socket 设备自动重启/断电 recv send阻塞的解决办法
(1)使用socket的KeepAlive机制，当突然断电后，recv函数阻塞超时退出，不会造成程序死锁，或者程序一直阻塞在recv函数的问题，方法如下：
//
BOOL bKeepAlive = TRUE;
nRet=setsockopt(listen_sock, SOL_SOCKET, SO_KEEPALIVE, (char *)&bKeepAlive, sizeof(bKeepAlive));
if (nRet==SOCKET_ERROR)
{
printf("setsockopt failed : %d\n", WSAGetLastError());
return FALSE;
}
struct tcp_keepalive alive_in;
struct tcp_keepalive alive_out;
alive_in.keepalivetime = 5000;
alive_in.keepaliveinterval = 2000;
alive_in.onoff = TRUE;
unsigned long ulBytesReturn = 0;
nRet=WSAIoctl(listen_sock, SIO_KEEPALIVE_VALS, &alive_in, sizeof(alive_in), &alive_out, sizeof(alive_out),
&ulBytesReturn, NULL, NULL);
if (nRet == SOCKET_ERROR)
{
printf("setsockopt failed : %d\n", WSAGetLastError());
return FALSE;
}
//
同时判断设备是否掉线的方法有2个，一个为心跳包，一个还是KeepAlive 机制