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

       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)