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Re: [lwip-users] Closing TCP connections

From: Sergio R. Caprile
Subject: Re: [lwip-users] Closing TCP connections
Date: Mon, 04 Aug 2014 10:19:59 -0300
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:24.0) Gecko/20100101 Thunderbird/24.6.0


if you do need to detect a particular situation on tcp_err(), then you
can use the arg parameter. You are not getting the pcb because, afaik,
it no longer exists; it has been removed and freed before calling the
callback function; see tcp.c line 393 for example

The tcp_err() callback that will be called is the one you provided as a
parameter when you set the environment for that pcb. How come you don't
know what you setup ?
        tcp_err(mypcb, myerr);

If you will reuse one tcp_err() function for many connections, you will
use the arg parameter, as shown in every example application:
    tcp_arg(mypcb, myapplicationstructurepointer);

A closure action is something like this:
static void myclose(struct tcp_pcb* pcb)
        state = myCLOSING;
        if(tcp_close(pcb) == ERR_OK){
                tcp_recv(pcb, NULL);
                state = myCLOSED;

You need to have a state (or equivalent) and try to do close later if
the call to tcp_close() fails for low memory or whatever. If the other
host sends RST, you are going to close anyway.
To try later, you can use tcp_poll():

        tcp_poll(mypcb, mypoll, POLL_TIME);

static err_t mypoll(void *arg, struct tcp_pcb* pcb)
        if(state == myCLOSING){
                // Retry closing the connection
        } else {
                // Retry sending data we couldn't send before
        return ERR_OK;


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