replace byte in nr

#include <stdio.h>
#include <stdint.h>

uint64_t replaceByte(uint64_t nr, uint8_t n, int p) {
  uint64_t shiftedN = n << (p * 8);
  uint64_t mask = 0xFF << (p * 8);
  return (~mask & nr) | shiftedN;
}

void showBits(uint64_t x) {
  for (int i = 63; i >= 0; i--) {
    printf("%llu", (x >> i) & 1);
    if (i % 4 == 0) {
      putchar(' ');
    }
  }
  puts("");
}

int main() {
  uint64_t nr;
  uint8_t n;
  int p;

  printf("nr="); scanf("%llu", &nr);
  printf("n="); scanf("%hhu", &n);
  printf("p="); scanf("%d", &p);

  showBits(nr);
  showBits(n << (p * 8ULL));
  showBits(replaceByte(nr, n, p));
  return 0;
}