RE: FW: [iPAQ] skiff tools question

Gang,

> Jamey is correct. I did not see the [1]. I thought it was [0].
>
> Please note that the netwinder uses a kernel handler for this
> issue. I have
> not looked at the code. Does anybody know more????
>
> Check out the ARM/NetWinder Structure Alignment FAQ
> (http://www.netwinder.org/~brianbr/alignment.html)

        Very interesting link. I had assumed that the original problem was due to
endian-ness. Are the x86 and SA1100 both the same endian? This behavior
sure screams big-endian/little-endian to me!

Steve

> > -----Original Message-----
> > From: Jamey Hicks [mailto:jamey@crl.dec.com]
> > Sent: Thursday, September 14, 2000 7:16 PM
> > To: 'Gordon McNutt'; ipaq@handhelds.org
> > Subject: RE: FW: [iPAQ] skiff tools question
> >
> >
> >
> > > I hope it's something that simple. But the following
> > example makes me
> > > worry that it is not, as this example does not use structs:
> > >
> > > u8 buf[4];
> > > u16 *a;
> > > a = &buf[1];
> > > *a = 4;
> >
> > This example shows a difference in behavior with unaligned
> > pointers. This
> > is another area of non-portability in C. You're doing a
> > 2-byte store on an
> > odd address. Depending on compilation and kernel options on
> > the ARM, you
> > will get different results. x86 is specified to work with
> any pointer
> > alignment, but it is one of the few CPU architectures that
> > behave this way.
> >
> > >
> > > Printing the contents of buf shows one thing on the ARM:
> > >
> > > 0xbffffc40 = 0x4
> > > 0xbffffc41 = 0x0
> > > 0xbffffc42 = 0x0
> > > 0xbffffc43 = 0x0
> > >
> > > and another on the i386:
> > > 0xbffff914 = 0x0
> > > 0xbffff915 = 0x4
> > > 0xbffff916 = 0x0
> > > 0xbffff917 = 0x0
> > >
> > > It appears that on the ARM, the address of a variable is
> > the last byte
> > > of the variable, but on the i386 it is the first. It seems
> that this
> > > makes overlaying structs non-portable.
> >
> > Overlaying structures is, in general, non-portable.
> > Sometimes it works, and
> > if you work hard you can sometimes make it work on all the
> > platforms you're
> > interested in.
> >
> > -Jamey
> >
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Received on Fri Sep 15 16:17:22 2000