Networking: Still waiting for mbuf cluster

Thomas Doerfler Thomas.Doerfler at
Fri Oct 24 07:43:40 UTC 2003

Hello Rolf,

the RTEMS networking stack produces this message, when it ran 
out of mbuf clusters. During startup, the stack allocates a 
certain amount of memory as mbufs. You can increase that 
amount with the 4. and 5. parameter of the 
"rtems_bsdnet_config" data structure. 

But I guess that this will only delay your problem. Maybe your 
application does not free the received mbufs after it has 
processed them? Or you have another mbuf leak in your system.


> My RTEMS application communicates with a Linux-PC via connectionless UDP 
> socket,
> receiving and replying UDP packets every ~20 milliseconds.
> After a few minutes the application stops communicating.
> The application is still running, but no more UDP packets are received.
>  From time to time a message is displayed at the console:
>     "Still waiting for mbuf cluster."
> With [ethereal] I see that correct UDP packets are coming from the PC.
> Additionally the PC is sending ARP requests:
>     "Who has ? Tell"
> which are also not answered by RTEMS.
> What does this mean ?
> How can the UDP communication be restarted ?
> Any suggestions are highly appreciated.
> Rolf.

IMD Ingenieurbuero fuer Microcomputertechnik
Thomas Doerfler           Herbststrasse 8
D-82178 Puchheim          Germany
email:    Thomas.Doerfler at
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