Ubuntu Manpage: tzfile - timezone information

NAME

       tzfile - timezone information

DESCRIPTION

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.

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-maintained timezone data (which is maintained by some other entity).

   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

COLOPHON

       This page is part of release 4.04 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)

NAME

DESCRIPTION

NOTES

SEE ALSO

COLOPHON