7.11.3 Preprocessing Fortran

It is possible to use Fortran which has cpp-style directives within it. Some Fortran compilers, such as g77, can process these directives internally, and so need no separate preprocessing stage; in other cases, the code must be compiled indirectly, with a preprocessor producing pure Fortran code which is only then passed to the compiler.

Because of the syntactical differences in the underlying languages, however it is not always possible to do this using the cpp program, and you may need help from a separate Fortran-specific preprocessor.¹

Automake's support for preprocessed Fortran is tightly bound to the Autoconf AC_PROG_FPP macro (introduced in version XXX), which determines if the Fortran compiler can cope with preprocessor directives, and if not finds a preprocessor which can. It looks for files with a .F extension (the extension is configurable).

The AC_PROG_FPP macro takes as argument a list of features that you require a preprocessor to support (handling of -I and -D options, and so on), and does one of two things:

1. checks whether your Fortran compiler can handle the required set of preprocessor directives internally; and if not
2. checks whether it can find a separate preprocessor which can turn the preprocessable code into a form which the Fortran compiler can handle.

     if FPPDIRECT
     ## do things appropriate for direct mode
     else !FPPDIRECT
     ## do things appropriate for indirect mode
     endif !FPPDIRECT

Handling Ratfor is a type of preprocessing, but this is rather more straightforward. The file N.f is made automatically from N.r by a rule which runs just the preprocessor to convert a Ratfor source file into a strict Fortran 77 source file. The precise command used is as follows:

.r

$(FC) $(AM_FCFLAGS) $(FCFLAGS) $(AM_RFLAGS) $(RFLAGS)

(or the corresponding alternatives for the F77 interface).