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Linux 2.6 - man page for tzfile (linux section 5)

TZFILE(5)			    Linux Programmer's Manual				TZFILE(5)

NAME
       tzfile - timezone information

DESCRIPTION
       This page describes the structure of the timezone files used by tzset(3).  These files are
       typically found under one of the directories /usr/lib/zoneinfo or /usr/share/zoneinfo.

       Timezone information files begin with the magic characters  "TZif"  to  identify  them  as
       timezone  information files, followed by a character identifying the version of the file's
       format (as of 2005, either an ASCII NUL ('\0') or a '2') followed by  fifteen  bytes  con-
       taining	zeros  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 "timezone 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 (with tzh_typecnt entries) that appear 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 timezone 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(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 timezone file is used in handling
       POSIX-style timezone 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 timezone file is used in handling POSIX-style time-
       zone environment variables.

       localtime(3)  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.

       For  version-2-format  timezone	files,	the above header and data is followed by a second
       header and data, identical in format except that eight bytes are used for each  transition
       time  or  leap-second  time.   After  the second header and data comes a newline-enclosed,
       POSIX-TZ-environment-variable-style string for use in handling  instants  after	the  last
       transition time stored in the file (with nothing between the newlines if there is no POSIX
       representation for such instants).

SEE ALSO
       ctime(3), tzset(3), tzselect(8),

       timezone/tzfile.h in the glibc source tree

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

					    2012-05-04					TZFILE(5)


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