On Mon, 20 Jun 2011, Nicolas Pitre wrote:
> On Mon, 20 Jun 2011, Alan Stern wrote:
>
> > On Mon, 20 Jun 2011, Alexander Holler wrote:
> >
> > > I see it that way: packed is needed to be sure that at least for struct
> > > ehci_regs there are no padding bytes inbetween the members.
> >
> > But is it _really_ needed?
> >
> > > It might
> > > work without, but that depends on the compiler (-version, architecture,
> > > whatever).
> >
> > Have there _ever_ been _any_ combinations of compiler, version,
> > architecture, whatever, that had unwanted padding bytes in this
> > structure?
>
> This can be determined by simple code inspection.
>
> If you must have struct members which are not aligned to their natural
> size then you need __packed. Example:
>
> struct foo {
> u8 a;
> u16 b;
> u32 c;
> u64 d;
> };
>
> Without __packed, there will be padding between a and b, and between c
> and d.
One byte of padding between a and b is enough. No more is needed, and
the compiler would have to be pretty stupid to add anything else.
> If the order of the members in this struct were reversed, then
> everything would be naturally aligned and no padding between members
> would be inserted.
>
> The size of structures is normally rounded up with padding to the size
> of the largest basic element it contains. Example:
>
> struct foo {
> u64 a;
> u8 b;
> };
>
> Here sizeof(struct foo) would return 16, even if the actual content
> occupies 9 bytes only. That's because the largest basic element is u64
> i.e. 8 bytes. Normally this trailing padding is not an issue, unless
> you have an array of such a struct or if it is a member of another
> struct. If you want to get rid of that padding, you need to use
> __packed again (which of course would make all subsequent instances of
> that structure in your array completely misaligned too).
>
> Two odd exceptions with the old ABI on ARM:
>
> - The alignment of a 64-bit value is always 4 bytes not 8.
>
> - The size of all structures are always rounded up to a 4-byte boundary,
> irrespective of their content.
>
> If you fall into none of the above issues, then you don't need any
> __packed, period.
We don't fall into any of these cases, and therefore as you say, we
don't need packed. Arnd and I have both explained this. So why do you
keep arguing that we do need it?
Alan Stern
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