TZFILE(5)TZFILE(5)NAMEtzfile - time zone information
SYNOPSIS
#include <tzfile.h>
DESCRIPTION
The time zone information files used by tzset(3) begin
with bytes reserved for future use, followed by four 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_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 val-
ues 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
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TZFILE(5)TZFILE(5)
tt_abbrind. In each structure, tt_gmtoff gives the number
of seconds to be added to GMT, 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 abbrevia-
tion 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.
Finally 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.
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.
SEE ALSOdate(1), ctime(3)
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