tzfile(4)                                               tzfile(4)



     NAME
          tzfile - time-zone information

     SYNOPSIS
          #include <tzfile.h>

     DESCRIPTION
          The time-zone information files used by tzset(3) begin with
          bytes reserved for future use, followed by four 4-byte
          values of type long, written in a standard byte order where
          the high-order byte of the value is written first.  These
          values are, in order:

          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 0)

          tzh_charcnt
               The number of characters of ``time-zone abbreviation
               strings'' stored in the file

          The above header is followed by tzh_timecnt 4-byte values of
          type long, sorted in ascending order.  These values are
          written in standard byte order.  Each is used as a transi-
          tion time (as returned by time(2)) where the rules for com-
          puting local time change.  Next come tzh_timecnt 1-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 struc-
          tures 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 4-byte value for tt_gmtoff of



     April, 1990                                                     1

     tzfile(4)                                               tzfile(4)



          type long, in a standard byte order, followed by a 1-byte
          value for tt_isdst and a 1-byte value for tt_abbrind.  In
          each structure, tt_gmtoff gives the number of seconds to be
          added to Greenwich mean time (GMT), 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 4-byte values, written
          in a 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.

          Finally, there are tzh_ttisstdcnt standard/wall indicators,
          each stored as a 1-byte value.  They tell whether the tran-
          sition times associated with local time types are specified
          as standard time or wall-clock time and are used when a
          time-zone file is used in handling POSIX-style time-zone en-
          vironment variables.

          localtime uses the first standard-time ttinfo structure in
          the file (or simply the first ttinfo structure in the ab-
          sence of a standard-time structure) if either tzh_timecnt is
          0 or the time argument is less than the first transition
          time recorded in the file.

     SEE ALSO
          ctime(3), tzic(1M), tzdump(1M).

























     2                                                     April, 1990