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/