linux-kernel - Re: kfree(0)

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Message-ID: <46C32366.2040900@zytor.com>
Date:	Wed, 15 Aug 2007 09:01:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
CC:	Rene Herman <rene.herman@...il.com>,
	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:
> 0 is all-bits-zero.
> NULL is 0. ("It is.", above)
> 
> Transitively, this would make NULL all-bits-zero.
> I might have missed something, though, perhaps that the cast to void* makes it
> intransitive.

It does -- a cast from integer to pointer isn't required to be a bitwise
noop.  Machines on which NULL isn't bitwise zero do exist, and the C
standard allows them.  What is almost certainly more common than "all
bits zero is not a NULL pointer" is "any NULL pointer is not necessarily
all bits zero"; there are quite a few machines on which there are
nonzero bitpatterns that are still valid NULL pointers, but an all-zero
memset() will still produce NULL pointers.

However, the particular supersets of the C standard that gcc provides
and which the kernel are written in (the latter being a proper subset of
the former) does not.

	-hpa
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