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

NAME
       asctime,	  ctime,   gmtime,   localtime,	 mktime,  asctime_r,  ctime_r,
       gmtime_r, localtime_r - 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);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       asctime_r(), ctime_r(), gmtime_r(), localtime_r():
	      _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
	      _SVID_SOURCE || _POSIX_SOURCE

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 the Epoch,	1970-01-01 00:00:00 +0000 (UTC).

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

       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 60 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  day-
		 light	saving time is in effect, zero if it is	not, and nega-
		 tive if the information is not	available.

       The call	ctime(t) is equivalent to asctime(localtime(t)).  It  converts
       the calendar time t into	a null-terminated 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 variables
       tzname,	timezone,  and	daylight (see tzset(3))	with information about
       the current timezone.  The reentrant version ctime_r() does  the	 same,
       but  stores the string in a user-supplied buffer	which should have room
       for at least 26 bytes.  It need not set tzname, timezone, and daylight.

       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-
       down time representation, expressed relative to	the  user's  specified
       timezone.   The	function  acts	as  if it called tzset(3) and sets the
       external	variables tzname with information about	the current  timezone,
       timezone	 with  the difference between Coordinated Universal Time (UTC)
       and local standard time in seconds, and daylight	to a nonzero 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, timezone, and daylight.

       The asctime() function converts the broken-down time value  tm  into  a
       null-terminated	string	with  the  same	format as ctime().  The	return
       value points to a statically allocated string which might be  overwrit-
       ten  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 which should have room for at least 26 bytes.

       The  mktime() function converts a broken-down time structure, expressed
       as local	time, to calendar time representation.	The  function  ignores
       the  values  supplied  by the caller in the tm_wday and tm_yday fields.
       The value specified in the tm_isdst field informs mktime()  whether  or
       not  daylight  saving  time (DST) is in effect for the time supplied in
       the tm structure: a positive value means	DST is in effect;  zero	 means
       that  DST  is  not  in effect; and a negative value means that mktime()
       should (use timezone information	and system databases  to)  attempt  to
       determine whether DST is	in effect at the specified time.

       The  mktime()  function modifies	the fields of the tm structure as fol-
       lows: tm_wday and tm_yday are set to values determined  from  the  con-
       tents of	the other fields; if structure members are outside their valid
       interval, they will be normalized (so that, for example,	40 October  is
       changed	into  9	 November); tm_isdst is	set (regardless	of its initial
       value) to a positive value or to	0, respectively, to  indicate  whether
       DST  is	or  is	not in effect at the specified time.  Calling mktime()
       also sets the external variable tzname with information about the  cur-
       rent timezone.

       If  the	specified  broken-down	time cannot be represented as calendar
       time (seconds since the Epoch), mktime()	returns	(time_t) -1  and  does
       not alter the 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.

CONFORMING TO
       POSIX.1-2001.  C89 and C99 specify asctime(), ctime(), gmtime(),	local-
       time(),	and  mktime().	 POSIX.1-2008  marks  asctime(),  asctime_r(),
       ctime(),	and ctime_r() as obsolete, recommending	the use	of strftime(3)
       instead.

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

       POSIX.1-2001 says: "The asctime(), ctime(), gmtime(),  and  localtime()
       functions  shall	 return	values in one of two static objects: a broken-
       down time structure and an array	of type	char.  Execution of any	of the
       functions  may  overwrite  the  information returned in either of these
       objects by any of the other functions."	This can occur	in  the	 glibc
       implementation.

       In many implementations,	including glibc, a 0 in	tm_mday	is interpreted
       as meaning the last day of the preceding	month.

       The glibc version of struct tm has additional fields

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

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

       According  to POSIX.1-2004, localtime() is required to behave as	though
       tzset(3)	was called, while localtime_r()	does not  have	this  require-
       ment.   For  portable  code  tzset(3)  should  be  called before	local-
       time_r().

SEE ALSO
       date(1),	gettimeofday(2),  time(2),  utime(2),  clock(3),  difftime(3),
       strftime(3), strptime(3), timegm(3), tzset(3), time(7)

COLOPHON
       This  page  is  part of release 3.53 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/.

				  2010-02-25			      CTIME(3)

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

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