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).
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.
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¶
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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/.