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今天搞tcp链接弄了一天，前面创建socket,绑定，监听等主要分清自己的参数，udp还是tcp的。好不容易调通了，然后就是一个需求，当客户端主动断开连接时，服务端也要断开连接，这样一下次客户端请求链接的时候才能成功链接。
然后就开始找各种方法。其中简单的是看recv()返回为0,表明断开了链接，但是recv函数始终返回SOCKET_ERROR,找不到原因。。。。。。。。。。。。
现在已经调通了，就是recv()==0时，断开连接。先主要是没有用accept()的socket，当然接收不到东西。。。
参考的方法： 下面来罗列一下判断远端已经断开的方法：
法一： 当recv()返回值小于等于0时，socket连接断开。但是还需要判断 errno是否等于 EINTR，如果errno == EINTR
则说明recv函数是由于程序接收到信号后返回的，socket连接还是正常的，不应close掉socket连接。
法二：
struct tcp_info info;
int len=sizeof(info);
getsockopt(sock, IPPROTO_TCP, TCP_INFO, &info, (socklen_t *)&len);
if((info.tcpi_state==TCP_ESTABLISHED)) 则说明未断开 else 断开
法三： 若使用了select等系统函数，若远端断开，则select返回1，recv返回0则断开。其他注意事项同法一。
法四：
int keepAlive = 1
