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

NAME
       tzfile - time zone information

SYNOPSIS
       #include <tzfile.h>

DESCRIPTION
       This  page  describes the structure of timezone files as commonly found
       in /usr/lib/zoneinfo or /usr/share/zoneinfo.

       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 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
       containing zeroes 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	"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  (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	 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 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 timezone environ‐
       ment 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.

NOTES
       This  manual page documents <tzfile.h> in the glibc source archive, see
       timezone/tzfile.h.

       It seems that timezone uses tzfile internally,  but  glibc  refuses  to
       expose  it  to userspace.  This is most likely because the standardised
       functions are more useful and  portable,	 and  actually	documented  by
       glibc.	It  may	 only  be in glibc just to support the non-glibc-main‐
       tained timezone data (which is maintained by some other entity).

SEE ALSO
       time(3), gettimeofday(3), tzset(3), ctime(3)

       For version-2-format timezone files, the above header and data is  fol‐
       lowed  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)

COLOPHON
       This page is part of release 3.27 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/.

				  2010-08-31			     TZFILE(5)
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