NAME
gfortran - GNU Fortran compiler
SYNOPSIS
gfortran [-c|-S|-E]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the
remainder.
DESCRIPTION
The gfortran command supports all the options supported by the gcc
command. Only options specific to GNU Fortran are documented here.
All GCC and GNU Fortran 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 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 -ffoo would be -fno-foo. This manual documents only one of
these two forms, whichever one is not the default.
OPTIONS
Here is a summary of all the options specific to GNU Fortran, grouped
by type. Explanations are in the following sections.
Fortran Language Options
-fall-intrinsics -ffree-form -fno-fixed-form -fdollar-ok
-fimplicit-none -fmax-identifier-length -std=std -fd-lines-as-code
-fd-lines-as-comments -ffixed-line-length-n
-ffixed-line-length-none -ffree-line-length-n
-ffree-line-length-none -fdefault-double-8 -fdefault-integer-8
-fdefault-real-8 -fcray-pointer -fopenmp -fno-range-check
-fbackslash -fmodule-private
Error and Warning Options
-fmax-errors=n -fsyntax-only -pedantic -pedantic-errors -Wall
-Waliasing -Wampersand -Wcharacter-truncation -Wconversion
-Wimplicit-interface -Wline-truncation -Wnonstd-intrinsics
-Wsurprising -Wno-tabs -Wunderflow -Wunused-parameter
Debugging Options
-fdump-parse-tree -ffpe-trap=list -fdump-core -fbacktrace
Directory Options
-Idir -Jdir -Mdir -fintrinsic-modules-path dir
Link Options
-static-libgfortran
Runtime Options
-fconvert=conversion -frecord-marker=length
-fmax-subrecord-length=length -fsign-zero
Code Generation Options
-fno-automatic -ff2c -fno-underscoring -fsecond-underscore
-fbounds-check -fmax-stack-var-size=n -fpack-derived
-frepack-arrays -fshort-enums -fexternal-blas
-fblas-matmul-limit=n -frecursive -finit-local-zero
-finit-integer=n -finit-real=<zero|inf|-inf|nan>
-finit-logical=<true|false> -finit-character=n
Options controlling Fortran dialect
The following options control the details of the Fortran dialect
accepted by the compiler:
-ffree-form
-ffixed-form
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.
-fall-intrinsics
Accept all of the intrinsic procedures provided in libgfortran
without regard to the setting of -std. In particular, this option
can be quite useful with -std=f95. Additionally, gfortran will
ignore -Wnonstd-intrinsics.
-fd-lines-as-code
-fd-lines-as-comments
Enable special treatment for lines beginning 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 comment
lines.
-fdefault-double-8
Set the "DOUBLE PRECISION" type to an 8 byte wide type.
-fdefault-integer-8
Set the default integer and logical types to an 8 byte wide type.
Do nothing if this is already the default.
-fdefault-real-8
Set the default real type to an 8 byte wide type. Do nothing if
this is already the default.
-fdollar-ok
Allow $ as a valid character in a symbol name.
-fbackslash
Change the interpretation of backslashes in string literals from a
single backslash character to "C-style" escape characters. The
following combinations are expanded \a, \b, \f, \n, \r, \t, \v, \,
and \0 to the ASCII characters alert, backspace, form feed,
newline, carriage return, horizontal tab, vertical tab, backslash,
and NUL, respectively. All other combinations of a character
preceded by \ are unexpanded.
-fmodule-private
Set the default accessibility of module entities to "PRIVATE".
Use-associated entities will not be accessible unless they are
explicitly declared as "PUBLIC".
-ffixed-line-length-n
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 (corresponding to "extended-source" options
in some popular compilers). n may also 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
-ffixed-line-length-none.
-ffree-line-length-n
Set column after which characters are ignored in typical free-form
lines in the source file. 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 -ffree-line-length-none.
-fmax-identifier-length=n
Specify the maximum allowed identifier length. Typical values are
31 (Fortran 95) and 63 (Fortran 2003).
-fimplicit-none
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.
-fcray-pointer
Enable the Cray pointer extension, which provides C-like pointer
functionality.
-fopenmp
Enable the OpenMP extensions. This includes OpenMP "!$omp"
directives in free form and "c$omp", *$omp and "!$omp" directives
in fixed form, "!$" conditional compilation sentinels in free form
and "c$", "*$" and "!$" sentinels in fixed form, and when linking
arranges for the OpenMP runtime library to be linked in. The
option -fopenmp implies -frecursive.
-fno-range-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 "a = 1. / 0". With this
option, no error will be given and "a" will be assigned the value
"+Infinity". If an expression evaluates to a value outside of the
relevant range of ["-HUGE()":"HUGE()"], then the expression will be
replaced by "-Inf" or "+Inf" as appropriate. Similarly, "DATA
i/Z'FFFFFFFF'/" will result in an integer overflow on most systems,
but with -fno-range-check the value will "wrap around" and "i" will
be initialized to -1 instead.
-std=std
Specify the standard to which the program is expected to conform,
which may be one of f95, f2003, gnu, or legacy. The default value
for std is gnu, which specifies a superset of the Fortran 95
standard that includes all of the extensions supported by GNU
Fortran, although warnings will be given for obsolete extensions
not recommended for use in new code. The legacy value is
equivalent but without the warnings for obsolete extensions, and
may be useful for old non-standard programs. The f95 and f2003
values specify strict conformance to the Fortran 95 and Fortran
2003 standards, respectively; errors are given for all extensions
beyond the relevant language standard, and warnings are given for
the Fortran 77 features that are permitted but obsolescent in later
standards.
Options to request or suppress errors and 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 -Werror is specified, they do not
prevent compilation of the program.
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 errors and warnings
produced by GNU Fortran:
-fmax-errors=n
Limits the maximum number of error messages to n, at which point
GNU Fortran bails out rather than attempting to continue processing
the source code. If n is 0, there is no limit on the number of
error messages produced.
-fsyntax-only
Check the code for syntax errors, but don’t actually compile it.
This will generate module files for each module present in the
code, but no other output file.
-pedantic
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
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
GNU Fortran in this area are welcome.
This should be used in conjunction with -std=f95 or -std=f2003.
-pedantic-errors
Like -pedantic, except that errors are produced rather than
warnings.
-Wall
Enables commonly used warning options pertaining to usage that we
recommend avoiding and that we believe are easy to avoid. This
currently includes -Waliasing, -Wampersand, -Wsurprising,
-Wnonstd-intrinsics, -Wno-tabs, and -Wline-truncation.
-Waliasing
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.
interface
subroutine bar(a,b)
integer, intent(in) :: a
integer, intent(out) :: b
end subroutine
end interface
integer :: a
call bar(a,a)
-Wampersand
Warn about missing ampersand in continued character constants. The
warning is given with -Wampersand, -pedantic, -std=f95, and
-std=f2003. Note: With no ampersand given in a continued character
constant, GNU Fortran assumes continuation at the first non-
comment, non-whitespace character after the ampersand that
initiated the continuation.
-Wcharacter-truncation
Warn when a character assignment will truncate the assigned string.
-Wconversion
Warn about implicit conversions between different types.
-Wimplicit-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.
-Wnonstd-intrinsics
Warn if the user tries to use an intrinsic that does not belong to
the standard the user has chosen via the -std option.
-Wsurprising
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.
· A TRANSFER specifies a source that is shorter than the
destination.
-Wtabs
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. -Wno-tabs
will cause a warning to be issued if a tab is encountered. Note,
-Wno-tabs is active for -pedantic, -std=f95, -std=f2003, and -Wall.
-Wunderflow
Produce a warning when numerical constant expressions are
encountered, which yield an UNDERFLOW during compilation.
-Wunused-parameter
Contrary to gcc’s meaning of -Wunused-parameter, gfortran’s
implementation of this option does not warn about unused dummy
arguments, but about unused "PARAMETER" values. -Wunused-parameter
is not included in -Wall but is implied by -Wall -Wextra.
-Werror
Turns all warnings into errors.
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 the GNU Fortran compiler.
-fdump-parse-tree
Output the internal parse tree before starting code generation.
Only really useful for debugging the GNU Fortran compiler itself.
-ffpe-trap=list
Specify a list of IEEE exceptions when a Floating Point Exception
(FPE) should be raised. On most systems, this will result in a
SIGFPE signal being sent and the program being interrupted,
producing a core file useful for debugging. 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 value).
Some of the routines in the Fortran runtime library, like CPU_TIME,
are likely to to trigger floating point exceptions when
"ffpe-trap=precision" is used. For this reason, the use of
"ffpe-trap=precision" is not recommended.
-fbacktrace
Specify that, when a runtime error is encountered or a deadly
signal is emitted (segmentation fault, illegal instruction, bus
error or floating-point exception), the Fortran runtime library
should output a backtrace of the error. This option only has
influence for compilation of the Fortran main program.
-fdump-core
Request that a core-dump file is written to disk when a runtime
error is encountered on systems that support core dumps. This
option is only effective for the compilation of the Fortran main
program.
Options for directory search
These options affect how GNU Fortran 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
Fortran source.
-Idir
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.
-Mdir
-Jdir
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"
statement.
The default is the current directory.
-J is an alias for -M to avoid conflicts with existing GCC options.
-fintrinsic-modules-path dir
This option specifies the location of pre-compiled intrinsic
modules, if they are not in the default location expected by the
compiler.
Influencing the linking step
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.
-static-libgfortran
On systems that provide libgfortran as a shared and a static
library, this option forces the use of the static version. If no
shared version of libgfortran was built when the compiler was
configured, this option has no effect.
Influencing runtime behavior
These options affect the runtime behavior of programs compiled with GNU
Fortran.
-fconvert=conversion
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.
-frecord-marker=length
Specify the length of record markers for unformatted files. Valid
values for length are 4 and 8. Default is 4. This is different
from previous versions of gfortran, which specified a default
record marker length of 8 on most systems. If you want to read or
write files compatible with earlier versions of gfortran, use
-frecord-marker=8.
-fmax-subrecord-length=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.
-fsign-zero
When writing zero values, show the negative sign if the sign bit is
set. "fno-sign-zero" does not print the negative sign of zero
values for compatibility with F77. Default behavior is to show the
negative sign.
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.
-fno-automatic
Treat each program unit (except those marked as RECURSIVE) as if
the "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 -static or
-save.) The default, which is -fautomatic, uses the stack for
local variables smaller than the value given by
-fmax-stack-var-size. Use the option -frecursive to use no static
memory.
-ff2c
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.
-fno-underscoring
Do not transform names of entities specified in the Fortran source
file by appending underscores to them.
With -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.
Caution: The default behavior of GNU Fortran is incompatible with
f2c and g77, please use the -ff2c option if you want object files
compiled with GNU Fortran to be compatible with object code created
with these tools.
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 so on).
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 GNU Fortran code
with other languages.
Note that just because the names match does 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 -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.
-fsecond-underscore
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 -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.
-fbounds-check
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.
Some checks require that -fbounds-check is set for the compilation
of the main program.
In the future this may also include other forms of checking, e.g.,
checking substring references.
-fmax-stack-var-size=n
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
-frecursive to allow for recursive procedures which do not have a
RECURSIVE attribute or for parallel programs. Use -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 n is 32768.
-fpack-derived
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.
-frepack-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.
-fshort-enums
This option is provided for interoperability with C code that was
compiled with the -fshort-enums option. It will make GNU Fortran
choose the smallest "INTEGER" kind a given enumerator set will fit
in, and give all its enumerators this kind.
-fexternal-blas
This option will make gfortran generate calls to BLAS functions for
some matrix operations like "MATMUL", instead of using our own
algorithms, if the size of the matrices involved is larger than a
given limit (see -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.
-fblas-matmul-limit=n
Only significant when -fexternal-blas is in effect. Matrix
multiplication of matrices with size larger than (or equal to) n
will be performed by calls to BLAS functions, while others will be
handled by 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 n is 30.
-frecursive
Allow indirect recursion by forcing all local arrays to be
allocated on the stack. This flag cannot be used together with
-fmax-stack-var-size= or -fno-automatic.
-finit-local-zero
-finit-integer=n
-finit-real=<zero|inf|-inf|nan>
-finit-logical=<true|false>
-finit-character=n
The -finit-local-zero option instructs the compiler to initialize
local "INTEGER", "REAL", and "COMPLEX" variables to zero, "LOGICAL"
variables to false, and "CHARACTER" variables to a string of null
bytes. Finer-grained initialization options are provided by the
-finit-integer=n, -finit-real=<zero|inf|-inf|nan> (which also
initializes the real and imaginary parts of local "COMPLEX"
variables), -finit-logical=<true|false>, and -finit-character=n
(where n is an ASCII character value) options. These options do
not initialize components of derived type variables, nor do they
initialize variables that appear in an "EQUIVALENCE" statement.
(This limitation may be removed in future releases).
Note that the -finit-real=nan option initializes "REAL" and
"COMPLEX" variables with a quiet NaN.
ENVIRONMENT
The gfortran compiler currently does not make use of any environment
variables to control its operation above and beyond those that affect
the operation of gcc.
BUGS
For instructions on reporting bugs, see
<file:///usr/share/doc/gcc-4.3/README.Bugs>.
SEE ALSO
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.
AUTHOR
See the Info entry for gfortran for contributors to GCC and GNU
Fortran.
COPYRIGHT
Copyright (c) 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding Free
Software", the Front-Cover texts being (a) (see below), and with the
Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF’s Front-Cover Text is:
A GNU Manual
(b) The FSF’s Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.