linux-kernel - Re: kfree(0)

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Date:	Wed, 15 Aug 2007 11:32:00 +0200
From:	"Giacomo A. Catenazzi" <cate@...eee.net>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
CC:	Jason Uhlenkott <jasonuhl@...onuhl.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Tim Bird <tim.bird@...sony.com>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: kfree(0) - ok?

Jan Engelhardt wrote:
> On Aug 14 2007 16:21, Jason Uhlenkott wrote:
>> On Tue, Aug 14, 2007 at 15:55:48 -0700, Arjan van de Ven wrote:
>>> NULL is not 0 though.
>> It is.  Its representation isn't guaranteed to be all-bits-zero,
> 
> C guarantees that.

Hmm. It depends on your interpretation of "representation".
On memory a null pointer can have some bit set.

No, see a very recent discussion on austin group list
(which list also few machines that don't have all 0-bits null pointer)

To clarify, from Rationale of C99, section 6.7.8 "Initialization":

: An implementation might conceivably have codes for floating zero
: and/or null pointer other than all bits zero. In such a case,
: the implementation must fill out an incomplete initializer with
: the various appropriate representations of zero; it may not just
: fill the area with zero bytes. As far as the committee knows,
: all machines treat all bits zero as a representation of
: floating-point zero. But, all bits zero might not be the
: canonical representation of zero.

Anyway, I think for kernel it is safe to assume all-zero bit
null pointer.

ciao
	cate
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