tzfile(5)
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
SEE ALSO
ctime(3), tzset(3), tzselect(8),
timezone/tzfile.h in the glibc source tree
COLOPHON
This page is part of release 4.09 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/.
2015-05-07 TZFILE(5)
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