Question

When I run valgrind on this function, it says I’ve definitely lost 4 bytes. I know it’s because I’m redirecting the pointer x in x=y, thus losing access to the initial memory allocated in the first line. How do I fix this? What’s the correct principle here? I’m just learning C, so I’m trying to get the hang of all this. Thanks!

int main() {
  int* x = malloc(sizeof(*x));
  int* y = malloc(sizeof(*y));
  *x = 2;
  *y = 5;
  x = y;
  *y = 6;
  *x = 4;
  printf("y = %d\n", *y);

  free(x);
  free(y);

  return 0;
}

Answer 1

x was pointing to an int, then that address was overwritten by x = y; (as you expected) but the space that the previous address pointed to was not released. x and y then contain the same address and therefore both point to the same space in memory, thus your free() calls are both trying to free the same location.

If you don’t want to leak those 4 bytes, put in free(x); just before x = y;