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5.10.8 Fortran Compiler Characteristics

The Autoconf Fortran support is divided into two categories: legacy Fortran 77 macros (F77), and modern Fortran macros (FC). The former are intended for traditional Fortran 77 code, and have output variables like F77, FFLAGS, and FLIBS. The latter are for newer programs that can (or must) compile under the newer Fortran standards, and have output variables like FC, FCFLAGS, and FCLIBS.

Except for the macros AC_FC_SRCEXT, AC_FC_FREEFORM, AC_FC_FIXEDFORM, and AC_FC_LINE_LENGTH (see below), the FC and F77 macros behave almost identically, and so they are documented together in this section.

Macro: AC_PROG_F77 ([compiler-search-list]

Determine a Fortran 77 compiler to use. If F77 is not already set in the environment, then check for g77 and f77, and then some other names. Set the output variable F77 to the name of the compiler found.

This macro may, however, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran 77 compilers to search for. This just gives the user an opportunity to specify an alternative search list for the Fortran 77 compiler. For example, if you didn’t like the default order, then you could invoke AC_PROG_F77 like this:

AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90])

If using g77 (the GNU Fortran 77 compiler), then set the shell variable G77 to ‘yes’. If the output variable FFLAGS was not already set in the environment, then set it to ‘-g -02’ for g77 (or ‘-O2’ where g77 does not accept ‘-g’). Otherwise, set FFLAGS to ‘-g’ for all other Fortran 77 compilers.

The result of the GNU test is cached in the ac_cv_f77_compiler_gnu variable, acceptance of ‘-g’ in the ac_cv_prog_f77_g variable.

Macro: AC_PROG_FC ([compiler-search-list] @ovar{dialect})

Determine a Fortran compiler to use. If FC is not already set in the environment, then dialect is a hint to indicate what Fortran dialect to search for; the default is to search for the newest available dialect. Set the output variable FC to the name of the compiler found.

By default, newer dialects are preferred over older dialects, but if dialect is specified then older dialects are preferred starting with the specified dialect. dialect can currently be one of Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of which compiler name to prefer (e.g., f90 or f95), and no attempt is made to guarantee that a particular language standard is actually supported. Thus, it is preferable that you avoid the dialect option, and use AC_PROG_FC only for code compatible with the latest Fortran standard.

This macro may, alternatively, be invoked with an optional first argument which, if specified, must be a blank-separated list of Fortran compilers to search for, just as in AC_PROG_F77.

If using gfortran or g77 (the GNU Fortran compilers), then set the shell variable GFC to ‘yes’. If the output variable FCFLAGS was not already set in the environment, then set it to ‘-g -02’ for GNU g77 (or ‘-O2’ where g77 does not accept ‘-g’). Otherwise, set FCFLAGS to ‘-g’ for all other Fortran compilers.

The result of the GNU test is cached in the ac_cv_fc_compiler_gnu variable, acceptance of ‘-g’ in the ac_cv_prog_fc_g variable.

Macro: AC_PROG_F77_C_O
Macro: AC_PROG_FC_C_O

Test whether the Fortran compiler accepts the options ‘-c’ and ‘-o’ simultaneously, and define F77_NO_MINUS_C_MINUS_O or FC_NO_MINUS_C_MINUS_O, respectively, if it does not.

The result of the test is cached in the ac_cv_prog_f77_c_o or ac_cv_prog_fc_c_o variable, respectively.

The following macros check for Fortran compiler characteristics. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see section Running the Compiler) or AC_RUN_IFELSE (see section Checking Runtime Behavior), making sure to first set the current language to Fortran 77 or Fortran via AC_LANG([Fortran 77]) or AC_LANG(Fortran) (see section Language Choice).

Macro: AC_F77_LIBRARY_LDFLAGS
Macro: AC_FC_LIBRARY_LDFLAGS

Determine the linker flags (e.g., ‘-L’ and ‘-l’) for the Fortran intrinsic and runtime libraries that are required to successfully link a Fortran program or shared library. The output variable FLIBS or FCLIBS is set to these flags (which should be included after LIBS when linking).

This macro is intended to be used in those situations when it is necessary to mix, e.g., C++ and Fortran source code in a single program or shared library (see Mixing Fortran 77 With C and C++ in GNU Automake).

For example, if object files from a C++ and Fortran compiler must be linked together, then the C++ compiler/linker must be used for linking (since special C++-ish things need to happen at link time like calling global constructors, instantiating templates, enabling exception support, etc.).

However, the Fortran intrinsic and runtime libraries must be linked in as well, but the C++ compiler/linker doesn’t know by default how to add these Fortran 77 libraries. Hence, this macro was created to determine these Fortran libraries.

The macros AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or AC_F77_MAIN and AC_FC_MAIN are probably also necessary to link C/C++ with Fortran; see below. Further, it is highly recommended that you use AC_CONFIG_HEADERS (see section Configuration Header Files) because the complex defines that the function wrapper macros create may not work with C/C++ compiler drivers.

These macros internally compute the flag needed to verbose linking output and cache it in ac_cv_prog_f77_v or ac_cv_prog_fc_v variables, respectively. The computed linker flags are cached in ac_cv_f77_libs or ac_cv_fc_libs, respectively.

Macro: AC_F77_DUMMY_MAIN ([action-if-found] @dvar{action-if-not-found, AC_MSG_FAILURE})
Macro: AC_FC_DUMMY_MAIN ([action-if-found] @dvar{action-if-not-found, AC_MSG_FAILURE})

With many compilers, the Fortran libraries detected by AC_F77_LIBRARY_LDFLAGS or AC_FC_LIBRARY_LDFLAGS provide their own main entry function that initializes things like Fortran I/O, and which then calls a user-provided entry function named (say) MAIN__ to run the user’s program. The AC_F77_DUMMY_MAIN and AC_FC_DUMMY_MAIN or AC_F77_MAIN and AC_FC_MAIN macros figure out how to deal with this interaction.

When using Fortran for purely numerical functions (no I/O, etc.) often one prefers to provide one’s own main and skip the Fortran library initializations. In this case, however, one may still need to provide a dummy MAIN__ routine in order to prevent linking errors on some systems. AC_F77_DUMMY_MAIN or AC_FC_DUMMY_MAIN detects whether any such routine is required for linking, and what its name is; the shell variable F77_DUMMY_MAIN or FC_DUMMY_MAIN holds this name, unknown when no solution was found, and none when no such dummy main is needed.

By default, action-if-found defines F77_DUMMY_MAIN or FC_DUMMY_MAIN to the name of this routine (e.g., MAIN__) if it is required. action-if-not-found defaults to exiting with an error.

In order to link with Fortran routines, the user’s C/C++ program should then include the following code to define the dummy main if it is needed:

#ifdef F77_DUMMY_MAIN
#  ifdef __cplusplus
     extern "C"
#  endif
   int F77_DUMMY_MAIN () { return 1; }
#endif

(Replace F77 with FC for Fortran instead of Fortran 77.)

Note that this macro is called automatically from AC_F77_WRAPPERS or AC_FC_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.

The result of this macro is cached in the ac_cv_f77_dummy_main or ac_cv_fc_dummy_main variable, respectively.

Macro: AC_F77_MAIN
Macro: AC_FC_MAIN

As discussed above, many Fortran libraries allow you to provide an entry point called (say) MAIN__ instead of the usual main, which is then called by a main function in the Fortran libraries that initializes things like Fortran I/O. The AC_F77_MAIN and AC_FC_MAIN macros detect whether it is possible to utilize such an alternate main function, and defines F77_MAIN and FC_MAIN to the name of the function. (If no alternate main function name is found, F77_MAIN and FC_MAIN are simply defined to main.)

Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and declare the "main" function like so:

#ifdef __cplusplus
  extern "C"
#endif
int F77_MAIN (int argc, char *argv[]);

(Again, replace F77 with FC for Fortran instead of Fortran 77.)

The result of this macro is cached in the ac_cv_f77_main or ac_cv_fc_main variable, respectively.

Macro: AC_F77_WRAPPERS
Macro: AC_FC_WRAPPERS

Defines C macros F77_FUNC (name, NAME), FC_FUNC (name, NAME), F77_FUNC_(name, NAME), and FC_FUNC_(name, NAME) to properly mangle the names of C/C++ identifiers, and identifiers with underscores, respectively, so that they match the name-mangling scheme used by the Fortran compiler.

Fortran is case-insensitive, and in order to achieve this the Fortran compiler converts all identifiers into a canonical case and format. To call a Fortran subroutine from C or to write a C function that is callable from Fortran, the C program must explicitly use identifiers in the format expected by the Fortran compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by AC_F77_WRAPPERS or AC_FC_WRAPPERS. For example, suppose you have the following Fortran 77 subroutine:

      subroutine foobar (x, y)
      double precision x, y
      y = 3.14159 * x
      return
      end

You would then declare its prototype in C or C++ as:

#define FOOBAR_F77 F77_FUNC (foobar, FOOBAR)
#ifdef __cplusplus
extern "C"  /* prevent C++ name mangling */
#endif
void FOOBAR_F77 (double *x, double *y);

Note that we pass both the lowercase and uppercase versions of the function name to F77_FUNC so that it can select the right one. Note also that all parameters to Fortran 77 routines are passed as pointers (see Mixing Fortran 77 With C and C++ in GNU Automake).

(Replace F77 with FC for Fortran instead of Fortran 77.)

Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran compiler, there may be Fortran compilers that it doesn’t support yet. In this case, the above code generates a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether F77_FUNC or FC_FUNC is defined.

Now, to call that routine from a C program, we would do something like:

{
    double x = 2.7183, y;
    FOOBAR_F77 (&x, &y);
}

If the Fortran identifier contains an underscore (e.g., foo_bar), you should use F77_FUNC_ or FC_FUNC_ instead of F77_FUNC or FC_FUNC (with the same arguments). This is because some Fortran compilers mangle names differently if they contain an underscore.

The name mangling scheme is encoded in the ac_cv_f77_mangling or ac_cv_fc_mangling cache variable, respectively, and also used for the AC_F77_FUNC and AC_FC_FUNC macros described below.

Macro: AC_F77_FUNC (name, [shellvar]
Macro: AC_FC_FUNC (name, [shellvar]

Given an identifier name, set the shell variable shellvar to hold the mangled version name according to the rules of the Fortran linker (see also AC_F77_WRAPPERS or AC_FC_WRAPPERS). shellvar is optional; if it is not supplied, the shell variable is simply name. The purpose of this macro is to give the caller a way to access the name-mangling information other than through the C preprocessor as above, for example, to call Fortran routines from some language other than C/C++.

Macro: AC_FC_SRCEXT (ext, [action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]
Macro: AC_FC_PP_SRCEXT (ext, [action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]

By default, the FC macros perform their tests using a ‘.f’ extension for source-code files. Some compilers, however, only enable newer language features for appropriately named files, e.g., Fortran 90 features only for ‘.f90’ files, or preprocessing only with ‘.F’ files or maybe other upper-case extensions. On the other hand, some other compilers expect all source files to end in ‘.f’ and require special flags to support other file name extensions. The AC_FC_SRCEXT and AC_FC_PP_SRCEXT macros deal with these issues.

The AC_FC_SRCEXT macro tries to get the FC compiler to accept files ending with the extension ‘.ext’ (i.e., ext does not contain the dot). If any special compiler flags are needed for this, it stores them in the output variable FCFLAGS_ext. This extension and these flags are then used for all subsequent FC tests (until AC_FC_SRCEXT or AC_FC_PP_SRCEXT is called another time).

For example, you would use AC_FC_SRCEXT(f90) to employ the ‘.f90’ extension in future tests, and it would set the FCFLAGS_f90 output variable with any extra flags that are needed to compile such files.

Similarly, the AC_FC_PP_SRCEXT macro tries to get the FC compiler to preprocess and compile files with the extension ‘.ext’. When both fpp and cpp style preprocessing are provided, the former is preferred, as the latter may treat continuation lines, // tokens, and white space differently from what some Fortran dialects expect. Conversely, if you do not want files to be preprocessed, use only lower-case characters in the file name extension. Like with AC_FC_SRCEXT(f90), any needed flags are stored in the FCFLAGS_ext variable.

The FCFLAGS_ext flags can not be simply absorbed into FCFLAGS, for two reasons based on the limitations of some compilers. First, only one FCFLAGS_ext can be used at a time, so files with different extensions must be compiled separately. Second, FCFLAGS_ext must appear immediately before the source-code file name when compiling. So, continuing the example above, you might compile a ‘foo.f90’ file in your makefile with the command:

foo.o: foo.f90
       $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90'

If AC_FC_SRCEXT or AC_FC_PP_SRCEXT succeeds in compiling files with the ext extension, it calls action-if-success (defaults to nothing). If it fails, and cannot find a way to make the FC compiler accept such files, it calls action-if-failure (defaults to exiting with an error message).

The AC_FC_SRCEXT and AC_FC_PP_SRCEXT macros cache their results in ac_cv_fc_srcext_ext and ac_cv_fc_pp_srcext_ext variables, respectively.

Macro: AC_FC_PP_DEFINE ([action-if-success] @dvar{action-if-failure, AC_MSG_FAILURE})

Find a flag to specify defines for preprocessed Fortran. Not all Fortran compilers use ‘-D’. Substitute FC_DEFINE with the result and call action-if-success (defaults to nothing) if successful, and action-if-failure (defaults to failing with an error message) if not.

This macro calls AC_FC_PP_SRCEXT([F]) in order to learn how to preprocess a ‘conftest.F’ file, but restores a previously used Fortran source file extension afterwards again.

The result of this test is cached in the ac_cv_fc_pp_define variable.

Macro: AC_FC_FREEFORM ([action-if-success] @dvar{action-if-failure, AC_MSG_FAILURE})

Try to ensure that the Fortran compiler ($FC) allows free-format source code (as opposed to the older fixed-format style from Fortran 77). If necessary, it may add some additional flags to FCFLAGS.

This macro is most important if you are using the default ‘.f’ extension, since many compilers interpret this extension as indicating fixed-format source unless an additional flag is supplied. If you specify a different extension with AC_FC_SRCEXT, such as ‘.f90’, then AC_FC_FREEFORM ordinarily succeeds without modifying FCFLAGS. For extensions which the compiler does not know about, the flag set by the AC_FC_SRCEXT macro might let the compiler assume Fortran 77 by default, however.

If AC_FC_FREEFORM succeeds in compiling free-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

The result of this test, or ‘none’ or ‘unknown’, is cached in the ac_cv_fc_freeform variable.

Macro: AC_FC_FIXEDFORM ([action-if-success] @dvar{action-if-failure, AC_MSG_FAILURE})

Try to ensure that the Fortran compiler ($FC) allows the old fixed-format source code (as opposed to free-format style). If necessary, it may add some additional flags to FCFLAGS.

This macro is needed for some compilers alias names like xlf95 which assume free-form source code by default, and in case you want to use fixed-form source with an extension like ‘.f90’ which many compilers interpret as free-form by default. If you specify a different extension with AC_FC_SRCEXT, such as ‘.f’, then AC_FC_FIXEDFORM ordinarily succeeds without modifying FCFLAGS.

If AC_FC_FIXEDFORM succeeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

The result of this test, or ‘none’ or ‘unknown’, is cached in the ac_cv_fc_fixedform variable.

Macro: AC_FC_LINE_LENGTH ([length] @ovar{action-if-success}, [action-if-failure = ‘AC_MSG_FAILURE]

Try to ensure that the Fortran compiler ($FC) accepts long source code lines. The length argument may be given as 80, 132, or unlimited, and defaults to 132. Note that line lengths above 254 columns are not portable, and some compilers do not accept more than 132 columns at least for fixed format source. If necessary, it may add some additional flags to FCFLAGS.

If AC_FC_LINE_LENGTH succeeds in compiling fixed-form source, it calls action-if-success (defaults to nothing). If it fails, it calls action-if-failure (defaults to exiting with an error message).

The result of this test, or ‘none’ or ‘unknown’, is cached in the ac_cv_fc_line_length variable.

Macro: AC_FC_CHECK_BOUNDS ([action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]

The AC_FC_CHECK_BOUNDS macro tries to enable array bounds checking in the Fortran compiler. If successful, the action-if-success is called and any needed flags are added to FCFLAGS. Otherwise, action-if-failure is called, which defaults to failing with an error message. The macro currently requires Fortran 90 or a newer dialect.

The result of the macro is cached in the ac_cv_fc_check_bounds variable.

Macro: AC_F77_IMPLICIT_NONE ([action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]
Macro: AC_FC_IMPLICIT_NONE ([action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]

Try to disallow implicit declarations in the Fortran compiler. If successful, action-if-success is called and any needed flags are added to FFLAGS or FCFLAGS, respectively. Otherwise, action-if-failure is called, which defaults to failing with an error message.

The result of these macros are cached in the ac_cv_f77_implicit_none and ac_cv_fc_implicit_none variables, respectively.

Macro: AC_FC_MODULE_EXTENSION

Find the Fortran 90 module file name extension. Most Fortran 90 compilers store module information in files separate from the object files. The module files are usually named after the name of the module rather than the source file name, with characters possibly turned to upper case, plus an extension, often ‘.mod’.

Not all compilers use module files at all, or by default. The Cray Fortran compiler requires ‘-e m’ in order to store and search module information in ‘.mod’ files rather than in object files. Likewise, the Fujitsu Fortran compilers uses the ‘-Am’ option to indicate how module information is stored.

The AC_FC_MODULE_EXTENSION macro computes the module extension without the leading dot, and stores that in the FC_MODEXT variable. If the compiler does not produce module files, or the extension cannot be determined, FC_MODEXT is empty. Typically, the result of this macro may be used in cleanup make rules as follows:

clean-modules:
        -test -z "$(FC_MODEXT)" || rm -f *.$(FC_MODEXT)

The extension, or ‘unknown’, is cached in the ac_cv_fc_module_ext variable.

Macro: AC_FC_MODULE_FLAG ([action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]

Find the compiler flag to include Fortran 90 module information from another directory, and store that in the FC_MODINC variable. Call action-if-success (defaults to nothing) if successful, and set FC_MODINC to empty and call action-if-failure (defaults to exiting with an error message) if not.

Most Fortran 90 compilers provide a way to specify module directories. Some have separate flags for the directory to write module files to, and directories to search them in, whereas others only allow writing to the current directory or to the first directory specified in the include path. Further, with some compilers, the module search path and the preprocessor search path can only be modified with the same flag. Thus, for portability, write module files to the current directory only and list that as first directory in the search path.

There may be no whitespace between FC_MODINC and the following directory name, but FC_MODINC may contain trailing white space. For example, if you use Automake and would like to search ‘../lib’ for module files, you can use the following:

AM_FCFLAGS = $(FC_MODINC). $(FC_MODINC)../lib

Inside configure tests, you can use:

if test -n "$FC_MODINC"; then
  FCFLAGS="$FCFLAGS $FC_MODINC. $FC_MODINC../lib"
fi

The flag is cached in the ac_cv_fc_module_flag variable. The substituted value of FC_MODINC may refer to the ac_empty dummy placeholder empty variable, to avoid losing the significant trailing whitespace in a ‘Makefile’.

Macro: AC_FC_MODULE_OUTPUT_FLAG ([action-if-success] [action-if-failure = ‘AC_MSG_FAILURE]

Find the compiler flag to write Fortran 90 module information to another directory, and store that in the FC_MODOUT variable. Call action-if-success (defaults to nothing) if successful, and set FC_MODOUT to empty and call action-if-failure (defaults to exiting with an error message) if not.

Not all Fortran 90 compilers write module files, and of those that do, not all allow writing to a directory other than the current one, nor do all have separate flags for writing and reading; see the description of AC_FC_MODULE_FLAG above. If you need to be able to write to another directory, for maximum portability use FC_MODOUT before any FC_MODINC and include both the current directory and the one you write to in the search path:

AM_FCFLAGS = $(FC_MODOUT)../mod $(FC_MODINC)../mod $(FC_MODINC). …

The flag is cached in the ac_cv_fc_module_output_flag variable. The substituted value of FC_MODOUT may refer to the ac_empty dummy placeholder empty variable, to avoid losing the significant trailing whitespace in a ‘Makefile’.


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