Re: need for packed attribute

Mikael Pettersson
2006-01-12T16:30:52Z

Russell King writes:
> > As fas as I can tell, the AAPCS document (v2.03 7th Oct 2005) requires
> > that a simple "struct foo { unsigned char c; };" should have both size
> > and alignment equal to 1, but gcc makes them both 4. Do you have any
> > information about why gcc is doing this on ARM/Linux? Is there an accurate
> > ABI document for ARM/Linux somewhere?
>
> That's the new EABI, which is a major change to the existing ABI which
> the kernel and all of userspace is currently built using.
>
> The old ABI has it's roots in 1993 when the kernel and userland was
> initially built using an ANSI C compiler, and the work being done to
> port GCC was to make it compliant with that version of the ABI. This
> ABI is documented only in dead-tree form.
>
> Due to lack of manpower on the Linux side (iow, more or less just me)
> this became the ABI of the early ARM Linux a.out toolchain. At that
> time, I did not consider this to be a problem - it wasn't a problem
> as far as the kernel was concerned.
>
> When ELF came along, other folk worked on the toolchain, but they stuck
> with that ABI - you could not transition between the a.out ABI to the
> ELF ABI without breaking the kernel - structure layouts would change.
>
> Hence, this is the existing ABI we have. Changing the padding or
> alignment of structures changes the kernel ABI, making it incompatible
> with current userland.

OK, thanks for this info. It means that GCC is the definitive authority
on calling conventions and data layouts, not the AAPCS; I wasn't aware of
that before.

(My interest in this issue comes from working on a port of a functional
programming language's JIT compiler and runtime system to XScale.)

/Mikael