linux-kernel - Re: [PATCH] Add "is_power_of

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Message-ID: <Pine.LNX.4.64.0701300753170.23302@CPE00045a9c397f-CM001225dbafb6>
Date:	Tue, 30 Jan 2007 07:56:20 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Paul Mackerras <paulus@...ba.org>, dhowells@...hat.com,
	galak@...nel.crashing.org
Subject: Re: [PATCH] Add "is_power_of_2" checking to log2.h.

On Tue, 30 Jan 2007, Nick Piggin wrote:

> Robert P. J. Day wrote:
> >   Add the inline function "is_power_of_2()" to log2.h, where the value
> > zero is *not* considered to be a power of two.
>
> >
> > Signed-off-by: Robert P. J. Day <rpjday@...dspring.com>
> >
> >  /*
> > + *  Determine whether some value is a power of two, where zero is
> > + * *not* considered a power of two.
> > + */
>
> Why the qualifier? Zero *is* not a power of 2, is it?

no, but it bears repeating since some developers might think it *is*.
if you peruse the current kernel code, you'll find some tests of the
simpler form:

	((n & (n - 1)) == 0))

which is clearly testing for "power of twoness" but which will return
true for a value of zero.  that's wrong, and it's why it's emphasized
in the comment.

rday

-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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