diff options
Diffstat (limited to 'tzfile.5.txt')
| -rw-r--r-- | tzfile.5.txt | 129 |
1 files changed, 129 insertions, 0 deletions
diff --git a/tzfile.5.txt b/tzfile.5.txt new file mode 100644 index 000000000000..d55ac47fcb27 --- /dev/null +++ b/tzfile.5.txt @@ -0,0 +1,129 @@ +TZFILE(5) File Formats Manual TZFILE(5) + +NAME + tzfile - time zone information + +DESCRIPTION + The time zone information files used by tzset(3) are typically found + under a directory with a name like /usr/share/zoneinfo. These files + begin with a 44-byte header containing the following fields: + + * The magic four-byte ASCII sequence "TZif" identifies the file as a + time zone information file. + + * A byte identifying the version of the file's format (as of 2017, + either an ASCII NUL, or "2", or "3"). + + * Fifteen bytes containing zeros reserved for future use. + + * Six four-byte integer values written in a standard byte order (the + high-order byte of the value is written first). These values are, in + order: + + tzh_ttisgmtcnt + The number of UT/local indicators stored in the file. + + tzh_ttisstdcnt + The number of standard/wall indicators stored in the file. + + tzh_leapcnt + The number of leap seconds for which data entries are stored + in the file. + + tzh_timecnt + The number of transition times for which data entries are + stored in the file. + + tzh_typecnt + The number of local time types for which data entries are + stored in the file (must not be zero). + + tzh_charcnt + The number of bytes of time zone abbreviation strings stored + in the file. + + The above header is followed by the following fields, whose lengths + depend on the contents of the header: + + * tzh_timecnt four-byte signed integer values 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. + + * tzh_timecnt one-byte unsigned integer values; each one tells which of + the different types of local time types described in the file is + associated with the time period starting with the same-indexed + transition time. These values serve as indices into the next field. + + * tzh_typecnt ttinfo entries, each defined as follows: + + struct ttinfo { + int32_t tt_gmtoff; + unsigned char tt_isdst; + unsigned char tt_abbrind; + }; + + Each structure is written as a four-byte signed integer value for + tt_gmtoff, 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 UT, 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 bytes + that follow the ttinfo structure(s) in the file. + + * tzh_leapcnt pairs of four-byte values, written in standard byte + order; the first value of each pair gives the nonnegative time (as + returned by time(2)) at which a leap second occurs; the second gives + the total number of leap seconds to be applied during the time period + starting at the given time. The pairs of values are sorted in + ascending order by time. Each transition is for one leap second, + either positive or negative; transitions always separated by at least + 28 days minus 1 second. + + * 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 time zone file is used in handling POSIX-style time + zone environment variables. + + * tzh_ttisgmtcnt UT/local indicators, each stored as a one-byte value; + they tell whether the transition times associated with local time + types were specified as UT or local time, and are used when a time + zone file is used in handling POSIX-style time zone environment + variables. + + The localtime(3) function 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. + + Version 2 format + For version-2-format time zone files, the above header and data are + followed by a second header and data, identical in format except that + eight bytes are used for each transition time or leap second time. + (Leap second counts remain four bytes.) After the second header and + data comes a newline-enclosed, POSIX-TZ-environment-variable-style + string for use in handling instants after the last transition time + stored in the file (with nothing between the newlines if there is no + POSIX representation for such instants). The POSIX-style string must + agree with the local time type after both data's last transition times; + for example, given the string "WET0WEST,M3.5.0,M10.5.0/3" then if a + last transition time is in July, the transition's local time type must + specify a daylight-saving time abbreviated "WEST" that is one hour east + of UT. + + Version 3 format + For version-3-format time zone files, the POSIX-TZ-style string may use + two minor extensions to the POSIX TZ format, as described in + newtzset(3). First, the hours part of its transition times may be + signed and range from 167 through 167 instead of the POSIX-required + unsigned values from 0 through 24. Second, DST is in effect all year + if it starts January 1 at 00:00 and ends December 31 at 24:00 plus the + difference between daylight saving and standard time. + + Future changes to the format may append more data. + +SEE ALSO + time(2), localtime(3), tzset(3), tzselect(8), zdump(8), zic(8) + + TZFILE(5) |