winsock select

MSDN中，有：

select

The select function determines the status of one or more sockets, waiting if necessary, to perform synchronous I/O.

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

Parameters

nfds

[in] Ignored. The nfds parameter is included only for compatibility with Berkeley sockets.

readfds

[in, out] Optional pointer to a set of sockets to be checked for readability.

writefds

[in, out] Optional pointer to a set of sockets to be checked for writability.

exceptfds

[in, out] Optional pointer to a set of sockets to be checked for errors.

timeout

[in] Maximum time for select to wait, provided in the form of a TIMEVAL structure. Set the timeout parameter to null for blocking operations.

http://hi.baidu.com/ytmayer/blog/item/602698667aa09422aa184cfd.html中，有：

struct timeval* timeout是select的超时时间，这个参数至关重要，它可以使select处于三种状态。

第一，若将NULL以形参传入，即不传入时间结构，就是将select置于阻塞状态，一定等到监视文件描述符集合中某个文件描述符发生变化为止；第二，若将时间值设为0秒0毫秒，就变成一个纯粹的非阻塞函数，不管文件描述符是否有变化，都立刻返回继续执行，文件无变化返回0，有变化返回一个正值；第三，timeout的值大于0，这就是等待的超时时间，即select在timeout时间内阻塞，超时时间之内有事件到来就返回了，否则在超时后不管怎样一定返回，返回值同上述。