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       gfortran - GNU Fortran compiler


       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


       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.


       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

       Error and Warning Options
           -fmax-errors=n -fsyntax-only  -pedantic  -pedantic-errors -Wall
           -Waliasing  -Wampersand  -Wcharacter-truncation  -Wconversion
           -Wimplicit-interface  -Wline-truncation  -Wnonstd-intrinsics
           -Wsurprising -Wno-tabs  -Wunderflow -Wunused-parameter

       Debugging Options
           -fdump-parse-tree  -ffpe-trap=list -fdump-core -fbacktrace

       Directory Options
           -Idir  -Jdir  -Mdir  -fintrinsic-modules-path dir

       Link Options

       Runtime Options
           -fconvert=conversion  -frecord-marker=length
           -fmax-subrecord-length=length  -fsign-zero

       Code Generation Options
           -fno-automatic  -ff2c  -fno-underscoring -fsecond-underscore
           -fbounds-check  -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

   Options controlling Fortran dialect
       The following options control the details of the Fortran dialect
       accepted by the compiler:

           Specify the layout used by the source file.  The free form layout
           was introduced in Fortran 90.  Fixed form was traditionally used in
           older Fortran programs.  When neither option is specified, the
           source form is determined by the file extension.

           Accept all of the intrinsic procedures provided in libgfortran
           without regard to the setting of -std.  In particular, this option
           can be quite useful with -std=f95.  Additionally, gfortran will
           ignore -Wnonstd-intrinsics.

           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

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

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

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

           Allow $ as a valid character in a symbol name.

           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.  All other combinations of a character
           preceded by \ are unexpanded.

           Set the default accessibility of module entities to "PRIVATE".
           Use-associated entities will not be accessible unless they are
           explicitly declared as "PUBLIC".

           Set column after which characters are ignored in typical fixed-form
           lines in the source file, and through which spaces are assumed (as
           if padded to that length) after the ends of short fixed-form lines.

           Popular values for n include 72 (the standard and the default), 80
           (card image), and 132 (corresponding to "extended-source" options
           in some popular compilers).  n may also be none, meaning that the
           entire line is meaningful and that continued character constants
           never have implicit spaces appended to them to fill out the line.
           -ffixed-line-length-0 means the same thing as

           Set column after which characters are ignored in typical free-form
           lines in the source file. The default value is 132.  n may be none,
           meaning that the entire line is meaningful.  -ffree-line-length-0
           means the same thing as -ffree-line-length-none.

           Specify the maximum allowed identifier length. Typical values are
           31 (Fortran 95) and 63 (Fortran 2003).

           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.

           Enable the Cray pointer extension, which provides C-like pointer

           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.

           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.

           Specify the standard to which the program is expected to conform,
           which may be one of f95, f2003, 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 and f2003
           values specify strict conformance to the Fortran 95 and Fortran
           2003 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

   Options to request or suppress errors and warnings
       Errors are diagnostic messages that report that the GNU Fortran
       compiler cannot compile the relevant piece of source code.  The
       compiler will continue to process the program in an attempt to report
       further errors to aid in debugging, but will not produce any compiled

       Warnings are diagnostic messages that report constructions which are
       not inherently erroneous but which are risky or suggest there is likely
       to be a bug in the program.  Unless -Werror is specified, they do not
       prevent compilation of the program.

       You can request many specific warnings with options beginning -W, for
       example -Wimplicit to request warnings on implicit declarations.  Each
       of these specific warning options also has a negative form beginning
       -Wno- to turn off warnings; for example, -Wno-implicit.  This manual
       lists only one of the two forms, whichever is not the default.

       These options control the amount and kinds of errors and warnings
       produced by GNU Fortran:

           Limits the maximum number of error messages to n, at which point
           GNU Fortran bails out rather than attempting to continue processing
           the source code.  If n is 0, there is no limit on the number of
           error messages produced.

           Check the code for syntax errors, but don’t actually compile it.
           This will generate module files for each module present in the
           code, but no other output file.

           Issue warnings for uses of extensions to Fortran 95.  -pedantic
           also applies to C-language constructs where they occur in GNU
           Fortran source files, such as use of \e in a character constant
           within a directive like "#include".

           Valid Fortran 95 programs should compile properly with or without
           this option.  However, without this option, certain GNU extensions
           and traditional Fortran features are supported as well.  With this
           option, many of them are rejected.

           Some users try to use -pedantic to check programs for conformance.
           They soon find that it does not do quite what they want---it finds
           some nonstandard practices, but not all.  However, improvements to
           GNU Fortran in this area are welcome.

           This should be used in conjunction with -std=f95 or -std=f2003.

           Like -pedantic, except that errors are produced rather than

           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,
           -Wnonstd-intrinsics, -Wno-tabs, and -Wline-truncation.

           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.

                       subroutine bar(a,b)
                         integer, intent(in) :: a
                         integer, intent(out) :: b
                       end subroutine
                     end interface
                     integer :: a

                     call bar(a,a)

           Warn about missing ampersand in continued character constants. The
           warning is given with -Wampersand, -pedantic, -std=f95, and
           -std=f2003. 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.

           Warn when a character assignment will truncate the assigned string.

           Warn about implicit conversions between different types.

           Warn if a procedure is called without an explicit interface.  Note
           this only checks that an explicit interface is present.  It does
           not check that the declared interfaces are consistent across
           program units.

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

           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

           ·   An INTEGER SELECT construct has a CASE that can never be
               matched as its lower value is greater than its upper value.

           ·   A LOGICAL SELECT construct has three CASE statements.

           ·   A TRANSFER specifies a source that is shorter than the

           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, and -Wall.

           Produce a warning when numerical constant expressions are
           encountered, which yield an UNDERFLOW during compilation.

           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.

           Turns all warnings into errors.

       Some of these have no effect when compiling programs written in

   Options for debugging your program or GNU Fortran
       GNU Fortran has various special options that are used for debugging
       either your program or the GNU Fortran compiler.

           Output the internal parse tree before starting code generation.
           Only really useful for debugging the GNU Fortran compiler itself.

           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 to trigger floating point exceptions when
           "ffpe-trap=precision" is used. For this reason, the use of
           "ffpe-trap=precision" is not recommended.

           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.

           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

   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

       It also affects the search paths used by cpp when used to preprocess
       Fortran source.

           These affect interpretation of the "INCLUDE" directive (as well as
           of the "#include" directive of the cpp preprocessor).

           Also note that the general behavior of -I and "INCLUDE" is pretty
           much the same as of -I with "#include" in the cpp preprocessor,
           with regard to looking for header.gcc files and other such things.

           This path is also used to search for .mod files when previously
           compiled modules are required by a "USE" statement.

           This option specifies where to put .mod files for compiled modules.
           It is also added to the list of directories to searched by an "USE"

           The default is the current directory.

           -J is an alias for -M 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

   Influencing the linking step
       These options come into play when the compiler links object files into
       an executable output file. They are meaningless if the compiler is not
       doing a link step.

           On systems that provide libgfortran as a shared and a static
           library, this option forces the use of the static version. If no
           shared version of libgfortran was built when the compiler was
           configured, this option has no effect.

   Influencing runtime behavior
       These options affect the runtime behavior of programs compiled with GNU

           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.

           Specify the length of record markers for unformatted files.  Valid
           values for length are 4 and 8.  Default is 4.  This is different
           from previous versions of gfortran, which specified a default
           record marker length of 8 on most systems.  If you want to read or
           write files compatible with earlier versions of gfortran, use

           Specify the maximum length for a subrecord.  The maximum permitted
           value for length is 2147483639, which is also the default.  Only
           really useful for use by the gfortran testsuite.

           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.

           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

           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.

           Do not transform names of entities specified in the Fortran source
           file by appending underscores to them.

           With -funderscoring in effect, GNU Fortran appends one underscore
           to external names with no underscores.  This is done to ensure
           compatibility with code produced by many UNIX Fortran compilers.

           Caution: The default behavior of GNU Fortran is incompatible with
           f2c and g77, please use the -ff2c option if you want object files
           compiled with GNU Fortran to be compatible with object code created
           with these tools.

           Use of -fno-underscoring is not recommended unless you are
           experimenting with issues such as integration of GNU Fortran into
           existing system environments (vis-@‘{a}-vis existing libraries,
           tools, and so on).

           For example, with -funderscoring, and assuming other defaults like
           -fcase-lower and that "j()" and "max_count()" are external
           functions while "my_var" and "lvar" are local variables, a
           statement like

                   I = J() + MAX_COUNT (MY_VAR, LVAR)

           is implemented as something akin to:

                   i = j_() + max_count__(&my_var__, &lvar);

           With -fno-underscoring, the same statement is implemented as:

                   i = j() + max_count(&my_var, &lvar);

           Use of -fno-underscoring allows direct specification of user-
           defined names while debugging and when interfacing GNU Fortran code
           with other languages.

           Note that just because the names match does not mean that the
           interface implemented by GNU Fortran for an external name matches
           the interface implemented by some other language for that same
           name.  That is, getting code produced by GNU Fortran to link to
           code produced by some other compiler using this or any other method
           can be only a small part of the overall solution---getting the code
           generated by both compilers to agree on issues other than naming
           can require significant effort, and, unlike naming disagreements,
           linkers normally cannot detect disagreements in these other areas.

           Also, note that with -fno-underscoring, the lack of appended
           underscores introduces the very real possibility that a user-
           defined external name will conflict with a name in a system
           library, which could make finding unresolved-reference bugs quite
           difficult in some cases---they might occur at program run time, and
           show up only as buggy behavior at run time.

           In future versions of GNU Fortran we hope to improve naming and
           linking issues so that debugging always involves using the names as
           they appear in the source, even if the names as seen by the linker
           are mangled to prevent accidental linking between procedures with
           incompatible interfaces.

           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

           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

           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.

           Some checks require that -fbounds-check is set for the compilation
           of the main program.

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

           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.

           This option tells GNU Fortran to pack derived type members as
           closely as possible.  Code compiled with this option is likely to
           be incompatible with code compiled without this option, and may
           execute slower.

           In some circumstances GNU Fortran may pass assumed shape array
           sections via a descriptor describing a noncontiguous area of
           memory.  This option adds code to the function prologue to repack
           the data into a contiguous block at runtime.

           This should result in faster accesses to the array.  However it can
           introduce significant overhead to the function call, especially
           when the passed data is noncontiguous.

           This option is provided for interoperability with C code that was
           compiled with the -fshort-enums option.  It will make GNU Fortran
           choose the smallest "INTEGER" kind a given enumerator set will fit
           in, and give all its enumerators this kind.

           This option will make gfortran generate calls to BLAS functions for
           some matrix operations like "MATMUL", instead of using our own
           algorithms, if the size of the matrices involved is larger than a
           given limit (see -fblas-matmul-limit).  This may be profitable if
           an optimized vendor BLAS library is available.  The BLAS library
           will have to be specified at link time.

           Only significant when -fexternal-blas is in effect.  Matrix
           multiplication of matrices with size larger than (or equal to) n
           will be performed by calls to BLAS functions, while others will be
           handled by gfortran internal algorithms. If the matrices involved
           are not square, the size comparison is performed using the
           geometric mean of the dimensions of the argument and result

           The default value for n is 30.

           Allow indirect recursion by forcing all local arrays to be
           allocated on the stack. This flag cannot be used together with
           -fmax-stack-var-size= or -fno-automatic.

           The -finit-local-zero option instructs the compiler to initialize
           local "INTEGER", "REAL", and "COMPLEX" variables to zero, "LOGICAL"
           variables to false, and "CHARACTER" variables to a string of null
           bytes.  Finer-grained initialization options are provided by the
           -finit-integer=n, -finit-real=<zero|inf|-inf|nan> (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.


       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.


       For instructions on reporting bugs, see


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


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


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