tzfile

TZFILE(5)                                                            TZFILE(5)



NAME
       tzfile - time zone information

SYNOPSIS
       #include <tzfile.h>

DESCRIPTION
       The  time  zone information files used by tzset(3) begin with the magic
       characters "TZif" to identify then as time zone information files, fol-
       lowed  by  sixteen bytes reserved for future use, followed by six four-
       byte values of type long, 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 ``stan-
       dard'' byte order.  Each is used as a transition time (as  returned  by
       time(2)) 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  stan-
       dard  byte  order;  the  first  value  of  each pair gives the time (as
       returned by time(2)) 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 envi-
       ronment variables.

       Localtime uses the first standard-time ttinfo structure in the file (or
       simply  the  first  ttinfo  structure 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.



                                                                     TZFILE(5)