[ntp:questions] Purpose of a leap second file?

David J Taylor david-taylor at blueyonder.co.uk.invalid
Thu Feb 16 15:13:41 UTC 2012


"Dave Hart" <hart at ntp.org> wrote in message 
news:CAMbSiYDGA1NK8KJLWfg9bDL+ih0CCNDJuJK1-A_GfDJfvbry_Q at mail.gmail.com...
> On Thu, Feb 16, 2012 at 07:25, David J Taylor
> <david-taylor at blueyonder.co.uk.invalid> wrote:
>> Ah, I found the instructions about using the file:
>>
>>  http://support.ntp.org/bin/view/Support/ConfiguringNTP#Section_6.14.
>
> I've updated that section to give a few more details.

Thanks, more information oftem helps!

>> I had the impression, falsely, from the way people were talking that it 
>> was
>> an OS thing, and not an NTP thing.
>
> It's handled by both ntpd and the OS in the common situation of a
> kernel with precision timekeeping extensions.  ntpd notifies the
> kernel loop discipline which takes care of implementing the insertion.

OK - that's what I have on the FreeBSD system.

>> Still don't know if Windows supports it.
>
> Windows doesn't have those extensions.  When the kernel loop
> discipline is unavailable or disabled, the daemon loop discipline
> implements insertion by stepping the clock backward one second on
> POSIX systems.  On Windows, the clock is run at half speed for two
> seconds.

Yes, but what I don't know if whether the two types of GPS receiver I have 
will tell about the leap second, or whether the message from the Internet 
servers will be enough.  I'll find out in June.

>>  I also see that entry format is
>> different for the current entry compared to all previous, so I hope the
>> white-space count isn't important....
>
> I'm not sure what you're referring to.  Here's the prior entry and the 
> latest:
>
> 3439756800 34 # 1 Jan 2009
> 3550089600 35 # 1 Jul 2012

Well, that's interesting, and thanks for checking.  Using two different 
editors (Notepad++ and Programmer's File Editor, PFE) I get different 
results!  PFE shows a consistent tab separated field all the way down the 
file, but Notepad++ shows a space before the tab on the last three entries 
(2014 & 2015).  The tabs are being expanded differently, sigh!

I'd also found a file: leap-seconds.3535228800  which didn't have those 
future, commented out, entries.

> You can verify the leapsecond file is loaded by seeing tai=34 among
> the output of ntpq -crv
>
> Cheers,
> Dave Hart

I will check that shortly.  Yes, looking good:

  tai=34, leapsec=201207010000, expire=201212280000

Thanks again!

Cheers,
David 



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