[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: address@hidden
Subject: Re: [lwip-users] p->payload == iphdr failed...
Date: Sat, 08 Nov 2008 17:05:38 +0100
User-agent: Thunderbird (Macintosh/20080914)

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...


reply via email to

[Prev in Thread] Current Thread [Next in Thread]