1 @c Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
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, 2008, 2009, 2010, 2011
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 1.3 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``Funding Free Software'', the Front-Cover
15 Texts being (a) (see below), and with the Back-Cover Texts being (b)
16 (see below). A copy of the license is included in the gfdl(7) man page.
18 (a) The FSF's Front-Cover Text is:
22 (b) The FSF's Back-Cover Text is:
24 You have freedom to copy and modify this GNU Manual, like GNU
25 software. Copies published by the Free Software Foundation raise
26 funds for GNU development.
28 @c Set file name and title for the man page.
30 @settitle GNU Fortran compiler.
32 gfortran [@option{-c}|@option{-S}|@option{-E}]
33 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
34 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
35 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
36 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
37 [@option{-f}@var{option}@dots{}]
38 [@option{-m}@var{machine-option}@dots{}]
39 [@option{-o} @var{outfile}] @var{infile}@dots{}
41 Only the most useful options are listed here; see below for the
45 gpl(7), gfdl(7), fsf-funding(7),
46 cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{gfortran}, @file{as},
48 @file{ld}, @file{binutils} and @file{gdb}.
51 For instructions on reporting bugs, see
55 See the Info entry for @command{gfortran} for contributors to GCC and
60 @node Invoking GNU Fortran
61 @chapter GNU Fortran Command Options
62 @cindex GNU Fortran command options
63 @cindex command options
64 @cindex options, @command{gfortran} command
66 @c man begin DESCRIPTION
68 The @command{gfortran} command supports all the options supported by the
69 @command{gcc} command. Only options specific to GNU Fortran are documented here.
71 @xref{Invoking GCC,,GCC Command Options,gcc,Using the GNU Compiler
72 Collection (GCC)}, for information
73 on the non-Fortran-specific aspects of the @command{gcc} command (and,
74 therefore, the @command{gfortran} command).
76 @cindex options, negative forms
77 All GCC and GNU Fortran options
78 are accepted both by @command{gfortran} and by @command{gcc}
79 (as well as any other drivers built at the same time,
80 such as @command{g++}),
81 since adding GNU Fortran to the GCC distribution
82 enables acceptance of GNU Fortran options
83 by all of the relevant drivers.
85 In some cases, options have positive and negative forms;
86 the negative form of @option{-ffoo} would be @option{-fno-foo}.
87 This manual documents only one of these two forms, whichever
88 one is not the default.
92 * Option Summary:: Brief list of all @command{gfortran} options,
94 * Fortran Dialect Options:: Controlling the variant of Fortran language
96 * Preprocessing Options:: Enable and customize preprocessing.
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 Preprocessing Options
127 @xref{Preprocessing Options,,Enable and customize preprocessing}.
128 @gccoptlist{-cpp -dD -dI -dM -dN -dU -fworking-directory @gol
129 -imultilib @var{dir} -iprefix @var{file} -isysroot @var{dir} @gol
130 -iquote -isystem @var{dir} -nocpp -nostdinc -undef @gol
131 -A@var{question}=@var{answer} -A-@var{question}@r{[}=@var{answer}@r{]} @gol
132 -C -CC -D@var{macro}@r{[}=@var{defn}@r{]} -U@var{macro} -H -P}
134 @item Error and Warning Options
135 @xref{Error and Warning Options,,Options to request or suppress errors
137 @gccoptlist{-fmax-errors=@var{n}
138 -fsyntax-only -pedantic -pedantic-errors -Wall @gol
139 -Waliasing -Wampersand -Warray-bounds -Wcharacter-truncation @gol
140 -Wconversion -Wimplicit-interface -Wimplicit-procedure -Wline-truncation @gol
141 -Wintrinsics-std -Wreal-q-constant -Wsurprising -Wno-tabs -Wunderflow @gol
142 -Wunused-parameter -Wintrinsic-shadow -Wno-align-commons @gol
143 -Wfunction-elimination}
145 @item Debugging Options
146 @xref{Debugging Options,,Options for debugging your program or GNU Fortran}.
147 @gccoptlist{-fdump-fortran-original -fdump-fortran-optimized @gol
148 -ffpe-trap=@var{list} -fdump-core -fbacktrace -fdump-parse-tree}
150 @item Directory Options
151 @xref{Directory Options,,Options for directory search}.
152 @gccoptlist{-I@var{dir} -J@var{dir} -fintrinsic-modules-path @var{dir}}
155 @xref{Link Options,,Options for influencing the linking step}.
156 @gccoptlist{-static-libgfortran}
158 @item Runtime Options
159 @xref{Runtime Options,,Options for influencing runtime behavior}.
160 @gccoptlist{-fconvert=@var{conversion} -fno-range-check
161 -frecord-marker=@var{length} @gol -fmax-subrecord-length=@var{length}
164 @item Code Generation Options
165 @xref{Code Gen Options,,Options for code generation conventions}.
166 @gccoptlist{-fno-automatic -ff2c -fno-underscoring @gol
167 -fwhole-file -fsecond-underscore @gol
168 -fbounds-check -fcheck-array-temporaries -fmax-array-constructor =@var{n} @gol
169 -fcheck=@var{<all|array-temps|bounds|do|mem|pointer|recursion>} @gol
170 -fcoarray=@var{<none|single|lib>} -fmax-stack-var-size=@var{n} @gol
172 -fpack-derived -frepack-arrays -fshort-enums -fexternal-blas @gol
173 -fblas-matmul-limit=@var{n} -frecursive -finit-local-zero @gol
174 -finit-integer=@var{n} -finit-real=@var{<zero|inf|-inf|nan|snan>} @gol
175 -finit-logical=@var{<true|false>} -finit-character=@var{n} @gol
176 -fno-align-commons -fno-protect-parens -frealloc-lhs @gol
177 -faggressive-function-elimination -ffrontend-optimize}
181 * Fortran Dialect Options:: Controlling the variant of Fortran language
183 * Preprocessing Options:: Enable and customize preprocessing.
184 * Error and Warning Options:: How picky should the compiler be?
185 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
186 * Directory Options:: Where to find module files
187 * Link Options :: Influencing the linking step
188 * Runtime Options:: Influencing runtime behavior
189 * Code Gen Options:: Specifying conventions for function calls, data layout
193 @node Fortran Dialect Options
194 @section Options controlling Fortran dialect
195 @cindex dialect options
196 @cindex language, dialect options
197 @cindex options, dialect
199 The following options control the details of the Fortran dialect
200 accepted by the compiler:
205 @opindex @code{ffree-form}
206 @opindex @code{fno-fixed-form}
207 @cindex options, fortran dialect
208 @cindex file format, free
209 @cindex file format, fixed
210 Specify the layout used by the source file. The free form layout
211 was introduced in Fortran 90. Fixed form was traditionally used in
212 older Fortran programs. When neither option is specified, the source
213 form is determined by the file extension.
215 @item -fall-intrinsics
216 @opindex @code{fall-intrinsics}
217 This option causes all intrinsic procedures (including the GNU-specific
218 extensions) to be accepted. This can be useful with @option{-std=f95} to
219 force standard-compliance but get access to the full range of intrinsics
220 available with @command{gfortran}. As a consequence, @option{-Wintrinsics-std}
221 will be ignored and no user-defined procedure with the same name as any
222 intrinsic will be called except when it is explicitly declared @code{EXTERNAL}.
224 @item -fd-lines-as-code
225 @itemx -fd-lines-as-comments
226 @opindex @code{fd-lines-as-code}
227 @opindex @code{fd-lines-as-comments}
228 Enable special treatment for lines beginning with @code{d} or @code{D}
229 in fixed form sources. If the @option{-fd-lines-as-code} option is
230 given they are treated as if the first column contained a blank. If the
231 @option{-fd-lines-as-comments} option is given, they are treated as
234 @item -fdefault-double-8
235 @opindex @code{fdefault-double-8}
236 Set the @code{DOUBLE PRECISION} type to an 8 byte wide type. If
237 @option{-fdefault-real-8} is given, @code{DOUBLE PRECISION} would
238 instead be promoted to 16 bytes if possible, and @option{-fdefault-double-8}
239 can be used to prevent this. The kind of real constants like @code{1.d0} will
240 not be changed by @option{-fdefault-real-8} though, so also
241 @option{-fdefault-double-8} does not affect it.
243 @item -fdefault-integer-8
244 @opindex @code{fdefault-integer-8}
245 Set the default integer and logical types to an 8 byte wide type.
246 Do nothing if this is already the default. This option also affects
247 the kind of integer constants like @code{42}.
249 @item -fdefault-real-8
250 @opindex @code{fdefault-real-8}
251 Set the default real type to an 8 byte wide type.
252 Do nothing if this is already the default. This option also affects
253 the kind of non-double real constants like @code{1.0}, and does promote
254 the default width of @code{DOUBLE PRECISION} to 16 bytes if possible, unless
255 @code{-fdefault-double-8} is given, too.
258 @opindex @code{fdollar-ok}
261 @cindex character set
262 Allow @samp{$} as a valid non-first character in a symbol name. Symbols
263 that start with @samp{$} are rejected since it is unclear which rules to
264 apply to implicit typing as different vendors implement different rules.
265 Using @samp{$} in @code{IMPLICIT} statements is also rejected.
268 @opindex @code{backslash}
270 @cindex escape characters
271 Change the interpretation of backslashes in string literals from a single
272 backslash character to ``C-style'' escape characters. The following
273 combinations are expanded @code{\a}, @code{\b}, @code{\f}, @code{\n},
274 @code{\r}, @code{\t}, @code{\v}, @code{\\}, and @code{\0} to the ASCII
275 characters alert, backspace, form feed, newline, carriage return,
276 horizontal tab, vertical tab, backslash, and NUL, respectively.
277 Additionally, @code{\x}@var{nn}, @code{\u}@var{nnnn} and
278 @code{\U}@var{nnnnnnnn} (where each @var{n} is a hexadecimal digit) are
279 translated into the Unicode characters corresponding to the specified code
280 points. All other combinations of a character preceded by \ are
283 @item -fmodule-private
284 @opindex @code{fmodule-private}
285 @cindex module entities
287 Set the default accessibility of module entities to @code{PRIVATE}.
288 Use-associated entities will not be accessible unless they are explicitly
289 declared as @code{PUBLIC}.
291 @item -ffixed-line-length-@var{n}
292 @opindex @code{ffixed-line-length-}@var{n}
293 @cindex file format, fixed
294 Set column after which characters are ignored in typical fixed-form
295 lines in the source file, and through which spaces are assumed (as
296 if padded to that length) after the ends of short fixed-form lines.
298 Popular values for @var{n} include 72 (the
299 standard and the default), 80 (card image), and 132 (corresponding
300 to ``extended-source'' options in some popular compilers).
301 @var{n} may also be @samp{none}, meaning that the entire line is meaningful
302 and that continued character constants never have implicit spaces appended
303 to them to fill out the line.
304 @option{-ffixed-line-length-0} means the same thing as
305 @option{-ffixed-line-length-none}.
307 @item -ffree-line-length-@var{n}
308 @opindex @code{ffree-line-length-}@var{n}
309 @cindex file format, free
310 Set column after which characters are ignored in typical free-form
311 lines in the source file. The default value is 132.
312 @var{n} may be @samp{none}, meaning that the entire line is meaningful.
313 @option{-ffree-line-length-0} means the same thing as
314 @option{-ffree-line-length-none}.
316 @item -fmax-identifier-length=@var{n}
317 @opindex @code{fmax-identifier-length=}@var{n}
318 Specify the maximum allowed identifier length. Typical values are
319 31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).
321 @item -fimplicit-none
322 @opindex @code{fimplicit-none}
323 Specify that no implicit typing is allowed, unless overridden by explicit
324 @code{IMPLICIT} statements. This is the equivalent of adding
325 @code{implicit none} to the start of every procedure.
328 @opindex @code{fcray-pointer}
329 Enable the Cray pointer extension, which provides C-like pointer
333 @opindex @code{fopenmp}
335 Enable the OpenMP extensions. This includes OpenMP @code{!$omp} directives
337 and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,
338 @code{!$} conditional compilation sentinels in free form
339 and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form,
340 and when linking arranges for the OpenMP runtime library to be linked
341 in. The option @option{-fopenmp} implies @option{-frecursive}.
343 @item -fno-range-check
344 @opindex @code{frange-check}
345 Disable range checking on results of simplification of constant
346 expressions during compilation. For example, GNU Fortran will give
347 an error at compile time when simplifying @code{a = 1. / 0}.
348 With this option, no error will be given and @code{a} will be assigned
349 the value @code{+Infinity}. If an expression evaluates to a value
350 outside of the relevant range of [@code{-HUGE()}:@code{HUGE()}],
351 then the expression will be replaced by @code{-Inf} or @code{+Inf}
353 Similarly, @code{DATA i/Z'FFFFFFFF'/} will result in an integer overflow
354 on most systems, but with @option{-fno-range-check} the value will
355 ``wrap around'' and @code{i} will be initialized to @math{-1} instead.
358 @opindex @code{std=}@var{std} option
359 Specify the standard to which the program is expected to conform, which
360 may be one of @samp{f95}, @samp{f2003}, @samp{f2008}, @samp{gnu}, or
361 @samp{legacy}. The default value for @var{std} is @samp{gnu}, which
362 specifies a superset of the Fortran 95 standard that includes all of the
363 extensions supported by GNU Fortran, although warnings will be given for
364 obsolete extensions not recommended for use in new code. The
365 @samp{legacy} value is equivalent but without the warnings for obsolete
366 extensions, and may be useful for old non-standard programs. The
367 @samp{f95}, @samp{f2003} and @samp{f2008} values specify strict
368 conformance to the Fortran 95, Fortran 2003 and Fortran 2008 standards,
369 respectively; errors are given for all extensions beyond the relevant
370 language standard, and warnings are given for the Fortran 77 features
371 that are permitted but obsolescent in later standards. @samp{-std=f2008tr}
372 allows the Fortran 2008 standard including the additions of the
373 technical report (TR) 29113.
377 @node Preprocessing Options
378 @section Enable and customize preprocessing
380 @cindex options, preprocessor
383 Preprocessor related options. See section
384 @ref{Preprocessing and conditional compilation} for more detailed
385 information on preprocessing in @command{gfortran}.
392 @cindex preprocessor, enable
393 @cindex preprocessor, disable
394 Enable preprocessing. The preprocessor is automatically invoked if
395 the file extension is @file{.fpp}, @file{.FPP}, @file{.F}, @file{.FOR},
396 @file{.FTN}, @file{.F90}, @file{.F95}, @file{.F03} or @file{.F08}. Use
397 this option to manually enable preprocessing of any kind of Fortran file.
399 To disable preprocessing of files with any of the above listed extensions,
400 use the negative form: @option{-nocpp}.
402 The preprocessor is run in traditional mode. Any restrictions of the
403 file-format, especially the limits on line length, apply for
404 preprocessed output as well, so it might be advisable to use the
405 @option{-ffree-line-length-none} or @option{-ffixed-line-length-none}
410 @cindex preprocessor, debugging
411 @cindex debugging, preprocessor
412 Instead of the normal output, generate a list of @code{'#define'}
413 directives for all the macros defined during the execution of the
414 preprocessor, including predefined macros. This gives you a way
415 of finding out what is predefined in your version of the preprocessor.
416 Assuming you have no file @file{foo.f90}, the command
418 touch foo.f90; gfortran -cpp -E -dM foo.f90
420 will show all the predefined macros.
424 @cindex preprocessor, debugging
425 @cindex debugging, preprocessor
426 Like @option{-dM} except in two respects: it does not include the
427 predefined macros, and it outputs both the @code{#define} directives
428 and the result of preprocessing. Both kinds of output go to the
429 standard output file.
433 @cindex preprocessor, debugging
434 @cindex debugging, preprocessor
435 Like @option{-dD}, but emit only the macro names, not their expansions.
439 @cindex preprocessor, debugging
440 @cindex debugging, preprocessor
441 Like @option{dD} except that only macros that are expanded, or whose
442 definedness is tested in preprocessor directives, are output; the
443 output is delayed until the use or test of the macro; and @code{'#undef'}
444 directives are also output for macros tested but undefined at the time.
448 @cindex preprocessor, debugging
449 @cindex debugging, preprocessor
450 Output @code{'#include'} directives in addition to the result
453 @item -fworking-directory
454 @opindex @code{fworking-directory}
455 @cindex preprocessor, working directory
456 Enable generation of linemarkers in the preprocessor output that will
457 let the compiler know the current working directory at the time of
458 preprocessing. When this option is enabled, the preprocessor will emit,
459 after the initial linemarker, a second linemarker with the current
460 working directory followed by two slashes. GCC will use this directory,
461 when it's present in the preprocessed input, as the directory emitted
462 as the current working directory in some debugging information formats.
463 This option is implicitly enabled if debugging information is enabled,
464 but this can be inhibited with the negated form
465 @option{-fno-working-directory}. If the @option{-P} flag is present
466 in the command line, this option has no effect, since no @code{#line}
467 directives are emitted whatsoever.
469 @item -idirafter @var{dir}
470 @opindex @code{idirafter @var{dir}}
471 @cindex preprocessing, include path
472 Search @var{dir} for include files, but do it after all directories
473 specified with @option{-I} and the standard system directories have
474 been exhausted. @var{dir} is treated as a system include directory.
475 If dir begins with @code{=}, then the @code{=} will be replaced by
476 the sysroot prefix; see @option{--sysroot} and @option{-isysroot}.
478 @item -imultilib @var{dir}
479 @opindex @code{imultilib @var{dir}}
480 @cindex preprocessing, include path
481 Use @var{dir} as a subdirectory of the directory containing target-specific
484 @item -iprefix @var{prefix}
485 @opindex @code{iprefix @var{prefix}}
486 @cindex preprocessing, include path
487 Specify @var{prefix} as the prefix for subsequent @option{-iwithprefix}
488 options. If the @var{prefix} represents a directory, you should include
489 the final @code{'/'}.
491 @item -isysroot @var{dir}
492 @opindex @code{isysroot @var{dir}}
493 @cindex preprocessing, include path
494 This option is like the @option{--sysroot} option, but applies only to
495 header files. See the @option{--sysroot} option for more information.
497 @item -iquote @var{dir}
498 @opindex @code{iquote @var{dir}}
499 @cindex preprocessing, include path
500 Search @var{dir} only for header files requested with @code{#include "file"};
501 they are not searched for @code{#include <file>}, before all directories
502 specified by @option{-I} and before the standard system directories. If
503 @var{dir} begins with @code{=}, then the @code{=} will be replaced by the
504 sysroot prefix; see @option{--sysroot} and @option{-isysroot}.
506 @item -isystem @var{dir}
507 @opindex @code{isystem @var{dir}}
508 @cindex preprocessing, include path
509 Search @var{dir} for header files, after all directories specified by
510 @option{-I} but before the standard system directories. Mark it as a
511 system directory, so that it gets the same special treatment as is
512 applied to the standard system directories. If @var{dir} begins with
513 @code{=}, then the @code{=} will be replaced by the sysroot prefix;
514 see @option{--sysroot} and @option{-isysroot}.
517 @opindex @code{nostdinc}
518 Do not search the standard system directories for header files. Only
519 the directories you have specified with @option{-I} options (and the
520 directory of the current file, if appropriate) are searched.
523 @opindex @code{undef}
524 Do not predefine any system-specific or GCC-specific macros.
525 The standard predefined macros remain defined.
527 @item -A@var{predicate}=@var{answer}
528 @opindex @code{A@var{predicate}=@var{answer}}
529 @cindex preprocessing, assertion
530 Make an assertion with the predicate @var{predicate} and answer @var{answer}.
531 This form is preferred to the older form -A predicate(answer), which is still
532 supported, because it does not use shell special characters.
534 @item -A-@var{predicate}=@var{answer}
535 @opindex @code{A-@var{predicate}=@var{answer}}
536 @cindex preprocessing, assertion
537 Cancel an assertion with the predicate @var{predicate} and answer @var{answer}.
541 @cindex preprocessing, keep comments
542 Do not discard comments. All comments are passed through to the output
543 file, except for comments in processed directives, which are deleted
544 along with the directive.
546 You should be prepared for side effects when using @option{-C}; it causes
547 the preprocessor to treat comments as tokens in their own right. For example,
548 comments appearing at the start of what would be a directive line have the
549 effect of turning that line into an ordinary source line, since the first
550 token on the line is no longer a @code{'#'}.
552 Warning: this currently handles C-Style comments only. The preprocessor
553 does not yet recognize Fortran-style comments.
557 @cindex preprocessing, keep comments
558 Do not discard comments, including during macro expansion. This is like
559 @option{-C}, except that comments contained within macros are also passed
560 through to the output file where the macro is expanded.
562 In addition to the side-effects of the @option{-C} option, the @option{-CC}
563 option causes all C++-style comments inside a macro to be converted to C-style
564 comments. This is to prevent later use of that macro from inadvertently
565 commenting out the remainder of the source line. The @option{-CC} option
566 is generally used to support lint comments.
568 Warning: this currently handles C- and C++-Style comments only. The
569 preprocessor does not yet recognize Fortran-style comments.
572 @opindex @code{D@var{name}}
573 @cindex preprocessing, define macros
574 Predefine name as a macro, with definition @code{1}.
576 @item -D@var{name}=@var{definition}
577 @opindex @code{D@var{name}=@var{definition}}
578 @cindex preprocessing, define macros
579 The contents of @var{definition} are tokenized and processed as if they
580 appeared during translation phase three in a @code{'#define'} directive.
581 In particular, the definition will be truncated by embedded newline
584 If you are invoking the preprocessor from a shell or shell-like program
585 you may need to use the shell's quoting syntax to protect characters such
586 as spaces that have a meaning in the shell syntax.
588 If you wish to define a function-like macro on the command line, write
589 its argument list with surrounding parentheses before the equals sign
590 (if any). Parentheses are meaningful to most shells, so you will need
591 to quote the option. With sh and csh, @code{-D'name(args...)=definition'}
594 @option{-D} and @option{-U} options are processed in the order they are
595 given on the command line. All -imacros file and -include file options
596 are processed after all -D and -U options.
600 Print the name of each header file used, in addition to other normal
601 activities. Each name is indented to show how deep in the @code{'#include'}
606 @cindex preprocessing, no linemarkers
607 Inhibit generation of linemarkers in the output from the preprocessor.
608 This might be useful when running the preprocessor on something that
609 is not C code, and will be sent to a program which might be confused
613 @opindex @code{U@var{name}}
614 @cindex preprocessing, undefine macros
615 Cancel any previous definition of @var{name}, either built in or provided
616 with a @option{-D} option.
620 @node Error and Warning Options
621 @section Options to request or suppress errors and warnings
622 @cindex options, warnings
623 @cindex options, errors
624 @cindex warnings, suppressing
625 @cindex messages, error
626 @cindex messages, warning
627 @cindex suppressing warnings
629 Errors are diagnostic messages that report that the GNU Fortran compiler
630 cannot compile the relevant piece of source code. The compiler will
631 continue to process the program in an attempt to report further errors
632 to aid in debugging, but will not produce any compiled output.
634 Warnings are diagnostic messages that report constructions which
635 are not inherently erroneous but which are risky or suggest there is
636 likely to be a bug in the program. Unless @option{-Werror} is specified,
637 they do not prevent compilation of the program.
639 You can request many specific warnings with options beginning @option{-W},
640 for example @option{-Wimplicit} to request warnings on implicit
641 declarations. Each of these specific warning options also has a
642 negative form beginning @option{-Wno-} to turn off warnings;
643 for example, @option{-Wno-implicit}. This manual lists only one of the
644 two forms, whichever is not the default.
646 These options control the amount and kinds of errors and warnings produced
650 @item -fmax-errors=@var{n}
651 @opindex @code{fmax-errors=}@var{n}
652 @cindex errors, limiting
653 Limits the maximum number of error messages to @var{n}, at which point
654 GNU Fortran bails out rather than attempting to continue processing the
655 source code. If @var{n} is 0, there is no limit on the number of error
659 @opindex @code{fsyntax-only}
660 @cindex syntax checking
661 Check the code for syntax errors, but don't actually compile it. This
662 will generate module files for each module present in the code, but no
666 @opindex @code{pedantic}
667 Issue warnings for uses of extensions to Fortran 95.
668 @option{-pedantic} also applies to C-language constructs where they
669 occur in GNU Fortran source files, such as use of @samp{\e} in a
670 character constant within a directive like @code{#include}.
672 Valid Fortran 95 programs should compile properly with or without
674 However, without this option, certain GNU extensions and traditional
675 Fortran features are supported as well.
676 With this option, many of them are rejected.
678 Some users try to use @option{-pedantic} to check programs for conformance.
679 They soon find that it does not do quite what they want---it finds some
680 nonstandard practices, but not all.
681 However, improvements to GNU Fortran in this area are welcome.
683 This should be used in conjunction with @option{-std=f95},
684 @option{-std=f2003} or @option{-std=f2008}.
686 @item -pedantic-errors
687 @opindex @code{pedantic-errors}
688 Like @option{-pedantic}, except that errors are produced rather than
694 @cindex warnings, all
695 Enables commonly used warning options pertaining to usage that
696 we recommend avoiding and that we believe are easy to avoid.
697 This currently includes @option{-Waliasing}, @option{-Wampersand},
698 @option{-Wconversion}, @option{-Wsurprising}, @option{-Wintrinsics-std},
699 @option{-Wno-tabs}, @option{-Wintrinsic-shadow}, @option{-Wline-truncation},
700 @option{-Wreal-q-constant} and @option{-Wunused}.
703 @opindex @code{Waliasing}
705 @cindex warnings, aliasing
706 Warn about possible aliasing of dummy arguments. Specifically, it warns
707 if the same actual argument is associated with a dummy argument with
708 @code{INTENT(IN)} and a dummy argument with @code{INTENT(OUT)} in a call
709 with an explicit interface.
711 The following example will trigger the warning.
715 integer, intent(in) :: a
716 integer, intent(out) :: b
725 @opindex @code{Wampersand}
726 @cindex warnings, ampersand
728 Warn about missing ampersand in continued character constants. The warning is
729 given with @option{-Wampersand}, @option{-pedantic}, @option{-std=f95},
730 @option{-std=f2003} and @option{-std=f2008}. Note: With no ampersand
731 given in a continued character constant, GNU Fortran assumes continuation
732 at the first non-comment, non-whitespace character after the ampersand
733 that initiated the continuation.
735 @item -Warray-temporaries
736 @opindex @code{Warray-temporaries}
737 @cindex warnings, array temporaries
738 Warn about array temporaries generated by the compiler. The information
739 generated by this warning is sometimes useful in optimization, in order to
740 avoid such temporaries.
742 @item -Wcharacter-truncation
743 @opindex @code{Wcharacter-truncation}
744 @cindex warnings, character truncation
745 Warn when a character assignment will truncate the assigned string.
747 @item -Wline-truncation
748 @opindex @code{Wline-truncation}
749 @cindex warnings, line truncation
750 Warn when a source code line will be truncated.
753 @opindex @code{Wconversion}
754 @cindex warnings, conversion
756 Warn about implicit conversions that are likely to change the value of
757 the expression after conversion. Implied by @option{-Wall}.
759 @item -Wconversion-extra
760 @opindex @code{Wconversion-extra}
761 @cindex warnings, conversion
763 Warn about implicit conversions between different types and kinds.
765 @item -Wimplicit-interface
766 @opindex @code{Wimplicit-interface}
767 @cindex warnings, implicit interface
768 Warn if a procedure is called without an explicit interface.
769 Note this only checks that an explicit interface is present. It does not
770 check that the declared interfaces are consistent across program units.
772 @item -Wimplicit-procedure
773 @opindex @code{Wimplicit-procedure}
774 @cindex warnings, implicit procedure
775 Warn if a procedure is called that has neither an explicit interface
776 nor has been declared as @code{EXTERNAL}.
778 @item -Wintrinsics-std
779 @opindex @code{Wintrinsics-std}
780 @cindex warnings, non-standard intrinsics
781 @cindex warnings, intrinsics of other standards
782 Warn if @command{gfortran} finds a procedure named like an intrinsic not
783 available in the currently selected standard (with @option{-std}) and treats
784 it as @code{EXTERNAL} procedure because of this. @option{-fall-intrinsics} can
785 be used to never trigger this behavior and always link to the intrinsic
786 regardless of the selected standard.
788 @item -Wreal-q-constant
789 @opindex @code{Wreal-q-constant}
790 @cindex warnings, @code{q} exponent-letter
791 Produce a warning if a real-literal-constant contains a @code{q}
795 @opindex @code{Wsurprising}
796 @cindex warnings, suspicious code
797 Produce a warning when ``suspicious'' code constructs are encountered.
798 While technically legal these usually indicate that an error has been made.
800 This currently produces a warning under the following circumstances:
804 An INTEGER SELECT construct has a CASE that can never be matched as its
805 lower value is greater than its upper value.
808 A LOGICAL SELECT construct has three CASE statements.
811 A TRANSFER specifies a source that is shorter than the destination.
814 The type of a function result is declared more than once with the same type. If
815 @option{-pedantic} or standard-conforming mode is enabled, this is an error.
818 A @code{CHARACTER} variable is declared with negative length.
822 @opindex @code{Wtabs}
823 @cindex warnings, tabs
825 By default, tabs are accepted as whitespace, but tabs are not members
826 of the Fortran Character Set. For continuation lines, a tab followed
827 by a digit between 1 and 9 is supported. @option{-Wno-tabs} will cause
828 a warning to be issued if a tab is encountered. Note, @option{-Wno-tabs}
829 is active for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003},
830 @option{-std=f2008} and @option{-Wall}.
833 @opindex @code{Wunderflow}
834 @cindex warnings, underflow
836 Produce a warning when numerical constant expressions are
837 encountered, which yield an UNDERFLOW during compilation.
839 @item -Wintrinsic-shadow
840 @opindex @code{Wintrinsic-shadow}
841 @cindex warnings, intrinsic
843 Warn if a user-defined procedure or module procedure has the same name as an
844 intrinsic; in this case, an explicit interface or @code{EXTERNAL} or
845 @code{INTRINSIC} declaration might be needed to get calls later resolved to
846 the desired intrinsic/procedure.
848 @item -Wunused-dummy-argument
849 @opindex @code{Wunused-dummy-argument}
850 @cindex warnings, unused dummy argument
851 @cindex unused dummy argument
852 @cindex dummy argument, unused
853 Warn about unused dummy arguments. This option is implied by @option{-Wall}.
855 @item -Wunused-parameter
856 @opindex @code{Wunused-parameter}
857 @cindex warnings, unused parameter
858 @cindex unused parameter
859 Contrary to @command{gcc}'s meaning of @option{-Wunused-parameter},
860 @command{gfortran}'s implementation of this option does not warn
861 about unused dummy arguments (see @option{-Wunused-dummy-argument}),
862 but about unused @code{PARAMETER} values. @option{-Wunused-parameter}
863 is not included in @option{-Wall} but is implied by @option{-Wall -Wextra}.
865 @item -Walign-commons
866 @opindex @code{Walign-commons}
867 @cindex warnings, alignment of @code{COMMON} blocks
868 @cindex alignment of @code{COMMON} blocks
869 By default, @command{gfortran} warns about any occasion of variables being
870 padded for proper alignment inside a @code{COMMON} block. This warning can be turned
871 off via @option{-Wno-align-commons}. See also @option{-falign-commons}.
873 @item -Wfunction-elimination
874 @opindex @code{Wfunction-elimination}
875 @cindex function elimination
876 @cindex warnings, function elimination
877 Warn if any calls to functions are eliminated by the optimizations
878 enabled by the @option{-ffrontend-optimize} option.
882 @opindex @code{Werror}
883 @cindex warnings, to errors
884 Turns all warnings into errors.
887 @xref{Warning Options,,Options to Request or Suppress Errors and
888 Warnings, gcc,Using the GNU Compiler Collection (GCC)}, for information on
889 more options offered by the GBE shared by @command{gfortran}, @command{gcc}
890 and other GNU compilers.
892 Some of these have no effect when compiling programs written in Fortran.
894 @node Debugging Options
895 @section Options for debugging your program or GNU Fortran
896 @cindex options, debugging
897 @cindex debugging information options
899 GNU Fortran has various special options that are used for debugging
900 either your program or the GNU Fortran compiler.
903 @item -fdump-fortran-original
904 @opindex @code{fdump-fortran-original}
905 Output the internal parse tree after translating the source program
906 into internal representation. Only really useful for debugging the
907 GNU Fortran compiler itself.
909 @item -fdump-optimized-tree
910 @opindex @code{fdump-fortran-optimized}
911 Output the parse tree after front-end optimization. Only really
912 useful for debugging the GNU Fortran compiler itself.
914 @opindex @code{fdump-parse-tree}
915 Output the internal parse tree after translating the source program
916 into internal representation. Only really useful for debugging the
917 GNU Fortran compiler itself. This option is deprecated; use
918 @code{-fdump-fortran-original} instead.
920 @item -ffpe-trap=@var{list}
921 @opindex @code{ffpe-trap=}@var{list}
922 Specify a list of IEEE exceptions when a Floating Point Exception
923 (FPE) should be raised. On most systems, this will result in a SIGFPE
924 signal being sent and the program being interrupted, producing a core
925 file useful for debugging. @var{list} is a (possibly empty) comma-separated
926 list of the following IEEE exceptions: @samp{invalid} (invalid floating
927 point operation, such as @code{SQRT(-1.0)}), @samp{zero} (division by
928 zero), @samp{overflow} (overflow in a floating point operation),
929 @samp{underflow} (underflow in a floating point operation),
930 @samp{precision} (loss of precision during operation) and @samp{denormal}
931 (operation produced a denormal value).
933 Some of the routines in the Fortran runtime library, like
934 @samp{CPU_TIME}, are likely to trigger floating point exceptions when
935 @code{ffpe-trap=precision} is used. For this reason, the use of
936 @code{ffpe-trap=precision} is not recommended.
939 @opindex @code{fbacktrace}
942 Specify that, when a runtime error is encountered or a deadly signal is
943 emitted (segmentation fault, illegal instruction, bus error or
944 floating-point exception), the Fortran runtime
945 library should output a backtrace of the error. This option
946 only has influence for compilation of the Fortran main program.
950 @opindex @code{fdump-core}
951 Request that a core-dump file is written to disk when a runtime error
952 is encountered on systems that support core dumps. This option is
953 only effective for the compilation of the Fortran main program.
956 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
957 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
960 @node Directory Options
961 @section Options for directory search
962 @cindex directory, options
963 @cindex options, directory search
965 @cindex @code{INCLUDE} directive
966 @cindex directive, @code{INCLUDE}
967 These options affect how GNU Fortran searches
968 for files specified by the @code{INCLUDE} directive and where it searches
969 for previously compiled modules.
971 It also affects the search paths used by @command{cpp} when used to preprocess
976 @opindex @code{I}@var{dir}
977 @cindex directory, search paths for inclusion
978 @cindex inclusion, directory search paths for
979 @cindex search paths, for included files
980 @cindex paths, search
981 @cindex module search path
982 These affect interpretation of the @code{INCLUDE} directive
983 (as well as of the @code{#include} directive of the @command{cpp}
986 Also note that the general behavior of @option{-I} and
987 @code{INCLUDE} is pretty much the same as of @option{-I} with
988 @code{#include} in the @command{cpp} preprocessor, with regard to
989 looking for @file{header.gcc} files and other such things.
991 This path is also used to search for @file{.mod} files when previously
992 compiled modules are required by a @code{USE} statement.
994 @xref{Directory Options,,Options for Directory Search,
995 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
999 @opindex @code{J}@var{dir}
1000 @opindex @code{M}@var{dir}
1001 @cindex paths, search
1002 @cindex module search path
1003 This option specifies where to put @file{.mod} files for compiled modules.
1004 It is also added to the list of directories to searched by an @code{USE}
1007 The default is the current directory.
1009 @item -fintrinsic-modules-path @var{dir}
1010 @opindex @code{fintrinsic-modules-path} @var{dir}
1011 @cindex paths, search
1012 @cindex module search path
1013 This option specifies the location of pre-compiled intrinsic modules, if
1014 they are not in the default location expected by the compiler.
1018 @section Influencing the linking step
1019 @cindex options, linking
1020 @cindex linking, static
1022 These options come into play when the compiler links object files into an
1023 executable output file. They are meaningless if the compiler is not doing
1027 @item -static-libgfortran
1028 @opindex @code{static-libgfortran}
1029 On systems that provide @file{libgfortran} as a shared and a static
1030 library, this option forces the use of the static version. If no
1031 shared version of @file{libgfortran} was built when the compiler was
1032 configured, this option has no effect.
1036 @node Runtime Options
1037 @section Influencing runtime behavior
1038 @cindex options, runtime
1040 These options affect the runtime behavior of programs compiled with GNU Fortran.
1043 @item -fconvert=@var{conversion}
1044 @opindex @code{fconvert=}@var{conversion}
1045 Specify the representation of data for unformatted files. Valid
1046 values for conversion are: @samp{native}, the default; @samp{swap},
1047 swap between big- and little-endian; @samp{big-endian}, use big-endian
1048 representation for unformatted files; @samp{little-endian}, use little-endian
1049 representation for unformatted files.
1051 @emph{This option has an effect only when used in the main program.
1052 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
1053 variable override the default specified by @option{-fconvert}.}
1056 @item -fno-range-check
1057 @opindex @code{fno-range-check}
1058 Disable range checking of input values during integer @code{READ} operations.
1059 For example, GNU Fortran will give an error if an input value is
1060 outside of the relevant range of [@code{-HUGE()}:@code{HUGE()}]. In other words,
1061 with @code{INTEGER (kind=4) :: i} , attempting to read @math{-2147483648} will
1062 give an error unless @option{-fno-range-check} is given.
1065 @item -frecord-marker=@var{length}
1066 @opindex @code{frecord-marker=}@var{length}
1067 Specify the length of record markers for unformatted files.
1068 Valid values for @var{length} are 4 and 8. Default is 4.
1069 @emph{This is different from previous versions of @command{gfortran}},
1070 which specified a default record marker length of 8 on most
1071 systems. If you want to read or write files compatible
1072 with earlier versions of @command{gfortran}, use @option{-frecord-marker=8}.
1074 @item -fmax-subrecord-length=@var{length}
1075 @opindex @code{fmax-subrecord-length=}@var{length}
1076 Specify the maximum length for a subrecord. The maximum permitted
1077 value for length is 2147483639, which is also the default. Only
1078 really useful for use by the gfortran testsuite.
1081 @opindex @code{fsign-zero}
1082 When enabled, floating point numbers of value zero with the sign bit set
1083 are written as negative number in formatted output and treated as
1084 negative in the @code{SIGN} intrinsic. @code{fno-sign-zero} does not
1085 print the negative sign of zero values and regards zero as positive
1086 number in the @code{SIGN} intrinsic for compatibility with F77.
1087 Default behavior is to show the negative sign.
1090 @node Code Gen Options
1091 @section Options for code generation conventions
1092 @cindex code generation, conventions
1093 @cindex options, code generation
1094 @cindex options, run-time
1096 These machine-independent options control the interface conventions
1097 used in code generation.
1099 Most of them have both positive and negative forms; the negative form
1100 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
1101 one of the forms is listed---the one which is not the default. You
1102 can figure out the other form by either removing @option{no-} or adding
1106 @item -fno-automatic
1107 @opindex @code{fno-automatic}
1108 @cindex @code{SAVE} statement
1109 @cindex statement, @code{SAVE}
1110 Treat each program unit (except those marked as RECURSIVE) as if the
1111 @code{SAVE} statement were specified for every local variable and array
1112 referenced in it. Does not affect common blocks. (Some Fortran compilers
1113 provide this option under the name @option{-static} or @option{-save}.)
1114 The default, which is @option{-fautomatic}, uses the stack for local
1115 variables smaller than the value given by @option{-fmax-stack-var-size}.
1116 Use the option @option{-frecursive} to use no static memory.
1120 @cindex calling convention
1121 @cindex @command{f2c} calling convention
1122 @cindex @command{g77} calling convention
1123 @cindex libf2c calling convention
1124 Generate code designed to be compatible with code generated
1125 by @command{g77} and @command{f2c}.
1127 The calling conventions used by @command{g77} (originally implemented
1128 in @command{f2c}) require functions that return type
1129 default @code{REAL} to actually return the C type @code{double}, and
1130 functions that return type @code{COMPLEX} to return the values via an
1131 extra argument in the calling sequence that points to where to
1132 store the return value. Under the default GNU calling conventions, such
1133 functions simply return their results as they would in GNU
1134 C---default @code{REAL} functions return the C type @code{float}, and
1135 @code{COMPLEX} functions return the GNU C type @code{complex}.
1136 Additionally, this option implies the @option{-fsecond-underscore}
1137 option, unless @option{-fno-second-underscore} is explicitly requested.
1139 This does not affect the generation of code that interfaces with
1140 the @command{libgfortran} library.
1142 @emph{Caution:} It is not a good idea to mix Fortran code compiled with
1143 @option{-ff2c} with code compiled with the default @option{-fno-f2c}
1144 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
1145 functions between program parts which were compiled with different
1146 calling conventions will break at execution time.
1148 @emph{Caution:} This will break code which passes intrinsic functions
1149 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
1150 the library implementations use the @option{-fno-f2c} calling conventions.
1152 @item -fno-underscoring
1153 @opindex @code{fno-underscoring}
1155 @cindex symbol names, underscores
1156 @cindex transforming symbol names
1157 @cindex symbol names, transforming
1158 Do not transform names of entities specified in the Fortran
1159 source file by appending underscores to them.
1161 With @option{-funderscoring} in effect, GNU Fortran appends one
1162 underscore to external names with no underscores. This is done to ensure
1163 compatibility with code produced by many UNIX Fortran compilers.
1165 @emph{Caution}: The default behavior of GNU Fortran is
1166 incompatible with @command{f2c} and @command{g77}, please use the
1167 @option{-ff2c} option if you want object files compiled with
1168 GNU Fortran to be compatible with object code created with these
1171 Use of @option{-fno-underscoring} is not recommended unless you are
1172 experimenting with issues such as integration of GNU Fortran into
1173 existing system environments (vis-@`{a}-vis existing libraries, tools,
1176 For example, with @option{-funderscoring}, and assuming other defaults like
1177 @option{-fcase-lower} and that @code{j()} and @code{max_count()} are
1178 external functions while @code{my_var} and @code{lvar} are local variables,
1181 I = J() + MAX_COUNT (MY_VAR, LVAR)
1184 is implemented as something akin to:
1186 i = j_() + max_count__(&my_var__, &lvar);
1189 With @option{-fno-underscoring}, the same statement is implemented as:
1192 i = j() + max_count(&my_var, &lvar);
1195 Use of @option{-fno-underscoring} allows direct specification of
1196 user-defined names while debugging and when interfacing GNU Fortran
1197 code with other languages.
1199 Note that just because the names match does @emph{not} mean that the
1200 interface implemented by GNU Fortran for an external name matches the
1201 interface implemented by some other language for that same name.
1202 That is, getting code produced by GNU Fortran to link to code produced
1203 by some other compiler using this or any other method can be only a
1204 small part of the overall solution---getting the code generated by
1205 both compilers to agree on issues other than naming can require
1206 significant effort, and, unlike naming disagreements, linkers normally
1207 cannot detect disagreements in these other areas.
1209 Also, note that with @option{-fno-underscoring}, the lack of appended
1210 underscores introduces the very real possibility that a user-defined
1211 external name will conflict with a name in a system library, which
1212 could make finding unresolved-reference bugs quite difficult in some
1213 cases---they might occur at program run time, and show up only as
1214 buggy behavior at run time.
1216 In future versions of GNU Fortran we hope to improve naming and linking
1217 issues so that debugging always involves using the names as they appear
1218 in the source, even if the names as seen by the linker are mangled to
1219 prevent accidental linking between procedures with incompatible
1223 @opindex @code{fwhole-file}
1224 By default, GNU Fortran parses, resolves and translates each procedure
1225 in a file separately. Using this option modifies this such that the
1226 whole file is parsed and placed in a single front-end tree. During
1227 resolution, in addition to all the usual checks and fixups, references
1228 to external procedures that are in the same file effect resolution of
1229 that procedure, if not already done, and a check of the interfaces. The
1230 dependences are resolved by changing the order in which the file is
1231 translated into the backend tree. Thus, a procedure that is referenced
1232 is translated before the reference and the duplication of backend tree
1233 declarations eliminated.
1235 @item -fsecond-underscore
1236 @opindex @code{fsecond-underscore}
1238 @cindex symbol names, underscores
1239 @cindex transforming symbol names
1240 @cindex symbol names, transforming
1241 @cindex @command{f2c} calling convention
1242 @cindex @command{g77} calling convention
1243 @cindex libf2c calling convention
1244 By default, GNU Fortran appends an underscore to external
1245 names. If this option is used GNU Fortran appends two
1246 underscores to names with underscores and one underscore to external names
1247 with no underscores. GNU Fortran also appends two underscores to
1248 internal names with underscores to avoid naming collisions with external
1251 This option has no effect if @option{-fno-underscoring} is
1252 in effect. It is implied by the @option{-ff2c} option.
1254 Otherwise, with this option, an external name such as @code{MAX_COUNT}
1255 is implemented as a reference to the link-time external symbol
1256 @code{max_count__}, instead of @code{max_count_}. This is required
1257 for compatibility with @command{g77} and @command{f2c}, and is implied
1258 by use of the @option{-ff2c} option.
1260 @item -fcoarray=@var{<keyword>}
1261 @opindex @code{fcoarray}
1266 Disable coarray support; using coarray declarations and image-control
1267 statements will produce a compile-time error. (Default)
1270 Single-image mode, i.e. @code{num_images()} is always one.
1273 Library-based coarray parallelization; a suitable GNU Fortran coarray
1274 library needs to be linked.
1278 @item -fcheck=@var{<keyword>}
1279 @opindex @code{fcheck}
1280 @cindex array, bounds checking
1281 @cindex bounds checking
1282 @cindex pointer checking
1283 @cindex memory checking
1284 @cindex range checking
1285 @cindex subscript checking
1286 @cindex checking subscripts
1287 @cindex run-time checking
1288 @cindex checking array temporaries
1290 Enable the generation of run-time checks; the argument shall be
1291 a comma-delimited list of the following keywords.
1295 Enable all run-time test of @option{-fcheck}.
1297 @item @samp{array-temps}
1298 Warns at run time when for passing an actual argument a temporary array
1299 had to be generated. The information generated by this warning is
1300 sometimes useful in optimization, in order to avoid such temporaries.
1302 Note: The warning is only printed once per location.
1305 Enable generation of run-time checks for array subscripts
1306 and against the declared minimum and maximum values. It also
1307 checks array indices for assumed and deferred
1308 shape arrays against the actual allocated bounds and ensures that all string
1309 lengths are equal for character array constructors without an explicit
1312 Some checks require that @option{-fcheck=bounds} is set for
1313 the compilation of the main program.
1315 Note: In the future this may also include other forms of checking, e.g.,
1316 checking substring references.
1319 Enable generation of run-time checks for invalid modification of loop
1320 iteration variables.
1323 Enable generation of run-time checks for memory allocation.
1324 Note: This option does not affect explicit allocations using the
1325 @code{ALLOCATE} statement, which will be always checked.
1327 @item @samp{pointer}
1328 Enable generation of run-time checks for pointers and allocatables.
1330 @item @samp{recursion}
1331 Enable generation of run-time checks for recursively called subroutines and
1332 functions which are not marked as recursive. See also @option{-frecursive}.
1333 Note: This check does not work for OpenMP programs and is disabled if used
1334 together with @option{-frecursive} and @option{-fopenmp}.
1338 @item -fbounds-check
1339 @opindex @code{fbounds-check}
1340 @c Note: This option is also referred in gcc's manpage
1341 Deprecated alias for @option{-fcheck=bounds}.
1343 @item -fcheck-array-temporaries
1344 @opindex @code{fcheck-array-temporaries}
1345 Deprecated alias for @option{-fcheck=array-temps}.
1347 @item -fmax-array-constructor=@var{n}
1348 @opindex @code{fmax-array-constructor}
1349 This option can be used to increase the upper limit permitted in
1350 array constructors. The code below requires this option to expand
1351 the array at compile time.
1357 integer, parameter :: n = 100000
1358 integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
1359 print '(10(I0,1X))', i
1363 @emph{Caution: This option can lead to long compile times and excessively
1364 large object files.}
1366 The default value for @var{n} is 65535.
1369 @item -fmax-stack-var-size=@var{n}
1370 @opindex @code{fmax-stack-var-size}
1371 This option specifies the size in bytes of the largest array that will be put
1372 on the stack; if the size is exceeded static memory is used (except in
1373 procedures marked as RECURSIVE). Use the option @option{-frecursive} to
1374 allow for recursive procedures which do not have a RECURSIVE attribute or
1375 for parallel programs. Use @option{-fno-automatic} to never use the stack.
1377 This option currently only affects local arrays declared with constant
1378 bounds, and may not apply to all character variables.
1379 Future versions of GNU Fortran may improve this behavior.
1381 The default value for @var{n} is 32768.
1383 @item -fstack-arrays
1384 @opindex @code{fstack-arrays}
1385 Adding this option will make the fortran compiler put all local arrays,
1386 even those of unknown size onto stack memory. If your program uses very
1387 large local arrays it's possible that you'll have to extend your runtime
1388 limits for stack memory on some operating systems.
1390 @item -fpack-derived
1391 @opindex @code{fpack-derived}
1392 @cindex structure packing
1393 This option tells GNU Fortran to pack derived type members as closely as
1394 possible. Code compiled with this option is likely to be incompatible
1395 with code compiled without this option, and may execute slower.
1397 @item -frepack-arrays
1398 @opindex @code{frepack-arrays}
1399 @cindex repacking arrays
1400 In some circumstances GNU Fortran may pass assumed shape array
1401 sections via a descriptor describing a noncontiguous area of memory.
1402 This option adds code to the function prologue to repack the data into
1403 a contiguous block at runtime.
1405 This should result in faster accesses to the array. However it can introduce
1406 significant overhead to the function call, especially when the passed data
1410 @opindex @code{fshort-enums}
1411 This option is provided for interoperability with C code that was
1412 compiled with the @option{-fshort-enums} option. It will make
1413 GNU Fortran choose the smallest @code{INTEGER} kind a given
1414 enumerator set will fit in, and give all its enumerators this kind.
1416 @item -fexternal-blas
1417 @opindex @code{fexternal-blas}
1418 This option will make @command{gfortran} generate calls to BLAS functions
1419 for some matrix operations like @code{MATMUL}, instead of using our own
1420 algorithms, if the size of the matrices involved is larger than a given
1421 limit (see @option{-fblas-matmul-limit}). This may be profitable if an
1422 optimized vendor BLAS library is available. The BLAS library will have
1423 to be specified at link time.
1425 @item -fblas-matmul-limit=@var{n}
1426 @opindex @code{fblas-matmul-limit}
1427 Only significant when @option{-fexternal-blas} is in effect.
1428 Matrix multiplication of matrices with size larger than (or equal to) @var{n}
1429 will be performed by calls to BLAS functions, while others will be
1430 handled by @command{gfortran} internal algorithms. If the matrices
1431 involved are not square, the size comparison is performed using the
1432 geometric mean of the dimensions of the argument and result matrices.
1434 The default value for @var{n} is 30.
1437 @opindex @code{frecursive}
1438 Allow indirect recursion by forcing all local arrays to be allocated
1439 on the stack. This flag cannot be used together with
1440 @option{-fmax-stack-var-size=} or @option{-fno-automatic}.
1442 @item -finit-local-zero
1443 @itemx -finit-integer=@var{n}
1444 @itemx -finit-real=@var{<zero|inf|-inf|nan|snan>}
1445 @itemx -finit-logical=@var{<true|false>}
1446 @itemx -finit-character=@var{n}
1447 @opindex @code{finit-local-zero}
1448 @opindex @code{finit-integer}
1449 @opindex @code{finit-real}
1450 @opindex @code{finit-logical}
1451 @opindex @code{finit-character}
1452 The @option{-finit-local-zero} option instructs the compiler to
1453 initialize local @code{INTEGER}, @code{REAL}, and @code{COMPLEX}
1454 variables to zero, @code{LOGICAL} variables to false, and
1455 @code{CHARACTER} variables to a string of null bytes. Finer-grained
1456 initialization options are provided by the
1457 @option{-finit-integer=@var{n}},
1458 @option{-finit-real=@var{<zero|inf|-inf|nan|snan>}} (which also initializes
1459 the real and imaginary parts of local @code{COMPLEX} variables),
1460 @option{-finit-logical=@var{<true|false>}}, and
1461 @option{-finit-character=@var{n}} (where @var{n} is an ASCII character
1462 value) options. These options do not initialize components of derived
1463 type variables, nor do they initialize variables that appear in an
1464 @code{EQUIVALENCE} statement. (This limitation may be removed in
1467 Note that the @option{-finit-real=nan} option initializes @code{REAL}
1468 and @code{COMPLEX} variables with a quiet NaN. For a signalling NaN
1469 use @option{-finit-real=snan}; note, however, that compile-time
1470 optimizations may convert them into quiet NaN and that trapping
1471 needs to be enabled (e.g. via @option{-ffpe-trap}).
1473 @item -falign-commons
1474 @opindex @code{falign-commons}
1475 @cindex alignment of @code{COMMON} blocks
1476 By default, @command{gfortran} enforces proper alignment of all variables in a
1477 @code{COMMON} block by padding them as needed. On certain platforms this is mandatory,
1478 on others it increases performance. If a @code{COMMON} block is not declared with
1479 consistent data types everywhere, this padding can cause trouble, and
1480 @option{-fno-align-commons} can be used to disable automatic alignment. The
1481 same form of this option should be used for all files that share a @code{COMMON} block.
1482 To avoid potential alignment issues in @code{COMMON} blocks, it is recommended to order
1483 objects from largest to smallest.
1485 @item -fno-protect-parens
1486 @opindex @code{fno-protect-parens}
1487 @cindex re-association of parenthesized expressions
1488 By default the parentheses in expression are honored for all optimization
1489 levels such that the compiler does not do any re-association. Using
1490 @option{-fno-protect-parens} allows the compiler to reorder @code{REAL} and
1491 @code{COMPLEX} expressions to produce faster code. Note that for the re-association
1492 optimization @option{-fno-signed-zeros} and @option{-fno-trapping-math}
1493 need to be in effect. The parentheses protection is enabled by default, unless
1494 @option{-Ofast} is given.
1497 @opindex @code{frealloc-lhs}
1498 @cindex Reallocate the LHS in assignments
1499 An allocatable left-hand side of an intrinsic assignment is automatically
1500 (re)allocated if it is either unallocated or has a different shape. The
1501 option is enabled by default except when @option{-std=f95} is given.
1503 @item -faggressive-function-elimination
1504 @opindex @code{faggressive-function-elimination}
1505 @cindex Elimination of functions with identical argument lists
1506 Functions with identical argument lists are eliminated within
1507 statements, regardless of whether these functions are marked
1508 @code{PURE} or not. For example, in
1512 there will only be a single call to @code{f}. This option only works
1513 if @option{-ffrontend-optimize} is in effect.
1515 @item -ffrontend-optimize
1516 @opindex @code{frontend-optimize}
1517 @cindex Front-end optimization
1518 This option performs front-end optimization, based on manipulating
1519 parts the Fortran parse tree. Enabled by default by any @option{-O}
1520 option. Optimizations enabled by this option include elimination of
1521 identical function calls within expressions, removing unnecessary
1522 calls to @code{TRIM} in comparisons and assignments and replacing
1523 @code{TRIM(a)} with @code{a(1:LEN_TRIM(a))}.
1524 It can be deselected by specifying @option{-fno-frontend-optimize}.
1527 @xref{Code Gen Options,,Options for Code Generation Conventions,
1528 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
1530 shared by @command{gfortran}, @command{gcc}, and other GNU compilers.
1534 @node Environment Variables
1535 @section Environment variables affecting @command{gfortran}
1536 @cindex environment variable
1538 @c man begin ENVIRONMENT
1540 The @command{gfortran} compiler currently does not make use of any environment
1541 variables to control its operation above and beyond those
1542 that affect the operation of @command{gcc}.
1544 @xref{Environment Variables,,Environment Variables Affecting GCC,
1545 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
1548 @xref{Runtime}, for environment variables that affect the
1549 run-time behavior of programs compiled with GNU Fortran.