Change endianess (e.g. 01101 to 10110 bit-wise)

swap.c

#include <stdio.h>

unsigned int reverse(unsigned char b) {
   b = (b & 0xF0) >> 4 | (b & 0x0F) << 4;
   b = (b & 0xCC) >> 2 | (b & 0x33) << 2;
   b = (b & 0xAA) >> 1 | (b & 0x55) << 1;
   return b;
}

int
main ()
{
  unsigned int c;
  while (1) {
    c = getchar();
    if (c == EOF)
      break;
    putchar(reverse(c));
  };
  return 0;
}

http://stackoverflow.com/questions/2602823/in-c-c-whats-the-simplest-way-to-reverse-the-order-of-bits-in-a-byte