tzfile

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

NAME
       tzfile - timezone information

DESCRIPTION
       The  timezone  information  files  used by tzset(3) are typically found
       under a directory with a name like  /usr/share/zoneinfo.   These  files
       begin with a 44-byte header containing the following fields:

       * The  magic  four-byte  ASCII sequence "TZif" identifies the file as a
         timezone information file.

       * A byte identifying the version of the  file's  format  (as  of  2017,
         either an ASCII NUL, or "2", or "3").

       * Fifteen bytes containing zeros reserved for future use.

       * Six  four-byte  integer  values 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 UT/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 entries are stored
                in the file.

         tzh_timecnt
                The number of transition times  for  which  data  entries  are
                stored in the file.

         tzh_typecnt
                The  number  of  local  time  types for which data entries are
                stored in the file (must not be zero).

         tzh_charcnt
                The number of bytes of timezone abbreviation strings stored in
                the file.

       The  above  header  is  followed by the following fields, whose lengths
       vary depend on the contents of the header:

       * tzh_timecnt four-byte  signed  integer  values  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.

       * tzh_timecnt one-byte unsigned integer values; each one tells which of
         the different types of local time types  described  in  the  file  is
         associated  with the time period starting with the same-indexed tran-
         sition time.  These values serve as indices into the next field.

       * tzh_typecnt ttinfo entries, each defined as follows:

           struct ttinfo {
               int32_t       tt_gmtoff;
               unsigned char tt_isdst;
               unsigned char tt_abbrind;
           };

       Each structure is written as  a  four-byte  signed  integer  value  for
       tt_gmtoff,  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 UT, 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 bytes that follow the
       ttinfo structure(s) in the file.

       *      tzh_leapcnt pairs of four-byte values, written in standard  byte
              order;  the  first value of each pair gives the nonnegative time
              (as returned by time(2)) at which a leap second occurs; the sec-
              ond  gives the total number of leap seconds to be applied during
              the time period starting at the given time.  The pairs of values
              are  sorted  in ascending order by time.  Each transition is for
              one leap second, either positive or negative; transitions always
              separated by at least 28 days minus 1 second.

       *      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  han-
              dling POSIX-style timezone environment variables.

       *      tzh_ttisgmtcnt  UT/local  indicators,  each stored as a one-byte
              value; they tell whether the transition  times  associated  with
              local  time  types  were  specified as UT or local time, and are
              used when a timezone file is used in handling POSIX-style  time-
              zone environment variables.

       The localtime(3) function 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).

   Version 2 format
       For version-2-format timezone files, the above header and data are 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.
       (Leap second counts remain four bytes.)  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).  The POSIX-style  string  must
       agree with the local time type after both data's last transition times;
       for example, given the string  "WET0WEST,M3.5.0,M10.5.0/3"  then  if  a
       last  transition time is in July, the transition's local time type must
       specify a daylight-saving time abbreviated "WEST" that is one hour east
       of UT.

   Version 3 format
       For  version-3-format timezone files, the POSIX-TZ-style string may use
       two  minor  extensions  to  the  POSIX  TZ  format,  as  described   in
       newtzset(3).   First,  the  hours  part  of its transition times may be
       signed and range from -167 through 167 instead  of  the  POSIX-required
       unsigned  values  from 0 through 24.  Second, DST is in effect all year
       if it starts January 1 at 00:00 and ends December 31 at 24:00 plus  the
       difference between daylight saving and standard time.

       Future changes to the format may append more data.

SEE ALSO
       time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8)

COLOPHON
       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

                                  2017-08-04                         TZFILE(5)
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