GFORTRAN(1) GNU 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 -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 -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 -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
Directory Options
-Idir -Jdir -fintrinsic-modules-path dir
Link Options
-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.
-fdefault-double-8
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.
-fdefault-integer-8
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 42.
-fdefault-real-8
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.
-fdollar-ok
Allow $ as a valid non-first character in a symbol name. Symbols that start with $ are rejected since it
is unclear which rules to apply to implicit typing as different vendors implement different rules. Using
$ in "IMPLICIT" statements is also rejected.
-fbackslash
Change the interpretation of backslashes in string literals from a single backslash character to "C-style"
escape characters. The following combinations are expanded "\a", "\b", "\f", "\n", "\r", "\t", "\v", "\\",
and "\0" to the ASCII characters alert, backspace, form feed, newline, carriage return, horizontal tab,
vertical tab, backslash, and NUL, respectively. Additionally, "\x"nn, "\u"nnnn and "\U"nnnnnnnn (where
each n is a hexadecimal digit) are translated into the Unicode characters corresponding to the specified
code points. All other combinations of a character preceded by \ are unexpanded.
-fmodule-private
Set the default accessibility of module entities to "PRIVATE". Use-associated entities will not be
accessible unless they are explicitly declared as "PUBLIC".
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.
-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.
-fcray-pointer
Enable the Cray pointer extension, which provides C-like pointer functionality.
-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.
-freal-4-real-8
-freal-4-real-10
-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
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.
-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.
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, -Wno-tabs, -Wintrinsic-shadow, -Wline-truncation, -Wtarget-lifetime,
-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.
-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.
-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.
-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".
-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.
warning to be issued if a tab is encountered. Note, -Wno-tabs is active for -pedantic, -std=f95,
-std=f2003, -std=f2008 and -Wall.
-Wunderflow
Produce a warning when numerical constant expressions are encountered, which yield an UNDERFLOW during
compilation.
-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.
-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 not included in -Wall but is implied by -Wall -Wextra.
-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.
-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
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.
-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
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
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.
-fno-whole-file
This flag causes the compiler to resolve and translate each procedure in a file separately.
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.
-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
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.
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.
-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.
-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.
-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
· 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 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 <http://bugzilla.redhat.com/bugzilla>.
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.
gcc-4.8.5 2015-06-23 GFORTRAN(1)
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