[ntp:hackers] smearing the leap second
hmurray at megapathdsl.net
Sat Jul 11 08:32:21 UTC 2015
john.stultz at linaro.org said:
>> No systems support TAI. There is no API to get time in TAI.
> Linux supports CLOCK_TAI via clock_gettime(). One can also get the
> tai_offset via adjtimex().
Thanks for the correction. I thought I checked, but either I fat-fingered
something or I tried it on an old-old system.
The man page for clock_gettime() doesn't mention CLOCK_TAI, at least not on
any of the boxes I tried. It is defined in /usr/include/linux/time.h (at
least for recent kernels)
Including time.h isn't good enough on many of the systems I tried. Maybe it
would work if I defined the right magic feature symbol.
It gives sensible results if somebody tells the kernel the TAI offset. ntpd
does that if it has a leap file.
If the TAI offset isn't setup, it lies rather than giving an error.
Do you know of any distros that setup and maintain a leap file out of the box?
I couldn't find CLOCK_TAI on NetBSD or FreeBSD.
Getting TAI using the tai_offset via adjtimex() isn't trivial, at least if
you expect your code to do the right thing when running over a leap second.
You have to do something like:
read time in UTC
read tai_offset again
If the tai_offsets differ, you were reading the clock close to the leap
second. Try again.
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