Re: A pettiness question.

[Eyal Lebedinsky <eyal@eyal.emu.id.au>]

Ustyugov Roman wrote:
>>Hi, All.
>>
>>    I found there are use double operator ! continuously sometimes in
>>kernel.
>>e.g:
>>
>>    static inline int is_page_cache_freeable(struct page *page)
>>    {
>>        return page_count(page) - !!PagePrivate(page) == 2;
>>    }
>>
>>    Who would like tell me why write like above?
>>
>>
>>    Thanks in advanced.
>>
>>
>>Liyu
> 
> 
> For example,
> 
> 	int test = 5;
> 	!test will be  0,  !!test will be 1.
> 
> This give a enum of {0,1}. If test is not 0, !!test will give 1, otherwise 0.
> 
> Am I right?

Yes, !! converts {zero,not-zero} to {0,1} which is useable
in arithmetic.

--
Eyal Lebedinsky (eyal@eyal.emu.id.au) <http://samba.org/eyal/>
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