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CTIME(3)		   Linux Programmers 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 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_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 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 reentrant 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  users  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  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.

       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 valid interval, they
       will  be normalized (so that, for example, 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.

CONFORMING TO
       POSIX.1-2001.  C89 and C99 specify asctime(), ctime(), gmtime(), local
       time(), and mktime().

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 3.05 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/.



				  2008-04-06			      CTIME(3)




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