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TZFILE(5)                   BSD File Formats Manual                  TZFILE(5)

NAME
     tzfile -- timezone information

SYNOPSIS
     #include <tzfile.h>

DESCRIPTION
     The time zone information files used by tzset(3) begin with the magic characters ``TZif'' to identify
     them as time zone information files, followed by sixteen bytes reserved for future use, followed by
     four four-byte values written in a ``standard'' byte order (the high-order byte of the value is written
     first).  These values are, in order:

     tzh_ttisgmtcnt  The number of UTC/local indicators stored in the file.
     tzh_ttisstdcnt  The number of standard/wall indicators stored in the file.
     tzh_leapcnt     The number of leap seconds for which data is stored in the file.
     tzh_timecnt     The number of ``transition times'' for which data is stored in the file.
     tzh_typecnt     The number of ``local time types'' for which data is stored in the file (must not be
                     zero).
     tzh_charcnt     The number of characters of ``time zone abbreviation strings'' stored in the file.

     The above header is followed by tzh_timecnt four-byte values of type long, sorted in ascending order.
     These values are written in ``standard'' byte order.  Each is used as a transition time (as returned by
     time(3)) at which the rules for computing local time change.  Next come tzh_timecnt one-byte values of
     type unsigned char; each one tells which of the different types of ``local time'' types described in
     the file is associated with the same-indexed transition time.  These values serve as indices into an
     array of ttinfo structures that appears next in the file; these structures are defined as follows:

           struct ttinfo {
                   long    tt_gmtoff;
                   int     tt_isdst;
                   unsigned int    tt_abbrind;
           };

     Each structure is written as a four-byte value for tt_gmtoff of type long, in a standard byte order,
     followed by a one-byte value for tt_isdst and a one-byte value for tt_abbrind.  In each structure,
     tt_gmtoff gives the number of seconds to be added to UTC, tt_isdst tells whether tm_isdst should be set
     by localtime(3) and tt_abbrind serves as an index into the array of time zone abbreviation characters
     that follow the ttinfo structure(s) in the file.

     Then there are tzh_leapcnt pairs of four-byte values, written in standard byte order; the first value
     of each pair gives the time (as returned by time(3)) at which a leap second occurs; the second gives
     the total number of leap seconds to be applied after the given time.  The pairs of values are sorted in
     ascending order by time.

     Then there are tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte value; they tell
     whether the transition times associated with local time types were specified as standard time or wall
     clock time, and are used when a time zone file is used in handling POSIX-style time zone environment
     variables.

     Finally there are tzh_ttisgmtcnt UTC/local indicators, each stored as a one-byte value; they tell
     whether the transition times associated with local time types were specified as UTC or local time, and
     are used when a time zone file is used in handling POSIX-style time zone environment variables.

     localtime uses the first standard-time ttinfo structure in the file (or simply the first ttinfo struc-ture structure
     ture in the absence of a standard-time structure) if either tzh_timecnt is zero or the time argument is
     less than the first transition time recorded in the file.

SEE ALSO
     ctime(3), time2posix(3), zic(8)

BSD                           September 13, 1994                           BSD