TZFILE(5) MachTen Programmer’s Manual TZFILE(5)

NAME
tzfile - time zone information

SYNOPSIS
#include <tzfile.h>

DESCRIPTION
The time zone information files used by tzset(3) begin with bytes re-
served 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 values of type
long, sorted in ascending order. These values are written in ‘‘stan-
dard’’ 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 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 abbreviation characters that follow the ttinfo struc-
ture(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 num-
ber 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.