tzfile.5.txt

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
     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(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 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(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 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 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 representation for such
     instants).

SEE ALSO
     newctime(3)