Who calls __glutSetFCB and __glutGetFCB?  The application?

gcc issues a lot of warnings when compiling this file:

glut_fbc.c: In function ‘__glutSetFCB’:
glut_fbc.c:27: warning: ISO C forbids conversion of object pointer to function pointer type
glut_fbc.c:30: warning: ISO C forbids conversion of object pointer to function pointer type
etc.

If we changed the parameter (and return type) of those functions to GLUTproc it would silence the errors, but I don't know that'd effect the Fortran ABI.

Any ideas?

__glutSetFCB and __glutGetFCB are called by f90gl.  The use of the Set function is (for example):

void APIENTRY fglutDisplayFunc(GLUTdisplayFCBUSR display)
{
    __glutSetFCB(GLUT_TMP_DISPLAY,(void *)display);
    glutDisplayFunc(fortranDisplayWrapper);
}

and the use of the Get function is:

void GLUTCALLBACK fortranDisplayWrapper(void)
{
  ((GLUTdisplayFCBUSR)__glutGetFCB(GLUT_TMP_DISPLAY))();
}

If that doesn't tell you enough about whether the change is safe, I can fill in more of the details, or you can look at the source in cwrapglt.c in the f90gl distribution at http://math.nist.gov/f90gl

commit 39fc89a24e778f3ff6d92e8513b652a6c598370e
Author: Brian <brian.paul@tungstengraphics.com>
Date:   Wed Sep 12 13:40:15 2007 -0600

    Add glut_fcb.c to the build and update to fix warnings (see bug 12405)