GFORTRAN(1) GNU GFORTRAN(1)
gfortran - GNU Fortran 95 compiler
[-g] [-pg] [-Olevel]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remain-
The gfortran command supports all the options supported by the gcc com-
mand. Only options specific to gfortran are documented here.
Gfortran is not yet a fully conformant Fortran 95 compiler. It can
generate code for most constructs and expressions, but work remains to
be done. In particular, there are known deficiencies with ENTRY,
NAMELIST, and sophisticated use of MODULES, POINTERS and DERIVED TYPES.
For those whose Fortran codes conform to either the Fortran 77 standard
or the GNU Fortran 77 language, we recommend to use g77 from GCC 3.4.
We recommend that distributors continue to provide packages of g77-3.4
until we announce that gfortran fully replaces g77. The gfortran
developers welcome any feedback on user experience with gfortran at
All gcc and gfortran options are accepted both by gfortran and by gcc
(as well as any other drivers built at the same time, such as g++),
since adding gfortran to the gcc distribution enables acceptance of
gfortran options by all of the relevant drivers.
In some cases, options have positive and negative forms; the negative
form of -ffoo would be -fno-foo. This manual documents only one of
these two forms, whichever one is not the default.
Here is a summary of all the options specific to GNU Fortran, grouped
by type. Explanations are in the following sections.
Fortran Language Options
-ffree-form -fno-fixed-form -fdollar-ok -fimplicit-none
-fmax-identifier-length -std=std -ffixed-line-length-n
-ffixed-line-length-none -fdefault-double-8 -fdefault-integer-8
-fsyntax-only -pedantic -pedantic-errors -w -Wall -Waliasing
-Wconversion -Wimplicit-interface -Wnonstd-intrinsics -Wsurpris-
ing -Wunderflow -Wunused-labels -Wline-truncation -W
Code Generation Options
-fno-automatic -ff2c -fno-underscoring -fsecond-underscore
-fbounds-check -fmax-stack-var-size=n -fpackderived
Options Controlling Fortran Dialect
The following options control the dialect of Fortran that the compiler
Specify the layout used by the the source file. The free form lay-
out was introduced in Fortran 90. Fixed form was traditionally
used in older Fortran programs.
Set the "DOUBLE PRECISION" type to an 8 byte wide.
Set the default integer and logical types to an 8 byte wide type.
Do nothing if this is already the default.
Set the default real type to an 8 byte wide type. Do nothing if
this is already the default.
Allow $ as a valid character in a symbol name.
B Compile switch to change the interpretation of a backslash from
‘‘C’’-style escape characters to a single backslash character.
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 n include 72 (the standard and the default), 80
(card image), and 132 (corresponds to ‘‘extended-source’’ options
in some popular compilers). n may be 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.
-ffixed-line-length-0 means the same thing as
Specify the maximum allowed identifier length. Typical values are
31 (Fortran 95) and 63 (Fortran 200x).
Specify that no implicit typing is allowed, unless overridden by
explicit IMPLICIT statements. This is the equivalent of adding
implicit none to the start of every procedure.
Conform to the specified standard. Allowed values for std are gnu,
f95, f2003 and legacy.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there might
have been an error.
You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This manual
lists only one of the two forms, whichever is not the default.
These options control the amount and kinds of warnings produced by GNU
Check the code for syntax errors, but don’t do anything beyond
Issue warnings for uses of extensions to FORTRAN 95. -pedantic
also applies to C-language constructs where they occur in GNU For-
tran source files, such as use of \e in a character constant within
a directive like #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 -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
gfortran in this area are welcome.
This should be used in conjunction with -std=std.
Like -pedantic, except that errors are produced rather than warn-
-w Inhibit all warning messages.
Enables commonly used warning options that which pertain to usage
that we recommend avoiding and that we believe is easy to avoid.
This currently includes -Wunused-labels, -Waliasing, -Wsurprising,
-Wnonstd-intrinsic and -Wline-truncation.
Warn about possible aliasing of dummy arguments. The following
example will trigger the warning as it would be illegal to "bar" to
modify either parameter.
Warn about implicit conversions between different types.
Warn about when procedure are called without an explicit interface.
Note this only checks that an explicit interface is present. It
does not check that the declared interfaces are consistent across
Warn if the user tries to use an intrinsic that does not belong to
the standard the user has chosen via the -std option.
Produce a warning when ‘‘suspicious’’ code constructs are encoun-
tered. While technically legal these usually indicate that an
error has been made.
This currently produces a warning under the following circum-
* An INTEGER SELECT construct has a CASE that can never be
matched as its lower value is greater than its upper value.
* A LOGICAL SELECT construct has three CASE statements.
Produce a warning when numerical constant expressions are encoun-
tered, which yield an UNDERFLOW during compilation.
Warn whenever a label is defined but never referenced.
Turns all warnings into errors.
-W Turns on ‘‘extra warnings’’ and, if optimization is specified via
-O, the -Wuninitialized option. (This might change in future ver-
sions of gfortran
Some of these have no effect when compiling programs written in For-
Options for Debugging Your Program or GNU Fortran
GNU Fortran has various special options that are used for debugging
either your program or gfortran
Output the internal parse tree before starting code generation.
Only really useful for debugging gfortran itself.
Options for Directory Search
There options affect how affect how gfortran searches for files speci-
fied via the "INCLUDE" directive, and where it searches for previously
It also affects the search paths used by cpp when used to preprocess
These affect interpretation of the "INCLUDE" directive (as well as
of the "#include" directive of the cpp preprocessor).
Also note that the general behavior of -I and "INCLUDE" is pretty
much the same as of -I with "#include" in the cpp preprocessor,
with regard to looking for header.gcc files and other such things.
This path is also used to search for .mod files when previously
compiled modules are required by a "USE" statement.
This option specifies where to put .mod files for compiled modules.
It is also added to the list of directories to searched by an "USE"
The default is the current directory.
-J is an alias for -M to avoid conflicts with existing GCC options.
Options for Code Generation Conventions
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 -ffoo would be -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 no- or adding it.
Treat each program unit as if the "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 -static.)
Generate code designed to be compatible with code generated by g77
The calling conventions used by g77 (originally implemented in f2c)
require functions that return type default "REAL" to actually
return the C type "double", and functions that return type "COM-
PLEX" 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 "REAL" functions return
the C type "float", and "COMPLEX" functions return the GNU C type
"complex". Additionally, this option implies the -fsecond-under-
score option, unless -fno-second-underscore is explicitly
This does not affect the generation of code that interfaces with
the libgfortran library.
Caution: It is not a good idea to mix Fortran code compiled with
"-ff2c" with code compiled with the default "-fno-f2c" calling con-
ventions as, calling "COMPLEX" or default "REAL" functions between
program parts which were compiled with different calling conven-
tions will break at execution time.
Caution: This will break code which passes intrinsic functions of
type default "REAL" or "COMPLEX" as actual arguments, as the
library implementations use the -fno-f2c calling conventions.
Do not transform names of entities specified in the Fortran source
file by appending underscores to them.
With -funderscoring in effect, gfortran appends one underscore to
external names with no underscores.
This is done to ensure compatibility with code produced by many
UNIX Fortran compilers.
Caution: The default behavior of gfortran is incompatible with f2c
and g77, please use the -ff2c option if you want object files com-
piled with gfortran to be compatible with object code created with
Use of -fno-underscoring is not recommended unless you are experi-
menting with issues such as integration of (GNU) Fortran into
existing system environments (vis-a-vis existing libraries, tools,
and so on).
For example, with -funderscoring, and assuming other defaults like
-fcase-lower and that j() and mmaaxx_ccoouunntt(()) are external functions
while my_var and lvar are local variables, a statement like
I = J() + MAX_COUNT (MY_VAR, LVAR)
is implemented as something akin to:
i = j_() + max_count__(&my_var__, &lvar);
With -fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
Use of -fno-underscoring allows direct specification of user-
defined names while debugging and when interfacing gfortran code
with other languages.
Note that just because the names match does not mean that the
interface implemented by gfortran for an external name matches the
interface implemented by some other language for that same name.
That is, getting code produced by gfortran to link to code produced
by some other compiler using this or any other method can be only a
small part of the overall solution---getting the code generated by
both compilers to agree on issues other than naming can require
significant effort, and, unlike naming disagreements, linkers nor-
mally cannot detect disagreements in these other areas.
Also, note that with -fno-underscoring, the lack of appended under-
scores 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 gfortran we hope to improve naming and link-
ing 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
By default, gfortran appends an underscore to external names. If
this option is used gfortran appends two underscores to names with
underscores and one underscore to external names with no under-
scores. (gfortran also appends two underscores to internal names
with underscores to avoid naming collisions with external names.
This option has no effect if -fno-underscoring is in effect. It is
implied by the -ff2c option.
Otherwise, with this option, an external name such as MAX_COUNT is
implemented as a reference to the link-time external symbol
max_count__, instead of max_count_. This is required for compati-
bility with g77 and f2c, and is implied by use of the -ff2c option.
Enable generation of run-time checks for array subscripts and
against the declared minimum and maximum values. It also checks
array indices for assumed and deferred shape arrays against the
actual allocated bounds.
In the future this may also include other forms of checking, eg.
checking substring references.
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 con-
stant bounds, and may not apply to all character variables. Future
versions of gfortran may improve this behavior.
The default value for n is 32768.
This option tells gfortran to pack derived type members as closely
as possible. Code compiled with this option is likely to be incom-
patible with code compiled without this option, and may execute
In some circumstances gfortran may pass assumed shape array sec-
tions via a descriptor describing a discontiguous area of memory.
This option adds code to the function prologue to repack the data
into a contiguous block at runtime.
This should result in faster accesses to the array. However it can
introduce significant overhead to the function call, especially
when the passed data is discontiguous.
GNU Fortran 95 currently does not make use of any environment variables
to control its operation above and beyond those that affect the opera-
tion of gcc.
For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>.
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 gcc, cpp, gfor-
tran, as, ld, binutils and gdb.
See the Info entry for gfortran for contributors to GCC and GFORTRAN.
Copyright (c) 2004 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
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gcc-4.0.2 2005-11-25 GFORTRAN(1)
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