/*  File: fptr3.c

    Exploring function pointers.
    This version does not compile with gcc.
*/

#include <stdio.h>

/* some simple functions */

void add3(int *iptr) {
   *iptr = *iptr + 3 ;
}


void add5(double *dptr) {
   *dptr = *dptr + 5.0 ;
}


/* Takes a function parameter */

void apply (void addx(void *), void *rptr) {
   
  addx(rptr) ;
}


main() {
   int m = 2 ;
   double x = 5.0 ;

   /* fptr is a variable that holds the address of a function
      which takes a void * parameter and returns nothing.
   */
   void (*fptr)(void *) ;

   add3(&m) ;
   printf("After add3(&m), m = %d\n", m) ;
   add5(&x) ;
   printf("After add5(&x), x = %lf\n", x) ;
   printf("\n") ;

   apply(&add3, &m) ;
   printf("Applying add3 to m, m = %d\n", m) ; 
   apply(&add5, &x) ;
   printf("Applying add5 to x, x = %lf\n",  x) ; 
   printf("\n") ;

   fptr = &add3 ;
   (*fptr) (&m) ; 
   printf("Applying fptr to m, m = %d\n", m) ; 
   fptr = &add5 ;
   (*fptr) (&x) ; 
   printf("Applying fptr to x, x = %lf\n",  x) ; 
}