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Date: Mon, 12 Mar 2007 21:52:02 +0800
From: "Cong WANG" <xiyou.wangcong@gmail.com>
To: davids@webmaster.com, linux-kernel@vger.kernel.org,
       "Jan Engelhardt" <jengelh@linux01.gwdg.de>
Subject: Re: Style Question
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2007/3/12, David Schwartz <davids@webmaster.com>:
>
> > NULL has the same bit pattern as the number zero. (I'm not saying the bit
> > pattern is all zeroes. And I am not even sure if NULL ought to
> > have the same
> > pattern as zero.) So C++ could use (void *)0, if it would let itself :p
>
> They don't have to have the same bit pattern. There's no logical reason a
> NULL pointer couldn't have all bits set and the number zero have all bits
> cleared.
>
> Casts are perrmited to change the bit pattern. For example '(float) 7' can
> result in a different bit pattern than '7' and similarly '(void *) 0' can
> result in a different bit pattern from '0'.
>
> As a trivial example, consider an LP64 system. NULL will have the bit
> pattern of 64 zero bits, while '0' will have the bit pattern of 32 zero
> bits.
>
> DS

I agree. C99 standard just says:

6.3.2.3 Pointers

"3  An integer constant expression with the value 0, or such an
expression cast to type void *, is called a null pointer constant. If
a null pointer constant is converted to a pointer type, the resulting
pointer, called a null pointer, is guaranteed to compare unequal to a
pointer to any object or function."