[lwip-users] Global reference to pbufs

[Enrico Murador - Research & Development - CET]

Hi all,

On my application (TCP-server based with callback API) I need to limit 
the number of TCP connections in this way:

the application keeps a list of connection information structures of 
limited size; when a client requests a connection,
the accept callback searches the list for an empty slot. If all the 
slots are filled (so the maximum number of connections
is reached) the callback searches for the oldest "inactive" connection 
(if present) and closes it; then assigns the slot
to the new connection (a connection is marked as "inactive" after every 
successful query-response cycle, or after a
timeout).
To make this working, I have to keep a reference to the connection pcb 
as a parameter of the connection info structure;
this reference will be used to close the connection from the accept 
callback of another connection, as I explained.

But how can I ensure that the reference is still valid when I use it to 
close the connection?
(Of course, I have to free the relative connection info structure when 
the receive callback is called for a pcb with NULL data...)

Another case in wich I must check the validity of the pcb reference is 
when I have to send data to the client outside of a callback
(in this case I think it is also necessary to know if the connection is 
in a state that allows transfer of data, but maybe I'm wrong...).

Is it possible to "ask" the TCP stack to check if a pcb reference is 
still "usable"?
Actually, when I have to close a pcb outside of one of its callbacks, I 
first check if the pcb is referenced in one of the
tcp_listen_pcbs, tcp_active_pcbs, tcp_bound_pcbs and tcp_tw_pcbs lists; 
if the reference is not found, I consider that the pcb is
already deallocated.
Similarly, before I have to call tcp_write outside callbacks, I check 
for a reference but only in tcp_active_pcbs list.
But I'm not sure that this approach is correct...

Thank you very much

Enrico