[lwip-users] Re: [lwip] sys_mbox_free() problem

[Adam Dunkels]

Hi Horst!

On Wednesday 14 November 2001 11:59, you wrote:
> Here is my question:
> sys_mbox_free(sys_mbox_t mbox) is called in some situations with a not
> empty mbox! I found no possibility of freeing the attached element (it
> seems to me a memp of type MEMP_NETBUF - but who knows?).

You have found a very serious bug since this can lead to memory leakage which 
eventually will render a device useless.

The problem is that when packets are recieved by TCP, a callback function is 
called from TCP. The callback function posts the incoming packet in a mailbox 
which will be read from by the netconn_recv() function. The problem occurs 
when the connection is shut down before all data has been processed by the 
application. To fix this, the queue must be drained when a connection is 
closed.

The way I see it, there are two ways to solve this. In the upcoming 0.5 
version, the sys_arch has a function that can be used to drain the queue; 
sys_arch_mbox_fetch(). Since it has a timeout argument, it can be called with 
a timeout of 1 which means "almost no waiting at all". The other way is to 
implement a new queue using semaphores for just this special case. The queue 
can then be freed in any way when the connection is closed.

Of those two alternatives, I prefer the first. The bad part is of course that 
it won't work with a sys_arch from 0.4.2.

> [I have running lwip on an motorola 68302 processor, SMSC 91C111
> ethernet controller and a propriatary os called srtx]

Great!

/adam
-- 
Adam Dunkels <address@hidden>
http://www.sics.se/~adam
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