Re: [lwip-users] Struct packing/alignment problems

[Timmy Brolin]

> Hello all,
>
> Timmy Brolin wrote:
>
> > Why would they be put on 4 byte boundaries? The natural boundary of 
> > u16s is 2 bytes, not 4 bytes. Putting a u16 on a 4 byte boundary 
> > changes nothing.
>
> >
> OK, we all seem to agree on the fact that types should always be 
> aligned on their natural alignment boundaries.
>
> The real question is whether the compiler is allowed to add *extra*
> padding (for performance reasons or whatever).

Agreed.
There is never a performance reason to add extra padding. As long as a 
type is aligned on it's natural boundary or higher, performance is 
allways the same.
I'am fairly certain the ANSI C standard specify that a compiler should 
pad struct fields to their natural boundaries. If we can only verify this...
I have been looking for a copy of the official ANSI C standard document.

> If a compiler is free
> to add extra padding (i.e. other than for alignment reasons) we need
> to keep it from doing this (through packing).
>
> Can we give live examples of extranous padding?
>
> I remember the packing stuff being added for *some* reason, although
> I cannot remember the specific compiler or platform. 

The reason was most probably the troublesome unalignment caused by the 
14 byte ethernet header.

Timmy