localtime

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.1-2001, 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)

COLOPHON
       This page is part of release 5.09 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)