程序代码：

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int main(void) {

    fd_set rfds;
    
    struct timeval tv;

    int retval;

    FD_ZERO(&rfds);

    FD_SET(0, &rfds);

    tv.tv_sec = 5;

    tv.tv_usec = 0;

    retval = select( 1, &rfds, NULL, NULL, &tv);

    if( retval == -1 )
        
        perror( "select()" );

    else if(retval)

        /* FD_ISSET(0, &rfds) will be true */ 
        printf("Data is available now.\n");
    
    else
        
        printf("No data within five seconds.\n");

    return 0;
}