1 @c Copyright (C) 2004, 2005, 2006, 2007
2 @c Free Software Foundation, Inc.
3 @c This is part of the GNU Fortran manual.
4 @c For copying conditions, see the file gfortran.texi.
8 Copyright @copyright{} 2004, 2005, 2006, 2007
9 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU Fortran compiler.
33 gfortran [@option{-c}|@option{-S}|@option{-E}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}]
39 [@option{-m}@var{machine-option}@dots{}]
40 [@option{-o} @var{outfile}] @var{infile}@dots{}
42 Only the most useful options are listed here; see below for the
46 gpl(7), gfdl(7), fsf-funding(7),
47 cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
48 and the Info entries for @file{gcc}, @file{cpp}, @file{gfortran}, @file{as},
49 @file{ld}, @file{binutils} and @file{gdb}.
52 For instructions on reporting bugs, see
53 @w{@uref{http://gcc.gnu.org/bugs.html}}.
56 See the Info entry for @command{gfortran} for contributors to GCC and
61 @node Invoking GNU Fortran
62 @chapter GNU Fortran Command Options
63 @cindex GNU Fortran command options
64 @cindex command options
65 @cindex options, @command{gfortran} command
67 @c man begin DESCRIPTION
69 The @command{gfortran} command supports all the options supported by the
70 @command{gcc} command. Only options specific to GNU Fortran are documented here.
72 @xref{Invoking GCC,,GCC Command Options,gcc,Using the GNU Compiler
73 Collection (GCC)}, for information
74 on the non-Fortran-specific aspects of the @command{gcc} command (and,
75 therefore, the @command{gfortran} command).
77 @cindex options, negative forms
78 All GCC and GNU Fortran options
79 are accepted both by @command{gfortran} and by @command{gcc}
80 (as well as any other drivers built at the same time,
81 such as @command{g++}),
82 since adding GNU Fortran to the GCC distribution
83 enables acceptance of GNU Fortran options
84 by all of the relevant drivers.
86 In some cases, options have positive and negative forms;
87 the negative form of @option{-ffoo} would be @option{-fno-foo}.
88 This manual documents only one of these two forms, whichever
89 one is not the default.
93 * Option Summary:: Brief list of all @command{gfortran} options,
95 * Fortran Dialect Options:: Controlling the variant of Fortran language
97 * Error and Warning Options:: How picky should the compiler be?
98 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
99 * Directory Options:: Where to find module files
100 * Link Options :: Influencing the linking step
101 * Runtime Options:: Influencing runtime behavior
102 * Code Gen Options:: Specifying conventions for function calls, data layout
104 * Environment Variables:: Environment variables that affect @command{gfortran}.
108 @section Option summary
112 Here is a summary of all the options specific to GNU Fortran, grouped
113 by type. Explanations are in the following sections.
116 @item Fortran Language Options
117 @xref{Fortran Dialect Options,,Options controlling Fortran dialect}.
118 @gccoptlist{-fall-intrinsics -ffree-form -fno-fixed-form @gol
119 -fdollar-ok -fimplicit-none -fmax-identifier-length @gol
120 -std=@var{std} -fd-lines-as-code -fd-lines-as-comments @gol
121 -ffixed-line-length-@var{n} -ffixed-line-length-none @gol
122 -ffree-line-length-@var{n} -ffree-line-length-none @gol
123 -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 @gol
124 -fcray-pointer -fopenmp -frange-check -fno-backslash }
126 @item Error and Warning Options
127 @xref{Error and Warning Options,,Options to request or suppress errors
129 @gccoptlist{-fmax-errors=@var{n} @gol
130 -fsyntax-only -pedantic -pedantic-errors @gol
131 -w -Wall -Waliasing -Wampersand -Wcharacter-truncation -Wconversion @gol
132 -Wimplicit-interface -Wline-truncation -Wnonstd-intrinsics -Wsurprising @gol
133 -Wno-tabs -Wunderflow}
135 @item Debugging Options
136 @xref{Debugging Options,,Options for debugging your program or GNU Fortran}.
137 @gccoptlist{-fdump-parse-tree -ffpe-trap=@var{list} @gol
138 -fdump-core -fbacktrace}
140 @item Directory Options
141 @xref{Directory Options,,Options for directory search}.
142 @gccoptlist{-I@var{dir} -J@var{dir} -M@var{dir} -fintrinsic-modules-path @var{dir}}
145 @xref{Link Options,,Options for influencing the linking step}.
146 @gccoptlist{-static-libgfortran}
148 @item Runtime Options
149 @xref{Runtime Options,,Options for influencing runtime behavior}.
150 @gccoptlist{-fconvert=@var{conversion} -frecord-marker=@var{length} @gol
151 -fmax-subrecord-length=@var{length}}
153 @item Code Generation Options
154 @xref{Code Gen Options,,Options for code generation conventions}.
155 @gccoptlist{-fno-automatic -ff2c -fno-underscoring
156 -fsecond-underscore @gol
157 -fbounds-check -fmax-stack-var-size=@var{n} @gol
158 -fpack-derived -frepack-arrays -fshort-enums -fexternal-blas @gol
159 -fblas-matmul-limit=@var{n}}
163 * Fortran Dialect Options:: Controlling the variant of Fortran language
165 * Error and Warning Options:: How picky should the compiler be?
166 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
167 * Directory Options:: Where to find module files
168 * Link Options :: Influencing the linking step
169 * Runtime Options:: Influencing runtime behavior
170 * Code Gen Options:: Specifying conventions for function calls, data layout
174 @node Fortran Dialect Options
175 @section Options controlling Fortran dialect
176 @cindex dialect options
177 @cindex language, dialect options
178 @cindex options, dialect
180 The following options control the details of the Fortran dialect
181 accepted by the compiler:
186 @opindex @code{ffree-form}
187 @opindex @code{fno-fixed-form}
188 @cindex options, fortran dialect
189 @cindex file format, free
190 @cindex file format, fixed
191 Specify the layout used by the source file. The free form layout
192 was introduced in Fortran 90. Fixed form was traditionally used in
193 older Fortran programs. When neither option is specified, the source
194 form is determined by the file extension.
196 @item -fall-intrinsics
197 @opindex @code{fall-intrinsics}
198 Accept all of the intrinsic procedures provided in libgfortran
199 without regard to the setting of @option{-std}. In particular,
200 this option can be quite useful with @option{-std=f95}. Additionally,
201 @command{gfortran} will ignore @option{-Wnonstd-intrinsics}.
203 @item -fd-lines-as-code
204 @item -fd-lines-as-comments
205 @opindex @code{fd-lines-as-code}
206 @opindex @code{fd-lines-as-comments}
207 Enable special treatment for lines beginning with @code{d} or @code{D}
208 in fixed form sources. If the @option{-fd-lines-as-code} option is
209 given they are treated as if the first column contained a blank. If the
210 @option{-fd-lines-as-comments} option is given, they are treated as
213 @item -fdefault-double-8
214 @opindex @code{fdefault-double-8}
215 Set the @code{DOUBLE PRECISION} type to an 8 byte wide type.
217 @item -fdefault-integer-8
218 @opindex @code{fdefault-integer-8}
219 Set the default integer and logical types to an 8 byte wide type.
220 Do nothing if this is already the default.
222 @item -fdefault-real-8
223 @opindex @code{fdefault-real-8}
224 Set the default real type to an 8 byte wide type.
225 Do nothing if this is already the default.
228 @opindex @code{fdollar-ok}
231 @cindex character set
232 Allow @samp{$} as a valid character in a symbol name.
235 @opindex @code{fno-backslash}
237 @cindex escape characters
238 Change the interpretation of backslashes in string literals from
239 ``C-style'' escape characters to a single backslash character.
241 @item -ffixed-line-length-@var{n}
242 @opindex @code{ffixed-line-length-}@var{n}
243 @cindex file format, fixed
244 Set column after which characters are ignored in typical fixed-form
245 lines in the source file, and through which spaces are assumed (as
246 if padded to that length) after the ends of short fixed-form lines.
248 Popular values for @var{n} include 72 (the
249 standard and the default), 80 (card image), and 132 (corresponding
250 to ``extended-source'' options in some popular compilers).
251 @var{n} may also be @samp{none}, meaning that the entire line is meaningful
252 and that continued character constants never have implicit spaces appended
253 to them to fill out the line.
254 @option{-ffixed-line-length-0} means the same thing as
255 @option{-ffixed-line-length-none}.
257 @item -ffree-line-length-@var{n}
258 @opindex @code{ffree-line-length-}@var{n}
259 @cindex file format, free
260 Set column after which characters are ignored in typical free-form
261 lines in the source file. The default value is 132.
262 @var{n} may be @samp{none}, meaning that the entire line is meaningful.
263 @option{-ffree-line-length-0} means the same thing as
264 @option{-ffree-line-length-none}.
266 @item -fmax-identifier-length=@var{n}
267 @opindex @code{fmax-identifier-length=}@var{n}
268 Specify the maximum allowed identifier length. Typical values are
269 31 (Fortran 95) and 63 (Fortran 2003).
271 @item -fimplicit-none
272 @opindex @code{fimplicit-none}
273 Specify that no implicit typing is allowed, unless overridden by explicit
274 @code{IMPLICIT} statements. This is the equivalent of adding
275 @code{implicit none} to the start of every procedure.
278 @opindex @code{fcray-pointer}
279 Enable the Cray pointer extension, which provides C-like pointer
283 @opindex @code{fopenmp}
285 Enable the OpenMP extensions. This includes OpenMP @code{!$omp} directives
287 and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,
288 @code{!$} conditional compilation sentinels in free form
289 and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form,
290 and when linking arranges for the OpenMP runtime library to be linked
294 @opindex @code{frange-check}
295 Enable range checking on results of simplification of constant
296 expressions during compilation. For example, by default, GNU Fortran
297 will give an overflow error at compile time when simplifying @code{a =
298 EXP(1000)}. With @option{-fno-range-check}, no error will be given and
299 the variable @code{a} will be assigned the value @code{+Infinity}.
300 Similarly, @code{DATA i/Z'FFFFFFFF'/} will result in an integer overflow
301 on most systems, but with @option{-fno-range-check} the value will
302 ``wrap around'' and @code{i} will be initialized to @math{-1} instead.
305 @opindex @code{std=}@var{std} option
306 Specify the standard to which the program is expected to conform, which
307 may be one of @samp{f95}, @samp{f2003}, @samp{gnu}, or @samp{legacy}.
308 The default value for @var{std} is @samp{gnu}, which specifies a
309 superset of the Fortran 95 standard that includes all of the extensions
310 supported by GNU Fortran, although warnings will be given for obsolete
311 extensions not recommended for use in new code. The @samp{legacy} value
312 is equivalent but without the warnings for obsolete extensions, and may
313 be useful for old non-standard programs. The @samp{f95} and
314 @samp{f2003} values specify strict conformance to the Fortran 95 and
315 Fortran 2003 standards, respectively; errors are given for all
316 extensions beyond the relevant language standard, and warnings are given
317 for the Fortran 77 features that are permitted but obsolescent in later
322 @node Error and Warning Options
323 @section Options to request or suppress errors and warnings
324 @cindex options, warnings
325 @cindex options, errors
326 @cindex warnings, suppressing
327 @cindex messages, error
328 @cindex messages, warning
329 @cindex suppressing warnings
331 Errors are diagnostic messages that report that the GNU Fortran compiler
332 cannot compile the relevant piece of source code. The compiler will
333 continue to process the program in an attempt to report further errors
334 to aid in debugging, but will not produce any compiled output.
336 Warnings are diagnostic messages that report constructions which
337 are not inherently erroneous but which are risky or suggest there is
338 likely to be a bug in the program. Unless @option{-Werror} is specified,
339 they do not prevent compilation of the program.
341 You can request many specific warnings with options beginning @option{-W},
342 for example @option{-Wimplicit} to request warnings on implicit
343 declarations. Each of these specific warning options also has a
344 negative form beginning @option{-Wno-} to turn off warnings;
345 for example, @option{-Wno-implicit}. This manual lists only one of the
346 two forms, whichever is not the default.
348 These options control the amount and kinds of errors and warnings produced
352 @item -fmax-errors-@var{n}
353 @opindex @code{fmax-errors-}@var{n}
354 @cindex errors, limiting
355 Limits the maximum number of error messages to @var{n}, at which point
356 GNU Fortran bails out rather than attempting to continue processing the
357 source code. If @var{n} is 0, there is no limit on the number of error
361 @opindex @code{fsyntax-only}
362 @cindex syntax checking
363 Check the code for syntax errors, but don't do anything beyond that.
366 @opindex @code{pedantic}
367 Issue warnings for uses of extensions to Fortran 95.
368 @option{-pedantic} also applies to C-language constructs where they
369 occur in GNU Fortran source files, such as use of @samp{\e} in a
370 character constant within a directive like @code{#include}.
372 Valid Fortran 95 programs should compile properly with or without
374 However, without this option, certain GNU extensions and traditional
375 Fortran features are supported as well.
376 With this option, many of them are rejected.
378 Some users try to use @option{-pedantic} to check programs for conformance.
379 They soon find that it does not do quite what they want---it finds some
380 nonstandard practices, but not all.
381 However, improvements to GNU Fortran in this area are welcome.
383 This should be used in conjunction with @option{-std=f95} or
386 @item -pedantic-errors
387 @opindex @code{pedantic-errors}
388 Like @option{-pedantic}, except that errors are produced rather than
394 @cindex warnings, all
395 Enables commonly used warning options pertaining to usage that
396 we recommend avoiding and that we believe are easy to avoid.
397 This currently includes @option{-Waliasing},
398 @option{-Wampersand}, @option{-Wsurprising}, @option{-Wnonstd-intrinsics},
399 @option{-Wno-tabs}, and @option{-Wline-truncation}.
402 @opindex @code{Waliasing}
404 @cindex warnings, aliasing
405 Warn about possible aliasing of dummy arguments. Specifically, it warns
406 if the same actual argument is associated with a dummy argument with
407 @code{INTENT(IN)} and a dummy argument with @code{INTENT(OUT)} in a call
408 with an explicit interface.
410 The following example will trigger the warning.
414 integer, intent(in) :: a
415 integer, intent(out) :: b
424 @opindex @code{Wampersand}
425 @cindex warnings, ampersand
427 Warn about missing ampersand in continued character constants. The warning is
428 given with @option{-Wampersand}, @option{-pedantic}, @option{-std=f95}, and
429 @option{-std=f2003}. Note: With no ampersand given in a continued character
430 constant, GNU Fortran assumes continuation at the first non-comment,
431 non-whitespace character after the ampersand that initiated the continuation.
433 @item -Wcharacter-truncation
434 @opindex @code{Wcharacter-truncation}
435 @cindex warnings, character truncation
436 Warn when a character assignment will truncate the assigned string.
439 @opindex @code{Wconversion}
440 @cindex warnings, conversion
442 Warn about implicit conversions between different types.
444 @item -Wimplicit-interface
445 @opindex @code{Wimplicit-interface}
446 @cindex warnings, implicit interface
447 Warn if a procedure is called without an explicit interface.
448 Note this only checks that an explicit interface is present. It does not
449 check that the declared interfaces are consistent across program units.
451 @item -Wnonstd-intrinsics
452 @opindex @code{Wnonstd-intrinsics}
453 @cindex warnings, non-stdandard intrinsics
454 Warn if the user tries to use an intrinsic that does not belong to the
455 standard the user has chosen via the @option{-std} option.
458 @opindex @code{Wsurprising}
459 @cindex warnings, suspicious code
460 Produce a warning when ``suspicious'' code constructs are encountered.
461 While technically legal these usually indicate that an error has been made.
463 This currently produces a warning under the following circumstances:
467 An INTEGER SELECT construct has a CASE that can never be matched as its
468 lower value is greater than its upper value.
471 A LOGICAL SELECT construct has three CASE statements.
475 @opindex @code{Wtabs}
476 @cindex warnings, tabs
478 By default, tabs are accepted as whitespace, but tabs are not members
479 of the Fortran Character Set. @option{-Wno-tabs} will cause a warning
480 to be issued if a tab is encountered. Note, @option{-Wno-tabs} is active
481 for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003}, and
485 @opindex @code{Wunderflow}
486 @cindex warnings, underflow
488 Produce a warning when numerical constant expressions are
489 encountered, which yield an UNDERFLOW during compilation.
492 @opindex @code{Werror}
493 @cindex warnings, to errors
494 Turns all warnings into errors.
497 @xref{Error and Warning Options,,Options to Request or Suppress Errors and
498 Warnings, gcc,Using the GNU Compiler Collection (GCC)}, for information on
499 more options offered by the GBE shared by @command{gfortran}, @command{gcc}
500 and other GNU compilers.
502 Some of these have no effect when compiling programs written in Fortran.
504 @node Debugging Options
505 @section Options for debugging your program or GNU Fortran
506 @cindex options, debugging
507 @cindex debugging information options
509 GNU Fortran has various special options that are used for debugging
510 either your program or the GNU Fortran compiler.
513 @item -fdump-parse-tree
514 @opindex @code{fdump-parse-tree}
515 Output the internal parse tree before starting code generation. Only
516 really useful for debugging the GNU Fortran compiler itself.
518 @item -ffpe-trap=@var{list}
519 @opindex @code{ffpe-trap=}@var{list}
520 Specify a list of IEEE exceptions when a Floating Point Exception
521 (FPE) should be raised. On most systems, this will result in a SIGFPE
522 signal being sent and the program being interrupted, producing a core
523 file useful for debugging. @var{list} is a (possibly empty) comma-separated
524 list of the following IEEE exceptions: @samp{invalid} (invalid floating
525 point operation, such as @code{SQRT(-1.0)}), @samp{zero} (division by
526 zero), @samp{overflow} (overflow in a floating point operation),
527 @samp{underflow} (underflow in a floating point operation),
528 @samp{precision} (loss of precision during operation) and @samp{denormal}
529 (operation produced a denormal value).
532 @opindex @code{fbacktrace}
535 Specify that, when a runtime error is encountered, the Fortran runtime
536 library should output a backtrace of the error. This option
537 only has influence for compilation of the Fortran main program.
541 @opindex @code{fdump-core}
542 Request that a core-dump file is written to disk when a runtime error
543 is encountered on systems that support core dumps. This option is
544 only effective for the compilation of the Fortran main program.
547 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
548 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
551 @node Directory Options
552 @section Options for directory search
553 @cindex directory, options
554 @cindex options, directory search
556 @cindex INCLUDE directive
557 @cindex directive, INCLUDE
558 These options affect how GNU Fortran searches
559 for files specified by the @code{INCLUDE} directive and where it searches
560 for previously compiled modules.
562 It also affects the search paths used by @command{cpp} when used to preprocess
567 @opindex @code{I}@var{dir}
568 @cindex directory, search paths for inclusion
569 @cindex inclusion, directory search paths for
570 @cindex search paths, for included files
571 @cindex paths, search
572 @cindex module search path
573 These affect interpretation of the @code{INCLUDE} directive
574 (as well as of the @code{#include} directive of the @command{cpp}
577 Also note that the general behavior of @option{-I} and
578 @code{INCLUDE} is pretty much the same as of @option{-I} with
579 @code{#include} in the @command{cpp} preprocessor, with regard to
580 looking for @file{header.gcc} files and other such things.
582 This path is also used to search for @file{.mod} files when previously
583 compiled modules are required by a @code{USE} statement.
585 @xref{Directory Options,,Options for Directory Search,
586 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
591 @opindex @code{M}@var{dir}
592 @opindex @code{J}@var{dir}
593 @cindex paths, search
594 @cindex module search path
595 This option specifies where to put @file{.mod} files for compiled modules.
596 It is also added to the list of directories to searched by an @code{USE}
599 The default is the current directory.
601 @option{-J} is an alias for @option{-M} to avoid conflicts with existing
604 @item -fintrinsic-modules-path @var{dir}
605 @opindex @code{fintrinsic-modules-path} @var{dir}
606 @cindex paths, search
607 @cindex module search path
608 This option specifies the location of pre-compiled intrinsic modules, if
609 they are not in the default location expected by the compiler.
613 @section Influencing the linking step
614 @cindex options, linking
615 @cindex linking, static
617 These options come into play when the compiler links object files into an
618 executable output file. They are meaningless if the compiler is not doing
622 @item -static-libgfortran
623 @opindex @code{static-libgfortran}
624 On systems that provide @file{libgfortran} as a shared and a static
625 library, this option forces the use of the static version. If no
626 shared version of @file{libgfortran} was built when the compiler was
627 configured, this option has no effect.
631 @node Runtime Options
632 @section Influencing runtime behavior
633 @cindex options, runtime
635 These options affect the runtime behavior of programs compiled with GNU Fortran.
637 @item -fconvert=@var{conversion}
638 @opindex @code{fconvert=}@var{conversion}
639 Specify the representation of data for unformatted files. Valid
640 values for conversion are: @samp{native}, the default; @samp{swap},
641 swap between big- and little-endian; @samp{big-endian}, use big-endian
642 representation for unformatted files; @samp{little-endian}, use little-endian
643 representation for unformatted files.
645 @emph{This option has an effect only when used in the main program.
646 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
647 variable override the default specified by @option{-fconvert}.}
649 @item -frecord-marker=@var{length}
650 @opindex @code{frecord-marker=}@var{length}
651 Specify the length of record markers for unformatted files.
652 Valid values for @var{length} are 4 and 8. Default is 4.
653 @emph{This is different from previous versions of @command{gfortran}},
654 which specified a default record marker length of 8 on most
655 systems. If you want to read or write files compatible
656 with earlier versions of @command{gfortran}, use @option{-frecord-marker=8}.
658 @item -fmax-subrecord-length=@var{length}
659 @opindex @code{fmax-subrecord-length=}@var{length}
660 Specify the maximum length for a subrecord. The maximum permitted
661 value for length is 2147483639, which is also the default. Only
662 really useful for use by the gfortran testsuite.
665 @node Code Gen Options
666 @section Options for code generation conventions
667 @cindex code generation, conventions
668 @cindex options, code generation
669 @cindex options, run-time
671 These machine-independent options control the interface conventions
672 used in code generation.
674 Most of them have both positive and negative forms; the negative form
675 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
676 one of the forms is listed---the one which is not the default. You
677 can figure out the other form by either removing @option{no-} or adding
682 @opindex @code{fno-automatic}
683 @cindex @code{SAVE} statement
684 @cindex statement, @code{SAVE}
685 Treat each program unit as if the @code{SAVE} statement was specified for
686 every local variable and array referenced in it. Does not affect common
687 blocks. (Some Fortran compilers provide this option under the name
692 @cindex calling convention
693 @cindex @command{f2c} calling convention
694 @cindex @command{g77} calling convention
695 @cindex libf2c calling convention
696 Generate code designed to be compatible with code generated
697 by @command{g77} and @command{f2c}.
699 The calling conventions used by @command{g77} (originally implemented
700 in @command{f2c}) require functions that return type
701 default @code{REAL} to actually return the C type @code{double}, and
702 functions that return type @code{COMPLEX} to return the values via an
703 extra argument in the calling sequence that points to where to
704 store the return value. Under the default GNU calling conventions, such
705 functions simply return their results as they would in GNU
706 C---default @code{REAL} functions return the C type @code{float}, and
707 @code{COMPLEX} functions return the GNU C type @code{complex}.
708 Additionally, this option implies the @option{-fsecond-underscore}
709 option, unless @option{-fno-second-underscore} is explicitly requested.
711 This does not affect the generation of code that interfaces with
712 the @command{libgfortran} library.
714 @emph{Caution:} It is not a good idea to mix Fortran code compiled with
715 @option{-ff2c} with code compiled with the default @option{-fno-f2c}
716 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
717 functions between program parts which were compiled with different
718 calling conventions will break at execution time.
720 @emph{Caution:} This will break code which passes intrinsic functions
721 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
722 the library implementations use the @option{-fno-f2c} calling conventions.
724 @item -fno-underscoring
725 @opindex @code{fno-underscoring}
727 @cindex symbol names, underscores
728 @cindex transforming symbol names
729 @cindex symbol names, transforming
730 Do not transform names of entities specified in the Fortran
731 source file by appending underscores to them.
733 With @option{-funderscoring} in effect, GNU Fortran appends one
734 underscore to external names with no underscores. This is done to ensure
735 compatibility with code produced by many UNIX Fortran compilers.
737 @emph{Caution}: The default behavior of GNU Fortran is
738 incompatible with @command{f2c} and @command{g77}, please use the
739 @option{-ff2c} option if you want object files compiled with
740 GNU Fortran to be compatible with object code created with these
743 Use of @option{-fno-underscoring} is not recommended unless you are
744 experimenting with issues such as integration of GNU Fortran into
745 existing system environments (vis-a-vis existing libraries, tools, and
748 For example, with @option{-funderscoring}, and assuming other defaults like
749 @option{-fcase-lower} and that @code{j()} and @code{max_count()} are
750 external functions while @code{my_var} and @code{lvar} are local variables,
753 I = J() + MAX_COUNT (MY_VAR, LVAR)
756 is implemented as something akin to:
758 i = j_() + max_count__(&my_var__, &lvar);
761 With @option{-fno-underscoring}, the same statement is implemented as:
764 i = j() + max_count(&my_var, &lvar);
767 Use of @option{-fno-underscoring} allows direct specification of
768 user-defined names while debugging and when interfacing GNU Fortran
769 code with other languages.
771 Note that just because the names match does @emph{not} mean that the
772 interface implemented by GNU Fortran for an external name matches the
773 interface implemented by some other language for that same name.
774 That is, getting code produced by GNU Fortran to link to code produced
775 by some other compiler using this or any other method can be only a
776 small part of the overall solution---getting the code generated by
777 both compilers to agree on issues other than naming can require
778 significant effort, and, unlike naming disagreements, linkers normally
779 cannot detect disagreements in these other areas.
781 Also, note that with @option{-fno-underscoring}, the lack of appended
782 underscores introduces the very real possibility that a user-defined
783 external name will conflict with a name in a system library, which
784 could make finding unresolved-reference bugs quite difficult in some
785 cases---they might occur at program run time, and show up only as
786 buggy behavior at run time.
788 In future versions of GNU Fortran we hope to improve naming and linking
789 issues so that debugging always involves using the names as they appear
790 in the source, even if the names as seen by the linker are mangled to
791 prevent accidental linking between procedures with incompatible
794 @item -fsecond-underscore
795 @opindex @code{fsecond-underscore}
797 @cindex symbol names, underscores
798 @cindex transforming symbol names
799 @cindex symbol names, transforming
800 @cindex @command{f2c} calling convention
801 @cindex @command{g77} calling convention
802 @cindex libf2c calling convention
803 By default, GNU Fortran appends an underscore to external
804 names. If this option is used GNU Fortran appends two
805 underscores to names with underscores and one underscore to external names
806 with no underscores. GNU Fortran also appends two underscores to
807 internal names with underscores to avoid naming collisions with external
810 This option has no effect if @option{-fno-underscoring} is
811 in effect. It is implied by the @option{-ff2c} option.
813 Otherwise, with this option, an external name such as @code{MAX_COUNT}
814 is implemented as a reference to the link-time external symbol
815 @code{max_count__}, instead of @code{max_count_}. This is required
816 for compatibility with @command{g77} and @command{f2c}, and is implied
817 by use of the @option{-ff2c} option.
820 @opindex @code{fbounds-check}
821 @cindex array, bounds checking
822 @cindex bounds checking
823 @cindex range checking
824 @cindex subscript checking
825 @cindex checking subscripts
826 Enable generation of run-time checks for array subscripts
827 and against the declared minimum and maximum values. It also
828 checks array indices for assumed and deferred
829 shape arrays against the actual allocated bounds.
831 In the future this may also include other forms of checking, e.g., checking
832 substring references.
834 @item -fmax-stack-var-size=@var{n}
835 @opindex @code{fmax-stack-var-size}
836 This option specifies the size in bytes of the largest array that will be put
839 This option currently only affects local arrays declared with constant
840 bounds, and may not apply to all character variables.
841 Future versions of GNU Fortran may improve this behavior.
843 The default value for @var{n} is 32768.
846 @opindex @code{fpack-derived}
847 @cindex structure packing
848 This option tells GNU Fortran to pack derived type members as closely as
849 possible. Code compiled with this option is likely to be incompatible
850 with code compiled without this option, and may execute slower.
852 @item -frepack-arrays
853 @opindex @code{frepack-arrays}
854 @cindex repacking arrays
855 In some circumstances GNU Fortran may pass assumed shape array
856 sections via a descriptor describing a noncontiguous area of memory.
857 This option adds code to the function prologue to repack the data into
858 a contiguous block at runtime.
860 This should result in faster accesses to the array. However it can introduce
861 significant overhead to the function call, especially when the passed data
865 @opindex @code{fshort-enums}
866 This option is provided for interoperability with C code that was
867 compiled with the @option{-fshort-enums} option. It will make
868 GNU Fortran choose the smallest @code{INTEGER} kind a given
869 enumerator set will fit in, and give all its enumerators this kind.
871 @item -fexternal-blas
872 @opindex @code{fexternal-blas}
873 This option will make @command{gfortran} generate calls to BLAS functions
874 for some matrix operations like @code{MATMUL}, instead of using our own
875 algorithms, if the size of the matrices involved is larger than a given
876 limit (see @option{-fblas-matmul-limit}). This may be profitable if an
877 optimized vendor BLAS library is available. The BLAS library will have
878 to be specified at link time.
880 @item -fblas-matmul-limit=@var{n}
881 @opindex @code{fblas-matmul-limit}
882 Only significant when @option{-fexternal-blas} is in effect.
883 Matrix multiplication of matrices with size larger than (or equal to) @var{n}
884 will be performed by calls to BLAS functions, while others will be
885 handled by @command{gfortran} internal algorithms. If the matrices
886 involved are not square, the size comparison is performed using the
887 geometric mean of the dimensions of the argument and result matrices.
889 The default value for @var{n} is 30.
893 @xref{Code Gen Options,,Options for Code Generation Conventions,
894 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
896 shared by @command{gfortran}, @command{gcc}, and other GNU compilers.
901 @node Environment Variables
902 @section Environment variables affecting @command{gfortran}
903 @cindex environment variable
905 @c man begin ENVIRONMENT
907 The @command{gfortran} compiler currently does not make use of any environment
908 variables to control its operation above and beyond those
909 that affect the operation of @command{gcc}.
911 @xref{Environment Variables,,Environment Variables Affecting GCC,
912 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
915 @xref{Runtime}, for environment variables that affect the
916 run-time behavior of programs compiled with GNU Fortran.