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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 information 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 overwritten 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-supplied 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 external 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 func-
       tions.  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  struc-
       ture  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 November).  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 specified 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), strf-
       time(3), strptime(3), tzset(3)

					    2001-12-13					 CTIME(3)
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