leap seconds and the stdc library Post: 94268

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Top Forums UNIX for Advanced & Expert Users leap seconds and the stdc library Post 94268 by jim mcnamara on Wednesday 28th of December 2005 02:50:01 PM

12-28-2005

Registered User

leap seconds and the stdc library

I understand the NTP protocol, so keeping system time updated is not a problem.

Standard C library routines like localtime() take a number of UTC seconds elapsed since the start of the epoch (Jan 1, 1970). These times in seconds can be a filetime, system time, or some other time in the past or the future.

In order to translate the seconds into years, months, days, etc. correctly the routines have to know about leap seconds. Including the one coming up this Dec 31, there are 23 leap seconds since 1970.

How is this accomplished? -- knowing about newly hatched leap seconds?
BTW HPUX 11.00 C runtine does not do this correction.... VAX C did.

Code:

/* leapsecond.c */
#include <time.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
	time_t when=strtoul(argv[1],(char **)0,10);
	struct tm *tmptr=gmtime(&when);
	
	printf("%s\n",asctime(tmptr));
	return 0;
}

gives this result:
kcsdev:/home/jmcnama> leapsecond 0
Thu Jan  1 00:00:00 1970

kcsdev:/home/jmcnama> leapsecond `echo "365*2*86400"|bc`
Sat Jan  1 00:00:00 1972

kcsdev:/home/jmcnama> leapsecond `echo "365*3*86400"|bc`
Sun Dec 31 00:00:00 1972

kcsdev:/home/jmcnama> leapsecond `echo "(365*3*86400)+86400"|bc`
Mon Jan  1 00:00:00 1973

There were two leap seconds in 1972, the translation does not take leap seconds into account.

jim mcnamara

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time

TIME(2) 						     Linux Programmer's Manual							   TIME(2)

NAME

       time - get time in seconds

SYNOPSIS

       #include <time.h>

       time_t time(time_t *t);

DESCRIPTION

       time() returns the time as the number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

       If t is non-NULL, the return value is also stored in the memory pointed to by t.

RETURN VALUE

       On success, the value of time in seconds since the Epoch is returned.  On error, ((time_t) -1) is returned, and errno is set appropriately.

ERRORS

       EFAULT t points outside your accessible address space.

CONFORMING TO

       SVr4, 4.3BSD, C89, C99, POSIX.1-2001.  POSIX does not specify any error conditions.

NOTES

       POSIX.1	defines  seconds  since  the  Epoch  as a value to be interpreted as the number of seconds between a specified time and the Epoch,
       according to a formula for conversion from 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.	The intention is that the interpretation  of  sec-
       onds since the Epoch values be consistent; see POSIX.1 Annex B 2.2.2 for further rationale.

SEE ALSO

       date(1), gettimeofday(2), ctime(3), ftime(3), time(7)

COLOPHON

       This  page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-02-25								   TIME(2)