Messages in this thread | | | Date | Fri, 15 Jun 2007 07:45:12 -0600 | From | Matthew Wilcox <> | Subject | Re: [PATCH] Introduce compat_u64 and compat_s64 types |
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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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