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CTIME(3)		   Linux Programmer's Manual		      CTIME(3)

NAME
       asctime,	 ctime,	gmtime,	localtime, mktime - transform date and time to
       broken-down time	or ASCII

SYNOPSIS
       #include	<time.h>

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

       char *ctime(const time_t	*timep);
       char *ctime_r(const time_t *timep, char *buf);

       struct tm *gmtime(const time_t *timep);
       struct tm *gmtime_r(const time_t	*timep,	struct tm *result);

       struct tm *localtime(const time_t *timep);
       struct tm *localtime_r(const time_t *timep, struct tm *result);

       time_t mktime(struct tm *tm);

DESCRIPTION
       The ctime(), gmtime() and localtime() functions all take	an argument of
       data  type  time_t which	represents calendar time.  When	interpreted as
       an absolute time	value, it represents the  number  of  seconds  elapsed
       since 00:00:00 on January 1, 1970, Coordinated Universal	Time (UTC).

       The asctime() and mktime() functions both take an argument representing
       broken-down time	which is a representation separated into year,	month,
       day, etc.

       Broken-down  time  is  stored  in  the structure	tm which is defined in
       _time.h_	as follows:

	      struct tm	{
		      int     tm_sec;	      /* seconds */
		      int     tm_min;	      /* minutes */
		      int     tm_hour;	      /* hours */
		      int     tm_mday;	      /* day of	the month */
		      int     tm_mon;	      /* month */
		      int     tm_year;	      /* year */
		      int     tm_wday;	      /* day of	the week */
		      int     tm_yday;	      /* day in	the year */
		      int     tm_isdst;	      /* daylight saving time */
	      };

       The members of the tm structure are:

       tm_sec The number of seconds after the minute, normally in the range  0
	      to 59, but can be	up to 61 to allow for leap seconds.

       tm_min The number of minutes after the hour, in the range 0 to 59.

       tm_hour
	      The number of hours past midnight, in the	range 0	to 23.

       tm_mday
	      The day of the month, in the range 1 to 31.

       tm_mon The number of months since January, in the range 0 to 11.

       tm_year
	      The number of years since	1900.

       tm_wday
	      The number of days since Sunday, in the range 0 to 6.

       tm_yday
	      The number of days since January 1, in the range 0 to 365.

       tm_isdst
	      A	 flag that indicates whether daylight saving time is in	effect
	      at the time described.  The value	is positive if daylight	saving
	      time is in effect, zero if it is not, and	negative if the	infor-
	      mation is	not available.

       The call	ctime(t) is equivalent to asctime(localtime(t)).  It  converts
       the calendar time t into	a string of the	form

	      "Wed Jun 30 21:49:08 1993\n"

       The  abbreviations  for	the  days of the week are `Sun', `Mon',	`Tue',
       `Wed', `Thu', `Fri', and	`Sat'.	The abbreviations for the  months  are
       `Jan',  `Feb',  `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep',	`Oct',
       `Nov', and `Dec'.  The return value points to  a	 statically  allocated
       string  which  might  be	 overwritten by	subsequent calls to any	of the
       date and	time functions.	 The function also sets	the external  variable
       tzname  (see  tzset(3))	with  information about	the current time zone.
       The re-entrant version ctime_r()	does the same, but stores  the	string
       in  a  user-supplied  buffer  of	 length	 at  least 26. It need not set
       tzname.

       The gmtime() function converts the calendar time	timep  to  broken-down
       time  representation, expressed in Coordinated Universal	Time (UTC). It
       may return NULL when the	year does not fit into an integer.  The	return
       value  points to	a statically allocated struct which might be overwrit-
       ten by subsequent calls to any of the date  and	time  functions.   The
       gmtime_r()  function  does the same, but	stores the data	in a user-sup-
       plied struct.

       The localtime() function	converts the calendar time  timep  to  broken-
       time  representation,  expressed	 relative to the user's	specified time
       zone.	The function acts as if	it called tzset(3) and sets the	exter-
       nal  variables  tzname  with  information  about	the current time zone,
       timezone	with the difference between Coordinated	Universal  Time	 (UTC)
       and local standard time in seconds, and daylight	to a non-zero value if
       daylight	savings	time rules apply during	some part of  the  year.   The
       return  value  points  to  a statically allocated struct	which might be
       overwritten by subsequent calls to any of the date and time  functions.
       The  localtime_r()  function  does  the	same, but stores the data in a
       user-supplied struct. It	need not set tzname.

       The asctime() function converts the broken-down time value  tm  into  a
       string  with  the same format as	ctime().  The return value points to a
       statically allocated string which might be  overwritten	by  subsequent
       calls  to any of	the date and time functions.  The asctime_r() function
       does the	same, but stores the  string  in  a  user-supplied  buffer  of
       length at least 26.

       The  mktime() function converts a broken-down time structure, expressed
       as local	time, to calendar time representation.	The  function  ignores
       the specified contents of the structure members tm_wday and tm_yday and
       recomputes them from the	other  information  in	the  broken-down  time
       structure.  If structure	members	are outside their legal	interval, they
       will be normalized (so that, e.g., 40 October is	changed	into 9	Novem-
       ber).   Calling	mktime()  also	sets the external variable tzname with
       information about the current time zone.	 If the	specified  broken-down
       time  cannot be represented as calendar time (seconds since the epoch),
       mktime()	returns	a value	of (time_t)(-1)	and does not alter the tm_wday
       and tm_yday members of the broken-down time structure.

RETURN VALUE
       Each  of	 these	functions  returns the value described,	or NULL	(-1 in
       case of mktime()) in case an error was detected.

NOTES
       The four	functions acstime(), ctime(), gmtime() and localtime()	return
       a  pointer  to  static data and hence are not thread-safe.  Thread-safe
       versions	acstime_r(), ctime_r(),	gmtime_r() and localtime_r() are spec-
       ified by	SUSv2, and available since libc	5.2.5.

       The glibc version of struct tm has additional fields

	      long tm_gmtoff;		/* Seconds east	of UTC */
	      const char *tm_tm_zone;	/* Timezone abbreviation */

       defined	when _BSD_SOURCE was set before	including _time.h_.  This is a
       BSD extension, present in 4.3BSD-Reno.

CONFORMING TO
       SVID 3, POSIX, BSD 4.3, ISO 9899

SEE ALSO
       date(1),	gettimeofday(2),  newctime(3),	time(2),  utime(2),  clock(3),
       difftime(3), strftime(3), strptime(3), tzset(3)

				  2001-12-13			      CTIME(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | CONFORMING TO | SEE ALSO

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