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

[K.J. Mansley]

On Mon, 2004-04-26 at 14:53, address@hidden wrote:
> Quoting "K.J. Mansley" <address@hidden>:
> 
> > On Mon, 2004-04-26 at 11:47, Jani Monoses wrote:
> > > > That's alot of work you have done there.
> > > > A decent solution to this alignment problem is really needed, because as
> > 
> > > > it is now, lwip is incompatible with most 32bit embedded CPUs.(!!) (Such
> > 
> > > > as ARM, TI DSPs, ...)
> > > 
> > > more correctly incompatible with some compilers. lwip works fine on ARM
> > using gcc.
> > 
> > Quite.  I wonder if the effort to get lwip to work with the compilers
> > that currently have problems could be better spent getting the compilers
> > to work with packed structs!  I doubt this is feasible though, so we are
> > going to have to address this recurrent problem sooner or later.
> > 
> > Kieran
> 
> gcc must do some very weird stuff to get unaligned packed structures to work 
> on 
> ARM.

gcc does weird things full stop!

> lwip currently depends highly on compiler specific extensions to compile. 
> Structure packing for unaligned memory access is not ANSI C. Don't blaim the 
> compilers when it is the code that is wrong!

If you can suggest a way to ensure the layout of a struct in ANSI C,
without reducing all structs to byte arrays and so moving the complexity
into the rest of the code, that would be great.  There have been many
suggestions so far on this topic, but none is perfect, so there will
have to be compromise somewhere.  On this issue I think I agree with you
(albeit from a practical standpoint) that we won't get support for
packed structs from all compilers, so we have to provide some means to
work around it.

> gcc is not the C language definition, ANSI C is. I believe good code should 
> be 
> written to comply with ANSI C, not gcc. Don't you think? :)

Sometimes, yes, other times no :)  (i.e. There are sometimes good
reasons for using extensions to a language.)

Kieran