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 * 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
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 @gol
132 -Wimplicit-interface -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.
294 @cindex -std=@var{std} option
295 @cindex option, -std=@var{std}
297 Conform to the specified standard. Allowed values for @var{std} are
298 @samp{gnu}, @samp{f95}, @samp{f2003} and @samp{legacy}.
302 @node Warning Options
303 @section Options to Request or Suppress Warnings
304 @cindex options, warnings
305 @cindex warnings, suppressing
306 @cindex messages, warning
307 @cindex suppressing warnings
309 Warnings are diagnostic messages that report constructions which
310 are not inherently erroneous but which are risky or suggest there
311 might have been an error.
313 You can request many specific warnings with options beginning @option{-W},
314 for example @option{-Wimplicit} to request warnings on implicit
315 declarations. Each of these specific warning options also has a
316 negative form beginning @option{-Wno-} to turn off warnings;
317 for example, @option{-Wno-implicit}. This manual lists only one of the
318 two forms, whichever is not the default.
320 These options control the amount and kinds of warnings produced by GNU
324 @cindex syntax checking
325 @cindex -fsyntax-only option
326 @cindex options, -fsyntax-only
328 Check the code for syntax errors, but don't do anything beyond that.
330 @cindex -pedantic option
331 @cindex options, -pedantic
333 Issue warnings for uses of extensions to FORTRAN 95.
334 @option{-pedantic} also applies to C-language constructs where they
335 occur in GNU Fortran source files, such as use of @samp{\e} in a
336 character constant within a directive like @samp{#include}.
338 Valid FORTRAN 95 programs should compile properly with or without
340 However, without this option, certain GNU extensions and traditional
341 Fortran features are supported as well.
342 With this option, many of them are rejected.
344 Some users try to use @option{-pedantic} to check programs for conformance.
345 They soon find that it does not do quite what they want---it finds some
346 nonstandard practices, but not all.
347 However, improvements to @command{gfortran} in this area are welcome.
349 This should be used in conjunction with -std=@var{std}.
351 @cindex -pedantic-errors option
352 @cindex options, -pedantic-errors
353 @item -pedantic-errors
354 Like @option{-pedantic}, except that errors are produced rather than
360 Inhibit all warning messages.
364 @cindex options, -Wall
367 @cindex warnings, all
368 Enables commonly used warning options that which pertain to usage that
369 we recommend avoiding and that we believe is easy to avoid.
370 This currently includes @option{-Wunused-labels}, @option{-Waliasing},
371 @option{-Wsurprising}, @option{-Wnonstd-intrinsic} and
372 @option{-Wline-truncation}.
375 @cindex -Waliasing option
376 @cindex options, -Waliasing
379 Warn about possible aliasing of dummy arguments. Specifically, it warns
380 if the same actual argument is associated with a dummy argument with
381 @code{intent(in)} and a dummy argument with @code{intent(out)} in a call
382 with an explicit interface.
384 The following example will trigger the warning.
388 integer, intent(in) :: a
389 integer, intent(out) :: b
398 @cindex -Wconversion option
399 @cindex options, -Wconversion
402 Warn about implicit conversions between different types.
405 @cindex -Wimplicit-interface option
406 @cindex options, -Wimplicit-interface
407 @item -Wimplicit-interface
408 Warn about when procedure are called without an explicit interface.
409 Note this only checks that an explicit interface is present. It does not
410 check that the declared interfaces are consistent across program units.
413 @cindex -Wnonstd-intrinsic option
414 @cindex options, -Wnonstd-intrinsic
415 @item -Wnonstd-intrinsic
416 Warn if the user tries to use an intrinsic that does not belong to the
417 standard the user has chosen via the -std option.
421 @cindex options, -Wsurprising
424 Produce a warning when ``suspicious'' code constructs are encountered.
425 While technically legal these usually indicate that an error has been made.
427 This currently produces a warning under the following circumstances:
431 An INTEGER SELECT construct has a CASE that can never be matched as its
432 lower value is greater than its upper value.
435 A LOGICAL SELECT construct has three CASE statements.
439 @cindex options, -Wunderflow
442 Produce a warning when numerical constant expressions are
443 encountered, which yield an UNDERFLOW during compilation.
446 @cindex -Wunused-labels option
447 @cindex options, -Wunused-labels
448 @item -Wunused-labels
449 @cindex unused labels
450 @cindex labels, unused
451 Warn whenever a label is defined but never referenced.
455 @cindex options, -Werror
457 Turns all warnings into errors.
463 @cindex extra warnings
464 @cindex warnings, extra
465 Turns on ``extra warnings'' and, if optimization is specified
466 via @option{-O}, the @option{-Wuninitialized} option.
467 (This might change in future versions of @command{gfortran}
470 @xref{Warning Options,,Options to Request or Suppress Warnings,
471 gcc,Using the GNU Compiler Collection (GCC)}, for information on more
472 options offered by the GBE shared by @command{gfortran}, @command{gcc} and
475 Some of these have no effect when compiling programs written in Fortran.
477 @node Debugging Options
478 @section Options for Debugging Your Program or GNU Fortran
479 @cindex options, debugging
480 @cindex debugging information options
482 GNU Fortran has various special options that are used for debugging
483 either your program or @command{gfortran}
486 @cindex -fdump-parse-tree option
487 @cindex option, -fdump-parse-tree
488 @item -fdump-parse-tree
489 Output the internal parse tree before starting code generation. Only
490 really useful for debugging gfortran itself.
494 @cindex -ffpe-trap=@var{list} option
495 @cindex option, -ffpe-trap=@var{list}
496 @item -ffpe-trap=@var{list}
497 Specify a list of IEEE exceptions when a Floating Point Exception
498 (FPE) should be raised. On most systems, this will result in a SIGFPE
499 signal being sent and the program being interrupted, producing a core
500 file useful for debugging. @var{list} is a (possibly empty) comma-separated
501 list of the following IEEE exceptions: @samp{invalid} (invalid floating
502 point operation, such as @code{sqrt(-1.0)}), @samp{zero} (division by
503 zero), @samp{overflow} (overflow in a floating point operation),
504 @samp{underflow} (underflow in a floating point operation),
505 @samp{precision} (loss of precision during operation) and @samp{denormal}
506 (operation produced a denormal denormal value).
509 @xref{Debugging Options,,Options for Debugging Your Program or GCC,
510 gcc,Using the GNU Compiler Collection (GCC)}, for more information on
513 @node Directory Options
514 @section Options for Directory Search
515 @cindex directory, options
516 @cindex options, directory search
519 @cindex INCLUDE directive
520 @cindex directive, INCLUDE
521 These options affect how @command{gfortran} searches
522 for files specified by the @code{INCLUDE} directive and where it searches
523 for previously compiled modules.
525 It also affects the search paths used by @command{cpp} when used to preprocess
530 @cindex options, -Idir
532 @cindex directory, search paths for inclusion
533 @cindex inclusion, directory search paths for
534 @cindex search paths, for included files
535 @cindex paths, search
536 @cindex module search path
537 These affect interpretation of the @code{INCLUDE} directive
538 (as well as of the @code{#include} directive of the @command{cpp}
541 Also note that the general behavior of @option{-I} and
542 @code{INCLUDE} is pretty much the same as of @option{-I} with
543 @code{#include} in the @command{cpp} preprocessor, with regard to
544 looking for @file{header.gcc} files and other such things.
546 This path is also used to search for @samp{.mod} files when previously
547 compiled modules are required by a @code{USE} statement.
549 @xref{Directory Options,,Options for Directory Search,
550 gcc,Using the GNU Compiler Collection (GCC)}, for information on the
554 @cindex option, -Mdir
557 This option specifies where to put @samp{.mod} files for compiled modules.
558 It is also added to the list of directories to searched by an @code{USE}
561 The default is the current directory.
563 @option{-J} is an alias for @option{-M} to avoid conflicts with existing
567 @node Runtime Options
568 @section Influencing runtime behavior
569 @cindex runtime, options
571 These options affect the runtime behavior of @command{gfortran}.
573 @cindex -fconvert=@var{conversion} option
574 @item -fconvert=@var{conversion}
575 Specify the representation of data for unformatted files. Valid
576 values for conversion are: @samp{native}, the default; @samp{swap},
577 swap between big- and little-endian; @samp{big-endian}, use big-endian
578 representation for unformatted files; @samp{little-endian}, use little-endian
579 representation for unformatted files.
581 @emph{This option has an effect only when used in the main program.
582 The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment
583 variable override the default specified by -fconvert.}
586 @node Code Gen Options
587 @section Options for Code Generation Conventions
588 @cindex code generation, conventions
589 @cindex options, code generation
590 @cindex run-time, options
592 These machine-independent options control the interface conventions
593 used in code generation.
595 Most of them have both positive and negative forms; the negative form
596 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
597 one of the forms is listed---the one which is not the default. You
598 can figure out the other form by either removing @option{no-} or adding
602 @cindex @option{-fno-automatic} option
603 @cindex options, @option{-fno-automatic}
605 @cindex SAVE statement
606 @cindex statements, SAVE
607 Treat each program unit as if the @code{SAVE} statement was specified for
608 every local variable and array referenced in it. Does not affect common
609 blocks. (Some Fortran compilers provide this option under the name
612 @cindex @option{-ff2c} option
613 @cindex options, @option{-ff2c}
615 @cindex calling convention
616 @cindex @command{f2c} calling convention
617 @cindex @command{g77} calling convention
618 @cindex libf2c calling convention
619 Generate code designed to be compatible with code generated
620 by @command{g77} and @command{f2c}.
622 The calling conventions used by @command{g77} (originally implemented
623 in @command{f2c}) require functions that return type
624 default @code{REAL} to actually return the C type @code{double}, and
625 functions that return type @code{COMPLEX} to return the values via an
626 extra argument in the calling sequence that points to where to
627 store the return value. Under the default GNU calling conventions, such
628 functions simply return their results as they would in GNU
629 C -- default @code{REAL} functions return the C type @code{float}, and
630 @code{COMPLEX} functions return the GNU C type @code{complex}.
631 Additionally, this option implies the @option{-fsecond-underscore}
632 option, unless @option{-fno-second-underscore} is explicitly requested.
634 This does not affect the generation of code that interfaces with
635 the @command{libgfortran} library.
637 @emph{Caution:} It is not a good idea to mix Fortran code compiled
638 with @code{-ff2c} with code compiled with the default @code{-fno-f2c}
639 calling conventions as, calling @code{COMPLEX} or default @code{REAL}
640 functions between program parts which were compiled with different
641 calling conventions will break at execution time.
643 @emph{Caution:} This will break code which passes intrinsic functions
644 of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
645 the library implementations use the @command{-fno-f2c} calling conventions.
647 @cindex @option{-fno-underscoring option}
648 @cindex options, @option{-fno-underscoring}
649 @item -fno-underscoring
651 @cindex symbol names, underscores
652 @cindex transforming symbol names
653 @cindex symbol names, transforming
654 Do not transform names of entities specified in the Fortran
655 source file by appending underscores to them.
657 With @option{-funderscoring} in effect, @command{gfortran} appends one
658 underscore to external names with no underscores. This is done to ensure
659 compatibility with code produced by many UNIX Fortran compilers.
661 @emph{Caution}: The default behavior of @command{gfortran} is
662 incompatible with @command{f2c} and @command{g77}, please use the
663 @option{-ff2c} option if you want object files compiled with
664 @option{gfortran} to be compatible with object code created with these
667 Use of @option{-fno-underscoring} is not recommended unless you are
668 experimenting with issues such as integration of (GNU) Fortran into
669 existing system environments (vis-a-vis existing libraries, tools, and
672 For example, with @option{-funderscoring}, and assuming other defaults like
673 @option{-fcase-lower} and that @samp{j()} and @samp{max_count()} are
674 external functions while @samp{my_var} and @samp{lvar} are local variables,
678 I = J() + MAX_COUNT (MY_VAR, LVAR)
682 is implemented as something akin to:
685 i = j_() + max_count__(&my_var__, &lvar);
688 With @option{-fno-underscoring}, the same statement is implemented as:
691 i = j() + max_count(&my_var, &lvar);
694 Use of @option{-fno-underscoring} allows direct specification of
695 user-defined names while debugging and when interfacing @command{gfortran}
696 code with other languages.
698 Note that just because the names match does @emph{not} mean that the
699 interface implemented by @command{gfortran} for an external name matches the
700 interface implemented by some other language for that same name.
701 That is, getting code produced by @command{gfortran} to link to code produced
702 by some other compiler using this or any other method can be only a
703 small part of the overall solution---getting the code generated by
704 both compilers to agree on issues other than naming can require
705 significant effort, and, unlike naming disagreements, linkers normally
706 cannot detect disagreements in these other areas.
708 Also, note that with @option{-fno-underscoring}, the lack of appended
709 underscores introduces the very real possibility that a user-defined
710 external name will conflict with a name in a system library, which
711 could make finding unresolved-reference bugs quite difficult in some
712 cases---they might occur at program run time, and show up only as
713 buggy behavior at run time.
715 In future versions of @command{gfortran} we hope to improve naming and linking
716 issues so that debugging always involves using the names as they appear
717 in the source, even if the names as seen by the linker are mangled to
718 prevent accidental linking between procedures with incompatible
721 @cindex @option{-fsecond-underscore option}
722 @cindex options, @option{-fsecond-underscore}
723 @item -fsecond-underscore
725 @cindex symbol names, underscores
726 @cindex transforming symbol names
727 @cindex symbol names, transforming
728 @cindex @command{f2c} calling convention
729 @cindex @command{g77} calling convention
730 @cindex libf2c calling convention
731 By default, @command{gfortran} appends an underscore to external
732 names. If this option is used @command{gfortran} appends two
733 underscores to names with underscores and one underscore to external names
734 with no underscores. (@command{gfortran} also appends two underscores to
735 internal names with underscores to avoid naming collisions with external
738 This option has no effect if @option{-fno-underscoring} is
739 in effect. It is implied by the @option{-ff2c} option.
741 Otherwise, with this option, an external name such as @samp{MAX_COUNT}
742 is implemented as a reference to the link-time external symbol
743 @samp{max_count__}, instead of @samp{max_count_}. This is required
744 for compatibility with @command{g77} and @command{f2c}, and is implied
745 by use of the @option{-ff2c} option.
748 @cindex -fbounds-check option
749 @cindex -ffortran-bounds-check option
751 @cindex bounds checking
752 @cindex range checking
753 @cindex array bounds checking
754 @cindex subscript checking
755 @cindex checking subscripts
756 Enable generation of run-time checks for array subscripts
757 and against the declared minimum and maximum values. It also
758 checks array indices for assumed and deferred
759 shape arrays against the actual allocated bounds.
761 In the future this may also include other forms of checking, eg. checking
762 substring references.
765 @cindex -fmax-stack-var-size option
766 @item -fmax-stack-var-size=@var{n}
767 This option specifies the size in bytes of the largest array that will be put
770 This option currently only affects local arrays declared with constant
771 bounds, and may not apply to all character variables.
772 Future versions of @command{gfortran} may improve this behavior.
774 The default value for @var{n} is 32768.
776 @cindex -fpackderived
778 @cindex Structure packing
779 This option tells gfortran to pack derived type members as closely as
780 possible. Code compiled with this option is likely to be incompatible
781 with code compiled without this option, and may execute slower.
783 @cindex -frepack-arrays option
784 @item -frepack-arrays
785 @cindex Repacking arrays
786 In some circumstances @command{gfortran} may pass assumed shape array
787 sections via a descriptor describing a discontiguous area of memory.
788 This option adds code to the function prologue to repack the data into
789 a contiguous block at runtime.
791 This should result in faster accesses to the array. However it can introduce
792 significant overhead to the function call, especially when the passed data
795 @cindex -fshort-enums
797 This option is provided for interoperability with C code that was
798 compiled with the @command{-fshort-enums} option. It will make
799 @command{gfortran} choose the smallest @code{INTEGER} kind a given
800 enumerator set will fit in, and give all its enumerators this kind.
803 @xref{Code Gen Options,,Options for Code Generation Conventions,
804 gcc,Using the GNU Compiler Collection (GCC)}, for information on more options
806 shared by @command{gfortran} @command{gcc} and other GNU compilers.
811 @node Environment Variables
812 @section Environment Variables Affecting GNU Fortran
813 @cindex environment variables
815 @c man begin ENVIRONMENT
817 GNU Fortran 95 currently does not make use of any environment
818 variables to control its operation above and beyond those
819 that affect the operation of @command{gcc}.
821 @xref{Environment Variables,,Environment Variables Affecting GCC,
822 gcc,Using the GNU Compiler Collection (GCC)}, for information on environment
825 @xref{Runtime}, for environment variables that affect the
826 run-time behavior of @command{gfortran} programs.