linux-kernel - Re: bug list: assigning negative values to unsigned variables

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Message-ID: <1264601569.10795.131.camel@localhost>
Date:	Wed, 27 Jan 2010 15:12:49 +0100
From:	Bernd Petrovitsch <bernd@...rovitsch.priv.at>
To:	Julia Lawall <julia@...u.dk>
Cc:	Dan Carpenter <error27@...il.com>, kernel-janitors@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: bug list: assigning negative values to unsigned variables

On Mit, 2010-01-27 at 13:30 +0100, Julia Lawall wrote:
> On Wed, 27 Jan 2010, Bernd Petrovitsch wrote:
> > On Mit, 2010-01-27 at 11:57 +0100, Julia Lawall wrote:
> > > On Wed, 27 Jan 2010, Dan Carpenter wrote:
> > > 
> > > > Fixing the places which assign negative values to unsigned variables is a good janitor task.
> > > 
> > > I had the impression that assignment to -1 was done sometimes as a 
> > > portable way to initialize the variable to 0xffff (for any number of f's).  
Hmm, perhaps some experienced language lawyer can comment on the
"portable".
> > > So perhaps it is not so trivial to fix.
> > Any particular reason that ~0U, ~0UL, and ~0ULL shouldn't do the same
> > (without relying on conversion from signed to unsigned)?
> 
> Then the constant specifies the type?
Yes. And it is necessary as "~0U" assigned to a "unsigned long long int"
won't give "~0ULL".
Otherwise "0" would be a signed int and from then on (starting with
"~0") we are in the C hell of type promotion/conversion from signed to
unsigned and/or back - at least in theory.

	Bernd
-- 
Bernd Petrovitsch                  Email : bernd@...rovitsch.priv.at
                     LUGA : http://www.luga.at

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