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

[Mountifield, Tony]

Hi All

I don't have a strong opinion one way or the other about removing the packing 
macros, but I wanted to reply to one point:

> The macros will have to remain in the code for the Ethernet
> ARP header, if I understand that correctly, so it really
> isn't worth worrying about.

This is no longer correct. Since the sipaddr and dipaddr fields were changed 
from struct ip_addr to struct ip_addr2, they now only have 16-bit alignment 
requirements (this is why they were changed) ...

... so long as compilers don't try to insert padding before and after struct A 
in struct B in the following example:

struct A {
        u16_t x[2];
};

struct B {
        u16_t p;
        struct A q;
        u16_t r;
};

Hopefully:

offsetof(p) is 0
offsetof(q.x[0]) is 2
offsetof(q.x[1]) is 4
offsetof(r) is 6
sizeof(struct B) is 8

Cheers,
Tony


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