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5.10.5 Fortran 77 Compiler Characteristics

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 space 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 AC_PROG_F77 will 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.

Macro: AC_PROG_F77_C_O
Test if the Fortran 77 compiler accepts the options `-c' and `-o' simultaneously, and define F77_NO_MINUS_C_MINUS_O if it does not.

The following macros check for Fortran 77 compiler characteristics. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see section 6.4 Running the Compiler) or AC_RUN_IFELSE (see section 6.6 Checking Run Time Behavior), making sure to first set the current language to Fortran 77 AC_LANG(Fortran 77) (see section 6.1 Language Choice).

Macro: AC_F77_LIBRARY_LDFLAGS
Determine the linker flags (e.g., `-L' and `-l') for the Fortran 77 intrinsic and run-time libraries that are required to successfully link a Fortran 77 program or shared library. The output variable FLIBS is set to these flags.

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

For example, if object files from a C++ and Fortran 77 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 77 intrinsic and run-time 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, the macro AC_F77_LIBRARY_LDFLAGS was created to determine these Fortran 77 libraries.

The macro AC_F77_DUMMY_MAIN or AC_F77_MAIN will probably also be necessary to link C/C++ with Fortran; see below.

Macro: AC_F77_DUMMY_MAIN ([action-if-found], [action-if-not-found])
With many compilers, the Fortran libraries detected by AC_F77_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 or AC_F77_MAIN macro figures 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 detects whether any such routine is required for linking, and what its name is; the shell variable F77_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 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

Note that AC_F77_DUMMY_MAIN is called automatically from AC_F77_WRAPPERS; there is generally no need to call it explicitly unless one wants to change the default actions.

Macro: AC_F77_MAIN
As discussed above for AC_F77_DUMMY_MAIN, 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 macro detects whether it is possible to utilize such an alternate main function, and defines F77_MAIN to the name of the function. (If no alternate main function name is found, F77_MAIN is simply defined to main.)

Thus, when calling Fortran routines from C that perform things like I/O, one should use this macro and name the "main" function F77_MAIN instead of main.

Macro: AC_F77_WRAPPERS
Defines C macros F77_FUNC(name,NAME) and F77_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 77 compiler.

Fortran 77 is case-insensitive, and in order to achieve this the Fortran 77 compiler converts all identifiers into a canonical case and format. To call a Fortran 77 subroutine from C or to write a C function that is callable from Fortran 77, the C program must explicitly use identifiers in the format expected by the Fortran 77 compiler. In order to do this, one simply wraps all C identifiers in one of the macros provided by AC_F77_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 section `Mixing Fortran 77 With C and C++' in GNU Automake).

Although Autoconf tries to be intelligent about detecting the name-mangling scheme of the Fortran 77 compiler, there may be Fortran 77 compilers that it doesn't support yet. In this case, the above code will generate a compile-time error, but some other behavior (e.g., disabling Fortran-related features) can be induced by checking whether the F77_FUNC macro 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 77 identifier contains an underscore (e.g., foo_bar), you should use F77_FUNC_ instead of F77_FUNC (with the same arguments). This is because some Fortran 77 compilers mangle names differently if they contain an underscore.

Macro: AC_F77_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 77 linker (see also AC_F77_WRAPPERS). shellvar is optional; if it is not supplied, the shell variable will be 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++.


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