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TZSET(3)                   Library Functions Manual                   TZSET(3)

NAME
     tzset, tzalloc, tzgetname, tzfree -- initialize time conversion
     information

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     timezone_t
     tzalloc(const char *zone);

     void
     tzfree(timezone_t restrict tz);

     const char *
     tzgetname(timezone_t restrict tz, int isdst);

     void
     tzset(void);

DESCRIPTION
     The tzalloc() function takes as an argument a timezone name and returns a
     timezone_t object suitable to be used in the ctime_rz(), localtime_rz(),
     and mktime_z() functions.

     If tz is not a valid time zone description, or if the object cannot be
     allocated, tzalloc() returns a NULL pointer and sets errno.

     A NULL pointer may be passed to tzalloc() instead of a timezone name, to
     refer to the current system timezone.  An empty timezone string indicates
     Coordinated Universal Time (UTC).

     Note that instead of setting the environment variable TZ, and globally
     changing the behavior of the calling program, one can use multiple
     timezones at the same time by using separate timezone_t objects allocated
     by tzalloc() and calling the ``z'' variants of the functions.  The
     tzfree() function deallocates tz, which was previously allocated by
     tzalloc().  This invalidates any tm_zone pointers that tz was used to
     set.  Finally, tzgetname() returns the name for the given tz.  If isdst
     is 0, the call is equivalent to tzname[0].  If isdst is set to 1 the call
     is equivalent to tzname[1].  The tzset() function acts like
     tzalloc(getenv("TZ")), except it saves any resulting time zone object
     into internal storage that is accessed by localtime(), localtime_r(), and
     mktime().  The anonymous shared time zone object is freed by the next
     call to tzset().  If the implied call to tzalloc() fails, tzset() falls
     back on UTC.  localtime(3).  If TZ is NULL, the best available
     approximation to local wall clock time, as specified by the tzfile(5)
     format file /etc/localtime is used by localtime(3).  If TZ appears in the
     environment but its value is the empty string, Universal Time (UT) is
     used, with the abbreviation ``UTC'' and without leap second correction;
     please see ctime(3).  If TZ is nonnull and nonempty:

     -   if the value begins with a colon, it is used as a pathname of a file
         from which to read the time conversion information;

     -   if the value does not begin with a colon, it is first used as the
         pathname of a file from which to read the time conversion
         information, and, if that file cannot be read, is used directly as a
         specification of the time conversion information.

     When TZ is used as a pathname, if it begins with a slash, it is used as
     an absolute pathname; otherwise, it is used as a pathname relative to
     /usr/share/zoneinfo.  The file must be in the format specified in
     tzfile(5).

     When TZ is used directly as a specification of the time conversion
     information, it must have the following syntax (spaces inserted for
     clarity):

           stdoffset[dst[offset][,rule]]

     where:
     std and dst  Three or more bytes that are the designation for the
                  standard (std) or summer (dst) time zone.  Only std is
                  required; if dst is missing, then summer time does not apply
                  in this locale.  Upper- and lowercase letters are explicitly
                  allowed.  Any characters except a leading colon (:), digits,
                  comma (,), minus (-), plus (+), and NUL bytes are allowed.
     offset       Indicates the value one must add to the local time to arrive
                  at Coordinated Universal Time.  The offset has the form:

                        hh[:mm[:ss]]

                  The minutes (mm) and seconds (ss) are optional.  The hour
                  (hh) is required and may be a single digit.  The offset
                  following std is required.  If no offset follows dst, summer
                  time is assumed to be one hour ahead of standard time.  One
                  or more digits may be used; the value is always interpreted
                  as a decimal number.  The hour must be between zero and 24,
                  and the minutes (and seconds)  if present  between zero and
                  59.  If preceded by a ``-'' the time zone shall be east of
                  the Prime Meridian; otherwise it shall be west (which may be
                  indicated by an optional preceding ``+'').
     rule         Indicates when to change to and back from summer time.  The
                  rule has the form:

                        date/time,date/time

                  where the first date describes when the change from standard
                  to summer time occurs and the second date describes when the
                  change back happens.  Each time field describes when, in
                  current local time, the change to the other time is made.
                  As an extension to POSIX, daylight saving is assumed to be
                  in effect all year if it begins January 1 at 00:00 and ends
                  December 31 at 24:00 plus the difference between daylight
                  saving and standard time, leaving no room for standard time
                  in the calendar.  The format of date is one of the
                  following:
                  Jn              The Julian day n (1 <= n <= 365).  Leap days
                                  are not counted; that is, in all years
                                  including leap years  February 28 is day 59
                                  and March 1 is day 60.  It is impossible to
                                  explicitly refer to the occasional February
                                  29.
                  n               The zero-based Julian day (0 <= n <= 365).
                                  Leap days are counted, and it is possible to
                                  refer to February 29.
                  Mm.n.d          The d'th day (0 <= d <= 6) of week n of
                                  month m of the year (1 <= n <= 5, 1 <= m
                                  <= 12, where week 5 means ``the last d
                                  day in month m'' which may occur in either
                                  the fourth or the fifth week).  Week 1 is
                                  the first week in which the d'th day occurs.
                                  Day zero is Sunday.
                  The time has the same format as offset except that POSIX
                  does not allow a leading sign ``-'' or ``+'' is allowed.  As
                  an extension to POSIX, the hours part of time can range from
                  -167 through 167; this allows for unusual rules such as
                  ``the Saturday before the first Sunday of March''.  The
                  default, if time is not given, is 02:00:00.

     Here are some examples of TZ values that directly specify the time zone
     rules; they use some of the extensions to POSIX.

     EST5   stands for US Eastern Standard Time (EST), 5 hours behind UTC,
            without daylight saving.

     FJT-12FJST,M11.1.0,M1.3.4/75
            stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours
            ahead of UTC, springing forward on November's first Sunday at
            02:00, and falling back on January's third Thursday at 75:00
            (i.e., 03:00 on the first Sunday on or after January 18).

     IST-2IDT,M3.4.4/26,M10.5.0
            stands for Israel Standard Time (IST) and Israel Daylight Time
            (IDT), 2 hours ahead of UTC, springing forward on March's fourth
            Thursday at 26:00 (i.e., 02:00 on the first Friday on or after
            March 23), and falling back on October's last Sunday at 02:00.

     WART4WARST,J1/0,J365/25
            stands for Western Argentina Summer Time (WARST), 3 hours behind
            UTC.  There is a dummy fall-back transition on December 31 at
            25:00 daylight saving time (i.e., 24:00 standard time, equivalent
            to January 1 at 00:00 standard time), and a simultaneous spring-
            forward transition on January 1 at 00:00 standard time, so
            daylight saving time is in effect all year and the initial WART is
            a placeholder.

     WGT3WGST,M3.5.0/-2,M10.5.0/-1
            stands for Western Greenland Time (WGT) and Western Greenland
            Summer Time (WGST), 3 hours behind UTC, where clocks follow the EU
            rules of springing forward on March's last Sunday at 01:00 UTC
            (-02:00 local time) and falling back on October's last Sunday at
            01:00 UTC (-01:00 local time).

     If no rule is present in TZ, the rules specified by the tzfile(5) format
     file posixrules in /usr/share/zoneinfo are used, with the standard and
     summer time offsets from UTC replaced by those specified by the offset
     values in TZ.

     For compatibility with System V Release 3.1, a semicolon (;) may be used
     to separate the rule from the rest of the specification.

FILES
     /etc/localtime                  local time zone file
     /usr/share/zoneinfo             time zone information directory
     /usr/share/zoneinfo/posixrules  used with POSIX-style TZ's
     /usr/share/zoneinfo/GMT         for UTC leap seconds

     If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
     /usr/share/zoneinfo/posixrules.

SEE ALSO
     ctime(3), getenv(3), strftime(3), time(3), tzfile(5)

STANDARDS
     The tzset() function conforms to IEEE Std 1003.1-1988 (``POSIX.1'').

NetBSD 7.1.2                   October 23, 2014                   NetBSD 7.1.2