tzfile — timezone information
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 transition 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 second 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 handling 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
timezone 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.
For version-2-format timezone files, the above header
and data are followed 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.
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.
This page is part of release 4.16 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/.
%%%LICENSE_START(PUBLIC_DOMAIN) This file is in the public domain, so clarified as of 1996-06-05 by Arthur David Olson <arthur_david_olsonnih.gov>. %%%LICENSE_END (#)tzfile.5 7.11 |