#include <stdio.h>#include <sys/time.h>#include <sys/types.h>#include <unistd.h>intmain(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");     return 0; }