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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
+.BR tzset (3)
+are typically found under a directory with a name like
+.IR /usr/share/zoneinfo .
+These files begin with a 44-byte header containing the following fields:
+.IP * 2
+The magic four-byte ASCII sequence
+.q "TZif"
+identifies the file as a time zone information file.
+.IP *
+A byte identifying the version of the file's format
+(as of 2017, either an ASCII NUL, or
+.q "2",
+or
+.q "3" ).
+.IP *
+Fifteen bytes containing zeros reserved for future use.
+.IP *
+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:
+.RS
.TP
.I tzh_ttisgmtcnt
The number of UT/local indicators stored in the file.
in the file (must not be zero).
.TP
.I tzh_charcnt
-The number of characters of time zone abbreviation strings
+The number of bytes of time zone abbreviation strings
stored in the file.
+.RE
.PP
-The above header is followed by
+The above header is followed by the following fields, whose lengths
+depend on the contents of the header:
+.IP * 2
.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))
+.BR time (2))
at which the rules for computing local time change.
-Next come
+.IP *
.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 time period
starting with the same-indexed transition time.
-These values serve as indices into an array of
-.I ttinfo
-structures (with
+These values serve as indices into the next field.
+.IP *
.I tzh_typecnt
-entries) that appears next in the file;
-these structures are defined as follows:
+.I ttinfo
+entries, each defined as follows:
.in +.5i
.sp
.nf
tells whether
.I tm_isdst
should be set by
-.I localtime (3)
+.BR localtime (3)
and
.I tt_abbrind
-serves as an index into the array of time zone abbreviation characters
+serves as an index into the array of time zone abbreviation bytes
that follow the
.I ttinfo
structure(s) in the file.
-.PP
-Then there are
+.IP *
.I tzh_leapcnt
pairs of four-byte values, written in standard byte order;
-the first value of each pair gives the time
+the first value of each pair gives the nonnegative time
(as returned by
-.IR time(2))
+.BR time (2))
at which a leap second occurs;
the second gives the
.I total
number of leap seconds to be applied during the time period
starting at the given time.
The pairs of values are sorted in ascending order by time.
-.PP
-Then there are
+Each transition is for one leap second, either positive or negative;
+transitions always separated by at least 28 days minus 1 second.
+.IP *
.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
+.IP *
.I tzh_ttisgmtcnt
UT/local indicators, each stored as a one-byte value;
they tell whether the transition times associated with local time types
and are used when a time zone file is used in handling POSIX-style
time zone environment variables.
.PP
-.I Localtime
+The
+.BR localtime (3)
+function
uses the first standard-time
.I ttinfo
structure in the file
.I tzh_timecnt
is zero or the time argument is less than the first transition time recorded
in the file.
-.PP
+.SS Version 2 format
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.
+(Leap second counts remain four bytes.)
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).
-The POSIX-style string must must agree with the local time type after
+The POSIX-style string must agree with the local time type after
both data's last transition times; for example, given the string
.q "WET0WEST,M3.5.0,M10.5.0/3"
then if a last transition time is in July, the transition's local time
type must specify a daylight-saving time abbreviated
.q "WEST"
that is one hour east of UT.
-.PP
+.SS Version 3 format
For version-3-format time zone files, the POSIX-TZ-style string may
use two minor extensions to the POSIX TZ format, as described in
-.IR newtzset (3).
+.BR newtzset (3).
First, the hours part of its transition times may be signed and range from
\-167 through 167 instead of the POSIX-required unsigned values
from 0 through 24. Second, DST is in effect all year if it starts
.PP
Future changes to the format may append more data.
.SH SEE ALSO
-newctime(3), newtzset(3), zdump(8), zic(8)
+.BR time (2),
+.BR localtime (3),
+.BR tzset (3),
+.BR tzselect (8),
+.BR zdump (8),
+.BR zic (8)
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson.
projects / platform / upstream / tzdata.git / blobdiff