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 

       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 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
        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-
       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 timezone,  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  overwrit-
       ten  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 a value of (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 .  This is a
       BSD extension, present in 4.3BSD-Reno.

       According to POSIX.1-2004, localtime() is required to behave as	though
       tzset() was called, while localtime_r() does not have this requirement.
       For portable code tzset() should be called before localtime_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 3.22 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/.



				  2009-03-15			      CTIME(3)