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gfortran - GNU Fortran compiler
[-g] [-pg] [-Olevel]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remainder.
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
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
-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 -fno-fixed-form -fno-range-check -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
-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 -Wcharacter-truncation -Wconversion
-Wfunction-elimination -Wimplicit-interface -Wimplicit-procedure -Wintrinsic-shadow
-Wintrinsics-std -Wline-truncation -Wno-align-commons -Wno-tabs -Wreal-q-constant
-Wsurprising -Wunderflow -Wunused-parameter -fmax-errors=n -fsyntax-only -pedantic
-fbacktrace -fdump-fortran-optimized -fdump-fortran-original -fdump-parse-tree
-Idir -Jdir -fintrinsic-modules-path dir
-fconvert=conversion -fmax-subrecord-length=length -fno-range-check
Code Generation Options
-faggressive-function-elimination -fblas-matmul-limit=n -fbounds-check
-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> -fmax-array-constructor=n -fmax-stack-var-size=n
-fno-align-commons -fno-automatic -fno-protect-parens -fno-underscoring
-fno-whole-file -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:
Specify the layout used by the source file. The free form layout was introduced in
Fortran 90. Fixed form was traditionally used in older Fortran programs. When
neither option is specified, the source form is determined by the file extension.
This option causes all intrinsic procedures (including the GNU-specific extensions) to
be accepted. This can be useful with -std=f95 to force standard-compliance but get
access to the full range of intrinsics available with gfortran. As a consequence,
-Wintrinsics-std will be ignored and no user-defined procedure with the same name as
any intrinsic will be called except when it is explicitly declared "EXTERNAL".
Enable special treatment for lines beginning with "d" or "D" in fixed form sources.
If the -fd-lines-as-code option is given they are treated as if the first column
contained a blank. If the -fd-lines-as-comments option is given, they are treated as
Set the "DOUBLE PRECISION" type to an 8 byte wide type. 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.
Set the default integer and logical types to an 8 byte wide type. Do nothing if this
is already the default. This option also affects the kind of integer constants like
Set the default real type to an 8 byte wide type. Do nothing if this is already the
default. 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.
Allow $ as a valid non-first character in a symbol name. Symbols that start with $ are
rejected since it is unclear which rules to apply to implicit typing as different
vendors implement different rules. Using $ in "IMPLICIT" statements is also rejected.
Change the interpretation of backslashes in string literals from a single backslash
character to "C-style" escape characters. The following combinations are expanded
"\a", "\b", "\f", "\n", "\r", "\t", "\v", "\\", and "\0" to the ASCII characters
alert, backspace, form feed, newline, carriage return, horizontal tab, vertical tab,
backslash, and NUL, respectively. Additionally, "\x"nn, "\u"nnnn and "\U"nnnnnnnn
(where each n is a hexadecimal digit) are translated into the Unicode characters
corresponding to the specified code points. All other combinations of a character
preceded by \ are unexpanded.
Set the default accessibility of module entities to "PRIVATE". Use-associated
entities will not be accessible unless they are explicitly declared as "PUBLIC".
Set column after which characters are ignored in typical fixed-form lines in the
source file, and through which spaces are assumed (as if padded to that length) after
the ends of short 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.
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.
Specify the maximum allowed identifier length. Typical values are 31 (Fortran 95) and
63 (Fortran 2003 and Fortran 2008).
Specify that no implicit typing is allowed, unless overridden by explicit "IMPLICIT"
statements. This is the equivalent of adding "implicit none" to the start of every
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
Enable the Cray pointer extension, which provides C-like pointer functionality.
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
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.
Promote all "REAL(KIND=M)" entities to "REAL(KIND=N)" entities. If "REAL(KIND=N)" is
unavailable, then an error will be issued. All other real kind types are unaffected
by this option. These options should be used with care and may not be suitable for
your codes. Areas of possible concern include calls to external procedures, alignment
in "EQUIVALENCE" and/or "COMMON", generic interfaces, BOZ literal constant conversion,
and I/O. Inspection of the intermediate representation of the translated Fortran
code, produced by -fdump-tree-original, is suggested.
Specify the standard to which the program is expected to conform, which may be one of
f95, f2003, f2008, gnu, or legacy. The default value for std is gnu, which specifies
a superset of the Fortran 95 standard that includes all of the extensions supported by
GNU Fortran, although warnings will be given for obsolete extensions not recommended
for use in new code. The legacy value is equivalent but without the warnings for
obsolete extensions, and may be useful for old non-standard programs. The f95, f2003
and f2008 values specify strict conformance to the Fortran 95, Fortran 2003 and
Fortran 2008 standards, respectively; errors are given for all extensions beyond the
relevant language standard, and warnings are given for the Fortran 77 features that
are permitted but obsolescent in later standards. -std=f2008ts allows the Fortran 2008
standard including the additions of the Technical Specification (TS) 29113 on Further
Interoperability of Fortran with C.
Enable and customize preprocessing
Preprocessor related options. See section Preprocessing and conditional compilation for
more detailed information on preprocessing in gfortran.
Enable preprocessing. The preprocessor is automatically invoked if the file extension
is .fpp, .FPP, .F, .FOR, .FTN, .F90, .F95, .F03 or .F08. Use this option to manually
enable preprocessing of any kind of Fortran file.
To disable preprocessing of files with any of the above listed extensions, use the
negative form: -nocpp.
The preprocessor is run in traditional mode. Any restrictions of the file-format,
especially the limits on line length, apply for preprocessed output as well, so it
might be advisable to use the -ffree-line-length-none or -ffixed-line-length-none
-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
-dI Output '#include' directives in addition to the result of preprocessing.
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.
Search dir for include files, but do it after all directories specified with -I and
the standard system directories have been exhausted. dir is treated as a system
include directory. If dir begins with "=", then the "=" will be replaced by the
sysroot prefix; see --sysroot and -isysroot.
Use dir as a subdirectory of the directory containing target-specific C++ headers.
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix
represents a directory, you should include the final '/'.
This option is like the --sysroot option, but applies only to header files. See the
--sysroot option for more information.
Search dir only for header files requested with "#include "file""; they are not
searched for "#include <file>", before all directories specified by -I and before the
standard system directories. If dir begins with "=", then the "=" will be replaced by
the sysroot prefix; see --sysroot and -isysroot.
Search dir for header files, after all directories specified by -I but before the
standard system directories. Mark it as a system directory, so that it gets the same
special treatment as is applied to the standard system directories. If dir begins with
"=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
Do not search the standard system directories for header files. Only the directories
you have specified with -I options (and the directory of the current file, if
appropriate) are searched.
Do not predefine any system-specific or GCC-specific macros. The standard predefined
macros remain defined.
Make an assertion with the predicate predicate and answer answer. This form is
preferred to the older form -A predicate(answer), which is still supported, because it
does not use shell special characters.
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.
Predefine name as a macro, with definition 1.
The contents of definition are tokenized and processed as if they appeared during
translation phase three in a '#define' directive. In particular, the definition will
be truncated by embedded 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 and -U options are processed in the order they are given on the command line. All
-imacros file and -include file options are processed after all -D and -U options.
-H Print the name of each header file used, in addition to other normal activities. Each
name is indented to show how deep in the '#include' stack it is.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be
useful when running the preprocessor on something that is not C code, and will be sent
to a program which might be confused by the linemarkers.
Cancel any previous definition of name, either built in or provided with a -D option.
Options to request or suppress errors and warnings
Errors are diagnostic messages that report that the GNU Fortran compiler cannot compile
the relevant piece of source code. The compiler will continue to process the program in
an attempt to report further errors to aid in debugging, but will not produce any compiled
Warnings are diagnostic messages that report constructions which are not inherently
erroneous but which are risky or suggest there is likely to be a bug in the program.
Unless -Werror is specified, they do not prevent compilation of the program.
You can request many specific warnings with options beginning -W, for example -Wimplicit
to request warnings on implicit declarations. Each of these specific warning options also
has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit.
This manual lists only one of the two forms, whichever is not the default.
These options control the amount and kinds of errors and warnings produced by GNU Fortran:
Limits the maximum number of error messages to n, at which point GNU Fortran bails out
rather than attempting to continue processing the source code. If n is 0, there is no
limit on the number of error messages produced.
Check the code for syntax errors, but do not actually compile it. This will generate
module files for each module present in the code, but no other output file.
Issue warnings for uses of extensions to Fortran 95. -pedantic also applies to
C-language constructs where they occur in GNU Fortran source files, such as use of \e
in a character constant within a directive like "#include".
Valid Fortran 95 programs should compile properly with or without this option.
However, without this option, certain GNU extensions and traditional Fortran features
are supported as well. With this option, many of them are rejected.
Some users try to use -pedantic to check programs for conformance. They soon find
that it does not do quite what they want---it finds some nonstandard practices, but
not all. However, improvements to GNU Fortran in this area are welcome.
This should be used in conjunction with -std=f95, -std=f2003 or -std=f2008.
Like -pedantic, except that errors are produced rather than warnings.
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, -Wintrinsics-std, -Wno-tabs,
-Wintrinsic-shadow, -Wline-truncation, -Wreal-q-constant and -Wunused.
Warn about possible aliasing of dummy arguments. Specifically, it warns if the same
actual argument is associated with a dummy argument with "INTENT(IN)" and a dummy
argument with "INTENT(OUT)" in a call with an explicit interface.
The following example will trigger the warning.
integer, intent(in) :: a
integer, intent(out) :: b
integer :: a
Warn about missing ampersand in continued character constants. The warning is given
with -Wampersand, -pedantic, -std=f95, -std=f2003 and -std=f2008. Note: With no
ampersand given in a continued character constant, GNU Fortran assumes continuation at
the first non-comment, non-whitespace character after the ampersand that initiated the
Warn about array temporaries generated by the compiler. The information generated by
this warning is sometimes useful in optimization, in order to avoid such temporaries.
Warn when a character assignment will truncate the assigned string.
Warn when a source code line will be truncated.
Warn about implicit conversions that are likely to change the value of the expression
after conversion. Implied by -Wall.
Warn about implicit conversions between different types and kinds.
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.
Warn if a procedure is called that has neither an explicit interface nor has been
declared as "EXTERNAL".
Warn if gfortran finds a procedure named like an intrinsic not available in the
currently selected standard (with -std) and treats it as "EXTERNAL" procedure because
of this. -fall-intrinsics can be used to never trigger this behavior and always link
to the intrinsic regardless of the selected standard.
Produce a warning if a real-literal-constant contains a "q" exponent-letter.
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.
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. -Wno-tabs will cause a warning to be issued if a tab is encountered. Note,
-Wno-tabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008 and -Wall.
Produce a warning when numerical constant expressions are encountered, which yield an
UNDERFLOW during compilation.
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
Warn about unused dummy arguments. This option is implied by -Wall.
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 not included in -Wall but is
implied by -Wall -Wextra.
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.
Warn if any calls to functions are eliminated by the optimizations enabled by the
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.
Output the internal parse tree after translating the source program into internal
representation. Only really useful for debugging the GNU Fortran compiler itself.
Output the parse tree after front-end optimization. Only really useful for debugging
the GNU Fortran compiler itself.
Output the internal parse tree after translating the source program into internal
representation. Only really useful for debugging the GNU Fortran compiler itself.
This option is deprecated; use "-fdump-fortran-original" instead.
Specify a list of floating point exception traps to enable. On most systems, if a
floating point exception occurs and the trap for that exception is enabled, a SIGFPE
signal will be sent and the program being aborted, producing a core file useful for
debugging. list is a (possibly empty) comma-separated list of the following
exceptions: invalid (invalid floating point operation, such as "SQRT(-1.0)"), zero
(division by zero), overflow (overflow in a floating point operation), underflow
(underflow in a floating point operation), inexact (loss of precision during
operation), and denormal (operation performed on a denormal value). The first five
exceptions correspond to the five IEEE 754 exceptions, whereas the last one (denormal)
is not part of the IEEE 754 standard but is available on some common architectures
such as x86.
The first three exceptions (invalid, zero, and overflow) often indicate serious
errors, and unless the program has provisions for dealing with these exceptions,
enabling traps for these three exceptions is probably a good idea.
Many, if not most, floating point operations incur loss of precision due to rounding,
and hence the "ffpe-trap=inexact" is likely to be uninteresting in practice.
By default no exception traps are enabled.
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.
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.
This option specifies where to put .mod files for compiled modules. It is also added
to the list of directories to searched by an "USE" statement.
The default is the current directory.
This option specifies the location of pre-compiled intrinsic modules, if they are not
in the default location expected by the 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.
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.
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
Disable range checking of input values during integer "READ" operations. For example,
GNU Fortran will give an error if an input value is outside of the relevant range of
["-HUGE()":"HUGE()"]. In other words, with "INTEGER (kind=4) :: i" , attempting to
read -2147483648 will give an error unless -fno-range-check is given.
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
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
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
Most of them have both positive and negative forms; the negative form of -ffoo would be
-fno-foo. In the table below, only one of the forms is listed---the one which is not the
default. You can figure out the other form by either removing no- or adding it.
Treat each program unit (except those marked as RECURSIVE) as if the "SAVE" statement
were specified for every local variable and array referenced in it. Does not affect
common blocks. (Some Fortran compilers provide this option under the name -static or
-save.) The default, which is -fautomatic, uses the stack for local variables smaller
than the value given by -fmax-stack-var-size. Use the option -frecursive to use no
Generate code designed to be compatible with code generated by g77 and f2c.
The calling conventions used by g77 (originally implemented in f2c) require functions
that return type default "REAL" to actually return the C type "double", and functions
that return type "COMPLEX" to return the values via an extra argument in the calling
sequence that points to where to store the return value. Under the default GNU
calling conventions, such functions simply return their results as they would in GNU
C---default "REAL" functions return the C type "float", and "COMPLEX" functions return
the GNU C type "complex". Additionally, this option implies the -fsecond-underscore
option, unless -fno-second-underscore is explicitly requested.
This does not affect the generation of code that interfaces with the libgfortran
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
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 other defaults like -fcase-lower and
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.
This flag causes the compiler to resolve and translate each procedure in a file
By default, the whole file is parsed and placed in a single front-end tree. During
resolution, in addition to all the usual checks and fixups, references to external
procedures that are in the same file effect resolution of that procedure, if not
already done, and a check of the interfaces. The dependences are resolved by changing
the order in which the file is translated into the backend tree. Thus, a procedure
that is referenced is translated before the reference and the duplication of backend
tree declarations eliminated.
The -fno-whole-file option is deprecated and may lead to wrong code.
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
Otherwise, with this option, an external name such as "MAX_COUNT" is implemented as a
reference to the link-time external symbol "max_count__", instead of "max_count_".
This is required for compatibility with g77 and f2c, and is implied by use of the
Disable coarray support; using coarray declarations and image-control statements
will produce a compile-time error. (Default)
Single-image mode, i.e. "num_images()" is always one.
lib Library-based coarray parallelization; a suitable GNU Fortran coarray library
needs to be linked.
Enable the generation of run-time checks; the argument shall be a comma-delimited list
of the following keywords.
all Enable all run-time test of -fcheck.
Warns at run time when for passing an actual argument a temporary array had to be
generated. The information generated by this warning is sometimes useful in
optimization, in order to avoid such temporaries.
Note: The warning is only printed once per location.
Enable generation of run-time checks for array subscripts and against the declared
minimum and maximum values. It also checks array indices for assumed and deferred
shape arrays against the actual allocated bounds and ensures that all string
lengths are equal for character array constructors without an explicit typespec.
Some checks require that -fcheck=bounds is set for the compilation of the main
Note: In the future this may also include other forms of checking, e.g., checking
do Enable generation of run-time checks for invalid modification of loop iteration
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
Enable generation of run-time checks for pointers and allocatables.
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.
Deprecated alias for -fcheck=bounds.
Deprecated alias for -fcheck=array-temps.
This option can be used to increase the upper limit permitted in array constructors.
The code below requires this option to expand the array at compile time.
integer, parameter :: n = 100000
integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
print '(10(I0,1X))', i
end program test
Caution: This option can lead to long compile times and excessively large object
The default value for n is 65535.
This option specifies the size in bytes of the largest array that will be put on the
stack; if the size is exceeded static memory is used (except in procedures marked as
RECURSIVE). Use the option -frecursive to allow for recursive procedures which do not
have a RECURSIVE attribute or for parallel programs. Use -fno-automatic to never use
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
The default value for n is 32768.
Adding this option will make the Fortran compiler put all local arrays, even those of
unknown size onto stack memory. If your program uses very large local arrays it is
possible that you will have to extend your runtime limits for stack memory on some
operating systems. This flag is enabled by default at optimization level -Ofast.
This option tells GNU Fortran to pack derived type members as closely as possible.
Code compiled with this option is likely to be incompatible with code compiled without
this option, and may execute slower.
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
This option is provided for interoperability with C code that was compiled with the
-fshort-enums option. It will make GNU Fortran choose the smallest "INTEGER" kind a
given enumerator set will fit in, and give all its enumerators this kind.
This option will make gfortran generate calls to BLAS functions for some matrix
operations like "MATMUL", instead of using our own algorithms, if the size of the
matrices involved is larger than a given limit (see -fblas-matmul-limit). This may be
profitable if an optimized vendor BLAS library is available. The BLAS library will
have to be specified at link time.
Only significant when -fexternal-blas is in effect. Matrix multiplication of matrices
with size larger than (or equal to) n will be performed by calls to BLAS functions,
while others will be handled by gfortran internal algorithms. If the matrices involved
are not square, the size comparison is performed using the geometric mean of the
dimensions of the argument and result matrices.
The default value for n is 30.
Allow indirect recursion by forcing all local arrays to be allocated on the stack.
This flag cannot be used together with -fmax-stack-var-size= or -fno-automatic.
The -finit-local-zero option instructs the compiler to initialize local "INTEGER",
"REAL", and "COMPLEX" variables to zero, "LOGICAL" variables to false, and "CHARACTER"
variables to a string of null bytes. Finer-grained initialization options are
provided by the -finit-integer=n, -finit-real=<zero|inf|-inf|nan|snan> (which also
initializes the real and imaginary parts of local "COMPLEX" variables),
-finit-logical=<true|false>, and -finit-character=n (where n is an ASCII character
value) options. These options do not 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.
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.
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
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.
Functions with identical argument lists are eliminated within statements, regardless
of whether these functions are marked "PURE" or not. For example, in
a = f(b,c) + f(b,c)
there will only be a single call to "f". This option only works if
-ffrontend-optimize is in effect.
This option performs front-end optimization, based on manipulating parts the Fortran
parse tree. Enabled by default by any -O option. Optimizations enabled by this
option include 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.
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.
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