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

[Leon Woestenberg]

Hello all,

> On Mon, 2004-04-26 at 15:02, address@hidden wrote:
>
>
> > Once the structs are nicely aligned I can see no reason to use struct packing 
> > compiler directives.
> > But of course, we can't be 100% certain until it have been tested.
>
>
> It's not just the initial alignment that is important, it's also the
> layout within the structure.  Without a packed struct (I think - could
> be wrong on this!) you can't guarantee that the compiler won't insert
> gaps (or reorder the elements) within the struct.  Normally this isn't a
> problem as the program is self-consistent, but when you try to access a
> network packet using a struct you would clearly be in all sorts of
> problems if there were gaps in the struct that aren't in the packet.
Exactly what I was thinking of...

and then I wondered if we can test for wrongly layed out header structs
by using compile-time or run-time assertions that do a sizeof(struct 
some_header) check against its intentional size, i.e.:

LWIP_ASSERT(sizeof(struct proto_header) == PROTO_HEADER_SIZE);

Regards,

Leon.