On 4/2/20 10:49 AM, Thomas Gleixner wrote:
> Michael,
>
> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes:
>> On 4/1/20 7:42 PM, Thomas Gleixner wrote:
>>> (b): Arming the timer in that case is indeed very questionable, but it
>>> could be argued that because the clock was set event happened with
>>> the old expiry value that the new expiry value is not affected.
>>>
>>> I'd be happy to change that and not arm the timer in the case of a
>>> pending cancel, but I fear that some user space already depends on
>>> that behaviour.
>>
>> Yes, that's the risk, of course. So, shall we just document all
>> this in the manual page?
>
> I think so.
>
> Thanks,
Okay. How is the following?
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.)
Thanks,
Michael
diff --git a/man2/timerfd_create.2 b/man2/timerfd_create.2
index ec137fbfe..98225dcad 100644
--- a/man2/timerfd_create.2
+++ b/man2/timerfd_create.2
@@ -477,6 +477,9 @@ is not a valid timerfd file descriptor.
.BR timerfd_settime ()
can also fail with the following errors:
.TP
+.B ECANCELED
+See NOTES.
+.TP
.B EINVAL
.I new_value
is not properly initialized (one of the
@@ -493,6 +496,52 @@ These system calls are available on Linux since kernel 2.6.
25.
Library support is provided by glibc since version 2.8.
.SH CONFORMING TO
These system calls are Linux-specific.
+.SH NOTES
+Suppose the following scenario for
+.BR CLOCK_REALTIME
+or
+.BR CLOCK_REALTIME_ALARM
+timer that was created with
+.BR timerfd_create ():
+.IP (a) 4
+The timer has been started
+.RB ( timerfd_settime ())
+with the
+.BR TFD_TIMER_ABSTIME
+and
+.BR TFD_TIMER_CANCEL_ON_SET
+flags;
+.IP (b)
+A discontinuous change (e.g.
+.BR settimeofday (2))
+is subsequently made to the
+.BR CLOCK_REALTIME
+clock; and
+.IP (c)
+the caller once more calls
+.BR timerfd_settime ()
+to rearm the timer (without first doing a
+.BR read (2)
+on the file descriptor).
+.PP
+In this case the following occurs:
+.IP \(bu 2
+The
+.BR timerfd_settime ()
+returns \-1 with
+.I errno
+set to
+.BR ECANCELED .
+(This enables the caller to know that the previous timer was affected
+by a discontinuous change to the clock.)
+.IP \(bu
+The timer
+.I "is successfully rearmed"
+with the settings provided in the second
+.BR timerfd_settime ()
+call.
+(This was probably an implementation accident, but won't be fixed now,
+in case there are applications that depend on this behaviour.)
.SH BUGS
Currently,
.\" 2.6.29
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
