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

[Timmy Brolin]

Jani Monoses wrote:

> > Oh but it does.
> > The fields in the IP and TCP headers are properly aligned relative to
> > the start of the headers by design. The problem is that the ethernet
> >    
>
> packing directives tell the compiler that it should not pad fields in
> the struct. So if you have an int16 it does not reserve 4 bytes for it
> to imporve performance because of alignedness.
> So padding solves a problem for some compilers but does not solve the
> other.
> Consider this struct:
> without the PACK directives all those u16s would be put at 4 byte
> boundaries, not what we want.
Why would they be put on 4 byte boundaries? The natural boundary of u16s 
is 2 bytes, not 4 bytes. Putting a u16 on a 4 byte boundary changes nothing.
There is however a special case problem in the etharp header: Either 
sipaddr or dipaddr will become unaligned. (Which one depends on the size 
of ethhdr).
But this special case has already been discussed and solved.

Timmy

> /** the ARP message */
> struct etharp_hdr {
>  PACK_STRUCT_FIELD(struct eth_hdr ethhdr);
>  PACK_STRUCT_FIELD(u16_t hwtype);
>  PACK_STRUCT_FIELD(u16_t proto);
>  PACK_STRUCT_FIELD(u16_t _hwlen_protolen);
>  PACK_STRUCT_FIELD(u16_t opcode);
>  PACK_STRUCT_FIELD(struct eth_addr shwaddr);
>  PACK_STRUCT_FIELD(struct ip_addr2 sipaddr);
>  PACK_STRUCT_FIELD(struct eth_addr dhwaddr);
>  PACK_STRUCT_FIELD(struct ip_addr2 dipaddr);
> } PACK_STRUCT_STRUCT;
> PACK_STRUCT_END
>
>
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