mirror of https://github.com/mkerrisk/man-pages
205 lines
6.3 KiB
Groff
205 lines
6.3 KiB
Groff
.\" %%%LICENSE_START(PUBLIC_DOMAIN)
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.\" This file is in the public domain, so clarified as of
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.\" 1996-06-05 by Arthur David Olson <arthur_david_olson@nih.gov>.
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.\" %%%LICENSE_END
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.\"
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.TH TZFILE 5 2019-03-06 "" "Linux Programmer's Manual"
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.SH NAME
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tzfile \- timezone information
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.SH DESCRIPTION
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.ie '\(lq'' .ds lq \&"\"
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.el .ds lq \(lq\"
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.ie '\(rq'' .ds rq \&"\"
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.el .ds rq \(rq\"
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.de q
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\\$3\*(lq\\$1\*(rq\\$2
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..
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The timezone information files used by
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.BR tzset (3)
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are typically found under a directory with a name like
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.IR /usr/share/zoneinfo .
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These files begin with a 44-byte header containing the following fields:
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.IP * 2
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The magic four-byte ASCII sequence
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.q "TZif"
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identifies the file as a timezone information file.
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.IP *
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A byte identifying the version of the file's format
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(as of 2017, either an ASCII NUL, or
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.q "2",
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or
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.q "3" ).
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.IP *
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Fifteen bytes containing zeros reserved for future use.
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.IP *
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Six four-byte integer values
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written in a standard byte order
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(the high-order byte of the value is written first).
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These values are,
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in order:
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.RS
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.TP
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.I tzh_ttisgmtcnt
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The number of UT/local indicators stored in the file.
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.TP
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.I tzh_ttisstdcnt
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The number of standard/wall indicators stored in the file.
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.TP
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.I tzh_leapcnt
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The number of leap seconds for which data entries are stored in the file.
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.TP
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.I tzh_timecnt
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The number of transition times for which data entries are stored
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in the file.
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.TP
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.I tzh_typecnt
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The number of local time types for which data entries are stored
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in the file (must not be zero).
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.TP
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.I tzh_charcnt
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The number of bytes of time zone abbreviation strings
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stored in the file.
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.RE
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.PP
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The above header is followed by the following fields, whose lengths
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depend on the contents of the header:
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.IP * 2
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.I tzh_timecnt
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four-byte signed integer values sorted in ascending order.
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These values are written in standard byte order.
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Each is used as a transition time (as returned by
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.BR time (2))
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at which the rules for computing local time change.
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.IP *
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.I tzh_timecnt
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one-byte unsigned integer values;
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each one but the last tells which of the different types of local time types
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described in the file is associated with the time period
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starting with the same-indexed transition time
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and continuing up to but not including the next transition time.
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(The last time type is present only for consistency checking with the
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POSIX-style TZ string described below.)
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These values serve as indices into the next field.
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.IP *
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.I tzh_typecnt
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.I ttinfo
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entries, each defined as follows:
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.in +.5i
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.sp
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.nf
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.ta .5i +\w'unsigned char\0\0'u
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struct ttinfo {
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int32_t tt_gmtoff;
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unsigned char tt_isdst;
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unsigned char tt_abbrind;
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};
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.in -.5i
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.fi
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.sp
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Each structure is written as a four-byte signed integer value for
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.IR tt_gmtoff ,
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in a standard byte order, followed by a one-byte value for
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.I tt_isdst
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and a one-byte value for
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.IR tt_abbrind .
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In each structure,
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.I tt_gmtoff
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gives the number of seconds to be added to UT,
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.I tt_isdst
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tells whether
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.I tm_isdst
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should be set by
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.BR localtime (3)
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and
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.I tt_abbrind
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serves as an index into the array of time zone abbreviation bytes
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that follow the
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.I ttinfo
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structure(s) in the file.
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.IP *
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.I tzh_leapcnt
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pairs of four-byte values, written in standard byte order;
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the first value of each pair gives the nonnegative time
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(as returned by
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.BR time (2))
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at which a leap second occurs;
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the second gives the
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.I total
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number of leap seconds to be applied during the time period
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starting at the given time.
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The pairs of values are sorted in ascending order by time.
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Each transition is for one leap second, either positive or negative;
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transitions always separated by at least 28 days minus 1 second.
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.IP *
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.I tzh_ttisstdcnt
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standard/wall indicators, each stored as a one-byte value;
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they tell whether the transition times associated with local time types
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were specified as standard time or wall clock time,
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and are used when a timezone file is used in handling POSIX-style
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timezone environment variables.
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.IP *
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.I tzh_ttisgmtcnt
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UT/local indicators, each stored as a one-byte value;
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they tell whether the transition times associated with local time types
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were specified as UT or local time,
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and are used when a timezone file is used in handling POSIX-style
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timezone environment variables.
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.PP
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The
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.BR localtime (3)
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function
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uses the first standard-time
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.I ttinfo
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structure in the file
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(or simply the first
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.I ttinfo
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structure in the absence of a standard-time structure)
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if either
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.I tzh_timecnt
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is zero or the time argument is less than the first transition time recorded
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in the file.
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.SS Version 2 format
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For version-2-format timezone files,
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the above header and data are followed by a second header and data,
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identical in format except that
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eight bytes are used for each transition time or leap second time.
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(Leap second counts remain four bytes.)
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After the second header and data comes a newline-enclosed,
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POSIX-TZ-environment-variable-style string for use in handling instants
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after the last transition time stored in the file
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or for all instants if the file has no transitions.
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The POSIX-style TZ string is empty (i.e., nothing between the newlines)
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if there is no POSIX representation for such instants.
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If nonempty, the POSIX-style TZ string must agree with the local time
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type after the last transition time if present in the eight-byte data;
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for example, given the string
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.q "WET0WEST,M3.5.0,M10.5.0/3"
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then if a last transition time is in July, the transition's local time
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type must specify a daylight-saving time abbreviated
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.q "WEST"
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that is one hour east of UT.
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Also, if there is at least one transition, time type 0 is associated
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with the time period from the indefinite past up to but not including
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the earliest transition time.
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.SS Version 3 format
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For version-3-format timezone files, the POSIX-TZ-style string may
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use two minor extensions to the POSIX TZ format, as described in
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.BR newtzset (3).
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First, the hours part of its transition times may be signed and range from
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\-167 through 167 instead of the POSIX-required unsigned values
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from 0 through 24.
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Second, DST is in effect all year if it starts
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January 1 at 00:00 and ends December 31 at 24:00 plus the difference
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between daylight saving and standard time.
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.PP
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Future changes to the format may append more data.
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.SH SEE ALSO
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.BR time (2),
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.BR localtime (3),
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.BR tzset (3),
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.BR tzselect (8),
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.BR zdump (8),
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.BR zic (8)
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.\" This file is in the public domain, so clarified as of
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.\" 1996-06-05 by Arthur David Olson.
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