Re: [lwip-users] Misalignment problems on PowerPC (little endian mode)

[Timmy Brolin]

> > I have the source which can be compiled if the option 
> > LWIP_MISALIGNMENT is set to 1. If anyone interested and have the same 
> > problem, I am more than willing to share and help. If you guys want 
> > to incorporate it to lwip as alternative solution to structure 
> > packing I am glad I can help. This lowers the stack performance a bit 
> > though.
>
>
> Not to offend you, but personally, I see this as a bug in the compiler 
> you are using and we don't support broken compilers: structure packing 
> _is_ a must for lwIP!
>
> Hope you are still having fun with lwIP!
>
> Simon
Actually, no. Structure packing is not necessarily a must for lwip.
In many cases lwip should work just fine without structure packing. I 
know that Texas Instruments DSP compilers will work with lwip without 
structure packing.
Patches made to lwip several years ago ensured that if ETH_PAD_SIZE is 
set to 2, then no variable will ever be unaligned. This solves most of 
the problems which would otherwise require structure packing.

There are two things a compiler may do which will still mess up the lwip 
structs:
1: Some compilers pad the size of all structs so that the size of a 
struct becomes a multiple of 4.
2: Some comiplers put all structs on an address boundary of 4, 
regardless of the actual boundary requirements of the members of the struct.
Compilers which does any of the above would still require packed 
structs. Compilers which does neither of the above should work without 
packed structs, if ETH_PAD_SIZE is set to 2.

Regards,
Timmy Brolin