1 @c Copyright (C) 2004-2017 Free Software Foundation, Inc.
2 @c This is part of the GNU Fortran manual.
3 @c For copying conditions, see the file gfortran.texi.
7 Copyright @copyright{} 2004-2017 Free Software Foundation, Inc.
9 Permission is granted to copy, distribute and/or modify this document
10 under the terms of the GNU Free Documentation License, Version 1.3 or
11 any later version published by the Free Software Foundation; with the
12 Invariant Sections being ``Funding Free Software'', the Front-Cover
13 Texts being (a) (see below), and with the Back-Cover Texts being (b)
14 (see below). A copy of the license is included in the gfdl(7) man page.
16 (a) The FSF's Front-Cover Text is:
20 (b) The FSF's Back-Cover Text is:
22 You have freedom to copy and modify this GNU Manual, like GNU
23 software. Copies published by the Free Software Foundation raise
24 funds for GNU development.
26 @c Set file name and title for the man page.
28 @settitle GNU Fortran compiler.
30 gfortran [@option{-c}|@option{-S}|@option{-E}]
31 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
32 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
33 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
34 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
35 [@option{-f}@var{option}@dots{}]
36 [@option{-m}@var{machine-option}@dots{}]
37 [@option{-o} @var{outfile}] @var{infile}@dots{}
39 Only the most useful options are listed here; see below for the
43 gpl(7), gfdl(7), fsf-funding(7),
44 cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
45 and the Info entries for @file{gcc}, @file{cpp}, @file{gfortran}, @file{as},
46 @file{ld}, @file{binutils} and @file{gdb}.
49 For instructions on reporting bugs, see
53 See the Info entry for @command{gfortran} for contributors to GCC and
58 @node Invoking GNU Fortran
59 @chapter GNU Fortran Command Options
60 @cindex GNU Fortran command options
61 @cindex command options
62 @cindex options, @command{gfortran} command
64 @c man begin DESCRIPTION
66 The @command{gfortran} command supports all the options supported by the
67 @command{gcc} command. Only options specific to GNU Fortran are documented
70 @xref{Invoking GCC,,GCC Command Options,gcc,Using the GNU Compiler
71 Collection (GCC)}, for information
72 on the non-Fortran-specific aspects of the @command{gcc} command (and,
73 therefore, the @command{gfortran} command).
75 @cindex options, negative forms
76 All GCC and GNU Fortran options
77 are accepted both by @command{gfortran} and by @command{gcc}
78 (as well as any other drivers built at the same time,
79 such as @command{g++}),
80 since adding GNU Fortran to the GCC distribution
81 enables acceptance of GNU Fortran options
82 by all of the relevant drivers.
84 In some cases, options have positive and negative forms;
85 the negative form of @option{-ffoo} would be @option{-fno-foo}.
86 This manual documents only one of these two forms, whichever
87 one is not the default.
91 * Option Summary:: Brief list of all @command{gfortran} options,
93 * Fortran Dialect Options:: Controlling the variant of Fortran language
95 * Preprocessing Options:: Enable and customize preprocessing.
96 * Error and Warning Options:: How picky should the compiler be?
97 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
98 * Directory Options:: Where to find module files
99 * Link Options :: Influencing the linking step
100 * Runtime Options:: Influencing runtime behavior
101 * Code Gen Options:: Specifying conventions for function calls, data layout
103 * Interoperability Options:: Options for interoperability with other
105 * Environment Variables:: Environment variables that affect @command{gfortran}.
109 @section Option summary
113 Here is a summary of all the options specific to GNU Fortran, grouped
114 by type. Explanations are in the following sections.
117 @item Fortran Language Options
118 @xref{Fortran Dialect Options,,Options controlling Fortran dialect}.
119 @gccoptlist{-fall-intrinsics -fbackslash -fcray-pointer -fd-lines-as-code @gol
120 -fd-lines-as-comments @gol
121 -fdec -fdec-structure -fdec-intrinsic-ints -fdec-static -fdec-math @gol
122 -fdefault-double-8 -fdefault-integer-8 @gol
123 -fdefault-real-8 -fdollar-ok -ffixed-line-length-@var{n} @gol
124 -ffixed-line-length-none -ffree-form -ffree-line-length-@var{n} @gol
125 -ffree-line-length-none -fimplicit-none -finteger-4-integer-8 @gol
126 -fmax-identifier-length -fmodule-private -ffixed-form -fno-range-check @gol
127 -fopenacc -fopenmp -freal-4-real-10 -freal-4-real-16 -freal-4-real-8 @gol
128 -freal-8-real-10 -freal-8-real-16 -freal-8-real-4 -std=@var{std}
132 @item Preprocessing Options
133 @xref{Preprocessing Options,,Enable and customize preprocessing}.
134 @gccoptlist{-A-@var{question}@r{[}=@var{answer}@r{]}
135 -A@var{question}=@var{answer} -C -CC -D@var{macro}@r{[}=@var{defn}@r{]}
137 -U@var{macro} -cpp -dD -dI -dM -dN -dU -fworking-directory
138 -imultilib @var{dir} @gol
139 -iprefix @var{file} -iquote -isysroot @var{dir} -isystem @var{dir} -nocpp
144 @item Error and Warning Options
145 @xref{Error and Warning Options,,Options to request or suppress errors
147 @gccoptlist{-Waliasing -Wall -Wampersand -Wargument-mismatch -Warray-bounds
148 -Wc-binding-type -Wcharacter-truncation @gol
149 -Wconversion -Wfunction-elimination -Wimplicit-interface @gol
150 -Wimplicit-procedure -Wintrinsic-shadow -Wuse-without-only -Wintrinsics-std @gol
151 -Wline-truncation -Wno-align-commons -Wno-tabs -Wreal-q-constant @gol
152 -Wsurprising -Wunderflow -Wunused-parameter -Wrealloc-lhs -Wrealloc-lhs-all @gol
153 -Wtarget-lifetime -fmax-errors=@var{n} -fsyntax-only -pedantic -pedantic-errors
156 @item Debugging Options
157 @xref{Debugging Options,,Options for debugging your program or GNU Fortran}.
158 @gccoptlist{-fbacktrace -fdump-fortran-optimized -fdump-fortran-original @gol
159 -fdump-parse-tree -ffpe-trap=@var{list} -ffpe-summary=@var{list}
162 @item Directory Options
163 @xref{Directory Options,,Options for directory search}.
164 @gccoptlist{-I@var{dir} -J@var{dir} -fintrinsic-modules-path @var{dir}}
167 @xref{Link Options,,Options for influencing the linking step}.
168 @gccoptlist{-static-libgfortran}
170 @item Runtime Options
171 @xref{Runtime Options,,Options for influencing runtime behavior}.
172 @gccoptlist{-fconvert=@var{conversion} -fmax-subrecord-length=@var{length} @gol
173 -frecord-marker=@var{length} -fsign-zero
176 @item Interoperability Options
177 @xref{Interoperability Options,,Options for interoperability}.
178 @gccoptlist{-fc-prototypes}
180 @item Code Generation Options
181 @xref{Code Gen Options,,Options for code generation conventions}.
182 @gccoptlist{-faggressive-function-elimination -fblas-matmul-limit=@var{n} @gol
183 -fbounds-check -fcheck-array-temporaries @gol
184 -fcheck=@var{<all|array-temps|bounds|do|mem|pointer|recursion>} @gol
185 -fcoarray=@var{<none|single|lib>} -fexternal-blas -ff2c
186 -ffrontend-optimize @gol
187 -finit-character=@var{n} -finit-integer=@var{n} -finit-local-zero @gol
189 -finit-logical=@var{<true|false>}
190 -finit-real=@var{<zero|inf|-inf|nan|snan>} @gol
191 -finline-matmul-limit=@var{n} @gol
192 -fmax-array-constructor=@var{n} -fmax-stack-var-size=@var{n}
193 -fno-align-commons @gol
194 -fno-automatic -fno-protect-parens -fno-underscoring @gol
195 -fsecond-underscore -fpack-derived -frealloc-lhs -frecursive @gol
196 -frepack-arrays -fshort-enums -fstack-arrays
200 @node Fortran Dialect Options
201 @section Options controlling Fortran dialect
202 @cindex dialect options
203 @cindex language, dialect options
204 @cindex options, dialect
206 The following options control the details of the Fortran dialect
207 accepted by the compiler:
212 @opindex @code{ffree-form}
213 @opindex @code{ffixed-form}
214 @cindex options, Fortran dialect
215 @cindex file format, free
216 @cindex file format, fixed
217 Specify the layout used by the source file. The free form layout
218 was introduced in Fortran 90. Fixed form was traditionally used in
219 older Fortran programs. When neither option is specified, the source
220 form is determined by the file extension.
222 @item -fall-intrinsics
223 @opindex @code{fall-intrinsics}
224 This option causes all intrinsic procedures (including the GNU-specific
225 extensions) to be accepted. This can be useful with @option{-std=f95} to
226 force standard-compliance but get access to the full range of intrinsics
227 available with @command{gfortran}. As a consequence, @option{-Wintrinsics-std}
228 will be ignored and no user-defined procedure with the same name as any
229 intrinsic will be called except when it is explicitly declared @code{EXTERNAL}.
231 @item -fd-lines-as-code
232 @itemx -fd-lines-as-comments
233 @opindex @code{fd-lines-as-code}
234 @opindex @code{fd-lines-as-comments}
235 Enable special treatment for lines beginning with @code{d} or @code{D}
236 in fixed form sources. If the @option{-fd-lines-as-code} option is
237 given they are treated as if the first column contained a blank. If the
238 @option{-fd-lines-as-comments} option is given, they are treated as
243 DEC compatibility mode. Enables extensions and other features that mimic
244 the default behavior of older compilers (such as DEC).
245 These features are non-standard and should be avoided at all costs.
246 For details on GNU Fortran's implementation of these extensions see the
249 Other flags enabled by this switch are:
250 @option{-fdollar-ok} @option{-fcray-pointer} @option{-fdec-structure}
251 @option{-fdec-intrinsic-ints} @option{-fdec-static} @option{-fdec-math}
253 If @option{-fd-lines-as-code}/@option{-fd-lines-as-comments} are unset, then
254 @option{-fdec} also sets @option{-fd-lines-as-comments}.
256 @item -fdec-structure
257 @opindex @code{fdec-structure}
258 Enable DEC @code{STRUCTURE} and @code{RECORD} as well as @code{UNION},
259 @code{MAP}, and dot ('.') as a member separator (in addition to '%'). This is
260 provided for compatibility only; Fortran 90 derived types should be used
261 instead where possible.
263 @item -fdec-intrinsic-ints
264 @opindex @code{fdec-intrinsic-ints}
265 Enable B/I/J/K kind variants of existing integer functions (e.g. BIAND, IIAND,
266 JIAND, etc...). For a complete list of intrinsics see the full documentation.
269 @opindex @code{fdec-math}
270 Enable legacy math intrinsics such as COTAN and degree-valued trigonometric
271 functions (e.g. TAND, ATAND, etc...) for compatability with older code.
274 @opindex @code{fdec-static}
275 Enable DEC-style STATIC and AUTOMATIC attributes to explicitly specify
276 the storage of variables and other objects.
279 @opindex @code{fdollar-ok}
282 @cindex character set
283 Allow @samp{$} as a valid non-first character in a symbol name. Symbols
284 that start with @samp{$} are rejected since it is unclear which rules to
285 apply to implicit typing as different vendors implement different rules.
286 Using @samp{$} in @code{IMPLICIT} statements is also rejected.
289 @opindex @code{backslash}
291 @cindex escape characters
292 Change the interpretation of backslashes in string literals from a single
293 backslash character to ``C-style'' escape characters. The following
294 combinations are expanded @code{\a}, @code{\b}, @code{\f}, @code{\n},
295 @code{\r}, @code{\t}, @code{\v}, @code{\\}, and @code{\0} to the ASCII
296 characters alert, backspace, form feed, newline, carriage return,
297 horizontal tab, vertical tab, backslash, and NUL, respectively.
298 Additionally, @code{\x}@var{nn}, @code{\u}@var{nnnn} and
299 @code{\U}@var{nnnnnnnn} (where each @var{n} is a hexadecimal digit) are
300 translated into the Unicode characters corresponding to the specified code
301 points. All other combinations of a character preceded by \ are
304 @item -fmodule-private
305 @opindex @code{fmodule-private}
306 @cindex module entities
308 Set the default accessibility of module entities to @code{PRIVATE}.
309 Use-associated entities will not be accessible unless they are explicitly
310 declared as @code{PUBLIC}.
312 @item -ffixed-line-length-@var{n}
313 @opindex @code{ffixed-line-length-}@var{n}
314 @cindex file format, fixed
315 Set column after which characters are ignored in typical fixed-form
316 lines in the source file, and through which spaces are assumed (as
317 if padded to that length) after the ends of short fixed-form lines.
319 Popular values for @var{n} include 72 (the
320 standard and the default), 80 (card image), and 132 (corresponding
321 to ``extended-source'' options in some popular compilers).
322 @var{n} may also be @samp{none}, meaning that the entire line is meaningful
323 and that continued character constants never have implicit spaces appended
324 to them to fill out the line.
325 @option{-ffixed-line-length-0} means the same thing as
326 @option{-ffixed-line-length-none}.
328 @item -ffree-line-length-@var{n}
329 @opindex @code{ffree-line-length-}@var{n}
330 @cindex file format, free
331 Set column after which characters are ignored in typical free-form
332 lines in the source file. The default value is 132.
333 @var{n} may be @samp{none}, meaning that the entire line is meaningful.
334 @option{-ffree-line-length-0} means the same thing as
335 @option{-ffree-line-length-none}.
337 @item -fmax-identifier-length=@var{n}
338 @opindex @code{fmax-identifier-length=}@var{n}
339 Specify the maximum allowed identifier length. Typical values are
340 31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).
342 @item -fimplicit-none
343 @opindex @code{fimplicit-none}
344 Specify that no implicit typing is allowed, unless overridden by explicit
345 @code{IMPLICIT} statements. This is the equivalent of adding
346 @code{implicit none} to the start of every procedure.
349 @opindex @code{fcray-pointer}
350 Enable the Cray pointer extension, which provides C-like pointer
354 @opindex @code{fopenacc}
356 Enable the OpenACC extensions. This includes OpenACC @code{!$acc}
357 directives in free form and @code{c$acc}, @code{*$acc} and
358 @code{!$acc} directives in fixed form, @code{!$} conditional
359 compilation sentinels in free form and @code{c$}, @code{*$} and
360 @code{!$} sentinels in fixed form, and when linking arranges for the
361 OpenACC runtime library to be linked in.
363 Note that this is an experimental feature, incomplete, and subject to
364 change in future versions of GCC. See
365 @w{@uref{https://gcc.gnu.org/wiki/OpenACC}} for more information.
368 @opindex @code{fopenmp}
370 Enable the OpenMP extensions. This includes OpenMP @code{!$omp} directives
372 and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,
373 @code{!$} conditional compilation sentinels in free form
374 and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form,
375 and when linking arranges for the OpenMP runtime library to be linked
376 in. The option @option{-fopenmp} implies @option{-frecursive}.
378 @item -fno-range-check
379 @opindex @code{frange-check}
380 Disable range checking on results of simplification of constant
381 expressions during compilation. For example, GNU Fortran will give
382 an error at compile time when simplifying @code{a = 1. / 0}.
383 With this option, no error will be given and @code{a} will be assigned
384 the value @code{+Infinity}. If an expression evaluates to a value
385 outside of the relevant range of [@code{-HUGE()}:@code{HUGE()}],
386 then the expression will be replaced by @code{-Inf} or @code{+Inf}
388 Similarly, @code{DATA i/Z'FFFFFFFF'/} will result in an integer overflow
389 on most systems, but with @option{-fno-range-check} the value will
390 ``wrap around'' and @code{i} will be initialized to @math{-1} instead.
392 @item -fdefault-integer-8
393 @opindex @code{fdefault-integer-8}
394 Set the default integer and logical types to an 8 byte wide type. This option
395 also affects the kind of integer constants like @code{42}. Unlike
396 @option{-finteger-4-integer-8}, it does not promote variables with explicit
399 @item -fdefault-real-8
400 @opindex @code{fdefault-real-8}
401 Set the default real type to an 8 byte wide type. This option also affects
402 the kind of non-double real constants like @code{1.0}, and does promote
403 the default width of @code{DOUBLE PRECISION} to 16 bytes if possible, unless
404 @code{-fdefault-double-8} is given, too. Unlike @option{-freal-4-real-8},
405 it does not promote variables with explicit kind declaration.
407 @item -fdefault-double-8
408 @opindex @code{fdefault-double-8}
409 Set the @code{DOUBLE PRECISION} type to an 8 byte wide type. Do nothing if this
410 is already the default. If @option{-fdefault-real-8} is given,
411 @code{DOUBLE PRECISION} would instead be promoted to 16 bytes if possible, and
412 @option{-fdefault-double-8} can be used to prevent this. The kind of real
413 constants like @code{1.d0} will not be changed by @option{-fdefault-real-8}
414 though, so also @option{-fdefault-double-8} does not affect it.
416 @item -finteger-4-integer-8
417 @opindex @code{finteger-4-integer-8}
418 Promote all @code{INTEGER(KIND=4)} entities to an @code{INTEGER(KIND=8)}
419 entities. If @code{KIND=8} is unavailable, then an error will be issued.
420 This option should be used with care and may not be suitable for your codes.
421 Areas of possible concern include calls to external procedures,
422 alignment in @code{EQUIVALENCE} and/or @code{COMMON}, generic interfaces,
423 BOZ literal constant conversion, and I/O. Inspection of the intermediate
424 representation of the translated Fortran code, produced by
425 @option{-fdump-tree-original}, is suggested.
427 @item -freal-4-real-8
428 @itemx -freal-4-real-10
429 @itemx -freal-4-real-16
430 @itemx -freal-8-real-4
431 @itemx -freal-8-real-10
432 @itemx -freal-8-real-16
433 @opindex @code{freal-4-real-8}
434 @opindex @code{freal-4-real-10}
435 @opindex @code{freal-4-real-16}
436 @opindex @code{freal-8-real-4}
437 @opindex @code{freal-8-real-10}
438 @opindex @code{freal-8-real-16}
439 @cindex options, real kind type promotion
440 Promote all @code{REAL(KIND=M)} entities to @code{REAL(KIND=N)} entities.
441 If @code{REAL(KIND=N)} is unavailable, then an error will be issued.
442 All other real kind types are unaffected by this option.
443 These options should be used with care and may not be suitable for your
444 codes. Areas of possible concern include calls to external procedures,
445 alignment in @code{EQUIVALENCE} and/or @code{COMMON}, generic interfaces,
446 BOZ literal constant conversion, and I/O. Inspection of the intermediate
447 representation of the translated Fortran code, produced by
448 @option{-fdump-tree-original}, is suggested.
451 @opindex @code{std=}@var{std} option
452 Specify the standard to which the program is expected to conform, which
453 may be one of @samp{f95}, @samp{f2003}, @samp{f2008}, @samp{gnu}, or
454 @samp{legacy}. The default value for @var{std} is @samp{gnu}, which
455 specifies a superset of the Fortran 95 standard that includes all of the
456 extensions supported by GNU Fortran, although warnings will be given for
457 obsolete extensions not recommended for use in new code. The
458 @samp{legacy} value is equivalent but without the warnings for obsolete
459 extensions, and may be useful for old non-standard programs. The
460 @samp{f95}, @samp{f2003} and @samp{f2008} values specify strict
461 conformance to the Fortran 95, Fortran 2003 and Fortran 2008 standards,
462 respectively; errors are given for all extensions beyond the relevant
463 language standard, and warnings are given for the Fortran 77 features
464 that are permitted but obsolescent in later standards. @samp{-std=f2008ts}
465 allows the Fortran 2008 standard including the additions of the
466 Technical Specification (TS) 29113 on Further Interoperability of Fortran
467 with C and TS 18508 on Additional Parallel Features in Fortran.
469 @item -ftest-forall-temp
470 @opindex @code{ftest-forall-temp}
471 Enhance test coverage by forcing most forall assignments to use temporary.
475 @node Preprocessing Options
476 @section Enable and customize preprocessing
478 @cindex options, preprocessor
481 Preprocessor related options. See section
482 @ref{Preprocessing and conditional compilation} for more detailed
483 information on preprocessing in @command{gfortran}.
490 @cindex preprocessor, enable
491 @cindex preprocessor, disable
492 Enable preprocessing. The preprocessor is automatically invoked if
493 the file extension is @file{.fpp}, @file{.FPP}, @file{.F}, @file{.FOR},
494 @file{.FTN}, @file{.F90}, @file{.F95}, @file{.F03} or @file{.F08}. Use
495 this option to manually enable preprocessing of any kind of Fortran file.
497 To disable preprocessing of files with any of the above listed extensions,
498 use the negative form: @option{-nocpp}.
500 The preprocessor is run in traditional mode. Any restrictions of the
501 file-format, especially the limits on line length, apply for
502 preprocessed output as well, so it might be advisable to use the
503 @option{-ffree-line-length-none} or @option{-ffixed-line-length-none}
508 @cindex preprocessor, debugging
509 @cindex debugging, preprocessor
510 Instead of the normal output, generate a list of @code{'#define'}
511 directives for all the macros defined during the execution of the
512 preprocessor, including predefined macros. This gives you a way
513 of finding out what is predefined in your version of the preprocessor.
514 Assuming you have no file @file{foo.f90}, the command
516 touch foo.f90; gfortran -cpp -E -dM foo.f90
518 will show all the predefined macros.
522 @cindex preprocessor, debugging
523 @cindex debugging, preprocessor
524 Like @option{-dM} except in two respects: it does not include the
525 predefined macros, and it outputs both the @code{#define} directives
526 and the result of preprocessing. Both kinds of output go to the
527 standard output file.
531 @cindex preprocessor, debugging
532 @cindex debugging, preprocessor
533 Like @option{-dD}, but emit only the macro names, not their expansions.
537 @cindex preprocessor, debugging
538 @cindex debugging, preprocessor
539 Like @option{dD} except that only macros that are expanded, or whose
540 definedness is tested in preprocessor directives, are output; the
541 output is delayed until the use or test of the macro; and @code{'#undef'}
542 directives are also output for macros tested but undefined at the time.
546 @cindex preprocessor, debugging
547 @cindex debugging, preprocessor
548 Output @code{'#include'} directives in addition to the result
551 @item -fworking-directory
552 @opindex @code{fworking-directory}
553 @cindex preprocessor, working directory
554 Enable generation of linemarkers in the preprocessor output that will
555 let the compiler know the current working directory at the time of
556 preprocessing. When this option is enabled, the preprocessor will emit,
557 after the initial linemarker, a second linemarker with the current
558 working directory followed by two slashes. GCC will use this directory,
559 when it is present in the preprocessed input, as the directory emitted
560 as the current working directory in some debugging information formats.
561 This option is implicitly enabled if debugging information is enabled,
562 but this can be inhibited with the negated form
563 @option{-fno-working-directory}. If the @option{-P} flag is present
564 in the command line, this option has no effect, since no @code{#line}
565 directives are emitted whatsoever.
567 @item -idirafter @var{dir}
568 @opindex @code{idirafter @var{dir}}
569 @cindex preprocessing, include path
570 Search @var{dir} for include files, but do it after all directories
571 specified with @option{-I} and the standard system directories have
572 been exhausted. @var{dir} is treated as a system include directory.
573 If dir begins with @code{=}, then the @code{=} will be replaced by
574 the sysroot prefix; see @option{--sysroot} and @option{-isysroot}.
576 @item -imultilib @var{dir}
577 @opindex @code{imultilib @var{dir}}
578 @cindex preprocessing, include path
579 Use @var{dir} as a subdirectory of the directory containing target-specific
582 @item -iprefix @var{prefix}
583 @opindex @code{iprefix @var{prefix}}
584 @cindex preprocessing, include path
585 Specify @var{prefix} as the prefix for subsequent @option{-iwithprefix}
586 options. If the @var{prefix} represents a directory, you should include
587 the final @code{'/'}.
589 @item -isysroot @var{dir}
590 @opindex @code{isysroot @var{dir}}
591 @cindex preprocessing, include path
592 This option is like the @option{--sysroot} option, but applies only to
593 header files. See the @option{--sysroot} option for more information.
595 @item -iquote @var{dir}
596 @opindex @code{iquote @var{dir}}
597 @cindex preprocessing, include path
598 Search @var{dir} only for header files requested with @code{#include "file"};
599 they are not searched for @code{#include <file>}, before all directories
600 specified by @option{-I} and before the standard system directories. If
601 @var{dir} begins with @code{=}, then the @code{=} will be replaced by the
602 sysroot prefix; see @option{--sysroot} and @option{-isysroot}.
604 @item -isystem @var{dir}
605 @opindex @code{isystem @var{dir}}
606 @cindex preprocessing, include path
607 Search @var{dir} for header files, after all directories specified by
608 @option{-I} but before the standard system directories. Mark it as a
609 system directory, so that it gets the same special treatment as is
610 applied to the standard system directories. If @var{dir} begins with
611 @code{=}, then the @code{=} will be replaced by the sysroot prefix;
612 see @option{--sysroot} and @option{-isysroot}.
615 @opindex @code{nostdinc}
616 Do not search the standard system directories for header files. Only
617 the directories you have specified with @option{-I} options (and the
618 directory of the current file, if appropriate) are searched.
621 @opindex @code{undef}
622 Do not predefine any system-specific or GCC-specific macros.
623 The standard predefined macros remain defined.
625 @item -A@var{predicate}=@var{answer}
626 @opindex @code{A@var{predicate}=@var{answer}}
627 @cindex preprocessing, assertion
628 Make an assertion with the predicate @var{predicate} and answer @var{answer}.
629 This form is preferred to the older form -A predicate(answer), which is still
630 supported, because it does not use shell special characters.
632 @item -A-@var{predicate}=@var{answer}
633 @opindex @code{A-@var{predicate}=@var{answer}}
634 @cindex preprocessing, assertion
635 Cancel an assertion with the predicate @var{predicate} and answer @var{answer}.
639 @cindex preprocessing, keep comments
640 Do not discard comments. All comments are passed through to the output
641 file, except for comments in processed directives, which are deleted
642 along with the directive.
644 You should be prepared for side effects when using @option{-C}; it causes
645 the preprocessor to treat comments as tokens in their own right. For example,
646 comments appearing at the start of what would be a directive line have the
647 effect of turning that line into an ordinary source line, since the first
648 token on the line is no longer a @code{'#'}.
650 Warning: this currently handles C-Style comments only. The preprocessor
651 does not yet recognize Fortran-style comments.
655 @cindex preprocessing, keep comments
656 Do not discard comments, including during macro expansion. This is like
657 @option{-C}, except that comments contained within macros are also passed
658 through to the output file where the macro is expanded.
660 In addition to the side-effects of the @option{-C} option, the @option{-CC}
661 option causes all C++-style comments inside a macro to be converted to C-style
662 comments. This is to prevent later use of that macro from inadvertently
663 commenting out the remainder of the source line. The @option{-CC} option
664 is generally used to support lint comments.
666 Warning: this currently handles C- and C++-Style comments only. The
667 preprocessor does not yet recognize Fortran-style comments.
670 @opindex @code{D@var{name}}
671 @cindex preprocessing, define macros
672 Predefine name as a macro, with definition @code{1}.
674 @item -D@var{name}=@var{definition}
675 @opindex @code{D@var{name}=@var{definition}}
676 @cindex preprocessing, define macros
677 The contents of @var{definition} are tokenized and processed as if they
678 appeared during translation phase three in a @code{'#define'} directive.
679 In particular, the definition will be truncated by embedded newline
682 If you are invoking the preprocessor from a shell or shell-like program
683 you may need to use the shell's quoting syntax to protect characters such
684 as spaces that have a meaning in the shell syntax.
686 If you wish to define a function-like macro on the command line, write
687 its argument list with surrounding parentheses before the equals sign
688 (if any). Parentheses are meaningful to most shells, so you will need
689 to quote the option. With sh and csh, @code{-D'name(args...)=definition'}
692 @option{-D} and @option{-U} options are processed in the order they are
693 given on the command line. All -imacros file and -include file options
694 are processed after all -D and -U options.
698 Print the name of each header file used, in addition to other normal
699 activities. Each name is indented to show how deep in the @code{'#include'}
704 @cindex preprocessing, no linemarkers
705 Inhibit generation of linemarkers in the output from the preprocessor.
706 This might be useful when running the preprocessor on something that
707 is not C code, and will be sent to a program which might be confused
711 @opindex @code{U@var{name}}
712 @cindex preprocessing, undefine macros
713 Cancel any previous definition of @var{name}, either built in or provided
714 with a @option{-D} option.
718 @node Error and Warning Options
719 @section Options to request or suppress errors and warnings
720 @cindex options, warnings
721 @cindex options, errors
722 @cindex warnings, suppressing
723 @cindex messages, error
724 @cindex messages, warning
725 @cindex suppressing warnings
727 Errors are diagnostic messages that report that the GNU Fortran compiler
728 cannot compile the relevant piece of source code. The compiler will
729 continue to process the program in an attempt to report further errors
730 to aid in debugging, but will not produce any compiled output.
732 Warnings are diagnostic messages that report constructions which
733 are not inherently erroneous but which are risky or suggest there is
734 likely to be a bug in the program. Unless @option{-Werror} is specified,
735 they do not prevent compilation of the program.
737 You can request many specific warnings with options beginning @option{-W},
738 for example @option{-Wimplicit} to request warnings on implicit
739 declarations. Each of these specific warning options also has a
740 negative form beginning @option{-Wno-} to turn off warnings;
741 for example, @option{-Wno-implicit}. This manual lists only one of the
742 two forms, whichever is not the default.
744 These options control the amount and kinds of errors and warnings produced
748 @item -fmax-errors=@var{n}
749 @opindex @code{fmax-errors=}@var{n}
750 @cindex errors, limiting
751 Limits the maximum number of error messages to @var{n}, at which point
752 GNU Fortran bails out rather than attempting to continue processing the
753 source code. If @var{n} is 0, there is no limit on the number of error
757 @opindex @code{fsyntax-only}
758 @cindex syntax checking
759 Check the code for syntax errors, but do not actually compile it. This
760 will generate module files for each module present in the code, but no
765 @opindex @code{pedantic}
766 @opindex @code{Wpedantic}
767 Issue warnings for uses of extensions to Fortran 95.
768 @option{-pedantic} also applies to C-language constructs where they
769 occur in GNU Fortran source files, such as use of @samp{\e} in a
770 character constant within a directive like @code{#include}.
772 Valid Fortran 95 programs should compile properly with or without
774 However, without this option, certain GNU extensions and traditional
775 Fortran features are supported as well.
776 With this option, many of them are rejected.
778 Some users try to use @option{-pedantic} to check programs for conformance.
779 They soon find that it does not do quite what they want---it finds some
780 nonstandard practices, but not all.
781 However, improvements to GNU Fortran in this area are welcome.
783 This should be used in conjunction with @option{-std=f95},
784 @option{-std=f2003} or @option{-std=f2008}.
786 @item -pedantic-errors
787 @opindex @code{pedantic-errors}
788 Like @option{-pedantic}, except that errors are produced rather than
794 @cindex warnings, all
795 Enables commonly used warning options pertaining to usage that
796 we recommend avoiding and that we believe are easy to avoid.
797 This currently includes @option{-Waliasing}, @option{-Wampersand},
798 @option{-Wconversion}, @option{-Wsurprising}, @option{-Wc-binding-type},
799 @option{-Wintrinsics-std}, @option{-Wtabs}, @option{-Wintrinsic-shadow},
800 @option{-Wline-truncation}, @option{-Wtarget-lifetime},
801 @option{-Winteger-division}, @option{-Wreal-q-constant}, @option{-Wunused}
802 and @option{-Wundefined-do-loop}.
805 @opindex @code{Waliasing}
807 @cindex warnings, aliasing
808 Warn about possible aliasing of dummy arguments. Specifically, it warns
809 if the same actual argument is associated with a dummy argument with
810 @code{INTENT(IN)} and a dummy argument with @code{INTENT(OUT)} in a call
811 with an explicit interface.
813 The following example will trigger the warning.
817 integer, intent(in) :: a
818 integer, intent(out) :: b
827 @opindex @code{Wampersand}
828 @cindex warnings, ampersand
830 Warn about missing ampersand in continued character constants. The warning is
831 given with @option{-Wampersand}, @option{-pedantic}, @option{-std=f95},
832 @option{-std=f2003} and @option{-std=f2008}. Note: With no ampersand
833 given in a continued character constant, GNU Fortran assumes continuation
834 at the first non-comment, non-whitespace character after the ampersand
835 that initiated the continuation.
837 @item -Wargument-mismatch
838 @opindex @code{Wargument-mismatch}
839 @cindex warnings, argument mismatch
840 @cindex warnings, parameter mismatch
841 @cindex warnings, interface mismatch
842 Warn about type, rank, and other mismatches between formal parameters and actual
843 arguments to functions and subroutines. These warnings are recommended and
844 thus enabled by default.
846 @item -Warray-temporaries
847 @opindex @code{Warray-temporaries}
848 @cindex warnings, array temporaries
849 Warn about array temporaries generated by the compiler. The information
850 generated by this warning is sometimes useful in optimization, in order to
851 avoid such temporaries.
853 @item -Wc-binding-type
854 @opindex @code{Wc-binding-type}
855 @cindex warning, C binding type
856 Warn if the a variable might not be C interoperable. In particular, warn if
857 the variable has been declared using an intrinsic type with default kind
858 instead of using a kind parameter defined for C interoperability in the
859 intrinsic @code{ISO_C_Binding} module. This option is implied by
862 @item -Wcharacter-truncation
863 @opindex @code{Wcharacter-truncation}
864 @cindex warnings, character truncation
865 Warn when a character assignment will truncate the assigned string.
867 @item -Wline-truncation
868 @opindex @code{Wline-truncation}
869 @cindex warnings, line truncation
870 Warn when a source code line will be truncated. This option is
871 implied by @option{-Wall}. For free-form source code, the default is
872 @option{-Werror=line-truncation} such that truncations are reported as
876 @opindex @code{Wconversion}
877 @cindex warnings, conversion
879 Warn about implicit conversions that are likely to change the value of
880 the expression after conversion. Implied by @option{-Wall}.
882 @item -Wconversion-extra
883 @opindex @code{Wconversion-extra}
884 @cindex warnings, conversion
886 Warn about implicit conversions between different types and kinds. This
887 option does @emph{not} imply @option{-Wconversion}.
890 @opindex @code{Wextra}
891 @cindex extra warnings
892 @cindex warnings, extra
893 Enables some warning options for usages of language features which
894 may be problematic. This currently includes @option{-Wcompare-reals}
895 and @option{-Wunused-parameter}.
897 @item -Wimplicit-interface
898 @opindex @code{Wimplicit-interface}
899 @cindex warnings, implicit interface
900 Warn if a procedure is called without an explicit interface.
901 Note this only checks that an explicit interface is present. It does not
902 check that the declared interfaces are consistent across program units.
904 @item -Wimplicit-procedure
905 @opindex @code{Wimplicit-procedure}
906 @cindex warnings, implicit procedure
907 Warn if a procedure is called that has neither an explicit interface
908 nor has been declared as @code{EXTERNAL}.
910 @item -Winteger-division
911 @opindex @code{Winteger-division}
912 @cindex warnings, integer division
913 @cindex warnings, division of integers
914 Warn if a constant integer division truncates it result.
915 As an example, 3/5 evaluates to 0.
917 @item -Wintrinsics-std
918 @opindex @code{Wintrinsics-std}
919 @cindex warnings, non-standard intrinsics
920 @cindex warnings, intrinsics of other standards
921 Warn if @command{gfortran} finds a procedure named like an intrinsic not
922 available in the currently selected standard (with @option{-std}) and treats
923 it as @code{EXTERNAL} procedure because of this. @option{-fall-intrinsics} can
924 be used to never trigger this behavior and always link to the intrinsic
925 regardless of the selected standard.
927 @item -Wreal-q-constant
928 @opindex @code{Wreal-q-constant}
929 @cindex warnings, @code{q} exponent-letter
930 Produce a warning if a real-literal-constant contains a @code{q}
934 @opindex @code{Wsurprising}
935 @cindex warnings, suspicious code
936 Produce a warning when ``suspicious'' code constructs are encountered.
937 While technically legal these usually indicate that an error has been made.
939 This currently produces a warning under the following circumstances:
943 An INTEGER SELECT construct has a CASE that can never be matched as its
944 lower value is greater than its upper value.
947 A LOGICAL SELECT construct has three CASE statements.
950 A TRANSFER specifies a source that is shorter than the destination.
953 The type of a function result is declared more than once with the same type. If
954 @option{-pedantic} or standard-conforming mode is enabled, this is an error.
957 A @code{CHARACTER} variable is declared with negative length.
961 @opindex @code{Wtabs}
962 @cindex warnings, tabs
964 By default, tabs are accepted as whitespace, but tabs are not members
965 of the Fortran Character Set. For continuation lines, a tab followed
966 by a digit between 1 and 9 is supported. @option{-Wtabs} will cause
967 a warning to be issued if a tab is encountered. Note, @option{-Wtabs}
968 is active for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003},
969 @option{-std=f2008}, @option{-std=f2008ts} and @option{-Wall}.
971 @item -Wundefined-do-loop
972 @opindex @code{Wundefined-do-loop}
973 @cindex warnings, undefined do loop
974 Warn if a DO loop with step either 1 or -1 yields an underflow or an overflow
975 during iteration of an induction variable of the loop.
976 This option is implied by @option{-Wall}.
979 @opindex @code{Wunderflow}
980 @cindex warnings, underflow
982 Produce a warning when numerical constant expressions are
983 encountered, which yield an UNDERFLOW during compilation. Enabled by default.
985 @item -Wintrinsic-shadow
986 @opindex @code{Wintrinsic-shadow}
987 @cindex warnings, intrinsic
989 Warn if a user-defined procedure or module procedure has the same name as an
990 intrinsic; in this case, an explicit interface or @code{EXTERNAL} or
991 @code{INTRINSIC} declaration might be needed to get calls later resolved to
992 the desired intrinsic/procedure. This option is implied by @option{-Wall}.
994 @item -Wuse-without-only
995 @opindex @code{Wuse-without-only}
996 @cindex warnings, use statements
998 Warn if a @code{USE} statement has no @code{ONLY} qualifier and
999 thus implicitly imports all public entities of the used module.
1001 @item -Wunused-dummy-argument
1002 @opindex @code{Wunused-dummy-argument}
1003 @cindex warnings, unused dummy argument
1004 @cindex unused dummy argument
1005 @cindex dummy argument, unused
1006 Warn about unused dummy arguments. This option is implied by @option{-Wall}.
1008 @item -Wunused-parameter
1009 @opindex @code{Wunused-parameter}
1010 @cindex warnings, unused parameter
1011 @cindex unused parameter
1012 Contrary to @command{gcc}'s meaning of @option{-Wunused-parameter},
1013 @command{gfortran}'s implementation of this option does not warn
1014 about unused dummy arguments (see @option{-Wunused-dummy-argument}),
1015 but about unused @code{PARAMETER} values. @option{-Wunused-parameter}
1016 is implied by @option{-Wextra} if also @option{-Wunused} or
1017 @option{-Wall} is used.
1019 @item -Walign-commons
1020 @opindex @code{Walign-commons}
1021 @cindex warnings, alignment of @code{COMMON} blocks
1022 @cindex alignment of @code{COMMON} blocks
1023 By default, @command{gfortran} warns about any occasion of variables being
1024 padded for proper alignment inside a @code{COMMON} block. This warning can be turned
1025 off via @option{-Wno-align-commons}. See also @option{-falign-commons}.
1027 @item -Wfunction-elimination
1028 @opindex @code{Wfunction-elimination}
1029 @cindex function elimination
1030 @cindex warnings, function elimination
1031 Warn if any calls to functions are eliminated by the optimizations
1032 enabled by the @option{-ffrontend-optimize} option.
1035 @opindex @code{Wrealloc-lhs}
1036 @cindex Reallocate the LHS in assignments, notification
1037 Warn when the compiler might insert code to for allocation or reallocation of
1038 an allocatable array variable of intrinsic type in intrinsic assignments. In
1039 hot loops, the Fortran 2003 reallocation feature may reduce the performance.
1040 If the array is already allocated with the correct shape, consider using a
1041 whole-array array-spec (e.g. @code{(:,:,:)}) for the variable on the left-hand
1042 side to prevent the reallocation check. Note that in some cases the warning
1043 is shown, even if the compiler will optimize reallocation checks away. For
1044 instance, when the right-hand side contains the same variable multiplied by
1045 a scalar. See also @option{-frealloc-lhs}.
1047 @item -Wrealloc-lhs-all
1048 @opindex @code{Wrealloc-lhs-all}
1049 Warn when the compiler inserts code to for allocation or reallocation of an
1050 allocatable variable; this includes scalars and derived types.
1052 @item -Wcompare-reals
1053 @opindex @code{Wcompare-reals}
1054 Warn when comparing real or complex types for equality or inequality.
1055 This option is implied by @option{-Wextra}.
1057 @item -Wtarget-lifetime
1058 @opindex @code{Wtargt-lifetime}
1059 Warn if the pointer in a pointer assignment might be longer than the its
1060 target. This option is implied by @option{-Wall}.
1063 @opindex @code{Wzerotrip}
1064 Warn if a @code{DO} loop is known to execute zero times at compile
1065 time. This option is implied by @option{-Wall}.
1068 @opindex @code{Werror}
1069 @cindex warnings, to errors
1070 Turns all warnings into errors.
1073 @xref{Warning Options,,Options to Request or Suppress Errors and
1074 Warnings, gcc,Using the GNU Compiler Collection (GCC)}, for information on
1075 more options offered by the GBE shared by @command{gfortran}, @command{gcc}
1076 and other GNU compilers.
1078 Some of these have no effect when compiling programs written in Fortran.
1080 @node Debugging Options
1081 @section Options for debugging your program or GNU Fortran
1082 @cindex options, debugging
1083 @cindex debugging information options
1085 GNU Fortran has various special options that are used for debugging
1086 either your program or the GNU Fortran compiler.
1089 @item -fdump-fortran-original
1090 @opindex @code{fdump-fortran-original}
1091 Output the internal parse tree after translating the source program
1092 into internal representation. Only really useful for debugging the
1093 GNU Fortran compiler itself.
1095 @item -fdump-fortran-optimized
1096 @opindex @code{fdump-fortran-optimized}
1097 Output the parse tree after front-end optimization. Only really
1098 useful for debugging the GNU Fortran compiler itself.
1100 @item -fdump-parse-tree
1101 @opindex @code{fdump-parse-tree}
1102 Output the internal parse tree after translating the source program
1103 into internal representation. Only really useful for debugging the
1104 GNU Fortran compiler itself. This option is deprecated; use
1105 @code{-fdump-fortran-original} instead.
1107 @item -ffpe-trap=@var{list}
1108 @opindex @code{ffpe-trap=}@var{list}
1109 Specify a list of floating point exception traps to enable. On most
1110 systems, if a floating point exception occurs and the trap for that
1111 exception is enabled, a SIGFPE signal will be sent and the program
1112 being aborted, producing a core file useful for debugging. @var{list}
1113 is a (possibly empty) comma-separated list of the following
1114 exceptions: @samp{invalid} (invalid floating point operation, such as
1115 @code{SQRT(-1.0)}), @samp{zero} (division by zero), @samp{overflow}
1116 (overflow in a floating point operation), @samp{underflow} (underflow
1117 in a floating point operation), @samp{inexact} (loss of precision
1118 during operation), and @samp{denormal} (operation performed on a
1119 denormal value). The first five exceptions correspond to the five
1120 IEEE 754 exceptions, whereas the last one (@samp{denormal}) is not
1121 part of the IEEE 754 standard but is available on some common
1122 architectures such as x86.
1124 The first three exceptions (@samp{invalid}, @samp{zero}, and
1125 @samp{overflow}) often indicate serious errors, and unless the program
1126 has provisions for dealing with these exceptions, enabling traps for
1127 these three exceptions is probably a good idea.
1129 Many, if not most, floating point operations incur loss of precision
1130 due to rounding, and hence the @code{ffpe-trap=inexact} is likely to
1131 be uninteresting in practice.
1133 By default no exception traps are enabled.
1135 @item -ffpe-summary=@var{list}
1136 @opindex @code{ffpe-summary=}@var{list}
1137 Specify a list of floating-point exceptions, whose flag status is printed
1138 to @code{ERROR_UNIT} when invoking @code{STOP} and @code{ERROR STOP}.
1139 @var{list} can be either @samp{none}, @samp{all} or a comma-separated list
1140 of the following exceptions: @samp{invalid}, @samp{zero}, @samp{overflow},
1141 @samp{underflow}, @samp{inexact} and @samp{denormal}. (See
1142 @option{-ffpe-trap} for a description of the exceptions.)
1144 By default, a summary for all exceptions but @samp{inexact} is shown.
1146 @item -fno-backtrace
1147 @opindex @code{fno-backtrace}
1150 When a serious runtime error is encountered or a deadly signal is
1151 emitted (segmentation fault, illegal instruction, bus error,
1152 floating-point exception, and the other POSIX signals that have the
1153 action @samp{core}), the Fortran runtime library tries to output a
1154 backtrace of the error. @code{-fno-backtrace} disables the backtrace
1155 generation. This option only has influence for compilation of the
1156 Fortran main program.
1160 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
1161 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
1164 @node Directory Options
1165 @section Options for directory search
1166 @cindex directory, options
1167 @cindex options, directory search
1169 @cindex @code{INCLUDE} directive
1170 @cindex directive, @code{INCLUDE}
1171 These options affect how GNU Fortran searches
1172 for files specified by the @code{INCLUDE} directive and where it searches
1173 for previously compiled modules.
1175 It also affects the search paths used by @command{cpp} when used to preprocess
1180 @opindex @code{I}@var{dir}
1181 @cindex directory, search paths for inclusion
1182 @cindex inclusion, directory search paths for
1183 @cindex search paths, for included files
1184 @cindex paths, search
1185 @cindex module search path
1186 These affect interpretation of the @code{INCLUDE} directive
1187 (as well as of the @code{#include} directive of the @command{cpp}
1190 Also note that the general behavior of @option{-I} and
1191 @code{INCLUDE} is pretty much the same as of @option{-I} with
1192 @code{#include} in the @command{cpp} preprocessor, with regard to
1193 looking for @file{header.gcc} files and other such things.
1195 This path is also used to search for @file{.mod} files when previously
1196 compiled modules are required by a @code{USE} statement.
1198 @xref{Directory Options,,Options for Directory Search,
1199 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
1203 @opindex @code{J}@var{dir}
1204 @opindex @code{M}@var{dir}
1205 @cindex paths, search
1206 @cindex module search path
1207 This option specifies where to put @file{.mod} files for compiled modules.
1208 It is also added to the list of directories to searched by an @code{USE}
1211 The default is the current directory.
1213 @item -fintrinsic-modules-path @var{dir}
1214 @opindex @code{fintrinsic-modules-path} @var{dir}
1215 @cindex paths, search
1216 @cindex module search path
1217 This option specifies the location of pre-compiled intrinsic modules, if
1218 they are not in the default location expected by the compiler.
1222 @section Influencing the linking step
1223 @cindex options, linking
1224 @cindex linking, static
1226 These options come into play when the compiler links object files into an
1227 executable output file. They are meaningless if the compiler is not doing
1231 @item -static-libgfortran
1232 @opindex @code{static-libgfortran}
1233 On systems that provide @file{libgfortran} as a shared and a static
1234 library, this option forces the use of the static version. If no
1235 shared version of @file{libgfortran} was built when the compiler was
1236 configured, this option has no effect.
1240 @node Runtime Options
1241 @section Influencing runtime behavior
1242 @cindex options, runtime
1244 These options affect the runtime behavior of programs compiled with GNU Fortran.
1247 @item -fconvert=@var{conversion}
1248 @opindex @code{fconvert=}@var{conversion}
1249 Specify the representation of data for unformatted files. Valid
1250 values for conversion are: @samp{native}, the default; @samp{swap},
1251 swap between big- and little-endian; @samp{big-endian}, use big-endian
1252 representation for unformatted files; @samp{little-endian}, use little-endian
1253 representation for unformatted files.
1255 @emph{This option has an effect only when used in the main program.
1256 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
1257 variable override the default specified by @option{-fconvert}.}
1259 @item -frecord-marker=@var{length}
1260 @opindex @code{frecord-marker=}@var{length}
1261 Specify the length of record markers for unformatted files.
1262 Valid values for @var{length} are 4 and 8. Default is 4.
1263 @emph{This is different from previous versions of @command{gfortran}},
1264 which specified a default record marker length of 8 on most
1265 systems. If you want to read or write files compatible
1266 with earlier versions of @command{gfortran}, use @option{-frecord-marker=8}.
1268 @item -fmax-subrecord-length=@var{length}
1269 @opindex @code{fmax-subrecord-length=}@var{length}
1270 Specify the maximum length for a subrecord. The maximum permitted
1271 value for length is 2147483639, which is also the default. Only
1272 really useful for use by the gfortran testsuite.
1275 @opindex @code{fsign-zero}
1276 When enabled, floating point numbers of value zero with the sign bit set
1277 are written as negative number in formatted output and treated as
1278 negative in the @code{SIGN} intrinsic. @option{-fno-sign-zero} does not
1279 print the negative sign of zero values (or values rounded to zero for I/O)
1280 and regards zero as positive number in the @code{SIGN} intrinsic for
1281 compatibility with Fortran 77. The default is @option{-fsign-zero}.
1284 @node Code Gen Options
1285 @section Options for code generation conventions
1286 @cindex code generation, conventions
1287 @cindex options, code generation
1288 @cindex options, run-time
1290 These machine-independent options control the interface conventions
1291 used in code generation.
1293 Most of them have both positive and negative forms; the negative form
1294 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
1295 one of the forms is listed---the one which is not the default. You
1296 can figure out the other form by either removing @option{no-} or adding
1300 @item -fno-automatic
1301 @opindex @code{fno-automatic}
1302 @cindex @code{SAVE} statement
1303 @cindex statement, @code{SAVE}
1304 Treat each program unit (except those marked as RECURSIVE) as if the
1305 @code{SAVE} statement were specified for every local variable and array
1306 referenced in it. Does not affect common blocks. (Some Fortran compilers
1307 provide this option under the name @option{-static} or @option{-save}.)
1308 The default, which is @option{-fautomatic}, uses the stack for local
1309 variables smaller than the value given by @option{-fmax-stack-var-size}.
1310 Use the option @option{-frecursive} to use no static memory.
1314 @cindex calling convention
1315 @cindex @command{f2c} calling convention
1316 @cindex @command{g77} calling convention
1317 @cindex libf2c calling convention
1318 Generate code designed to be compatible with code generated
1319 by @command{g77} and @command{f2c}.
1321 The calling conventions used by @command{g77} (originally implemented
1322 in @command{f2c}) require functions that return type
1323 default @code{REAL} to actually return the C type @code{double}, and
1324 functions that return type @code{COMPLEX} to return the values via an
1325 extra argument in the calling sequence that points to where to
1326 store the return value. Under the default GNU calling conventions, such
1327 functions simply return their results as they would in GNU
1328 C---default @code{REAL} functions return the C type @code{float}, and
1329 @code{COMPLEX} functions return the GNU C type @code{complex}.
1330 Additionally, this option implies the @option{-fsecond-underscore}
1331 option, unless @option{-fno-second-underscore} is explicitly requested.
1333 This does not affect the generation of code that interfaces with
1334 the @command{libgfortran} library.
1336 @emph{Caution:} It is not a good idea to mix Fortran code compiled with
1337 @option{-ff2c} with code compiled with the default @option{-fno-f2c}
1338 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
1339 functions between program parts which were compiled with different
1340 calling conventions will break at execution time.
1342 @emph{Caution:} This will break code which passes intrinsic functions
1343 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
1344 the library implementations use the @option{-fno-f2c} calling conventions.
1346 @item -fno-underscoring
1347 @opindex @code{fno-underscoring}
1349 @cindex symbol names, underscores
1350 @cindex transforming symbol names
1351 @cindex symbol names, transforming
1352 Do not transform names of entities specified in the Fortran
1353 source file by appending underscores to them.
1355 With @option{-funderscoring} in effect, GNU Fortran appends one
1356 underscore to external names with no underscores. This is done to ensure
1357 compatibility with code produced by many UNIX Fortran compilers.
1359 @emph{Caution}: The default behavior of GNU Fortran is
1360 incompatible with @command{f2c} and @command{g77}, please use the
1361 @option{-ff2c} option if you want object files compiled with
1362 GNU Fortran to be compatible with object code created with these
1365 Use of @option{-fno-underscoring} is not recommended unless you are
1366 experimenting with issues such as integration of GNU Fortran into
1367 existing system environments (vis-@`{a}-vis existing libraries, tools,
1370 For example, with @option{-funderscoring}, and assuming that @code{j()} and
1371 @code{max_count()} are external functions while @code{my_var} and
1372 @code{lvar} are local variables, a statement like
1374 I = J() + MAX_COUNT (MY_VAR, LVAR)
1377 is implemented as something akin to:
1379 i = j_() + max_count__(&my_var__, &lvar);
1382 With @option{-fno-underscoring}, the same statement is implemented as:
1385 i = j() + max_count(&my_var, &lvar);
1388 Use of @option{-fno-underscoring} allows direct specification of
1389 user-defined names while debugging and when interfacing GNU Fortran
1390 code with other languages.
1392 Note that just because the names match does @emph{not} mean that the
1393 interface implemented by GNU Fortran for an external name matches the
1394 interface implemented by some other language for that same name.
1395 That is, getting code produced by GNU Fortran to link to code produced
1396 by some other compiler using this or any other method can be only a
1397 small part of the overall solution---getting the code generated by
1398 both compilers to agree on issues other than naming can require
1399 significant effort, and, unlike naming disagreements, linkers normally
1400 cannot detect disagreements in these other areas.
1402 Also, note that with @option{-fno-underscoring}, the lack of appended
1403 underscores introduces the very real possibility that a user-defined
1404 external name will conflict with a name in a system library, which
1405 could make finding unresolved-reference bugs quite difficult in some
1406 cases---they might occur at program run time, and show up only as
1407 buggy behavior at run time.
1409 In future versions of GNU Fortran we hope to improve naming and linking
1410 issues so that debugging always involves using the names as they appear
1411 in the source, even if the names as seen by the linker are mangled to
1412 prevent accidental linking between procedures with incompatible
1415 @item -fsecond-underscore
1416 @opindex @code{fsecond-underscore}
1418 @cindex symbol names, underscores
1419 @cindex transforming symbol names
1420 @cindex symbol names, transforming
1421 @cindex @command{f2c} calling convention
1422 @cindex @command{g77} calling convention
1423 @cindex libf2c calling convention
1424 By default, GNU Fortran appends an underscore to external
1425 names. If this option is used GNU Fortran appends two
1426 underscores to names with underscores and one underscore to external names
1427 with no underscores. GNU Fortran also appends two underscores to
1428 internal names with underscores to avoid naming collisions with external
1431 This option has no effect if @option{-fno-underscoring} is
1432 in effect. It is implied by the @option{-ff2c} option.
1434 Otherwise, with this option, an external name such as @code{MAX_COUNT}
1435 is implemented as a reference to the link-time external symbol
1436 @code{max_count__}, instead of @code{max_count_}. This is required
1437 for compatibility with @command{g77} and @command{f2c}, and is implied
1438 by use of the @option{-ff2c} option.
1440 @item -fcoarray=@var{<keyword>}
1441 @opindex @code{fcoarray}
1446 Disable coarray support; using coarray declarations and image-control
1447 statements will produce a compile-time error. (Default)
1450 Single-image mode, i.e. @code{num_images()} is always one.
1453 Library-based coarray parallelization; a suitable GNU Fortran coarray
1454 library needs to be linked.
1458 @item -fcheck=@var{<keyword>}
1459 @opindex @code{fcheck}
1460 @cindex array, bounds checking
1461 @cindex bounds checking
1462 @cindex pointer checking
1463 @cindex memory checking
1464 @cindex range checking
1465 @cindex subscript checking
1466 @cindex checking subscripts
1467 @cindex run-time checking
1468 @cindex checking array temporaries
1470 Enable the generation of run-time checks; the argument shall be
1471 a comma-delimited list of the following keywords. Prefixing a check with
1472 @option{no-} disables it if it was activated by a previous specification.
1476 Enable all run-time test of @option{-fcheck}.
1478 @item @samp{array-temps}
1479 Warns at run time when for passing an actual argument a temporary array
1480 had to be generated. The information generated by this warning is
1481 sometimes useful in optimization, in order to avoid such temporaries.
1483 Note: The warning is only printed once per location.
1486 Enable generation of run-time checks for array subscripts
1487 and against the declared minimum and maximum values. It also
1488 checks array indices for assumed and deferred
1489 shape arrays against the actual allocated bounds and ensures that all string
1490 lengths are equal for character array constructors without an explicit
1493 Some checks require that @option{-fcheck=bounds} is set for
1494 the compilation of the main program.
1496 Note: In the future this may also include other forms of checking, e.g.,
1497 checking substring references.
1500 Enable generation of run-time checks for invalid modification of loop
1501 iteration variables.
1504 Enable generation of run-time checks for memory allocation.
1505 Note: This option does not affect explicit allocations using the
1506 @code{ALLOCATE} statement, which will be always checked.
1508 @item @samp{pointer}
1509 Enable generation of run-time checks for pointers and allocatables.
1511 @item @samp{recursion}
1512 Enable generation of run-time checks for recursively called subroutines and
1513 functions which are not marked as recursive. See also @option{-frecursive}.
1514 Note: This check does not work for OpenMP programs and is disabled if used
1515 together with @option{-frecursive} and @option{-fopenmp}.
1518 Example: Assuming you have a file @file{foo.f90}, the command
1520 gfortran -fcheck=all,no-array-temps foo.f90
1522 will compile the file with all checks enabled as specified above except
1523 warnings for generated array temporaries.
1526 @item -fbounds-check
1527 @opindex @code{fbounds-check}
1528 @c Note: This option is also referred in gcc's manpage
1529 Deprecated alias for @option{-fcheck=bounds}.
1531 @item -fcheck-array-temporaries
1532 @opindex @code{fcheck-array-temporaries}
1533 Deprecated alias for @option{-fcheck=array-temps}.
1535 @item -fmax-array-constructor=@var{n}
1536 @opindex @code{fmax-array-constructor}
1537 This option can be used to increase the upper limit permitted in
1538 array constructors. The code below requires this option to expand
1539 the array at compile time.
1545 integer, parameter :: n = 100000
1546 integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
1547 print '(10(I0,1X))', i
1551 @emph{Caution: This option can lead to long compile times and excessively
1552 large object files.}
1554 The default value for @var{n} is 65535.
1557 @item -fmax-stack-var-size=@var{n}
1558 @opindex @code{fmax-stack-var-size}
1559 This option specifies the size in bytes of the largest array that will be put
1560 on the stack; if the size is exceeded static memory is used (except in
1561 procedures marked as RECURSIVE). Use the option @option{-frecursive} to
1562 allow for recursive procedures which do not have a RECURSIVE attribute or
1563 for parallel programs. Use @option{-fno-automatic} to never use the stack.
1565 This option currently only affects local arrays declared with constant
1566 bounds, and may not apply to all character variables.
1567 Future versions of GNU Fortran may improve this behavior.
1569 The default value for @var{n} is 32768.
1571 @item -fstack-arrays
1572 @opindex @code{fstack-arrays}
1573 Adding this option will make the Fortran compiler put all local arrays,
1574 even those of unknown size onto stack memory. If your program uses very
1575 large local arrays it is possible that you will have to extend your runtime
1576 limits for stack memory on some operating systems. This flag is enabled
1577 by default at optimization level @option{-Ofast}.
1580 @item -fpack-derived
1581 @opindex @code{fpack-derived}
1582 @cindex structure packing
1583 This option tells GNU Fortran to pack derived type members as closely as
1584 possible. Code compiled with this option is likely to be incompatible
1585 with code compiled without this option, and may execute slower.
1587 @item -frepack-arrays
1588 @opindex @code{frepack-arrays}
1589 @cindex repacking arrays
1590 In some circumstances GNU Fortran may pass assumed shape array
1591 sections via a descriptor describing a noncontiguous area of memory.
1592 This option adds code to the function prologue to repack the data into
1593 a contiguous block at runtime.
1595 This should result in faster accesses to the array. However it can introduce
1596 significant overhead to the function call, especially when the passed data
1600 @opindex @code{fshort-enums}
1601 This option is provided for interoperability with C code that was
1602 compiled with the @option{-fshort-enums} option. It will make
1603 GNU Fortran choose the smallest @code{INTEGER} kind a given
1604 enumerator set will fit in, and give all its enumerators this kind.
1606 @item -fexternal-blas
1607 @opindex @code{fexternal-blas}
1608 This option will make @command{gfortran} generate calls to BLAS functions
1609 for some matrix operations like @code{MATMUL}, instead of using our own
1610 algorithms, if the size of the matrices involved is larger than a given
1611 limit (see @option{-fblas-matmul-limit}). This may be profitable if an
1612 optimized vendor BLAS library is available. The BLAS library will have
1613 to be specified at link time.
1615 @item -fblas-matmul-limit=@var{n}
1616 @opindex @code{fblas-matmul-limit}
1617 Only significant when @option{-fexternal-blas} is in effect.
1618 Matrix multiplication of matrices with size larger than (or equal to) @var{n}
1619 will be performed by calls to BLAS functions, while others will be
1620 handled by @command{gfortran} internal algorithms. If the matrices
1621 involved are not square, the size comparison is performed using the
1622 geometric mean of the dimensions of the argument and result matrices.
1624 The default value for @var{n} is 30.
1626 @item -finline-matmul-limit=@var{n}
1627 @opindex @code{finline-matmul-limit}
1628 When front-end optimiztion is active, some calls to the @code{MATMUL}
1629 intrinsic function will be inlined. This may result in code size
1630 increase if the size of the matrix cannot be determined at compile
1631 time, as code for both cases is generated. Setting
1632 @code{-finline-matmul-limit=0} will disable inlining in all cases.
1633 Setting this option with a value of @var{n} will produce inline code
1634 for matrices with size up to @var{n}. If the matrices involved are not
1635 square, the size comparison is performed using the geometric mean of
1636 the dimensions of the argument and result matrices.
1638 The default value for @var{n} is 30. The @code{-fblas-matmul-limit}
1639 can be used to change this value.
1642 @opindex @code{frecursive}
1643 Allow indirect recursion by forcing all local arrays to be allocated
1644 on the stack. This flag cannot be used together with
1645 @option{-fmax-stack-var-size=} or @option{-fno-automatic}.
1647 @item -finit-local-zero
1648 @itemx -finit-derived
1649 @itemx -finit-integer=@var{n}
1650 @itemx -finit-real=@var{<zero|inf|-inf|nan|snan>}
1651 @itemx -finit-logical=@var{<true|false>}
1652 @itemx -finit-character=@var{n}
1653 @opindex @code{finit-local-zero}
1654 @opindex @code{finit-derived}
1655 @opindex @code{finit-integer}
1656 @opindex @code{finit-real}
1657 @opindex @code{finit-logical}
1658 @opindex @code{finit-character}
1659 The @option{-finit-local-zero} option instructs the compiler to
1660 initialize local @code{INTEGER}, @code{REAL}, and @code{COMPLEX}
1661 variables to zero, @code{LOGICAL} variables to false, and
1662 @code{CHARACTER} variables to a string of null bytes. Finer-grained
1663 initialization options are provided by the
1664 @option{-finit-integer=@var{n}},
1665 @option{-finit-real=@var{<zero|inf|-inf|nan|snan>}} (which also initializes
1666 the real and imaginary parts of local @code{COMPLEX} variables),
1667 @option{-finit-logical=@var{<true|false>}}, and
1668 @option{-finit-character=@var{n}} (where @var{n} is an ASCII character
1669 value) options. Components of derived type variables will be initialized
1670 according to these flags only with @option{-finit-derived}. These options do
1674 objects with the POINTER attribute
1678 variables that appear in an @code{EQUIVALENCE} statement.
1680 (These limitations may be removed in future releases).
1682 Note that the @option{-finit-real=nan} option initializes @code{REAL}
1683 and @code{COMPLEX} variables with a quiet NaN. For a signalling NaN
1684 use @option{-finit-real=snan}; note, however, that compile-time
1685 optimizations may convert them into quiet NaN and that trapping
1686 needs to be enabled (e.g. via @option{-ffpe-trap}).
1688 Finally, note that enabling any of the @option{-finit-*} options will
1689 silence warnings that would have been emitted by @option{-Wuninitialized}
1690 for the affected local variables.
1692 @item -falign-commons
1693 @opindex @code{falign-commons}
1694 @cindex alignment of @code{COMMON} blocks
1695 By default, @command{gfortran} enforces proper alignment of all variables in a
1696 @code{COMMON} block by padding them as needed. On certain platforms this is mandatory,
1697 on others it increases performance. If a @code{COMMON} block is not declared with
1698 consistent data types everywhere, this padding can cause trouble, and
1699 @option{-fno-align-commons} can be used to disable automatic alignment. The
1700 same form of this option should be used for all files that share a @code{COMMON} block.
1701 To avoid potential alignment issues in @code{COMMON} blocks, it is recommended to order
1702 objects from largest to smallest.
1704 @item -fno-protect-parens
1705 @opindex @code{fno-protect-parens}
1706 @cindex re-association of parenthesized expressions
1707 By default the parentheses in expression are honored for all optimization
1708 levels such that the compiler does not do any re-association. Using
1709 @option{-fno-protect-parens} allows the compiler to reorder @code{REAL} and
1710 @code{COMPLEX} expressions to produce faster code. Note that for the re-association
1711 optimization @option{-fno-signed-zeros} and @option{-fno-trapping-math}
1712 need to be in effect. The parentheses protection is enabled by default, unless
1713 @option{-Ofast} is given.
1716 @opindex @code{frealloc-lhs}
1717 @cindex Reallocate the LHS in assignments
1718 An allocatable left-hand side of an intrinsic assignment is automatically
1719 (re)allocated if it is either unallocated or has a different shape. The
1720 option is enabled by default except when @option{-std=f95} is given. See
1721 also @option{-Wrealloc-lhs}.
1723 @item -faggressive-function-elimination
1724 @opindex @code{faggressive-function-elimination}
1725 @cindex Elimination of functions with identical argument lists
1726 Functions with identical argument lists are eliminated within
1727 statements, regardless of whether these functions are marked
1728 @code{PURE} or not. For example, in
1732 there will only be a single call to @code{f}. This option only works
1733 if @option{-ffrontend-optimize} is in effect.
1735 @item -ffrontend-optimize
1736 @opindex @code{frontend-optimize}
1737 @cindex Front-end optimization
1738 This option performs front-end optimization, based on manipulating
1739 parts the Fortran parse tree. Enabled by default by any @option{-O}
1740 option. Optimizations enabled by this option include inlining calls
1741 to @code{MATMUL}, elimination of identical function calls within
1742 expressions, removing unnecessary calls to @code{TRIM} in comparisons
1743 and assignments and replacing @code{TRIM(a)} with
1744 @code{a(1:LEN_TRIM(a))}. It can be deselected by specifying
1745 @option{-fno-frontend-optimize}.
1748 @xref{Code Gen Options,,Options for Code Generation Conventions,
1749 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
1751 shared by @command{gfortran}, @command{gcc}, and other GNU compilers.
1755 @node Interoperability Options
1756 @section Options for interoperability with other languages
1760 @item -fc-prototypes
1761 @opindex @code{c-prototypes}
1762 @cindex Generating C prototypes from Fortran source code
1763 This option will generate C prototypes from @code{BIND(C)} variable
1764 declarations, types and procedure interfaces and writes them to
1765 standard output. @code{ENUM} is not yet supported.
1767 The generated prototypes may need inclusion of an appropriate header,
1768 such as @code{<stdint.h>} or @code{<stdlib.h>}. For types which are
1769 not specified using the appropriate kind from the @code{iso_c_binding}
1770 module, a warning is added as a comment to the code.
1772 For function pointers, a pointer to a function returning @code{int}
1773 without an explicit argument list is generated.
1777 $ gfortran -fc-prototypes -fsyntax-only foo.f90 > foo.h
1779 where the C code intended for interoperating with the Fortran code
1780 then uses @code{#include "foo.h"}.
1783 @node Environment Variables
1784 @section Environment variables affecting @command{gfortran}
1785 @cindex environment variable
1787 @c man begin ENVIRONMENT
1789 The @command{gfortran} compiler currently does not make use of any environment
1790 variables to control its operation above and beyond those
1791 that affect the operation of @command{gcc}.
1793 @xref{Environment Variables,,Environment Variables Affecting GCC,
1794 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
1797 @xref{Runtime}, for environment variables that affect the
1798 run-time behavior of programs compiled with GNU Fortran.