tzfile - Time zone information
#include <tzfile.h>
The time zone information files used by tzset(3) begin
with bytes reserved for future use, followed by six fourbyte
values, written in a standard byte order (that is,
with the high-order byte of the value written first).
These values are, in order: The number of GMT/local indicators
stored in the file. The number of standard/wall
indicators stored in the file. The number of leap seconds
for which data is stored in the file. The number of transition
times for which data is stored in the file. The
number of local time types for which data is stored in the
file. (This value must not be zero.) The number of characters
of time zone abbreviation strings stored in the
file.
The above header is followed by tzh_timecnt four-byte values,
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 value tells which of the different
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 structure(s) in the
file.
Next, 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.
Fnally, there are tzh_ttisgmtcnt GMT/local indicators,
each stored as a one-byte value; they tell whether the
transition times associated with local time types were
specified as GMT or local time, and are used when a time
zone file is used in handling POSIX-style time zone environment
variables.
The localtime(3) call 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.
ctime(3) delim off
tzfile(4)
[ Back ] |