hello i have a question about select()

int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *timeout);

#include <stdio.h>
       #include <stdlib.h>
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>
       int
       main(void)
       {
           fd_set rfds;
           struct timeval tv;
           int retval;
           /* Watch stdin (fd 0) to see when it has input. */
           FD_ZERO(&rfds);
           FD_SET(0, &rfds);
           /* Wait up to five seconds. */
           tv.tv_sec = 5;
           tv.tv_usec = 0;
           retval = select(1, &rfds, NULL, NULL, &tv);
           /* Don't rely on the value of tv now! */
           if (retval == -1)
               perror("select()");
           else if (retval)
               printf("Data is available now.\n");
               /* FD_ISSET(0, &rfds) will be true. */
           else
               printf("No data within five seconds.\n");
           exit(EXIT_SUCCESS);
       }