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NAME

       asctime,  ctime,  gmtime,  localtime,  mktime, asctime_r, ctime_r, gmtime_r, localtime_r -
       transform date and time to broken-down time or ASCII

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <time.h>

       char *asctime(const struct tm *tm);
       char *asctime_r(const struct tm *restrict tm,
                           char buf[restrict 26]);

       char *ctime(const time_t *timep);
       char *ctime_r(const time_t *restrict timep,
                           char buf[restrict 26]);

       struct tm *gmtime(const time_t *timep);
       struct tm *gmtime_r(const time_t *restrict timep,
                           struct tm *restrict result);

       struct tm *localtime(const time_t *timep);
       struct tm *localtime_r(const time_t *restrict timep,
                           struct tm *restrict 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 <= 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, described in tm(3type).

       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 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-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  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 (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 current 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 functions return NULL on
       error.  On error, errno is set to indicate the error.

ERRORS

       EOVERFLOW
              The result cannot be represented.

ATTRIBUTES

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

       ┌────────────────────────┬───────────────┬────────────────────────────────────────────────┐
       │InterfaceAttributeValue                                          │
       ├────────────────────────┼───────────────┼────────────────────────────────────────────────┤
       │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(), gmtime_r(),  │ Thread safety │ MT-Safe env locale                             │
       │localtime_r(), mktime() │               │                                                │
       ├────────────────────────┼───────────────┼────────────────────────────────────────────────┤
       │gmtime(), localtime()   │ Thread safety │ MT-Unsafe race:tmbuf env locale                │
       └────────────────────────┴───────────────┴────────────────────────────────────────────────┘

STANDARDS

       POSIX.1-2001.   C99  specifies  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.

       POSIX  doesn't  specify  the  parameters  of ctime_r() to be restrict; that is specific to
       glibc.

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.

       According  to  POSIX.1-2001,  localtime()  is  required  to  behave as though tzset(3) was
       called, while localtime_r() does not have this requirement.  For portable  code,  tzset(3)
       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)