quoted

struct {
      u64 fielda
	u32 field;
};

In this case: On 64 bit: the u64 is aligned to 8 and the u32 
is aligned to 4. So
the structure is aligned to 8. A pad is inserted at the end of the
struct to bring it out. On 32 bit, the u64 is aligned to 4, so the
struct is aligned to 4, so no pad is added.

That is true for 32bit i386, some 32bit ABI may require 64bit
alignment for 64bit items.

...

The aligned attribute overrides the automatic determination of the
alignment based on the contents and just forces it.

So, as an example, if you have this hardware layout:

struct {
  u32 fielda;
  u64 fieldb;
} attribute ((aligned(4));

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.

I was actually suggesting defining (modulo syntax errors):
typedef u64 u64_aligned_8 attribute ((aligned(8));

Then you can define:
struct foo {
	u32 fielda;
	u64_aligned_8 fieldb;
};
and know that the aligment should be the same on all systems.
(But I'd still add the explicit pad in the above.)

This is actually more useful when you need to process structures
that have misaligned 64bit items - as happens when doing 'compat 32'
code in a 64bit kernel. Or trying to map the x86 disk mbr.

For hardware facing structures I'd combine this with a static assert
to verify structure size at compile time.

Also useful if you need to ensure a structure doesn't accidentally
change size when you need to preserve a userspace interface.

So..

1) Avoid using attributes unless the structure has unaligned members.
2) Avoid creating structures with unaligned members (eg for userspace
   communication)
3) Frown at hardware/firmware developers who make communication
   structures with unaligned members :)
4) Be explicit about padding in your layout for 64/32
   compatibility.

Agreed.

	David
 


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