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BSD 2.11 - man page for tzfile (bsd section 5)

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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 bytes reserved for future use,
       followed by three 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_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(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  GMT,  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.

       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.

SEE ALSO
       ctime(3)

System V										TZFILE(5)
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