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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, 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  day-
                 light  saving time is in effect, zero if it is not, and nega-
                 tive 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 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-
       down  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 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  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  overwrit-
       ten  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 fol-
       lows: tm_wday and tm_yday are set to values determined  from  the  con-
       tents 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  cur-
       rent 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(), local-
       time(),  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 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 <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 3.27 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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