Originální popis anglicky:
tzfile - time zone information
Návod, kniha: File Formats Manual
#include <tzfile.h>
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.
