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    SubjectRe: need for packed attribute
    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
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