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@c Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010

@c Free Software Foundation, Inc.

@c This is part of the GNU Fortran manual.

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

@ignore

@c man begin COPYRIGHT

Copyright @copyright{} 2004, 2005, 2006, 2007, 2008, 2009

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 ``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 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{@value{BUGURL}}.

@c man end

@c man begin AUTHOR

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

GNU Fortran.

@c man end

@end ignore

@node Invoking GNU Fortran

@chapter GNU Fortran Command Options

@cindex GNU Fortran command options

@cindex command options

@cindex options, @command{gfortran} command

@c man begin DESCRIPTION

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

@command{gcc} command.  Only options specific to GNU Fortran 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

All GCC and GNU Fortran 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 GNU Fortran to the GCC distribution

enables acceptance of GNU Fortran 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.

* Preprocessing Options::  Enable and customize preprocessing.

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

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

* Directory Options::   Where to find module files

* Link Options ::       Influencing the linking step

* Runtime Options::     Influencing runtime behavior

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

                        and register usage.

* Environment Variables:: Environment variables that affect @command{gfortran}.

@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{-fall-intrinsics  -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  -fopenmp  -fno-range-check -fbackslash -fmodule-private}

@item Preprocessing Options

@xref{Preprocessing Options,,Enable and customize preprocessing}.

@gccoptlist{-cpp -dD -dI -dM -dN -dU -fworking-directory @gol

-imultilib @var{dir} -iprefix @var{file} -isysroot @var{dir} @gol

-iquote -isystem @var{dir} -nocpp -nostdinc -undef @gol

-A@var{question}=@var{answer} -A-@var{question}@r{[}=@var{answer}@r{]} @gol

-C -CC -D@var{macro}@r{[}=@var{defn}@r{]} -U@var{macro} -H -P}

@item Error and Warning Options

@xref{Error and Warning Options,,Options to request or suppress errors

and warnings}.

@gccoptlist{-fmax-errors=@var{n} @gol

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

-Wall  -Waliasing  -Wampersand  -Warray-bounds -Wcharacter-truncation @gol

-Wconversion -Wimplicit-interface  -Wimplicit-procedure  -Wline-truncation @gol

-Wintrinsics-std  -Wsurprising  -Wno-tabs  -Wunderflow  -Wunused-parameter @gol

-Wintrinsics-shadow  -Wno-align-commons}

@item Debugging Options

@xref{Debugging Options,,Options for debugging your program or GNU Fortran}.

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

-fdump-core -fbacktrace}

@item Directory Options

@xref{Directory Options,,Options for directory search}.

@gccoptlist{-I@var{dir}  -J@var{dir} -fintrinsic-modules-path @var{dir}}

@item Link Options

@xref{Link Options,,Options for influencing the linking step}.

@gccoptlist{-static-libgfortran}

@item Runtime Options

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

@gccoptlist{-fconvert=@var{conversion}  -fno-range-check

-frecord-marker=@var{length} @gol  -fmax-subrecord-length=@var{length}

-fsign-zero}

@item Code Generation Options

@xref{Code Gen Options,,Options for code generation conventions}.

@gccoptlist{-fno-automatic  -ff2c  -fno-underscoring @gol

-fwhole-file -fsecond-underscore @gol

-fbounds-check -fcheck-array-temporaries  -fmax-array-constructor =@var{n} @gol

-fcheck=@var{<all|array-temps|bounds|do|mem|pointer|recursion>}

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

-fpack-derived  -frepack-arrays  -fshort-enums  -fexternal-blas @gol

-fblas-matmul-limit=@var{n} -frecursive -finit-local-zero @gol

-finit-integer=@var{n} -finit-real=@var{<zero|inf|-inf|nan|snan>} @gol

-finit-logical=@var{<true|false>} -finit-character=@var{n} @gol

-fno-align-commons -fno-protect-parens}

@end table

@menu

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

                             compiled.

* Preprocessing Options::  Enable and customize preprocessing.

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

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

* Directory Options::   Where to find module files

* Link Options ::       Influencing the linking step

* 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 details of the Fortran dialect

accepted by the compiler:

@table @gcctabopt

@item -ffree-form

@item -ffixed-form

@opindex @code{ffree-form}

@opindex @code{fno-fixed-form}

@cindex options, fortran dialect

@cindex file format, free

@cindex file format, fixed

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.  When neither option is specified, the source

form is determined by the file extension.

@item -fall-intrinsics

@opindex @code{fall-intrinsics}

This option causes all intrinsic procedures (including the GNU-specific

extensions) to be accepted.  This can be useful with @option{-std=f95} to

force standard-compliance but get access to the full range of intrinsics

available with @command{gfortran}.  As a consequence, @option{-Wintrinsics-std}

will be ignored and no user-defined procedure with the same name as any

intrinsic will be called except when it is explicitly declared @code{EXTERNAL}.

@item -fd-lines-as-code

@item -fd-lines-as-comments

@opindex @code{fd-lines-as-code}

@opindex @code{fd-lines-as-comments}

Enable special treatment for lines beginning with @code{d} or @code{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.

@item -fdefault-double-8

@opindex @code{fdefault-double-8}

Set the @code{DOUBLE PRECISION} type to an 8 byte wide type.  If

@option{-fdefault-real-8} is given, @code{DOUBLE PRECISION} would

instead be promoted to 16 bytes if possible, and @option{-fdefault-double-8}

can be used to prevent this.  The kind of real constants like @code{1.d0} will

not be changed by @option{-fdefault-real-8} though, so also

@option{-fdefault-double-8} does not affect it.

@item -fdefault-integer-8

@opindex @code{fdefault-integer-8}

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

Do nothing if this is already the default.  This option also affects

the kind of integer constants like @code{42}.

@item -fdefault-real-8

@opindex @code{fdefault-real-8}

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

Do nothing if this is already the default.  This option also affects

the kind of non-double real constants like @code{1.0}, and does promote

the default width of @code{DOUBLE PRECISION} to 16 bytes if possible, unless

@code{-fdefault-double-8} is given, too.

@item -fdollar-ok

@opindex @code{fdollar-ok}

@cindex $

@cindex symbol names

@cindex character set

Allow @samp{$} as a valid non-first character in a symbol name. Symbols

that start with @samp{$} are rejected since it is unclear which rules to

apply to implicit typing as different vendors implement different rules.

Using @samp{$} in @code{IMPLICIT} statements is also rejected.

@item -fbackslash

@opindex @code{backslash}

@cindex backslash

@cindex escape characters

Change the interpretation of backslashes in string literals from a single

backslash character to ``C-style'' escape characters. The following

combinations are expanded @code{\a}, @code{\b}, @code{\f}, @code{\n},

@code{\r}, @code{\t}, @code{\v}, @code{\\}, and @code{\0} to the ASCII

characters alert, backspace, form feed, newline, carriage return,

horizontal tab, vertical tab, backslash, and NUL, respectively.

Additionally, @code{\x}@var{nn}, @code{\u}@var{nnnn} and

@code{\U}@var{nnnnnnnn} (where each @var{n} is a hexadecimal digit) are

translated into the Unicode characters corresponding to the specified code

points. All other combinations of a character preceded by \ are

unexpanded.

@item -fmodule-private

@opindex @code{fmodule-private}

@cindex module entities

@cindex private

Set the default accessibility of module entities to @code{PRIVATE}.

Use-associated entities will not be accessible unless they are explicitly

declared as @code{PUBLIC}.

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

@opindex @code{ffixed-line-length-}@var{n}

@cindex file format, fixed

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.

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

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

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

@var{n} may also 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}.

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

@opindex @code{ffree-line-length-}@var{n}

@cindex file format, free

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

lines in the source file. 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}.

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

@opindex @code{fmax-identifier-length=}@var{n}

Specify the maximum allowed identifier length. Typical values are

31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).

@item -fimplicit-none

@opindex @code{fimplicit-none}

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

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

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

@item -fcray-pointer

@opindex @code{fcray-pointer}

Enable the Cray pointer extension, which provides C-like pointer

functionality.

@item -fopenmp

@opindex @code{fopenmp}

@cindex OpenMP

Enable the OpenMP extensions.  This includes OpenMP @code{!$omp} directives

in free form

and @code{c$omp}, @code{*$omp} and @code{!$omp} directives in fixed form,

@code{!$} conditional compilation sentinels in free form

and @code{c$}, @code{*$} and @code{!$} sentinels in fixed form,

and when linking arranges for the OpenMP runtime library to be linked

in.  The option @option{-fopenmp} implies @option{-frecursive}.

@item -fno-range-check

@opindex @code{frange-check}

Disable range checking on results of simplification of constant

expressions during compilation.  For example, GNU Fortran will give

an error at compile time when simplifying @code{a = 1. / 0}.

With this option, no error will be given and @code{a} will be assigned

the value @code{+Infinity}.  If an expression evaluates to a value

outside of the relevant range of [@code{-HUGE()}:@code{HUGE()}],

then the expression will be replaced by @code{-Inf} or @code{+Inf}

as appropriate.

Similarly, @code{DATA i/Z'FFFFFFFF'/} will result in an integer overflow

on most systems, but with @option{-fno-range-check} the value will

``wrap around'' and @code{i} will be initialized to @math{-1} instead.

@item -std=@var{std}

@opindex @code{std=}@var{std} option

Specify the standard to which the program is expected to conform, which

may be one of @samp{f95}, @samp{f2003}, @samp{f2008}, @samp{gnu}, or

@samp{legacy}.  The default value for @var{std} is @samp{gnu}, which

specifies a superset of the Fortran 95 standard that includes all of the

extensions supported by GNU Fortran, although warnings will be given for

obsolete extensions not recommended for use in new code.  The

@samp{legacy} value is equivalent but without the warnings for obsolete

extensions, and may be useful for old non-standard programs.  The

@samp{f95}, @samp{f2003} and @samp{f2008} values specify strict

conformance to the Fortran 95, Fortran 2003 and Fortran 2008 standards,

respectively; errors are given for all extensions beyond the relevant

language standard, and warnings are given for the Fortran 77 features

that are permitted but obsolescent in later standards.

@end table

@node Preprocessing Options

@section Enable and customize preprocessing

@cindex preprocessor

@cindex options, preprocessor

@cindex CPP

Preprocessor related options. See section

@ref{Preprocessing and conditional compilation} for more detailed

information on preprocessing in @command{gfortran}.

@table @gcctabopt

@item -cpp

@item -nocpp

@opindex @code{cpp}

@opindex @code{fpp}

@cindex preprocessor, enable

@cindex preprocessor, disable

Enable preprocessing. The preprocessor is automatically invoked if

the file extension is @file{.fpp}, @file{.FPP},  @file{.F}, @file{.FOR},

@file{.FTN}, @file{.F90}, @file{.F95}, @file{.F03} or @file{.F08}. Use

this option to manually enable preprocessing of any kind of Fortran file.

To disable preprocessing of files with any of the above listed extensions,

use the negative form: @option{-nocpp}.

The preprocessor is run in traditional mode, be aware that any

restrictions of the file-format, e.g. fixed-form line width,

apply for preprocessed output as well.

@item -dM

@opindex @code{dM}

@cindex preprocessor, debugging

@cindex debugging, preprocessor

Instead of the normal output, generate a list of @code{'#define'}

directives for all the macros defined during the execution of the

preprocessor, including predefined macros. This gives you a way

of finding out what is predefined in your version of the preprocessor.

Assuming you have no file @file{foo.f90}, the command

@smallexample

  touch foo.f90; gfortran -cpp -dM foo.f90

@end smallexample

will show all the predefined macros.

@item -dD

@opindex @code{dD}

@cindex preprocessor, debugging

@cindex debugging, preprocessor

Like @option{-dM} except in two respects: it does not include the

predefined macros, and it outputs both the @code{#define} directives

and the result of preprocessing. Both kinds of output go to the

standard output file.

@item -dN

@opindex @code{dN}

@cindex preprocessor, debugging

@cindex debugging, preprocessor

Like @option{-dD}, but emit only the macro names, not their expansions.

@item -dU

@opindex @code{dU}

@cindex preprocessor, debugging

@cindex debugging, preprocessor

Like @option{dD} except that only macros that are expanded, or whose

definedness is tested in preprocessor directives, are output; the

output is delayed until the use or test of the macro; and @code{'#undef'}

directives are also output for macros tested but undefined at the time.

@item -dI

@opindex @code{dI}

@cindex preprocessor, debugging

@cindex debugging, preprocessor

Output @code{'#include'} directives in addition to the result

of preprocessing.

@item -fworking-directory

@opindex @code{fworking-directory}

@cindex preprocessor, working directory

Enable generation of linemarkers in the preprocessor output that will

let the compiler know the current working directory at the time of

preprocessing. When this option is enabled, the preprocessor will emit,

after the initial linemarker, a second linemarker with the current

working directory followed by two slashes. GCC will use this directory,

when it's present in the preprocessed input, as the directory emitted

as the current working directory in some debugging information formats.

This option is implicitly enabled if debugging information is enabled,

but this can be inhibited with the negated form

@option{-fno-working-directory}. If the @option{-P} flag is present

in the command line, this option has no effect, since no @code{#line}

directives are emitted whatsoever.

@item -idirafter @var{dir}

@opindex @code{idirafter @var{dir}}

@cindex preprocessing, include path

Search @var{dir} for include files, but do it after all directories

specified with @option{-I} and the standard system directories have

been exhausted. @var{dir} is treated as a system include directory.

If dir begins with @code{=}, then the @code{=} will be replaced by

the sysroot prefix; see @option{--sysroot} and @option{-isysroot}.

@item -imultilib @var{dir}

@opindex @code{imultilib @var{dir}}

@cindex preprocessing, include path

Use @var{dir} as a subdirectory of the directory containing target-specific

C++ headers.

@item -iprefix @var{prefix}

@opindex @code{iprefix @var{prefix}}

@cindex preprocessing, include path

Specify @var{prefix} as the prefix for subsequent @option{-iwithprefix}

options. If the @var{prefix} represents a directory, you should include

the final @code{'/'}.

@item -isysroot @var{dir}

@opindex @code{isysroot @var{dir}}

@cindex preprocessing, include path

This option is like the @option{--sysroot} option, but applies only to

header files. See the @option{--sysroot} option for more information.

@item -iquote @var{dir}

@opindex @code{iquote @var{dir}}

@cindex preprocessing, include path

Search @var{dir} only for header files requested with @code{#include "file"};

they are not searched for @code{#include <file>}, before all directories

specified by @option{-I} and before the standard system directories. If

@var{dir} begins with @code{=}, then the @code{=} will be replaced by the

sysroot prefix; see @option{--sysroot} and @option{-isysroot}.

@item -isystem @var{dir}

@opindex @code{isystem @var{dir}}

@cindex preprocessing, include path

Search @var{dir} for header files, after all directories specified by

@option{-I} but before the standard system directories. Mark it as a

system directory, so that it gets the same special treatment as is

applied to the standard system directories. If @var{dir} begins with

@code{=}, then the @code{=} will be replaced by the sysroot prefix;

see @option{--sysroot} and @option{-isysroot}.

@item -nostdinc

@opindex @code{nostdinc}

Do not search the standard system directories for header files. Only

the directories you have specified with @option{-I} options (and the

directory of the current file, if appropriate) are searched.

@item -undef

@opindex @code{undef}

Do not predefine any system-specific or GCC-specific macros.

The standard predefined macros remain defined.

@item -A@var{predicate}=@var{answer}

@opindex @code{A@var{predicate}=@var{answer}}

@cindex preprocessing, assertation

Make an assertion with the predicate @var{predicate} and answer @var{answer}.

This form is preferred to the older form -A predicate(answer), which is still

supported, because it does not use shell special characters.

@item -A-@var{predicate}=@var{answer}

@opindex @code{A-@var{predicate}=@var{answer}}

@cindex preprocessing, assertation

Cancel an assertion with the predicate @var{predicate} and answer @var{answer}.

@item -C

@opindex @code{C}

@cindex preprocessing, keep comments

Do not discard comments. All comments are passed through to the output

file, except for comments in processed directives, which are deleted

along with the directive.

You should be prepared for side effects when using @option{-C}; it causes

the preprocessor to treat comments as tokens in their own right. For example,

comments appearing at the start of what would be a directive line have the

effect of turning that line into an ordinary source line, since the first

token on the line is no longer a @code{'#'}.

Warning: this currently handles C-Style comments only. The preprocessor

does not yet recognize Fortran-style comments.

@item -CC

@opindex @code{CC}

@cindex preprocessing, keep comments

Do not discard comments, including during macro expansion. This is like

@option{-C}, except that comments contained within macros are also passed

through to the output file where the macro is expanded.

In addition to the side-effects of the @option{-C} option, the @option{-CC}

option causes all C++-style comments inside a macro to be converted to C-style

comments. This is to prevent later use of that macro from inadvertently

commenting out the remainder of the source line. The @option{-CC} option

is generally used to support lint comments.

Warning: this currently handles C- and C++-Style comments only. The

preprocessor does not yet recognize Fortran-style comments.

@item -D@var{name}

@opindex @code{D@var{name}}

@cindex preprocessing, define macros

Predefine name as a macro, with definition @code{1}.

@item -D@var{name}=@var{definition}

@opindex @code{D@var{name}=@var{definition}}

@cindex preprocessing, define macros

The contents of @var{definition} are tokenized and processed as if they

appeared during translation phase three in a @code{'#define'} directive.

In particular, the definition will be truncated by embedded newline

characters.

If you are invoking the preprocessor from a shell or shell-like program

you may need to use the shell's quoting syntax to protect characters such

as spaces that have a meaning in the shell syntax.

If you wish to define a function-like macro on the command line, write

its argument list with surrounding parentheses before the equals sign

(if any). Parentheses are meaningful to most shells, so you will need

to quote the option. With sh and csh, @code{-D'name(args...)=definition'}

works.

@option{-D} and @option{-U} options are processed in the order they are

given on the command line. All -imacros file and -include file options

are processed after all -D and -U options.

@item -H

@opindex @code{H}

Print the name of each header file used, in addition to other normal

activities. Each name is indented to show how deep in the @code{'#include'}

stack it is.

@item -P

@opindex @code{P}

@cindex preprocessing, no linemarkers

Inhibit generation of linemarkers in the output from the preprocessor.

This might be useful when running the preprocessor on something that

is not C code, and will be sent to a program which might be confused

by the linemarkers.

@item -U@var{name}

@opindex @code{U@var{name}}

@cindex preprocessing, undefine macros

Cancel any previous definition of @var{name}, either built in or provided

with a @option{-D} option.

@end table

@node Error and Warning Options

@section Options to request or suppress errors and warnings

@cindex options, warnings

@cindex options, errors

@cindex warnings, suppressing

@cindex messages, error

@cindex messages, warning

@cindex suppressing warnings

Errors are diagnostic messages that report that the GNU Fortran compiler

cannot compile the relevant piece of source code.  The compiler will

continue to process the program in an attempt to report further errors

to aid in debugging, but will not produce any compiled output.

Warnings are diagnostic messages that report constructions which

are not inherently erroneous but which are risky or suggest there is

likely to be a bug in the program.  Unless @option{-Werror} is specified,

they do not prevent compilation of the program.

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 errors and warnings produced

by GNU Fortran:

@table @gcctabopt

@item -fmax-errors=@var{n}

@opindex @code{fmax-errors=}@var{n}

@cindex errors, limiting

Limits the maximum number of error messages to @var{n}, at which point

GNU Fortran bails out rather than attempting to continue processing the

source code.  If @var{n} is 0, there is no limit on the number of error

messages produced.

@item -fsyntax-only

@opindex @code{fsyntax-only}

@cindex syntax checking

Check the code for syntax errors, but don't actually compile it.  This

will generate module files for each module present in the code, but no

other output file.

@item -pedantic

@opindex @code{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 @code{#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 GNU Fortran in this area are welcome.

This should be used in conjunction with @option{-std=f95},

@option{-std=f2003} or @option{-std=f2008}.

@item -pedantic-errors

@opindex @code{pedantic-errors}

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

warnings.

@item -Wall

@opindex @code{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{-Waliasing},

@option{-Wampersand}, @option{-Wsurprising}, @option{-Wintrinsics-std},

@option{-Wno-tabs}, @option{-Wintrinsic-shadow} and @option{-Wline-truncation}.

@item -Waliasing

@opindex @code{Waliasing}

@cindex aliasing

@cindex warnings, 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

@item -Wampersand

@opindex @code{Wampersand}

@cindex warnings, ampersand

@cindex &

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

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

@option{-std=f2003} and @option{-std=f2008}. Note: With no ampersand

given in a continued character constant, GNU Fortran assumes continuation

at the first non-comment, non-whitespace character after the ampersand

that initiated the continuation.

@item -Warray-temporaries

@opindex @code{Warray-temporaries}

@cindex warnings, array temporaries

Warn about array temporaries generated by the compiler.  The information

generated by this warning is sometimes useful in optimization, in order to

avoid such temporaries.

@item -Wcharacter-truncation

@opindex @code{Wcharacter-truncation}

@cindex warnings, character truncation

Warn when a character assignment will truncate the assigned string.

@item -Wline-truncation

@opindex @code{Wline-truncation}

@cindex warnings, line truncation

Warn when a source code line will be truncated.

@item -Wconversion

@opindex @code{Wconversion}

@cindex warnings, conversion

@cindex conversion

Warn about implicit conversions between different types.

@item -Wimplicit-interface

@opindex @code{Wimplicit-interface}

@cindex warnings, implicit interface

Warn if a procedure is 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.

@item -Wimplicit-procedure

@opindex @code{Wimplicit-procedure}

@cindex warnings, implicit procedure

Warn if a procedure is called that has neither an explicit interface

nor has been declared as @code{EXTERNAL}.

@item -Wintrinsics-std

@opindex @code{Wintrinsics-std}

@cindex warnings, non-standard intrinsics

@cindex warnings, intrinsics of other standards

Warn if @command{gfortran} finds a procedure named like an intrinsic not

available in the currently selected standard (with @option{-std}) and treats

it as @code{EXTERNAL} procedure because of this.  @option{-fall-intrinsics} can

be used to never trigger this behavior and always link to the intrinsic

regardless of the selected standard.

@item -Wsurprising

@opindex @code{Wsurprising}

@cindex warnings, suspicious code

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.

@item

A TRANSFER specifies a source that is shorter than the destination.

@item

The type of a function result is declared more than once with the same type.  If

@option{-pedantic} or standard-conforming mode is enabled, this is an error.

@item

A @code{CHARACTER} variable is declared with negative length.

@end itemize

@item -Wtabs

@opindex @code{Wtabs}

@cindex warnings, tabs

@cindex tabulators

By default, tabs are accepted as whitespace, but tabs are not members

of the Fortran Character Set.  For continuation lines, a tab followed

by a digit between 1 and 9 is supported.  @option{-Wno-tabs} will cause

a warning to be issued if a tab is encountered. Note, @option{-Wno-tabs}

is active for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003},

@option{-std=f2008} and @option{-Wall}.

@item -Wunderflow

@opindex @code{Wunderflow}

@cindex warnings, underflow

@cindex underflow

Produce a warning when numerical constant expressions are

encountered, which yield an UNDERFLOW during compilation.

@item -Wintrinsic-shadow

@opindex @code{Wintrinsic-shadow}

@cindex warnings, intrinsic

@cindex intrinsic

Warn if a user-defined procedure or module procedure has the same name as an

intrinsic; in this case, an explicit interface or @code{EXTERNAL} or

@code{INTRINSIC} declaration might be needed to get calls later resolved to

the desired intrinsic/procedure.

@item -Wunused-parameter

@opindex @code{Wunused-parameter}

@cindex warnings, unused parameter

@cindex unused parameter

Contrary to @command{gcc}'s meaning of @option{-Wunused-parameter},

@command{gfortran}'s implementation of this option does not warn

about unused dummy arguments, but about unused @code{PARAMETER} values.

@option{-Wunused-parameter} is not included in @option{-Wall} but is

implied by @option{-Wall -Wextra}.

@item -Walign-commons

@opindex @code{Walign-commons}

@cindex warnings, alignment of COMMON blocks

@cindex alignment of COMMON blocks

By default, @command{gfortran} warns about any occasion of variables being

padded for proper alignment inside a COMMON block. This warning can be turned

off via @option{-Wno-align-commons}. See also @option{-falign-commons}.

@item -Werror

@opindex @code{Werror}

@cindex warnings, to errors

Turns all warnings into errors.

@end table

@xref{Error and Warning Options,,Options to Request or Suppress Errors and

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 the GNU Fortran compiler.

@table @gcctabopt

@item -fdump-parse-tree

@opindex @code{fdump-parse-tree}

Output the internal parse tree before starting code generation.  Only

really useful for debugging the GNU Fortran compiler itself.

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

@opindex @code{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 value).

Some of the routines in the Fortran runtime library, like

@samp{CPU_TIME}, are likely to trigger floating point exceptions when

@code{ffpe-trap=precision} is used. For this reason, the use of

@code{ffpe-trap=precision} is not recommended.

@item -fbacktrace

@opindex @code{fbacktrace}

@cindex backtrace

@cindex trace

Specify that, when a runtime error is encountered or a deadly signal is

emitted (segmentation fault, illegal instruction, bus error or

floating-point exception), the Fortran runtime

library should output a backtrace of the error.  This option

only has influence for compilation of the Fortran main program.

@item -fdump-core

@cindex core, dump

@opindex @code{fdump-core}

Request that a core-dump file is written to disk when a runtime error

is encountered on systems that support core dumps. This option is

only effective for the compilation of the Fortran main program.

@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 GNU Fortran 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

@item -I@var{dir}

@opindex @code{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