#include <stdio.h>
#include <stdlib.h>

int sub(int a, int b) {
  return a-b;
}

int add(int a, int b) {
  return a+b;
}

int main(int argc, char *argv[]) {

  // This is the declaration of a variable, op, which is a pointer to a function
  int (*op)(int, int);  // allocates 8 bytes to hold a pointer to a function
                               // that takes two ints and returns an int
  int x = 10;
  int y = 100;

  op = add;
  printf("(op <- add) => op(x, y) = %d\n", op(x, y));  // this is an invocation of the function
                                                                                   // pointed at by op

  op = sub;
  printf("(op <- sub) => op(x, y) = %d\n", op(x, y));

  return 0;
}