SV: SV: [lwip-users] Alignment problem i pbuf_alloc()

[Lars Thorup]

Fra: K.J. Mansley [mailto:address@hidden 
> On Tue, 2004-05-25 at 07:05, Lars Thorup wrote:
> > This is the original code from pbuf_alloc():
> > 
> > case PBUF_RAM:
> >   /* If pbuf is to be allocated in RAM, allocate memory for it. */
> >   p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf) + length + 
> > offset));
> > 
> > I have changed the third line to
> > 
> >   p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf) + offset) + 
> > MEM_ALIGN_SIZE(length));
> 
> Would it be better to have:
> 
> p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf)) + 
> MEM_ALIGN_SIZE(offset + length));
> 
> This makes more sense to me, as "offset" and "length" go 
> together to make the payload of the pbuf, but it may mean the 
> payload pointer starts off pointing at a non-word aligned 
> address, which might cause problems?

No that would not work; I just tried it out and got the same memory
overwrite as with the original code. The explanation is that the "offset"
bytes is not included in the part of the buffer that the payload pointer is
set to point at, but rather in the part before the payload pointer. This can
be seen four lines ahead in the source code:

     p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf) +
offset));

So for now I still vote for my own suggestion :-)

Best regards
Lars Thorup