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Timezone information files begin with a 44-byte header structured as follows:
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 (with tzh_typecnt entries) that appear 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 timezone 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 timezone file is used in handling POSIX-style timezone 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 timezone file is used in handling POSIX-style timezone environment variables.
localtime(3) 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.
The second section of the timezone file consists of another 44-byte header record, identical in structure to the one at the beginning of the file, except that it applies to the data that follows, which is also identical in structure to the first section of the timezone file, with the following differences:
In all cases, the eight-byte time values are given in the "standard" byte order, the high-order byte first.
The POSIX timezone string defines a rule for computing transition times that follow the last transition time explicitly specified in the timezone information file.
Four-byte value section (header version 0x00 or 0x32) Header record Four-byte transition times Transition time index ttinfo structures Timezone abbreviation array Leap second records Standard/Wall array UTC/Local array Eight-byte value section (only if first header version is 0x32, the second header's version is also 0x32) Header record Eight-byte transition times Transition time index ttinfo structures Timezone abbreviation array Leap second records Standard/Wall array UTC/Local array Third section (optional, only in 0x32 version files) Newline character Timezone string Newline character
timezone/tzfile.h in the glibc source tree