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

[David Aldrich]

Hi Leon

Thanks for your response.  So I conclude that if I want to use a simple
global variable for my connection identifier (leaving aside the
arguments about globals, handles etc) I should reassign the global
variable with the value passed to tcp_accept().  Am I correct?

I wonder whether this could be made clearer in rawapi.txt?  This
document explains that tcp_listen returns a new connection identifier,
but it does not explain that the identifier will change again when a
connection is established.

Thanks again

David

> -----Original Message-----
> From: Leon Woestenberg [mailto:address@hidden 
> Sent: Thursday, February 26, 2004 9:49 PM
> To: Mailing list for lwIP users
> Subject: Re: [lwip-users] Newbie question (tcp_pcb* allocation)
> 
> 
> 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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> http://mail.gnu.org/mailman/listinfo/lwip-> users
> 
> 
> 
> 
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