ctime(3)



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
                  || /* Glibc versions <= 2.19: */ _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 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, and so on.

       Broken-down time is stored in the structure tm,  which  is  defined  in
       <time.h> as follows:

           struct tm {
               int tm_sec;    /* Seconds (0-60) */
               int tm_min;    /* Minutes (0-59) */
               int tm_hour;   /* Hours (0-23) */
               int tm_mday;   /* Day of the month (1-31) */
               int tm_mon;    /* Month (0-11) */
               int tm_year;   /* Year - 1900 */
               int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
               int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
               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"
           ,in

       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 (time_t) -1 and does
       not alter the members of the broken-down time structure.

RETURN VALUE
       On success, gmtime() and localtime() return a pointer to a struct tm.

       On success, gmtime_r() and localtime_r()  return  the  address  of  the
       structure pointed to by result.

       On success, asctime() and ctime() return a pointer to a string.

       On  success,  asctime_r()  and ctime_r() return a pointer to the string
       pointed to by buf.

       On success, mktime() returns  the  calendar  time  (seconds  since  the
       Epoch), expressed as a value of type time_t.

       On  error, mktime() returns the value (time_t) -1.  The remaining func-
       tions return NULL on error.  On error, errno is  set  to  indicate  the
       cause of the error.

ERRORS
       EOVERFLOW
              The result cannot be represented.

ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(7).

       +---------------+---------------+---------------------------------+
       |Interface      | Attribute     | Value                           |
       +---------------+---------------+---------------------------------+
       |asctime()      | Thread safety | MT-Unsafe race:asctime locale   |
       +---------------+---------------+---------------------------------+
       |asctime_r()    | Thread safety | MT-Safe locale                  |
       +---------------+---------------+---------------------------------+
       |ctime()        | Thread safety | MT-Unsafe race:tmbuf            |
       |               |               | race:asctime env locale         |
       +---------------+---------------+---------------------------------+
       |ctime_r(),     | Thread safety | MT-Safe env locale              |
       |gmtime_r(),    |               |                                 |
       |localtime_r(), |               |                                 |
       |mktime()       |               |                                 |
       +---------------+---------------+---------------------------------+
       |gmtime(),      | Thread safety | MT-Unsafe race:tmbuf env locale |
       |localtime()    |               |                                 |
       +---------------+---------------+---------------------------------+
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.   The  thread-
       safe  versions,  asctime_r(),  ctime_r(), gmtime_r() and localtime_r(),
       are specified by SUSv2.

       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

           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(3) was called, while localtime_r() does not  have  this  require-
       ment.   For  portable  code,  tzset(3)  should  be called before local-
       time_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 4.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

                                  2017-09-15                          CTIME(3)

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