[lwip-users] Re: [lwip] Packed structures

[Florian Schulze]

> I guess this depends on the compiler, but in my experience it is only the
> arp_hdr that needs to be packet. The other headers are aligned in such a way
> that 16-bit word accesses are on a 16-bit word boundary, 32-bit accesses are
> on a 32-bit boundary, etc. I guess the protocol designers had this in mind
> when they designed the header layouts.

I will try that out, I hope it will not be aligned on 32bit boundaries.

> When it comes to the best way to pack the structure, I would think that
> something along the lines of a PACK_STRUCT macro and a PACK_STRUCT_FIELD
> macro would be the way to go. The actual #definition of the macro could be
> put in the arch/cc.h file and the structs that needs packing could be defined
> as:
>
> PACK_STRUCT(
> struct a_struct_that_needs_to_be_packed {
>    PACK_STRUCT_FIELD(u8_t a);
>    PACK_STRUCT_FIELD(u32_t b);
> }
> )
>
> But it would looks rather cluttred...

But it would only be for very few headers. The only problem I see with this is
that some compilers don't like #'s in #defines.

Florian

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