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Date: Thu, 23 Sep 2004 18:57:00 -0000
From: Joe Buck <Joe.Buck@synopsys.COM>
To: Dave Korn <dk@artimi.com>
Cc: "'Andreas Schwab'" <schwab@suse.de>,
	"'Morten Welinder'" <terra@gnome.org>, gcc@gcc.gnu.org
Subject: Re: signed vs unsigned pointer warning
Message-ID: <20040923091118.A2943@synopsys.com>
References: <jeu0tqhsre.fsf@sykes.suse.de> <NUTMEGL3tAkqzwUUAPk00000053@NUTMEG.CAM.ARTIMI.COM>
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In-Reply-To: <NUTMEGL3tAkqzwUUAPk00000053@NUTMEG.CAM.ARTIMI.COM>; from dk@artimi.com on Thu, Sep 23, 2004 at 12:35:22PM +0100
X-SW-Source: 2004-09/txt/msg01348.txt.bz2

On Thu, Sep 23, 2004 at 12:35:22PM +0100, Dave Korn wrote:
> > "Dave Korn" writes:
> > 
> > > Which I don't think you can, since you can't store negative numbers
> > > in an unsigned type. 
> > 
> > Actually you can, due to the modulo behaviour of unsigned integers.
> > 
> > Andreas.
> 
>   Well, yes, it is physically possible, but it's a kind of type-punning, it
> defies the aliasing rules, and we get into some very deeply
> language-lawyerly issues here, but it's not a valid representation IIUIC and
> therefore invokes undefined behaviour in many circumstances.

The C aliasing rules specifically bless accessing ints as unsigneds, and,
more generally, punning accesses between any type and the unsigned version
of that type.