tzfile(5)
Name
tzfile − time zone information
Syntax
#include <tzfile.h>
Description
The time zone information files used by tzset begin with bytes reserved for future use, followed by three 4-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_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.
This header is followed by tzh_timecnt 4-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 at which the rules for computing local time change). Next come tzh_timecnt 1-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 4-byte value for tt_gmtoff of type “long”, in a standard byte order, followed by a 1-byte value for tt_isdst and a 1-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 and tt_abbrind serves as an index into the array of time zone abbreviation characters that follows the ttinfo structure or structures in the file.
The localtime 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.