @code{complex}.
757 Additionally, this option implies the @option{-fsecond-underscore}
758 option, unless @option{-fno-second-underscore} is explicitly requested.
760 This does not affect the generation of code that interfaces with
761 the @command{libgfortran} library.
763 @emph{Caution:} It is not a good idea to mix Fortran code compiled with
764 @option{-ff2c} with code compiled with the default @option{-fno-f2c}
765 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
766 functions between program parts which were compiled with different
767 calling conventions will break at execution time.
769 @emph{Caution:} This will break code which passes intrinsic functions
770 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
771 the library implementations use the @option{-fno-f2c} calling conventions.
773 @item -fno-underscoring
774 @opindex @code{fno-underscoring}
776 @cindex symbol names, underscores
777 @cindex transforming symbol names
778 @cindex symbol names, transforming
779 Do not transform names of entities specified in the Fortran
780 source file by appending underscores to them.
782 With @option{-funderscoring} in effect, GNU Fortran appends one
783 underscore to external names with no underscores. This is done to ensure
784 compatibility with code produced by many UNIX Fortran compilers.
786 @emph{Caution}: The default behavior of GNU Fortran is
787 incompatible with @command{f2c} and @command{g77}, please use the
788 @option{-ff2c} option if you want object files compiled with
789 GNU Fortran to be compatible with object code created with these
792 Use of @option{-fno-underscoring} is not recommended unless you are
793 experimenting with issues such as integration of GNU Fortran into
794 existing system environments (vis-@`{a}-vis existing libraries, tools,
797 For example, with @option{-funderscoring}, and assuming other defaults like
798 @option{-fcase-lower} and that @code{j()} and @code{max_count()} are
799 external functions while @code{my_var} and @code{lvar} are local variables,
802 I = J() + MAX_COUNT (MY_VAR, LVAR)
805 is implemented as something akin to:
807 i = j_() + max_count__(&my_var__, &lvar);
810 With @option{-fno-underscoring}, the same statement is implemented as:
813 i = j() + max_count(&my_var, &lvar);
816 Use of @option{-fno-underscoring} allows direct specification of
817 user-defined names while debugging and when interfacing GNU Fortran
818 code with other languages.
820 Note that just because the names match does @emph{not} mean that the
821 interface implemented by GNU Fortran for an external name matches the
822 interface implemented by some other language for that same name.
823 That is, getting code produced by GNU Fortran to link to code produced
824 by some other compiler using this or any other method can be only a
825 small part of the overall solution---getting the code generated by
826 both compilers to agree on issues other than naming can require
827 significant effort, and, unlike naming disagreements, linkers normally
828 cannot detect disagreements in these other areas.
830 Also, note that with @option{-fno-underscoring}, the lack of appended
831 underscores introduces the very real possibility that a user-defined
832 external name will conflict with a name in a system library, which
833 could make finding unresolved-reference bugs quite difficult in some
834 cases---they might occur at program run time, and show up only as
835 buggy behavior at run time.
837 In future versions of GNU Fortran we hope to improve naming and linking
838 issues so that debugging always involves using the names as they appear
839 in the source, even if the names as seen by the linker are mangled to
840 prevent accidental linking between procedures with incompatible
843 @item -fsecond-underscore
844 @opindex @code{fsecond-underscore}
846 @cindex symbol names, underscores
847 @cindex transforming symbol names
848 @cindex symbol names, transforming
849 @cindex @command{f2c} calling convention
850 @cindex @command{g77} calling convention
851 @cindex libf2c calling convention
852 By default, GNU Fortran appends an underscore to external
853 names. If this option is used GNU Fortran appends two
854 underscores to names with underscores and one underscore to external names
855 with no underscores. GNU Fortran also appends two underscores to
856 internal names with underscores to avoid naming collisions with external
859 This option has no effect if @option{-fno-underscoring} is
860 in effect. It is implied by the @option{-ff2c} option.
862 Otherwise, with this option, an external name such as @code{MAX_COUNT}
863 is implemented as a reference to the link-time external symbol
864 @code{max_count__}, instead of @code{max_count_}. This is required
865 for compatibility with @command{g77} and @command{f2c}, and is implied
866 by use of the @option{-ff2c} option.
869 @opindex @code{fbounds-check}
870 @cindex array, bounds checking
871 @cindex bounds checking
872 @cindex range checking
873 @cindex subscript checking
874 @cindex checking subscripts
875 Enable generation of run-time checks for array subscripts
876 and against the declared minimum and maximum values. It also
877 checks array indices for assumed and deferred
878 shape arrays against the actual allocated bounds.
880 Some checks require that @option{-fbounds-check} is set for
881 the compilation of the main program.
883 In the future this may also include other forms of checking, e.g., checking
884 substring references.
886 @item -fmax-stack-var-size=@var{n}
887 @opindex @code{fmax-stack-var-size}
888 This option specifies the size in bytes of the largest array that will be put
889 on the stack; if the size is exceeded static memory is used (except in
890 procedures marked as RECURSIVE). Use the option @option{-frecursive} to
891 allow for recursive procedures which do not have a RECURSIVE attribute or
892 for parallel programs. Use @option{-fno-automatic} to never use the stack.
894 This option currently only affects local arrays declared with constant
895 bounds, and may not apply to all character variables.
896 Future versions of GNU Fortran may improve this behavior.
898 The default value for @var{n} is 32768.
901 @opindex @code{fpack-derived}
902 @cindex structure packing
903 This option tells GNU Fortran to pack derived type members as closely as
904 possible. Code compiled with this option is likely to be incompatible
905 with code compiled without this option, and may execute slower.
907 @item -frepack-arrays
908 @opindex @code{frepack-arrays}
909 @cindex repacking arrays
910 In some circumstances GNU Fortran may pass assumed shape array
911 sections via a descriptor describing a noncontiguous area of memory.
912 This option adds code to the function prologue to repack the data into
913 a contiguous block at runtime.
915 This should result in faster accesses to the array. However it can introduce
916 significant overhead to the function call, especially when the passed data
920 @opindex @code{fshort-enums}
921 This option is provided for interoperability with C code that was
922 compiled with the @option{-fshort-enums} option. It will make
923 GNU Fortran choose the smallest @code{INTEGER} kind a given
924 enumerator set will fit in, and give all its enumerators this kind.
926 @item -fexternal-blas
927 @opindex @code{fexternal-blas}
928 This option will make @command{gfortran} generate calls to BLAS functions
929 for some matrix operations like @code{MATMUL}, instead of using our own
930 algorithms, if the size of the matrices involved is larger than a given
931 limit (see @option{-fblas-matmul-limit}). This may be profitable if an
932 optimized vendor BLAS library is available. The BLAS library will have
933 to be specified at link time.
935 @item -fblas-matmul-limit=@var{n}
936 @opindex @code{fblas-matmul-limit}
937 Only significant when @option{-fexternal-blas} is in effect.
938 Matrix multiplication of matrices with size larger than (or equal to) @var{n}
939 will be performed by calls to BLAS functions, while others will be
940 handled by @command{gfortran} internal algorithms. If the matrices
941 involved are not square, the size comparison is performed using the
942 geometric mean of the dimensions of the argument and result matrices.
944 The default value for @var{n} is 30.
947 @opindex @code{frecursive}
948 Allow indirect recursion by forcing all local arrays to be allocated
949 on the stack. This flag cannot be used together with
950 @option{-fmax-stack-var-size=} or @option{-fno-automatic}.
952 @item -finit-local-zero
953 @item -finit-integer=@var{n}
954 @item -finit-real=@var{<zero|inf|-inf|nan>}
955 @item -finit-logical=@var{<true|false>}
956 @item -finit-character=@var{n}
957 @opindex @code{finit-local-zero}
958 @opindex @code{finit-integer}
959 @opindex @code{finit-real}
960 @opindex @code{finit-logical}
961 @opindex @code{finit-character}
962 The @option{-finit-local-zero} option instructs the compiler to
963 initialize local @code{INTEGER}, @code{REAL}, and @code{COMPLEX}
964 variables to zero, @code{LOGICAL} variables to false, and
965 @code{CHARACTER} variables to a string of null bytes. Finer-grained
966 initialization options are provided by the
967 @option{-finit-integer=@var{n}},
968 @option{-finit-real=@var{<zero|inf|-inf|nan>}} (which also initializes
969 the real and imaginary parts of local @code{COMPLEX} variables),
970 @option{-finit-logical=@var{<true|false>}}, and
971 @option{-finit-character=@var{n}} (where @var{n} is an ASCII character
972 value) options. These options do not initialize components of derived
973 type variables, nor do they initialize variables that appear in an
974 @code{EQUIVALENCE} statement. (This limitation may be removed in
977 Note that the @option{-finit-real=nan} option initializes @code{REAL}
978 and @code{COMPLEX} variables with a quiet NaN.
981 @xref{Code Gen Options,,Options for Code Generation Conventions,
982 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
984 shared by @command{gfortran}, @command{gcc}, and other GNU compilers.
989 @node Environment Variables
990 @section Environment variables affecting @command{gfortran}
991 @cindex environment variable
993 @c man begin ENVIRONMENT
995 The @command{gfortran} compiler currently does not make use of any environment
996 variables to control its operation above and beyond those
997 that affect the operation of @command{gcc}.
999 @xref{Environment Variables,,Environment Variables Affecting GCC,
1000 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
1003 @xref{Runtime}, for environment variables that affect the
1004 run-time behavior of programs compiled with GNU Fortran.