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 -fbackslash -fcray-pointer -fd-lines-as-code
-fd-lines-as-comments -fdec -fdec-structure
-fdec-intrinsic-ints -fdec-static -fdec-math -fdec-include
-fdefault-double-8 -fdefault-integer-8 -fdefault-real-8
-fdefault-real-10 -fdefault-real-16 -fdollar-ok
-ffixed-line-length-n -ffixed-line-length-none -fpad-source
-ffree-form -ffree-line-length-n -ffree-line-length-none
-fimplicit-none -finteger-4-integer-8 -fmax-identifier-length
-fmodule-private -ffixed-form -fno-range-check -fopenacc
-fopenmp -freal-4-real-10 -freal-4-real-16 -freal-4-real-8
-freal-8-real-10 -freal-8-real-16 -freal-8-real-4 -std=std
-ftest-forall-temp
Preprocessing Options
-A-question[=answer] -Aquestion=answer -C -CC -Dmacro[=defn]
-H -P -Umacro -cpp -dD -dI -dM -dN -dU -fworking-directory
-imultilib dir -iprefix file -iquote -isysroot dir -isystem
dir -nocpp -nostdinc -undef
Error and Warning Options
-Waliasing -Wall -Wampersand -Wargument-mismatch
-Warray-bounds -Wc-binding-type -Wcharacter-truncation
-Wconversion -Wdo-subscript -Wfunction-elimination
-Wimplicit-interface -Wimplicit-procedure -Wintrinsic-shadow
-Wuse-without-only -Wintrinsics-std -Wline-truncation
-Wno-align-commons -Wno-tabs -Wreal-q-constant -Wsurprising
-Wunderflow -Wunused-parameter -Wrealloc-lhs
-Wrealloc-lhs-all -Wfrontend-loop-interchange
-Wtarget-lifetime -fmax-errors=n -fsyntax-only -pedantic
-pedantic-errors
Debugging Options
-fbacktrace -fdump-fortran-optimized -fdump-fortran-original
-fdump-fortran-global -fdump-parse-tree -ffpe-trap=list
-ffpe-summary=list
Directory Options
-Idir -Jdir -fintrinsic-modules-path dir
Link Options
-static-libgfortran
Runtime Options
-fconvert=conversion -fmax-subrecord-length=length
-frecord-marker=length -fsign-zero
Interoperability Options
-fc-prototypes -fc-prototypes-external
Code Generation Options
-faggressive-function-elimination -fblas-matmul-limit=n
-fbounds-check -ftail-call-workaround
-ftail-call-workaround=n -fcheck-array-temporaries
-fcheck=<all|array-temps|bounds|do|mem|pointer|recursion>
-fcoarray=<none|single|lib> -fexternal-blas -ff2c
-ffrontend-loop-interchange -ffrontend-optimize
-finit-character=n -finit-integer=n -finit-local-zero
-finit-derived -finit-logical=<true|false>
-finit-real=<zero|inf|-inf|nan|snan> -finline-matmul-limit=n
-fmax-array-constructor=n -fmax-stack-var-size=n
-fno-align-commons -fno-automatic -fno-protect-parens
-fno-underscoring -fsecond-underscore -fpack-derived
-frealloc-lhs -frecursive -frepack-arrays -fshort-enums
-fstack-arrays
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
This option causes all intrinsic procedures (including the
GNU-specific extensions) to be accepted. This can be useful
with -std=f95 to force standard-compliance but get access to
the full range of intrinsics available with gfortran. As a
consequence, -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 "EXTERNAL".
-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.
-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: -fdollar-ok
-fcray-pointer -fdec-structure -fdec-intrinsic-ints
-fdec-static -fdec-math
If -fd-lines-as-code/-fd-lines-as-comments are unset, then
-fdec also sets -fd-lines-as-comments.
-fdec-structure
Enable DEC "STRUCTURE" and "RECORD" as well as "UNION",
"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.
-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.
-fdec-math
Enable legacy math intrinsics such as COTAN and degree-valued
trigonometric functions (e.g. TAND, ATAND, etc...) for
compatability with older code.
-fdec-static
Enable DEC-style STATIC and AUTOMATIC attributes to
explicitly specify the storage of variables and other
objects.
-fdec-include
Enable parsing of INCLUDE as a statement in addition to
parsing it as INCLUDE line. When parsed as INCLUDE
statement, INCLUDE does not have to be on a single line and
can use line continuations.
-fdollar-ok
Allow $ as a valid non-first character in a symbol name.
Symbols that start with $ are rejected since it is unclear
which rules to apply to implicit typing as different vendors
implement different rules. Using $ in "IMPLICIT" statements
is also rejected.
-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. Additionally, "\x"nn, "\u"nnnn and
"\U"nnnnnnnn (where each 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.
-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, unless
"-fno-pad-source", 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.
-fno-pad-source
By default fixed-form lines have spaces assumed (as if padded
to that length) after the ends of short fixed-form lines.
This is not done either if -ffixed-line-length-0,
-ffixed-line-length-none or if -fno-pad-source option is
used. With any of those options continued character
constants never have implicit spaces appended to them to fill
out the line.
-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 and Fortran 2008).
-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.
-fopenacc
Enable the OpenACC extensions. This includes OpenACC "!$acc"
directives in free form and "c$acc", *$acc and "!$acc"
directives in fixed form, "!$" conditional compilation
sentinels in free form and "c$", "*$" and "!$" 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
<https://gcc.gnu.org/wiki/OpenACC > for more information.
-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.
-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 42. Unlike -finteger-4-integer-8, it does not promote
variables with explicit kind declaration.
-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 1.0. This option promotes the default width of "DOUBLE
PRECISION" and double real constants like "1.d0" to 16 bytes
if possible. If "-fdefault-double-8" is given along with
"fdefault-real-8", "DOUBLE PRECISION" and double real
constants are not promoted. Unlike -freal-4-real-8,
"fdefault-real-8" does not promote variables with explicit
kind declarations.
-fdefault-real-10
Set the default real type to an 10 byte wide type. This
option also affects the kind of non-double real constants
like 1.0. This option promotes the default width of "DOUBLE
PRECISION" and double real constants like "1.d0" to 16 bytes
if possible. If "-fdefault-double-8" is given along with
"fdefault-real-10", "DOUBLE PRECISION" and double real
constants are not promoted. Unlike -freal-4-real-10,
"fdefault-real-10" does not promote variables with explicit
kind declarations.
-fdefault-real-16
Set the default real type to an 16 byte wide type. This
option also affects the kind of non-double real constants
like 1.0. This option promotes the default width of "DOUBLE
PRECISION" and double real constants like "1.d0" to 16 bytes
if possible. If "-fdefault-double-8" is given along with
"fdefault-real-16", "DOUBLE PRECISION" and double real
constants are not promoted. Unlike -freal-4-real-16,
"fdefault-real-16" does not promote variables with explicit
kind declarations.
-fdefault-double-8
Set the "DOUBLE PRECISION" type and double real constants
like "1.d0" to an 8 byte wide type. Do nothing if this is
already the default. This option prevents -fdefault-real-8,
-fdefault-real-10, and -fdefault-real-16, from promoting
"DOUBLE PRECISION" and double real constants like "1.d0" to
16 bytes.
-finteger-4-integer-8
Promote all "INTEGER(KIND=4)" entities to an
"INTEGER(KIND=8)" entities. If "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 "EQUIVALENCE" and/or "COMMON", generic
interfaces, BOZ literal constant conversion, and I/O.
Inspection of the intermediate representation of the
translated Fortran code, produced by -fdump-tree-original, is
suggested.
-freal-4-real-8
-freal-4-real-10
-freal-4-real-16
-freal-8-real-4
-freal-8-real-10
-freal-8-real-16
Promote all "REAL(KIND=M)" entities to "REAL(KIND=N)"
entities. If "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
"EQUIVALENCE" and/or "COMMON", generic interfaces, BOZ
literal constant conversion, and I/O. Inspection of the
intermediate representation of the translated Fortran code,
produced by -fdump-tree-original, is suggested.
-std=std
Specify the standard to which the program is expected to
conform, which may be one of f95, f2003, f2008, f2018, gnu,
or legacy. The default value for std is 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 legacy value is equivalent but without the
warnings for obsolete extensions, and may be useful for old
non-standard programs. The f95, f2003, f2008, and 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. The deprecated option -std=f2008ts acts as an
alias for -std=f2018. It is only present for backwards
compatibility with earlier gfortran versions and should not
be used any more.
-ftest-forall-temp
Enhance test coverage by forcing most forall assignments to
use temporary.
Enable and customize preprocessing
Preprocessor related options. See section Preprocessing and
conditional compilation for more detailed information on
preprocessing in gfortran.
-cpp
-nocpp
Enable preprocessing. The preprocessor is automatically
invoked if the file extension is .fpp, .FPP, .F, .FOR, .FTN,
.F90, .F95, .F03 or .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: -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 -ffree-line-length-none or
-ffixed-line-length-none options.
-dM Instead of the normal output, generate a list of '#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 foo.f90, the
command
touch foo.f90; gfortran -cpp -E -dM foo.f90
will show all the predefined macros.
-dD Like -dM except in two respects: it does not include the
predefined macros, and it outputs both the "#define"
directives and the result of preprocessing. Both kinds of
output go to the standard output file.
-dN Like -dD, but emit only the macro names, not their
expansions.
-dU Like 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
'#undef' directives are also output for macros tested but
undefined at the time.
-dI Output '#include' directives in addition to the result of
preprocessing.
-fworking-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 -fno-working-directory. If the -P flag
is present in the command line, this option has no effect,
since no "#line" directives are emitted whatsoever.
-idirafter dir
Search dir for include files, but do it after all directories
specified with -I and the standard system directories have
been exhausted. dir is treated as a system include directory.
If dir begins with "=", then the "=" will be replaced by the
sysroot prefix; see --sysroot and -isysroot.
-imultilib dir
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should
include the final '/'.
-isysroot dir
This option is like the --sysroot option, but applies only to
header files. See the --sysroot option for more information.
-iquote dir
Search dir only for header files requested with "#include
"file""; they are not searched for "#include <file>", before
all directories specified by -I and before the standard
system directories. If dir begins with "=", then the "=" will
be replaced by the sysroot prefix; see --sysroot and
-isysroot.
-isystem dir
Search dir for header files, after all directories specified
by -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 dir begins with "=", then the "=" will be replaced by the
sysroot prefix; see --sysroot and -isysroot.
-nostdinc
Do not search the standard system directories for header
files. Only the directories you have specified with -I
options (and the directory of the current file, if
appropriate) are searched.
-undef
Do not predefine any system-specific or GCC-specific macros.
The standard predefined macros remain defined.
-Apredicate=answer
Make an assertion with the predicate predicate and answer
answer. This form is preferred to the older form -A
predicate(answer), which is still supported, because it does
not use shell special characters.
-A-predicate=answer
Cancel an assertion with the predicate predicate and answer
answer.
-C 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 -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 '#'.
Warning: this currently handles C-Style comments only. The
preprocessor does not yet recognize Fortran-style comments.
-CC Do not discard comments, including during macro expansion.
This is like -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 -C option, the -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 -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.
-Dname
Predefine name as a macro, with definition 1.
-Dname=definition
The contents of definition are tokenized and processed as if
they appeared during translation phase three in a '#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, "-D'name(args...)=definition'" works.
-D and -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.
-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 '#include' stack it is.
-P 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.
-Uname
Cancel any previous definition of name, either built in or
provided with a -D option.
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 do not actually compile
it. This will generate module files for each module present
in the code, but no other output file.
-Wpedantic
-pedantic
Issue warnings for uses of extensions to Fortran. -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 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, -std=f2003,
-std=f2008 or -std=f2018.
-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,
-Wconversion, -Wsurprising, -Wc-binding-type,
-Wintrinsics-std, -Wtabs, -Wintrinsic-shadow,
-Wline-truncation, -Wtarget-lifetime, -Winteger-division,
-Wreal-q-constant, -Wunused and -Wundefined-do-loop.
-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, -std=f2003, -std=f2008 and -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.
-Wargument-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.
-Warray-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.
-Wc-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
"ISO_C_Binding" module. This option is implied by -Wall.
-Wcharacter-truncation
Warn when a character assignment will truncate the assigned
string.
-Wline-truncation
Warn when a source code line will be truncated. This option
is implied by -Wall. For free-form source code, the default
is -Werror=line-truncation such that truncations are reported
as error.
-Wconversion
Warn about implicit conversions that are likely to change the
value of the expression after conversion. Implied by -Wall.
-Wconversion-extra
Warn about implicit conversions between different types and
kinds. This option does not imply -Wconversion.
-Wextra
Enables some warning options for usages of language features
which may be problematic. This currently includes
-Wcompare-reals, -Wunused-parameter and -Wdo-subscript.
-Wfrontend-loop-interchange
Enable warning for loop interchanges performed by the
-ffrontend-loop-interchange option.
-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.
-Wimplicit-procedure
Warn if a procedure is called that has neither an explicit
interface nor has been declared as "EXTERNAL".
-Winteger-division
Warn if a constant integer division truncates it result. As
an example, 3/5 evaluates to 0.
-Wintrinsics-std
Warn if gfortran finds a procedure named like an intrinsic
not available in the currently selected standard (with -std)
and treats it as "EXTERNAL" procedure because of this.
-fall-intrinsics can be used to never trigger this behavior
and always link to the intrinsic regardless of the selected
standard.
-Wreal-q-constant
Produce a warning if a real-literal-constant contains a "q"
exponent-letter.
-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.
* The type of a function result is declared more than once
with the same type. If -pedantic or standard-conforming
mode is enabled, this is an error.
* A "CHARACTER" variable is declared with negative length.
-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. -Wtabs will cause a warning to be issued if a tab
is encountered. Note, -Wtabs is active for -pedantic,
-std=f95, -std=f2003, -std=f2008, -std=f2018 and -Wall.
-Wundefined-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 -Wall.
-Wunderflow
Produce a warning when numerical constant expressions are
encountered, which yield an UNDERFLOW during compilation.
Enabled by default.
-Wintrinsic-shadow
Warn if a user-defined procedure or module procedure has the
same name as an intrinsic; in this case, an explicit
interface or "EXTERNAL" or "INTRINSIC" declaration might be
needed to get calls later resolved to the desired
intrinsic/procedure. This option is implied by -Wall.
-Wuse-without-only
Warn if a "USE" statement has no "ONLY" qualifier and thus
implicitly imports all public entities of the used module.
-Wunused-dummy-argument
Warn about unused dummy arguments. This option is implied by
-Wall.
-Wunused-parameter
Contrary to gcc's meaning of -Wunused-parameter, gfortran's
implementation of this option does not warn about unused
dummy arguments (see -Wunused-dummy-argument), but about
unused "PARAMETER" values. -Wunused-parameter is implied by
-Wextra if also -Wunused or -Wall is used.
-Walign-commons
By default, gfortran warns about any occasion of variables
being padded for proper alignment inside a "COMMON" block.
This warning can be turned off via -Wno-align-commons. See
also -falign-commons.
-Wfunction-elimination
Warn if any calls to impure functions are eliminated by the
optimizations enabled by the -ffrontend-optimize option.
This option is implied by -Wextra.
-Wrealloc-lhs
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. "(:,:,:)") 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 -frealloc-lhs.
-Wrealloc-lhs-all
Warn when the compiler inserts code to for allocation or
reallocation of an allocatable variable; this includes
scalars and derived types.
-Wcompare-reals
Warn when comparing real or complex types for equality or
inequality. This option is implied by -Wextra.
-Wtarget-lifetime
Warn if the pointer in a pointer assignment might be longer
than the its target. This option is implied by -Wall.
-Wzerotrip
Warn if a "DO" loop is known to execute zero times at compile
time. This option is implied by -Wall.
-Wdo-subscript
Warn if an array subscript inside a DO loop could lead to an
out-of-bounds access even if the compiler cannot prove that
the statement is actually executed, in cases like
real a(3)
do i=1,4
if (condition(i)) then
a(i) = 1.2
end if
end do
This option is implied by -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-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.
-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.
-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 "-fdump-fortran-original" instead.
-fdump-fortran-global
Output a list of the global identifiers after translating
into middle-end 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.
-ffpe-trap=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. list is a (possibly empty) comma-
separated list of the following 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), inexact (loss of precision during operation), and
denormal (operation performed on a denormal value). The
first five exceptions correspond to the five IEEE 754
exceptions, whereas the last one (denormal) is not part of
the IEEE 754 standard but is available on some common
architectures such as x86.
The first three exceptions (invalid, zero, and 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.
If the option is used more than once in the command line, the
lists will be joined: '"ffpe-trap="list1 "ffpe-trap="list2'
is equivalent to "ffpe-trap="list1,list2.
Note that once enabled an exception cannot be disabled (no
negative form).
Many, if not most, floating point operations incur loss of
precision due to rounding, and hence the "ffpe-trap=inexact"
is likely to be uninteresting in practice.
By default no exception traps are enabled.
-ffpe-summary=list
Specify a list of floating-point exceptions, whose flag
status is printed to "ERROR_UNIT" when invoking "STOP" and
"ERROR STOP". list can be either none, all or a comma-
separated list of the following exceptions: invalid, zero,
overflow, underflow, inexact and denormal. (See -ffpe-trap
for a description of the exceptions.)
If the option is used more than once in the command line,
only the last one will be used.
By default, a summary for all exceptions but inexact is
shown.
-fno-backtrace
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 core), the Fortran runtime
library tries to output a backtrace of the error.
"-fno-backtrace" disables the backtrace generation. This
option only has influence for 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.
-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.
-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 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 "SIGN" intrinsic.
-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 "SIGN" intrinsic for compatibility
with Fortran 77. The default is -fsign-zero.
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.
Local variables or arrays having an explicit "SAVE" attribute
are silently ignored unless the -pedantic option is added.
-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 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.
-fcoarray=<keyword>
none
Disable coarray support; using coarray declarations and
image-control statements will produce a compile-time
error. (Default)
single
Single-image mode, i.e. "num_images()" is always one.
lib Library-based coarray parallelization; a suitable GNU
Fortran coarray library needs to be linked.
-fcheck=<keyword>
Enable the generation of run-time checks; the argument shall
be a comma-delimited list of the following keywords.
Prefixing a check with no- disables it if it was activated by
a previous specification.
all Enable all run-time test of -fcheck.
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.
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 -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.
do Enable generation of run-time checks for invalid
modification of loop iteration variables.
mem Enable generation of run-time checks for memory
allocation. Note: This option does not affect explicit
allocations using the "ALLOCATE" statement, which will be
always checked.
pointer
Enable generation of run-time checks for pointers and
allocatables.
recursion
Enable generation of run-time checks for recursively
called subroutines and functions which are not marked as
recursive. See also -frecursive. Note: This check does
not work for OpenMP programs and is disabled if used
together with -frecursive and -fopenmp.
Example: Assuming you have a file foo.f90, the command
gfortran -fcheck=all,no-array-temps foo.f90
will compile the file with all checks enabled as specified
above except warnings for generated array temporaries.
-fbounds-check
Deprecated alias for -fcheck=bounds.
-ftail-call-workaround
-ftail-call-workaround=n
Some C interfaces to Fortran codes violate the gfortran ABI
by omitting the hidden character length arguments as
described in
This can lead to crashes because pushing arguments for tail
calls can overflow the stack.
To provide a workaround for existing binary packages, this
option disables tail call optimization for gfortran
procedures with character arguments. With
-ftail-call-workaround=2 tail call optimization is disabled
in all gfortran procedures with character arguments, with
-ftail-call-workaround=1 or equivalent -ftail-call-workaround
only in gfortran procedures with character arguments that
call implicitly prototyped procedures.
Using this option can lead to problems including crashes due
to insufficient stack space.
It is very strongly recommended to fix the code in question.
The -fc-prototypes-external option can be used to generate
prototypes which conform to gfortran's ABI, for inclusion in
the source code.
Support for this option will likely be withdrawn in a future
release of gfortran.
The negative form, -fno-tail-call-workaround or equivalent
-ftail-call-workaround=0, can be used to disable this option.
Default is currently -ftail-call-workaround, this will change
in future releases.
-fcheck-array-temporaries
Deprecated alias for -fcheck=array-temps.
-fmax-array-constructor=n
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.
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
Caution: This option can lead to long compile times and
excessively large object files.
The default value for n is 65535.
-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.
-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 -Ofast unless
-fmax-stack-var-size is specified.
-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.
-finline-matmul-limit=n
When front-end optimiztion is active, some calls to the
"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 "-finline-matmul-limit=0" will disable
inlining in all cases. Setting this option with a value of n
will produce inline code for matrices with size up to 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 n is 30. The "-fblas-matmul-limit" can
be used to change this value.
-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-derived
-finit-integer=n
-finit-real=<zero|inf|-inf|nan|snan>
-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|snan> (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.
With -finit-derived, components of derived type variables
will be initialized according to these flags. Components
whose type is not covered by an explicit -finit-* flag will
be treated as described above with -finit-local-zero.
These options do not initialize
* objects with the POINTER attribute
* allocatable arrays
* variables that appear in an "EQUIVALENCE" statement.
(These limitations may be removed in future releases).
Note that the -finit-real=nan option initializes "REAL" and
"COMPLEX" variables with a quiet NaN. For a signalling NaN
use -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 -ffpe-trap).
The -finit-integer option will parse the value into an
integer of type "INTEGER(kind=C_LONG)" on the host. Said
value is then assigned to the integer variables in the
Fortran code, which might result in wraparound if the value
is too large for the kind.
Finally, note that enabling any of the -finit-* options will
silence warnings that would have been emitted by
-Wuninitialized for the affected local variables.
-falign-commons
By default, gfortran enforces proper alignment of all
variables in a "COMMON" block by padding them as needed. On
certain platforms this is mandatory, on others it increases
performance. If a "COMMON" block is not declared with
consistent data types everywhere, this padding can cause
trouble, and -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 "COMMON" block. To avoid
potential alignment issues in "COMMON" blocks, it is
recommended to order objects from largest to smallest.
-fno-protect-parens
By default the parentheses in expression are honored for all
optimization levels such that the compiler does not do any
re-association. Using -fno-protect-parens allows the compiler
to reorder "REAL" and "COMPLEX" expressions to produce faster
code. Note that for the re-association optimization
-fno-signed-zeros and -fno-trapping-math need to be in
effect. The parentheses protection is enabled by default,
unless -Ofast is given.
-frealloc-lhs
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 -std=f95 is given. See also -Wrealloc-lhs.
-faggressive-function-elimination
Functions with identical argument lists are eliminated within
statements, regardless of whether these functions are marked
"PURE" or not. For example, in
a = f(b,c) + f(b,c)
there will only be a single call to "f". This option only
works if -ffrontend-optimize is in effect.
-ffrontend-optimize
This option performs front-end optimization, based on
manipulating parts the Fortran parse tree. Enabled by
default by any -O option except -O0 and -Og. Optimizations
enabled by this option include:
*<inlining calls to "MATMUL",>
*<elimination of identical function calls within
expressions,>
*<removing unnecessary calls to "TRIM" in comparisons and
assignments,>
*<replacing TRIM(a) with "a(1:LEN_TRIM(a))" and>
*<short-circuiting of logical operators (".AND." and
".OR.").>
It can be deselected by specifying -fno-frontend-optimize.
-ffrontend-loop-interchange
Attempt to interchange loops in the Fortran front end where
profitable. Enabled by default by any -O option. At the
moment, this option only affects "FORALL" and "DO CONCURRENT"
statements with several forall triplets.
