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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);

   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
       <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  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
       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
       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 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, 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 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
       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 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(),
       localtime(),  and mktime().  POSIX.1-2008 marks asctime(), asctime_r(),
       ctime(), and ctime_r() as obsolete.

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() 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 3.15 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/.

                                  2008-10-29                          CTIME(3)