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

[Leon Woestenberg]

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.