tzfile
—
timezone information
#include
"/usr/src/contrib/tzcode/stdtime/tzfile.h"
The time zone information files used by
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 2005, either an ASCII NUL
or a '2') followed by fifteen bytes containing zeroes reserved for future use,
followed by four four-byte values 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(3))
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 (with
tzh_typecnt entries) 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(3))
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.
For version-2-format time zone files, the above header and data is 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).
ctime(3),
time2posix(3),
zic(8)