Function pointer example

#include <stdio.h>

int square(int a) { return a * a; }
typedef int (*square_fp)(int);

typedef struct {
  int x;
  square_fp fp;
} data;

int main(void) {
  data test;
  test.x = 40;
  test.fp = square;
  printf("%d", test.fp(test.x));
  return 0;
}