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

[Timmy Brolin]

I just ran nested struct tests on gcc+arm. And unfourtunately, it did 
not turn out very well.
gcc+arm does indeed pad structs so that nested structs align to a 4 byte 
boundary. Also, it pads structs at the end to ensure the struct size is 
a factor of 4.
This causes problems in both etharp and eth_hdr.
The eth_addr struct is nested in the etharp struct and the eth_hdr 
struct. Size of the eth_addr struct is obviously 6, so this struct will 
be padded to 8 by gcc.
In etharp there is also the nested "sipaddr" struct. This nested struct 
is not on a 4 byte boundary, so there will be 2 bytes of padding before it.

I see two solutions.
1: Remove the nested ip_addr2 and eth_addr structs, and use the arrays 
directly.
2: Put the PACK_STRUCT directives back in etharp .

I can see a potential problem with the second solution if there is a ARM 
compiler out there who does not support pack directives.
I guess the decision is up to you Leon?

Stupid thing of me not to consider the nested structs in the first place.
Timmy

Leon Woestenberg wrote:

> Hello Jani,
>
> could you please test run the program below to see how the nested 
> struct is aligned under arm-gcc?
>
> > >
> > Actually, I think you are on to something here... Someone said that 
> > gcc for arm puts all structs on 4 byte boundarys in order to use the 
> > efficient load and store multiple instructions... I have not 
> > considered this. It would surely break the etharp struct.
>
>
> This piece of code should show the actual alignments under GCC when 
> using nested structs.
>
> #include <stdio.h>
> #include <stddef.h>
>
> #if 1
> #define PACKED __attribute__ ((packed))
> #else
> #define PACKED
> #endif
>
> struct A {
>     u16_t x[2];
> } PACKED;
>
> struct B {
>     u16_t p;
>     struct A q;
>     u16_t r;
> } PACKED;
>
> int main ()
> {
>         printf ("offsetof(p) = %d\n", offsetof(p));
>         printf ("offsetof(q.x[0]) = %d\n", offsetof(q.x[0]));
>         printf ("offsetof(q.x[1]) = %d\n", offsetof(q.x[1]));
>         printf ("offsetof(r) = %d\n", offsetof(r));
>         printf ("sizeof(struct B) = %d\n", sizeof(struct B));
>         return 0;
> }
>
> /*
> 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
> */
>
>
>
> Thanks, Leon.
>
>
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