linux-kernel - Re: somebody dropped a (warning) bomb

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Message-ID: <Pine.LNX.4.61.0702091330380.31955@yvahk01.tjqt.qr>
Date:	Fri, 9 Feb 2007 13:34:41 +0100 (MET)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	David Rientjes <rientjes@...gle.com>,
	Jeff Garzik <jeff@...zik.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: somebody dropped a (warning) bomb


On Feb 8 2007 16:42, Linus Torvalds wrote:
>
>Most C types don't, and some you can't even tell (do pointers generate 
>"signed" or "unsigned" comparisons?

I'd say "neither", because both

    signed void *ptr; and
    unsigned void *xyz;

are impossible (and dull at that). That's why you explicitly will
have to cast one operand in a comparison to make it evident
what sort of comparison is intended, i.e.

    if((unsigned long)ptr < PAGE_OFFSET)

Further, giving again answer to the question whether they generate signed or
unsigned comparisons: Have you ever seen a computer which addresses memory with
negative numbers? Since the answer is most likely no, signed comparisons would
not make sense for me.

> I'll argue that a compiler that 
>generates signed comparisons for them is broken, but it tends to be 
>something you can only see with a standards- conforming proghram if you 
>can allocate memory across the sign boundary, which may or may not be 
>true..)

Jan
-- 
ft: http://freshmeat.net/p/chaostables/
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