Passing functions from C++ DLD to Fortran and vise versa

[John W. Eaton]

On  6-Jun-2007, Thomas Treichl wrote:

| I've a (hopefully simple) problem that I can't figure out. I'm trying to pass 
a 
| function pointer from C++ to a Fortran routine and vise versa. But at the 
step 
| of compilation I fail and don't know what to do at the moment:

| extern "C" void F77_FUNC (ffuncall, FFUNCALL) (int *vn, double *vd, void *vp);

Here you are claiming that funcall expects a void pointer, but
you are passing it something else:

| extern "C" void F77_FUNC (ffun, FFUN) (int *vint, double *vdbl) {
|   printf ("   Example 4a: C side: Fortran called C function ffon with numbers 
%d %f\n",
|         *vint, *vdbl);
| }

This declares a function with the type

  void (*)(int*, double*)

and the compiler is telling you the types don't match.

You should declare ffuncall as

  extern "C" void
  F77_FUNC (ffuncall, FFUNCALL) (int *vn, double *vd, void (*)(int*, double*));

or better,

  extern "C" F77_RET_T
  F77_FUNC (ffuncall, FFUNCALL) (int *vn, double *vd, void (*)(int*, double*));

You might consider using const where appropriate.

You can also declare scalar Fortran parameters that are not modified
as const reference arguments to avoid needing to do things like this:

  extern "C" F77_RET_T F77_FUNC (f, F) (int*);
  ...
  int dummy = 42;
  F77_FUNC (f, F) (&dummy);

and instead write

  extern "C" F77_RET_T F77_FUNC (f, F) (const int&);
  ...
  F77_FUNC (f, F) (42);

There are plenty of examples of this kind of thing in the Octave
sources.

jwe