Provided by: manpages-dev_4.16-1_all bug

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 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-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  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 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).

       ┌───────────────┬───────────────┬─────────────────────────────────┐
       │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(),     │ 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(),  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.  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 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)

COLOPHON

       This page is part of release 4.16 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)