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153 lines
4.3 KiB
Groff
153 lines
4.3 KiB
Groff
.\" $FreeBSD$
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.Dd September 13, 1994
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.Dt TZFILE 5
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.Os
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.Sh NAME
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.Nm tzfile
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.Nd timezone information
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.Sh SYNOPSIS
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.Fd #include \&"/usr/src/lib/libc/stdtime/tzfile.h\&"
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.Sh DESCRIPTION
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The time zone information files used by
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.Xr tzset 3
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begin with the magic characters
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.Dq Li TZif
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to identify them as
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time zone information files,
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followed by a character identifying the version of the file's format
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(as of 2005, either an ASCII NUL or a '2')
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followed by fifteen bytes containing zeroes reserved for future use,
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followed by four four-byte 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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.Pp
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.Bl -tag -compact -width tzh_ttisstdcnt
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.It Va tzh_ttisgmtcnt
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The number of UTC/local indicators stored in the file.
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.It Va tzh_ttisstdcnt
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The number of standard/wall indicators stored in the file.
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.It Va tzh_leapcnt
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The number of leap seconds for which data is stored in the file.
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.It Va tzh_timecnt
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The number of ``transition times'' for which data is stored
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in the file.
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.It Va tzh_typecnt
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The number of ``local time types'' for which data is stored
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in the file (must not be zero).
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.It Va tzh_charcnt
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The number of characters of ``time zone abbreviation strings''
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stored in the file.
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.El
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.Pp
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The above header is followed by
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.Va tzh_timecnt
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four-byte values of type
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.Fa long ,
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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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.Xr time 3 )
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at which the rules for computing local time change.
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Next come
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.Va tzh_timecnt
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one-byte values of type
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.Fa "unsigned char" ;
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each one tells which of the different types of ``local time'' types
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described in the file is associated with the same-indexed transition time.
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These values serve as indices into an array of
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.Fa ttinfo
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structures (with
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.Fa tzh_typecnt
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entries) that appears next in the file;
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these structures are defined as follows:
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.Pp
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.Bd -literal -offset indent
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struct ttinfo {
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long tt_gmtoff;
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int tt_isdst;
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unsigned int tt_abbrind;
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};
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.Ed
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.Pp
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Each structure is written as a four-byte value for
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.Va tt_gmtoff
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of type
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.Fa long ,
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in a standard byte order, followed by a one-byte value for
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.Va tt_isdst
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and a one-byte value for
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.Va tt_abbrind .
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In each structure,
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.Va tt_gmtoff
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gives the number of seconds to be added to UTC,
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.Li tt_isdst
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tells whether
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.Li tm_isdst
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should be set by
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.Xr localtime 3
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and
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.Va tt_abbrind
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serves as an index into the array of time zone abbreviation characters
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that follow the
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.Li ttinfo
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structure(s) in the file.
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.Pp
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Then there are
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.Va 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 time
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(as returned by
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.Xr time 3 )
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at which a leap second occurs;
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the second gives the
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.Em total
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number of leap seconds to be applied after the given time.
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The pairs of values are sorted in ascending order by time.
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.Pp
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Then there are
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.Va 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 time zone file is used in handling POSIX-style
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time zone environment variables.
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.Pp
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Finally there are
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.Va tzh_ttisgmtcnt
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UTC/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 UTC or local time,
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and are used when a time zone file is used in handling POSIX-style
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time zone environment variables.
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.Pp
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.Nm localtime
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uses the first standard-time
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.Li ttinfo
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structure in the file
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(or simply the first
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.Li ttinfo
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structure in the absence of a standard-time structure)
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if either
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.Li 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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.Pp
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For version-2-format time zone files,
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the above header and data is followed by a second header and data,
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identical in format except that eight bytes are used for each
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transition time or leap second time.
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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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(with nothing between the newlines if there is no POSIX representation for
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such instants).
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.Sh SEE ALSO
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.Xr ctime 3 ,
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.Xr time2posix 3 ,
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.Xr zic 8
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.\" @(#)tzfile.5 8.3
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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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