Re: memcpy to an unaligned address

[Shaun Jackman <sjackman at gmail dot com>]

On 8/2/05, Dave Korn <dave.korn@artimi.com> wrote:
>   There are two separate issues here:
> 
> 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.
 
> 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 = 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