GNU Fortran Command Options
***************************

The gfortran command supports all the options supported by the gcc command.  Only options specific to GNU Fortran are documented here.

See GCC Command Options (Using the GNU Compiler Collection (GCC)), for information on the non-Fortran-specific aspects of the gcc command (and, therefore, the gfortran command).

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.

Option summary
==============

Here is a summary of all the options specific to GNU Fortran, grouped by type.  Explanations are in the following sections.

Fortran Language Options
     See Options controlling Fortran dialect. -fall-intrinsics  -ffree-form  -fno-fixed-form -fdollar-ok  -fimplicit-none  -fmax-identifier-length -std=STD -fd-lines-as-code  -fd-lines-as-comments -ffixed-line-length-N  -ffixed-line-length-none -ffree-line-length-N  -ffree-line-length-none -fdefault-double-8  -fdefault-integer-8  -fdefault-real-8 -fcray-pointer  -fopenmp  -fno-range-check -fbackslash -fmodule-private

Preprocessing Options
     See Enable and customize preprocessing. -cpp -dD -dI -dM -dN -dU -fworking-directory -imultilib DIR -iprefix FILE -isysroot DIR -iquote -isystem DIR -nocpp -nostdinc -undef -AQUESTION=ANSWER -A-QUESTION[=ANSWER] -C -CC -DMACRO[=DEFN] -UMACRO -H -P

Error and Warning Options
     See Options to request or suppress errors and warnings. -fmax-errors=N -fsyntax-only -pedantic -pedantic-errors -Wall -Waliasing -Wampersand -Warray-bounds -Wcharacter-truncation -Wconversion -Wimplicit-interface -Wimplicit-procedure -Wline-truncation -Wintrinsics-std -Wreal-q-constant -Wsurprising -Wno-tabs -Wunderflow -Wunused-parameter -Wintrinsic-shadow -Wno-align-commons -Wfunction-elimination

Debugging Options
     See Options for debugging your program or GNU Fortran. -fdump-fortran-original  -fdump-fortran-optimized -ffpe-trap=LIST -fbacktrace -fdump-parse-tree

Directory Options
     See Options for directory search. -IDIR  -JDIR -fintrinsic-modules-path DIR

Link Options
     See Options for influencing the linking step. -static-libgfortran

Runtime Options
     See Options for influencing runtime behavior. -fconvert=CONVERSION  -fno-range-check -frecord-marker=LENGTH -fmax-subrecord-length=LENGTH -fsign-zero

Code Generation Options
     See Options for code generation conventions. -fno-automatic  -ff2c  -fno-underscoring -fwhole-file -fsecond-underscore -fbounds-check -fcheck-array-temporaries  -fmax-array-constructor =N -fcheck=<ALL|ARRAY-TEMPS|BOUNDS|DO|MEM|POINTER|RECURSION> -fcoarray=<NONE|SINGLE|LIB> -fmax-stack-var-size=N -fstack-arrays -fpack-derived  -frepack-arrays  -fshort-enums  -fexternal-blas -fblas-matmul-limit=N -frecursive -finit-local-zero -finit-integer=N -finit-real=<ZERO|INF|-INF|NAN|SNAN> -finit-logical=<TRUE|FALSE> -finit-character=N -fno-align-commons -fno-protect-parens -frealloc-lhs -faggressive-function-elimination -ffrontend-optimize

Options controlling Fortran dialect
===================================

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

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

-fall-intrinsics
     This option causes all intrinsic procedures (including the 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.

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

-fdefault-double-8
     Set the DOUBLE PRECISION type to an 8 byte wide type.  If -fdefault-real-8 is given, DOUBLE PRECISION would instead be promoted to 16 bytes if possible, and -fdefault-double-8 can be used to prevent this.  The kind of real constants like 1.d0 will not be changed by -fdefault-real-8 though, so also -fdefault-double-8 does not affect it.

-fdefault-integer-8
     Set the default integer and logical types to an 8 byte wide type. Do nothing if this is already the default.  This option also affects the kind of integer constants like 42.

-fdefault-real-8
     Set the default real type to an 8 byte wide type. Do nothing if this is already the default.  This option also affects the kind of non-double real constants like 1.0, and does promote the default width of DOUBLE PRECISION to 16 bytes if possible, unless -fdefault-double-8 is given, too.

-fdollar-ok
     Allow $ as a valid 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.

-fbackslash
     Change the interpretation of backslashes in string literals from a single backslash character to ``C-style'' escape characters. The following combinations are expanded \a, \b, \f, \n, \r, \t, \v, \\, and \0 to the ASCII characters alert, backspace, form feed, newline, carriage return, horizontal tab, vertical tab, backslash, and NUL, respectively. Additionally, \xNN, \uNNNN and \UNNNNNNNN (where each N is a hexadecimal digit) are translated into the Unicode characters corresponding to the specified code points. All other combinations of a character preceded by \ are unexpanded.

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

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

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

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

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

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

-fcray-pointer
     Enable the Cray pointer extension, which provides C-like pointer functionality.

-fopenmp
     Enable the OpenMP extensions.  This includes OpenMP !$omp directives in free form and c$omp, *$omp and !$omp directives in fixed form, !$ conditional compilation sentinels in free form and c$, *$ and !$ sentinels in fixed form, and when linking arranges for the OpenMP runtime library to be linked in.  The option -fopenmp implies -frecursive.

-fno-range-check
     Disable range checking on results of simplification of constant expressions during compilation.  For example, GNU Fortran will give an error at compile time when simplifying a = 1. / 0. With this option, no error will be given and a will be assigned the value +Infinity.  If an expression evaluates to a value outside of the relevant range of [-HUGE():HUGE()], then the expression will be replaced by -Inf or +Inf as appropriate. Similarly, DATA i/Z'FFFFFFFF'/ will result in an integer overflow on most systems, but with -fno-range-check the value will ``wrap around'' and i will be initialized to -1 instead.

-std=STD
     Specify the standard to which the program is expected to conform, which may be one of f95, f2003, f2008, gnu, or legacy.  The default value for STD is gnu, which specifies a superset of the Fortran 95 standard that includes all of the extensions supported by GNU Fortran, although warnings will be given for obsolete extensions not recommended for use in new code.  The legacy value is equivalent but without the warnings for obsolete extensions, and may be useful for old non-standard programs.  The f95, f2003 and f2008 values specify strict conformance to the Fortran 95, Fortran 2003 and Fortran 2008 standards, respectively; errors are given for all extensions beyond the relevant language standard, and warnings are given for the Fortran 77 features that are permitted but obsolescent in later standards. -std=f2008tr allows the Fortran 2008 standard including the additions of the technical report (TR) 29113.

Enable and customize preprocessing
==================================

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

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

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

     The preprocessor is run in traditional mode. 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 preprocessing.

-fworking-directory
     Enable generation of linemarkers in the preprocessor output that will let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC will use this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever.

-idirafter DIR
     Search DIR for include files, but do it after all directories specified with -I and the standard system directories have been exhausted. DIR is treated as a system include directory. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot.

-imultilib DIR
     Use DIR as a subdirectory of the directory containing 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.

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

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

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

-A-PREDICATE=ANSWER
     Cancel an assertion with the predicate PREDICATE and answer ANSWER.

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

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

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

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

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

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

-DNAME
     Predefine name as a macro, with definition 1.

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

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

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

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

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

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

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

Options to request or suppress errors and warnings
==================================================

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

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

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

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

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

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

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

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

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

     This should be used in conjunction with -std=f95, -std=f2003 or -std=f2008.

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

-Wall
     Enables commonly used warning options pertaining to usage that we recommend avoiding and that we believe are easy to avoid. This currently includes -Waliasing, -Wampersand, -Wconversion, -Wsurprising, -Wintrinsics-std, -Wno-tabs, -Wintrinsic-shadow, -Wline-truncation, -Wreal-q-constant and -Wunused.

-Waliasing
     Warn about possible aliasing of dummy arguments. Specifically, it warns if the same actual argument is associated with a dummy argument with INTENT(IN) and a dummy argument with INTENT(OUT) in a call with an explicit interface.

     The following example will trigger the warning.

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

            call bar(a,a)

-Wampersand
     Warn about missing ampersand in continued character constants. The warning is given with -Wampersand, -pedantic, -std=f95, -std=f2003 and -std=f2008. Note: With no ampersand given in a continued character constant, GNU Fortran assumes continuation at the first non-comment, non-whitespace character after the ampersand that initiated the continuation.

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

-Wcharacter-truncation
     Warn when a character assignment will truncate the assigned string.

-Wline-truncation
     Warn when a source code line will be truncated.

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

-Wconversion-extra
     Warn about implicit conversions between different types and kinds.

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

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

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

-Wreal-q-constant
     Produce a warning if a real-literal-constant contains a q exponent-letter.

-Wsurprising
     Produce a warning when ``suspicious'' code constructs are encountered. While technically legal these usually indicate that an error has been made.

     This currently produces a warning under the following circumstances:

       * 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 destination.

       * 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.

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

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

-Wintrinsic-shadow
     Warn if a user-defined procedure or module procedure has the same name as an intrinsic; in this case, an explicit interface or EXTERNAL or INTRINSIC declaration might be needed to get calls later resolved to the desired intrinsic/procedure.

-Wunused-dummy-argument
     Warn about unused dummy arguments. This option is implied by -Wall.

-Wunused-parameter
     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 not included in -Wall but is implied by -Wall -Wextra.

-Walign-commons
     By default, gfortran warns about any occasion of variables being padded for proper alignment inside a COMMON block. This warning can be turned off via -Wno-align-commons. See also -falign-commons.

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

-Werror
     Turns all warnings into errors.

See Options to Request or Suppress Errors and Warnings (Using the GNU Compiler Collection (GCC)), for information on more options offered by the GBE shared by gfortran, gcc and other GNU compilers.

Some of these have no effect when compiling programs written in Fortran.

Options for debugging your program or GNU Fortran
=================================================

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

-fdump-fortran-original
     Output the internal parse tree after translating the source program into internal representation.  Only really useful for debugging the GNU Fortran compiler itself.

-fdump-optimized-tree
     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.

-ffpe-trap=LIST
     Specify a list of IEEE exceptions when a Floating Point Exception (FPE) should be raised.  On most systems, this will result in a SIGFPE signal being sent and the program being interrupted, producing a core file useful for debugging.  LIST is a (possibly empty) comma-separated list of the following IEEE exceptions: invalid (invalid floating point operation, such as SQRT(-1.0)), zero (division by zero), overflow (overflow in a floating point operation), underflow (underflow in a floating point operation), precision (loss of precision during operation) and denormal (operation produced a denormal value).

     Some of the routines in the Fortran runtime library, like CPU_TIME, are likely to trigger floating point exceptions when ffpe-trap=precision is used. For this reason, the use of ffpe-trap=precision is not recommended.

-fno-backtrace
     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.

See Options for Debugging Your Program or GCC (Using the GNU Compiler Collection (GCC)), for more information on debugging options.

Options for directory search
============================

These options affect how GNU Fortran searches for files specified by the INCLUDE directive and where it searches for previously compiled modules.

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

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

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

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

     See Options for Directory Search (Using the GNU Compiler Collection (GCC)), for information on the -I option.

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

     The default is the current directory.

-fintrinsic-modules-path DIR
     This option specifies the location of pre-compiled intrinsic modules, if they are not in the default location expected by the compiler.

Influencing the linking step
============================

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

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

Influencing runtime behavior
============================

These options affect the runtime behavior of programs compiled with GNU Fortran.

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

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

-fno-range-check
     Disable range checking of input values during integer READ operations. For example, GNU Fortran will give an error if an input value is outside of the relevant range of [-HUGE():HUGE()]. In other words, with INTEGER (kind=4) :: i , attempting to read -2147483648 will give an error unless -fno-range-check is given.

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

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

-fsign-zero
     When 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 and regards zero as positive number in the SIGN intrinsic for compatibility with F77. Default behavior is to show the negative sign.

Options for code generation conventions
=======================================

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo.  In the table below, only one of the forms is listed---the one which is not the default.  You can figure out the other form by either removing no- or adding it.

-fno-automatic
     Treat each program unit (except those marked as RECURSIVE) as if the SAVE statement were specified for every local variable and array referenced in it. Does not affect common blocks. (Some Fortran compilers provide this option under the name -static or -save.) The default, which is -fautomatic, uses the stack for local variables smaller than the value given by -fmax-stack-var-size. Use the option -frecursive to use no static memory.

-ff2c
     Generate code designed to be compatible with code generated by g77 and f2c.

     The calling conventions used by g77 (originally implemented in f2c) require functions that return type default REAL to actually return the C type double, and functions that return type COMPLEX to return the values via an extra argument in the calling sequence that points to where to store the return value.  Under the default GNU calling conventions, such functions simply return their results as they would in GNU C---default REAL functions return the C type float, and COMPLEX functions return the GNU C type complex. Additionally, this option implies the -fsecond-underscore option, unless -fno-second-underscore is explicitly requested.

     This does not affect the generation of code that interfaces with the libgfortran library.

     Caution: It is not a good idea to mix Fortran code compiled with -ff2c with code compiled with the default -fno-f2c calling conventions as, calling COMPLEX or default REAL functions between program parts which were compiled with different calling conventions will break at execution time.

     Caution: This will break code which passes intrinsic functions of type default REAL or COMPLEX as actual arguments, as the library implementations use the -fno-f2c calling conventions.

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

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

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

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

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

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

     is implemented as something akin to:

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

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

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

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

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

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

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

-fwhole-file
     By default, GNU Fortran parses, resolves and translates each procedure in a file separately.  Using this option modifies this such that the whole file is parsed and placed in a single front-end tree.  During resolution, in addition to all the usual checks and fixups, references to external procedures that are in the same file effect resolution of that procedure, if not already done, and a check of the interfaces. The dependences are resolved by changing the order in which the file is translated into the backend tree.  Thus, a procedure that is referenced is translated before the reference and the duplication of backend tree declarations eliminated.

-fsecond-underscore
     By default, GNU Fortran appends an underscore to external names.  If this option is used GNU Fortran appends two underscores to names with underscores and one underscore to external names with no underscores.  GNU Fortran also appends two underscores to internal names with underscores to avoid naming collisions with external names.

     This option has no effect if -fno-underscoring is in effect.  It is implied by the -ff2c option.

     Otherwise, with this option, an external name such as MAX_COUNT is implemented as a reference to the link-time external symbol max_count__, instead of max_count_.  This is required for compatibility with g77 and f2c, and is implied by use of the -ff2c option.

-fcoarray=<KEYWORD>

     none
          Disable coarray support; using coarray declarations and image-control statements will produce a compile-time error. (Default)

     single
          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.

-fcheck=<KEYWORD>

     Enable the generation of run-time checks; the argument shall be a comma-delimited list of the following keywords.

     all
          Enable all run-time test of -fcheck.

     array-temps
          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.

     bounds
          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.

     pointer
          Enable generation of run-time checks for pointers and allocatables.

     recursion
          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.

-fbounds-check
     Deprecated alias for -fcheck=bounds.

-fcheck-array-temporaries
     Deprecated alias for -fcheck=array-temps.

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

          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.

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

     This option currently only affects local arrays declared with constant bounds, and may not apply to all character variables. Future versions of GNU Fortran may improve this behavior.

     The default value for N is 32768.

-fstack-arrays
     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's possible that you'll have to extend your runtime limits for stack memory on some operating systems.

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

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

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

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

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

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

     The default value for N is 30.

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

-finit-local-zero
-finit-integer=N
-finit-real=<ZERO|INF|-INF|NAN|SNAN>
-finit-logical=<TRUE|FALSE>
-finit-character=N
     The -finit-local-zero option instructs the compiler to initialize local INTEGER, REAL, and COMPLEX variables to zero, LOGICAL variables to false, and CHARACTER variables to a string of null bytes.  Finer-grained initialization options are provided by the -finit-integer=N, -finit-real=<ZERO|INF|-INF|NAN|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 components of derived type variables, nor do they initialize variables that appear in an EQUIVALENCE statement.  (This limitation may be removed in future releases).

     Note that the -finit-real=nan option initializes REAL and COMPLEX variables with a quiet NaN. 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).

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

-fno-protect-parens
     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.

-frealloc-lhs
     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.

-faggressive-function-elimination
     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.

-ffrontend-optimize
     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 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.

See Options for Code Generation Conventions (Using the GNU Compiler Collection (GCC)), for information on more options offered by the GBE shared by gfortran, gcc, and other GNU compilers.

Environment variables affecting gfortran
========================================

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.

See Environment Variables Affecting GCC (Using the GNU Compiler Collection (GCC)), for information on environment variables.

See Runtime, for environment variables that affect the run-time behavior of programs compiled with GNU Fortran.