RE: [PATCH v2] netlink: align attributes on 64-bits
From: David Laight <hidden>
Date: 2012-12-18 16:58:56
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).
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)).
David