linux-kernel - Re: timer_settime() and ECANCELED

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Date:   Thu, 2 Apr 2020 21:48:04 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     mtk.manpages@...il.com, linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, arul.jeniston@...il.com,
        "devi R.K" <devi.feb27@...il.com>,
        Marc Lehmann <debian-reportbug@...n9.de>,
        John Stultz <john.stultz@...aro.org>,
        Andrei Vagin <avagin@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>
Subject: Re: timer_settime() and ECANCELED

On 4/2/20 3:35 PM, Thomas Gleixner wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>> NOTES
>>        Suppose  the  following scenario for CLOCK_REALTIME or CLOCK_REAL‐
>>        TIME_ALARM timer that was created with timerfd_create():
>>
>>        (a) The  timer  has  been  started  (timerfd_settime())  with  the
>>            TFD_TIMER_ABSTIME and TFD_TIMER_CANCEL_ON_SET flags;
>>
>>        (b) A discontinuous change (e.g.  settimeofday(2)) is subsequently
>>            made to the CLOCK_REALTIME clock; and
>>
>>        (c) the caller once more  calls  timerfd_settime()  to  rearm  the
>>            timer (without first doing a read(2) on the file descriptor).
>>
>>        In this case the following occurs:
>>
>>        · The  timerfd_settime()  returns  -1 with errno set to ECANCELED.
>>          (This enables the caller to know that  the  previous  timer  was
>>          affected by a discontinuous change to the clock.)
>>
>>        · The  timer is successfully rearmed with the settings provided in
>>          the second timerfd_settime() call.  (This was probably an imple‐
>>          mentation  accident,  but  won't be fixed now, in case there are
>>          applications that depend on this behaviour.)
> 
> Clear enough.
> 
> Thanks Michael!

Thanks. Committed. (But, next time you change the API. maybe a 
man-pages patch to go with that? :-).)

Cheers,

Michael

 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/