[PATCH] epoll_wait.2: Clarify that the timeout is measured against CLOCK_MONOTONIC

[PATCH] epoll_wait.2: Clarify that the timeout is measured against CLOCK_MONOTONIC

[Mike Crowe]

The existing page left the choice of clock as ambiguous. My reading of the
kernel implementation is that CLOCK_MONOTONIC is always used since ep_poll
calls schedule_hrtimeout_range, which calls schedule_hrtimeout_range_clock
passing CLOCK_MONOTONIC.

References:
 http://lxr.free-electrons.com/source/fs/eventpoll.c?v=4.7#L1614
 http://lxr.free-electrons.com/source/kernel/time/hrtimer.c?v=4.7#L1785

The "measured against" terminology was borrowed from nanosleep.2.

I've checked all the way back to Linux v2.6.12 where the calculation was
done using jiffies so I think that this has effectively always been true.

Signed-off-by: Mike Crowe <mac-6JwzNyXSCK3QT0dZR+AlfA@public.gmane.org>
---
 man2/epoll_wait.2 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/man2/epoll_wait.2 b/man2/epoll_wait.2
index 9637906..48bb281 100644
--- a/man2/epoll_wait.2
+++ b/man2/epoll_wait.2
@@ -55,6 +55,9 @@ The
 argument specifies the number of milliseconds that
 .BR epoll_wait ()
 will block.
+Time is measured against the
+.B CLOCK_MONOTONIC
+clock.
 The call will block until either:
 .IP * 3
 a file descriptor delivers an event;
-- 
2.1.4

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Re: [PATCH] epoll_wait.2: Clarify that the timeout is measured against CLOCK_MONOTONIC

[Michael Kerrisk (man-pages)]

On 09/15/2016 11:17 AM, Mike Crowe wrote:
> The existing page left the choice of clock as ambiguous. My reading of the
> kernel implementation is that CLOCK_MONOTONIC is always used since ep_poll
> calls schedule_hrtimeout_range, which calls schedule_hrtimeout_range_clock
> passing CLOCK_MONOTONIC.
> 
> References:
>  http://lxr.free-electrons.com/source/fs/eventpoll.c?v=4.7#L1614
>  http://lxr.free-electrons.com/source/kernel/time/hrtimer.c?v=4.7#L1785
> 
> The "measured against" terminology was borrowed from nanosleep.2.
> 
> I've checked all the way back to Linux v2.6.12 where the calculation was
> done using jiffies so I think that this has effectively always been true.

Thanks, Mike. I've applied this patch.

Cheers,

Michael

> Signed-off-by: Mike Crowe <mac-6JwzNyXSCK3QT0dZR+AlfA@public.gmane.org>
> ---
>  man2/epoll_wait.2 | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/man2/epoll_wait.2 b/man2/epoll_wait.2
> index 9637906..48bb281 100644
> --- a/man2/epoll_wait.2
> +++ b/man2/epoll_wait.2
> @@ -55,6 +55,9 @@ The
>  argument specifies the number of milliseconds that
>  .BR epoll_wait ()
>  will block.
> +Time is measured against the
> +.B CLOCK_MONOTONIC
> +clock.
>  The call will block until either:
>  .IP * 3
>  a file descriptor delivers an event;
> 


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