[C] How to check number

char usage[]="Usage: %s [-p port] \n";


     case 'p': /* port number */
      for (cp=optarg; *cp; cp++)
          if (!isdigit((int)*cp)){
              printf("Wrong: port number must be an unsigned integer\n");
              printf(usage,argv[0]);
              exit(1);
          }
      portnum = atoi(optarg);
      break;