tcp长连接判断对方断开的方法

1、心跳包

2、keepalive检测，对于设置了keepalive来说，当tcp检测到对端socket不再可用时(不能发出探测包,或探测包没有收到ACK的响应包),select会返回socket可读,并且在recv时返回-1,同时置上errno为ETIMEDOUT。此时TCP的状态是断开的。

 struct TCP_KEEPALIVE {
     unsigned long onoff;
     unsigned long keepalivetime;
     unsigned long keepaliveinterval;
 } ;

 #define SIO_KEEPALIVE_VALS _WSAIOW(IOC_VENDOR,4)

 /* KeepAlive实现 */
 void set_keepalive (SOCKET s)
 {
     BOOL bKeepAlive = TRUE;
     int nRet = ::setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, (char*)&bKeepAlive, sizeof(bKeepAlive));
     if (nRet == SOCKET_ERROR)
     {
         return ;
     }
     /* 输入参数 */
     struct TCP_KEEPALIVE inKeepAlive = {0};
     unsigned long ulInLen = sizeof(struct TCP_KEEPALIVE);

     /* 输出参数 */
     struct TCP_KEEPALIVE outKeepAlive = {0};
     unsigned long ulOutLen = sizeof(struct TCP_KEEPALIVE);
     unsigned long ulBytesReturn = 0;
     int ret = 0;

     /* 设置keepalive 为5秒，并且发送次数为3次 */
     inKeepAlive.onoff             = 1;
     /* 2次keepalive探测间隔时间 */
     inKeepAlive.keepaliveinterval = 5000;
     /* 开始首次keepalive探测前的tcp空闲时间 */
     inKeepAlive.keepalivetime     = 5000;

     ret = WSAIoctl((unsigned int)s,
             SIO_KEEPALIVE_VALS,
             (LPVOID)&inKeepAlive,
             ulInLen,
             (LPVOID)&outKeepAlive,
             ulOutLen,
             &ulBytesReturn,
             NULL,
             NULL);
     if (ret == SOCKET_ERROR)
     {
         printf ("error: %d\n", WSAGetLastError());
     }
 }