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

[Timmy Brolin]

K.J. Mansley wrote:

> 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.
Easy. I did some research and found theese two ANSI C requirements:
1: It isn't permitted for a type to have an alignment requirement larger 
than its own size.
2: Struct members must be laid out in the declared order.

Hence, if we make the structs properly aligned (ie. all structs on 4byte 
alignments), and no "gaps" in the structs, then the layout will be the 
same on all ANSI C compatible compilers/systems who support 8bit types. 
(lwip does not work on systems without support for 8bit types anyway).

>  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.
It's just a matter of aligning the structs properly.

Timmy