On Fri, Mar 23, 2012 at 07:03:37AM -0700, parav.pan...@emulex.com wrote:
> > David is saying you will get a 12 byte struct and fieldb will be unaligned.
> > Since
> > 12 is aligned to 4 no padding is added.
>
> So I decided to experiment above example before implementing in
> driver. However I find structure of 16 bytes (instead of 12) with
> padding after fielda in below example. Am I missing some compiler
> option or syntax error in attribute? Sorry to ask this silly
> question. I tried __attribute__((__aligned__(4))); too based on
> usage in other kernel code.
I got the syntax wrong for that specific case (it is a little
unintuitive.. IMHO, capping the alignment of a container should cap
the alignment of all members, otherwise it is nonsense!):
typedef uint64_t u64_unaligned_8 __attribute__((__aligned__(4)));
struct foo {
uint32_t fielda;
u64_unaligned_8 fieldb;
};
struct foo2 {
uint32_t fielda;
uint64_t fieldb;
};
int main(int argc,const char *argv[])
{
printf("sizeof(foo) = %zu, fieldb = %zu\n",sizeof(struct foo),
offsetof(struct foo,fieldb));
printf("sizeof(foo2) = %zu, fieldb = %zu\n",sizeof(struct foo2),
offsetof(struct foo2,fieldb));
return 0;
}
sizeof(foo) = 12, fieldb = 4
sizeof(foo2) = 16, fieldb = 8
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)
Jason
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