Re: CVS commit: src/sys/dev/scsipi

[khym%azeotrope.org@localhost]

On Fri, Dec 01, 2006 at 08:36:23PM +0100, Christian Biere wrote:
> I suggest using STATIC_ASSERT(sizeof(struct whatever) == whatever) wherever
> size matters.

Interesting... I didn't think that the aligned attribute would change
the sizeof the struct, but it does :( The gcc documentation (info gcc)
doesn't seem to make any mention of that (although I suppose if you set
the attribute on a structure member, it'd obviously have to increase
the sizeof the struct. I didn't think it'd do it when aligning the
struct itself.)

So how does one ensure that the start of the struct is aligned to a
32-bit boundary, but the sizeof the struct stays unchanged?

In an off-list email, Martin Husemann had suggested using a union to
force the appropriate alignment, but it looks like that doesn't work:

#include <stdio.h>

struct odd {
        char x[3];
} __attribute__((__packed__));

struct evenunion {
        union {
                int i_x;
                char x[4];
        } u_x;
        char y[4];
} __attribute__((__packed__));

int main(void)
{
        struct odd o;
        struct evenunion e;

        printf("odd is at %p, size is %d\n", &o, sizeof(o));
        printf("evenunion is at %p, size is %d\n", &e, sizeof(e));

        return 0;
}

odd is at 0x1ffffe638, size is 3
evenunion is at 0x1ffffe63b, size is 8

However, removing the __packed__ attribute from struct evenunion does
make it work. Is __packed__ necessary when all the members are chars?