amiga-tz/tzfile.5

.TH TZFILE 5
.SH NAME
tzfile \- time zone information
.SH SYNOPSIS
.B
#include <tzfile.h>
.SH DESCRIPTION
The time zone information files used by
.IR tzset (3)
begin with the magic characters "TZif" to identify them as
time zone information files,
followed by a character identifying the version of the file's format
(as of 2013, either an ASCII NUL, or '2', or '3')
followed by fifteen bytes containing zeroes reserved for future use,
followed by 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:
.TP
.I tzh_ttisgmtcnt
The number of UT/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 "time zone abbreviation strings"
stored in the file.
.PP
The above header is followed by
.I 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
.IR time (2))
at which the rules for computing local time change.
Next come
.I 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 same-indexed transition time.
These values serve as indices into an array of
.I ttinfo
structures (with
.I tzh_typecnt
entries) that appears next in the file;
these structures are defined as follows:
.in +.5i
.sp
.nf
.ta .5i +\w'unsigned char\0\0'u
struct ttinfo {
	int32_t	tt_gmtoff;
	unsigned char	tt_isdst;
	unsigned char	tt_abbrind;
};
.in -.5i
.fi
.sp
Each structure is written as a four-byte signed integer value for
.IR tt_gmtoff ,
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 UT,
.I tt_isdst
tells whether
.I tm_isdst
should be set by
.I localtime (3)
and
.I tt_abbrind
serves as an index into the array of time zone 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
.IR 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 time zone file is used in handling POSIX-style
time zone environment variables.
.PP
Finally there are
.I 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 time zone file is used in handling POSIX-style
time zone environment variables.
.PP
.I Localtime
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.
.PP
For version-2-format time zone 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.
After the second header and data,
and just before the end of the file, 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
Version-3-format time zone files have the following additions:
.IP
The POSIX-TZ-style string may use two minor extensions to the
POSIX TZ format, as described in
.IR newtzset (3).
First, the hours part of its transition times may be signed and range from
\(mi167 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.
.IP
The newline-enclosed POSIX-TZ-style string is preceded by a section
containing auxiliary meta-information that is not needed to process
time stamps.  This section consists of another copy of the
newline-enclosed POSIX-TZ-style string (this is for the benefit of
version-2-only clients), followed by a four-byte integer size value,
followed by zero or more NUL-terminated byte strings, followed by an
additional NUL.  The size value is the total number of bytes in all
the byte strings, including the trailing NULs at the end of the
strings, but not including the additional NUL.  Each byte string
consists of a name-value pair separated by "=".  Names consist of
ASCII letters, digits and underscores, and start with a letter;
duplicate names are not allowed.  Two common names are "name", the
Zone name for the data, and "version", the data's version number.
Values can contain any bytes except NUL.
.PP
Future additions to the format may insert more data just before the
newline-enclosed POSIX-TZ-style string at the end of the file, so
clients should not assume that this string immediately follows
the auxiliary meta-information.
.SH SEE ALSO
newctime(3), newtzset(3)
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson.