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gfortran - GNU Fortran 95 compiler
[-g] [-pg] [-Olevel]
[-o outfile] infile...
Only the most useful options are listed here; see below for the
The gfortran command supports all the options supported by the
gcc command. Only options specific to gfortran are documented here.
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
-fdollar-ok -fimplicit-none -fmax-identifier-length
-std=std -fd-lines-as-code -fd-lines-as-comments
-fdefault-double-8 -fdefault-integer-8 -fdefault-real-8
-fcray-pointer -fopenmp -frange-check
- Warning Options
-fsyntax-only -pedantic -pedantic-errors
-w -Wall -Waliasing -Wampersand -Wconversion -Wimplicit-interface
-Wnonstd-intrinsics -Wsurprising -Wunderflow
-Wunused-labels -Wline-truncation -W
- Debugging Options
- Directory Options
- Runtime Options
- Code Generation Options
-fno-automatic -ff2c -fno-underscoring -fsecond-underscore
-fpackderived -frepack-arrays -fshort-enums
Options Controlling Fortran Dialect
The following options control the dialect of Fortran
that the compiler accepts:
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.
Enables special treating for lines with d or D in fixed
form sources. If the -fd-lines-as-code option is given
they are treated as if the first column contained a blank. If the
-fd-lines-as-comments option is given, they are treated as
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.
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
Set column after which characters are ignored in typical free-form
lines in the source file. For free-form, the default value is 132.
n may be none, meaning that the entire line is meaningful.
-ffree-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.
Enables the Cray pointer extension, which provides a C-like pointer.
Enables handling of OpenMP "!$omp" directives in free form
and "c$omp", *$omp and "!$omp" directives in fixed form,
enables "!$" conditional compilation sentinels in free form
and "c$", "*$" and "!$" sentinels in fixed form
and when linking arranges for the OpenMP runtime library to be linked
Enable range checking on results of simplification of constant expressions
during compilation. For example, by default, gfortran will give
an overflow error at compile time when simplifying "a = EXP(1000)".
With -fno-range-check, no error will be given and the variable "a"
will be assigned the value "+Infinity".
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 that.
Issue warnings for uses of extensions to FORTRAN 95.
-pedantic also applies to C-language constructs where they
occur in GNU Fortran 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
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
Inhibit all warning messages.
Enables commonly used warning options pertaining to usage that
we recommend avoiding and that we believe are easy to avoid. This currently
includes -Wunused-labels, -Waliasing, -Wampersand,
Warn about possible aliasing of dummy arguments. Specifically, it warns
if the same actual argument is associated with a dummy argument with
"intent(in)" and a dummy argument with "intent(out)" in a call
with an explicit interface.
The following example will trigger the warning.
integer, intent(in) :: a
integer, intent(out) :: b
integer :: a
Warn about missing ampersand in continued character literals. The warning is
given with -Wampersand, -pedantic, and -std=f95.
Note: With no ampersand given in a continued character literal, gfortran
assumes continuation at the first non-comment, non-whitespace character.
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 program units.
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 encountered.
While technically legal these usually indicate that an error has been made.
This currently produces a warning under the following circumstances:
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
encountered, which yield an UNDERFLOW during compilation.
Warn whenever a label is defined but never referenced.
Turns all warnings into errors.
Turns on ``extra warnings'' and, if optimization is specified
via -O, the -Wuninitialized option.
(This might change in future versions of gfortran
Some of these have no effect when compiling programs written in Fortran.
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.
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. list is a (possibly empty) comma-separated
list of the following IEEE exceptions: invalid (invalid floating
point operation, such as "sqrt(-1.0)"), zero (division by
zero), overflow (overflow in a floating point operation),
underflow (underflow in a floating point operation),
precision (loss of precision during operation) and denormal
(operation produced a denormal denormal value).
Options for Directory Search
These options affect how gfortran searches
for files specified by the "INCLUDE" directive and where it searches
for previously compiled modules.
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
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
Influencing runtime behavior
These options affect the runtime behavior of gfortran.
Specify the representation of data for unformatted files. Valid
values for conversion are: native, the default; swap,
swap between big- and little-endian; big-endian, use big-endian
representation for unformatted files; little-endian, use little-endian
representation for unformatted files.
This option has an effect only when used in the main program.
The "CONVERT" specifier and the GFORTRAN_CONVERT_UNIT environment
variable override the default specified by -fconvert.
Specify the length of record markers for unformatted files.
Valid values for length are 4 and 8. Default is whatever
"off_t" is specified to be on that particular system.
Note that specifying length as 4 limits the record
length of unformatted files to 2 GB. This option does not
extend the maximum possible record length on systems where
"off_t" is a four_byte quantity.
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
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
Generate code designed to be compatible with code generated
by g77 and f2c.
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 "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 "REAL" functions return the C type "float", and
"COMPLEX" functions return the GNU C type "complex".
Additionally, this option implies the -fsecond-underscore
option, unless -fno-second-underscore is explicitly requested.
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 conventions as, calling "COMPLEX" or default "REAL"
functions between program parts which were compiled with different
calling conventions 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 compiled with
gfortran to be compatible with object code created with these
Use of -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
For example, with -funderscoring, and assuming other defaults like
-fcase-lower and that j() and max_count() 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 normally
cannot detect disagreements in these other areas.
Also, note that with -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 gfortran we hope to improve naming and linking
issues so that debugging always involves using the names as they appear
in the source, even if the names as seen by the linker are mangled to
prevent accidental linking between procedures with incompatible
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 underscores. (gfortran also appends two underscores to
internal names with underscores to avoid naming collisions with external
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 compatibility 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
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 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 incompatible
with code compiled without this option, and may execute slower.
In some circumstances gfortran may pass assumed shape array
sections via a descriptor describing a discontiguous area of memory.
This option adds code to the function prologue to repack the data into
a contiguous block at runtime.
This should result in faster accesses to the array. However it can introduce
significant overhead to the function call, especially when the passed data
This option is provided for interoperability with C code that was
compiled with the -fshort-enums option. It will make
gfortran choose the smallest "INTEGER" kind a given
enumerator set will fit in, and give all its enumerators this kind.
GNU Fortran 95 currently does not make use of any environment
variables to control its operation above and beyond those
that affect the operation of gcc.
For instructions on reporting bugs, see
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, gfortran, as,
ld, binutils and gdb.
See the Info entry for gfortran for contributors to GCC and
Copyright (c) 2004, 2005, 2006
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.
- Options Controlling Fortran Dialect
- Options to Request or Suppress Warnings
- Options for Debugging Your Program or GNU Fortran
- Options for Directory Search
- Influencing runtime behavior
- Options for Code Generation Conventions
- SEE ALSO
This document was created by
using the manual pages.
Time: 09:52:21 GMT, June 23, 2018