linux select 学习

#include <stdio.h>   
#include <stdlib.h>   
#include <sys/time.h>  
#include <sys/types.h>  
#include <unistd.h>  
#include <sys/stat.h>  
#include <fcntl.h>  
  
int  
main(void)  
{  
    fd_set rfds;  
    struct timeval tv;  
    int retval;  
  
    int rof = open("aa.tt", O_RDONLY);  
    printf("open aa.tt with fd is %d\n", rof);  
  
    /* Watch stdin (fd 0) to see when it has input. */  
    FD_ZERO(&rfds);  
    FD_SET(0, &rfds);  
    FD_SET(rof, &rfds);  
    printf("FD_ISET: %d\n", FD_ISSET(0, &rfds));  
  
    /* Wait up to five seconds. */  
    tv.tv_sec = 5;  
    tv.tv_usec = 0;  
  
    retval = select(rof + 1, &rfds, NULL, NULL, &tv);  
    /* Don't rely on the value of tv now! */  
  
    if (retval == -1)  
        perror("select()");  
    else if (retval)  
    {  
        printf("retval is %d\n", retval);  
        printf("Data is available now.\n");  
        char cc[20]={0};  
        int rreaded = read(0, cc, 19);  
        printf("rreaded is :%d\n", rreaded);  
        read(rof, cc+rreaded, 19-rreaded);  
        printf("data is %s\n", cc);  
        /* FD_ISSET(0, &rfds) will be true. */  
    }  
    else  
        printf("No data within five seconds.\n");  
  
    close(rof);  
  
    exit(EXIT_SUCCESS);  
}