linux-kernel - Re: double exclamation (!!) suckage in the kernel

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Message-ID: <Pine.LNX.4.61.0705261535240.7344@yvahk01.tjqt.qr>
Date:	Sat, 26 May 2007 15:41:16 +0200 (MEST)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	David Miller <davem@...emloft.net>
cc:	satyam.sharma@...il.com, linux-kernel@...r.kernel.org
Subject: Re: double exclamation (!!) suckage in the kernel


On May 25 2007 14:14, David Miller wrote:
>
>WHat is with multiple people asking about "!!" all of a
>sudden today?
>
>> Are all these occurrences merely the debris of
>> s/something/!notsomething/g kind of patches or is there some
>> dark, unknown C / gcc wizardry I have absolutely no clue of?
>
>"!!" is used in contexts where pointers might be being
>tested as well as plain integers, the "!!" turns a pointer
>into the equivalent integer boolean for testing.
>
>NULL pointers become 0
>non-NULL pointers become 1

Though,
	if(!!ptr)
is effectively the same as
	if(ptr)

!! is mostly used where non-zero values need to be mapped to 1.
Not so much in logic expressions (if,while,etc.) but in arithmetic
(who'd do that?).

	Jan
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