tzfile — time zone information
#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_ttisgmtcntThe number of UTC/local indicators stored in the file.
tzh_ttisstdcntThe number of standard/wall indicators stored in the file.
tzh_leapcntThe number of leap seconds for which data is stored in the file.
tzh_timecntThe number of "transition times" for which data is stored in the file.
tzh_typecntThe number of "local time types" for which data is stored in the file (must not be zero).
tzh_charcntThe 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_gmtoffof typelong, in a standard byte order, followed by a one-byte value fortt_isdstand a one-byte value fortt_abbrind. In each structure,tt_gmtoffgives the number of seconds to be added to UTC,tt_isdsttells whethertm_isdstshould be set by localtime(3), andtt_abbrindserves as an index into the array of time zone abbreviation characters that follow thettinfostructure(s) in the file.Then there are
tzh_leapcntpairs 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 thetotalnumber 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_ttisstdcntstandard/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_ttisgmtcntUTC/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.
Localtimeuses the first standard-timettinfostructure in the file (or simply the firstttinfostructure in the absence of a standard-time structure) if eithertzh_timecntis zero or the time argument is less than the first transition time recorded in the file.
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