On 5/26/07, Al Viro <[email protected]> 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
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