Messages in this thread | | | Date | Sat, 26 May 2007 02:26:11 +0530 | From | "Satyam Sharma" <> | Subject | Re: double exclamation (!!) suckage in the kernel |
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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.
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