Re: [PATCH v2] netlink: align attributes on 64-bits

On 12/18/12 at 04:50pm, David Laight wrote:

quoted

2/ Suppose that the attribute is:

   struct foo {
   	__u64 bar1;
   	__u32 bar2;
   }
   => sizeof(struct foo) = 12 (= payload)

That is only true if the host architecture aligns 64bit items
on 32 it boundaries (as i386 does).
Otherwise there are 4 bytes of padding at the end and the
size is 16.

Actually it is worse than that.
Consider the structure:
	struct bar {
		__u32 foo1;
		__u64 foo2;
	}
On i386 it will have size 12 and foo2 will be at offset 4.
On sparc32 (and most 64bit) it will have size 16 with foo2
at offset 8 (and 4 bytes of pad after foo1).

This is a known problem and I can't think of anything
that can be done about it except for memcpy()ing the
data before accessing it.

If you have ideas, I'm more that willing to listen :)

Do these messages move between systems?
If they do then any 64bit items need an explicit alignment
eg tag with __attribute__((aligned(8))) (or aligned(4)).

They don't. Netlink has and will be host bound. It also
uses host byte order for that reason.