linux-kernel - Re: Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009

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Message-ID: <496479EA.1020207@davidnewall.com>
Date:	Wed, 07 Jan 2009 20:16:18 +1030
From:	David Newall <davidn@...idnewall.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Nick Andrew <nick@...k-andrew.net>,
	Linas Vepstas <linasvepstas@...il.com>, david@...g.hm,
	Kyle Moffett <kyle@...fetthome.net>,
	Ben Goodger <goodgerster@...il.com>,
	Robert Hancock <hancockr@...w.ca>,
	linux-kernel@...r.kernel.org,
	"Jeffrey J. Kosowsky" <jeff@...owsky.org>,
	MentalMooMan <slashdot@...eshallam.info>,
	Travis Crump <pretzalz@...hhouse.org>, burdell@...ntheinter.net
Subject: Re: Bug: Status/Summary of slashdot leap-second crash on new years
 2008-2009

Alan Cox wrote:
>>        UTC  equivalent  to  conversion  on  the naive basis that leap seconds are ignored and all
>>        years divisible by 4 are leap years.  This value is not the same as the actual  number  of
>>        seconds between the time and the Epoch, because of leap seconds and because clocks are not
>>        required to be synchronized to a standard reference.
>>     
>
> I'm not sure what you are quoting from but it is out of date on the
> subject of leap years.
>   

The range of signed 32-bit times is 1901 through 2039, which has only
one century, 2000, which is a leap year.  So the caveat for leap years
is correct but unnecessary.


So I've discoverd, at least on Ubuntu, something wonderful and
reassuring.  It already works exactly the way I think is correct.  Look:
I create a test timezone with no daylight saving and one leap second:

    davidn@...auji:~/timetest$ cat tz
    Zone testzone 0:00 0 XXX/YYY
    davidn@...auji:~/timetest$ cat leapseconds
    Leap 2008 Dec 31 23:59:59 + S
    davidn@...auji:~/timetest$ zic -d . -L leapseconds tz

Then the test program, which makes a time_t (what time() returns) for a
few seconds before the leap second, then counts off seconds...

    davidn@...auji:~/timetest$ cat timetest.c
    #include <time.h>
    #include <stdio.h>

    main() {

            setenv("TZ", ":/home/davidn/timetest/testzone", 1);

            struct tm tm1 = { 55, 59, 23, 31, 11, 108 };
            time_t t1 = mktime(&tm1);
            int i;
            for (i = 10; --i; t1++) printf("ctime(%ld) = %s", t1, ctime(&t1));

            return 0;
      

    }


Observe two 23:59:59's.  Apparently it could be better if the second
23:59:59 was 23:59:60, but I prefer it this way.

    davidn@...auji:~/timetest$ ./timetest
    ctime(1230767995) = Wed Dec 31 23:59:55 2008
    ctime(1230767996) = Wed Dec 31 23:59:56 2008
    ctime(1230767997) = Wed Dec 31 23:59:57 2008
    ctime(1230767998) = Wed Dec 31 23:59:58 2008
    ctime(1230767999) = Wed Dec 31 23:59:59 2008
    ctime(1230768000) = Wed Dec 31 23:59:59 2008
    ctime(1230768001) = Thu Jan 1 00:00:00 2009
    ctime(1230768002) = Thu Jan 1 00:00:01 2009
    ctime(1230768003) = Thu Jan 1 00:00:02 2009


Perhaps this is distribution-dependent, but even so, there's no need for
the kernel to drop the second (and it's wrong if it does.)
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