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CTIME(3)		 BSD Library Functions Manual		      CTIME(3)

     asctime, asctime_r, ctime,	ctime_r, difftime, gmtime, gmtime_r,
     localtime,	localtime_r, mktime, timegm -- transform binary	date and time

     Standard C	Library	(libc, -lc)

     #include <time.h>

     extern char *tzname[2];

     char *
     ctime(const time_t	*clock);

     difftime(time_t time1, time_t time0);

     char *
     asctime(const struct tm *tm);

     struct tm *
     localtime(const time_t *clock);

     struct tm *
     gmtime(const time_t *clock);

     mktime(struct tm *tm);

     timegm(struct tm *tm);

     char *
     ctime_r(const time_t *clock, char *buf);

     struct tm *
     localtime_r(const time_t *clock, struct tm	*result);

     struct tm *
     gmtime_r(const time_t *clock, struct tm *result);

     char *
     asctime_r(const struct tm *tm, char *buf);

     The functions ctime(), gmtime() and localtime() all take as an argument a
     time value	representing the time in seconds since the Epoch (00:00:00
     UTC, January 1, 1970; see time(3)).

     The function localtime() converts the time	value pointed at by clock, and
     returns a pointer to a "struct tm"	(described below) which	contains the
     broken-out	time information for the value after adjusting for the current
     time zone (and any	other factors such as Daylight Saving Time).  Time
     zone adjustments are performed as specified by the	TZ environment vari-
     able (see tzset(3)).  The function	localtime() uses tzset(3) to initial-
     ize time conversion information if	tzset(3) has not already been called
     by	the process.

     After filling in the tm structure,	localtime() sets the tm_isdst'th ele-
     ment of tzname to a pointer to an ASCII string that's the time zone ab-
     breviation	to be used with	localtime()'s return value.

     The function gmtime() similarly converts the time value, but without any
     time zone adjustment, and returns a pointer to a tm structure (described

     The ctime() function adjusts the time value for the current time zone in
     the same manner as	localtime(), and returns a pointer to a	26-character
     string of the form:

	   Thu Nov 24 18:22:48 1986\n\0

     All the fields have constant width.

     ctime_r() provides	the same functionality as ctime() except the caller
     must provide the output buffer buf	to store the result, which must	be at
     least 26 characters long.	localtime_r() and gmtime_r() provide the same
     functionality as localtime() and gmtime() respectively, except the	caller
     must provide the output buffer result.

     The asctime() function converts the broken	down time in the structure tm
     pointed at	by *tm to the form shown in the	example	above.

     asctime_r() provides the same functionality as asctime() except the
     caller provide the	output buffer buf to store the result, which must be
     at	least 26 characters long.

     The functions mktime() and	timegm() convert the broken-down time in the
     structure pointed to by tm	into a time value with the same	encoding as
     that of the values	returned by the	time(3)	function (that is, seconds
     from the Epoch, UTC).  mktime() interprets	the input structure according
     to	the current timezone setting (see tzset(3)).  timegm() interprets the
     input structure as	representing Universal Coordinated Time	(UTC).

     The original values of the	tm_wday	and tm_yday components of the struc-
     ture are ignored, and the original	values of the other components are not
     restricted	to their normal	ranges,	and will be normalized if needed.  For
     example, October 40 is changed into November 9, a tm_hour of -1 means 1
     hour before midnight, tm_mday of 0	means the day preceding	the current
     month, and	tm_mon of -2 means 2 months before January of tm_year.	(A
     positive or zero value for	tm_isdst causes	mktime() to presume initially
     that summer time (for example, Daylight Saving Time) is or	is not in ef-
     fect for the specified time, respectively.	 A negative value for tm_isdst
     causes the	mktime() function to attempt to	divine whether summer time is
     in	effect for the specified time.	The tm_isdst and tm_gmtoff members are
     forced to zero by timegm().)

     On	successful completion, the values of the tm_wday and tm_yday compo-
     nents of the structure are	set appropriately, and the other components
     are set to	represent the specified	calendar time, but with	their values
     forced to their normal ranges; the	final value of tm_mday is not set un-
     til tm_mon	and tm_year are	determined.  Mktime() returns the specified
     calendar time; if the calendar time cannot	be represented,	it returns -1;

     The difftime() function returns the difference between two	calendar
     times, (time1 - time0), expressed in seconds.

     External declarations as well as the tm structure definition are in the
     <time.h> include file.  The tm structure includes at least	the following

	   int tm_sec;	   /* seconds (0 - 60) */
	   int tm_min;	   /* minutes (0 - 59) */
	   int tm_hour;	   /* hours (0 - 23) */
	   int tm_mday;	   /* day of month (1 -	31) */
	   int tm_mon;	   /* month of year (0 - 11) */
	   int tm_year;	   /* year - 1900 */
	   int tm_wday;	   /* day of week (Sunday = 0) */
	   int tm_yday;	   /* day of year (0 - 365) */
	   int tm_isdst;   /* is summer	time in	effect?	*/
	   char	*tm_zone;  /* abbreviation of timezone name */
	   long	tm_gmtoff; /* offset from UTC in seconds */

     The field tm_isdst	is non-zero if summer time is in effect.

     The field tm_gmtoff is the	offset (in seconds) of the time	represented
     from UTC, with positive values indicating east of the Prime Meridian.

     date(1), gettimeofday(2), getenv(3), time(3), tzset(3), tzfile(5)

     The asctime(), ctime(), difftime(), gmtime(), localtime(),	and mktime()
     functions conform to ISO/IEC 9899:1990 ("ISO C90"), and conform to
     ISO/IEC 9945-1:1996 ("POSIX.1") provided the selected local timezone does
     not contain a leap-second table (see zic(8)).

     The asctime_r(), ctime_r(), gmtime_r(), and localtime_r() functions are
     expected to conform to ISO/IEC 9945-1:1996	("POSIX.1") (again provided
     the selected local	timezone does not contain a leap-second	table).

     The timegm() function is not specified by any standard; its function can-
     not be completely emulated	using the standard functions described above.

     This manual page is derived from the time package contributed to Berkeley
     by	Arthur Olson and which appeared	in 4.3BSD.

     Except for	difftime(), mktime(), and the _r() variants of the other func-
     tions, these functions leaves their result	in an internal static object
     and return	a pointer to that object.  Subsequent calls to these function
     will modify the same object.

     The C Standard provides no	mechanism for a	program	to modify its current
     local timezone setting, and the POSIX-standard method is not reentrant.
     (However, thread-safe implementations are provided	in the POSIX threaded

     The tm_zone field of a returned tm	structure points to a static array of
     characters, which will also be overwritten	by any subsequent calls	(as
     well as by	subsequent calls to tzset(3) and tzsetwall(3)).

     Use of the	external variable tzname is discouraged; the tm_zone entry in
     the tm structure is preferred.

BSD				January	2, 1999				   BSD


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