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ctime

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 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 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 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 exter-
       nal  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  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 legal interval, they
       will be normalized (so that, e.g., 40 October is changed into 9  Novem-
       ber).   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 spec-
       ified 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), strftime(3), strptime(3), tzset(3)



                                  2001-12-13                          CTIME(3)