In this article
I shall summarize the experience about
calling C from Fortran and vice versa.
Over the past two years the originally Fortran-based library package KUIP has been gradually rewritten in C. (KUIP stands for ``Kit for a User Interface Package'' and provides the user interface for PAW and other applications from the CERN program library. KUIP has been ported to VMS, VM/CMS, MVS, MS-DOS, Windows/NT, and a large number of Unix flavours.)
The rewrite was not a pure ``exercise of style'' but it appeared to be the best choice in order to improve the quality of the package. Key elements for a user interface package such as dynamic string handling, recursivity, and function pointers are inherent to the C language. Using C allowed the removal of most of the limitations with respect to maximum string lengths built into the original Fortran code. It also facilitated the interface to Motif which is only accessible through C libraries.
The Fortran routines of KUIP have been replaced by C functions. The C code emulates the former Fortran entry points and is therefore completely compatible.
When doing inter-language calls between Fortran and C we have to
realize that the Fortran part is fixed, and that the C code has to
adapt to the characteristics employed by the Fortran compiler.
The cfortran.h
header file by Burkhard Burow provides
convenient preprocessor macros for interfacing Fortran and C.
For reasons explained below it does, however,
not cover the IBM mainframe systems.
Also, cfortran.h exercises cpp to the full
which always involves the risk of stretching the C preprocessor
beyond its limits.
By no means we do want to discourage the use of cfortran.h.
However, if cfortran.h cannot be used (because the target
platform is not supported or the overhead is considered too large),
here you should find the necessary information for doing the Fortran-C
interface ``by hand''.