Macro to swap nibble of BYTE source code

It is similar. Just use the appropriate masks (0x000F and 0xF000) and shifts (12) to swap the highest and lowest nibble of a 16 bit value. But don't forget to let the middle bits untouched (return them too):

#define swap(v) ((((v) & 0x000F) << 12) | (((v) & 0xF000) >> 12) | ((v) & 0x0FF0))

Because the assignment is to use a macro and these don't know about the type of the passed argument, it requires also adding a cast to get the desired short result and casting the operand for the right shift to perform an unsigned shift:

#define swap(v) ((short int)((((v) & 0x000F) << 12) | (((unsigned)(v) & 0xF000) >> 12) | ((v) & 0x0FF0)))

Will work whether 16 bit or 32 bit or 64 bit processor

#include<stdio.h>
#define HALFWIDTH() (sizeof(short int) << 2)
#define BITSWAP(p)     (unsigned short)(p << HALFWIDTH()) | (unsigned short)(p >> HALFWIDTH())
int main() {
  unsigned short i;
  printf("half short: %d\n", HALFWIDTH());
  printf("read the number: \n");
  scanf("%hu", &i);
  printf("shift left: %hu\n", (unsigned short)(i << HALFWIDTH()));
  printf("shift right: %hu\n", (unsigned short)(i >> HALFWIDTH()));
  printf("%hu\n", BITSWAP(i));
  return 0;
}