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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  cal-
       endar  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 representa-
       tion 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-termi-
       nated 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 abbre-
       viations 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 Coordi-
       nated 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  for-
       mat  as ctime().  The return value points to a statically allocated string which might be overwritten by subse-
       quent 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 repre-
       sentation.  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 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(), localtime(), 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 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 preced-
       ing 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                  2009-03-15                          CTIME(3)