观察select超时的表现

#include <sys/time.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
#include <sys/select.h>

int main()
{
    int keyboard;
    int ret,i;
    char c;
    fd_set readfd;
    struct timeval timeout;
    keyboard = open("/dev/tty",O_RDONLY|O_NONBLOCK);
    assert(keyboard>0);
    while(1){
        timeout.tv_sec = 5;
        timeout.tv_usec = 0;
        FD_ZERO(&readfd);
        FD_SET(keyboard,&readfd);
        ret = select(keyboard+1,&readfd,NULL,NULL,&timeout);
        if(ret == -1){
            perror("select error!
");
        }
        else if(ret){
            if(FD_ISSET(keyboard,&readfd)){
                i = read(keyboard,&c,1);
                if('
'==c)
                    continue;
                printf("The input is %c
",c);
                if('q'==c)
                    break;
            }
        }
        else if(ret == 0){
            printf("time out !
");
        }
    }
    return 0;
}