swap() without any extra variable in C

Suppose x = 10, y = 8
x = x + y ( 10 + 8 = 18 )
y = x - y ( 18 - 8 = 10 )
x = x - y ( 18 - 10 = 8 )
Now x = 8, y = 10

In Code : 

1. // call by reference
2. #include <stdio.h>
3. int a, b;
4. void swap1(void)
5. {
6. int tmp = b;
7. b = a;
8. a = tmp;
9. }
10. void swap2(void)
11. {
12. a ^= b;
13. b ^= a;
14. a ^= b;
15. }
16. void swap3(void)
17. {
18. a = a + b;
19. b = a - b;
20. a = a - b;
21. }
22. int main(void)
23. {
24. scanf("%d%d",&a,&b);
25. printf("%d%d\n",a,b);
26. swap1();
27. printf("%d%d\n",a,b);
28. swap2();
29. printf("%d%d\n",a,b);
30. swap3();
31. printf("%d%d\n",a,b);
32. }

This is correct for any value.