关闭套接字。
The FIN indicates that no more data is to be sent by this endpoint, but until the peer closes its end of the socket, it may continue to transmit data.
FIN说明这个端点不会再发送数据，但是在这一端关闭自己这端的套接字之前，它一直可以继续传输数据。
The FIN indicates that no more data is to be sent by this endpoint, but until the peer closes its end of the socket, it may continue to transmit data.
FIN说明这个端点不会再发送数据，但是在这一端关闭自己这端的套接字之前，它一直可以继续传输数据。
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