GFORTRAN(1)                          GNU                         GFORTRAN(1)

NAME         top

       gfortran - GNU Fortran compiler

SYNOPSIS         top

       gfortran [-c|-S|-E]
                [-g] [-pg] [-Olevel]
                [-Wwarn...] [-pedantic]
                [-Idir...] [-Ldir...]
                [-Dmacro[=defn]...] [-Umacro]
                [-o outfile] infile...

       Only the most useful options are listed here; see below for the

DESCRIPTION         top

       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         top

       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 -fdec -fdec-structure -fdec-intrinsic-ints
           -fdec-static -fdec-math -fdefault-double-8 -fdefault-integer-8
           -fdefault-real-8 -fdefault-real-10 -fdefault-real-16 -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
           -ffixed-form -fno-range-check -fopenacc -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 -ftest-forall-temp

       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 -Wargument-mismatch -Warray-bounds
           -Wc-binding-type -Wcharacter-truncation -Wconversion
           -Wdo-subscript -Wfunction-elimination -Wimplicit-interface
           -Wimplicit-procedure -Wintrinsic-shadow -Wuse-without-only
           -Wintrinsics-std -Wline-truncation -Wno-align-commons -Wno-tabs
           -Wreal-q-constant -Wsurprising -Wunderflow -Wunused-parameter
           -Wrealloc-lhs -Wrealloc-lhs-all -Wfrontend-loop-interchange
           -Wtarget-lifetime -fmax-errors=n -fsyntax-only -pedantic

       Debugging Options
           -fbacktrace -fdump-fortran-optimized -fdump-fortran-original
           -fdump-parse-tree -ffpe-trap=list -ffpe-summary=list

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

       Link Options

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

       Interoperability Options

       Code Generation Options
           -faggressive-function-elimination -fblas-matmul-limit=n
           -fbounds-check -fcheck-array-temporaries
           -fcoarray=<none|single|lib> -fexternal-blas -ff2c
           -ffrontend-loop-interchange -ffrontend-optimize
           -finit-character=n -finit-integer=n -finit-local-zero
           -finit-derived -finit-logical=<true|false>
           -finit-real=<zero|inf|-inf|nan|snan> -finline-matmul-limit=n
           -fmax-array-constructor=n -fmax-stack-var-size=n
           -fno-align-commons -fno-automatic -fno-protect-parens
           -fno-underscoring -fsecond-underscore -fpack-derived
           -frealloc-lhs -frecursive -frepack-arrays -fshort-enums

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

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

           This option causes all intrinsic procedures (including the GNU-
           specific extensions) to be accepted.  This can be useful with
           -std=f95 to force standard-compliance but get access to the full
           range of intrinsics available with gfortran.  As a consequence,
           -Wintrinsics-std will be ignored and no user-defined procedure
           with the same name as any intrinsic will be called except when it
           is explicitly declared "EXTERNAL".

           Enable special treatment for lines beginning with "d" or "D" in
           fixed form sources.  If the -fd-lines-as-code option is given
           they are treated as if the first column contained a blank.  If
           the -fd-lines-as-comments option is given, they are treated as
           comment lines.

           DEC compatibility mode. Enables extensions and other features
           that mimic the default behavior of older compilers (such as DEC).
           These features are non-standard and should be avoided at all
           costs.  For details on GNU Fortran's implementation of these
           extensions see the full documentation.

           Other flags enabled by this switch are: -fdollar-ok
           -fcray-pointer -fdec-structure -fdec-intrinsic-ints -fdec-static

           If -fd-lines-as-code/-fd-lines-as-comments are unset, then -fdec
           also sets -fd-lines-as-comments.

           Enable DEC "STRUCTURE" and "RECORD" as well as "UNION", "MAP",
           and dot ('.') as a member separator (in addition to '%'). This is
           provided for compatibility only; Fortran 90 derived types should
           be used instead where possible.

           Enable B/I/J/K kind variants of existing integer functions (e.g.
           BIAND, IIAND, JIAND, etc...). For a complete list of intrinsics
           see the full documentation.

           Enable legacy math intrinsics such as COTAN and degree-valued
           trigonometric functions (e.g. TAND, ATAND, etc...) for
           compatability with older code.

           Enable DEC-style STATIC and AUTOMATIC attributes to explicitly
           specify the storage of variables and other objects.

           Allow $ as a valid non-first character in a symbol name. Symbols
           that start with $ are rejected since it is unclear which rules to
           apply to implicit typing as different vendors implement different
           rules.  Using $ in "IMPLICIT" statements is also rejected.

           Change the interpretation of backslashes in string literals from
           a single backslash character to "C-style" escape characters. The
           following combinations are expanded "\a", "\b", "\f", "\n", "\r",
           "\t", "\v", "\\", and "\0" to the ASCII characters alert,
           backspace, form feed, newline, carriage return, horizontal tab,
           vertical tab, backslash, and NUL, respectively.  Additionally,
           "\x"nn, "\u"nnnn and "\U"nnnnnnnn (where each n is a hexadecimal
           digit) are translated into the Unicode characters corresponding
           to the specified code points. All other combinations of a
           character preceded by \ are unexpanded.

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

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

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

           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

           Specify the maximum allowed identifier length. Typical values are
           31 (Fortran 95) and 63 (Fortran 2003 and Fortran 2008).

           Specify that no implicit typing is allowed, unless overridden by
           explicit "IMPLICIT" statements.  This is the equivalent of adding
           "implicit none" to the start of every procedure.

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

           Enable the OpenACC extensions.  This includes OpenACC "!$acc"
           directives in free form and "c$acc", *$acc and "!$acc" directives
           in fixed form, "!$" conditional compilation sentinels in free
           form and "c$", "*$" and "!$" sentinels in fixed form, and when
           linking arranges for the OpenACC runtime library to be linked in.

           Note that this is an experimental feature, incomplete, and
           subject to change in future versions of GCC.  See
           < > for more information.

           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.

           Set the default integer and logical types to an 8 byte wide type.
           This option also affects the kind of integer constants like 42.
           Unlike -finteger-4-integer-8, it does not promote variables with
           explicit kind declaration.

           Set the default real type to an 8 byte wide type. 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. Unlike
           -freal-4-real-8, it does not promote variables with explicit kind

           Set the default real type to a 10 byte wide type. 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. Unlike
           -freal-4-real-10, it does not promote variables with explicit
           kind declaration.

           Set the default real type to a 16 byte wide type. 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. Unlike
           -freal-4-real-16, it does not promote variables with explicit
           kind declaration.

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

           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

           Promote all "REAL(KIND=M)" entities to "REAL(KIND=N)" entities.
           If "REAL(KIND=N)" is unavailable, then an error will be issued.
           All other real kind types are unaffected by this option.  These
           options should be used with care and may not be suitable for your
           codes.  Areas of possible concern include calls to external
           procedures, alignment in "EQUIVALENCE" and/or "COMMON", generic
           interfaces, BOZ literal constant conversion, and I/O.  Inspection
           of the intermediate representation of the translated Fortran
           code, produced by -fdump-tree-original, is suggested.

           Specify the standard to which the program is expected to conform,
           which may be one of f95, f2003, f2008, f2018, gnu, or legacy.
           The default value for std is gnu, which specifies a superset of
           the latest Fortran 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, f2008, and f2018 values specify strict conformance to
           the Fortran 95, Fortran 2003, Fortran 2008 and Fortran 2018
           standards, respectively; errors are given for all extensions
           beyond the relevant language standard, and warnings are given for
           the Fortran 77 features that are permitted but obsolescent in
           later standards. -std=f2008ts allows the Fortran 2008 standard
           including the additions of the Technical Specification (TS) 29113
           on Further Interoperability of Fortran with C and TS 18508 on
           Additional Parallel Features in Fortran.

           Enhance test coverage by forcing most forall assignments to use

   Enable and customize preprocessing
       Preprocessor related options. See section Preprocessing and
       conditional compilation for more detailed information on
       preprocessing in gfortran.

           Enable preprocessing. The preprocessor is automatically invoked
           if the file extension is .fpp, .FPP,  .F, .FOR, .FTN, .F90, .F95,
           .F03 or .F08. Use this option to manually enable preprocessing of
           any kind of Fortran file.

           To disable preprocessing of files with any of the above listed
           extensions, use the negative form: -nocpp.

           The preprocessor is run in traditional mode. Any restrictions of
           the file-format, especially the limits on line length, apply for
           preprocessed output as well, so it might be advisable to use the
           -ffree-line-length-none or -ffixed-line-length-none 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

           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.

       -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.

           Do not search the standard system directories for header files.
           Only the directories you have specified with -I options (and the
           directory of the current file, if appropriate) are searched.

           Do not predefine any system-specific or GCC-specific macros.  The
           standard predefined macros remain defined.

           Make an assertion with the predicate predicate and answer answer.
           This form is preferred to the older form -A predicate(answer),
           which is still supported, because it does not use shell special

           Cancel an assertion with the predicate predicate and answer

       -C  Do not discard comments. All comments are passed through to the
           output file, except for comments in processed directives, which
           are deleted along with the directive.

           You should be prepared for side effects when using -C; it causes
           the preprocessor to treat comments as tokens in their own right.
           For example, comments appearing at the start of what would be a
           directive line have the effect of turning that line into an
           ordinary source line, since the first token on the line is no
           longer a '#'.

           Warning: this currently handles C-Style comments only. The
           preprocessor does not yet recognize Fortran-style comments.

       -CC Do not discard comments, including during macro expansion. This
           is like -C, except that comments contained within macros are also
           passed through to the output file where the macro is expanded.

           In addition to the side-effects of the -C option, the -CC option
           causes all C++-style comments inside a macro to be converted to
           C-style comments. This is to prevent later use of that macro from
           inadvertently commenting out the remainder of the source line.
           The -CC option is generally used to support lint comments.

           Warning: this currently handles C- and C++-Style comments only.
           The preprocessor does not yet recognize Fortran-style comments.

           Predefine name as a macro, with definition 1.

           The contents of definition are tokenized and processed as if they
           appeared during translation phase three in a '#define' directive.
           In particular, the definition will be truncated by embedded
           newline characters.

           If you are invoking the preprocessor from a shell or shell-like
           program you may need to use the shell's quoting syntax to protect
           characters such as spaces that have a meaning in the shell

           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.

           Cancel any previous definition of name, either built in or
           provided with a -D option.

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

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

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

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

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

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

           Issue warnings for uses of extensions to Fortran.  -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 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,
           -std=f2008 or -std=f2018.

           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, -Wconversion,
           -Wsurprising, -Wc-binding-type, -Wintrinsics-std, -Wtabs,
           -Wintrinsic-shadow, -Wline-truncation, -Wtarget-lifetime,
           -Winteger-division, -Wreal-q-constant, -Wunused and

           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,
           -std=f2003, -std=f2008 and -std=f2018. 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 about type, rank, and other mismatches between formal
           parameters and actual arguments to functions and subroutines.
           These warnings are recommended and thus enabled by default.

           Warn about array temporaries generated by the compiler.  The
           information generated by this warning is sometimes useful in
           optimization, in order to avoid such temporaries.

           Warn 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.

           Warn when a character assignment will truncate the assigned

           Warn when a source code line will be truncated.  This option is
           implied by -Wall.  For free-form source code, the default is
           -Werror=line-truncation such that truncations are reported as

           Warn about implicit conversions that are likely to change the
           value of the expression after conversion. Implied by -Wall.

           Warn about implicit conversions between different types and
           kinds. This option does not imply -Wconversion.

           Enables some warning options for usages of language features
           which may be problematic. This currently includes
           -Wcompare-reals, -Wunused-parameter and -Wdo-subscript.

           Enable warning for loop interchanges performed by the
           -ffrontend-loop-interchange option.

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

           Warn if a procedure is called that has neither an explicit
           interface nor has been declared as "EXTERNAL".

           Warn if a constant integer division truncates it result.  As an
           example, 3/5 evaluates to 0.

           Warn if gfortran finds a procedure named like an intrinsic not
           available in the currently selected standard (with -std) and
           treats it as "EXTERNAL" procedure because of this.
           -fall-intrinsics can be used to never trigger this behavior and
           always link to the intrinsic regardless of the selected standard.

           Produce a warning if a real-literal-constant contains a "q"

           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

           *   The type of a function result is declared more than once with
               the same type.  If -pedantic or standard-conforming mode is
               enabled, this is an error.

           *   A "CHARACTER" variable is declared with negative length.

           By default, tabs are accepted as whitespace, but tabs are not
           members of the Fortran Character Set.  For continuation lines, a
           tab followed by a digit between 1 and 9 is supported.  -Wtabs
           will cause a warning to be issued if a tab is encountered. Note,
           -Wtabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008,
           -std=f2008ts, -std=f2018 and -Wall.

           Warn if a DO loop with step either 1 or -1 yields an underflow or
           an overflow during iteration of an induction variable of the
           loop.  This option is implied by -Wall.

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

           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.

           Warn if a "USE" statement has no "ONLY" qualifier and thus
           implicitly imports all public entities of the used module.

           Warn about unused dummy arguments. This option is implied by

           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 implied by -Wextra if
           also -Wunused or -Wall is used.

           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

           Warn if any calls to functions are eliminated by the
           optimizations enabled by the -ffrontend-optimize option.

           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

           Warn when the compiler inserts code to for allocation or
           reallocation of an allocatable variable; this includes scalars
           and derived types.

           Warn when comparing real or complex types for equality or
           inequality.  This option is implied by -Wextra.

           Warn if the pointer in a pointer assignment might be longer than
           the its target. This option is implied by -Wall.

           Warn if a "DO" loop is known to execute zero times at compile
           time.  This option is implied by -Wall.

           Warn if an array subscript inside a DO loop could lead to an out-
           of-bounds access even if the compiler can not prove that the
           statement is actually executed, in cases like

                     real a(3)
                     do i=1,4
                       if (condition(i)) then
                         a(i) = 1.2
                       end if
                     end do

           This option is implied by -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 after translating the source
           program into internal representation.  This option is mostly
           useful for debugging the GNU Fortran compiler itself. The output
           generated by this option might change between releases. This
           option may also generate internal compiler errors for features
           which have only recently been added.

           Output the parse tree after front-end optimization.  Mostly
           useful for debugging the GNU Fortran compiler itself. The output
           generated by this option might change between releases.  This
           option may also generate internal compiler errors for features
           which have only recently been added.

           Output the internal parse tree after translating the source
           program into internal representation.  Mostly useful for
           debugging the GNU Fortran compiler itself. The output generated
           by this option might change between releases. This option may
           also generate internal compiler errors for features which have
           only recently been added. This option is deprecated; use
           "-fdump-fortran-original" instead.

           Specify a list of floating point exception traps to enable.  On
           most systems, if a floating point exception occurs and the trap
           for that exception is enabled, a SIGFPE signal will be sent and
           the program being aborted, producing a core file useful for
           debugging.  list is a (possibly empty) comma-separated list of
           the following exceptions: invalid (invalid floating point
           operation, such as "SQRT(-1.0)"), zero (division by zero),
           overflow (overflow in a floating point operation), underflow
           (underflow in a floating point operation), inexact (loss of
           precision during operation), and denormal (operation performed on
           a denormal value).  The first five exceptions correspond to the
           five IEEE 754 exceptions, whereas the last one (denormal) is not
           part of the IEEE 754 standard but is available on some common
           architectures such as x86.

           The first three exceptions (invalid, zero, and overflow) often
           indicate serious errors, and unless the program has provisions
           for dealing with these exceptions, enabling traps for these three
           exceptions is probably a good idea.

           Many, if not most, floating point operations incur loss of
           precision due to rounding, and hence the "ffpe-trap=inexact" is
           likely to be uninteresting in practice.

           By default no exception traps are enabled.

           Specify a list of floating-point exceptions, whose flag status is
           printed to "ERROR_UNIT" when invoking "STOP" and "ERROR STOP".
           list can be either none, all or a comma-separated list of the
           following exceptions: invalid, zero, overflow, underflow, inexact
           and denormal. (See -ffpe-trap for a description of the

           By default, a summary for all exceptions but inexact is shown.

           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

       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

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

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

           The default is the current directory.

       -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 Fortran.

           Specify the representation of data for unformatted files.  Valid
           values for conversion are: native, the default; swap, swap
           between big- and little-endian; big-endian, use big-endian
           representation for unformatted files; little-endian, use little-
           endian representation for unformatted files.

           This option has an effect only when used in the main program.
           The "CONVERT" specifier and the GFORTRAN_CONVERT_UNIT environment
           variable override the default specified by -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 -frecord-marker=8.

           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 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

   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 memory.

           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 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 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.

               Disable coarray support; using coarray declarations and
               image-control statements will produce a compile-time error.

               Single-image mode, i.e. "num_images()" is always one.

           lib Library-based coarray parallelization; a suitable GNU Fortran
               coarray library needs to be linked.

           Enable the generation of run-time checks; the argument shall be a
           comma-delimited list of the following keywords.  Prefixing a
           check with no- disables it if it was activated by a previous

           all Enable all run-time test of -fcheck.

               Warns at run time when for passing an actual argument a
               temporary array had to be generated. The information
               generated by this warning is sometimes useful in
               optimization, in order to avoid such temporaries.

               Note: The warning is only printed once per location.

               Enable generation of run-time checks for array subscripts and
               against the declared minimum and maximum values.  It also
               checks array indices for assumed and deferred shape arrays
               against the actual allocated bounds and ensures that all
               string lengths are equal for character array constructors
               without an explicit typespec.

               Some checks require that -fcheck=bounds is set for the
               compilation of the main 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.

               Enable generation of run-time checks for pointers and

               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.

           Example: Assuming you have a file foo.f90, the command

                     gfortran -fcheck=all,no-array-temps foo.f90

           will compile the file with all checks enabled as specified above
           except warnings for generated array temporaries.

           Deprecated alias for -fcheck=bounds.

           Deprecated alias for -fcheck=array-temps.

           This option can be used to increase the upper limit permitted in
           array constructors.  The code below requires this option to
           expand the array at compile time.

                   program test
                   implicit none
                   integer j
                   integer, parameter :: n = 100000
                   integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
                   print '(10(I0,1X))', i
                   end program test

           Caution:  This option can lead to long compile times and
           excessively large object files.

           The default value for n is 65535.

           This option specifies the size in bytes of the largest array that
           will be put on the stack; if the size is exceeded static memory
           is used (except in procedures marked as RECURSIVE). Use the
           option -frecursive to allow for recursive procedures which do not
           have a RECURSIVE attribute or for parallel programs. Use
           -fno-automatic to never use 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.

           Adding this option will make the Fortran compiler put all arrays
           of unknown size and array temporaries 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 unless -fmax-stack-var-size is

           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.

           When front-end optimiztion is active, some calls to the "MATMUL"
           intrinsic function will be inlined.  This may result in code size
           increase if the size of the matrix cannot be determined at
           compile time, as code for both cases is generated.  Setting
           "-finline-matmul-limit=0" will disable inlining in all cases.
           Setting this option with a value of n will produce inline code
           for matrices with size up to n. 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.  The "-fblas-matmul-limit" can be
           used to change this value.

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

           The -finit-local-zero option instructs the compiler to initialize
           local "INTEGER", "REAL", and "COMPLEX" variables to zero,
           "LOGICAL" variables to false, and "CHARACTER" variables to a
           string of null bytes.  Finer-grained initialization options are
           provided by the -finit-integer=n,
           -finit-real=<zero|inf|-inf|nan|snan> (which also initializes the
           real and imaginary parts of local "COMPLEX" variables),
           -finit-logical=<true|false>, and -finit-character=n (where n is
           an ASCII character value) options.

           With -finit-derived, components of derived type variables will be
           initialized according to these flags.  Components whose type is
           not covered by an explicit -finit-* flag will be treated as
           described above with -finit-local-zero.

           These options do not initialize

           *   objects with the POINTER attribute

           *   allocatable arrays

           *   variables that appear in an "EQUIVALENCE" statement.

           (These limitations may be removed in future releases).

           Note that the -finit-real=nan option initializes "REAL" and
           "COMPLEX" variables with a quiet NaN. For a signalling NaN use
           -finit-real=snan; note, however, that compile-time optimizations
           may convert them into quiet NaN and that trapping needs to be
           enabled (e.g. via -ffpe-trap).

           Finally, note that enabling any of the -finit-* options will
           silence warnings that would have been emitted by -Wuninitialized
           for the affected local variables.

           By default, gfortran enforces proper alignment of all variables
           in a "COMMON" block by padding them as needed. On certain
           platforms this is mandatory, on others it increases performance.
           If a "COMMON" block is not declared with consistent data types
           everywhere, this padding can cause trouble, and
           -fno-align-commons can be used to disable automatic alignment.
           The same form of this option should be used for all files that
           share a "COMMON" block.  To avoid potential alignment issues in
           "COMMON" blocks, it is recommended to order objects from largest
           to smallest.

           By default the parentheses in expression are honored for all
           optimization levels such that the compiler does not do any re-
           association. Using -fno-protect-parens allows the compiler to
           reorder "REAL" and "COMPLEX" expressions to produce faster code.
           Note that for the re-association optimization -fno-signed-zeros
           and -fno-trapping-math need to be in effect. The parentheses
           protection is enabled by default, unless -Ofast is given.

           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.

           Functions with identical argument lists are eliminated within
           statements, regardless of whether these functions are marked
           "PURE" or not. For example, in

                     a = f(b,c) + f(b,c)

           there will only be a single call to "f".  This option only works
           if -ffrontend-optimize is in effect.

           This option performs front-end optimization, based on
           manipulating parts the Fortran parse tree.  Enabled by default by
           any -O option.  Optimizations enabled by this option include
           inlining calls to "MATMUL", 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

           Attempt to interchange loops in the Fortran front end where
           profitable.  Enabled by default by any -O option.  At the moment,
           this option only affects "FORALL" and "DO CONCURRENT" statements
           with several forall triplets.

ENVIRONMENT         top

       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         top

       For instructions on reporting bugs, see < >.

SEE ALSO         top

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

AUTHOR         top

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

COPYRIGHT         top

       Copyright (c) 2004-2018 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 1.3 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

       (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.

COLOPHON         top

       This page is part of the gcc (GNU Compiler Collection) project.
       Information about the project can be found at ⟨⟩.
       If you have a bug report for this manual page, see
       ⟨⟩.  This page was obtained from the tarball
       gcc-8.2.0.tar.gz fetched from
       ⟨⟩ on 2018-10-29.
       If you discover any rendering problems in this HTML version of the
       page, or you believe there is a better or more up-to-date source for
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       send a mail to

gcc-8.2.0                        2018-07-26                      GFORTRAN(1)