tzfile.5 revision 35922 
1.Dd September 13, 1994
2.Dt TZFILE 5
3.Os FreeBSD 2.0
4.Sh NAME
5.Nm tzfile
6.Nd timezone information
7.Sh SYNOPSIS
8.Fd #include <tzfile.h>
9.Sh DESCRIPTION
10The time zone information files used by
11.Xr tzset 3
12begin with bytes reserved for future use,
13followed by four four-byte values
14written in a ``standard'' byte order
15(the high-order byte of the value is written first).
16These values are,
17in order:
18.Pp
19.Bl -tag -compact -width tzh_ttisstdcnt
20.It Li tzh_ttisstdcnt
21The number of standard/wall indicators stored in the file.
22.It Li tzh_leapcnt
23The number of leap seconds for which data is stored in the file.
24.It Li tzh_timecnt
25The number of ``transition times'' for which data is stored
26in the file.
27.It Li tzh_typecnt
28The number of ``local time types'' for which data is stored
29in the file (must not be zero).
30.It Li tzh_charcnt
31The number of characters of ``time zone abbreviation strings''
32stored in the file.
33.El
34.Pp
35The above header is followed by
36.Li tzh_timecnt
37four-byte values of type
38.Fa long ,
39sorted in ascending order.
40These values are written in ``standard'' byte order.
41Each is used as a transition time (as returned by
42.Xr time 3 )
43at which the rules for computing local time change.
44Next come
45.Li tzh_timecnt
46one-byte values of type
47.Fa "unsigned char" ;
48each one tells which of the different types of ``local time'' types
49described in the file is associated with the same-indexed transition time.
50These values serve as indices into an array of
51.Fa ttinfo
52structures that appears next in the file;
53these structures are defined as follows:
54.Pp
55.Bd -literal -offset indent
56struct ttinfo {
57	long	tt_gmtoff;
58	int	tt_isdst;
59	unsigned int	tt_abbrind;
60};
61.Ed
62.Pp
63Each structure is written as a four-byte value for
64.Li tt_gmtoff
65of type
66.Fa long ,
67in a standard byte order, followed by a one-byte value for
68.Li tt_isdst
69and a one-byte value for
70.Li tt_abbrind .
71In each structure,
72.Li tt_gmtoff
73gives the number of seconds to be added to GMT,
74.Li tt_isdst
75tells whether
76.Li tm_isdst
77should be set by
78.Xr localtime 3
79and
80.Li tt_abbrind
81serves as an index into the array of time zone abbreviation characters
82that follow the
83.Li ttinfo
84structure(s) in the file.
85.Pp
86Then there are
87.Li tzh_leapcnt
88pairs of four-byte values, written in standard byte order;
89the first value of each pair gives the time
90(as returned by
91.Xr time 3 )
92at which a leap second occurs;
93the second gives the
94.Em total
95number of leap seconds to be applied after the given time.
96The pairs of values are sorted in ascending order by time.
97.Pp
98Finally there are
99.Li tzh_ttisstdcnt
100standard/wall indicators, each stored as a one-byte value;
101they tell whether the transition times associated with local time types
102were specified as standard time or wall clock time,
103and are used when a time zone file is used in handling POSIX-style
104time zone environment variables.
105.Pp
106.Nm localtime
107uses the first standard-time
108.Li ttinfo
109structure in the file
110(or simply the first
111.Li ttinfo
112structure in the absence of a standard-time structure)
113if either
114.Li tzh_timecnt
115is zero or the time argument is less than the first transition time recorded
116in the file.
117.Sh SEE ALSO
118.Xr ctime 3 ,
119.Xr time2posix 3 ,
120.Xr zic 8
121.\" @(#)tzfile.5	7.2
122