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

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Message-ID: <20070525204127.GK4095@ftp.linux.org.uk>
Date:	Fri, 25 May 2007 21:41:27 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Satyam Sharma <satyam.sharma@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: double exclamation (!!) suckage in the kernel

On Sat, May 26, 2007 at 01:53:59AM +0530, Satyam Sharma wrote:
> Hello,
> 
> Grepping through the sources I found 500+ occurrences of double
> exclamation marks before identifier names (such as !!x -- I took care
> to ignore occurrences of !! inside comment blocks, because there
> are plenty of that sort too).
> 
> !! are to be found even in the definitions of common macros such as
> likely() and unlikely(), which hundreds of eyes must have seen over
> the last year or so ...
> 
> 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?

That's a question for a quiz in introductory course on C:
	what type should x have for !!x to be a valid expression?
	what will be the type of result?
	what are the possible values of the result?
	describe which values of x correspond to each possible value of !!x

You have 10 minutes (and that's a fairly generous, actually).
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