GFORTRAN
Section: GNU (1)
Updated: 2021-04-05
Page Index
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
-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 -fdump-fortran-optimized -fdump-fortran-original
-fdebug-aux-vars -fdump-fortran-global -fdump-parse-tree -ffpe-trap=list
-ffpe-summary=list
- Directory Options
-
-Idir -Jdir -fintrinsic-modules-path dir
- Link Options
-
-static-libgfortran
- Runtime Options
-
-fconvert=conversion -fmax-subrecord-length=length
-frecord-marker=length -fsign-zero
- Interoperability Options
-
-fc-prototypes -fc-prototypes-external
- Code Generation Options
-
-faggressive-function-elimination -fblas-matmul-limit=n
-fbounds-check -ftail-call-workaround -ftail-call-workaround=n
-fcheck-array-temporaries
-fcheck=<all|array-temps|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
Options controlling Fortran dialect
The following options control the details of the Fortran dialect
accepted by the compiler:
- -ffree-form
-
- -ffixed-form
-
Specify the layout used by the source file. The free form layout
was introduced in Fortran 90. Fixed form was traditionally used in
older Fortran programs. When neither option is specified, the source
form is determined by the file extension.
- -fall-intrinsics
-
This option causes all intrinsic procedures (including the GNU-specific
extensions) to be accepted. This can be useful with -std=f95 to
force standard-compliance but get access to the full range of intrinsics
available with gfortran. As a consequence, -Wintrinsics-std
will be ignored and no user-defined procedure with the same name as any
intrinsic will be called except when it is explicitly declared "EXTERNAL".
- -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
-
Enable legacy math intrinsics such as COTAN and degree-valued trigonometric
functions (e.g. TAND, ATAND, etc...) for compatability with older code.
- -fdec-static
-
Enable DEC-style STATIC and AUTOMATIC attributes to explicitly specify
the storage of variables and other objects.
- -fdec-include
-
Enable parsing of INCLUDE as a statement in addition to parsing it as
INCLUDE line. When parsed as INCLUDE statement, INCLUDE does not have to
be on a single line and can use line continuations.
- -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 Fortran 2008).
- -fimplicit-none
-
Specify that no implicit typing is allowed, unless overridden by explicit
"IMPLICIT" statements. This is the equivalent of adding
"implicit none" to the start of every procedure.
- -fcray-pointer
-
Enable the Cray pointer extension, which provides C-like pointer
functionality.
- -fopenacc
-
Enable the OpenACC extensions. This includes OpenACC "!$acc"
directives in free form and "c$acc", *$acc and
"!$acc" directives in fixed form, "!$" conditional
compilation sentinels in free form and "c$", "*$" and
"!$" sentinels in fixed form, and when linking arranges for the
OpenACC runtime library to be linked in.
- -fopenmp
-
Enable the OpenMP extensions. This includes OpenMP "!$omp" directives
in free form
and "c$omp", *$omp and "!$omp" directives in fixed form,
"!$" conditional compilation sentinels in free form
and "c$", "*$" and "!$" sentinels in fixed form,
and when linking arranges for the OpenMP runtime library to be linked
in. The option -fopenmp implies -frecursive.
- -fno-range-check
-
Disable range checking on results of simplification of constant
expressions during compilation. For example, GNU Fortran will give
an error at compile time when simplifying "a = 1. / 0".
With this option, no error will be given and "a" will be assigned
the value "+Infinity". If an expression evaluates to a value
outside of the relevant range of ["-HUGE()":"HUGE()"],
then the expression will be replaced by "-Inf" or "+Inf"
as appropriate.
Similarly, "DATA i/Z'FFFFFFFF'/" will result in an integer overflow
on most systems, but with -fno-range-check the value will
``wrap around'' and "i" will be initialized to -1 instead.
- -fdefault-integer-8
-
Set the default integer and logical types to an 8 byte wide type. This option
also affects the kind of integer constants like 42. Unlike
-finteger-4-integer-8, it does not promote variables with explicit
kind declaration.
- -fdefault-real-8
-
Set the default real type to an 8 byte wide type. This option also affects
the kind of non-double real constants like 1.0. This option promotes
the default width of "DOUBLE PRECISION" and double real constants
like "1.d0" to 16 bytes if possible. If "-fdefault-double-8"
is given along with "fdefault-real-8", "DOUBLE PRECISION"
and double real constants are not promoted. Unlike -freal-4-real-8,
"fdefault-real-8" does not promote variables with explicit kind
declarations.
- -fdefault-real-10
-
Set the default real type to an 10 byte wide type. This option also affects
the kind of non-double real constants like 1.0. This option promotes
the default width of "DOUBLE PRECISION" and double real constants
like "1.d0" to 16 bytes if possible. If "-fdefault-double-8"
is given along with "fdefault-real-10", "DOUBLE PRECISION"
and double real constants are not promoted. Unlike -freal-4-real-10,
"fdefault-real-10" does not promote variables with explicit kind
declarations.
- -fdefault-real-16
-
Set the default real type to an 16 byte wide type. This option also affects
the kind of non-double real constants like 1.0. This option promotes
the default width of "DOUBLE PRECISION" and double real constants
like "1.d0" to 16 bytes if possible. If "-fdefault-double-8"
is given along with "fdefault-real-16", "DOUBLE PRECISION"
and double real constants are not promoted. Unlike -freal-4-real-16,
"fdefault-real-16" does not promote variables with explicit kind
declarations.
- -fdefault-double-8
-
Set the "DOUBLE PRECISION" type and double real constants
like "1.d0" to an 8 byte wide type. Do nothing if this
is already the default. This option prevents -fdefault-real-8,
-fdefault-real-10, and -fdefault-real-16,
from promoting "DOUBLE PRECISION" and double real constants like
"1.d0" to 16 bytes.
- -finteger-4-integer-8
-
Promote all "INTEGER(KIND=4)" entities to an "INTEGER(KIND=8)"
entities. If "KIND=8" is unavailable, then an error will be issued.
This option should be used with care and may not be suitable for your codes.
Areas of possible concern include calls to external procedures,
alignment in "EQUIVALENCE" and/or "COMMON", generic interfaces,
BOZ literal constant conversion, and I/O. Inspection of the intermediate
representation of the translated Fortran code, produced by
-fdump-tree-original, is suggested.
- -freal-4-real-8
-
- -freal-4-real-10
-
- -freal-4-real-16
-
- -freal-8-real-4
-
- -freal-8-real-10
-
- -freal-8-real-16
-
Promote all "REAL(KIND=M)" entities to "REAL(KIND=N)" entities.
If "REAL(KIND=N)" is unavailable, then an error will be issued.
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, gnu, or legacy. The default value for
std is gnu, which specifies a superset of the latest
Fortran standard that includes all of the extensions supported by GNU
Fortran, although warnings will be given for obsolete extensions not
recommended for use in new code. The legacy value is
equivalent but without the warnings for obsolete extensions, and may
be useful for old non-standard programs. The f95,
f2003, f2008, and f2018 values specify strict
conformance to the Fortran 95, Fortran 2003, Fortran 2008 and Fortran
2018 standards, respectively; errors are given for all extensions
beyond the relevant language standard, and warnings are given for the
Fortran 77 features that are permitted but obsolescent in later
standards. The deprecated option -std=f2008ts acts as an alias for
-std=f2018. It is only present for backwards compatibility with
earlier gfortran versions and should not be used any more.
- -ftest-forall-temp
-
Enhance test coverage by forcing most forall assignments to use temporary.
Enable and customize preprocessing
Preprocessor related options. See section
Preprocessing and conditional compilation for more detailed
information on preprocessing in
gfortran.
- -cpp
-
- -nocpp
-
Enable preprocessing. The preprocessor is automatically invoked if
the file extension is .fpp, .FPP, .F, .FOR,
.FTN, .F90, .F95, .F03 or .F08. Use
this option to manually enable preprocessing of any kind of Fortran file.
To disable preprocessing of files with any of the above listed extensions,
use the negative form: -nocpp.
The preprocessor is run in traditional mode. Any restrictions of the
file-format, especially the limits on line length, apply for
preprocessed output as well, so it might be advisable to use the
-ffree-line-length-none or -ffixed-line-length-none
options.
- -dM
-
Instead of the normal output, generate a list of '#define'
directives for all the macros defined during the execution of the
preprocessor, including predefined macros. This gives you a way
of finding out what is predefined in your version of the preprocessor.
Assuming you have no file foo.f90, the command
touch foo.f90; gfortran -cpp -E -dM foo.f90
will show all the predefined macros.
- -dD
-
Like -dM except in two respects: it does not include the
predefined macros, and it outputs both the "#define" directives
and the result of preprocessing. Both kinds of output go to the
standard output file.
- -dN
-
Like -dD, but emit only the macro names, not their expansions.
- -dU
-
Like dD except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output; the
output is delayed until the use or test of the macro; and '#undef'
directives are also output for macros tested but undefined at the time.
- -dI
-
Output '#include' directives in addition to the result
of preprocessing.
- -fworking-directory
-
Enable generation of linemarkers in the preprocessor output that will
let the compiler know the current working directory at the time of
preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current
working directory followed by two slashes. GCC will use this directory,
when it is present in the preprocessed input, as the directory emitted
as the current working directory in some debugging information formats.
This option is implicitly enabled if debugging information is enabled,
but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present
in the command line, this option has no effect, since no "#line"
directives are emitted whatsoever.
- -idirafter dir
-
Search dir for include files, but do it after all directories
specified with -I and the standard system directories have
been exhausted. dir is treated as a system include directory.
If dir begins with "=", then the "=" will be replaced by
the sysroot prefix; see --sysroot and -isysroot.
- -imultilib dir
-
Use dir as a subdirectory of the directory containing target-specific
C++ headers.
- -iprefix prefix
-
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should include
the final '/'.
- -isysroot dir
-
This option is like the --sysroot option, but applies only to
header files. See the --sysroot option for more information.
- -iquote dir
-
Search dir only for header files requested with "#include "file"";
they are not searched for "#include <file>", before all directories
specified by -I and before the standard system directories. If
dir begins with "=", then the "=" will be replaced by the
sysroot prefix; see --sysroot and -isysroot.
- -isystem dir
-
Search dir for header files, after all directories specified by
-I but before the standard system directories. Mark it as a
system directory, so that it gets the same special treatment as is
applied to the standard system directories. If dir begins with
"=", then the "=" will be replaced by the sysroot prefix;
see --sysroot and -isysroot.
- -nostdinc
-
Do not search the standard system directories for header files. Only
the directories you have specified with -I options (and the
directory of the current file, if appropriate) are searched.
- -undef
-
Do not predefine any system-specific or GCC-specific macros.
The standard predefined macros remain defined.
- -Apredicate=answer
-
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -A predicate(answer), which is still
supported, because it does not use shell special characters.
- -A-predicate=answer
-
Cancel an assertion with the predicate predicate and answer answer.
- -C
-
Do not discard comments. All comments are passed through to the output
file, except for comments in processed directives, which are deleted
along with the directive.
You should be prepared for side effects when using -C; it causes
the preprocessor to treat comments as tokens in their own right. For example,
comments appearing at the start of what would be a directive line have the
effect of turning that line into an ordinary source line, since the first
token on the line is no longer a '#'.
Warning: this currently handles C-Style comments only. The preprocessor
does not yet recognize Fortran-style comments.
- -CC
-
Do not discard comments, including during macro expansion. This is like
-C, except that comments contained within macros are also passed
through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC
option causes all C++-style comments inside a macro to be converted to C-style
comments. This is to prevent later use of that macro from inadvertently
commenting out the remainder of the source line. The -CC option
is generally used to support lint comments.
Warning: this currently handles C- and C++-Style comments only. The
preprocessor does not yet recognize Fortran-style comments.
- -Dname
-
Predefine name as a macro, with definition 1.
- -Dname=definition
-
The contents of definition are tokenized and processed as if they
appeared during translation phase three in a '#define' directive.
In particular, the definition will be truncated by embedded newline
characters.
If you are invoking the preprocessor from a shell or shell-like program
you may need to use the shell's quoting syntax to protect characters such
as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh, "-D'name(args...)=definition'"
works.
-D and -U options are processed in the order they are
given on the command line. All -imacros file and -include file options
are processed after all -D and -U options.
- -H
-
Print the name of each header file used, in addition to other normal
activities. Each name is indented to show how deep in the '#include'
stack it is.
- -P
-
Inhibit generation of linemarkers in the output from the preprocessor.
This might be useful when running the preprocessor on something that
is not C code, and will be sent to a program which might be confused
by the linemarkers.
- -Uname
-
Cancel any previous definition of name, either built in or provided
with a -D option.
Options to request or suppress errors and warnings
Errors are diagnostic messages that report that the
GNU Fortran compiler
cannot compile the relevant piece of source code. The compiler will
continue to process the program in an attempt to report further errors
to aid in debugging, but will not produce any compiled output.
Warnings are diagnostic messages that report constructions which
are not inherently erroneous but which are risky or suggest there is
likely to be a bug in the program. Unless -Werror is specified,
they do not prevent compilation of the program.
You can request many specific warnings with options beginning -W,
for example -Wimplicit to request warnings on implicit
declarations. Each of these specific warning options also has a
negative form beginning -Wno- to turn off warnings;
for example, -Wno-implicit. This manual lists only one of the
two forms, whichever is not the default.
These options control the amount and kinds of errors and warnings produced
by GNU Fortran:
- -fmax-errors=n
-
Limits the maximum number of error messages to n, at which point
GNU Fortran bails out rather than attempting to continue processing the
source code. If n is 0, there is no limit on the number of error
messages produced.
- -fsyntax-only
-
Check the code for syntax errors, but do not actually compile it. This
will generate module files for each module present in the code, but no
other output file.
- -Wpedantic
-
- -pedantic
-
Issue warnings for uses of extensions to Fortran.
-pedantic also applies to C-language constructs where they
occur in GNU Fortran source files, such as use of \e in a
character constant within a directive like "#include".
Valid Fortran programs should compile properly with or without
this option.
However, without this option, certain GNU extensions and traditional
Fortran features are supported as well.
With this option, many of them are rejected.
Some users try to use -pedantic to check programs for conformance.
They soon find that it does not do quite what they want---it finds some
nonstandard practices, but not all.
However, improvements to GNU Fortran in this area are welcome.
This should be used in conjunction with -std=f95,
-std=f2003, -std=f2008 or -std=f2018.
- -pedantic-errors
-
Like -pedantic, except that errors are produced rather than
warnings.
- -Wall
-
Enables commonly used warning options pertaining to usage that
we recommend avoiding and that we believe are easy to avoid.
This currently includes -Waliasing, -Wampersand,
-Wconversion, -Wsurprising, -Wc-binding-type,
-Wintrinsics-std, -Wtabs, -Wintrinsic-shadow,
-Wline-truncation, -Wtarget-lifetime,
-Winteger-division, -Wreal-q-constant, -Wunused
and -Wundefined-do-loop.
- -Waliasing
-
Warn about possible aliasing of dummy arguments. Specifically, it warns
if the same actual argument is associated with a dummy argument with
"INTENT(IN)" and a dummy argument with "INTENT(OUT)" in a call
with an explicit interface.
The following example will trigger the warning.
interface
subroutine bar(a,b)
integer, intent(in) :: a
integer, intent(out) :: b
end subroutine
end interface
integer :: a
call bar(a,a)
- -Wampersand
-
Warn about missing ampersand in continued character constants. The
warning is given with -Wampersand, -pedantic,
-std=f95, -std=f2003, -std=f2008 and
-std=f2018. Note: With no ampersand given in a continued
character constant, GNU Fortran assumes continuation at the first
non-comment, non-whitespace character after the ampersand that
initiated the continuation.
- -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.
-
- -Wtabs
-
By default, tabs are accepted as whitespace, but tabs are not members
of the Fortran Character Set. For continuation lines, a tab followed
by a digit between 1 and 9 is supported. -Wtabs will cause a
warning to be issued if a tab is encountered. Note, -Wtabs is
active for -pedantic, -std=f95, -std=f2003,
-std=f2008, -std=f2018 and
-Wall.
- -Wundefined-do-loop
-
Warn if a DO loop with step either 1 or -1 yields an underflow or an overflow
during iteration of an induction variable of the loop.
This option is implied by -Wall.
- -Wunderflow
-
Produce a warning when numerical constant expressions are
encountered, which yield an UNDERFLOW during compilation. Enabled by default.
- -Wintrinsic-shadow
-
Warn if a user-defined procedure or module procedure has the same name as an
intrinsic; in this case, an explicit interface or "EXTERNAL" or
"INTRINSIC" declaration might be needed to get calls later resolved to
the desired intrinsic/procedure. This option is implied by -Wall.
- -Wuse-without-only
-
Warn if a "USE" statement has no "ONLY" qualifier and
thus implicitly imports all public entities of the used module.
- -Wunused-dummy-argument
-
Warn about unused dummy arguments. This option is implied by -Wall.
- -Wunused-parameter
-
Contrary to gcc's meaning of -Wunused-parameter,
gfortran's implementation of this option does not warn
about unused dummy arguments (see -Wunused-dummy-argument),
but about unused "PARAMETER" values. -Wunused-parameter
is implied by -Wextra if also -Wunused or
-Wall is used.
- -Walign-commons
-
By default, gfortran warns about any occasion of variables being
padded for proper alignment inside a "COMMON" block. This warning can be turned
off via -Wno-align-commons. See also -falign-commons.
- -Wfunction-elimination
-
Warn if any calls to impure functions are eliminated by the optimizations
enabled by the -ffrontend-optimize option.
This option is implied by -Wextra.
- -Wrealloc-lhs
-
Warn when the compiler might insert code to for allocation or reallocation of
an allocatable array variable of intrinsic type in intrinsic assignments. In
hot loops, the Fortran 2003 reallocation feature may reduce the performance.
If the array is already allocated with the correct shape, consider using a
whole-array array-spec (e.g. "(:,:,:)") for the variable on the left-hand
side to prevent the reallocation check. Note that in some cases the warning
is shown, even if the compiler will optimize reallocation checks away. For
instance, when the right-hand side contains the same variable multiplied by
a scalar. See also -frealloc-lhs.
- -Wrealloc-lhs-all
-
Warn when the compiler inserts code to for allocation or reallocation of an
allocatable variable; this includes scalars and derived types.
- -Wcompare-reals
-
Warn when comparing real or complex types for equality or inequality.
This option is implied by -Wextra.
- -Wtarget-lifetime
-
Warn if the pointer in a pointer assignment might be longer than the its
target. This option is implied by -Wall.
- -Wzerotrip
-
Warn if a "DO" loop is known to execute zero times at compile
time. This option is implied by -Wall.
- -Wdo-subscript
-
Warn if an array subscript inside a DO loop could lead to an
out-of-bounds access even if the compiler cannot prove that the
statement is actually executed, in cases like
real a(3)
do i=1,4
if (condition(i)) then
a(i) = 1.2
end if
end do
This option is implied by -Wextra.
- -Werror
-
Turns all warnings into errors.
Some of these have no effect when compiling programs written in Fortran.
Options for debugging your program or GNU Fortran
GNU Fortran has various special options that are used for debugging
either your program or the
GNU Fortran compiler.
- -fdump-fortran-original
-
Output the internal parse tree after translating the source program
into internal representation. This option is mostly useful for
debugging the GNU Fortran compiler itself. The output generated by
this option might change between releases. This option may also
generate internal compiler errors for features which have only
recently been added.
- -fdump-fortran-optimized
-
Output the parse tree after front-end optimization. Mostly useful for
debugging the GNU Fortran compiler itself. The output generated by
this option might change between releases. This option may also
generate internal compiler errors for features which have only
recently been added.
- -fdump-parse-tree
-
Output the internal parse tree after translating the source program
into internal representation. Mostly useful for debugging the GNU
Fortran compiler itself. The output generated by this option might
change between releases. This option may also generate internal
compiler errors for features which have only recently been added. This
option is deprecated; use "-fdump-fortran-original" instead.
- -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".
- -fdump-fortran-global
-
Output a list of the global identifiers after translating into
middle-end representation. Mostly useful for debugging the GNU Fortran
compiler itself. The output generated by this option might change
between releases. This option may also generate internal compiler
errors for features which have only recently been added.
- -ffpe-trap=list
-
Specify a list of floating point exception traps to enable. On most
systems, if a floating point exception occurs and the trap for that
exception is enabled, a SIGFPE signal will be sent and the program
being aborted, producing a core file useful for debugging. list
is a (possibly empty) comma-separated list of the following
exceptions: invalid (invalid floating point operation, such as
"SQRT(-1.0)"), zero (division by zero), overflow
(overflow in a floating point operation), underflow (underflow
in a floating point operation), inexact (loss of precision
during operation), and denormal (operation performed on a
denormal value). The first five exceptions correspond to the five
IEEE 754 exceptions, whereas the last one (denormal) is not
part of the IEEE 754 standard but is available on some common
architectures such as x86.
The first three exceptions (invalid, zero, and
overflow) often indicate serious errors, and unless the program
has provisions for dealing with these exceptions, enabling traps for
these three exceptions is probably a good idea.
If the option is used more than once in the command line, the lists will
be joined: '"ffpe-trap="list1 "ffpe-trap="list2'
is equivalent to "ffpe-trap="list1,list2.
Note that once enabled an exception cannot be disabled (no negative form).
Many, if not most, floating point operations incur loss of precision
due to rounding, and hence the "ffpe-trap=inexact" is likely to
be uninteresting in practice.
By default no exception traps are enabled.
- -ffpe-summary=list
-
Specify a list of floating-point exceptions, whose flag status is printed
to "ERROR_UNIT" when invoking "STOP" and "ERROR STOP".
list can be either none, all or a comma-separated list
of the following exceptions: invalid, zero, overflow,
underflow, inexact and denormal. (See
-ffpe-trap for a description of the exceptions.)
If the option is used more than once in the command line, only the
last one will be used.
By default, a summary for all exceptions but inexact is shown.
- -fno-backtrace
-
When a serious runtime error is encountered or a deadly signal is
emitted (segmentation fault, illegal instruction, bus error,
floating-point exception, and the other POSIX signals that have the
action core), the Fortran runtime library tries to output a
backtrace of the error. "-fno-backtrace" disables the backtrace
generation. This option only has influence for compilation of the
Fortran main program.
Options for directory search
These options affect how
GNU Fortran searches
for files specified by the
"INCLUDE" directive and where it searches
for previously compiled modules.
It also affects the search paths used by cpp when used to preprocess
Fortran source.
- -Idir
-
These affect interpretation of the "INCLUDE" directive
(as well as of the "#include" directive of the cpp
preprocessor).
Also note that the general behavior of -I and
"INCLUDE" is pretty much the same as of -I with
"#include" in the cpp preprocessor, with regard to
looking for header.gcc files and other such things.
This path is also used to search for .mod files when previously
compiled modules are required by a "USE" statement.
- -Jdir
-
This option specifies where to put .mod files for compiled modules.
It is also added to the list of directories to searched by an "USE"
statement.
The default is the current directory.
- -fintrinsic-modules-path dir
-
This option specifies the location of pre-compiled intrinsic modules, if
they are not in the default location expected by the compiler.
Influencing the linking step
These options come into play when the compiler links object files into an
executable output file. They are meaningless if the compiler is not doing
a link step.
- -static-libgfortran
-
On systems that provide libgfortran as a shared and a static
library, this option forces the use of the static version. If no
shared version of libgfortran was built when the compiler was
configured, this option has no effect.
Influencing runtime behavior
These options affect the runtime behavior of programs compiled with
GNU Fortran.
- -fconvert=conversion
-
Specify the representation of data for unformatted files. Valid
values for conversion are: native, the default; swap,
swap between big- and little-endian; big-endian, use big-endian
representation for unformatted files; little-endian, use little-endian
representation for unformatted files.
This option has an effect only when used in the main program.
The "CONVERT" specifier and the GFORTRAN_CONVERT_UNIT environment
variable override the default specified by -fconvert.
- -frecord-marker=length
-
Specify the length of record markers for unformatted files.
Valid values for length are 4 and 8. Default is 4.
This is different from previous versions of gfortran,
which specified a default record marker length of 8 on most
systems. If you want to read or write files compatible
with earlier versions of gfortran, use -frecord-marker=8.
- -fmax-subrecord-length=length
-
Specify the maximum length for a subrecord. The maximum permitted
value for length is 2147483639, which is also the default. Only
really useful for use by the gfortran testsuite.
- -fsign-zero
-
When enabled, floating point numbers of value zero with the sign bit set
are written as negative number in formatted output and treated as
negative in the "SIGN" intrinsic. -fno-sign-zero does not
print the negative sign of zero values (or values rounded to zero for I/O)
and regards zero as positive number in the "SIGN" intrinsic for
compatibility with Fortran 77. The default is -fsign-zero.
Options for code generation conventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo would be -fno-foo. In the table below, only
one of the forms is listed---the one which is not the default. You
can figure out the other form by either removing no- or adding
it.
- -fno-automatic
-
Treat each program unit (except those marked as RECURSIVE) as if the
"SAVE" statement were specified for every local variable and array
referenced in it. Does not affect common blocks. (Some Fortran compilers
provide this option under the name -static or -save.)
The default, which is -fautomatic, uses the stack for local
variables smaller than the value given by -fmax-stack-var-size.
Use the option -frecursive to use no static memory.
Local variables or arrays having an explicit "SAVE" attribute are
silently ignored unless the -pedantic option is added.
- -ff2c
-
Generate code designed to be compatible with code generated
by g77 and f2c.
The calling conventions used by g77 (originally implemented
in f2c) require functions that return type
default "REAL" to actually return the C type "double", and
functions that return type "COMPLEX" to return the values via an
extra argument in the calling sequence that points to where to
store the return value. Under the default GNU calling conventions, such
functions simply return their results as they would in GNU
C---default "REAL" functions return the C type "float", and
"COMPLEX" functions return the GNU C type "complex".
Additionally, this option implies the -fsecond-underscore
option, unless -fno-second-underscore is explicitly requested.
This does not affect the generation of code that interfaces with
the libgfortran library.
Caution: It is not a good idea to mix Fortran code compiled with
-ff2c with code compiled with the default -fno-f2c
calling conventions as, calling "COMPLEX" or default "REAL"
functions between program parts which were compiled with different
calling conventions will break at execution time.
Caution: This will break code which passes intrinsic functions
of type default "REAL" or "COMPLEX" as actual arguments, as
the library implementations use the -fno-f2c calling conventions.
- -fno-underscoring
-
Do not transform names of entities specified in the Fortran
source file by appending underscores to them.
With -funderscoring in effect, GNU Fortran appends one
underscore to external names with no underscores. This is done to ensure
compatibility with code produced by many UNIX Fortran compilers.
Caution: The default behavior of GNU Fortran is
incompatible with f2c and g77, please use the
-ff2c option if you want object files compiled with
GNU Fortran to be compatible with object code created with these
tools.
Use of -fno-underscoring is not recommended unless you are
experimenting with issues such as integration of GNU Fortran into
existing system environments (vis-a-vis existing libraries, tools,
and so on).
For example, with -funderscoring, and assuming that "j()" and
"max_count()" are external functions while "my_var" and
"lvar" are local variables, a statement like
I = J() + MAX_COUNT (MY_VAR, LVAR)
is implemented as something akin to:
i = j_() + max_count__(&my_var__, &lvar);
With -fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
Use of -fno-underscoring allows direct specification of
user-defined names while debugging and when interfacing GNU Fortran
code with other languages.
Note that just because the names match does not mean that the
interface implemented by GNU Fortran for an external name matches the
interface implemented by some other language for that same name.
That is, getting code produced by GNU Fortran to link to code produced
by some other compiler using this or any other method can be only a
small part of the overall solution---getting the code generated by
both compilers to agree on issues other than naming can require
significant effort, and, unlike naming disagreements, linkers normally
cannot detect disagreements in these other areas.
Also, note that with -fno-underscoring, the lack of appended
underscores introduces the very real possibility that a user-defined
external name will conflict with a name in a system library, which
could make finding unresolved-reference bugs quite difficult in some
cases---they might occur at program run time, and show up only as
buggy behavior at run time.
In future versions of GNU Fortran we hope to improve naming and linking
issues so that debugging always involves using the names as they appear
in the source, even if the names as seen by the linker are mangled to
prevent accidental linking between procedures with incompatible
interfaces.
- -fsecond-underscore
-
By default, GNU Fortran appends an underscore to external
names. If this option is used GNU Fortran appends two
underscores to names with underscores and one underscore to external names
with no underscores. GNU Fortran also appends two underscores to
internal names with underscores to avoid naming collisions with external
names.
This option has no effect if -fno-underscoring is
in effect. It is implied by the -ff2c option.
Otherwise, with this option, an external name such as "MAX_COUNT"
is implemented as a reference to the link-time external symbol
"max_count__", instead of "max_count_". This is required
for compatibility with g77 and f2c, and is implied
by use of the -ff2c option.
- -fcoarray=<keyword>
-
-
- none
-
Disable coarray support; using coarray declarations and image-control
statements will produce a compile-time error. (Default)
- single
-
Single-image mode, i.e. "num_images()" is always one.
- lib
-
Library-based coarray parallelization; a suitable GNU Fortran coarray
library needs to be linked.
-
- -fcheck=<keyword>
-
Enable the generation of run-time checks; the argument shall be
a comma-delimited list of the following keywords. Prefixing a check with
no- disables it if it was activated by a previous specification.
-
- all
-
Enable all run-time test of -fcheck.
- array-temps
-
Warns at run time when for passing an actual argument a temporary array
had to be generated. The information generated by this warning is
sometimes useful in optimization, in order to avoid such temporaries.
Note: The warning is only printed once per location.
- 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 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
<
http://bugzilla.redhat.com/bugzilla>.
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) 2004-2021 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.