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

[Timmy Brolin]

Ok, I found "something" here: 
http://std.dkuug.dk/jtc1/sc22/open/n2794/n2794.txt
It is not the final ANSI C99 document, but the latest publicaly 
available version.
"Each non-bit-field member of a structure or union object is aligned in 
an implementation-defined manner appropriate to its type."

Not exactly a very precise statement, but it does indicate that fields 
are aligned to the natural boundary of it's type.
What do you think?

Timmy


Leon Woestenberg wrote:

> 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). 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.
>
> Regards,
>
> Leon.
>
>
>
> _______________________________________________
> lwip-users mailing list
> address@hidden
> http://mail.gnu.org/mailman/listinfo/lwip-users