GNU Fortran 95 Command Options
******************************

The gfortran command supports all the options supported by the gcc command.  Only options specific to gfortran 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 gfortran 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 gfortran to the gcc distribution enables acceptance of gfortran 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  -frange-check

Warning Options
     See Options to Request or Suppress Warnings.

     -fsyntax-only  -pedantic  -pedantic-errors -w  -Wall  -Waliasing -Wampersand -Wconversion -Wimplicit-interface -Wtabs -Wnonstd-intrinsics -Wsurprising -Wunderflow -Wunused-labels -Wline-truncation -W

Debugging Options
     See Options for Debugging Your Program or GCC.

     -fdump-parse-tree -ffpe-trap=LIST

Directory Options
     See Options for Directory Search.

     -IDIR  -MDIR

Runtime Options
     See Options for influencing runtime behavior.

     -fconvert=CONVERSION -frecord-marker=LENGTH

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

Options Controlling Fortran Dialect
===================================

The following options control the dialect of Fortran that the compiler accepts:

-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.

-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-comment
     Enables special treating for lines 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.

-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.

-fno-backslash
     Compile switch to change the interpretation of a backslash from ``C''-style escape characters to a single backslash character.

-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 (corresponds to ``extended-source'' options in some popular compilers). N may 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. For free-form, 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 200x).

-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
     Enables the Cray pointer extension, which provides a C-like pointer.

-fopenmp
     Enables handling of OpenMP !$omp directives in free form and c$omp, *$omp and !$omp directives in fixed form, enables !$ 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.

-frange-check
     Enable range checking on results of simplification of constant expressions during compilation.  For example, by default, gfortran will give an overflow error at compile time when simplifying a = EXP(1000). With -fno-range-check, no error will be given and the variable a will be assigned the value +Infinity.

-std=STD
     Conform to the specified standard.  Allowed values for STD are gnu, f95, f2003 and legacy.

Options to Request or Suppress Warnings
=======================================

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there might have been an error.

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 warnings produced by GNU Fortran:

-fsyntax-only
     Check the code for syntax errors, but don't do anything beyond that.

-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 gfortran in this area are welcome.

     This should be used in conjunction with -std=STD.

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

-w
     Inhibit all warning messages.

-Wall
     Enables commonly used warning options pertaining to usage that we recommend avoiding and that we believe are easy to avoid. This currently includes -Wunused-labels, -Waliasing, -Wampersand, -Wsurprising, -Wnonstd-intrinsic, -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, and -std=f2003. Note: With no ampersand given in a continued character constant, gfortran assumes continuation at the first non-comment, non-whitespace character after the ampersand that initiated the continuation.

-Wconversion
     Warn about implicit conversions between different types.

-Wimplicit-interface
     Warn about when procedure are 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-intrinsic
     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.

-Wtabs
     By default, tabs are accepted as whitespace, but tabs are not members of the Fortran Character Set.  -Wno-tabs will cause a warning to be issued if a tab is encountered. Note, -Wno-tabs is active for -pedantic, -std=f95, and -Wall.

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

-Wunused-labels
     Warn whenever a label is defined but never referenced.

-Werror
     Turns all warnings into errors.

-W
     Turns on ``extra warnings'' and, if optimization is specified via -O, the -Wuninitialized option. (This might change in future versions of gfortran

See Options to Request or Suppress 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 gfortran

-fdump-parse-tree
     Output the internal parse tree before starting code generation.  Only really useful for debugging gfortran 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 denormal value).

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 gfortran 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.

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

These options affect the runtime behavior of gfortran.

-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 whatever off_t is specified to be on that particular system. Note that specifying LENGTH as 4 limits the record length of unformatted files to 2 GB.  This option does not extend the maximum possible record length on systems where off_t is a four_byte quantity.

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 as if the SAVE statement was 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.)

-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, gfortran 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 gfortran is incompatible with f2c and g77, please use the -ff2c option if you want object files compiled with gfortran to be compatible with object code created with these tools.

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

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

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

     is implemented as something akin to:

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

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

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

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

     Note that just because the names match does not mean that the interface implemented by gfortran for an external name matches the interface implemented by some other language for that same name. That is, getting code produced by gfortran 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 gfortran 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, gfortran appends an underscore to external names.  If this option is used gfortran appends two underscores to names with underscores and one underscore to external names with no underscores.  (gfortran 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.

     In the future this may also include other forms of checking, eg. 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.

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

     The default value for N is 32768.

-fpack-derived
     This option tells gfortran 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 gfortran may pass assumed shape array sections via a descriptor describing a discontiguous 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 discontiguous.

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

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 GNU Fortran
===========================================

GNU Fortran 95 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 gfortran programs.