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 -ffree-form -fno-fixed-form -fdollar-ok -fimplicit-none -fmax-identifier-length -std=std -fd-lines-as-code -fd-lines-as-comments -ffixed-line-length-n -ffixed-line-length-none -ffree-line-length-n -ffree-line-length-none -fdefault-double-8 -fdefault-integer-8 -fdefault-real-8 -fcray-pointer -fopenmp -fno-range-check -fbackslash -fmodule-private
Preprocessing Options: -cpp -dD -dI -dM -dN -dU -fworking-directory -imultilib dir -iprefix file -isysroot dir -iquote -isystem dir -nocpp -nostdinc -undef -Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -Umacro -H -P
Error and Warning Options: -fmax-errors=n -fsyntax-only -pedantic -pedantic-errors -Wall -Waliasing -Wampersand -Warray-bounds -Wcharacter-truncation -Wconversion -Wimplicit-interface -Wline-truncation -Wintrinsics-std -Wsurprising -Wno-tabs -Wunderflow -Wunused-parameter -Wintrinsics-shadow -Wno-align-commons
Debugging Options: -fdump-parse-tree -ffpe-trap=list -fdump-core -fbacktrace
Directory Options: -Idir -Jdir -Mdir -fintrinsic-modules-path dir
Link Options: -static-libgfortran
Runtime Options: -fconvert=conversion -fno-range-check -frecord-marker=length -fmax-subrecord-length=length -fsign-zero
Code Generation Options: -fno-automatic -ff2c -fno-underscoring -fsecond-underscore -fbounds-check -fcheck-array-temporaries -fmax-array-constructor =n -fmax-stack-var-size=n -fpack-derived -frepack-arrays -fshort-enums -fexternal-blas -fblas-matmul-limit=n -frecursive -finit-local-zero -finit-integer=n -finit-real=<zero|inf|-inf|nan> -finit-logical=<true|false> -finit-character=n -fno-align-commons
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 GNUspecific 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 character in a symbol name.; -fbackslash
Change the interpretation of backslashes in string literals from a single backslash character to "C-style" escape characters. The
following combinations are expanded "\a", "\b", "\f", "\n", "\r",
"\t", "\v", "\\", and "\0" to the ASCII characters alert,
backspace, form feed, newline, carriage return, horizontal tab,
vertical tab, backslash, and NUL, respectively. Additionally,
"\x"nn, "\u"nnnn and "\U"nnnnnnnn (where each n is a hexadecimal digit) are translated into the Unicode characters corresponding to the specified code points. All other combinations of a character
preceded by \ are unexpanded.; -fmodule-private
Set the default accessibility of module entities to "PRIVATE".
Use-associated entities will not be accessible unless they are
explicitly declared as "PUBLIC".; -ffixed-line-length-n
Set column after which characters are ignored in typical fixed-form lines in the source file, and 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.; -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.; -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.
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, be aware that any
restrictions of the file-format, e.g. fixed-form line width, apply for preprocessed output as well.; -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 -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's 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 targetspecific 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 don't 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, -Wsurprising, -Wintrinsics-std, -Wno-tabs, -Wintrinsic-shadow and -Wline-truncation.; -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.
-Wcharacter-truncation: Warn when a character assignment will truncate the assigned string.
-Wline-truncation: Warn when a source code line will be truncated.
-Wconversion: Warn about implicit conversions between different types.
-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.
-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 behaviour and always link to the
intrinsic regardless of the selected standard.
-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:; o An INTEGER SELECT construct has a CASE that can never be
matched as its lower value is greater than its upper value.; o A LOGICAL SELECT construct has three CASE statements.; o A TRANSFER specifies a source that is shorter than the
destination.; o 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.
-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. -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.
-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.
-Wunused-parameter: Contrary to gcc's meaning of -Wunused-parameter, gfortran's implementation of this option does not warn about unused dummy
arguments, 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.
-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-parse-tree
Output the internal parse tree before starting code generation.
Only really useful for debugging the GNU Fortran compiler itself.; -ffpe-trap=list
Specify a list of IEEE exceptions when a Floating Point Exception
(FPE) should be raised. On most systems, this will result in a
SIGFPE signal being sent and the program being interrupted,
producing a core file useful for debugging. list is a (possibly empty) comma-separated list of the following IEEE 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), precision (loss of precision during operation) and denormal (operation produced a denormal value).

Some of the routines in the Fortran runtime library, like CPU_TIME, are likely to trigger floating point exceptions when
"ffpe-trap=precision" is used. For this reason, the use of
"ffpe-trap=precision" is not recommended.; -fbacktrace
Specify that, when a runtime error is encountered or a deadly
signal is emitted (segmentation fault, illegal instruction, bus
error or floating-point exception), the Fortran runtime library
should output a backtrace of the error. This option only has
influence for compilation of the Fortran main program.; -fdump-core
Request that a core-dump file is written to disk when a runtime
error is encountered on systems that support core dumps. This
option is only effective for the 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
-Mdir
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.

-M is deprecated to avoid conflicts with existing GCC options.; -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 --ffccoonnvveerrtt.; -fno-range-check
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.; -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 writing zero values, show the negative sign if the sign bit is set. "fno-sign-zero" does not print the negative sign of zero
values for compatibility with F77. Default behavior is to show the negative sign.
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 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 userdefined 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 userdefined 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.
-fbounds-check: 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 -fbounds-check 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.
fcheck-array-temporaries: 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.
-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.

C<program test>
C<implicit none>
C<integer j>
C<integer, parameter :: n = 100000>
C<integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)>
C<print '(10(I0,1X))', i>
C<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.
-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
-finit-real=<zero|inf|-inf|nan> -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> (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 components of derived type variables, nor do they
initialize variables that appear in an "EQUIVALENCE" statement.
(This limitation may be removed in future releases).; Note that the -finit-real=nan option initializes "REAL" and "COMPLEX" variables with a quiet NaN.
-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 largests to smallest.

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-4.4/README.Bugs>.

AUTHOR

See the Info entry for gfortran for contributors to GCC and GNU Fortran.

COPYRIGHT

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 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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