Never swap with yourself

#include <stdio.h>

#define SWAP(x, y) x ^= y ^= x ^= y

main()

{

  int i = 3;

  int j = 5;

  printf("%d\n", i);

  printf("%d\n", j);

  SWAP(i, j);

  printf("%d\n", i);

  printf("%d\n", j);

  SWAP(i, i);

  printf("%d\n", i);

}

The output.

3
5
5
3
0