怎么判断一个socket连接是否关闭

如何判断一个socket连接是否关闭？
如何判断一个socket连接是否关闭？

------解决方案--------------------
发心跳包
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解决了别忘了告诉我………
------解决方案--------------------
struct tcp_keepalive inKeepAlive, outKeepAlive;
unsigned long ulInLen, ulOutLen, ulBytesReturn;
inKeepAlive.onoff = 1;
inKeepAlive.keepaliveinterval = 30000;
inKeepAlive.keepalivetime = 300000;
ulInLen = sizeof(tcp_keepalive);
ulOutLen = sizeof(tcp_keepalive);
ulBytesReturn = 0;
memset( &outKeepAlive, 0, sizeof(tcp_keepalive) );
if (WSAIoctl(sock, SIO_KEEPALIVE_VALS, (LPVOID)&inKeepAlive, ulInLen,
(LPVOID)&outKeepAlive, ulOutLen, &ulBytesReturn, NULL, NULL) == SOCKET_ERROR){ writelog( "WSAIoctl(keep alive) failed. error code = " + WSAGetLastError()) ;
}

如果 socket 关闭你就会接到通知。
p.s. 我的程序是 IOCP 的，通知从 GetQueuedCompletionStatus() 返回中得到。估计要是用 select() 的话，也可以从 errorlist 中得到；