[-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 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.
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 -fdefault-double-8-fdefault-integer-8 -fdefault-real-8 -fdollar-ok-ffixed-line-length-n-ffixed-line-length-none -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=stdPreprocessing 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 -undefError and Warning Options-Waliasing -Wall -Wampersand -Warray-bounds -Wc-binding-type-Wcharacter-truncation -Wconversion -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-Wtarget-lifetime -fmax-errors=n-fsyntax-only -pedantic-pedantic-errorsDebugging Options-fbacktrace -fdump-fortran-optimized -fdump-fortran-original-fdump-parse-tree -ffpe-trap=list-ffpe-summary=listDirectory Options-Idir-Jdir-fintrinsic-modules-path dirLink Options-static-libgfortranRuntime Options-fconvert=conversion-fmax-subrecord-length=length-frecord-marker=length-fsign-zeroCode Generation Options-faggressive-function-elimination -fblas-matmul-limit=n-fbounds-check -fcheck-array-temporaries-fcheck=<all|array-temps|bounds|do|mem|pointer|recursion>-fcoarray=<none|single|lib>-fexternal-blas -ff2c-ffrontend-optimize -finit-character=n-finit-integer=n-finit-local-zero -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-arraysOptions controlling Fortran dialect
The following options control the details of the Fortran dialect
accepted by the compiler:
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.
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".
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
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-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.
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.
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.
Set the default accessibility of module entities to "PRIVATE".
Use-associated entities will not be accessible unless they are
explicitly declared as "PUBLIC".
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
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
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
Specify the maximum allowed identifier length. Typical values are
31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).
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.
Enable the Cray pointer extension, which provides C-like pointer
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.
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.
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.
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.
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, and
does promote the default width of "DOUBLE PRECISION" to 16 bytes
if possible, unless "-fdefault-double-8" is given, too. Unlike
-freal-4-real-8, it does not promote variables with explicit kind
Set the "DOUBLE PRECISION" type to an 8 byte wide type. Do
nothing if this is already the default. If -fdefault-real-8 is
given, "DOUBLE PRECISION" would instead be promoted to 16 bytes
if possible, and -fdefault-double-8 can be used to prevent this.
The kind of real constants like "1.d0" will not be changed by
-fdefault-real-8 though, so also -fdefault-double-8 does not
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
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.
Specify the standard to which the program is expected to conform,
which may be one of f95, f2003, f2008, 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, f2003
and f2008 values specify strict conformance to the Fortran 95,
Fortran 2003 and Fortran 2008 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. -std=f2008ts allows
the Fortran 2008 standard including the additions of the
Technical Specification (TS) 29113 on Further Interoperability of
Fortran with C and TS 18508 on Additional Parallel Features in
Enable and customize preprocessing
Preprocessor related options. See section Preprocessing andconditional compilation for more detailed information on
preprocessing in gfortran.
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
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
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.
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
Specify prefix as the prefix for subsequent -iwithprefix options.
If the prefix represents a directory, you should include the
This option is like the --sysroot option, but applies only to
header files. See the --sysroot option for more information.
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.
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.
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.
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
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
Cancel an assertion with the predicate predicate and 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.
Predefine name as a macro, with definition 1.
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
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
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 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.
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
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
These options control the amount and kinds of errors and warnings
produced by GNU Fortran:
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.
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.
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, -std=f2003 or
Like -pedantic, except that errors are produced rather than
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 and -Wunused.
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 constants.
The warning is given with -Wampersand, -pedantic, -std=f95,
-std=f2003 and -std=f2008. 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.
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.
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.
Warn when a character assignment will truncate the assigned
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
Warn about implicit conversions that are likely to change the
value of the expression after conversion. Implied by -Wall.
Warn about implicit conversions between different types and
kinds. This option does not imply -Wconversion.
Enables some warning options for usages of language features
which may be problematic. This currently includes -Wcompare-reals
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
Warn if a procedure is called that has neither an explicit
interface nor has been declared as "EXTERNAL".
Warn if a constant integer division truncates it result. As an
example, 3/5 evaluates to 0.
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.
Produce a warning if a real-literal-constant contains a "q"
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
* 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
* 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.
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=f2008ts and -Wall.
Produce a warning when numerical constant expressions are
encountered, which yield an UNDERFLOW during compilation. Enabled
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.
Warn if a "USE" statement has no "ONLY" qualifier and thus
implicitly imports all public entities of the used module.
Warn about unused dummy arguments. This option is implied by
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.
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
Warn if any calls to functions are eliminated by the
optimizations enabled by the -ffrontend-optimize option.
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
Warn when the compiler inserts code to for allocation or
reallocation of an allocatable variable; this includes scalars
and derived types.
Warn when comparing real or complex types for equality or
inequality. This option is implied by -Wextra.
Warn if the pointer in a pointer assignment might be longer than
the its target. This option is implied by -Wall.
Warn if a "DO" loop is known to execute zero times at compile
time. This option is implied by -Wall.
Turns all warnings into errors.
Some of these have no effect when compiling programs written in
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.
Output the internal parse tree after translating the source
program into internal representation. Only really useful for
debugging the GNU Fortran compiler itself.
Output the parse tree after front-end optimization. Only really
useful for debugging the GNU Fortran compiler itself.
Output the internal parse tree after translating the source
program into internal representation. Only really useful for
debugging the GNU Fortran compiler itself. This option is
deprecated; use "-fdump-fortran-original" instead.
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.
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.
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
By default, a summary for all exceptions but inexact is shown.
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
It also affects the search paths used by cpp when used to preprocess
These affect interpretation of the "INCLUDE" directive (as well
as of the "#include" directive of the cpp preprocessor).
Also note that the general behavior of -I and "INCLUDE" is pretty
much the same as of -I with "#include" in the cpp preprocessor,
with regard to looking for header.gcc files and other such
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" statement.
The default is the current directory.
This option specifies the location of pre-compiled intrinsic
modules, if they are not in the default location expected by the
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.
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
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 environmentvariable 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 isdifferent from previous versions ofgfortran, 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.
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.
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
Options for code generation conventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo would be -fno-foo. In the table below, only one of the
forms is listed---the one which is not the default. You can figure
out the other form by either removing no- or adding it.
Treat each program unit (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
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, 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.
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
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
Disable coarray support; using coarray declarations and
image-control statements will produce a compile-time error.
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.
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
all Enable all run-time test of -fcheck.
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.
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.
Enable generation of run-time checks for pointers and
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.
Deprecated alias for -fcheck=bounds.
Deprecated alias for -fcheck=array-temps.
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.
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 andexcessively large object files.
The default value for n is 65535.
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.
Adding this option will make the Fortran compiler put all local
arrays, even those of unknown size 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.
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
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.
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.
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.
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
The default value for n is 30.
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 the value specified for
"-fblas-matmul-limit" if this option is specified, or unlimitited
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.
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. These options do not
* allocatable arrays
* components of derived type variables
* 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).
Finally, note that enabling any of the -finit-* options will
silence warnings that would have been emitted by -Wuninitialized
for the affected local variables.
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
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.
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.
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.
This option performs front-end optimization, based on
manipulating parts the Fortran parse tree. Enabled by default by
any -O option. 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 and replacing TRIM(a) with
"a(1:LEN_TRIM(a))". It can be deselected by specifying
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