libc/time/tzfile.5

tzfile.5 revision 1.12  $NetBSD: tzfile.5,v 1.12 2003/04/16 13:34:58 wiz Exp $

 @(#)tzfile.5 7.11
 This file is in the public domain, so clarified as of
 1996-06-05 by Arthur David Olson (arthur_david_olson (at] nih.gov).
.Dd September 16, 2001
.Os
.Dt TZFILE 5
.Sh NAME
.Nm tzfile
.Nd time zone information
.Sh SYNOPSIS
n tzfile.h .Sh DESCRIPTION
The time zone information files used by
.Xr tzset 3
begin with the magic characters
.Dq TZif
to identify them as time zone information files,
followed by sixteen bytes reserved for future use,
followed by six four-byte values of type
.Fa long ,
written in a
.Dq standard
byte order (the high-order byte of the value is written first).
These values are, in order:
l -tag -width XXXXXX -compact t Va tzh_ttisgmtcnt The number of UTC/local indicators stored in the file.
t Va tzh_ttisstdcnt The number of standard/wall indicators stored in the file.
t Va tzh_leapcnt The number of leap seconds for which data is stored in the file.
t Va tzh_timecnt The number of
.Dq transition times
for which data is stored in the file.
t Va tzh_typecnt The number of
.Dq local time types
for which data is stored in the file (must not be zero).
t Va tzh_charcnt The number of characters of "time zone abbreviation strings"
stored in the file.
.El
p
The above header is followed by
.Va 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 (as returned by
.Xr time 3 )
at which the rules for computing local time change.
Next come
.Va 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 that appears next in the file;
these structures are defined as follows:
d -literal struct ttinfo {
 long tt_gmtoff;
 int tt_isdst;
 unsigned int tt_abbrind;
};
.Ed
Each structure is written as a four-byte value for
.Va tt_gmtoff
of type
.Fa long ,
in a standard byte order, followed by a one-byte value for
.Va tt_isdst
and a one-byte value for
.Va tt_abbrind .
In each structure,
.Va tt_gmtoff
gives the number of seconds to be added to UTC,
.Va tt_isdst
tells whether
.Va tm_isdst
should be set by
.Xr localtime 3
and
.Va tt_abbrind
serves as an index into the array of time zone abbreviation characters
that follow the
.Va ttinfo
structure(s) in the file.
p
Then there are
.Va tzh_leapcnt
pairs of four-byte values, written in standard byte order;
the first value of each pair gives the time
(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.
p
Then there are
.Va 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.
p
Finally there are
.Va 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.
p
.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
.Va tzh_timecnt
is zero or the time argument is less than the first transition time recorded
in the file.
.Sh SEE ALSO
.Xr ctime 3 ,
.Xr localtime 3 ,
.Xr time 3