LINUX，select

       int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *timeout);

       timeout  is  an  upper bound on the amount of time elapsed before select() returns.  If

       both fields of the timeval stucture are zero, then select() returns immediately.  (This

       is  useful  for  polling.)  If timeout is NULL (no timeout), select() can block indefinitely.

select阻塞超过timeout时间，就返回0。

如果timeout是0，则select直接返回0；

如果timeout是NULL，则select会阻塞，时间不确定。