LwIP raw api下使用tcp keep alive

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

// 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 = ;      // ms
    tpcb->keep_intvl = ;     // ms
    tpcb->keep_cnt = ;          // report error after 2 KA without response
}