Re: [lwip-users] PBUF_REF: udp_send() has pbuf_dechain() removed. Why?

[David Haas]

Hi Leon,

The problem I was/am having has to do with the way the output driver 
frees pbufs and the differences between tcp and udp. When frames are 
output from tcp, pbuf_dechain() is not called. Yet from UDP it is 
called. In both cases there is a chain of buffers.

When I made this change, I also changed the way pbuf_free() works. It 
now goes through the entire buffer chain decrementing ref on each 
buffer. When I tested this change, I tested it with UDP through the 
socket layer (which does use a no-copy approach) and it seemed to work. 
How is your use of the interface different from the way sockets works?

David.

address@hidden wrote:

> Hello David,
>
> testing our code against the latest CVS version, we ended up with an
> assertion failure all the time.
>
> in udp.c 1.24 to 1.25 udp_send() was changed to have the call to
> pbuf_dechain() removed:
>
>   /* dechain and free the header pbuf */
>   if(hdr != NULL) {
>     ////////   pbuf_dechain(hdr);
>     pbuf_free(hdr);
>   }
>
> Now, the whole chain is freed, after which the application again frees
> its pointer to its own pbuf, resulting in an assertion failure in 
> pbuf_free()!
>
> David, why did you remove the dechain()?
>
> Regards,
>
> Leon Woestenberg
>
> Axon Digital Design
> Phone: +31 13 511 6666
> Fax: +31 13 511 4151
> web: www.axon.tv
>
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