tzfile — timezone information
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_ttisgmtcntThe number of UTC/local indicators stored in the file.
tzh_ttisstdcntThe number of standard/wall indicators stored in the file.
tzh_leapcntThe number of leap seconds for which data is stored in the file.
tzh_timecntThe number of "transition times" for which data is stored in the file.
tzh_typecntThe number of "local time types" for which data is stored in the file (must not be zero).
tzh_charcntThe 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.
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.
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.
Four-byte value section (header version0x00or0x32) Header record Four-byte transition times Transition time indexttinfostructures Timezone abbreviation array Leap second records Standard/Wall array UTC/Local array Eight-byte value section (only if first header version is0x32, the second header's version is also0x32) Header record Eight-byte transition times Transition time indexttinfostructures Timezone abbreviation array Leap second records Standard/Wall array UTC/Local array Third section (optional, only in0x32version files) Newline character Timezone string Newline character
This page is part of release 4.07 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/.
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%%%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 |