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,
       followed	 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	 val-
       ues  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
       environment 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)