tzfile(5) - NetBSD Manual Pages

TZFILE(5)                 NetBSD 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, fol-
     lowed by a character identifying the version 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 ascending order.  These values are written in
     ``standard'' byte order.  Each is used as a transition 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 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 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 struc-
     ture(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 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 transi-
     tion time stored in the file (with nothing between the newlines if there
     is no POSIX representation for such instants).


SEE ALSO

     ctime(3), localtime(3), time(3)

NetBSD 6.0                    September 16, 2001                    NetBSD 6.0