#include <stdio.h>

void swap1 (int *x, int *y);

int main ()
{
  int a, b;
  a = 10; 
  b = 20;
  printf ("Original a = %d    b = %d\n", a, b);
  swap1 (&a, &b);
  printf ("After calling swap1, a = %d b = %d\n", a, b);
  return 0;
}

void swap1 (int *x, int *y)         //Compiled without error but did not work!
{
printf("Before swap, the values of x= %p, y = %p\n", x, y);
printf("Before swap, the contents of x= %d, y = %d\n\n", *x, *y);

  int t;
  t = *x;
  x = y;           // Line 23 Correct way is *x = *y; 
  y = &t;           //Line 24 Correct way is *y = t; But it did change the value of y, cf. the ./a.out part:  y = 0x7fffc542206c.

printf("After swap, the values of x= %p, y = %p\n", x, y);
printf("After swap, the contents of x= %d, y = %d\n\n", *x, *y);

}

yifangt@mint $ ./a.out
Original a = 10    b = 20
Before swap, the values of x= 0x7fffc5422088, y = 0x7fffc542208c
Before swap, the contents of x= 10, y = 20

After swap, the values of x= 0x7fffc542208c, y = 0x7fffc542206c             //because y = &t Line 24
After swap, the contents of x= 20, y = 10

After calling swap1, a = 10 b = 20
yifangt@mint $