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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 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 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  day-
       light  (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 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-down time representa-
       tion, 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 func-
       tions.  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 overwritten by subsequent calls to any of the date and time func-
       tions.  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 sys-
       tem 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 follows: tm_wday and
       tm_yday are set to values determined from the contents 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 current 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(),	localtime(),  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 specified 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 requirement.  For portable code tzset(3)
       should be called before localtime_r().

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

COLOPHON
       This  page  is  part of release 3.55 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)
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