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

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Date:	Fri, 25 May 2007 22:37:30 +0200
From:	Björn Steinbrink <B.Steinbrink@....de>
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 2007.05.26 01:53:59 +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?

It avoids useless warnings. If you have a pointer and want to store if
it is NULL/non-NULL in an integer variables, you have to use !x or !!x.
Just using x will result in a warning that you convert a pointer to an
integer without a cast.

Given this example:

int main()
{
	int ret;
	void *foo;

	ret = foo;
	ret = !!foo;

	return 0;
}

gcc will emit this warning:
ttt.c:6: warning: assignment makes integer from pointer without a cast

HTH
Björn
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