用getsockopt来判断，还是蛮准确的
int SocketConnected(int sock)
{
if(sock<=0)
return 0;
struct tcp_info info;
int len=sizeof(info);
getsockopt(sock, IPPROTO_TCP, TCP_INFO, &info, (socklen_t *)&len);
if((info.tcpi_state==TCP_ESTABLISHED))
{
//myprintf("socket connected\n");
return 1;
}
else
{
//myprintf("socket disconnected\n");
return 0;
}
}

判断socket断开