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。

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1. struct TCP_KEEPALIVE {  
2.     unsigned long onoff;  
3.     unsigned long keepalivetime;  
4.     unsigned long keepaliveinterval;  
5. } ;  
6.   
7. #define SIO_KEEPALIVE_VALS _WSAIOW(IOC_VENDOR,4)  
8.   
9. /* KeepAlive实现 */  
10. void set_keepalive (SOCKET s)  
11. {  
12.     BOOL bKeepAlive = TRUE;  
13.     int nRet = ::setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, (char*)&bKeepAlive, sizeof(bKeepAlive));  
14.     if (nRet == SOCKET_ERROR)  
15.     {  
16.         return ;  
17.     }  
18.     /* 输入参数 */  
19.     struct TCP_KEEPALIVE inKeepAlive = {0};   
20.     unsigned long ulInLen = sizeof(struct TCP_KEEPALIVE);  
21.   
22.     /* 输出参数 */  
23.     struct TCP_KEEPALIVE outKeepAlive = {0};   
24.     unsigned long ulOutLen = sizeof(struct TCP_KEEPALIVE);  
25.     unsigned long ulBytesReturn = 0;  
26.     int ret = 0;  
27.   
28.     /* 设置keepalive 为5秒，并且发送次数为3次 */  
29.     inKeepAlive.onoff             = 1;  
30.     /* 2次keepalive探测间隔时间 */  
31.     inKeepAlive.keepaliveinterval = 5000;   
32.     /* 开始首次keepalive探测前的tcp空闲时间 */  
33.     inKeepAlive.keepalivetime     = 5000;   
34.   
35.     ret = WSAIoctl((unsigned int)s,  
36.             SIO_KEEPALIVE_VALS,  
37.             (LPVOID)&inKeepAlive,   
38.             ulInLen,  
39.             (LPVOID)&outKeepAlive,   
40.             ulOutLen,  
41.             &ulBytesReturn,   
42.             NULL,   
43.             NULL);  
44.     if (ret == SOCKET_ERROR)  
45.     {  
46.         printf ("error: %d\n", WSAGetLastError());  
47.     }  
48. }