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Re: [lwip-devel] [bug #27034] Invalid ASSERT in pbuf_alloc()

From: address@hidden
Subject: Re: [lwip-devel] [bug #27034] Invalid ASSERT in pbuf_alloc()
Date: Wed, 15 Jul 2009 21:09:08 +0200
User-agent: Thunderbird (Macintosh/20090605)

Bill Auerbach wrote:
However, I'd like to know what the purpose of

 pbuf_alloc(PBUF_RAW, 0, PBUF_POOL);

is? Since there is no API that supports increasing a pbuf's length
moving the payload to the front (which you can't in this case, because
PBUF_RAW is used), you'll end up with a pbuf of zero length, which is
bound to
lead to problems in other parts of the stack. There are checks for pbuf-
not being zero somewhere... (although I don't remember where and why).

The above is used in PPP,

To me, that's not a reason to allow zero-length pbufs: the PPP code is unfortunately not very actively maintained and thus may (beside this one) contain other bugs or invalid code: lines like

     *((u_char*)nb->payload + nb->len++) = PPP_ESCAPE;

in ppp.c are, strictly seen, bugs, as the pbuf->len value may not be changed by code external to pbuf.c. I'd rather fix that than allowing zero-length pbufs throughout the stack (which is in most cases a bug).

 and I use:

pbuf_alloc( PBUF_RAW, 0, PBUF_REF )

in my application.  It's useful for zero-copy output operations.

So you're fiddling around with pbuf->len, too? Code like that is not supported! In general, application code playing around with lwIP struct members without using API functions provided for that use is (except for some rare cases) not allowed/supported. Unfortunately, with C, it can't really be avoided by our code/files in a decent way. [There are some rare cases like netif->flags= where most available code uses the internal struct members, but I'd vote for introducing API functions/defines for these places, too.]

You should know the length before calling pbuf_alloc() and pass it instead of 0.


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