interchanging the values of string in C

main ( )
{
 char * s1 = “abcd”;
 char  s2[] = “efgh”; 
printf( “%s %16lu \n, s1, s1); 
printf( “%s %16lu \n, s2, s2);  
 s1 = s2; 
printf( “%s %16lu \n, s1, s1);   
printf( “%s %16lu \n, s2, s2);  
}