#include <stdio.h>

void swapbytes(char * p1, char *p2, int numbytes)
{
   for (int i = 0; i < numbytes; i++) 
      *(p2+numbytes-i-1) = *(p1+i);
}

int main(void)
{
   int i = 20, j = 0;
   void *pi, *pj;
   pi = &i, pj = &j;

   printf("i=%d j=%d\n", i, j);

   swapbytes((char *) pi, (char *) pj, 4);

   printf("i=%d j=%d\n", i, j);

   int k = 0;
   void *pk = &k;

   swapbytes((char *) pj, (char *) pk, 4);

   printf("i=%d j=%d k = %d\n", i, j, k);
 
   return 0;
}