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

[Lars Thorup]

Fra: K.J. Mansley [mailto:address@hidden 
> Looks like it, but I'm not sure which bit is wrong.  Out of 
> interest, where does the "6" bytes in the offset come from?  
> I'm guessing some physical layer header?

Yes, we are running on top of an ARCNET, and we use a 6 byte header on the
physical layer.

> I can think of a number of work-arounds, but does anyone have 
> a suggestion for a solid fix?

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));

In this part of the code a single buffer is allocated to hold two parts. The
second part is put on an alignment boundary, so the size of the first part
must be rounded up with a call to MEM_ALIGN_SIZE(sizeof(struct
pbuf)+offset). To make sure the size of the entire buffer is aligned, also
the size of the second part must be rounded up with a call to
MEM_ALIGN_SIZE(length). 

And that seems to solve our problem. Can you confirm this fix and make sure
it gets into CVS?

Best regards
Lars Thorup