gfortran



GFORTRAN(1)                           GNU                          GFORTRAN(1)




NAME

       gfortran - GNU Fortran 95 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 remain-
       der.


DESCRIPTION

       The gfortran command supports all the options supported by the gcc com-
       mand.  Only options specific to gfortran are documented here.

       Gfortran is not yet a fully conformant Fortran 95 compiler.  It can
       generate code for most constructs and expressions, but work remains to
       be done.  In particular, there are known deficiencies with ENTRY,
       NAMELIST, and sophisticated use of MODULES, POINTERS and DERIVED TYPES.
       For those whose Fortran codes conform to either the Fortran 77 standard
       or the GNU Fortran 77 language, we recommend to use g77 from GCC 3.4.
       We recommend that distributors continue to provide packages of g77-3.4
       until we announce that gfortran fully replaces g77.  The gfortran
       developers welcome any feedback on user experience with gfortran at
       <fortran@gcc.gnu.org>.

       All gcc and gfortran 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 gfortran to the gcc distribution enables acceptance of
       gfortran 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
           -ffree-form  -fno-fixed-form -fdollar-ok  -fimplicit-none
           -fmax-identifier-length -std=std -ffixed-line-length-n
           -ffixed-line-length-none -fdefault-double-8  -fdefault-integer-8
           -fdefault-real-8

       Warning Options
           -fsyntax-only  -pedantic  -pedantic-errors -w  -Wall  -Waliasing
           -Wconversion -Wimplicit-interface  -Wnonstd-intrinsics  -Wsurpris-
           ing  -Wunderflow -Wunused-labels -Wline-truncation -W

       Debugging Options
           -fdump-parse-tree

       Directory Options
           -Idir  -Mdir

       Code Generation Options
           -fno-automatic -ff2c -fno-underscoring  -fsecond-underscore
           -fbounds-check  -fmax-stack-var-size=n -fpackderived
           -frepack-arrays

       Options Controlling Fortran Dialect

       The following options control the dialect of Fortran that the compiler
       accepts:

       -ffree-form
       -ffixed-form
           Specify the layout used by the the source file. The free form lay-
           out was introduced in Fortran 90.  Fixed form was traditionally
           used in older Fortran programs.

       -fdefault-double-8
           Set the "DOUBLE PRECISION" type to an 8 byte wide.

       -fdefault-integer-8
           Set the default integer and logical types to an 8 byte wide type.
           Do nothing if this is already the default.

       -fdefault-real-8
           Set the default real type to an 8 byte wide type.  Do nothing if
           this is already the default.

       -fdollar-ok
           Allow $ as a valid character in a symbol name.

       -fno-backslash
       B   Compile switch to change the interpretation of a backslash from
           ‘‘C’’-style escape characters to a single backslash character.

       "-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 (corresponds to ‘‘extended-source’’ options
           in some popular compilers).  n may 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.

       "-fmax-identifier-length=n"
           Specify the maximum allowed identifier length. Typical values are
           31 (Fortran 95) and 63 (Fortran 200x).

       "-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.

       "-std=std"
           Conform to the specified standard.  Allowed values for std are gnu,
           f95, f2003 and legacy.

       Options to Request or Suppress Warnings

       Warnings are diagnostic messages that report constructions which are
       not inherently erroneous but which are risky or suggest there might
       have been an error.

       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 warnings produced by GNU
       Fortran:

       -fsyntax-only
           Check the code for syntax errors, but don’t do anything beyond
           that.

       -pedantic
           Issue warnings for uses of extensions to FORTRAN 95.  -pedantic
           also applies to C-language constructs where they occur in GNU For-
           tran 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
           gfortran in this area are welcome.

           This should be used in conjunction with -std=std.

       -pedantic-errors
           Like -pedantic, except that errors are produced rather than warn-
           ings.

       -w  Inhibit all warning messages.

       -Wall
           Enables commonly used warning options that which pertain to usage
           that we recommend avoiding and that we believe is easy to avoid.
           This currently includes -Wunused-labels, -Waliasing, -Wsurprising,
           -Wnonstd-intrinsic and -Wline-truncation.

       -Waliasing
           Warn about possible aliasing of dummy arguments. The following
           example will trigger the warning as it would be illegal to "bar" to
           modify either parameter.

                     INTEGER A
                     CALL BAR(A,A)

       -Wconversion
           Warn about implicit conversions between different types.

       -Wimplicit-interface
           Warn about when procedure are 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.

       -Wnonstd-intrinsic
           Warn if the user tries to use an intrinsic that does not belong to
           the standard the user has chosen via the -std option.

       -Wsurprising
           Produce a warning when ‘‘suspicious’’ code constructs are encoun-
           tered.  While technically legal these usually indicate that an
           error has been made.

           This currently produces a warning under the following circum-
           stances:

           *   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.

       -Wunderflow
           Produce a warning when numerical constant expressions are encoun-
           tered, which yield an UNDERFLOW during compilation.

       -Wunused-labels
           Warn whenever a label is defined but never referenced.

       -Werror
           Turns all warnings into errors.

       -W  Turns on ‘‘extra warnings’’ and, if optimization is specified via
           -O, the -Wuninitialized option.  (This might change in future ver-
           sions of gfortran

       Some of these have no effect when compiling programs written in For-
       tran.

       Options for Debugging Your Program or GNU Fortran

       GNU Fortran has various special options that are used for debugging
       either your program or gfortran

       -fdump-parse-tree
           Output the internal parse tree before starting code generation.
           Only really useful for debugging gfortran itself.

       Options for Directory Search

       There options affect how affect how gfortran searches for files speci-
       fied via 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.

       -Mdir
       -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.

           -J is an alias for -M to avoid conflicts with existing GCC options.

       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 as if the "SAVE" statement was 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.)

       -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 "COM-
           PLEX" 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-under-
           score 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 con-
           ventions as, calling "COMPLEX" or default "REAL" functions between
           program parts which were compiled with different calling conven-
           tions 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, gfortran 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 gfortran is incompatible with f2c
           and g77, please use the -ff2c option if you want object files com-
           piled with gfortran to be compatible with object code created with
           these tools.

           Use of -fno-underscoring is not recommended unless you are experi-
           menting 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 mmaaxx_ccoouunntt(()) 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 gfortran code
           with other languages.

           Note that just because the names match does not mean that the
           interface implemented by gfortran for an external name matches the
           interface implemented by some other language for that same name.
           That is, getting code produced by gfortran 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 nor-
           mally cannot detect disagreements in these other areas.

           Also, note that with -fno-underscoring, the lack of appended under-
           scores 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 gfortran we hope to improve naming and link-
           ing 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, gfortran appends an underscore to external names.  If
           this option is used gfortran appends two underscores to names with
           underscores and one underscore to external names with no under-
           scores.  (gfortran 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 compati-
           bility 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.

           In the future this may also include other forms of checking, eg.
           checking substring references.

       -fmax-stack-var-size=n
           This option specifies the size in bytes of the largest array that
           will be put on the stack.

           This option currently only affects local arrays declared with con-
           stant bounds, and may not apply to all character variables.  Future
           versions of gfortran may improve this behavior.

           The default value for n is 32768.

       -fpackderived
           This option tells gfortran to pack derived type members as closely
           as possible.  Code compiled with this option is likely to be incom-
           patible with code compiled without this option, and may execute
           slower.

       -frepack-arrays
           In some circumstances gfortran may pass assumed shape array sec-
           tions via a descriptor describing a discontiguous 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 discontiguous.


ENVIRONMENT

       GNU Fortran 95 currently does not make use of any environment variables
       to control its operation above and beyond those that affect the opera-
       tion of gcc.


BUGS

       For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>.


SEE ALSO

       gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), gcc(1), as(1), ld(1),
       gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, gfor-
       tran, as, ld, binutils and gdb.


AUTHOR

       See the Info entry for gfortran for contributors to GCC and GFORTRAN.


COPYRIGHT

       Copyright (c) 2004 Free Software Foundation, Inc.

       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2 or
       any later version published by the Free Software Foundation; with the
       Invariant Sections being ‘‘GNU General Public License’’ and ‘‘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-4.0.2                         2005-11-25                       GFORTRAN(1)

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