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CTIME(3)                   Linux Programmer's Manual                  CTIME(3)



NAME
       asctime,  ctime,  gmtime,  localtime,  mktime, asctime_r, ctime_r, gmtime_r, local-
       time_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 fol-
       lows:

           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 avail-
                 able.

       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 repre-
       sentation, 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  represen-
       tation,  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, time-
       zone, and daylight.

       The  asctime()  function  converts the broken-down time value tm into a null-termi-
       nated string with the same format as ctime().  The return value points to a  stati-
       cally 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 (regard-
       less 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, 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  infor-
       mation  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 exten-
       sion, 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 information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



                                  2009-03-15                          CTIME(3)

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