Messages in this thread | | | Date | Wed, 21 Sep 2005 11:30:07 +0200 | From | Andrea Arcangeli <> | Subject | Re: A pettiness question. |
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On Wed, Sep 21, 2005 at 02:01:11PM +0500, Fawad Lateef wrote: > On 9/21/05, Ustyugov Roman <dr_unique@ymg.ru> 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? > > > > 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, but what abt the above case/example ??? PagePrivate is defined as > test_bit and test_bit will return 0 or 1 only ...... So y there is ( > !! ) ??
Note that gcc should optimize it away as long as the asm*/bitops is doing "return something != 0" like most archs do.
Most of the time test_bit retval is checked against zero only, here it's one of the few cases where it's required to be 1 or 0. If you audit all archs then you can as well remove the !! from above.
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