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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);

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 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
       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 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.

       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.

CONFORMING TO

       SVID 3, POSIX, 4.3BSD, ISO 9899

SEE ALSO

       date(1), gettimeofday(2),  time(2),  utime(2),  clock(3),  difftime(3),
       strftime(3), strptime(3), tzset(3)

                                  2004-11-16                          CTIME(3)