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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 <&lt;time.h>&gt;

       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
       &lt;time.h&gt; 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 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 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 &lt;time.h&gt;.  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)