TZFILE(5)

NAME
tzfile - time zone information

SYNOPSIS
#include <tzfile.h>

DESCRIPTION

The time zone information files used by tzset (see ctime(3V)) 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 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 (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 gettimeofday(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 (see ctime(3V)) 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.

SEE ALSO
gettimeofday(2), ctime(3V)