Re: [lwip-users] p->payload == iphdr failed...

[address@hidden]

Ed Sutter wrote:
> Ok, I see what you're talking about in pbuf_header().  I don't know the history
> of the code to comment on whether or not the change should be in the master
> code base; but it would seem to me that if this is forcing a driver to use
> the memcpy loop to build the pbuf chain, then it would be a good improvement.
>   
The reason pbuf_header can't expand to the front is that it doesn't know 
where the memory area that the PBUF_REF points to starts. While 
shrinking works as long as p->len is > 0, expanding to the front doesn't 
work without another pointer in the pbuf struct that tells us the 
original ->payload pointer.

For this reason, it is not a good thing to give this 'feature' to 
everyone as this could very easily lead to memory corruption.

The way to prevent memcpy is to allocate a PBUF_POOL (of the maximum 
size) and pass the ->payload pointer to the MAC which can store the 
receive data directly to that location. This method has some 
disadvantages though: a) with most MACs, you cannot use chained pbufs 
for receiving and b) you cannot use this method with MACs that have 
internal receive buffers.

Just so you know: changing this _is_ on the to-do-list somewhere, but I 
think don't even know whether there is a feature-request for this 
somewhere...

Simon