Re: [lwip-users] Limiting the number of connections: Possible bug with MEMP_NUM_TCP_PCB?

[address@hidden]

Tony wrote:
> The aim is to only allow n open TCP connection at a time, and reject 
> all further connection requests. This worked reasonably well in 1.3.2, 
> but now fails in 2.0.2.
> [..]
> HOWEVER: this n+1 connection terminates the very 1st connection (takes 
> over the 1st PCB?).

Although it's strange it's like that, this is expected behaviour of 
tcp_alloc(). I compared 2.0.2 to 1.3.2 and I don't really see a 
difference though...
Anyway, the pcb limitation might not be the correct solution. A listen 
backlog might better do what you want.
If you want to stay with your pcb limitation, try calling 
"tcp_setprio(newpcb, TCP_PRIO_MAX);" after you allocate the first n 
pcbs. That should prevent tcp_alloc from reusing them.

> Now, if I leave all previous connections open and make a n+1 
> connection (the 6th connection in this example) I hit an exception 
> (the processor tries to access memory that does not exists and I hit 
> the exception handler in the MCU).

Now this indeed looks like a bug in your port, not in lwIP.

> I narrowed down the origin of the fatal exception in tcp_alloc() to 
> the code following this comment:
> /* zero out the whole pcb, so there is no need to initialize members 
> to zero */
>
> The problem at that point seems to be that a struct tcp_pcb_listen 
> (that is still in use) is reused and overwritten as a struct tcp_pcb...

Ehrm, the pcb that is used there should *NOT* be a listen pcb. It comes 
from the MEMP_TCP_PCB pool, so it's a standard pcb. Why do you think 
it's a listen pcb which is still in use? This lets me think you have a 
port problem...

> Is there a way to simply "tell" LWIP to "refuse" further connections?

As I wrote above, give your other connections a higher prio so new 
connections won't kill the existing ones.

> Are there things that I might have missed that need to be changed in 
> my application, when going from 1.3.2 to 2.0.2?

None that I know of. If you find any, please let us know!

Cheers,
Simon