ctime

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
                 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"

       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).

       ┌───────────────┬───────────────┬─────────────────────────────────┐
       │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(),
       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 5.02 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/.



                                  2019-03-06                          CTIME(3)