linux-kernel - Re: [RFC] Documentation about unaligned memory access

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Date:	Sat, 24 Nov 2007 18:35:25 +0100
From:	Pierre Ossman <drzeus-list@...eus.cx>
To:	Luciano Rocha <strange@....no-ip.org>
Cc:	Daniel Drake <dsd@...too.org>, linux-kernel@...r.kernel.org,
	davem@...emloft.net, kune@...ne-taler.de, johannes@...solutions.net
Subject: Re: [RFC] Documentation about unaligned memory access

On Sat, 24 Nov 2007 17:22:36 +0000
Luciano Rocha <strange@....no-ip.org> wrote:

> On Sat, Nov 24, 2007 at 05:19:31PM +0100, Pierre Ossman wrote:
> > It most certainly does not. gcc will assume that an int* has int alignment. memcpy() is a builtin, which gcc can translate to pretty much anything. And C specifies that a pointer to foo, will point to a real object of type foo, so gcc can't be blamed for the unsafe typecasts. I have tested this the hard way, so this is not just speculation.
> 
> Yes, on *int and other assumed aligned pointers, gcc uses its internal
> version.
> 
> However, my point is that those pointers, unless speaking of packed
> structures, can safely be assumed aligned, while char*/void* can't.
> 

I get the sensation we're violently in agreement here, just misunderstanding each other. :)

_My_ point was that the documentation should mention that normal, unpacked C objects have alignments that influence the code generated by __builtin_memcpy(). As such, one should always make sure to have either src or dst be char*/void* when alignment cannot be guaranteed. The example in the documentation has this, but it isn't explicit that this is required.

Rgds
-- 
     -- Pierre Ossman

  Linux kernel, MMC maintainer        http://www.kernel.org
  PulseAudio, core developer          http://pulseaudio.org
  rdesktop, core developer          http://www.rdesktop.org
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