linux-kernel - Re: Is __ffs64 supposed to be zero based?

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Date:	Wed, 06 Nov 2013 12:52:43 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Ben Greear <greearb@...delatech.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Is __ffs64 supposed to be zero based?

Ben Greear wrote:
> Similarly named methods elsewhere seem to indicate it is supposed to be
> ones-based counting (ie, bit (1<<0) would be considred 'bit 1'.

ffs() is defined to use one-based counting:
<http://pubs.opengroup.org/onlinepubs/9699919799/functions/ffs.html>

__ffs() uses zero-based counting.  On gcc, it's likely to be implemented
with__builtin_ctz*(), whose documentation says:
| Returns the number of trailing 0-bits in x, starting at the least
| significant bit position.


Regards,
Clemens
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