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

[Leon Woestenberg]

Hello David,

David Aldrich wrote:
> At the risk of hogging the mailing list, I would like to ask another
>
There is no such thing as too much intelligent questions :-)

> my_pcb = tcp_new();
> [my_pcb = ValueA]
>
> tcp_bind(my_pcb);
>
> my_pcb = tcp_listen(my_pcb);
> [my_pcb = ValueB]
>
> tcp_accept(my_pcb);
>
> 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?
Without looking at the code, we happens is that lwIP (re)allocates a
bigger structure. However, if realloc() has free memory available
after the existing structure, you might get the same pointer back; with 
the (unvisible) difference that it now points to a bigger area.

> 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()?
static tcp_pcb *my_pcb = tcp_new();

< do not use the pcb here, for things other than cleaning it up >

my_pcb  = tcp_listen(my_pcb);

< now have your app use this pcb >


If the above is incorrect, pls list, correct me.

However, thing twice about using a static variable; why not have a
connection handle that has all the state information about this
TCP connection, that you pass to all functions?

It is much more scalable and allows for multiple connections in the
future, more state variables you might want to take account of in
the future, etc...

Unless you are really really deeply embedded where every byte counts,
I usually allow my code to scale up.

Leon.