IP Address to String Conversion in C

ip_to_str.c

//convert an integer IPv4 address to a string OR a string to an integer IPv4 address


//STR to IP
static unsigned int str_to_ip(char *str) {
  //assumes str is properly formatted IPv4 address
  register unsigned int ip, octet;
  register char *tail = str;
  while(*tail++)
    ; //work backwards to track decimal position
  tail -= 2;

  ip = *tail-- - '0';
  ip += (*tail == '.') ? 0 : (*tail-- - '0') * 10;
  ip += (*tail == '.') ? 0 : (*tail-- - '0') * 100;
  tail -= 1;
  octet = *tail-- - '0';
  octet += (*tail == '.') ? 0 : (*tail-- - '0') * 10;
  octet += (*tail == '.') ? 0 : (*tail-- - '0') * 100;
  tail -= 1;
  ip |= octet << 8;
  octet = *tail-- - '0';
  octet += (*tail == '.') ? 0 : (*tail-- - '0') * 10;
  octet += (*tail == '.') ? 0 : (*tail-- - '0') * 100;
  tail -= 1;
  ip |= octet << 16;
  octet = *tail-- - '0';
  octet += (tail < str) ? 0 : (*tail-- - '0') * 10;
  octet += (tail < str) ? 0 : (*tail - '0') * 100;
  ip |= octet << 24;

  //IP in network byte order (big-endian)
  return ip;
}


//IP to STR
static unsigned int ip_to_str(unsigned int ip, unsigned char* buff) {
  //IP in network byte order (big-endian)
  //buffer must be at least 16 char/bytes
  register unsigned char *tail = buff, tmp;
  register unsigned int n, num, count = 0;
  unsigned char octet[4];
  //compute octets in reverse order, because they will need to be reversed later
  octet[0] = ip & 255;
  octet[1] = (ip >> 8) & 255;
  octet[2] = (ip >> 16) & 255;
  octet[3] = ip >> 24;

  for(int i = 0; i < 4; i++) {
    num = octet[i];
    *tail = '0';
    tail += !num; //in case of octet == 0
    count += !num;
    //print decimal values backwards. required for decimal position tracking
    while(num > 0) {
      n = num % 10;
      num /= 10;
      *tail++ = n + '0';
      count += 1;
    }
    *tail++ = '.';
  }
  count += 3;
  tail -= 1;
  *tail-- = 0; //overwrite trailing '.' with null terminator

  //reverse whole string, decimal values and octet order.
  while(buff < tail) {
    tmp = *buff;
    *buff++ = *tail;
    *tail-- = tmp;
  }

  return count;
}