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NetBSD 6.1.5 - man page for tzfile (netbsd section 5)

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TZFILE(5)			     BSD File Formats Manual				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 them as time zone information files, followed by a character identifying the ver-
     sion of the file's format (as of 2005, either an ASCII NUL 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 "time zone abbreviation strings" stored in the file.

     The above header is followed by tzh_timecnt four-byte values of type long, sorted in ascend-
     ing order.  These values are written in ``standard'' byte order.  Each is used as a transi-
     tion time (as returned by time(3)) 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 differ-
     ent 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 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 standard byte order; the
     first value of each pair gives the time (as returned by time(3)) 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(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 time zone 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 representa-
     tion for such instants).

SEE ALSO
     ctime(3), localtime(3), time(3)

BSD					September 16, 2001				      BSD
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