1 @c Copyright (C) 2004, 2005
2 @c Free Software Foundation, Inc.
3 @c This is part of the GFORTRAN manual.
4 @c For copying conditions, see the file gfortran.texi.
8 Copyright @copyright{} 2004, 2005
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 95 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 GFORTRAN
62 @chapter GNU Fortran 95 Command Options
63 @cindex GNU Fortran 95 command options
64 @cindex command options
65 @cindex options, GNU Fortran 95 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 gfortran 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 @cindex negative forms of options
79 All @command{gcc} and @command{gfortran} options
80 are accepted both by @command{gfortran} and by @command{gcc}
81 (as well as any other drivers built at the same time,
82 such as @command{g++}),
83 since adding @command{gfortran} to the @command{gcc} distribution
84 enables acceptance of @command{gfortran} options
85 by all of the relevant drivers.
87 In some cases, options have positive and negative forms;
88 the negative form of @option{-ffoo} would be @option{-fno-foo}.
89 This manual documents only one of these two forms, whichever
90 one is not the default.
94 * Option Summary:: Brief list of all @command{gfortran} options,
96 * Fortran Dialect Options:: Controlling the variant of Fortran language
98 * Warning Options:: How picky should the compiler be?
99 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
100 * Directory Options:: Where to find module files
101 * Code Gen Options:: Specifying conventions for function calls, data layout
103 * Environment Variables:: Env vars that affect GNU Fortran.
107 @section Option Summary
111 Here is a summary of all the options specific to GNU Fortran, grouped
112 by type. Explanations are in the following sections.
115 @item Fortran Language Options
116 @xref{Fortran Dialect Options,,Options Controlling Fortran Dialect}.
118 -ffree-form -fno-fixed-form @gol
119 -fdollar-ok -fimplicit-none -fmax-identifier-length @gol
121 -ffixed-line-length-@var{n} -ffixed-line-length-none @gol
122 -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 }
124 @item Warning Options
125 @xref{Warning Options,,Options to Request or Suppress Warnings}.
127 -fsyntax-only -pedantic -pedantic-errors @gol
128 -w -Wall -Waliasing -Wconversion @gol
129 -Wimplicit-interface -Wnonstd-intrinsics -Wsurprising -Wunderflow @gol
130 -Wunused-labels -Wline-truncation @gol
133 @item Debugging Options
134 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
138 @item Directory Options
139 @xref{Directory Options,,Options for Directory Search}.
141 -I@var{dir} -M@var{dir}}
143 @item Code Generation Options
144 @xref{Code Gen Options,,Options for Code Generation Conventions}.
146 -fno-underscoring -fno-second-underscore @gol
147 -fbounds-check -fmax-stack-var-size=@var{n} @gol
148 -fpackderived -frepack-arrays}
152 * Fortran Dialect Options:: Controlling the variant of Fortran language
154 * Warning Options:: How picky should the compiler be?
155 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
156 * Directory Options:: Where to find module files
157 * Code Gen Options:: Specifying conventions for function calls, data layout
161 @node Fortran Dialect Options
162 @section Options Controlling Fortran Dialect
163 @cindex dialect options
164 @cindex language, dialect options
165 @cindex options, dialect
167 The following options control the dialect of Fortran
168 that the compiler accepts:
171 @cindex -ffree-form option
172 @cindex options, -ffree-form
173 @cindex -fno-fixed-form option
174 @cindex options, -fno-fixed-form
175 @cindex source file format
179 @cindex Fortran 90, features
182 Specify the layout used by the the source file. The free form layout
183 was introduced in Fortran 90. Fixed form was traditionally used in
184 older Fortran programs.
186 @cindex option, -fdefault-double-8
187 @cindex -fdefault-double-8, option
188 @item -fdefault-double-8
189 Set the "DOUBLE PRECISION" type to an 8 byte wide.
191 @cindex option, -fdefault-integer-8
192 @cindex -fdefault-integer-8, option
193 @item -fdefault-integer-8
194 Set the default integer and logical types to an 8 byte wide type.
195 Do nothing if this is already the default.
197 @cindex option, -fdefault-real-8
198 @cindex -fdefault-real-8, option
199 @item -fdefault-real-8
200 Set the default real type to an 8 byte wide type.
201 Do nothing if this is already the default.
203 @cindex -fdollar-ok option
204 @cindex options, -fdollar-ok
208 @cindex character set
209 Allow @samp{$} as a valid character in a symbol name.
211 @cindex -ffixed-line-length-@var{n} option
212 @cindex options, -ffixed-line-length-@var{n}
213 @item -ffixed-line-length-@var{n}
214 @cindex source file format
215 @cindex lines, length
216 @cindex length of source lines
218 @cindex limits, lengths of source lines
219 Set column after which characters are ignored in typical fixed-form
220 lines in the source file, and through which spaces are assumed (as
221 if padded to that length) after the ends of short fixed-form lines.
224 @cindex extended-source option
225 Popular values for @var{n} include 72 (the
226 standard and the default), 80 (card image), and 132 (corresponds
227 to ``extended-source'' options in some popular compilers).
228 @var{n} may be @samp{none}, meaning that the entire line is meaningful
229 and that continued character constants never have implicit spaces appended
230 to them to fill out the line.
231 @option{-ffixed-line-length-0} means the same thing as
232 @option{-ffixed-line-length-none}.
234 @cindex -fmax-identifier-length=@var{n} option
235 @cindex option -fmax-identifier-length=@var{n}
236 @item -fmax-identifier-length=@var{n}
237 Specify the maximum allowed identifier length. Typical values are
238 31 (Fortran 95) and 63 (Fortran 200x).
240 @cindex -fimplicit-none option
241 @cindex options, -fimplicit-none
242 @item -fimplicit-none
243 Specify that no implicit typing is allowed, unless overridden by explicit
244 @samp{IMPLICIT} statements. This is the equivalent of adding
245 @samp{implicit none} to the start of every procedure.
247 @cindex -std=@var{std} option
248 @cindex option, -std=@var{std}
250 Conform to the specified standard. Allowed values for @var{std} are
251 @samp{gnu} and @samp{f95}.
255 @node Warning Options
256 @section Options to Request or Suppress Warnings
257 @cindex options, warnings
258 @cindex warnings, suppressing
259 @cindex messages, warning
260 @cindex suppressing warnings
262 Warnings are diagnostic messages that report constructions which
263 are not inherently erroneous but which are risky or suggest there
264 might have been an error.
266 You can request many specific warnings with options beginning @option{-W},
267 for example @option{-Wimplicit} to request warnings on implicit
268 declarations. Each of these specific warning options also has a
269 negative form beginning @option{-Wno-} to turn off warnings;
270 for example, @option{-Wno-implicit}. This manual lists only one of the
271 two forms, whichever is not the default.
273 These options control the amount and kinds of warnings produced by GNU
277 @cindex syntax checking
278 @cindex -fsyntax-only option
279 @cindex options, -fsyntax-only
281 Check the code for syntax errors, but don't do anything beyond that.
283 @cindex -pedantic option
284 @cindex options, -pedantic
286 Issue warnings for uses of extensions to FORTRAN 95.
287 @option{-pedantic} also applies to C-language constructs where they
288 occur in GNU Fortran source files, such as use of @samp{\e} in a
289 character constant within a directive like @samp{#include}.
291 Valid FORTRAN 95 programs should compile properly with or without
293 However, without this option, certain GNU extensions and traditional
294 Fortran features are supported as well.
295 With this option, many of them are rejected.
297 Some users try to use @option{-pedantic} to check programs for conformance.
298 They soon find that it does not do quite what they want---it finds some
299 nonstandard practices, but not all.
300 However, improvements to @command{gfortran} in this area are welcome.
302 This should be used in conjunction with -std=@var{std}.
304 @cindex -pedantic-errors option
305 @cindex options, -pedantic-errors
306 @item -pedantic-errors
307 Like @option{-pedantic}, except that errors are produced rather than
313 Inhibit all warning messages.
317 @cindex options, -Wall
320 @cindex warnings, all
321 Enables commonly used warning options that which pertain to usage that
322 we recommend avoiding and that we believe is easy to avoid.
323 This currently includes @option{-Wunused-labels}, @option{-Waliasing},
324 @option{-Wsurprising}, @option{-Wnonstd-intrinsic} and
325 @option{-Wline-truncation}.
328 @cindex -Waliasing option
329 @cindex options, -Waliasing
332 Warn about possible aliasing of dummy arguments. Specifically, it warns
333 if the same actual argument is associated with a dummy argument with
334 @code{intent(in)} and a dummy argument with @code{intent(out)} in a call
335 with an explicit interface.
337 The following example will trigger the warning.
341 integer, intent(in) :: a
342 integer, intent(out) :: b
351 @cindex -Wconversion option
352 @cindex options, -Wconversion
355 Warn about implicit conversions between different types.
358 @cindex -Wimplicit-interface option
359 @cindex options, -Wimplicit-interface
360 @item -Wimplicit-interface
361 Warn about when procedure are called without an explicit interface.
362 Note this only checks that an explicit interface is present. It does not
363 check that the declared interfaces are consistent across program units.
366 @cindex -Wnonstd-intrinsic option
367 @cindex options, -Wnonstd-intrinsic
368 @item -Wnonstd-intrinsic
369 Warn if the user tries to use an intrinsic that does not belong to the
370 standard the user has chosen via the -std option.
374 @cindex options, -Wsurprising
377 Produce a warning when ``suspicious'' code constructs are encountered.
378 While technically legal these usually indicate that an error has been made.
380 This currently produces a warning under the following circumstances:
384 An INTEGER SELECT construct has a CASE that can never be matched as its
385 lower value is greater than its upper value.
388 A LOGICAL SELECT construct has three CASE statements.
392 @cindex options, -Wunderflow
395 Produce a warning when numerical constant expressions are
396 encountered, which yield an UNDERFLOW during compilation.
399 @cindex -Wunused-labels option
400 @cindex options, -Wunused-labels
401 @item -Wunused-labels
402 @cindex unused labels
403 @cindex labels, unused
404 Warn whenever a label is defined but never referenced.
408 @cindex options, -Werror
410 Turns all warnings into errors.
416 @cindex extra warnings
417 @cindex warnings, extra
418 Turns on ``extra warnings'' and, if optimization is specified
419 via @option{-O}, the @option{-Wuninitialized} option.
420 (This might change in future versions of @command{gfortran}
423 @xref{Warning Options,,Options to Request or Suppress Warnings,
424 gcc,Using the GNU Compiler Collection (GCC)}, for information on more
425 options offered by the GBE shared by @command{gfortran}, @command{gcc} and
428 Some of these have no effect when compiling programs written in Fortran.
430 @node Debugging Options
431 @section Options for Debugging Your Program or GNU Fortran
432 @cindex options, debugging
433 @cindex debugging information options
435 GNU Fortran has various special options that are used for debugging
436 either your program or @command{gfortran}
439 @cindex -fdump-parse-tree option
440 @cindex option, -fdump-parse-tree
441 @item -fdump-parse-tree
442 Output the internal parse tree before starting code generation. Only
443 really useful for debugging gfortran itself.
446 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
447 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
450 @node Directory Options
451 @section Options for Directory Search
452 @cindex directory, options
453 @cindex options, directory search
456 @cindex INCLUDE directive
457 @cindex directive, INCLUDE
458 There options affect how affect how @command{gfortran} searches
459 for files specified via the @code{INCLUDE} directive, and where it searches
460 for previously compiled modules.
462 It also affects the search paths used by @command{cpp} when used to preprocess
467 @cindex options, -Idir
469 @cindex directory, search paths for inclusion
470 @cindex inclusion, directory search paths for
471 @cindex search paths, for included files
472 @cindex paths, search
473 @cindex module search path
474 These affect interpretation of the @code{INCLUDE} directive
475 (as well as of the @code{#include} directive of the @command{cpp}
478 Also note that the general behavior of @option{-I} and
479 @code{INCLUDE} is pretty much the same as of @option{-I} with
480 @code{#include} in the @command{cpp} preprocessor, with regard to
481 looking for @file{header.gcc} files and other such things.
483 This path is also used to search for @samp{.mod} files when previously
484 compiled modules are required by a @code{USE} statement.
486 @xref{Directory Options,,Options for Directory Search,
487 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
491 @cindex option, -Mdir
494 This option specifies where to put @samp{.mod} files for compiled modules.
495 It is also added to the list of directories to searched by an @code{USE}
498 The default is the current directory.
500 @option{-J} is an alias for @option{-M} to avoid conflicts with existing
504 @node Code Gen Options
505 @section Options for Code Generation Conventions
506 @cindex code generation, conventions
507 @cindex options, code generation
508 @cindex run-time, options
510 These machine-independent options control the interface conventions
511 used in code generation.
513 Most of them have both positive and negative forms; the negative form
514 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
515 one of the forms is listed---the one which is not the default. You
516 can figure out the other form by either removing @option{no-} or adding
521 @cindex -fno-underscoring option
522 @cindex options, -fno-underscoring
523 @item -fno-underscoring
525 @cindex symbol names, underscores
526 @cindex transforming symbol names
527 @cindex symbol names, transforming
528 Do not transform names of entities specified in the Fortran
529 source file by appending underscores to them.
531 With @option{-funderscoring} in effect, @command{gfortran} appends two
532 underscores to names with underscores and one underscore to external names
533 with no underscores. (@command{gfortran} also appends two underscores to
534 internal names with underscores to avoid naming collisions with external
535 names. The @option{-fno-second-underscore} option disables appending of the
536 second underscore in all cases.)
538 This is done to ensure compatibility with code produced by many
539 UNIX Fortran compilers, including @command{f2c} which perform the
540 same transformations.
542 Use of @option{-fno-underscoring} is not recommended unless you are
543 experimenting with issues such as integration of (GNU) Fortran into
544 existing system environments (vis-a-vis existing libraries, tools, and
547 For example, with @option{-funderscoring}, and assuming other defaults like
548 @option{-fcase-lower} and that @samp{j()} and @samp{max_count()} are
549 external functions while @samp{my_var} and @samp{lvar} are local variables,
553 I = J() + MAX_COUNT (MY_VAR, LVAR)
557 is implemented as something akin to:
560 i = j_() + max_count__(&my_var__, &lvar);
563 With @option{-fno-underscoring}, the same statement is implemented as:
566 i = j() + max_count(&my_var, &lvar);
569 Use of @option{-fno-underscoring} allows direct specification of
570 user-defined names while debugging and when interfacing @command{gfortran}
571 code with other languages.
573 Note that just because the names match does @emph{not} mean that the
574 interface implemented by @command{gfortran} for an external name matches the
575 interface implemented by some other language for that same name.
576 That is, getting code produced by @command{gfortran} to link to code produced
577 by some other compiler using this or any other method can be only a
578 small part of the overall solution---getting the code generated by
579 both compilers to agree on issues other than naming can require
580 significant effort, and, unlike naming disagreements, linkers normally
581 cannot detect disagreements in these other areas.
583 Also, note that with @option{-fno-underscoring}, the lack of appended
584 underscores introduces the very real possibility that a user-defined
585 external name will conflict with a name in a system library, which
586 could make finding unresolved-reference bugs quite difficult in some
587 cases---they might occur at program run time, and show up only as
588 buggy behavior at run time.
590 In future versions of @command{gfortran} we hope to improve naming and linking
591 issues so that debugging always involves using the names as they appear
592 in the source, even if the names as seen by the linker are mangled to
593 prevent accidental linking between procedures with incompatible
596 @cindex -fno-second-underscore option
597 @cindex options, -fno-second-underscore
598 @item -fno-second-underscore
600 @cindex symbol names, underscores
601 @cindex transforming symbol names
602 @cindex symbol names, transforming
603 Do not append a second underscore to names of entities specified
604 in the Fortran source file.
606 This option has no effect if @option{-fno-underscoring} is
609 Otherwise, with this option, an external name such as @samp{MAX_COUNT}
610 is implemented as a reference to the link-time external symbol
611 @samp{max_count_}, instead of @samp{max_count__}.
614 @cindex -fbounds-check option
615 @cindex -ffortran-bounds-check option
617 @cindex bounds checking
618 @cindex range checking
619 @cindex array bounds checking
620 @cindex subscript checking
621 @cindex checking subscripts
622 Enable generation of run-time checks for array subscripts
623 and against the declared minimum and maximum values. It also
624 checks array indices for assumed and deferred
625 shape arrays against the actual allocated bounds.
627 In the future this may also include other forms of checking, eg. checking
628 substring references.
631 @cindex -fmax-stack-var-size option
632 @item -fmax-stack-var-size=@var{n}
633 This option specifies the size in bytes of the largest array that will be put
636 This option currently only affects local arrays declared with constant
637 bounds, and may not apply to all character variables.
638 Future versions of @command{gfortran} may improve this behavior.
640 The default value for @var{n} is 32768.
642 @cindex -fpackderived
644 @cindex Structure packing
645 This option tells gfortran to pack derived type members as closely as
646 possible. Code compiled with this option is likely to be incompatible
647 with code compiled without this option, and may execute slower.
649 @cindex -frepack-arrays option
650 @item -frepack-arrays
651 @cindex Repacking arrays
652 In some circumstances @command{gfortran} may pass assumed shape array
653 sections via a descriptor describing a discontiguous area of memory.
654 This option adds code to the function prologue to repack the data into
655 a contiguous block at runtime.
657 This should result in faster accesses to the array. However it can introduce
658 significant overhead to the function call, especially when the passed data
662 @xref{Code Gen Options,,Options for Code Generation Conventions,
663 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
665 shared by @command{gfortran} @command{gcc} and other GNU compilers.
670 @node Environment Variables
671 @section Environment Variables Affecting GNU Fortran
672 @cindex environment variables
674 @c man begin ENVIRONMENT
676 GNU Fortran 95 currently does not make use of any environment
677 variables to control its operation above and beyond those
678 that affect the operation of @command{gcc}.
680 @xref{Environment Variables,,Environment Variables Affecting GCC,
681 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment