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

struct Point {
  int x, y;
};

void DoubleXBroken(struct Point p) {
  p.x *= 2;
}

void DoubleXWorks(struct Point *p) {
  p->x *= 2;
}

int main(int argc, char **argv) {
  struct Point a = {1, 1};

  DoubleXBroken(a);
  printf("(%d,%d)\n", a.x, a.y);   // prints: (1,1)

  DoubleXWorks(&a);
  printf("(%d,%d)\n", a.x, a.y);   // prints: (2,1)

  return EXIT_SUCCESS;
}