1 @c Copyright (C) 2004, 2005, 2006
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, 2006
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 * Runtime Options:: Influencing runtime behavior
102 * Code Gen Options:: Specifying conventions for function calls, data layout
104 * Environment Variables:: Env vars that affect GNU Fortran.
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}.
119 -ffree-form -fno-fixed-form @gol
120 -fdollar-ok -fimplicit-none -fmax-identifier-length @gol
121 -std=@var{std} -fd-lines-as-code -fd-lines-as-comments @gol
122 -ffixed-line-length-@var{n} -ffixed-line-length-none @gol
123 -ffree-line-length-@var{n} -ffree-line-length-none @gol
124 -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 @gol
125 -fcray-pointer -fopenmp }
127 @item Warning Options
128 @xref{Warning Options,,Options to Request or Suppress Warnings}.
130 -fsyntax-only -pedantic -pedantic-errors @gol
131 -w -Wall -Waliasing -Wconversion -Wimplicit-interface @gol
132 -Wtabs -Wnonstd-intrinsics -Wsurprising -Wunderflow @gol
133 -Wunused-labels -Wline-truncation -W}
135 @item Debugging Options
136 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
138 -fdump-parse-tree -ffpe-trap=@var{list}}
140 @item Directory Options
141 @xref{Directory Options,,Options for Directory Search}.
143 -I@var{dir} -M@var{dir}}
145 @item Runtime Options
146 @xref{Runtime Options,,Options for influencing runtime behavior}.
148 -fconvert=@var{conversion}}
150 @item Code Generation Options
151 @xref{Code Gen Options,,Options for Code Generation Conventions}.
153 -fno-automatic -ff2c -fno-underscoring -fsecond-underscore @gol
154 -fbounds-check -fmax-stack-var-size=@var{n} @gol
155 -fpackderived -frepack-arrays -fshort-enums}
159 * Fortran Dialect Options:: Controlling the variant of Fortran language
161 * Warning Options:: How picky should the compiler be?
162 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
163 * Directory Options:: Where to find module files
164 * Runtime Options:: Influencing runtime behavior
165 * Code Gen Options:: Specifying conventions for function calls, data layout
169 @node Fortran Dialect Options
170 @section Options Controlling Fortran Dialect
171 @cindex dialect options
172 @cindex language, dialect options
173 @cindex options, dialect
175 The following options control the dialect of Fortran
176 that the compiler accepts:
179 @cindex -ffree-form option
180 @cindex options, -ffree-form
181 @cindex -fno-fixed-form option
182 @cindex options, -fno-fixed-form
183 @cindex source file format
187 @cindex Fortran 90, features
190 Specify the layout used by the source file. The free form layout
191 was introduced in Fortran 90. Fixed form was traditionally used in
192 older Fortran programs.
194 @cindex option, -fd-lines-as-code
195 @cindex -fd-lines-as-code, option
196 @cindex option, -fd-lines-as-comments
197 @cindex -fd-lines-as-comments, option
198 @item -fd-lines-as-code
199 @item -fd-lines-as-comment
200 Enables special treating for lines with @samp{d} or @samp{D} in fixed
201 form sources. If the @option{-fd-lines-as-code} option is given
202 they are treated as if the first column contained a blank. If the
203 @option{-fd-lines-as-comments} option is given, they are treated as
206 @cindex option, -fdefault-double-8
207 @cindex -fdefault-double-8, option
208 @item -fdefault-double-8
209 Set the "DOUBLE PRECISION" type to an 8 byte wide.
211 @cindex option, -fdefault-integer-8
212 @cindex -fdefault-integer-8, option
213 @item -fdefault-integer-8
214 Set the default integer and logical types to an 8 byte wide type.
215 Do nothing if this is already the default.
217 @cindex option, -fdefault-real-8
218 @cindex -fdefault-real-8, option
219 @item -fdefault-real-8
220 Set the default real type to an 8 byte wide type.
221 Do nothing if this is already the default.
223 @cindex -fdollar-ok option
224 @cindex options, -fdollar-ok
228 @cindex character set
229 Allow @samp{$} as a valid character in a symbol name.
231 @cindex -fno-backslash option
232 @cindex options, -fno-backslash
235 @cindex escape characters
236 Compile switch to change the interpretation of a backslash from
237 ``C''-style escape characters to a single backslash character.
239 @cindex -ffixed-line-length-@var{n} option
240 @cindex options, -ffixed-line-length-@var{n}
241 @item -ffixed-line-length-@var{n}
242 @cindex source file format
243 @cindex lines, length
244 @cindex length of source lines
246 @cindex limits, lengths of source lines
247 Set column after which characters are ignored in typical fixed-form
248 lines in the source file, and through which spaces are assumed (as
249 if padded to that length) after the ends of short fixed-form lines.
252 @cindex extended-source option
253 Popular values for @var{n} include 72 (the
254 standard and the default), 80 (card image), and 132 (corresponds
255 to ``extended-source'' options in some popular compilers).
256 @var{n} may be @samp{none}, meaning that the entire line is meaningful
257 and that continued character constants never have implicit spaces appended
258 to them to fill out the line.
259 @option{-ffixed-line-length-0} means the same thing as
260 @option{-ffixed-line-length-none}.
262 @cindex -ffree-line-length-@var{n} option
263 @cindex options, -ffree-line-length-@var{n}
264 @item -ffree-line-length-@var{n}
265 @cindex source file format
266 @cindex lines, length
267 @cindex length of source lines
269 @cindex limits, lengths of source lines
270 Set column after which characters are ignored in typical free-form
271 lines in the source file. For free-form, the default value is 132.
272 @var{n} may be @samp{none}, meaning that the entire line is meaningful.
273 @option{-ffree-line-length-0} means the same thing as
274 @option{-ffree-line-length-none}.
276 @cindex -fmax-identifier-length=@var{n} option
277 @cindex option -fmax-identifier-length=@var{n}
278 @item -fmax-identifier-length=@var{n}
279 Specify the maximum allowed identifier length. Typical values are
280 31 (Fortran 95) and 63 (Fortran 200x).
282 @cindex -fimplicit-none option
283 @cindex options, -fimplicit-none
284 @item -fimplicit-none
285 Specify that no implicit typing is allowed, unless overridden by explicit
286 @samp{IMPLICIT} statements. This is the equivalent of adding
287 @samp{implicit none} to the start of every procedure.
289 @cindex -fcray-pointer option
290 @cindex options, -fcray-pointer
292 Enables the Cray pointer extension, which provides a C-like pointer.
295 @cindex options, -fopenmp
297 Enables handling of OpenMP @code{!$omp} directives in free form
298 and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,
299 enables @code{!$} conditional compilation sentinels in free form
300 and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form
301 and when linking arranges for the OpenMP runtime library to be linked
304 @cindex -std=@var{std} option
305 @cindex option, -std=@var{std}
307 Conform to the specified standard. Allowed values for @var{std} are
308 @samp{gnu}, @samp{f95}, @samp{f2003} and @samp{legacy}.
312 @node Warning Options
313 @section Options to Request or Suppress Warnings
314 @cindex options, warnings
315 @cindex warnings, suppressing
316 @cindex messages, warning
317 @cindex suppressing warnings
319 Warnings are diagnostic messages that report constructions which
320 are not inherently erroneous but which are risky or suggest there
321 might have been an error.
323 You can request many specific warnings with options beginning @option{-W},
324 for example @option{-Wimplicit} to request warnings on implicit
325 declarations. Each of these specific warning options also has a
326 negative form beginning @option{-Wno-} to turn off warnings;
327 for example, @option{-Wno-implicit}. This manual lists only one of the
328 two forms, whichever is not the default.
330 These options control the amount and kinds of warnings produced by GNU
334 @cindex syntax checking
335 @cindex -fsyntax-only option
336 @cindex options, -fsyntax-only
338 Check the code for syntax errors, but don't do anything beyond that.
340 @cindex -pedantic option
341 @cindex options, -pedantic
343 Issue warnings for uses of extensions to FORTRAN 95.
344 @option{-pedantic} also applies to C-language constructs where they
345 occur in GNU Fortran source files, such as use of @samp{\e} in a
346 character constant within a directive like @samp{#include}.
348 Valid FORTRAN 95 programs should compile properly with or without
350 However, without this option, certain GNU extensions and traditional
351 Fortran features are supported as well.
352 With this option, many of them are rejected.
354 Some users try to use @option{-pedantic} to check programs for conformance.
355 They soon find that it does not do quite what they want---it finds some
356 nonstandard practices, but not all.
357 However, improvements to @command{gfortran} in this area are welcome.
359 This should be used in conjunction with -std=@var{std}.
361 @cindex -pedantic-errors option
362 @cindex options, -pedantic-errors
363 @item -pedantic-errors
364 Like @option{-pedantic}, except that errors are produced rather than
370 Inhibit all warning messages.
374 @cindex options, -Wall
377 @cindex warnings, all
378 Enables commonly used warning options that which pertain to usage that
379 we recommend avoiding and that we believe is easy to avoid.
380 This currently includes @option{-Wunused-labels}, @option{-Waliasing},
381 @option{-Wsurprising}, @option{-Wnonstd-intrinsic}, @option{-Wno-tabs},
382 and @option{-Wline-truncation}.
385 @cindex -Waliasing option
386 @cindex options, -Waliasing
389 Warn about possible aliasing of dummy arguments. Specifically, it warns
390 if the same actual argument is associated with a dummy argument with
391 @code{intent(in)} and a dummy argument with @code{intent(out)} in a call
392 with an explicit interface.
394 The following example will trigger the warning.
398 integer, intent(in) :: a
399 integer, intent(out) :: b
408 @cindex -Wconversion option
409 @cindex options, -Wconversion
412 Warn about implicit conversions between different types.
415 @cindex -Wimplicit-interface option
416 @cindex options, -Wimplicit-interface
417 @item -Wimplicit-interface
418 Warn about when procedure are called without an explicit interface.
419 Note this only checks that an explicit interface is present. It does not
420 check that the declared interfaces are consistent across program units.
423 @cindex -Wnonstd-intrinsic option
424 @cindex options, -Wnonstd-intrinsic
425 @item -Wnonstd-intrinsic
426 Warn if the user tries to use an intrinsic that does not belong to the
427 standard the user has chosen via the -std option.
431 @cindex options, -Wsurprising
434 Produce a warning when ``suspicious'' code constructs are encountered.
435 While technically legal these usually indicate that an error has been made.
437 This currently produces a warning under the following circumstances:
441 An INTEGER SELECT construct has a CASE that can never be matched as its
442 lower value is greater than its upper value.
445 A LOGICAL SELECT construct has three CASE statements.
449 @cindex options, -Wtabs
452 By default, tabs are accepted as whitespace, but tabs are not members
453 of the Fortran Character Set. @option{-Wno-tabs} will cause a warning
454 to be issued if a tab is encountered. Note, @option{-Wno-tabs} is active
455 for @option{-pedantic}, @option{-std=f95}, and @option{-Wall}.
458 @cindex options, -Wunderflow
461 Produce a warning when numerical constant expressions are
462 encountered, which yield an UNDERFLOW during compilation.
465 @cindex -Wunused-labels option
466 @cindex options, -Wunused-labels
467 @item -Wunused-labels
468 @cindex unused labels
469 @cindex labels, unused
470 Warn whenever a label is defined but never referenced.
474 @cindex options, -Werror
476 Turns all warnings into errors.
482 @cindex extra warnings
483 @cindex warnings, extra
484 Turns on ``extra warnings'' and, if optimization is specified
485 via @option{-O}, the @option{-Wuninitialized} option.
486 (This might change in future versions of @command{gfortran}
489 @xref{Warning Options,,Options to Request or Suppress Warnings,
490 gcc,Using the GNU Compiler Collection (GCC)}, for information on more
491 options offered by the GBE shared by @command{gfortran}, @command{gcc} and
494 Some of these have no effect when compiling programs written in Fortran.
496 @node Debugging Options
497 @section Options for Debugging Your Program or GNU Fortran
498 @cindex options, debugging
499 @cindex debugging information options
501 GNU Fortran has various special options that are used for debugging
502 either your program or @command{gfortran}
505 @cindex -fdump-parse-tree option
506 @cindex option, -fdump-parse-tree
507 @item -fdump-parse-tree
508 Output the internal parse tree before starting code generation. Only
509 really useful for debugging gfortran itself.
513 @cindex -ffpe-trap=@var{list} option
514 @cindex option, -ffpe-trap=@var{list}
515 @item -ffpe-trap=@var{list}
516 Specify a list of IEEE exceptions when a Floating Point Exception
517 (FPE) should be raised. On most systems, this will result in a SIGFPE
518 signal being sent and the program being interrupted, producing a core
519 file useful for debugging. @var{list} is a (possibly empty) comma-separated
520 list of the following IEEE exceptions: @samp{invalid} (invalid floating
521 point operation, such as @code{sqrt(-1.0)}), @samp{zero} (division by
522 zero), @samp{overflow} (overflow in a floating point operation),
523 @samp{underflow} (underflow in a floating point operation),
524 @samp{precision} (loss of precision during operation) and @samp{denormal}
525 (operation produced a denormal denormal value).
528 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
529 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
532 @node Directory Options
533 @section Options for Directory Search
534 @cindex directory, options
535 @cindex options, directory search
538 @cindex INCLUDE directive
539 @cindex directive, INCLUDE
540 These options affect how @command{gfortran} searches
541 for files specified by the @code{INCLUDE} directive and where it searches
542 for previously compiled modules.
544 It also affects the search paths used by @command{cpp} when used to preprocess
549 @cindex options, -Idir
551 @cindex directory, search paths for inclusion
552 @cindex inclusion, directory search paths for
553 @cindex search paths, for included files
554 @cindex paths, search
555 @cindex module search path
556 These affect interpretation of the @code{INCLUDE} directive
557 (as well as of the @code{#include} directive of the @command{cpp}
560 Also note that the general behavior of @option{-I} and
561 @code{INCLUDE} is pretty much the same as of @option{-I} with
562 @code{#include} in the @command{cpp} preprocessor, with regard to
563 looking for @file{header.gcc} files and other such things.
565 This path is also used to search for @samp{.mod} files when previously
566 compiled modules are required by a @code{USE} statement.
568 @xref{Directory Options,,Options for Directory Search,
569 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
573 @cindex option, -Mdir
576 This option specifies where to put @samp{.mod} files for compiled modules.
577 It is also added to the list of directories to searched by an @code{USE}
580 The default is the current directory.
582 @option{-J} is an alias for @option{-M} to avoid conflicts with existing
586 @node Runtime Options
587 @section Influencing runtime behavior
588 @cindex runtime, options
590 These options affect the runtime behavior of @command{gfortran}.
592 @cindex -fconvert=@var{conversion} option
593 @item -fconvert=@var{conversion}
594 Specify the representation of data for unformatted files. Valid
595 values for conversion are: @samp{native}, the default; @samp{swap},
596 swap between big- and little-endian; @samp{big-endian}, use big-endian
597 representation for unformatted files; @samp{little-endian}, use little-endian
598 representation for unformatted files.
600 @emph{This option has an effect only when used in the main program.
601 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
602 variable override the default specified by -fconvert.}
605 @node Code Gen Options
606 @section Options for Code Generation Conventions
607 @cindex code generation, conventions
608 @cindex options, code generation
609 @cindex run-time, options
611 These machine-independent options control the interface conventions
612 used in code generation.
614 Most of them have both positive and negative forms; the negative form
615 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
616 one of the forms is listed---the one which is not the default. You
617 can figure out the other form by either removing @option{no-} or adding
621 @cindex @option{-fno-automatic} option
622 @cindex options, @option{-fno-automatic}
624 @cindex SAVE statement
625 @cindex statements, SAVE
626 Treat each program unit as if the @code{SAVE} statement was specified for
627 every local variable and array referenced in it. Does not affect common
628 blocks. (Some Fortran compilers provide this option under the name
631 @cindex @option{-ff2c} option
632 @cindex options, @option{-ff2c}
634 @cindex calling convention
635 @cindex @command{f2c} calling convention
636 @cindex @command{g77} calling convention
637 @cindex libf2c calling convention
638 Generate code designed to be compatible with code generated
639 by @command{g77} and @command{f2c}.
641 The calling conventions used by @command{g77} (originally implemented
642 in @command{f2c}) require functions that return type
643 default @code{REAL} to actually return the C type @code{double}, and
644 functions that return type @code{COMPLEX} to return the values via an
645 extra argument in the calling sequence that points to where to
646 store the return value. Under the default GNU calling conventions, such
647 functions simply return their results as they would in GNU
648 C -- default @code{REAL} functions return the C type @code{float}, and
649 @code{COMPLEX} functions return the GNU C type @code{complex}.
650 Additionally, this option implies the @option{-fsecond-underscore}
651 option, unless @option{-fno-second-underscore} is explicitly requested.
653 This does not affect the generation of code that interfaces with
654 the @command{libgfortran} library.
656 @emph{Caution:} It is not a good idea to mix Fortran code compiled
657 with @code{-ff2c} with code compiled with the default @code{-fno-f2c}
658 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
659 functions between program parts which were compiled with different
660 calling conventions will break at execution time.
662 @emph{Caution:} This will break code which passes intrinsic functions
663 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
664 the library implementations use the @command{-fno-f2c} calling conventions.
666 @cindex @option{-fno-underscoring option}
667 @cindex options, @option{-fno-underscoring}
668 @item -fno-underscoring
670 @cindex symbol names, underscores
671 @cindex transforming symbol names
672 @cindex symbol names, transforming
673 Do not transform names of entities specified in the Fortran
674 source file by appending underscores to them.
676 With @option{-funderscoring} in effect, @command{gfortran} appends one
677 underscore to external names with no underscores. This is done to ensure
678 compatibility with code produced by many UNIX Fortran compilers.
680 @emph{Caution}: The default behavior of @command{gfortran} is
681 incompatible with @command{f2c} and @command{g77}, please use the
682 @option{-ff2c} option if you want object files compiled with
683 @option{gfortran} to be compatible with object code created with these
686 Use of @option{-fno-underscoring} is not recommended unless you are
687 experimenting with issues such as integration of (GNU) Fortran into
688 existing system environments (vis-a-vis existing libraries, tools, and
691 For example, with @option{-funderscoring}, and assuming other defaults like
692 @option{-fcase-lower} and that @samp{j()} and @samp{max_count()} are
693 external functions while @samp{my_var} and @samp{lvar} are local variables,
697 I = J() + MAX_COUNT (MY_VAR, LVAR)
701 is implemented as something akin to:
704 i = j_() + max_count__(&my_var__, &lvar);
707 With @option{-fno-underscoring}, the same statement is implemented as:
710 i = j() + max_count(&my_var, &lvar);
713 Use of @option{-fno-underscoring} allows direct specification of
714 user-defined names while debugging and when interfacing @command{gfortran}
715 code with other languages.
717 Note that just because the names match does @emph{not} mean that the
718 interface implemented by @command{gfortran} for an external name matches the
719 interface implemented by some other language for that same name.
720 That is, getting code produced by @command{gfortran} to link to code produced
721 by some other compiler using this or any other method can be only a
722 small part of the overall solution---getting the code generated by
723 both compilers to agree on issues other than naming can require
724 significant effort, and, unlike naming disagreements, linkers normally
725 cannot detect disagreements in these other areas.
727 Also, note that with @option{-fno-underscoring}, the lack of appended
728 underscores introduces the very real possibility that a user-defined
729 external name will conflict with a name in a system library, which
730 could make finding unresolved-reference bugs quite difficult in some
731 cases---they might occur at program run time, and show up only as
732 buggy behavior at run time.
734 In future versions of @command{gfortran} we hope to improve naming and linking
735 issues so that debugging always involves using the names as they appear
736 in the source, even if the names as seen by the linker are mangled to
737 prevent accidental linking between procedures with incompatible
740 @cindex @option{-fsecond-underscore option}
741 @cindex options, @option{-fsecond-underscore}
742 @item -fsecond-underscore
744 @cindex symbol names, underscores
745 @cindex transforming symbol names
746 @cindex symbol names, transforming
747 @cindex @command{f2c} calling convention
748 @cindex @command{g77} calling convention
749 @cindex libf2c calling convention
750 By default, @command{gfortran} appends an underscore to external
751 names. If this option is used @command{gfortran} appends two
752 underscores to names with underscores and one underscore to external names
753 with no underscores. (@command{gfortran} also appends two underscores to
754 internal names with underscores to avoid naming collisions with external
757 This option has no effect if @option{-fno-underscoring} is
758 in effect. It is implied by the @option{-ff2c} option.
760 Otherwise, with this option, an external name such as @samp{MAX_COUNT}
761 is implemented as a reference to the link-time external symbol
762 @samp{max_count__}, instead of @samp{max_count_}. This is required
763 for compatibility with @command{g77} and @command{f2c}, and is implied
764 by use of the @option{-ff2c} option.
767 @cindex -fbounds-check option
768 @cindex -ffortran-bounds-check option
770 @cindex bounds checking
771 @cindex range checking
772 @cindex array bounds checking
773 @cindex subscript checking
774 @cindex checking subscripts
775 Enable generation of run-time checks for array subscripts
776 and against the declared minimum and maximum values. It also
777 checks array indices for assumed and deferred
778 shape arrays against the actual allocated bounds.
780 In the future this may also include other forms of checking, eg. checking
781 substring references.
784 @cindex -fmax-stack-var-size option
785 @item -fmax-stack-var-size=@var{n}
786 This option specifies the size in bytes of the largest array that will be put
789 This option currently only affects local arrays declared with constant
790 bounds, and may not apply to all character variables.
791 Future versions of @command{gfortran} may improve this behavior.
793 The default value for @var{n} is 32768.
795 @cindex -fpackderived
797 @cindex Structure packing
798 This option tells gfortran to pack derived type members as closely as
799 possible. Code compiled with this option is likely to be incompatible
800 with code compiled without this option, and may execute slower.
802 @cindex -frepack-arrays option
803 @item -frepack-arrays
804 @cindex Repacking arrays
805 In some circumstances @command{gfortran} may pass assumed shape array
806 sections via a descriptor describing a discontiguous area of memory.
807 This option adds code to the function prologue to repack the data into
808 a contiguous block at runtime.
810 This should result in faster accesses to the array. However it can introduce
811 significant overhead to the function call, especially when the passed data
814 @cindex -fshort-enums
816 This option is provided for interoperability with C code that was
817 compiled with the @command{-fshort-enums} option. It will make
818 @command{gfortran} choose the smallest @code{INTEGER} kind a given
819 enumerator set will fit in, and give all its enumerators this kind.
822 @xref{Code Gen Options,,Options for Code Generation Conventions,
823 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
825 shared by @command{gfortran} @command{gcc} and other GNU compilers.
830 @node Environment Variables
831 @section Environment Variables Affecting GNU Fortran
832 @cindex environment variables
834 @c man begin ENVIRONMENT
836 GNU Fortran 95 currently does not make use of any environment
837 variables to control its operation above and beyond those
838 that affect the operation of @command{gcc}.
840 @xref{Environment Variables,,Environment Variables Affecting GCC,
841 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
844 @xref{Runtime}, for environment variables that affect the
845 run-time behavior of @command{gfortran} programs.