Re: nanosleep() uses CLOCK_MONOTONIC, should be CLOCK_REALTIME?

["Bart Van Assche" <bart.vanassche@gmail.com>]

On Mon, Jun 23, 2008 at 11:48 AM, Michael Kerrisk
<mtk.manpages@googlemail.com> wrote:
> On Mon, Jun 23, 2008 at 10:34 AM, Bart Van Assche
> <bart.vanassche@gmail.com> wrote:
>> On Sun, Jun 22, 2008 at 9:35 AM, Michael Kerrisk <mtk.manpages@gmail.com> wrote:
>>> 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?

Sorry, but I don't think that a nanosleep() based on CLOCK_REALTIME
would have reasonable semantics. The first line of the description in
nanosleep()'s manpage says:
"nanosleep()  delays  the  execution  of  the program for at least the
time specified in *req". So you really need CLOCK_MONOTONIC and not
CLOCK_REALTIME.

The reason why CLOCK_REALTIME is mentioned is probably because other
POSIX man pages define three types of clocks: real, virtual and
profiiling. See e.g. the getitimer() man page
(http://www.opengroup.org/onlinepubs/009695399/functions/setitimer.html).
And an overview of all clock types defined by POSIX can be found here:
http://www.opengroup.org/onlinepubs/000095399/functions/clock_getres.html.

Bart.