I'm looking at "TCP echo server example using raw API" and trying to
undrestand it. It creates a listening TCP connection, receives a
packet, sends it back and then closes the connection.
In the initialization function, accept callback is registered in lwip
like this:
void
echo_init(void)
{
echo_pcb = tcp_new();
<...>
echo_pcb = tcp_listen(echo_pcb);
tcp_accept(echo_pcb, echo_accept);
Connection is closed by the server after each echo session,
like this:
void
echo_close(struct tcp_pcb *tpcb, struct echo_state *es)
{
tcp_arg(tpcb, NULL);
tcp_sent(tpcb, NULL);
tcp_recv(tpcb, NULL);
tcp_err(tpcb, NULL);
tcp_poll(tpcb, NULL, 0);
<...>
tcp_close(tpcb);
Documentation says that tcp_close will free pcb structure
(eventually). All of the callbacks that are used for tcp server are
registered with this structure.
But when client sends new packet and starts a new connection, accept
callback is called! Even though tcp_accept(echo_pcb, echo_accept);
(i.e. callback registration) is done only once in the init function
and that echo_pcb structure is already freed after tcp_close.
So I'm confused. I thought callback registration is done on a pcb basis,
i.e. function pointer is copied into pcb. So when pcb is freed all
callbacks should be registered again for a new pcb.
But this doesn't happen for accept callback.
Please tell me, what am I missing here?
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