A UDP TIME Client

• The TIME service is for computers

• returns seconds since 1-1-1900

• Useful for synchronizing and time-setting

• TCP and UDP versions return time as an integer

• Need to use ntohl to convert

  #define UNIXEPOCH       2208988800      /* UNIX epoch, in UCT secs      */
  /* UNIX epoch starts on 1-1-1970 */
  #define MSG             "what time is it?\n"
  
  int main(int argc, char *argv[])
  {
      char *host = "localhost"; /* host to use if none supplied */
      char *service = "time";   /* default service name         */
      time_t now;               /* 32-bit integer to hold time  */ 
      int s, n;                 /* socket descriptor, read count*/
  
      switch (argc) {
      case 1:
          host = "localhost";
          break;
      case 3:
          service = argv[2];
          /* FALL THROUGH */
      case 2:
          host = argv[1];
          break;
      default:
          fprintf(stderr, "usage: UDPtime [host [port]]\n");
          exit(1);
      }
  
      s = connectUDP(host, service);
  
      (void) write(s, MSG, strlen(MSG));
  
      /* Read the time */
  
      n = read(s, (char *)&now, sizeof(now));
      if (n < 0)
          errexit("read failed: %s\n", strerror(errno));
      now = ntohl((u_long)now);   /* put in host byte order       */
      now -= UNIXEPOCH;           /* convert UCT to UNIX epoch    */
      printf("%s", ctime(&now));
      exit(0);
  }