re PR fortran/32634 (renamed, use associated generic interface rejected)

[gcc.git] / gcc / fortran / invoke.texi

@c Copyright (C) 2004, 2005, 2006, 2007
@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
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 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
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.
* 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
* Runtime Options::     Influencing runtime behavior
* Code Gen Options::    Specifying conventions for function calls, data layout
                        and register usage.
* Environment Variables:: Env vars 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  -frange-check -fno-backslash }

@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
-w  -Wall  -Waliasing  -Wampersand  -Wcharacter-truncation  -Wconversion @gol
-Wimplicit-interface  -Wline-truncation  -Wnonstd-intrinsics  -Wsurprising @gol
-Wno-tabs  -Wunderflow  -W}

@item Debugging Options
@xref{Debugging Options,,Options for Debugging Your Program or GCC}.
@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}  -M@var{dir}  -fintrinsic-modules-path @var{dir}}

@item Runtime Options
@xref{Runtime Options,,Options for influencing runtime behavior}.
@gccoptlist{-fconvert=@var{conversion}  -frecord-marker=@var{length} @gol
-fmax-subrecord-length=@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
-fpack-derived  -frepack-arrays  -fshort-enums  -fexternal-blas @gol
-fblas-matmul-limit=@var{n}}
@end table

@menu
* Fortran Dialect Options::  Controlling the variant of Fortran language
                             compiled.
* 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
* 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}
Accept all of the intrinsic procedures provided in libgfortran 
without regard to the setting of @option{-std}.  In particular, 
this option can be quite useful with @option{-std=f95}.  Additionally,
@command{gfortran} will ignore @option{-Wnonstd-intrinsics}.

@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.

@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.

@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.

@item -fdollar-ok
@opindex @code{fdollar-ok}
@cindex $
@cindex symbol names
@cindex character set
Allow @samp{$} as a valid character in a symbol name.

@item -fno-backslash
@opindex @code{fno-backslash}
@cindex backslash
@cindex escape characters
Change the interpretation of backslashes in string literals from
``C-style'' escape characters to a single backslash character.

@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).

@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.

@item -frange-check
@opindex @code{frange-check}
Enable range checking on results of simplification of constant
expressions during compilation.  For example, by default, GNU Fortran
will give an overflow error at compile time when simplifying @code{a =
EXP(1000)}. With @option{-fno-range-check}, no error will be given and
the variable @code{a} will be assigned the value @code{+Infinity}.
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{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} and
@samp{f2003} values specify strict conformance to the Fortran 95 and
Fortran 2003 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 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 do anything beyond that.

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

@item -pedantic-errors
@opindex @code{pedantic-errors}
Like @option{-pedantic}, except that errors are produced rather than
warnings.

@item -w
@opindex @code{w}
@cindex warnings, none
Inhibit all warning messages.

@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{-Wnonstd-intrinsics},
@option{-Wno-tabs}, 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}, and
@option{-std=f2003}. 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 -Wcharacter-truncation
@opindex @code{Wcharacter-truncation}
@cindex warnings, character truncation
Warn when a character assignment will truncate the assigned string.

@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 -Wnonstd-intrinsics
@opindex @code{Wnonstd-intrinsics}
@cindex warnings, non-stdandard intrinsics
Warn if the user tries to use an intrinsic that does not belong to the 
standard the user has chosen via the @option{-std} option.

@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.
@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.  @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}, 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 -Werror
@opindex @code{Werror}
@cindex warnings, to errors
Turns all warnings into errors.

@item -W
@opindex @code{W}
@cindex warnings, extra
@cindex extra warnings
Turns on ``extra warnings'' and, if optimization is specified
via @option{-O}, the @option{-Wuninitialized} option.
(This might change in future versions of GNU Fortran.)
@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).

@item -fbacktrace
@opindex @code{fbacktrace}
@cindex backtrace
@cindex trace
Specify that, when a runtime error is encountered, 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
@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 @file{.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.

@item -M@var{dir}
@item -J@var{dir}
@opindex @code{M}@var{dir}
@opindex @code{J}@var{dir}
@cindex paths, search
@cindex module search path
This option specifies where to put @file{.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.

@item -fintrinsic-modules-path @var{dir}
@opindex @code{fintrinsic-modules-path} @var{dir}
@cindex paths, search
@cindex module search path
This option specifies the location of pre-compiled intrinsic modules, if
they are not in the default location expected by the compiler.
@end table

@node Runtime Options
@section Influencing runtime behavior
@cindex options, runtime

These options affect the runtime behavior of programs compiled with GNU Fortran.
@table @gcctabopt
@item -fconvert=@var{conversion}
@opindex @code{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 @option{-fconvert}.}

@item -frecord-marker=@var{length}
@opindex @code{frecord-marker=}@var{length}
Specify the length of record markers for unformatted files.
Valid values for @var{length} are 4 and 8.  Default is 4.
@emph{This is different from previous versions of gfortran},
which specified a default record marker length of 8 on most
systems.  If you want to read or write files compatible
with earlier versions of gfortran, use @option{-frecord-marker=8}.

@item -fmax-subrecord-length=@var{length}
@opindex @code{fmax-subrecord-length=}@var{length}
Specify the maximum length for a subrecord.  The maximum permitted
value for length is 2147483639, which is also the default.  Only
really useful for use by the gfortran testsuite.
@end table

@node Code Gen Options
@section Options for Code Generation Conventions
@cindex code generation, conventions
@cindex options, code generation
@cindex options, run-time

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
@item -fno-automatic
@opindex @code{fno-automatic}
@cindex @code{SAVE} statement
@cindex statement, @code{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}.)

@item -ff2c
@opindex 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
@option{-ff2c} with code compiled with the default @option{-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 @option{-fno-f2c} calling conventions.

@item -fno-underscoring
@opindex @code{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, GNU Fortran 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 GNU Fortran is
incompatible with @command{f2c} and @command{g77}, please use the
@option{-ff2c} option if you want object files compiled with
GNU Fortran 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 @code{j()} and @code{max_count()} are
external functions while @code{my_var} and @code{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 GNU Fortran
code with other languages.

Note that just because the names match does @emph{not} mean that the
interface implemented by GNU Fortran for an external name matches the
interface implemented by some other language for that same name.
That is, getting code produced by GNU Fortran 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 GNU Fortran 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.

@item -fsecond-underscore
@opindex @code{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, GNU Fortran appends an underscore to external
names.  If this option is used GNU Fortran appends two
underscores to names with underscores and one underscore to external names
with no underscores.  GNU Fortran 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 @code{MAX_COUNT}
is implemented as a reference to the link-time external symbol
@code{max_count__}, instead of @code{max_count_}.  This is required
for compatibility with @command{g77} and @command{f2c}, and is implied
by use of the @option{-ff2c} option.

@item -fbounds-check
@opindex @code{fbounds-check}
@cindex array, bounds checking
@cindex bounds checking
@cindex range 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, e.g., checking
substring references.

@item -fmax-stack-var-size=@var{n}
@opindex @code{fmax-stack-var-size}
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 GNU Fortran may improve this behavior.

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

@item -fpack-derived
@opindex @code{fpack-derived}
@cindex structure packing
This option tells GNU Fortran 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.

@item -frepack-arrays
@opindex @code{frepack-arrays}
@cindex repacking arrays
In some circumstances GNU Fortran may pass assumed shape array
sections via a descriptor describing a noncontiguous 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 noncontiguous.

@item -fshort-enums
@opindex @code{fshort-enums}
This option is provided for interoperability with C code that was
compiled with the @option{-fshort-enums} option.  It will make
GNU Fortran choose the smallest @code{INTEGER} kind a given
enumerator set will fit in, and give all its enumerators this kind.

@item -fexternal-blas
@opindex @code{fexternal-blas}
This option will make gfortran generate calls to BLAS functions for some
matrix operations like @code{MATMUL}, instead of using our own
algorithms, if the size of the matrices involved is larger than a given
limit (see @option{-fblas-matmul-limit}).  This may be profitable if an
optimized vendor BLAS library is available.  The BLAS library will have
to be specified at link time.

@item -fblas-matmul-limit=@var{n}
@opindex @code{fblas-matmul-limit}
Only significant when @option{-fexternal-blas} is in effect.
Matrix multiplication of matrices with size larger than (or equal to) @var{n}
will be performed by calls to BLAS functions, while others will be
handled by @command{gfortran} internal algorithms. If the matrices
involved are not square, the size comparison is performed using the
geometric mean of the dimensions of the argument and result matrices.

The default value for @var{n} is 30.

@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 @command{gfortran}
@cindex environment variable

@c man begin ENVIRONMENT

The @command{gfortran} compiler 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 programs compiled with GNU Fortran.
@c man end