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

[Al Viro <viro@ftp.linux.org.uk>]

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).