@c Copyright (C) 2004-2018 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{} 2004-2018 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 1.3 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), dbx(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.
* Interoperability Options::  Options for interoperability with other
                              languages.
* 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 -fbackslash -fcray-pointer -fd-lines-as-code @gol
-fd-lines-as-comments @gol
-fdec -fdec-structure -fdec-intrinsic-ints -fdec-static -fdec-math @gol
-fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 @gol
-fdefault-real-10 -fdefault-real-16 -fdollar-ok -ffixed-line-length-@var{n} @gol
-ffixed-line-length-none -ffree-form -ffree-line-length-@var{n} @gol
-ffree-line-length-none -fimplicit-none -finteger-4-integer-8 @gol
-fmax-identifier-length -fmodule-private -ffixed-form -fno-range-check @gol
-fopenacc -fopenmp -freal-4-real-10 -freal-4-real-16 -freal-4-real-8 @gol
-freal-8-real-10 -freal-8-real-16 -freal-8-real-4 -std=@var{std}
-ftest-forall-temp
}

@item Preprocessing Options
@xref{Preprocessing Options,,Enable and customize preprocessing}.
@gccoptlist{-A-@var{question}@r{[}=@var{answer}@r{]}
-A@var{question}=@var{answer} -C -CC -D@var{macro}@r{[}=@var{defn}@r{]}
-H -P @gol
-U@var{macro} -cpp -dD -dI -dM -dN -dU -fworking-directory
-imultilib @var{dir} @gol
-iprefix @var{file} -iquote -isysroot @var{dir} -isystem @var{dir} -nocpp 
-nostdinc @gol
-undef
}

@item Error and Warning Options
@xref{Error and Warning Options,,Options to request or suppress errors
and warnings}.
@gccoptlist{-Waliasing -Wall -Wampersand -Wargument-mismatch -Warray-bounds
-Wc-binding-type -Wcharacter-truncation -Wconversion @gol
-Wdo-subscript -Wfunction-elimination -Wimplicit-interface @gol
-Wimplicit-procedure -Wintrinsic-shadow -Wuse-without-only -Wintrinsics-std @gol
-Wline-truncation -Wno-align-commons -Wno-tabs -Wreal-q-constant @gol
-Wsurprising -Wunderflow -Wunused-parameter -Wrealloc-lhs @gol
-Wrealloc-lhs-all -Wfrontend-loop-interchange -Wtarget-lifetime @gol
-fmax-errors=@var{n} -fsyntax-only -pedantic -pedantic-errors @gol
}

@item Debugging Options
@xref{Debugging Options,,Options for debugging your program or GNU Fortran}.
@gccoptlist{-fbacktrace -fdump-fortran-optimized -fdump-fortran-original @gol
-fdump-parse-tree -ffpe-trap=@var{list} -ffpe-summary=@var{list}
}

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

@item Interoperability Options
@xref{Interoperability Options,,Options for interoperability}.
@gccoptlist{-fc-prototypes}

@item Code Generation Options
@xref{Code Gen Options,,Options for code generation conventions}.
@gccoptlist{-faggressive-function-elimination -fblas-matmul-limit=@var{n} @gol
-fbounds-check -fcheck-array-temporaries @gol
-fcheck=@var{<all|array-temps|bounds|do|mem|pointer|recursion>} @gol
-fcoarray=@var{<none|single|lib>} -fexternal-blas -ff2c
-ffrontend-loop-interchange @gol
-ffrontend-optimize @gol
-finit-character=@var{n} -finit-integer=@var{n} -finit-local-zero @gol
-finit-derived @gol
-finit-logical=@var{<true|false>}
-finit-real=@var{<zero|inf|-inf|nan|snan>} @gol
-finline-matmul-limit=@var{n} @gol
-fmax-array-constructor=@var{n} -fmax-stack-var-size=@var{n}
-fno-align-commons @gol
-fno-automatic -fno-protect-parens -fno-underscoring @gol
-fsecond-underscore -fpack-derived -frealloc-lhs -frecursive @gol
-frepack-arrays -fshort-enums -fstack-arrays
}
@end table

@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
@itemx -ffixed-form
@opindex @code{ffree-form}
@opindex @code{ffixed-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
@itemx -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 -fdec
@opindex @code{fdec}
DEC compatibility mode. Enables extensions and other features that mimic
the default behavior of older compilers (such as DEC).
These features are non-standard and should be avoided at all costs.
For details on GNU Fortran's implementation of these extensions see the
full documentation.

Other flags enabled by this switch are:
@option{-fdollar-ok} @option{-fcray-pointer} @option{-fdec-structure}
@option{-fdec-intrinsic-ints} @option{-fdec-static} @option{-fdec-math}

If @option{-fd-lines-as-code}/@option{-fd-lines-as-comments} are unset, then
@option{-fdec} also sets @option{-fd-lines-as-comments}.

@item -fdec-structure
@opindex @code{fdec-structure}
Enable DEC @code{STRUCTURE} and @code{RECORD} as well as @code{UNION},
@code{MAP}, and dot ('.') as a member separator (in addition to '%'). This is
provided for compatibility only; Fortran 90 derived types should be used
instead where possible.

@item -fdec-intrinsic-ints
@opindex @code{fdec-intrinsic-ints}
Enable B/I/J/K kind variants of existing integer functions (e.g. BIAND, IIAND,
JIAND, etc...). For a complete list of intrinsics see the full documentation.

@item -fdec-math
@opindex @code{fdec-math}
Enable legacy math intrinsics such as COTAN and degree-valued trigonometric
functions (e.g. TAND, ATAND, etc...) for compatability with older code.

@item -fdec-static
@opindex @code{fdec-static}
Enable DEC-style STATIC and AUTOMATIC attributes to explicitly specify
the storage of variables and other objects.

@item -fdollar-ok
@opindex @code{fdollar-ok}
@cindex @code{$}
@cindex symbol names
@cindex character set
Allow @samp{$} 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 -fopenacc
@opindex @code{fopenacc}
@cindex OpenACC
Enable the OpenACC extensions.  This includes OpenACC @code{!$acc}
directives in free form and @code{c$acc}, @code{*$acc} and
@code{!$acc} 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
OpenACC runtime library to be linked in.

Note that this is an experimental feature, incomplete, and subject to
change in future versions of GCC.  See
@w{@uref{https://gcc.gnu.org/wiki/OpenACC}} for more information.

@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 -fdefault-integer-8
@opindex @code{fdefault-integer-8}
Set the default integer and logical types to an 8 byte wide type.  This option
also affects the kind of integer constants like @code{42}. Unlike
@option{-finteger-4-integer-8}, it does not promote variables with explicit
kind declaration.

@item -fdefault-real-8
@opindex @code{fdefault-real-8}
Set the default real type to an 8 byte wide type. 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. Unlike @option{-freal-4-real-8},
it does not promote variables with explicit kind declaration.

@item -fdefault-real-10
@opindex @code{fdefault-real-10}
Set the default real type to a 10 byte wide type. 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. Unlike @option{-freal-4-real-10},
it does not promote variables with explicit kind declaration.

@item -fdefault-real-16
@opindex @code{fdefault-real-16}
Set the default real type to a 16 byte wide type. 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. Unlike @option{-freal-4-real-16},
it does not promote variables with explicit kind declaration.

@item -fdefault-double-8
@opindex @code{fdefault-double-8}
Set the @code{DOUBLE PRECISION} type to an 8 byte wide type.  Do nothing if this
is already the default.  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 -finteger-4-integer-8
@opindex @code{finteger-4-integer-8}
Promote all @code{INTEGER(KIND=4)} entities to an @code{INTEGER(KIND=8)}
entities.  If @code{KIND=8} is unavailable, then an error will be issued.
This option should be used with care and may not be suitable for your codes.
Areas of possible concern include calls to external procedures,
alignment in @code{EQUIVALENCE} and/or @code{COMMON}, generic interfaces,
BOZ literal constant conversion, and I/O.  Inspection of the intermediate
representation of the translated Fortran code, produced by
@option{-fdump-tree-original}, is suggested.

@item  -freal-4-real-8
@itemx -freal-4-real-10
@itemx -freal-4-real-16
@itemx -freal-8-real-4
@itemx -freal-8-real-10
@itemx -freal-8-real-16
@opindex @code{freal-4-real-8}
@opindex @code{freal-4-real-10}
@opindex @code{freal-4-real-16}
@opindex @code{freal-8-real-4}
@opindex @code{freal-8-real-10}
@opindex @code{freal-8-real-16}
@cindex options, real kind type promotion
Promote all @code{REAL(KIND=M)} entities to @code{REAL(KIND=N)} entities.
If @code{REAL(KIND=N)} is unavailable, then an error will be issued.
All other real kind types are unaffected by this option.
These options should be used with care and may not be suitable for your
codes.  Areas of possible concern include calls to external procedures,
alignment in @code{EQUIVALENCE} and/or @code{COMMON}, generic interfaces,
BOZ literal constant conversion, and I/O.  Inspection of the intermediate
representation of the translated Fortran code, produced by
@option{-fdump-tree-original}, is suggested.

@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{f2018}, @samp{gnu}, or @samp{legacy}.  The default value for
@var{std} is @samp{gnu}, which specifies a superset of the latest
Fortran 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}, @samp{f2008}, and @samp{f2018} values specify strict
conformance to the Fortran 95, Fortran 2003, Fortran 2008 and Fortran
2018 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. @samp{-std=f2008ts} allows the Fortran 2008 standard
including the additions of the Technical Specification (TS) 29113 on
Further Interoperability of Fortran with C and TS 18508 on Additional
Parallel Features in Fortran.

@item -ftest-forall-temp
@opindex @code{ftest-forall-temp}
Enhance test coverage by forcing most forall assignments to use temporary.

@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
@itemx -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. Any restrictions of the
file-format, especially the limits on line length, apply for
preprocessed output as well, so it might be advisable to use the
@option{-ffree-line-length-none} or @option{-ffixed-line-length-none}
options.

@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 -E -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 is 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, assertion
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, assertion
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 do not actually compile it.  This
will generate module files for each module present in the code, but no
other output file.

@item -Wpedantic
@itemx -pedantic
@opindex @code{pedantic}
@opindex @code{Wpedantic}
Issue warnings for uses of extensions to Fortran.
@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 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}, @option{-std=f2008} or @option{-std=f2018}.

@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{-Wconversion}, @option{-Wsurprising}, @option{-Wc-binding-type},
@option{-Wintrinsics-std}, @option{-Wtabs}, @option{-Wintrinsic-shadow},
@option{-Wline-truncation}, @option{-Wtarget-lifetime},
@option{-Winteger-division}, @option{-Wreal-q-constant}, @option{-Wunused}
and @option{-Wundefined-do-loop}.

@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 @code{&}
Warn about missing ampersand in continued character constants. The
warning is given with @option{-Wampersand}, @option{-pedantic},
@option{-std=f95}, @option{-std=f2003}, @option{-std=f2008} and
@option{-std=f2018}. 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 -Wargument-mismatch
@opindex @code{Wargument-mismatch}
@cindex warnings, argument mismatch
@cindex warnings, parameter mismatch
@cindex warnings, interface mismatch
Warn about type, rank, and other mismatches between formal parameters and actual
arguments to functions and subroutines.  These warnings are recommended and
thus enabled by default.

@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 -Wc-binding-type
@opindex @code{Wc-binding-type}
@cindex warning, C binding type
Warn if the a variable might not be C interoperable.  In particular, warn if 
the variable has been declared using an intrinsic type with default kind
instead of using a kind parameter defined for C interoperability in the
intrinsic @code{ISO_C_Binding} module.  This option is implied by
@option{-Wall}.

@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.  This option is
implied by @option{-Wall}.  For free-form source code, the default is
@option{-Werror=line-truncation} such that truncations are reported as
error.

@item -Wconversion
@opindex @code{Wconversion}
@cindex warnings, conversion
@cindex conversion
Warn about implicit conversions that are likely to change the value of 
the expression after conversion. Implied by @option{-Wall}.

@item -Wconversion-extra
@opindex @code{Wconversion-extra}
@cindex warnings, conversion
@cindex conversion
Warn about implicit conversions between different types and kinds. This
option does @emph{not} imply @option{-Wconversion}.

@item -Wextra
@opindex @code{Wextra}
@cindex extra warnings
@cindex warnings, extra
Enables some warning options for usages of language features which
may be problematic. This currently includes @option{-Wcompare-reals},
@option{-Wunused-parameter} and @option{-Wdo-subscript}.

@item -Wfrontend-loop-interchange
@opindex @code{Wfrontend-loop-interchange}
@cindex warnings, loop interchange
@cindex loop interchange, warning
Enable warning for loop interchanges performed by the
@option{-ffrontend-loop-interchange} option.

@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 -Winteger-division
@opindex @code{Winteger-division}
@cindex warnings, integer division
@cindex warnings, division of integers
Warn if a constant integer division truncates it result.
As an example, 3/5 evaluates to 0.

@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 -Wreal-q-constant
@opindex @code{Wreal-q-constant}
@cindex warnings, @code{q} exponent-letter
Produce a warning if a real-literal-constant contains a @code{q}
exponent-letter.

@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{-Wtabs} will cause a
warning to be issued if a tab is encountered. Note, @option{-Wtabs} is
active for @option{-pedantic}, @option{-std=f95}, @option{-std=f2003},
@option{-std=f2008}, @option{-std=f2008ts}, @option{-std=f2018} and
@option{-Wall}.

@item -Wundefined-do-loop
@opindex @code{Wundefined-do-loop}
@cindex warnings, undefined do loop
Warn if a DO loop with step either 1 or -1 yields an underflow or an overflow
during iteration of an induction variable of the loop.
This option is implied by @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. Enabled by default.

@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.  This option is implied by @option{-Wall}.

@item -Wuse-without-only
@opindex @code{Wuse-without-only}
@cindex warnings, use statements
@cindex intrinsic
Warn if a @code{USE} statement has no @code{ONLY} qualifier and 
thus implicitly imports all public entities of the used module.

@item -Wunused-dummy-argument
@opindex @code{Wunused-dummy-argument}
@cindex warnings, unused dummy argument
@cindex unused dummy argument
@cindex dummy argument, unused
Warn about unused dummy arguments. This option is implied by @option{-Wall}.

@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 (see @option{-Wunused-dummy-argument}),
but about unused @code{PARAMETER} values. @option{-Wunused-parameter}
is implied by @option{-Wextra} if also @option{-Wunused} or
@option{-Wall} is used.

@item -Walign-commons
@opindex @code{Walign-commons}
@cindex warnings, alignment of @code{COMMON} blocks
@cindex alignment of @code{COMMON} blocks
By default, @command{gfortran} warns about any occasion of variables being
padded for proper alignment inside a @code{COMMON} block. This warning can be turned
off via @option{-Wno-align-commons}. See also @option{-falign-commons}.

@item -Wfunction-elimination
@opindex @code{Wfunction-elimination}
@cindex function elimination
@cindex warnings, function elimination
Warn if any calls to functions are eliminated by the optimizations
enabled by the @option{-ffrontend-optimize} option.

@item -Wrealloc-lhs
@opindex @code{Wrealloc-lhs}
@cindex Reallocate the LHS in assignments, notification
Warn when the compiler might insert code to for allocation or reallocation of
an allocatable array variable of intrinsic type in intrinsic assignments.  In
hot loops, the Fortran 2003 reallocation feature may reduce the performance.
If the array is already allocated with the correct shape, consider using a
whole-array array-spec (e.g. @code{(:,:,:)}) for the variable on the left-hand
side to prevent the reallocation check. Note that in some cases the warning
is shown, even if the compiler will optimize reallocation checks away.  For
instance, when the right-hand side contains the same variable multiplied by
a scalar.  See also @option{-frealloc-lhs}.

@item -Wrealloc-lhs-all
@opindex @code{Wrealloc-lhs-all}
Warn when the compiler inserts code to for allocation or reallocation of an
allocatable variable; this includes scalars and derived types.

@item -Wcompare-reals
@opindex @code{Wcompare-reals}
Warn when comparing real or complex types for equality or inequality.
This option is implied by @option{-Wextra}.

@item -Wtarget-lifetime
@opindex @code{Wtargt-lifetime}
Warn if the pointer in a pointer assignment might be longer than the its
target. This option is implied by @option{-Wall}.

@item -Wzerotrip
@opindex @code{Wzerotrip}
Warn if a @code{DO} loop is known to execute zero times at compile
time.  This option is implied by @option{-Wall}.

@item -Wdo-subscript
@opindex @code{Wdo-subscript}
Warn if an array subscript inside a DO loop could lead to an
out-of-bounds access even if the compiler can not prove that the
statement is actually executed, in cases like
@smallexample
  real a(3)
  do i=1,4
    if (condition(i)) then
      a(i) = 1.2
    end if
  end do
@end smallexample
This option is implied by @option{-Wextra}.

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

@xref{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-fortran-original
@opindex @code{fdump-fortran-original}
Output the internal parse tree after translating the source program
into internal representation.  This option is mostly useful for
debugging the GNU Fortran compiler itself. The output generated by
this option might change between releases. This option may also
generate internal compiler errors for features which have only
recently been added.

@item -fdump-fortran-optimized
@opindex @code{fdump-fortran-optimized}
Output the parse tree after front-end optimization.  Mostly useful for
debugging the GNU Fortran compiler itself. The output generated by
this option might change between releases.  This option may also
generate internal compiler errors for features which have only
recently been added.

@item -fdump-parse-tree
@opindex @code{fdump-parse-tree}
Output the internal parse tree after translating the source program
into internal representation.  Mostly useful for debugging the GNU
Fortran compiler itself. The output generated by this option might
change between releases. This option may also generate internal
compiler errors for features which have only recently been added. This
option is deprecated; use @code{-fdump-fortran-original} instead.

@item -ffpe-trap=@var{list}
@opindex @code{ffpe-trap=}@var{list}
Specify a list of floating point exception traps to enable.  On most
systems, if a floating point exception occurs and the trap for that
exception is enabled, a SIGFPE signal will be sent and the program
being aborted, producing a core file useful for debugging.  @var{list}
is a (possibly empty) comma-separated list of the following
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{inexact} (loss of precision
during operation), and @samp{denormal} (operation performed on a
denormal value).  The first five exceptions correspond to the five
IEEE 754 exceptions, whereas the last one (@samp{denormal}) is not
part of the IEEE 754 standard but is available on some common
architectures such as x86.

The first three exceptions (@samp{invalid}, @samp{zero}, and
@samp{overflow}) often indicate serious errors, and unless the program
has provisions for dealing with these exceptions, enabling traps for
these three exceptions is probably a good idea.

Many, if not most, floating point operations incur loss of precision
due to rounding, and hence the @code{ffpe-trap=inexact} is likely to
be uninteresting in practice.

By default no exception traps are enabled.

@item -ffpe-summary=@var{list}
@opindex @code{ffpe-summary=}@var{list}
Specify a list of floating-point exceptions, whose flag status is printed
to @code{ERROR_UNIT} when invoking @code{STOP} and @code{ERROR STOP}.
@var{list} can be either @samp{none}, @samp{all} or a comma-separated list
of the following exceptions: @samp{invalid}, @samp{zero}, @samp{overflow},
@samp{underflow}, @samp{inexact} and @samp{denormal}. (See
@option{-ffpe-trap} for a description of the exceptions.)

By default, a summary for all exceptions but @samp{inexact} is shown.

@item -fno-backtrace
@opindex @code{fno-backtrace}
@cindex backtrace
@cindex trace
When a serious runtime error is encountered or a deadly signal is
emitted (segmentation fault, illegal instruction, bus error,
floating-point exception, and the other POSIX signals that have the
action @samp{core}), the Fortran runtime library tries to output a
backtrace of the error. @code{-fno-backtrace} disables the backtrace
generation. This option only has influence for 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 @code{INCLUDE} directive
@cindex directive, @code{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 -J@var{dir}
@opindex @code{J}@var{dir}
@opindex @code{M}@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.

@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 Link Options
@section Influencing the linking step
@cindex options, linking
@cindex linking, static

These options come into play when the compiler links object files into an 
executable output file. They are meaningless if the compiler is not doing 
a link step.

@table @gcctabopt
@item -static-libgfortran
@opindex @code{static-libgfortran}
On systems that provide @file{libgfortran} as a shared and a static
library, this option forces the use of the static version. If no
shared version of @file{libgfortran} was built when the compiler was
configured, this option has no effect.
@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 @command{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 @command{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.

@item -fsign-zero
@opindex @code{fsign-zero}
When enabled, floating point numbers of value zero with the sign bit set
are written as negative number in formatted output and treated as
negative in the @code{SIGN} intrinsic.  @option{-fno-sign-zero} does not
print the negative sign of zero values (or values rounded to zero for I/O)
and regards zero as positive number in the @code{SIGN} intrinsic for
compatibility with Fortran 77. The default is @option{-fsign-zero}.
@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 (except those marked as RECURSIVE) as if the
@code{SAVE} statement were 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} or @option{-save}.)
The default, which is @option{-fautomatic}, uses the stack for local
variables smaller than the value given by @option{-fmax-stack-var-size}.
Use the option @option{-frecursive} to use no static memory. 

@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 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 -fcoarray=@var{<keyword>}
@opindex @code{fcoarray}
@cindex coarrays

@table @asis
@item @samp{none}
Disable coarray support; using coarray declarations and image-control
statements will produce a compile-time error. (Default)

@item @samp{single}
Single-image mode, i.e. @code{num_images()} is always one.

@item @samp{lib}
Library-based coarray parallelization; a suitable GNU Fortran coarray
library needs to be linked.
@end table


@item -fcheck=@var{<keyword>}
@opindex @code{fcheck}
@cindex array, bounds checking
@cindex bounds checking
@cindex pointer checking
@cindex memory checking
@cindex range checking
@cindex subscript checking
@cindex checking subscripts
@cindex run-time checking
@cindex checking array temporaries

Enable the generation of run-time checks; the argument shall be
a comma-delimited list of the following keywords.  Prefixing a check with
@option{no-} disables it if it was activated by a previous specification.

@table @asis
@item @samp{all}
Enable all run-time test of @option{-fcheck}.

@item @samp{array-temps}
Warns at run time when for passing an actual argument a temporary array
had to be generated. The information generated by this warning is
sometimes useful in optimization, in order to avoid such temporaries.

Note: The warning is only printed once per location.

@item @samp{bounds}
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 and ensures that all string
lengths are equal for character array constructors without an explicit
typespec.

Some checks require that @option{-fcheck=bounds} is set for
the compilation of the main program.

Note: In the future this may also include other forms of checking, e.g.,
checking substring references.

@item @samp{do}
Enable generation of run-time checks for invalid modification of loop
iteration variables.

@item @samp{mem}
Enable generation of run-time checks for memory allocation.
Note: This option does not affect explicit allocations using the
@code{ALLOCATE} statement, which will be always checked.

@item @samp{pointer}
Enable generation of run-time checks for pointers and allocatables.

@item @samp{recursion}
Enable generation of run-time checks for recursively called subroutines and
functions which are not marked as recursive. See also @option{-frecursive}.
Note: This check does not work for OpenMP programs and is disabled if used
together with @option{-frecursive} and @option{-fopenmp}.
@end table

Example: Assuming you have a file @file{foo.f90}, the command
@smallexample
  gfortran -fcheck=all,no-array-temps foo.f90
@end smallexample
will compile the file with all checks enabled as specified above except
warnings for generated array temporaries.


@item -fbounds-check
@opindex @code{fbounds-check}
@c Note: This option is also referred in gcc's manpage
Deprecated alias for @option{-fcheck=bounds}.

@item -fcheck-array-temporaries
@opindex @code{fcheck-array-temporaries}
Deprecated alias for @option{-fcheck=array-temps}.

@item -fmax-array-constructor=@var{n}
@opindex @code{fmax-array-constructor}
This option can be used to increase the upper limit permitted in 
array constructors.  The code below requires this option to expand
the array at compile time.

@smallexample
program test
implicit none
integer j
integer, parameter :: n = 100000
integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
print '(10(I0,1X))', i
end program test
@end smallexample

@emph{Caution:  This option can lead to long compile times and excessively
large object files.}

The default value for @var{n} is 65535.


@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; if the size is exceeded static memory is used (except in
procedures marked as RECURSIVE). Use the option @option{-frecursive} to
allow for recursive procedures which do not have a RECURSIVE attribute or
for parallel programs. Use @option{-fno-automatic} to never use 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 -fstack-arrays
@opindex @code{fstack-arrays}
Adding this option will make the Fortran compiler put all arrays of
unknown size and array temporaries onto stack memory.  If your program uses very
large local arrays it is possible that you will have to extend your runtime
limits for stack memory on some operating systems. This flag is enabled
by default at optimization level @option{-Ofast} unless
@option{-fmax-stack-var-size} is specified.

@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 @command{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.

@item -finline-matmul-limit=@var{n}
@opindex @code{finline-matmul-limit}
When front-end optimiztion is active, some calls to the @code{MATMUL}
intrinsic function will be inlined.  This may result in code size
increase if the size of the matrix cannot be determined at compile
time, as code for both cases is generated.  Setting
@code{-finline-matmul-limit=0} will disable inlining in all cases.
Setting this option with a value of @var{n} will produce inline code
for matrices with size up to @var{n}. 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.  The @code{-fblas-matmul-limit}
can be used to change this value.

@item -frecursive
@opindex @code{frecursive}
Allow indirect recursion by forcing all local arrays to be allocated
on the stack. This flag cannot be used together with
@option{-fmax-stack-var-size=} or @option{-fno-automatic}.

@item -finit-local-zero
@itemx -finit-derived
@itemx -finit-integer=@var{n}
@itemx -finit-real=@var{<zero|inf|-inf|nan|snan>}
@itemx -finit-logical=@var{<true|false>}
@itemx -finit-character=@var{n}
@opindex @code{finit-local-zero}
@opindex @code{finit-derived}
@opindex @code{finit-integer}
@opindex @code{finit-real}
@opindex @code{finit-logical}
@opindex @code{finit-character}
The @option{-finit-local-zero} option instructs the compiler to
initialize local @code{INTEGER}, @code{REAL}, and @code{COMPLEX}
variables to zero, @code{LOGICAL} variables to false, and
@code{CHARACTER} variables to a string of null bytes.  Finer-grained
initialization options are provided by the
@option{-finit-integer=@var{n}},
@option{-finit-real=@var{<zero|inf|-inf|nan|snan>}} (which also initializes
the real and imaginary parts of local @code{COMPLEX} variables),
@option{-finit-logical=@var{<true|false>}}, and
@option{-finit-character=@var{n}} (where @var{n} is an ASCII character
value) options.

With @option{-finit-derived}, components of derived type variables will be
initialized according to these flags.  Components whose type is not covered by
an explicit @option{-finit-*} flag will be treated as described above with
@option{-finit-local-zero}.

These options do not initialize
@itemize @bullet
@item
objects with the POINTER attribute
@item
allocatable arrays
@item
variables that appear in an @code{EQUIVALENCE} statement.
@end itemize
(These limitations may be removed in future releases).

Note that the @option{-finit-real=nan} option initializes @code{REAL}
and @code{COMPLEX} variables with a quiet NaN. For a signalling NaN
use @option{-finit-real=snan}; note, however, that compile-time
optimizations may convert them into quiet NaN and that trapping
needs to be enabled (e.g. via @option{-ffpe-trap}).

Finally, note that enabling any of the @option{-finit-*} options will
silence warnings that would have been emitted by @option{-Wuninitialized}
for the affected local variables.

@item -falign-commons
@opindex @code{falign-commons}
@cindex alignment of @code{COMMON} blocks
By default, @command{gfortran} enforces proper alignment of all variables in a
@code{COMMON} block by padding them as needed. On certain platforms this is mandatory,
on others it increases performance. If a @code{COMMON} block is not declared with
consistent data types everywhere, this padding can cause trouble, and
@option{-fno-align-commons} can be used to disable automatic alignment. The
same form of this option should be used for all files that share a @code{COMMON} block.
To avoid potential alignment issues in @code{COMMON} blocks, it is recommended to order
objects from largest to smallest.

@item -fno-protect-parens
@opindex @code{fno-protect-parens}
@cindex re-association of parenthesized expressions
By default the parentheses in expression are honored for all optimization
levels such that the compiler does not do any re-association. Using
@option{-fno-protect-parens} allows the compiler to reorder @code{REAL} and
@code{COMPLEX} expressions to produce faster code. Note that for the re-association
optimization @option{-fno-signed-zeros} and @option{-fno-trapping-math}
need to be in effect. The parentheses protection is enabled by default, unless
@option{-Ofast} is given.

@item -frealloc-lhs
@opindex @code{frealloc-lhs}
@cindex Reallocate the LHS in assignments
An allocatable left-hand side of an intrinsic assignment is automatically
(re)allocated if it is either unallocated or has a different shape. The
option is enabled by default except when @option{-std=f95} is given. See
also @option{-Wrealloc-lhs}.

@item -faggressive-function-elimination
@opindex @code{faggressive-function-elimination}
@cindex Elimination of functions with identical argument lists
Functions with identical argument lists are eliminated within
statements, regardless of whether these functions are marked
@code{PURE} or not. For example, in
@smallexample
  a = f(b,c) + f(b,c)
@end smallexample
there will only be a single call to @code{f}.  This option only works
if @option{-ffrontend-optimize} is in effect.

@item -ffrontend-optimize
@opindex @code{frontend-optimize}
@cindex Front-end optimization
This option performs front-end optimization, based on manipulating
parts the Fortran parse tree.  Enabled by default by any @option{-O}
option.  Optimizations enabled by this option include inlining calls
to @code{MATMUL}, elimination of identical function calls within
expressions, removing unnecessary calls to @code{TRIM} in comparisons
and assignments and replacing @code{TRIM(a)} with
@code{a(1:LEN_TRIM(a))}.  It can be deselected by specifying
@option{-fno-frontend-optimize}.

@item -ffrontend-loop-interchange
@opindex @code{frontend-loop-interchange}
@cindex loop interchange, Fortran
Attempt to interchange loops in the Fortran front end where
profitable.  Enabled by default by any @option{-O} option.
At the moment, this option only affects @code{FORALL} and
@code{DO CONCURRENT} statements with several forall triplets.
@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 Interoperability Options
@section Options for interoperability with other languages

@table @asis

@item -fc-prototypes
@opindex @code{c-prototypes}
@cindex Generating C prototypes from Fortran source code
This option will generate C prototypes from @code{BIND(C)} variable
declarations, types and procedure interfaces and writes them to
standard output.  @code{ENUM} is not yet supported.

The generated prototypes may need inclusion of an appropriate header,
such as @code{<stdint.h>} or @code{<stdlib.h>}.  For types which are
not specified using the appropriate kind from the @code{iso_c_binding}
module, a warning is added as a comment to the code.

For function pointers, a pointer to a function returning @code{int}
without an explicit argument list is generated.

Example of use:
@smallexample
$ gfortran -fc-prototypes -fsyntax-only foo.f90 > foo.h
@end smallexample
where the C code intended for interoperating with the Fortran code
then  uses @code{#include "foo.h"}.
@end table

@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