On Fri, Jun 15, 2007 at 02:42:23PM +0200, Arnd Bergmann wrote:
> That's what I thought as well at first, since this is how the gcc
> documentation seems to describe it. However, recent version of gcc
> complain about this:
>
> gcc-4.1 -Wall -O2 test.c -c
> test.c:1: warning: 'packed' attribute ignored
>
> I have tested versions 2.95, 3.3 and 4.1, an they all ignore do the
> right thing when you do not specify the packed attribute.
...
> We might ask the gcc developers to clarify the documentation, which as of 4.1
> states:
>
> The `aligned' attribute can only increase the alignment; but you
> can decrease it by specifying `packed' as well. See below.
>
> My understanding is that this only applies to statically allocated variables,
> but not to automatic stack variables and to usage of the type inside of
> a data structure.
Here's a program which illustrates the source of confusion:
#include <stdio.h>
#include <stddef.h>
typedef unsigned long long __attribute__((aligned(4))) compat_u64;
struct foo {
int y;
unsigned long long __attribute__((aligned(4))) x;
};
struct bar {
int y;
compat_u64 x;
};
int main(void)
{
printf("offset of foo->x is %lu\n", offsetof(struct foo, x));
printf("offset of bar->x is %lu\n", offsetof(struct bar, x));
return 0;
}
output (on ia64, and I'm told other 64-bit platforms) is:
$ ./test
offset of foo->x is 8
offset of bar->x is 4
I'll try and come up with some wording that works for the GCC manual.
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