linux-kernel - Re: nanosleep() uses CLOCK_MONOTONIC, should be CLOCK

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Message-ID: <cfd18e0f0806230248g3a2ab795s63de2b96538ebdd0@mail.gmail.com>
Date:	Mon, 23 Jun 2008 11:48:00 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Bart Van Assche" <bart.vanassche@...il.com>
Cc:	"Michael Kerrisk" <mtk.manpages@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"john stultz" <johnstul@...ibm.com>, "Ingo Molnar" <mingo@...e.hu>,
	"Roman Zippel" <zippel@...ux-m68k.org>
Subject: Re: nanosleep() uses CLOCK_MONOTONIC, should be CLOCK_REALTIME?

Bart,

On Mon, Jun 23, 2008 at 10:34 AM, Bart Van Assche
<bart.vanassche@...il.com> wrote:
> On Sun, Jun 22, 2008 at 9:35 AM, Michael Kerrisk <mtk.manpages@...il.com> wrote:
>> Thomas,
>>
>> (I gues you are the right target for this?)
>>
>> The POSIX.1 specification of nanosleep() says:
>>
>>       But, except for the case of being interrupted by a signal, the
>>       suspension time shall not be less than the time  specified  by
>>       rqtp, as measured by the system clock CLOCK_REALTIME.
>>
>>
>> However, reading kernel/hrtimer.c:sys_nanosleep(), it appears that
>> CLOCK_MONOTONIC is used.
>>
>>    return hrtimer_nanosleep(&tu, rmtp, HRTIMER_MODE_REL, CLOCK_MONOTONIC);
>>
>> Is there a reason to use CLOCK_MONOTONIC, instead of CLOCK_REALTIME?  Is it
>> intentional?  If yes, then I should document this in the man-pages.  If not,
>> then it should be fixed.
>
> CLOCK_MONOTONIC works fine even if ntpd steps the clock forward or
> backward, CLOCK_REALTIME not. So the man page should be fixed.

Thanks for your reply, but I'm not quite convinced yet.  The things
is: the Solaris man page also says "CLOCK_REALTIME".  (Of course that
man page may just be parroting the standard.)  Could there not be some
reasonable semantics for a nanosleep() that was based on
CLOCK_REALTIME?

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
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