发生WSAEWOULDBLOCK时如何判断链接是否断开

非阻塞socket，recv返回-1，WSAGetLastError返回WSAEWOULDBLOCK，无法判断对方是否宕机（因为网络繁忙的时候或者对方没有发送数据的时候也会返回WSAEWOULDBLOCK），此时需要使用其他方法来判断网络是否断开：
recv() will return 0 upon a graceful disconnect, ie the peer shutdown its end of the connection and its socket stack sent a FIN packet to your socket stack. You are pretty much guaranteed to get this result immediately, whether you use a blocking or non-blocking socket.

recv() will return -1 on any other error, including an abnormal connection loss. You need to use WSAGetLastError() to find out what actually happened. For a lost connection on a blocking socket, you will usually get an error code such as WSAECONNRESET or WSAECONNABORTED. For a lost connection on a non-blocking socket, it is possible that recv() may report an WSAEWOULDBLOCK error immediately and then report the actual error at some later time, maybe via select() with an exception fd_set, or an asynchronous notification, depending on how you implement your non-blocking logic.

However, either way, you are NOT guaranteed to get a failure result on a lost connection in any timely manner! It MAY take some time (seconds, minutes, can even be hours in rare cases) before the OS deems the connection is actually lost and invalidates the socket connection. TCP is designed to recover lost connections when possible, so it has to account for temporary network outages and such, so there are internal timeouts. You don’t see that in your code, it happens in the background.

If you don’t want to wait for the OS to timeout internally, you can always use your own timeout in your code, such as via select(), setsocktopt(SO_RCVTIMEO), TCP keep-alives (setsockopt(SO_KEEPALIVE) or WSAIoCtl(SIO_KEEPALIVE_VALS)), etc. You may still not get a failure immediately, but you will get it sooner rather than later.
参考网址：https://stackoverflow.com/questions/54787490/socket-recv-function
windows系统下，可使用如下方法判断：
bool IsSocketClosed(SOCKET clientSocket)
{
bool ret = false;
HANDLE closeEvent = WSACreateEvent();
WSAEventSelect(clientSocket, closeEvent, FD_CLOSE);

DWORD dwRet = WaitForSingleObject(closeEvent, 0);

if(dwRet == WSA_WAIT_EVENT_0)
ret = true;
else if(dwRet == WSA_WAIT_TIMEOUT)
ret = false;

WSACloseEvent(closeEvent);
return ret;
}
参考：https://blog.youkuaiyun.com/e_wsq/article/details/12878885