Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!PT.CS.CMU.EDU!K.GP.CS.CMU.EDU!bww
From: bww@K.GP.CS.CMU.EDU (Bradley White)
Newsgroups: comp.bugs.4bsd
Subject: ctime(3) and leap seconds :-)
Keywords: ctime, leap second, epoch
Message-ID: <604@PT.CS.CMU.EDU>
Date: 7 Jan 88 18:35:41 GMT
Sender: netnews@PT.CS.CMU.EDU
Organization: Carnegie-Mellon University, CS/RI
Lines: 118

Index:	lib/libc/gen/ctime.c 4.3BSD

Description:
	The time routines in the C library know nothing about leap
	seconds.  This means that the correspondence between the
	number of seconds since the epoch and the actual time of
	day is broken.
Repeat-By:
	Count the actual number of seconds since January 1, 1970 and
	run the answer through ctime(3).  Notice that the answer is
	fourteen seconds in the future.
Fix:
	Apply the following patch and then check the output of the
	test program.
		main()
		{
			long t;

			t = 567993612;  printf("%s", ctime(&t));
			t = 567993613;  printf("%s", ctime(&t));
			t = 567993614;  printf("%s", ctime(&t));
		}

	It should look like this (in EST at least).
		Thu Dec 31 18:59:59 1987
		Thu Dec 31 18:59:60 1987
		Thu Dec 31 19:00:00 1987

RCS file: RCS/ctime.c,v
retrieving revision 1.1
diff -c -r1.1 ctime.c
*** /tmp/,RCSt1011358	Wed Jan  6 00:08:34 1988
--- ctime.c	Wed Jan  6 00:03:48 1988
***************
*** 321,326 ****
--- 321,362 ----
  	return tmp;
  }
  
+ /* THE LEAP SECOND TABLE SHOULD BE LOADED FROM A FILE */
+ 
+ static struct leapsec {
+ 	time_t	ls_when;
+ 	long	ls_corr;
+ } leapsecs[] = {
+ 	78796800,	1,	/* 30 Jun 1972 */
+ 	94694401,	2,	/* 31 Dec 1972 */
+ 	126230402,	3,	/* 31 Dec 1973 */
+ 	157766403,	4,	/* 31 Dec 1974 */
+ 	189302404,	5,	/* 31 Dec 1975 */
+ 	220924805,	6,	/* 31 Dec 1976 */
+ 	252460806,	7,	/* 31 Dec 1977 */
+ 	283996807,	8,	/* 31 Dec 1978 */
+ 	315532808,	9,	/* 31 Dec 1979 */
+ 	362793609,	10,	/* 30 Jun 1981 */
+ 	394329610,	11,	/* 30 Jun 1982 */
+ 	425865611,	12,	/* 30 Jun 1983 */
+ 	489024012,	13,	/* 30 Jun 1985 */
+ 	567993613,	14	/* 31 Dec 1987 */
+ };
+ 
+ #define	NLEAPSECS	(sizeof(leapsecs)/sizeof(leapsecs[0]))
+ 
+ struct leapsec *
+ lssearch(t)
+ 	time_t t;
+ {
+ 	register int i = NLEAPSECS;
+ 
+ 	while (--i >= 0)
+ 		if (t >= leapsecs[i].ls_when)
+ 			return &leapsecs[i];
+ 	return 0;
+ }
+ 
  static int	mon_lengths[2][MONS_PER_YEAR] = {
  	31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
  	31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
***************
*** 341,348 ****
--- 377,392 ----
  	register int		y;
  	register int		yleap;
  	register int *		ip;
+ 	int			ls_flag;
+ 	struct leapsec		*l;
  	static struct tm	tm;
  
+ 	ls_flag = 0;
+ 	if (l = lssearch(*clock)) {
+ 		if (*clock == l->ls_when)
+ 			ls_flag = 1;
+ 		*clock -= l->ls_corr;
+ 	}
  	tmp = &tm;
  	days = *clock / SECS_PER_DAY;
  	rem = *clock % SECS_PER_DAY;
***************
*** 359,364 ****
--- 403,415 ----
  	rem = rem % SECS_PER_HOUR;
  	tmp->tm_min = (int) (rem / SECS_PER_MIN);
  	tmp->tm_sec = (int) (rem % SECS_PER_MIN);
+ 	if (ls_flag)
+ 		/*
+ 		 * How should a leap second be represented?
+ 		 * We choose to add one to tm_sec which will
+ 		 * usually result in "... ??:59:60".
+ 		 */
+ 		 tmp->tm_sec += 1;
  	tmp->tm_wday = (int) ((EPOCH_WDAY + days) % DAYS_PER_WEEK);
  	if (tmp->tm_wday < 0)
  		tmp->tm_wday += DAYS_PER_WEEK;

-- 
--
Bradley White <bww@cs.cmu.edu>         +1-412-268-3060
CMU Computer Science Department  40 26'33"N 79 56'48"W