Messages in this thread | | | Subject | Re: [RFC-PATCH 1/5] unaligned: introduce common header | From | Harvey Harrison <> | Date | Mon, 10 Nov 2008 10:51:24 -0800 |
| |
On Mon, 2008-11-10 at 18:35 +0000, Will Newton wrote: > On Mon, Nov 10, 2008 at 4:51 PM, Harvey Harrison > <harvey.harrison@gmail.com> wrote: > > > > In this particular case, packed isn't right as you know big_data is > > aligned (as long as you can guarantee the struct alignment), so you'd > > probably want: > > > > struct foo { > > u64 big_data; > > u8 small_data; > > u32 medium_data __attribute__((__packed__)); > > } > > > > But that's not what we're talking about in the kernel's case. > > Perhaps that would be a neater way of expressing what is required in > my simple example, but it's fairly common to use packed on the whole > struct which could be because a field that is "packed" by default on > one architecture might not be on another. You could mark every field > as packed but few people seem to do that and as far as I am aware > there is no documented difference between packing all members and the > whole struct. The gcc documentation for packed is pretty short:
Actually it's documented that putting attribute(packed) on the struct is equivalent to putting attribute(packed) on _every_ member of the struct.
> The packed attribute specifies that a variable or structure field > should have the smallest > possible alignment—one byte for a variable, and one bit for a field, > unless you specify a > larger value with the aligned attribute. > > I'd love to know if the pointer alignment behaviour is widespread and > then maybe write a patch for the gcc manual.
Well, it's kind of the whole point of __packed isn't it? Otherwise the struct members get naturally (or some arch-dependent value) aligned, which the compiler can rely on unless you say __packed.
So in my example above, the compiler _knows_ how it has aligned big_data and small_data and can use whatever access is most efficient, but it can't make any assumptions about medium_data, so access through a pointer _must_ be done unaligned.
struct foo *bar; bar->medium_data; // compiler must do this unaligned
Harvey
-- 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/
| |