On Friday 15 June 2007, Matthew Wilcox wrote:
> 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.
I just talked to Ulrich Weigand, who explained to me that
__attribute__((packed)) should not be specified on a typedef that is
not also a struct/union/enum definition, because it can not change the
type anyway.
Also, the attribute((aligned(x))) works differently in a typedef than
in a field or variable declaration:
In your struct foo, __attribute__((aligned(4))) does not have any
effect because the attribute on a field declaration will only increase
the alignment if you specify a larger value than the default alignment
for the member type.
In struct bar, you have two members that both have type with a default
alignment of 4, because the typedef overwrote the default alignment
for the compat_u64 type.
Arnd <><
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
