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

[Mountifield, Tony]

> I considered solving the problem by changing the ip_addr struct from:
> struct ip_addr {
>   u32_t addr;
> }
> To something like:
> struct ip_addr {
>   u8_t addr0;
>   u8_t addr1;
>   u8_t addr2;
>   u8_t addr3;
> }
> 
> But I found that this struct is used in tons of places. 
> That's alot of code to 
> rewrite..
> Have anyone solved this problem without introducing padding?

Yes, I had to use a similar approach to what you suggested:

struct ip_addr {
  u8_t addrb[4];
}

I changed the name to ensure the compiler would catch all occurrences.

I then used macros to access the address. When copying, I used structure copy, 
i.e. instead of "p->addr = x->y.addr;" I did "*p = x->y;". I defined htonlb() 
and ntohlb() functions too.

A lot of the required changes are actually in LWIP_DEBUGF statements.

The next thing I found was that similar changes were needed for the seqno and 
ackno in the TCP header, which are also unaligned, and so need redefinition as 
u8_t arrays. It is then necessary to be careful about byte order. Instead of 
rewriting the header in the received buffer, I left the seqno and ackno in 
network byte order in the header, but transformed to native order when copying 
to local variables.

I have hesitated to submit my changes yet, because there are still a few 
kludges and also I haven't yet ensured portability back to CPUs and compilers 
that don't need all this trickery. Also because I've been doing it as paid work.

Cheers,
Tony


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