使用select为描述符设置超时

int readable_timeo(int fd, int sec)
{
    fd_set            rset;
    struct timeval    tv;

    FD_ZERO(&rset);
    FD_SET(fd, &rset);

    tv.tv_sec = sec;
    tv.tv_usec = 0;

    return (select(fd+1, &rset, NULL, NULL, &tv));
}

int writeable_timeo(int fd, int sec)
{
    fd_set            wset;
    struct timeval    tv;

    FD_ZERO(&rset);
    FD_SET(fd, &rset);

    tv.tv_sec = sec;
    tv.tv_usec = 0;

    return (select(fd+1, NULL, &wset, NULL, &tv));
}