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BSD 2.11 - man page for tzfile (bsd section 5)

TZFILE(5)					 File Formats Manual					    TZFILE(5)

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 three four-byte values of type long, written in a ``standard'' byte order (the high-order byte of the value is writ- ten first). These values are, in order: 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 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. 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.
ctime(3) System V TZFILE(5)

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