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@c Copyright (C) 2004, 2005, 2006

@c Free Software Foundation, Inc.

@c This is part of the GFORTRAN manual.

@c For copying conditions, see the file gfortran.texi.

@ignore

@c man begin COPYRIGHT

Copyright @copyright{} 2004, 2005

Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.2 or

any later version published by the Free Software Foundation; with the

Invariant Sections being ``GNU General Public License'' and ``Funding

Free Software'', the Front-Cover texts being (a) (see below), and with

the Back-Cover Texts being (b) (see below).  A copy of the license is

included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU

     software.  Copies published by the Free Software Foundation raise

     funds for GNU development.

@c man end

@c Set file name and title for the man page.

@setfilename gfortran

@settitle GNU Fortran 95 compiler.

@c man begin SYNOPSIS

gfortran [@option{-c}|@option{-S}|@option{-E}]

         [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]

         [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]

         [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]

         [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]

         [@option{-f}@var{option}@dots{}]

         [@option{-m}@var{machine-option}@dots{}]

         [@option{-o} @var{outfile}] @var{infile}@dots{}

Only the most useful options are listed here; see below for the

remainder.

@c man end

@c man begin SEEALSO

gpl(7), gfdl(7), fsf-funding(7),

cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)

and the Info entries for @file{gcc}, @file{cpp}, @file{gfortran}, @file{as},

@file{ld}, @file{binutils} and @file{gdb}.

@c man end

@c man begin BUGS

For instructions on reporting bugs, see

@w{@uref{http://gcc.gnu.org/bugs.html}}.

@c man end

@c man begin AUTHOR

See the Info entry for @command{gfortran} for contributors to GCC and

GFORTRAN@.

@c man end

@end ignore

@node Invoking GFORTRAN

@chapter GNU Fortran 95 Command Options

@cindex GNU Fortran 95 command options

@cindex command options

@cindex options, GNU Fortran 95 command

@c man begin DESCRIPTION

The @command{gfortran} command supports all the options supported by the

@command{gcc} command.  Only options specific to gfortran are documented here.

@xref{Invoking GCC,,GCC Command Options,gcc,Using the GNU Compiler

Collection (GCC)}, for information

on the non-Fortran-specific aspects of the @command{gcc} command (and,

therefore, the @command{gfortran} command).

@cindex options, negative forms

@cindex negative forms of options

All @command{gcc} and @command{gfortran} options

are accepted both by @command{gfortran} and by @command{gcc}

(as well as any other drivers built at the same time,

such as @command{g++}),

since adding @command{gfortran} to the @command{gcc} distribution

enables acceptance of @command{gfortran} options

by all of the relevant drivers.

In some cases, options have positive and negative forms;

the negative form of @option{-ffoo} would be @option{-fno-foo}.

This manual documents only one of these two forms, whichever

one is not the default.

@c man end

@menu

* Option Summary::      Brief list of all @command{gfortran} options,

                        without explanations.

* Fortran Dialect Options::  Controlling the variant of Fortran language

                             compiled.

* Warning Options::     How picky should the compiler be?

* Debugging Options::   Symbol tables, measurements, and debugging dumps.

* Directory Options::   Where to find module files

* Runtime Options::     Influencing runtime behavior

* Code Gen Options::    Specifying conventions for function calls, data layout

                        and register usage.

* Environment Variables:: Env vars that affect GNU Fortran.

@end menu

@node Option Summary

@section Option Summary

@c man begin OPTIONS

Here is a summary of all the options specific to GNU Fortran, grouped

by type.  Explanations are in the following sections.

@table @emph

@item Fortran Language Options

@xref{Fortran Dialect Options,,Options Controlling Fortran Dialect}.

@gccoptlist{

-ffree-form  -fno-fixed-form @gol

-fdollar-ok  -fimplicit-none  -fmax-identifier-length @gol

-std=@var{std} -fd-lines-as-code -fd-lines-as-comments @gol

-ffixed-line-length-@var{n}  -ffixed-line-length-none @gol

-ffree-line-length-@var{n}  -ffree-line-length-none @gol

-fdefault-double-8  -fdefault-integer-8  -fdefault-real-8 @gol

-fcray-pointer }

@item Warning Options

@xref{Warning Options,,Options to Request or Suppress Warnings}.

@gccoptlist{

-fsyntax-only  -pedantic  -pedantic-errors @gol

-w  -Wall  -Waliasing -Wampersand -Wconversion -Wimplicit-interface @gol

-Wnonstd-intrinsics -Wsurprising -Wunderflow @gol

-Wunused-labels -Wline-truncation -W}

@item Debugging Options

@xref{Debugging Options,,Options for Debugging Your Program or GCC}.

@gccoptlist{

-fdump-parse-tree -ffpe-trap=@var{list}}

@item Directory Options

@xref{Directory Options,,Options for Directory Search}.

@gccoptlist{

-I@var{dir}  -M@var{dir}}

@item Runtime Options

@xref{Runtime Options,,Options for influencing runtime behavior}.

@gccoptlist{

-fconvert=@var{conversion} -frecord-marker=@var{length}}

@item Code Generation Options

@xref{Code Gen Options,,Options for Code Generation Conventions}.

@gccoptlist{

-fno-automatic -ff2c -fno-underscoring  -fsecond-underscore @gol

-fbounds-check  -fmax-stack-var-size=@var{n} @gol

-fpackderived  -frepack-arrays  -fshort-enums}

@end table

@menu

* Fortran Dialect Options::  Controlling the variant of Fortran language

                             compiled.

* Warning Options::     How picky should the compiler be?

* Debugging Options::   Symbol tables, measurements, and debugging dumps.

* Directory Options::   Where to find module files

* Runtime Options::     Influencing runtime behavior

* Code Gen Options::    Specifying conventions for function calls, data layout

                        and register usage.

@end menu

@node Fortran Dialect Options

@section Options Controlling Fortran Dialect

@cindex dialect options

@cindex language, dialect options

@cindex options, dialect

The following options control the dialect of Fortran

that the compiler accepts:

@table @gcctabopt

@cindex -ffree-form option

@cindex options, -ffree-form

@cindex -fno-fixed-form option

@cindex options, -fno-fixed-form

@cindex source file format

@cindex free form

@cindex fixed form

@cindex Source Form

@cindex Fortran 90, features

@item -ffree-form

@item -ffixed-form

Specify the layout used by the source file.  The free form layout

was introduced in Fortran 90.  Fixed form was traditionally used in

older Fortran programs.

@cindex option, -fd-lines-as-code

@cindex -fd-lines-as-code, option

@cindex option, -fd-lines-as-comments

@cindex -fd-lines-as-comments, option

@item -fd-lines-as-code

@item -fd-lines-as-comment

Enables special treating for lines with @samp{d} or @samp{D} in fixed

form sources.  If the @option{-fd-lines-as-code} option is given

they are treated as if the first column contained a blank.  If the

@option{-fd-lines-as-comments} option is given, they are treated as

comment lines.

@cindex option, -fdefault-double-8

@cindex -fdefault-double-8, option

@item -fdefault-double-8

Set the "DOUBLE PRECISION" type to an 8 byte wide.

@cindex option, -fdefault-integer-8

@cindex -fdefault-integer-8, option

@item -fdefault-integer-8

Set the default integer and logical types to an 8 byte wide type.

Do nothing if this is already the default.

@cindex option, -fdefault-real-8

@cindex -fdefault-real-8, option

@item -fdefault-real-8

Set the default real type to an 8 byte wide type.

Do nothing if this is already the default.

@cindex -fdollar-ok option

@cindex options, -fdollar-ok

@item -fdollar-ok

@cindex dollar sign

@cindex symbol names

@cindex character set

Allow @samp{$} as a valid character in a symbol name.

@cindex -fno-backslash option

@cindex options, -fno-backslash

@item -fno-backslash

@cindex backslash

@cindex escape characters

Compile switch to change the interpretation of a backslash from

``C''-style escape characters to a single backslash character.

@cindex -ffixed-line-length-@var{n} option

@cindex options, -ffixed-line-length-@var{n}

@item -ffixed-line-length-@var{n}

@cindex source file format

@cindex lines, length

@cindex length of source lines

@cindex fixed form

@cindex limits, lengths of source lines

Set column after which characters are ignored in typical fixed-form

lines in the source file, and through which spaces are assumed (as

if padded to that length) after the ends of short fixed-form lines.

@cindex card image

@cindex extended-source option

Popular values for @var{n} include 72 (the

standard and the default), 80 (card image), and 132 (corresponds

to ``extended-source'' options in some popular compilers).

@var{n} may be @samp{none}, meaning that the entire line is meaningful

and that continued character constants never have implicit spaces appended

to them to fill out the line.

@option{-ffixed-line-length-0} means the same thing as

@option{-ffixed-line-length-none}.

@cindex -ffree-line-length-@var{n} option

@cindex options, -ffree-line-length-@var{n}

@item -ffree-line-length-@var{n}

@cindex source file format

@cindex lines, length

@cindex length of source lines

@cindex free form

@cindex limits, lengths of source lines

Set column after which characters are ignored in typical free-form

lines in the source file. For free-form, the default value is 132.

@var{n} may be @samp{none}, meaning that the entire line is meaningful.

@option{-ffree-line-length-0} means the same thing as

@option{-ffree-line-length-none}.

@cindex -fmax-identifier-length=@var{n} option

@cindex option -fmax-identifier-length=@var{n}

@item -fmax-identifier-length=@var{n}

Specify the maximum allowed identifier length. Typical values are

31 (Fortran 95) and 63 (Fortran 200x).

@cindex -fimplicit-none option

@cindex options, -fimplicit-none

@item -fimplicit-none

Specify that no implicit typing is allowed, unless overridden by explicit

@samp{IMPLICIT} statements.  This is the equivalent of adding

@samp{implicit none} to the start of every procedure.

@cindex -fcray-pointer option

@cindex options, -fcray-pointer

@item -fcray-pointer

Enables the Cray pointer extension, which provides a C-like pointer.

@cindex -std=@var{std} option

@cindex option, -std=@var{std}

@item -std=@var{std}

Conform to the specified standard.  Allowed values for @var{std} are

@samp{gnu}, @samp{f95}, @samp{f2003} and @samp{legacy}.

@end table

@node Warning Options

@section Options to Request or Suppress Warnings

@cindex options, warnings

@cindex warnings, suppressing

@cindex messages, warning

@cindex suppressing warnings

Warnings are diagnostic messages that report constructions which

are not inherently erroneous but which are risky or suggest there

might have been an error.

You can request many specific warnings with options beginning @option{-W},

for example @option{-Wimplicit} to request warnings on implicit

declarations.  Each of these specific warning options also has a

negative form beginning @option{-Wno-} to turn off warnings;

for example, @option{-Wno-implicit}.  This manual lists only one of the

two forms, whichever is not the default.

These options control the amount and kinds of warnings produced by GNU

Fortran:

@table @gcctabopt

@cindex syntax checking

@cindex -fsyntax-only option

@cindex options, -fsyntax-only

@item -fsyntax-only

Check the code for syntax errors, but don't do anything beyond that.

@cindex -pedantic option

@cindex options, -pedantic

@item -pedantic

Issue warnings for uses of extensions to FORTRAN 95.

@option{-pedantic} also applies to C-language constructs where they

occur in GNU Fortran source files, such as use of @samp{\e} in a

character constant within a directive like @samp{#include}.

Valid FORTRAN 95 programs should compile properly with or without

this option.

However, without this option, certain GNU extensions and traditional

Fortran features are supported as well.

With this option, many of them are rejected.

Some users try to use @option{-pedantic} to check programs for conformance.

They soon find that it does not do quite what they want---it finds some

nonstandard practices, but not all.

However, improvements to @command{gfortran} in this area are welcome.

This should be used in conjunction with -std=@var{std}.

@cindex -pedantic-errors option

@cindex options, -pedantic-errors

@item -pedantic-errors

Like @option{-pedantic}, except that errors are produced rather than

warnings.

@cindex -w option

@cindex options, -w

@item -w

Inhibit all warning messages.

@cindex -Wall option

@cindex options, -Wall

@item -Wall

@cindex all warnings

@cindex warnings, all

Enables commonly used warning options pertaining to usage that

we recommend avoiding and that we believe are easy to avoid. This currently

includes @option{-Wunused-labels}, @option{-Waliasing}, @option{-Wampersand},

@option{-Wsurprising}, @option{-Wnonstd-intrinsic},

and @option{-Wline-truncation}.

@cindex -Waliasing option

@cindex options, -Waliasing

@item -Waliasing

@cindex aliasing

Warn about possible aliasing of dummy arguments. Specifically, it warns

if the same actual argument is associated with a dummy argument with

@code{intent(in)} and a dummy argument with @code{intent(out)} in a call

with an explicit interface.

The following example will trigger the warning.

@smallexample

  interface

    subroutine bar(a,b)

      integer, intent(in) :: a

      integer, intent(out) :: b

    end subroutine

  end interface

  integer :: a

  call bar(a,a)

@end smallexample

@cindex -Wampersand option

@cindex options, -Wampersand

@item -Wampersand

@cindex ampersand

Warn about missing ampersand in continued character literals. The warning is

given with @option{-Wampersand}, @option{-pedantic}, and @option{-std=f95}.

Note: With no ampersand given in a continued character literal, gfortran

assumes continuation at the first non-comment, non-whitespace character.

@cindex -Wconversion option

@cindex options, -Wconversion

@item -Wconversion

@cindex conversion

Warn about implicit conversions between different types.

@cindex -Wimplicit-interface option

@cindex options, -Wimplicit-interface

@item -Wimplicit-interface

Warn about when procedure are called without an explicit interface.

Note this only checks that an explicit interface is present.  It does not

check that the declared interfaces are consistent across program units.

@cindex -Wnonstd-intrinsic option

@cindex options, -Wnonstd-intrinsic

@item -Wnonstd-intrinsic

Warn if the user tries to use an intrinsic that does not belong to the

standard the user has chosen via the -std option.

@cindex -Wsurprising

@cindex options, -Wsurprising

@item -Wsurprising

@cindex Suspicious

Produce a warning when ``suspicious'' code constructs are encountered.

While technically legal these usually indicate that an error has been made.

This currently produces a warning under the following circumstances:

@itemize @bullet

@item

An INTEGER SELECT construct has a CASE that can never be matched as its

lower value is greater than its upper value.

@item

A LOGICAL SELECT construct has three CASE statements.

@end itemize

@cindex -Wunderflow

@cindex options, -Wunderflow

@item -Wunderflow

@cindex UNDERFLOW

Produce a warning when numerical constant expressions are

encountered, which yield an UNDERFLOW during compilation.

@cindex -Wunused-labels option

@cindex options, -Wunused-labels

@item -Wunused-labels

@cindex unused labels

@cindex labels, unused

Warn whenever a label is defined but never referenced.

@cindex -Werror

@cindex options, -Werror

@item -Werror

Turns all warnings into errors.

@cindex -W option

@cindex options, -W

@item -W

@cindex extra warnings

@cindex warnings, extra

Turns on ``extra warnings'' and, if optimization is specified

via @option{-O}, the @option{-Wuninitialized} option.

(This might change in future versions of @command{gfortran}

@end table

@xref{Warning Options,,Options to Request or Suppress Warnings,

gcc,Using the GNU Compiler Collection (GCC)}, for information on more

options offered by the GBE shared by @command{gfortran}, @command{gcc} and

other GNU compilers.

Some of these have no effect when compiling programs written in Fortran.

@node Debugging Options

@section Options for Debugging Your Program or GNU Fortran

@cindex options, debugging

@cindex debugging information options

GNU Fortran has various special options that are used for debugging

either your program or @command{gfortran}

@table @gcctabopt

@cindex -fdump-parse-tree option

@cindex option, -fdump-parse-tree

@item -fdump-parse-tree

Output the internal parse tree before starting code generation.  Only

really useful for debugging gfortran itself.

@end table

@table @gcctabopt

@cindex -ffpe-trap=@var{list} option

@cindex option, -ffpe-trap=@var{list}

@item -ffpe-trap=@var{list}

Specify a list of IEEE exceptions when a Floating Point Exception

(FPE) should be raised.  On most systems, this will result in a SIGFPE

signal being sent and the program being interrupted, producing a core

file useful for debugging.  @var{list} is a (possibly empty) comma-separated

list of the following IEEE exceptions: @samp{invalid} (invalid floating

point operation, such as @code{sqrt(-1.0)}), @samp{zero} (division by

zero), @samp{overflow} (overflow in a floating point operation),

@samp{underflow} (underflow in a floating point operation),

@samp{precision} (loss of precision during operation) and @samp{denormal}

(operation produced a denormal denormal value).

@end table

@xref{Debugging Options,,Options for Debugging Your Program or GCC,

gcc,Using the GNU Compiler Collection (GCC)}, for more information on

debugging options.

@node Directory Options

@section Options for Directory Search

@cindex directory, options

@cindex options, directory search

@cindex search path

@cindex INCLUDE directive

@cindex directive, INCLUDE

These options affect how @command{gfortran} searches

for files specified by the @code{INCLUDE} directive and where it searches

for previously compiled modules.

It also affects the search paths used by @command{cpp} when used to preprocess

Fortran source.

@table @gcctabopt

@cindex -Idir option

@cindex options, -Idir

@item -I@var{dir}

@cindex directory, search paths for inclusion

@cindex inclusion, directory search paths for

@cindex search paths, for included files

@cindex paths, search

@cindex module search path

These affect interpretation of the @code{INCLUDE} directive

(as well as of the @code{#include} directive of the @command{cpp}

preprocessor).

Also note that the general behavior of @option{-I} and

@code{INCLUDE} is pretty much the same as of @option{-I} with

@code{#include} in the @command{cpp} preprocessor, with regard to

looking for @file{header.gcc} files and other such things.

This path is also used to search for @samp{.mod} files when previously

compiled modules are required by a @code{USE} statement.

@xref{Directory Options,,Options for Directory Search,

gcc,Using the GNU Compiler Collection (GCC)}, for information on the

@option{-I} option.

@cindex -Mdir option

@cindex option, -Mdir

@item -M@var{dir}

@item -J@var{dir}

This option specifies where to put @samp{.mod} files for compiled modules.

It is also added to the list of directories to searched by an @code{USE}

statement.

The default is the current directory.

@option{-J} is an alias for @option{-M} to avoid conflicts with existing

GCC options.

@end table

@node Runtime Options

@section Influencing runtime behavior

@cindex runtime, options

These options affect the runtime behavior of @command{gfortran}.

@table @gcctabopt

@cindex -fconvert=@var{conversion} option

@item -fconvert=@var{conversion}

Specify the representation of data for unformatted files.  Valid

values for conversion are: @samp{native}, the default; @samp{swap},

swap between big- and little-endian; @samp{big-endian}, use big-endian

representation for unformatted files; @samp{little-endian}, use little-endian

representation for unformatted files.

@emph{This option has an effect only when used in the main program.

The @code{CONVERT} specifier and the GFORTRAN_CONVERT_UNIT environment

variable override the default specified by -fconvert.}

@cindex -frecord-marker=@var{length}

@item -frecord-marker=@var{length}

Specify the length of record markers for unformatted files.

Valid values for @var{length} are 4 and 8.  Default is whatever

@code{off_t} is specified to be on that particular system.

Note that specifying @var{length} as 4 limits the record

length of unformatted files to 2 GB.  This option does not

extend the maximum possible record length on systems where

@code{off_t} is a four_byte quantity.

@end table

@node Code Gen Options

@section Options for Code Generation Conventions

@cindex code generation, conventions

@cindex options, code generation

@cindex run-time, options

These machine-independent options control the interface conventions

used in code generation.

Most of them have both positive and negative forms; the negative form

of @option{-ffoo} would be @option{-fno-foo}.  In the table below, only

one of the forms is listed---the one which is not the default.  You

can figure out the other form by either removing @option{no-} or adding

it.

@table @gcctabopt

@cindex @option{-fno-automatic} option

@cindex options, @option{-fno-automatic}

@item -fno-automatic

@cindex SAVE statement

@cindex statements, SAVE

Treat each program unit as if the @code{SAVE} statement was specified for

every local variable and array referenced in it. Does not affect common

blocks. (Some Fortran compilers provide this option under the name

@option{-static}.)

@cindex @option{-ff2c} option

@cindex options, @option{-ff2c}

@item -ff2c

@cindex calling convention

@cindex @command{f2c} calling convention

@cindex @command{g77} calling convention

@cindex libf2c calling convention

Generate code designed to be compatible with code generated

by @command{g77} and @command{f2c}.

The calling conventions used by @command{g77} (originally implemented

in @command{f2c}) require functions that return type

default @code{REAL} to actually return the C type @code{double}, and

functions that return type @code{COMPLEX} to return the values via an

extra argument in the calling sequence that points to where to

store the return value.  Under the default GNU calling conventions, such

functions simply return their results as they would in GNU

C -- default @code{REAL} functions return the C type @code{float}, and

@code{COMPLEX} functions return the GNU C type @code{complex}.

Additionally, this option implies the @option{-fsecond-underscore}

option, unless @option{-fno-second-underscore} is explicitly requested.

This does not affect the generation of code that interfaces with

the @command{libgfortran} library.

@emph{Caution:} It is not a good idea to mix Fortran code compiled

with @code{-ff2c} with code compiled with the default @code{-fno-f2c}

calling conventions as, calling @code{COMPLEX} or default @code{REAL}

functions between program parts which were compiled with different

calling conventions will break at execution time.

@emph{Caution:} This will break code which passes intrinsic functions

of type default @code{REAL} or @code{COMPLEX} as actual arguments, as

the library implementations use the @command{-fno-f2c} calling conventions.

@cindex @option{-fno-underscoring option}

@cindex options, @option{-fno-underscoring}

@item -fno-underscoring

@cindex underscore

@cindex symbol names, underscores

@cindex transforming symbol names

@cindex symbol names, transforming

Do not transform names of entities specified in the Fortran

source file by appending underscores to them.

With @option{-funderscoring} in effect, @command{gfortran} appends one

underscore to external names with no underscores.  This is done to ensure

compatibility with code produced by many UNIX Fortran compilers.

@emph{Caution}: The default behavior of @command{gfortran} is

incompatible with @command{f2c} and @command{g77}, please use the

@option{-ff2c} option if you want object files compiled with

@option{gfortran} to be compatible with object code created with these

tools.

Use of @option{-fno-underscoring} is not recommended unless you are

experimenting with issues such as integration of (GNU) Fortran into

existing system environments (vis-a-vis existing libraries, tools, and

so on).

For example, with @option{-funderscoring}, and assuming other defaults like

@option{-fcase-lower} and that @samp{j()} and @samp{max_count()} are

external functions while @samp{my_var} and @samp{lvar} are local variables,

a statement like

@smallexample

I = J() + MAX_COUNT (MY_VAR, LVAR)

@end smallexample

@noindent

is implemented as something akin to:

@smallexample

i = j_() + max_count__(&my_var__, &lvar);

@end smallexample

With @option{-fno-underscoring}, the same statement is implemented as:

@smallexample

i = j() + max_count(&my_var, &lvar);

@end smallexample

Use of @option{-fno-underscoring} allows direct specification of

user-defined names while debugging and when interfacing @command{gfortran}

code with other languages.

Note that just because the names match does @emph{not} mean that the

interface implemented by @command{gfortran} for an external name matches the

interface implemented by some other language for that same name.

That is, getting code produced by @command{gfortran} to link to code produced

by some other compiler using this or any other method can be only a

small part of the overall solution---getting the code generated by

both compilers to agree on issues other than naming can require

significant effort, and, unlike naming disagreements, linkers normally

cannot detect disagreements in these other areas.

Also, note that with @option{-fno-underscoring}, the lack of appended

underscores introduces the very real possibility that a user-defined

external name will conflict with a name in a system library, which

could make finding unresolved-reference bugs quite difficult in some

cases---they might occur at program run time, and show up only as

buggy behavior at run time.

In future versions of @command{gfortran} we hope to improve naming and linking

issues so that debugging always involves using the names as they appear

in the source, even if the names as seen by the linker are mangled to

prevent accidental linking between procedures with incompatible

interfaces.

@cindex @option{-fsecond-underscore option}

@cindex options, @option{-fsecond-underscore}

@item -fsecond-underscore

@cindex underscore

@cindex symbol names, underscores

@cindex transforming symbol names

@cindex symbol names, transforming

@cindex @command{f2c} calling convention

@cindex @command{g77} calling convention

@cindex libf2c calling convention

By default, @command{gfortran} appends an underscore to external

names.  If this option is used @command{gfortran} appends two

underscores to names with underscores and one underscore to external names

with no underscores.  (@command{gfortran} also appends two underscores to

internal names with underscores to avoid naming collisions with external

names.

This option has no effect if @option{-fno-underscoring} is

in effect.  It is implied by the @option{-ff2c} option.

Otherwise, with this option, an external name such as @samp{MAX_COUNT}

is implemented as a reference to the link-time external symbol

@samp{max_count__}, instead of @samp{max_count_}.  This is required

for compatibility with @command{g77} and @command{f2c}, and is implied

by use of the @option{-ff2c} option.

@cindex -fbounds-check option

@cindex -ffortran-bounds-check option

@item -fbounds-check

@cindex bounds checking

@cindex range checking

@cindex array bounds checking

@cindex subscript checking

@cindex checking subscripts

Enable generation of run-time checks for array subscripts

and against the declared minimum and maximum values.  It also

checks array indices for assumed and deferred

shape arrays against the actual allocated bounds.

In the future this may also include other forms of checking, eg. checking

substring references.

@cindex -fmax-stack-var-size option

@item -fmax-stack-var-size=@var{n}

This option specifies the size in bytes of the largest array that will be put

on the stack.

This option currently only affects local arrays declared with constant

bounds, and may not apply to all character variables.

Future versions of @command{gfortran} may improve this behavior.

The default value for @var{n} is 32768.

@cindex -fpackderived

@item -fpackderived

@cindex Structure packing

This option tells gfortran to pack derived type members as closely as

possible.  Code compiled with this option is likely to be incompatible

with code compiled without this option, and may execute slower.

@cindex -frepack-arrays option

@item -frepack-arrays

@cindex Repacking arrays

In some circumstances @command{gfortran} may pass assumed shape array

sections via a descriptor describing a discontiguous area of memory.

This option adds code to the function prologue to repack the data into

a contiguous block at runtime.

This should result in faster accesses to the array.  However it can introduce

significant overhead to the function call, especially  when the passed data

is discontiguous.

@cindex -fshort-enums

@item -fshort-enums

This option is provided for interoperability with C code that was

compiled with the @command{-fshort-enums} option.  It will make

@command{gfortran} choose the smallest @code{INTEGER} kind a given

enumerator set will fit in, and give all its enumerators this kind.

@end table

@xref{Code Gen Options,,Options for Code Generation Conventions,

gcc,Using the GNU Compiler Collection (GCC)}, for information on more options

offered by the GBE

shared by @command{gfortran} @command{gcc} and other GNU compilers.

@c man end

@node Environment Variables

@section Environment Variables Affecting GNU Fortran

@cindex environment variables

@c man begin ENVIRONMENT

GNU Fortran 95 currently does not make use of any environment

variables to control its operation above and beyond those

that affect the operation of @command{gcc}.

@xref{Environment Variables,,Environment Variables Affecting GCC,

gcc,Using the GNU Compiler Collection (GCC)}, for information on environment

variables.

@xref{Runtime}, for environment variables that affect the

run-time behavior of @command{gfortran} programs.

@c man end