tzfile(4) — File Formats
NAME
tzfile − Time zone information
SYNOPSIS
#include <tzfile.h>
DESCRIPTION
The time zone information file used by tzset begins with bytes reserved for future use, followed by three 4-byte values written in a “standard” byte order (that is, with the high-order byte of the value written 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. (This value must not be zero.)
tzh_charcnt
The number of characters of “time zone abbreviation strings” stored in the file.
This header is followed by tzh_timecnt 4-byte values sorted in ascending order. These values are written in “standard” byte order. Each is used as a transition time (as returned by the time at which the rules for computing local time change). Next come the tzh_timecnt 1-byte values of type unsigned char. Each value tells which of the different local time types described in the file is associated with the same-indexed transition time. These values serve as indexes 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.