man-pages/man5/tzfile.5

.\" %%%LICENSE_START(PUBLIC_DOMAIN)
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson <arthur_david_olson@nih.gov>.
.\" %%%LICENSE_END
.\"
.\" @(#)tzfile.5	7.11
.\"
.TH TZFILE 5 2015-05-07 "" "Linux Programmer's Manual"
.SH NAME
tzfile \- timezone information
.SH DESCRIPTION
This page describes the structure of the timezone files used by
.BR tzset (3).
These files are typically found under one of the directories
.IR /usr/lib/zoneinfo
or
.IR /usr/share/zoneinfo .

Timezone information files begin with a 44-byte header structured as follows:
.IP * 3
The magic four-byte sequence
"TZif" identifying this as a
timezone information file.
.IP *
A single character identifying the version of the file's format:
either an ASCII NUL (\(aq\\0\(aq) or a \(aq2\(aq (\fB0x32\fP).
.IP *
Fifteen bytes containing zeros reserved for future use.
.IP *
Six four-byte values of type
.IR long ,
written in a "standard" byte order
(the high-order byte of the value is written first).
These values are,
in order:
.RS
.TP
.I tzh_ttisgmtcnt
The number of UTC/local indicators stored in the file.
.TP
.I tzh_ttisstdcnt
The number of standard/wall indicators stored in the file.
.TP
.I tzh_leapcnt
The number of leap seconds for which data is stored in the file.
.TP
.I tzh_timecnt
The number of "transition times" for which data is stored
in the file.
.TP
.I tzh_typecnt
The number of "local time types" for which data is stored
in the file (must not be zero).
.TP
.I tzh_charcnt
The number of characters of "timezone abbreviation strings"
stored in the file.
.RE
.PP
The above header is followed by
.I tzh_timecnt
four-byte values of type
.IR long ,
sorted in ascending order.
These values are written in "standard" byte order.
Each is used as a transition time (as returned by
.BR time (2))
at which the rules for computing local time change.
Next come
.I tzh_timecnt
one-byte values of type
.IR "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
.I ttinfo
structures (with
.I tzh_typecnt
entries) that appear next in the file;
these structures are defined as follows:
.in +4n
.sp
.nf
struct ttinfo {
    long         tt_gmtoff;
    int          tt_isdst;
    unsigned int tt_abbrind;
};
.in
.fi
.sp
Each structure is written as a four-byte value for
.I tt_gmtoff
of type
.IR long ,
in a standard byte order, followed by a one-byte value for
.I tt_isdst
and a one-byte value for
.IR tt_abbrind .
In each structure,
.I tt_gmtoff
gives the number of seconds to be added to UTC,
.I tt_isdst
tells whether
.I tm_isdst
should be set by
.BR localtime (3),
and
.I tt_abbrind
serves as an index into the array of timezone abbreviation characters
that follow the
.I ttinfo
structure(s) in the file.
.PP
Then there are
.I tzh_leapcnt
pairs of four-byte values, written in standard byte order;
the first value of each pair gives the time
(as returned by
.BR time (2))
at which a leap second occurs;
the second gives the
.I total
number of leap seconds to be applied after the given time.
The pairs of values are sorted in ascending order by time.
.PP
Then there are
.I 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.
.PP
Finally, there are
.I 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.
.PP
.BR localtime (3)
uses the first standard-time
.I ttinfo
structure in the file
(or simply the first
.I ttinfo
structure in the absence of a standard-time structure)
if either
.I tzh_timecnt
is zero or the time argument is less than the first transition time recorded
in the file.
.SS 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
\fB0x32\fP rather than \fB0x00\fP).
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).
.PP
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:
.IP * 3
The transition time values, after the header, are eight-byte values.
.IP *
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.
.PP
In all cases, the eight-byte time values are given in
the "standard" byte order,
the high-order byte first.
.SS POSIX timezone string
The second eight-byte time value section is followed by an optional
third section:
a single ASCII newline character (\(aq\\n\(aq),
then a text string followed by a second
newline character.
The text string is a POSIX timezone string, whose format is described in the
.BR tzset (3)
manual page.
.PP
The POSIX timezone string defines a rule for computing transition times
that follow the last transition time explicitly specified in the timezone
information file.
.SS Summary of the timezone information file format
\&
.RS
.nf
Four-byte value section
(header version \fB0x00\fP or \fB0x32\fP)
        Header record
        Four-byte transition times
        Transition time index
        \fBttinfo\fP structures
        Timezone abbreviation array
        Leap second records
        Standard/Wall array
        UTC/Local array

Eight-byte value section
(only if first header version is \fB0x32\fP,
the second header's version is also \fB0x32\fP)
        Header record
        Eight-byte transition times
        Transition time index
        \fBttinfo\fP structures
        Timezone abbreviation array
        Leap second records
        Standard/Wall array
        UTC/Local array

Third section
(optional, only in \fB0x32\fP version files)
        Newline character
        Timezone string
        Newline character
.fi
.RE
.\"
.SH SEE ALSO
.BR ctime (3),
.BR tzset (3),
.BR tzselect (8),

.I timezone/tzfile.h
in the glibc source tree