libc/time/tzfile.5

.\"	$OpenBSD: tzfile.5,v 1.15 2015/03/14 22:15:09 millert Exp $
.Dd $Mdocdate: March 14 2015 $
.Dt TZFILE 5
.Os
.Sh NAME
.Nm tzfile
.Nd time zone information
.Sh DESCRIPTION
The time zone information files used by
.Xr tzset 3
begin with the magic characters "TZif" to identify themselves as
time zone information files,
followed by a character identifying the version of the file's format
(as of 2005, either an ASCII NUL or a '2')
followed by fifteen bytes containing zeroes reserved for future use,
followed by six four-byte values of type
.Fa int
written in a
.Dq big endian
byte order
.Po
the most significant byte of the value is written first
.Pc .
These values are,
in order:
.Bl -tag -width "tzh_ttisgmtcnt"
.It Fa tzh_ttisgmtcnt
The number of UTC/local indicators stored in the file.
.It Fa tzh_ttisstdcnt
The number of standard/wall indicators stored in the file.
.It Fa tzh_leapcnt
The number of leap seconds for which data is stored in the file.
.It Fa tzh_timecnt
The number of "transition times" for which data is stored
in the file.
.It Fa tzh_typecnt
The number of "local time types" for which data is stored
in the file (must not be zero).
.It Fa tzh_charcnt
The number of characters of "time zone abbreviation strings"
stored in the file.
.El
.Pp
The above header is followed by
.Fa tzh_timecnt
four-byte values of type
.Fa long ,
sorted in ascending order.
These values are written in
.Dq standard
byte order.
Each is used as a transition time
.Pq as returned by Xr time 3
at which the rules for computing local time change.
Next come
.Fa tzh_timecnt
one-byte values of type
.Fa unsigned char ;
each one tells which of the different types of
.Dq local time
types
described in the file is associated with the same-indexed transition time.
These values serve as indices into an array of
.Fa ttinfo
structures (with
.Fa tzh_typecnt
entries) that appears next in the file;
these structures are defined as follows:
.Bd -literal -offset indent
.ta .5i +\w'unsigned int\0\0'u
struct ttinfo {
	long	tt_gmtoff;	/* UTC offset in seconds */
	int	tt_isdst;	/* used to set tm_isdst */
	int	tt_abbrind;	/* abbreviation list index */
};
.Ed
.Pp
Each structure is written as a four-byte value for
.Fa tt_gmtoff
of type
.Fa long ,
in
.Dq big endian
byte order, followed by a one-byte value for
.Fa tt_isdst
and a one-byte value for
.Fa tt_abbrind .
In each structure,
.Fa tt_gmtoff
gives the number of seconds to be added to UTC,
.Fa tt_isdst
tells whether
.Fa tm_isdst
should be set by
.Xr localtime 3
and
.Fa tt_abbrind
serves as an index into the array of time zone abbreviation characters
that follow the
.Fa ttinfo
structure(s) in the file.
.Pp
Then there are
.Fa tzh_leapcnt
pairs of four-byte values, written in standard byte order;
the first value of each pair gives the time
.Pq as returned by Xr time 3
at which a leap second occurs;
the second gives the
.Em 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
.Fa 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
.Fa 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 time zone file is used in handling POSIX-style
time zone environment variables.
.Pp
.Xr localtime 3
uses the first standard-time
.Fa ttinfo
structure in the file,
or simply the first
.Fa ttinfo
structure in the absence of a standard-time structure,
if either
.Fa 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 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).
.Sh SEE ALSO
.Xr ctime 3
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson.