swapnib.c

#include <stdio.h>
#include <limits.h>

// works for 8 bits
unsigned swapnib8(unsigned n)
{
   return (n << 4) & 0xFF | n >> 4;
}

unsigned swapnib(unsigned n)
{

  if (n == 0) return n;
  unsigned lastbyte = swapnib8(n & 0xFF);
  unsigned rest = swapnib(n >> 8) << 8;
  return rest | lastbyte;
  /*
  printf("%x\n", n);
  return n ?  (swapnib(n >> 8) << 8) | swapnib8(n & 0xff) : n;
  */
}

unsigned swapnib2(unsigned n)
{
  unsigned r = 0;
  for (unsigned m4 = 0xF; m4; m4 <<= 4) {
    r |= ((n & m4) << 4);
    m4 <<= 4;
    r |= (n & m4) >> 4;
  }
  return r;
}

int main(void)
{
    unsigned n = 0xdeadbeef;
    printf("%x %x\n", n, swapnib(n));
    return 0;
}