NAME
tzfile —
timezone information
SYNOPSIS
#include
<tzfile.h>
DESCRIPTION
The time zone information files used by
tzset(3)
begin with the magic characters “TZif”
to identify them as time zone information files, followed by a character
identifying the version of the file's format (as of 2005, either an ASCII
NUL or a '2') followed by fifteen bytes containing zeroes reserved for
future use, followed by four four-byte values 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(3)) 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 (with tzh_typecnt entries) 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(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.
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.
For version-2-format time zone files, the above header and data is followed by a second header and data, identical in format except that eight bytes are used for each transition time or leap second time. After the second header and data comes a newline-enclosed, POSIX-TZ-environment-variable-style string for use in handling instants after the last transition time stored in the file (with nothing between the newlines if there is no POSIX representation for such instants).