LwIP raw api下使用tcp keep alive

// First, we should enable this macro in lwipopts.h
#define LWIP_TCP_KEEPALIVE                 1

// The following code is implemented after tcp_new() or in tcp_connected call back
... xxx_connected(..., struct tcp_pcb* tpcb, ...)
{
    tpcb->so_options |= SOF_KEEPALIVE;
    tpcb->keep_idle = 5000;      // ms
    tpcb->keep_intvl = 5000;     // ms
    tpcb->keep_cnt = 2;          // report error after 2 KA without response
}