Re: [lwip-users] Newbie question (tcp_pcb* allocation)

[Leon Woestenberg]

Hello David,

pls. ignore my earlier reply. It did contain a wrong explanation.

I just looked in the code as I became curious what goes on.


David Aldrich wrote:

> my_pcb = tcp_new();
> [my_pcb = ValueA]
>
> tcp_bind(my_pcb);
>
> my_pcb = tcp_listen(my_pcb);
> [my_pcb = ValueB]
>
> tcp_accept(my_pcb);

tcp_new() returns a full TCP protocol control block (PCB).
tcp_listen() returns a minimal PCB, as it does not have to keep 
connection state.

During listening, when a connect is received (SYN | ACK),
tcp_listen_inputs() gets called, which once again, allocates a
full TCP PCB.

Now, as the first (ValueA) had been de-allocated earlier, that
same memory area makes a good chance of being allocated again.

So, your callback is called with ValueC, that happens to be
equal to ValueA. Do not count on it.

Co-developers, is there a way to allocate a listening TCP PCB
from start? The above looks quite over-the-head for simply
accepting a connection, although it does hide the gory details.

Leon.





> Now I understand that tcp_listen reallocates the connection id, so all
> is fine so far,  However, when the connection is established, my accept
> callback gets called with pcb = ValueA.  Surely, pcb should have ValueB
> at this point.  Can anyone explain this please?
>
> I want to keep my connection identifier as a static variable that my
> functions can access to pass to tcp_write etc.  However, presently its
> not clear to me how to assign this variable: use the value returned by
> tcp_new() or tcp_listen()?
>
> Thank you
>
> David
>
>
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