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Message-ID: <7f45d93905080213152a54106c@mail.gmail.com>
Date: Tue, 02 Aug 2005 20:15:00 -0000
From: Shaun Jackman <sjackman@gmail.com>
Reply-To: Shaun Jackman <sjackman@gmail.com>
To: Dave Korn <dave.korn@artimi.com>
Subject: Re: memcpy to an unaligned address
Cc: Paul Koning <pkoning@equallogic.com>, gcc@sources.redhat.com
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On 8/2/05, Dave Korn <dave.korn@artimi.com> wrote:
>   There are two separate issues here:
>=20
> 1)  Is the base of the struct aligned to the natural alignment, or can the
> struct be based at any address

The base of the struct is aligned to the natural alignment, four bytes
in this case.
=20
> 2)  Is there padding between the struct members to maintain their natural
> alignments (on the assumption that the struct's base address is aligned.)

There is no padding. The structure is defined as
__attribute__((packed)) to explicitly remove the padding. The result
is that gcc knows the unaligned four byte member is at an offset of
two bytes from the base of the struct, but uses a four byte load at
the unaligned address of base+2. I don't expect...
	p->unaligned =3D n;
... to work, but I definitely expect
	memcpy(&p->unaligned, &n, sizeof p->unaligned);
to work. The second case is being optimised to the first case though
and generating and unaligned store.

Cheers,
Shaun