NAME | DESCRIPTION | SEE ALSO | COLOPHON

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

NAME         top

       tzfile - timezone information

DESCRIPTION         top

       This page describes the structure of the timezone files used by
       tzset(3).  These files are typically found under one of the
       directories /usr/lib/zoneinfo or /usr/share/zoneinfo.

       Timezone information files begin with a 44-byte header structured as
       follows:

       *  The magic four-byte sequence "TZif" identifying this as a timezone
          information file.

       *  A single character identifying the version of the file's format:
          either an ASCII NUL ('\0') or a '2' (0x32).

       *  Fifteen bytes containing zeros reserved for future use.

       *  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 "timezone 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 appear 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 timezone 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 timezone file is used in
       handling POSIX-style timezone 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 timezone file is used in handling POSIX-style
       timezone 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.

   Version 2 format
       For version-2-format timezone 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
       (and that the version byte in the header record is 0x32 rather than
       0x00).  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 second section of the timezone file consists of another 44-byte
       header record, identical in structure to the one at the beginning of
       the file, except that it applies to the data that follows, which is
       also identical in structure to the first section of the timezone
       file, with the following differences:

       *  The transition time values, after the header, are eight-byte
          values.

       *  In each leap second record, the leap second value is an eight-byte
          value.  The accumulated leap second count is still a four-byte
          value.

       In all cases, the eight-byte time values are given in the "standard"
       byte order, the high-order byte first.

   POSIX timezone string
       The second eight-byte time value section is followed by an optional
       third section: a single ASCII newline character ('\n'), then a text
       string followed by a second newline character.  The text string is a
       POSIX timezone string, whose format is described in the tzset(3)
       manual page.

       The POSIX timezone string defines a rule for computing transition
       times that follow the last transition time explicitly specified in
       the timezone information file.

   Summary of the timezone information file format

              Four-byte value section
              (header version 0x00 or 0x32)
                      Header record
                      Four-byte transition times
                      Transition time index
                      ttinfo structures
                      Timezone abbreviation array
                      Leap second records
                      Standard/Wall array
                      UTC/Local array

              Eight-byte value section
              (only if first header version is 0x32,
              the second header's version is also 0x32)
                      Header record
                      Eight-byte transition times
                      Transition time index
                      ttinfo structures
                      Timezone abbreviation array
                      Leap second records
                      Standard/Wall array
                      UTC/Local array

              Third section
              (optional, only in 0x32 version files)
                      Newline character
                      Timezone string
                      Newline character

SEE ALSO         top

       ctime(3), tzset(3), tzselect(8),

       timezone/tzfile.h in the glibc source tree

COLOPHON         top

       This page is part of release 4.00 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
       http://www.kernel.org/doc/man-pages/.

                                 2015-05-07                        TZFILE(5)