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

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Message-ID: <527A798F.7020809@candelatech.com>
Date:	Wed, 06 Nov 2013 09:17:03 -0800
From:	Ben Greear <greearb@...delatech.com>
To:	Clemens Ladisch <clemens@...isch.de>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Is __ffs64 supposed to be zero based?

On 11/06/2013 03:52 AM, Clemens Ladisch wrote:
> 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.

Thanks for the information!

Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com

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