Generally, C code written to link with
g77 code—calling and/or being
called from Fortran—should `#include <g2c.h>' to define the C
versions of the Fortran types.
Don't assume Fortran
correspond to C
ints, for instance; instead, declare them as
integer, a type defined by g2c.h.
g2c.h is installed where gcc will find it by
default, assuming you use a copy of gcc compatible with
g77, probably built at the same time as g77.