Name
tzfile — time zone information
DESCRIPTION
The time zone information files used by tzset(3) begin with the
magic characters "TZif" to identify then as time zone
information files, followed by sixteen bytes reserved for
future use, followed by six four-byte values of type
long, written in a
``standard'' byte order (the high-order byte of the value is
written first). These values are, in order:
tzh_ttisgmtcnt-
The number of UTC/local indicators stored in the
file.
tzh_ttisstdcnt-
The number of standard/wall indicators stored in the
file.
tzh_leapcnt-
The number of leap seconds for which data is stored
in the file.
tzh_timecnt-
The number of "transition times" for which data is
stored in the file.
tzh_typecnt-
The number of "local time types" for which data is
stored in the file (must not be zero).
tzh_charcnt-
The number of characters of "time zone abbreviation
strings" stored in the file.
The above header is followed by tzh_timecnt four-byte values
of type long,
sorted in ascending order. These values are written in
``standard'' byte order. Each is used as a transition time
(as returned by time(2)) at which the rules
for computing local time change. Next come tzh_timecnt one-byte values
of type 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 ttinfo structures that
appears next in the file; these structures are defined as
follows:
| struct |
ttinfo { |
| |
long |
|
tt_gmtoff; |
|
| |
int |
|
tt_isdst; |
|
| |
unsigned int |
|
tt_abbrind; |
|
| }; |
Each structure is written as a four-byte value for
tt_gmtoff of
type long, in a
standard byte order, followed by a one-byte value for
tt_isdst and a
one-byte value for tt_abbrind. In each
structure, tt_gmtoff gives the
number of seconds to be added to UTC, tt_isdst tells whether
tm_isdst should
be set by localtime(3), and
tt_abbrind
serves as an index into the array of time zone
abbreviation characters that follow the ttinfo structure(s) in
the file.
Then there are tzh_leapcnt pairs of
four-byte values, written in standard byte order; the
first value of each pair gives the time (as returned by
time(2)) at which a
leap second occurs; the second gives the total number of leap
seconds to be applied after the given time. The pairs of
values are sorted in ascending order by time.
Then there are 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.
Finally, there are 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.
Localtime
uses the first standard-time ttinfo structure in the
file (or simply the first ttinfo structure in the
absence of a standard-time structure) if either
tzh_timecnt is
zero or the time argument is less than the first
transition time recorded in the file.
@(#)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@nih.gov>.
|
