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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LEARN ABOUT OPENSOLARIS

tcl_gettime

Tcl_GetTime(3TCL)					      Tcl Library Procedures						 Tcl_GetTime(3TCL)

__________________________________________________________________________________________________________________________________________________

NAME

       Tcl_GetTime - get date and time

SYNOPSIS

       #include <tcl.h>

       Tcl_GetTime( timePtr )

ARGUMENTS

       Tcl_Time *   timePtr   (out)	Points to memory in which to store the date and time information.
_________________________________________________________________

DESCRIPTION

       The  Tcl_GetTime function retrieves the current time as a Tcl_Time structure in memory the caller provides.  This structure has the follow-
       ing definition:
	      typedef struct Tcl_Time {
		  long sec;
		  long usec;
	      } Tcl_Time;

       On return, the sec member of the structure is filled in with the number of seconds that have elapsed since the  epoch:  the  epoch  is  the
       point  in time of 00:00 UTC, 1 January 1970.  This number does not count leap seconds - an interval of one day advances it by 86400 seconds
       regardless of whether a leap second has been inserted.

       The usec member of the structure is filled in with the number of microseconds that have elapsed since the start of the second designated by
       sec.  The Tcl library makes every effort to keep this number as precise as possible, subject to the limitations of the computer system.	On
       multiprocessor variants of Windows, this number may be limited to the 10- or 20-ms granularity of the system clock.   (On  single-processor
       Windows systems, the usec field is derived from a performance counter and is highly precise.)

SEE ALSO

       clock

KEYWORDS

       date, time

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +--------------------+-----------------+
       |  ATTRIBUTE TYPE    | ATTRIBUTE VALUE |
       +--------------------+-----------------+
       |Availability	    | SUNWTcl	      |
       +--------------------+-----------------+
       |Interface Stability | Uncommitted     |
       +--------------------+-----------------+
NOTES

       Source for Tcl is available on http://opensolaris.org.

Tcl									8.4							 Tcl_GetTime(3TCL)