Provided by: gfortran-14-x86-64-linux-gnux32_14.2.0-4ubuntu2cross1_amd64 
NAME
gfortran - GNU Fortran compiler
SYNOPSIS
gfortran [-c|-S|-E]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...]
[-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remainder.
DESCRIPTION
The gfortran command supports all the options supported by the gcc command. Only options
specific to GNU Fortran are documented here.
All GCC and GNU Fortran options are accepted both by gfortran and by gcc (as well as any
other drivers built at the same time, such as g++), since adding GNU Fortran to the GCC
distribution enables acceptance of GNU Fortran options by all of the relevant drivers.
In some cases, options have positive and negative forms; the negative form of -ffoo would
be -fno-foo. This manual documents only one of these two forms, whichever one is not the
default.
OPTIONS
Here is a summary of all the options specific to GNU Fortran, grouped by type.
Explanations are in the following sections.
Fortran Language Options
-fall-intrinsics -fallow-argument-mismatch -fallow-invalid-boz -fbackslash
-fcray-pointer -fd-lines-as-code -fd-lines-as-comments -fdec -fdec-char-conversions
-fdec-structure -fdec-intrinsic-ints -fdec-static -fdec-math -fdec-include
-fdec-format-defaults -fdec-blank-format-item -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 -fopenmp-allocators
-fopenmp-simd -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 -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-overwrite-recursive -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 -fdebug-aux-vars -ffpe-trap=list -ffpe-summary=list
Directory Options
-Idir -Jdir -fintrinsic-modules-path dir
Link Options
-static-libgfortran -static-libquadmath
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|bits|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 -finline-arg-packing -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
Developer Options
-fdump-fortran-global -fdump-fortran-optimized -fdump-fortran-original
-fdump-parse-tree -save-temps
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= 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".
-fallow-argument-mismatch
Some code contains calls to external procedures with mismatches between the calls and
the procedure definition, or with mismatches between different calls. Such code is
non-conforming, and will usually be flagged with an error. This options degrades the
error to a warning, which can only be disabled by disabling all warnings via -w. Only
a single occurrence per argument is flagged by this warning.
-fallow-argument-mismatch is implied by -std=legacy.
Using this option is strongly discouraged. It is possible to provide standard-
conforming code which allows different types of arguments by using an explicit
interface and TYPE(*).
-fallow-invalid-boz
A BOZ literal constant can occur in a limited number of contexts in standard
conforming Fortran. This option degrades an error condition to a warning, and allows
a BOZ literal constant to appear where the Fortran standard would otherwise prohibit
its use.
-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-char-conversions -fdec-structure -fdec-intrinsic-ints -fdec-static -fdec-math
-fdec-include -fdec-blank-format-item -fdec-format-defaults
If -fd-lines-as-code/-fd-lines-as-comments are unset, then -fdec also sets
-fd-lines-as-comments.
-fdec-char-conversions
Enable the use of character literals in assignments and "DATA" statements for non-
character variables.
-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
Obsolete flag. The purpose of this option was to enable legacy math intrinsics such
as COTAN and degree-valued trigonometric functions (e.g. TAND, ATAND, etc...) for
compatability with older code. This option is no longer operable. The trigonometric
functions are now either part of Fortran 2023 or GNU extensions.
-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.
-fdec-format-defaults
Enable format specifiers F, G and I to be used without width specifiers, default
widths will be used instead.
-fdec-blank-format-item
Enable a blank format item at the end of a format specification i.e. nothing following
the final comma.
-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 later).
-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 handling of OpenACC directives !$acc in free-form Fortran and !$acc, c$acc and
*$acc in fixed-form Fortran. When -fopenacc is specified, the compiler generates
accelerated code according to the OpenACC Application Programming Interface v2.6
<https://www.openacc.org>. This option implies -pthread, and thus is only supported
on targets that have support for -pthread. The option -fopenacc implies -frecursive.
-fopenmp
Enable handling of OpenMP directives !$omp in Fortran. It additionally enables the
conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the
sentinels can also start with c or *. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application Program Interface v4.5
<https://www.openmp.org>. This option implies -pthread, and thus is only supported on
targets that have support for -pthread. -fopenmp implies -fopenmp-simd and
-frecursive.
-fopenmp-allocators
Enables handling of allocation, reallocation and deallocation of Fortran allocatable
and pointer variables that are allocated using the !$omp allocators and !$omp allocate
constructs. Files containing either directive have to be compiled with this option in
addition to -fopenmp. Additionally, all files that might deallocate or reallocate a
variable that has been allocated with an OpenMP allocator have to be compiled with
this option. This includes intrinsic assignment to allocatable variables when
reallocation may occur and deallocation due to either of the following: end of scope,
explicit deallocation, intent(out), deallocation of allocatable components etc. Files
not changing the allocation status or only for components of a derived type that have
not been allocated using those two directives do not need to be compiled with this
option. Nor do files that handle such variables after they have been deallocated or
allocated by the normal Fortran allocator.
-fopenmp-simd
Enable handling of OpenMP's "simd", "declare simd", "declare reduction", "assume",
"ordered", "scan" and "loop" directive, and of combined or composite directives with
"simd" as constituent with "!$omp" in Fortran. It additionally enables the
conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the
sentinels can also start with c or *. Other OpenMP directives are ignored. Unless
-fopenmp is additionally specified, the "loop" region binds to the current task
region, independent of the specified "bind" clause.
-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. The "-freal-4-" flags also affect the
default real kind and the "-freal-8-" flags also the double-precision real kind. All
other real-kind types are unaffected by this option. The promotion is also applied to
real literal constants of default and double-precision kind and a specified kind
number of 4 or 8, respectively. However, "-fdefault-real-8", "-fdefault-real-10",
"-fdefault-real-10", and "-fdefault-double-8" take precedence for the default and
double-precision real kinds, both for real literal constants and for declarations
without a kind number. Note that for "REAL(KIND=KIND(1.0))" the literal may get
promoted and then the result may get promoted again. 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 and calls to intrinsic procedures
when passing a value to the "kind=" dummy argument. Inspection of the intermediate
representation of the translated Fortran code, produced by -fdump-fortran-original or
-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, f2023, 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, f2018, and f2023 values specify strict conformance
to the Fortran 95, Fortran 2003, Fortran 2008, Fortran 2018 and Fortran 2023
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
Many Fortran compilers including GNU Fortran allow passing the source code through a C
preprocessor (CPP; sometimes also called the Fortran preprocessor, FPP) to allow for
conditional compilation. In the case of GNU Fortran, this is the GNU C Preprocessor in
the traditional mode. On systems with case-preserving file names, the preprocessor is
automatically invoked if the filename extension is .F, .FOR, .FTN, .fpp, .FPP, .F90, .F95,
.F03 or .F08. To manually invoke the preprocessor on any file, use -cpp, to disable
preprocessing on files where the preprocessor is run automatically, use -nocpp.
If a preprocessed file includes another file with the Fortran "INCLUDE" statement, the
included file is not preprocessed. To preprocess included files, use the equivalent
preprocessor statement "#include".
If GNU Fortran invokes the preprocessor, "__GFORTRAN__" is defined. The macros
"__GNUC__", "__GNUC_MINOR__" and "__GNUC_PATCHLEVEL__" can be used to determine the
version of the compiler. See Top,,Overview,cpp,The C Preprocessor for details.
GNU Fortran supports a number of "INTEGER" and "REAL" kind types in additional to the kind
types required by the Fortran standard. The availability of any given kind type is
architecture dependent. The following pre-defined preprocessor macros can be used to
conditionally include code for these additional kind types: "__GFC_INT_1__",
"__GFC_INT_2__", "__GFC_INT_8__", "__GFC_INT_16__", "__GFC_REAL_10__", and
"__GFC_REAL_16__".
While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran
95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely
used and not directly supported by the GNU Fortran compiler. You can use the program coco
to preprocess such files (<http://www.daniellnagle.com/coco.html>).
The following options control preprocessing of Fortran code:
-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, -std=f2018
or -std=f2023.
-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, -std=f2018 and
-std=f2023. 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.
-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
Warn when using -ffrontend-loop-interchange for performing loop interchanges.
-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 its 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.
-Wno-overwrite-recursive
Do not warn when -fno-automatic is used with -frecursive. Recursion will be broken if
the relevant local variables do not have the attribute "AUTOMATIC" explicitly
declared. This option can be used to suppress the warning when it is known that
recursion is not broken. Useful for build environments that use -Werror.
-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.
* With -fopenmp, for fixed-form source code, when an "omx" vendor-extension sentinel
is encountered. (The equivalent "ompx", used in free-form source code, is
diagnosed by default.)
-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,
-std=f2023 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
GNU Fortran has various special options that are used for debugging your program.
-fdebug-aux-vars
Renames internal variables created by the gfortran front end and makes them accessible
to a debugger. The name of the internal variables then start with upper-case letters
followed by an underscore. This option is useful for debugging the compiler's code
generation together with "-fdump-tree-original" and enabling debugging of the
executable program by using "-g" or "-ggdb3".
-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 either none (to clear
the set of exceptions to be trapped), or 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), except by
clearing all traps by specifying none.
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.
-static-libquadmath
On systems that provide libquadmath as a shared and a static library, this option
forces the use of the static version. If no shared version of libquadmath was built
when the compiler was configured, this option has no effect.
Please note that the libquadmath runtime library is licensed under the GNU Lesser
General Public License (LGPL), and linking it statically introduces requirements when
redistributing the resulting binaries.
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
on most systems 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.
On POWER systems which suppport -mabi=ieeelongdouble, there are additional options,
which can be combined with others with commas. Those are
@w<-fconvert=r16_ieee Use IEEE 128-bit format for>
REAL(KIND=16).
@w<-fconvert=r16_ibm Use IBM long double format for>
REAL(KIND=16).
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.
GNU Fortran Developer Options
GNU Fortran has various special options that are used for debugging the GNU Fortran
compiler.
-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.
-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-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-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.
-save-temps
Store the usual "temporary" intermediate files permanently; name them as auxiliary
output files, as specified described under GCC -dumpbase and -dumpdir.
gfortran -save-temps -c foo.F90
preprocesses input file foo.F90 to foo.fii, compiles to an intermediate foo.s, and
then assembles to the (implied) output file foo.o, whereas:
gfortran -save-temps -S foo.F
saves the preprocessor output in foo.fi, and then compiles to the (implied) output
file foo.s.
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.
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-à-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.
bits
Enable generation of run-time checks for invalid arguments to the bit manipulation
intrinsics.
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 65536.
-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.
-finline-arg-packing
When passing an assumed-shape argument of a procedure as actual argument to an
assumed-size or explicit size or as argument to a procedure that does not have an
explicit interface, the argument may have to be packed, that is put into contiguous
memory. An example is the call to "foo" in
subroutine foo(a)
real, dimension(*) :: a
end subroutine foo
subroutine bar(b)
real, dimension(:) :: b
call foo(b)
end subroutine bar
When -finline-arg-packing is in effect, this packing will be performed by inline code.
This allows for more optimization while increasing code size.
-finline-arg-packing is implied by any of the -O options except when optimizing for
size via -Os. If the code contains a very large number of argument that have to be
packed, code size and also compilation time may become excessive. If that is the
case, it may be better to disable this option. Instances of packing can be found by
using -Warray-temporaries.
-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 optimization 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.
ENVIRONMENT
The gfortran compiler currently does not make use of any environment variables to control
its operation above and beyond those that affect the operation of gcc.
BUGS
For instructions on reporting bugs, see <file:///usr/share/doc/gcc-14/README.Bugs>.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), dbx(1) and
the Info entries for gcc, cpp, gfortran, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gfortran for contributors to GCC and GNU Fortran.
COPYRIGHT
Copyright (c) 2004-2024 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of
the GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with the Invariant Sections being "Funding Free Software", the Front-
Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A
copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.