struct S {
    int i;
};
void cfunc(struct S *p) {
/* the definition of the C function to be called from C++ */
    p->i += 5;
}

struct S {            // has no base classes 
                      // or virtual functions
    S(int s) : i(s) { }
    int i;
}; extern "C" void cfunc(S *); 
// declare the C function to be called from C++
int f(){
    S s(2);           // initialize 's'
    cfunc(&s);        // call 'cfunc' so it can change 's'
    return s.i * 3;
}