gfortran(1)
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 -fbackslash -fcray-pointer -fd-lines-as-code
-fd-lines-as-comments -fdec -fdec-structure -fdefault-double-8
-fdefault-integer-8 -fdefault-real-8 -fdollar-ok
-ffixed-line-length-n -ffixed-line-length-none -ffree-form
-ffree-line-length-n -ffree-line-length-none -fimplicit-none
-finteger-4-integer-8 -fmax-identifier-length -fmodule-private
-ffixed-form -fno-range-check -fopenacc -fopenmp -freal-4-real-10
-freal-4-real-16 -freal-4-real-8 -freal-8-real-10 -freal-8-real-16
-freal-8-real-4 -std=std
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 -Wfunction-elimination
-Wimplicit-interface -Wimplicit-procedure -Wintrinsic-shadow
-Wuse-without-only -Wintrinsics-std -Wline-truncation
-Wno-align-commons -Wno-tabs -Wreal-q-constant -Wsurprising
-Wunderflow -Wunused-parameter -Wrealloc-lhs -Wrealloc-lhs-all
-Wtarget-lifetime -fmax-errors=n -fsyntax-only -pedantic
-pedantic-errors
Debugging Options
-fbacktrace -fdump-fortran-optimized -fdump-fortran-original
-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
Code Generation Options
-faggressive-function-elimination -fblas-matmul-limit=n
-fbounds-check -fcheck-array-temporaries
-fcheck=<all|array-temps|bounds|do|mem|pointer|recursion>
-fcoarray=<none|single|lib> -fexternal-blas -ff2c
-ffrontend-optimize -finit-character=n -finit-integer=n
-finit-local-zero -finit-logical=<true|false>
-finit-real=<zero|inf|-inf|nan|snan> -finline-matmul-limit=n
-fmax-array-constructor=n -fmax-stack-var-size=n -fno-align-commons
-fno-automatic -fno-protect-parens -fno-underscoring
-fsecond-underscore -fpack-derived -frealloc-lhs -frecursive
-frepack-arrays -fshort-enums -fstack-arrays
Options controlling Fortran dialect
The following options control the details of the Fortran dialect
accepted by the compiler:
-ffree-form
-ffixed-form
Specify the layout used by the source file. The free form layout
was introduced in Fortran 90. Fixed form was traditionally used in
older Fortran programs. When neither option is specified, the
source form is determined by the file extension.
-fall-intrinsics
This option causes all intrinsic procedures (including the GNU-
specific extensions) to be accepted. This can be useful with
-std=f95 to force standard-compliance but get access to the full
range of intrinsics available with gfortran. As a consequence,
-Wintrinsics-std will be ignored and no user-defined procedure with
the same name as any intrinsic will be called except when it is
explicitly declared "EXTERNAL".
-fd-lines-as-code
-fd-lines-as-comments
Enable special treatment for lines beginning with "d" or "D" in
fixed form sources. If the -fd-lines-as-code option is given they
are treated as if the first column contained a blank. If the
-fd-lines-as-comments option is given, they are treated as comment
lines.
-fdec
DEC compatibility mode. Enables extensions and other features that
mimic the default behavior of older compilers (such as DEC). These
features are non-standard and should be avoided at all costs. For
details on GNU Fortran's implementation of these extensions see the
full documentation.
Other flags enabled by this switch are: -fdollar-ok -fcray-pointer
-fdec-structure
-fdec-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.
-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 "
", " ", "\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 through which spaces are assumed (as
if padded to that length) after the ends of short fixed-form lines.
Popular values for n include 72 (the standard and the default), 80
(card image), and 132 (corresponding to "extended-source" options
in some popular compilers). n may also be none, meaning that the
entire line is meaningful and that continued character constants
never have implicit spaces appended to them to fill out the line.
-ffixed-line-length-0 means the same thing as
-ffixed-line-length-none.
-ffree-line-length-n
Set column after which characters are ignored in typical free-form
lines in the source file. The default value is 132. n may be none,
meaning that the entire line is meaningful. -ffree-line-length-0
means the same thing as -ffree-line-length-none.
-fmax-identifier-length=n
Specify the maximum allowed identifier length. Typical values are
31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).
-fimplicit-none
Specify that no implicit typing is allowed, unless overridden by
explicit "IMPLICIT" statements. This is the equivalent of adding
"implicit none" to the start of every procedure.
-fcray-pointer
Enable the Cray pointer extension, which provides C-like pointer
functionality.
-fopenacc
Enable the OpenACC extensions. This includes OpenACC "!$acc"
directives in free form and "c$acc", *$acc and "!$acc" directives
in fixed form, "!$" conditional compilation sentinels in free form
and "c$", "*$" and "!$" sentinels in fixed form, and when linking
arranges for the OpenACC runtime library to be linked in.
Note that this is an experimental feature, incomplete, and subject
to change in future versions of GCC. See
<https://gcc.gnu.org/wiki/OpenACC> for more information.
-fopenmp
Enable the OpenMP extensions. This includes OpenMP "!$omp"
directives in free form and "c$omp", *$omp and "!$omp" directives
in fixed form, "!$" conditional compilation sentinels in free form
and "c$", "*$" and "!$" sentinels in fixed form, and when linking
arranges for the OpenMP runtime library to be linked in. The
option -fopenmp implies -frecursive.
-fno-range-check
Disable range checking on results of simplification of constant
expressions during compilation. For example, GNU Fortran will give
an error at compile time when simplifying "a = 1. / 0". With this
option, no error will be given and "a" will be assigned the value
"+Infinity". If an expression evaluates to a value outside of the
relevant range of ["-HUGE()":"HUGE()"], then the expression will be
replaced by "-Inf" or "+Inf" as appropriate. Similarly, "DATA
i/Z'FFFFFFFF'/" will result in an integer overflow on most systems,
but with -fno-range-check the value will "wrap around" and "i" will
be initialized to -1 instead.
-fdefault-integer-8
Set the default integer and logical types to an 8 byte wide type.
This option also affects the kind of integer constants like 42.
Unlike -finteger-4-integer-8, it does not promote variables with
explicit kind declaration.
-fdefault-real-8
Set the default real type to an 8 byte wide type. This option also
affects the kind of non-double real constants like 1.0, and does
promote the default width of "DOUBLE PRECISION" to 16 bytes if
possible, unless "-fdefault-double-8" is given, too. Unlike
-freal-4-real-8, it does not promote variables with explicit kind
declaration.
-fdefault-double-8
Set the "DOUBLE PRECISION" type to an 8 byte wide type. Do nothing
if this is already the default. If -fdefault-real-8 is given,
"DOUBLE PRECISION" would instead be promoted to 16 bytes if
possible, and -fdefault-double-8 can be used to prevent this. The
kind of real constants like "1.d0" will not be changed by
-fdefault-real-8 though, so also -fdefault-double-8 does not affect
it.
-finteger-4-integer-8
Promote all "INTEGER(KIND=4)" entities to an "INTEGER(KIND=8)"
entities. If "KIND=8" is unavailable, then an error will be
issued. This option should be used with care and may not be
suitable for your codes. Areas of possible concern include calls
to external procedures, alignment in "EQUIVALENCE" and/or "COMMON",
generic interfaces, BOZ literal constant conversion, and I/O.
Inspection of the intermediate representation of the translated
Fortran code, produced by -fdump-tree-original, is suggested.
-freal-4-real-8
-freal-4-real-10
-freal-4-real-16
-freal-8-real-4
-freal-8-real-10
-freal-8-real-16
Promote all "REAL(KIND=M)" entities to "REAL(KIND=N)" entities. If
"REAL(KIND=N)" is unavailable, then an error will be issued. All
other real kind types are unaffected by this option. These options
should be used with care and may not be suitable for your codes.
Areas of possible concern include calls to external procedures,
alignment in "EQUIVALENCE" and/or "COMMON", generic interfaces, BOZ
literal constant conversion, and I/O. Inspection of the
intermediate representation of the translated Fortran code,
produced by -fdump-tree-original, is suggested.
-std=std
Specify the standard to which the program is expected to conform,
which may be one of f95, f2003, f2008, gnu, or legacy. The default
value for std is gnu, which specifies a superset of the Fortran 95
standard that includes all of the extensions supported by GNU
Fortran, although warnings will be given for obsolete extensions
not recommended for use in new code. The legacy value is
equivalent but without the warnings for obsolete extensions, and
may be useful for old non-standard programs. The f95, f2003 and
f2008 values specify strict conformance to the Fortran 95, Fortran
2003 and Fortran 2008 standards, respectively; errors are given for
all extensions beyond the relevant language standard, and warnings
are given for the Fortran 77 features that are permitted but
obsolescent in later standards. -std=f2008ts allows the Fortran
2008 standard including the additions of the Technical
Specification (TS) 29113 on Further Interoperability of Fortran
with C and TS 18508 on Additional Parallel Features in Fortran.
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.
-pedantic
Issue warnings for uses of extensions to Fortran 95. -pedantic
also applies to C-language constructs where they occur in GNU
Fortran source files, such as use of \e in a character constant
within a directive like "#include".
Valid Fortran 95 programs should compile properly with or without
this option. However, without this option, certain GNU extensions
and traditional Fortran features are supported as well. With this
option, many of them are rejected.
Some users try to use -pedantic to check programs for conformance.
They soon find that it does not do quite what they want---it finds
some nonstandard practices, but not all. However, improvements to
GNU Fortran in this area are welcome.
This should be used in conjunction with -std=f95, -std=f2003 or
-std=f2008.
-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 and -Wunused.
-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
and -std=f2008. Note: With no ampersand given in a continued
character constant, GNU Fortran assumes continuation at the first
non-comment, non-whitespace character after the ampersand that
initiated the continuation.
-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 and
-Wunused-parameter.
-Wimplicit-interface
Warn if a procedure is called without an explicit interface. Note
this only checks that an explicit interface is present. It does
not check that the declared interfaces are consistent across
program units.
-Wimplicit-procedure
Warn if a procedure is called that has neither an explicit
interface nor has been declared as "EXTERNAL".
-Winteger-division
Warn if a constant integer division truncates it result. As an
example, 3/5 evaluates to 0.
-Wintrinsics-std
Warn if gfortran finds a procedure named like an intrinsic not
available in the currently selected standard (with -std) and treats
it as "EXTERNAL" procedure because of this. -fall-intrinsics can
be used to never trigger this behavior and always link to the
intrinsic regardless of the selected standard.
-Wreal-q-constant
Produce a warning if a real-literal-constant contains a "q"
exponent-letter.
-Wsurprising
Produce a warning when "suspicious" code constructs are
encountered. While technically legal these usually indicate that
an error has been made.
This currently produces a warning under the following
circumstances:
* An INTEGER SELECT construct has a CASE that can never be
matched as its lower value is greater than its upper value.
* A LOGICAL SELECT construct has three CASE statements.
* A TRANSFER specifies a source that is shorter than the
destination.
* The type of a function result is declared more than once with
the same type. If -pedantic or standard-conforming mode is
enabled, this is an error.
* A "CHARACTER" variable is declared with negative length.
-Wtabs
By default, tabs are accepted as whitespace, but tabs are not
members of the Fortran Character Set. For continuation lines, a
tab followed by a digit between 1 and 9 is supported. -Wtabs will
cause a warning to be issued if a tab is encountered. Note, -Wtabs
is active for -pedantic, -std=f95, -std=f2003, -std=f2008,
-std=f2008ts and -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 functions are eliminated by the optimizations
enabled by the -ffrontend-optimize option.
-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.
-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. Only really useful for debugging the
GNU Fortran compiler itself.
-fdump-fortran-optimized
Output the parse tree after front-end optimization. Only really
useful for debugging the GNU Fortran compiler itself.
-fdump-parse-tree
Output the internal parse tree after translating the source program
into internal representation. Only really useful for debugging the
GNU Fortran compiler itself. This option is deprecated; use
"-fdump-fortran-original" instead.
-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.
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.)
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.
-ff2c
Generate code designed to be compatible with code generated by g77
and f2c.
The calling conventions used by g77 (originally implemented in f2c)
require functions that return type default "REAL" to actually
return the C type "double", and functions that return type
"COMPLEX" to return the values via an extra argument in the calling
sequence that points to where to store the return value. Under the
default GNU calling conventions, such functions simply return their
results as they would in GNU C---default "REAL" functions return
the C type "float", and "COMPLEX" functions return the GNU C type
"complex". Additionally, this option implies the
-fsecond-underscore option, unless -fno-second-underscore is
explicitly requested.
This does not affect the generation of code that interfaces with
the libgfortran library.
Caution: It is not a good idea to mix Fortran code compiled with
-ff2c with code compiled with the default -fno-f2c calling
conventions as, calling "COMPLEX" or default "REAL" functions
between program parts which were compiled with different calling
conventions will break at execution time.
Caution: This will break code which passes intrinsic functions of
type default "REAL" or "COMPLEX" as actual arguments, as the
library implementations use the -fno-f2c calling conventions.
-fno-underscoring
Do not transform names of entities specified in the Fortran source
file by appending underscores to them.
With -funderscoring in effect, GNU Fortran appends one underscore
to external names with no underscores. This is done to ensure
compatibility with code produced by many UNIX Fortran compilers.
Caution: The default behavior of GNU Fortran is incompatible with
f2c and g77, please use the -ff2c option if you want object files
compiled with GNU Fortran to be compatible with object code created
with these tools.
Use of -fno-underscoring is not recommended unless you are
experimenting with issues such as integration of GNU Fortran into
existing system environments (vis-a-vis existing libraries, tools,
and so on).
For example, with -funderscoring, and assuming that "j()" and
"max_count()" are external functions while "my_var" and "lvar" are
local variables, a statement like
I = J() + MAX_COUNT (MY_VAR, LVAR)
is implemented as something akin to:
i = j_() + max_count__(&my_var__, &lvar);
With -fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
Use of -fno-underscoring allows direct specification of user-
defined names while debugging and when interfacing GNU Fortran code
with other languages.
Note that just because the names match does not mean that the
interface implemented by GNU Fortran for an external name matches
the interface implemented by some other language for that same
name. That is, getting code produced by GNU Fortran to link to
code produced by some other compiler using this or any other method
can be only a small part of the overall solution---getting the code
generated by both compilers to agree on issues other than naming
can require significant effort, and, unlike naming disagreements,
linkers normally cannot detect disagreements in these other areas.
Also, note that with -fno-underscoring, the lack of appended
underscores introduces the very real possibility that a user-
defined external name will conflict with a name in a system
library, which could make finding unresolved-reference bugs quite
difficult in some cases---they might occur at program run time, and
show up only as buggy behavior at run time.
In future versions of GNU Fortran we hope to improve naming and
linking issues so that debugging always involves using the names as
they appear in the source, even if the names as seen by the linker
are mangled to prevent accidental linking between procedures with
incompatible interfaces.
-fsecond-underscore
By default, GNU Fortran appends an underscore to external names.
If this option is used GNU Fortran appends two underscores to names
with underscores and one underscore to external names with no
underscores. GNU Fortran also appends two underscores to internal
names with underscores to avoid naming collisions with external
names.
This option has no effect if -fno-underscoring is in effect. It is
implied by the -ff2c option.
Otherwise, with this option, an external name such as "MAX_COUNT"
is implemented as a reference to the link-time external symbol
"max_count__", instead of "max_count_". This is required for
compatibility with g77 and f2c, and is implied by use of the -ff2c
option.
-fcoarray=<keyword>
none
Disable coarray support; using coarray declarations and image-
control statements will produce a compile-time error. (Default)
single
Single-image mode, i.e. "num_images()" is always one.
lib Library-based coarray parallelization; a suitable GNU Fortran
coarray library needs to be linked.
-fcheck=<keyword>
Enable the generation of run-time checks; the argument shall be a
comma-delimited list of the following keywords. Prefixing a check
with no- disables it if it was activated by a previous
specification.
all Enable all run-time test of -fcheck.
array-temps
Warns at run time when for passing an actual argument a
temporary array had to be generated. The information generated
by this warning is sometimes useful in optimization, in order
to avoid such temporaries.
Note: The warning is only printed once per location.
bounds
Enable generation of run-time checks for array subscripts and
against the declared minimum and maximum values. It also
checks array indices for assumed and deferred shape arrays
against the actual allocated bounds and ensures that all string
lengths are equal for character array constructors without an
explicit typespec.
Some checks require that -fcheck=bounds is set for the
compilation of the main program.
Note: In the future this may also include other forms of
checking, e.g., checking substring references.
do Enable generation of run-time checks for invalid modification
of loop iteration variables.
mem Enable generation of run-time checks for memory allocation.
Note: This option does not affect explicit allocations using
the "ALLOCATE" statement, which will be always checked.
pointer
Enable generation of run-time checks for pointers and
allocatables.
recursion
Enable generation of run-time checks for recursively called
subroutines and functions which are not marked as recursive.
See also -frecursive. Note: This check does not work for
OpenMP programs and is disabled if used together with
-frecursive and -fopenmp.
Example: Assuming you have a file foo.f90, the command
gfortran -fcheck=all,no-array-temps foo.f90
will compile the file with all checks enabled as specified above
except warnings for generated array temporaries.
-fbounds-check
Deprecated alias for -fcheck=bounds.
-fcheck-array-temporaries
Deprecated alias for -fcheck=array-temps.
-fmax-array-constructor=n
This option can be used to increase the upper limit permitted in
array constructors. The code below requires this option to expand
the array at compile time.
program test
implicit none
integer j
integer, parameter :: n = 100000
integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
print '(10(I0,1X))', i
end program test
Caution: This option can lead to long compile times and
excessively large object files.
The default value for n is 65535.
-fmax-stack-var-size=n
This option specifies the size in bytes of the largest array that
will be put on the stack; if the size is exceeded static memory is
used (except in procedures marked as RECURSIVE). Use the option
-frecursive to allow for recursive procedures which do not have a
RECURSIVE attribute or for parallel programs. Use -fno-automatic to
never use the stack.
This option currently only affects local arrays declared with
constant bounds, and may not apply to all character variables.
Future versions of GNU Fortran may improve this behavior.
The default value for n is 32768.
-fstack-arrays
Adding this option will make the Fortran compiler put all local
arrays, even those of unknown size onto stack memory. If your
program uses very large local arrays it is possible that you will
have to extend your runtime limits for stack memory on some
operating systems. This flag is enabled by default at optimization
level -Ofast.
-fpack-derived
This option tells GNU Fortran to pack derived type members as
closely as possible. Code compiled with this option is likely to
be incompatible with code compiled without this option, and may
execute slower.
-frepack-arrays
In some circumstances GNU Fortran may pass assumed shape array
sections via a descriptor describing a noncontiguous area of
memory. This option adds code to the function prologue to repack
the data into a contiguous block at runtime.
This should result in faster accesses to the array. However it can
introduce significant overhead to the function call, especially
when the passed data is noncontiguous.
-fshort-enums
This option is provided for interoperability with C code that was
compiled with the -fshort-enums option. It will make GNU Fortran
choose the smallest "INTEGER" kind a given enumerator set will fit
in, and give all its enumerators this kind.
-fexternal-blas
This option will make gfortran generate calls to BLAS functions for
some matrix operations like "MATMUL", instead of using our own
algorithms, if the size of the matrices involved is larger than a
given limit (see -fblas-matmul-limit). This may be profitable if
an optimized vendor BLAS library is available. The BLAS library
will have to be specified at link time.
-fblas-matmul-limit=n
Only significant when -fexternal-blas is in effect. Matrix
multiplication of matrices with size larger than (or equal to) n
will be performed by calls to BLAS functions, while others will be
handled by gfortran internal algorithms. If the matrices involved
are not square, the size comparison is performed using the
geometric mean of the dimensions of the argument and result
matrices.
The default value for n is 30.
-finline-matmul-limit=n
When front-end optimiztion is active, some calls to the "MATMUL"
intrinsic function will be inlined. This may result in code size
increase if the size of the matrix cannot be determined at compile
time, as code for both cases is generated. Setting
"-finline-matmul-limit=0" will disable inlining in all cases.
Setting this option with a value of n will produce inline code for
matrices with size up to n. If the matrices involved are not
square, the size comparison is performed using the geometric mean
of the dimensions of the argument and result matrices.
The default value for n is the value specified for
"-fblas-matmul-limit" if this option is specified, or unlimitited
otherwise.
-frecursive
Allow indirect recursion by forcing all local arrays to be
allocated on the stack. This flag cannot be used together with
-fmax-stack-var-size= or -fno-automatic.
-finit-local-zero
-finit-integer=n
-finit-real=<zero|inf|-inf|nan|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. These options do
not initialize
* allocatable arrays
* components of derived type variables
* variables that appear in an "EQUIVALENCE" statement.
(These limitations may be removed in future releases).
Note that the -finit-real=nan option initializes "REAL" and
"COMPLEX" variables with a quiet NaN. For a signalling NaN use
-finit-real=snan; note, however, that compile-time optimizations
may convert them into quiet NaN and that trapping needs to be
enabled (e.g. via -ffpe-trap).
Finally, note that enabling any of the -finit-* options will
silence warnings that would have been emitted by -Wuninitialized
for the affected local variables.
-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.
Optimizations enabled by this option include inlining calls to
"MATMUL", elimination of identical function calls within
expressions, removing unnecessary calls to "TRIM" in comparisons
and assignments and replacing TRIM(a) with "a(1:LEN_TRIM(a))". It
can be deselected by specifying -fno-frontend-optimize.
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-6/README.Bugs>.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), gcc(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, gfortran, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gfortran for contributors to GCC and GNU Fortran.
COPYRIGHT
Copyright (c) 2004-2016 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.
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