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

["Satyam Sharma" <satyam.sharma@gmail.com>]

On 5/26/07, Al Viro <viro@ftp.linux.org.uk> wrote:
> 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:

Ugh ... ok, I've embarrassed myself publicly already, so I'll
also be brave enough to take a C quiz here :-)

>         what type should x have for !!x to be a valid expression?

Any integer type (includes pointers)

>         what will be the type of result?

int (I guess boolean for C99?)

>         what are the possible values of the result?

{0, 1}

>         describe which values of x correspond to each possible value of !!x

Russell's mail has already answered that ...

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

Hmmm, looking through the grep output, actually, it seems some of the
occurrences of !! are indeed debris, but yes, most are actually a way to
force the output to {0, 1} when dealing with individual bits in code.

Satyam