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

Error and Warning Options
     See Options to request or suppress errors and warnings. -fmax-errors=N -fsyntax-only  -pedantic  -pedantic-errors -Wall  -Waliasing  -Wampersand  -Wcharacter-truncation  -Wconversion -Wimplicit-interface  -Wline-truncation  -Wnonstd-intrinsics  -Wsurprising -Wno-tabs  -Wunderflow -Wunused-parameter

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

Directory Options
     See Options for directory search. -IDIR  -JDIR  -MDIR  -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  -frecord-marker=LENGTH -fmax-subrecord-length=LENGTH  -fsign-zero

Code Generation Options
     See Options for code generation conventions. -fno-automatic  -ff2c  -fno-underscoring -fsecond-underscore -fbounds-check  -fmax-stack-var-size=N -fpack-derived  -frepack-arrays  -fshort-enums  -fexternal-blas -fblas-matmul-limit=N -frecursive -finit-local-zero -finit-integer=N -finit-real=<ZERO|INF|-INF|NAN> -finit-logical=<TRUE|FALSE> -finit-character=N

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

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

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

-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.

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

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

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

-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.  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.

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

-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.

-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 between different types.

-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.

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

-Wsurprising
     Produce a warning when ``suspicious'' code constructs are 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.

-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.

-Wunused-parameter
     Contrary to gcc's meaning of -Wunused-parameter, gfortran's implementation of this option does not warn about unused dummy arguments, but about unused PARAMETER values. -Wunused-parameter is not included in -Wall but is implied by -Wall -Wextra.

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

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

-fbacktrace
     Specify that, when a runtime error is encountered or a deadly signal is emitted (segmentation fault, illegal instruction, bus error or floating-point exception), the Fortran runtime library should output a backtrace of the error.  This option only has influence for compilation of the Fortran main program.

-fdump-core
     Request that a core-dump file is written to disk when a runtime error is encountered on systems that support core dumps. This option is only effective for the compilation of the Fortran main 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.

-MDIR

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

     The default is the current directory.

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

-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.

-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 writing zero values, show the negative sign if the sign bit is set. fno-sign-zero does not print the negative sign of zero values for compatibility with F77.  Default behavior is to show the negative sign.

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

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

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

-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.

-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.

-fbounds-check
     Enable generation of run-time checks for array subscripts and against the declared minimum and maximum values.  It also checks array indices for assumed and deferred shape arrays against the actual allocated bounds.

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

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

-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.

-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>

-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> (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.

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.