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    SubjectRe: Problem with find_first_bit function and kin



    --- On Sun, 8/17/08, Alexander van Heukelum <heukelum@fastmail.fm> wrote:

    > From: Alexander van Heukelum <heukelum@fastmail.fm>
    > Subject: Re: Problem with find_first_bit function and kin
    > To: oakad@yahoo.com, "Linux kernel mailing list" <linux-kernel@vger.kernel.org>
    > Date: Sunday, August 17, 2008, 4:58 AM
    > On Sat, 16 Aug 2008 12:26:54 -0700 (PDT), "Alex
    > Dubov" <oakad@yahoo.com>
    > said:
    > > It's well may be that I'm just missing
    > something obvious.
    > >
    > > It seems to me that find_first_bit/find_next_bit
    > functions return their
    > > offsets "base 1" - first set bit is
    > "1" and last is "bitmap size". This
    > > means that if only the last bit in the bitmap is set,
    > the returned value
    > > will be indistinguishable from no bits set situation.
    > Moreover, bit
    > > manipulation functions appear to use "base
    > 0" bit addresses, adding to
    > > the
    > > inconvenience.
    > >
    > > Is this a desired behavior? And, if yes, how is one
    > supposed to deal with
    > > last bit of the bitmap?
    >
    > Hi Alex,
    >
    > If this is the behaviour you observe, it's a bug. How
    > did you
    > find out?
    >
    > The intended behaviour is that the bits are enumerated in
    > "base 0"
    > style. If only the last bit in the bitmap is set it should
    > return
    > bitmapsize-1 and if no bit is set it should return
    > bitmapsize.
    > Some architecture-specific code gets the last detail wrong,
    > they
    > return a value that is slightly larger than the bitmap size
    > in
    > some cases.
    >
    > Greetings,
    > Alexander
    > --

    I was compiling some stuff out-of-the tree and got "ffs" linked in instead
    of "__ffs", which caused all this weird behavior (on 64b platform). Sorry
    for the noise.

    But then, why "ffs" behaves differently from "__ffs" and whats the reason
    they both exist?






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