gfortran - GNU Fortran compiler


   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


   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 -fbackslash -fcray-pointer -fd-lines-as-code
       -fd-lines-as-comments -fdec -fdec-structure -fdefault-double-8
       -fdefault-integer-8 -fdefault-real-8 -fdollar-ok
       -ffixed-line-length-n -ffixed-line-length-none -ffree-form
       -ffree-line-length-n -ffree-line-length-none -fimplicit-none
       -finteger-4-integer-8 -fmax-identifier-length -fmodule-private
       -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

   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

   Error and Warning Options
       -Waliasing -Wall -Wampersand -Warray-bounds -Wc-binding-type
       -Wcharacter-truncation -Wconversion -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
       -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

   Code Generation Options
       -faggressive-function-elimination -fblas-matmul-limit=n
       -fbounds-check -fcheck-array-temporaries
       -fcoarray=<none|single|lib> -fexternal-blas -ff2c
       -ffrontend-optimize -finit-character=n -finit-integer=n
       -finit-local-zero -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 -fstack-arrays

   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

       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

       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.

       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 "
", "	", "\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 -ffree-line-length-none.

       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 "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

       Promote all "INTEGER(KIND=4)" entities to an "INTEGER(KIND=8)"
       entities.  If "KIND=8" is unavailable, then an error will be
       issued.  This option should be used with care and may not be
       suitable for your codes.  Areas of possible concern include calls
       to external procedures, alignment in "EQUIVALENCE" and/or "COMMON",
       generic interfaces, BOZ literal constant conversion, and I/O.
       Inspection of the intermediate representation of the translated
       Fortran code, produced by -fdump-tree-original, is suggested.

       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, gnu, or legacy.  The default
       value for std is gnu, which specifies a superset of the Fortran 95
       standard that includes all of the extensions supported by GNU
       Fortran, although warnings will be given for obsolete extensions
       not recommended for use in new code.  The legacy value is
       equivalent but without the warnings for obsolete extensions, and
       may be useful for old non-standard programs.  The f95, f2003 and
       f2008 values specify strict conformance to the Fortran 95, Fortran
       2003 and Fortran 2008 standards, respectively; errors are given for
       all extensions beyond the relevant language standard, and warnings
       are given for the Fortran 77 features that are permitted but
       obsolescent in later standards. -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.

   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

   -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

   -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

       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 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

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

       If you wish to define a function-like macro on the command line,
       write its argument list with surrounding parentheses before the
       equals sign (if any). Parentheses are meaningful to most shells, so
       you will need to quote the option. With sh and csh,
       "-D'name(args...)=definition'" works.

       -D and -U options are processed in the order they are given on the
       command line. All -imacros file and -include file options are
       processed after all -D and -U options.

   -H  Print the name of each header file used, in addition to other
       normal activities. Each name is indented to show how deep in the
       '#include' stack it is.

   -P  Inhibit generation of linemarkers in the output from the
       preprocessor.  This might be useful when running the preprocessor
       on something that is not C code, and will be sent to a program
       which might be confused by the linemarkers.

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

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

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

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

       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 and -Wunused.

       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
       and -std=f2008. Note: With no ampersand given in a continued
       character constant, GNU Fortran assumes continuation at the first
       non-comment, non-whitespace character after the ampersand that
       initiated the continuation.

       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 string.

       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 and

       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 and -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 -Wall.

       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 -falign-commons.

       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 -frealloc-lhs.

       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.

       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.  Only really useful for debugging the
       GNU Fortran compiler itself.

       Output the parse tree after front-end optimization.  Only really
       useful for debugging the GNU Fortran compiler itself.

       Output the internal parse tree after translating the source program
       into internal representation.  Only really useful for debugging the
       GNU Fortran compiler itself.  This option is deprecated; use
       "-fdump-fortran-original" instead.

       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 exceptions.)

       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 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.

   -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 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

       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

       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

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

           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 local
       arrays, even those of unknown size onto stack memory.  If your
       program uses very large local arrays it is possible that you will
       have to extend your runtime limits for stack memory on some
       operating systems. This flag is enabled by default at optimization
       level -Ofast.

       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 the value specified for
       "-fblas-matmul-limit" if this option is specified, or unlimitited

       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.  These options do
       not initialize

       *   allocatable arrays

       *   components of derived type variables

       *   variables that appear in an "EQUIVALENCE" statement.

       (These limitations may be removed in future releases).

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

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

       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 -fno-frontend-optimize.


   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


   Copyright (c) 2004-2016 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 page.

   (a) The FSF's Front-Cover Text is:

        A GNU Manual

   (b) The FSF's Back-Cover Text is:

        You have freedom to copy and modify this GNU Manual, like GNU
        software.  Copies published by the Free Software Foundation raise
        funds for GNU development.


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