stdtime/tzfile.5

   1 .\" $FreeBSD: src/lib/libc/stdtime/tzfile.5,v 1.10 2001/07/10 13:41:23 ru Exp $
   2 .Dd September 13, 1994
   3 .Dt TZFILE 5
   4 .Os
   5 .Sh NAME
   6 .Nm tzfile
   7 .Nd timezone information
   8 .Sh SYNOPSIS
   9 .In tzfile.h
  10 .Sh DESCRIPTION
  11 The time zone information files used by
  12 .Xr tzset 3
  13 begin with the magic characters
  14 .Dq Li TZif
  15 to identify them as
  16 time zone information files,
  17 followed by sixteen bytes reserved for future use,
  18 followed by four four-byte values
  19 written in a ``standard'' byte order
  20 (the high-order byte of the value is written first).
  21 These values are,
  22 in order:
  23 .Pp
  24 .Bl -tag -compact -width tzh_ttisstdcnt
  25 .It Va tzh_ttisgmtcnt
  26 The number of UTC/local indicators stored in the file.
  27 .It Va tzh_ttisstdcnt
  28 The number of standard/wall indicators stored in the file.
  29 .It Va tzh_leapcnt
  30 The number of leap seconds for which data is stored in the file.
  31 .It Va tzh_timecnt
  32 The number of ``transition times'' for which data is stored
  33 in the file.
  34 .It Va tzh_typecnt
  35 The number of ``local time types'' for which data is stored
  36 in the file (must not be zero).
  37 .It Va tzh_charcnt
  38 The number of characters of ``time zone abbreviation strings''
  39 stored in the file.
  40 .El
  41 .Pp
  42 The above header is followed by
  43 .Va tzh_timecnt
  44 four-byte values of type
  45 .Fa long ,
  46 sorted in ascending order.
  47 These values are written in ``standard'' byte order.
  48 Each is used as a transition time (as returned by
  49 .Xr time 3 )
  50 at which the rules for computing local time change.
  51 Next come
  52 .Va tzh_timecnt
  53 one-byte values of type
  54 .Fa "unsigned char" ;
  55 each one tells which of the different types of ``local time'' types
  56 described in the file is associated with the same-indexed transition time.
  57 These values serve as indices into an array of
  58 .Fa ttinfo
  59 structures that appears next in the file;
  60 these structures are defined as follows:
  61 .Pp
  62 .Bd -literal -offset indent
  63 struct ttinfo {
  64         long    tt_gmtoff;
  65         int     tt_isdst;
  66         unsigned int    tt_abbrind;
  67 };
  68 .Ed
  69 .Pp
  70 Each structure is written as a four-byte value for
  71 .Va tt_gmtoff
  72 of type
  73 .Fa long ,
  74 in a standard byte order, followed by a one-byte value for
  75 .Va tt_isdst
  76 and a one-byte value for
  77 .Va tt_abbrind .
  78 In each structure,
  79 .Va tt_gmtoff
  80 gives the number of seconds to be added to UTC,
  81 .Li tt_isdst
  82 tells whether
  83 .Li tm_isdst
  84 should be set by
  85 .Xr localtime 3
  86 and
  87 .Va tt_abbrind
  88 serves as an index into the array of time zone abbreviation characters
  89 that follow the
  90 .Li ttinfo
  91 structure(s) in the file.
  92 .Pp
  93 Then there are
  94 .Va tzh_leapcnt
  95 pairs of four-byte values, written in standard byte order;
  96 the first value of each pair gives the time
  97 (as returned by
  98 .Xr time 3 )
  99 at which a leap second occurs;
 100 the second gives the
 101 .Em total
 102 number of leap seconds to be applied after the given time.
 103 The pairs of values are sorted in ascending order by time.
 104 .Pp
 105 Then there are
 106 .Va tzh_ttisstdcnt
 107 standard/wall indicators, each stored as a one-byte value;
 108 they tell whether the transition times associated with local time types
 109 were specified as standard time or wall clock time,
 110 and are used when a time zone file is used in handling POSIX-style
 111 time zone environment variables.
 112 .Pp
 113 Finally there are
 114 .Va tzh_ttisgmtcnt
 115 UTC/local indicators, each stored as a one-byte value;
 116 they tell whether the transition times associated with local time types
 117 were specified as UTC or local time,
 118 and are used when a time zone file is used in handling POSIX-style
 119 time zone environment variables.
 120 .Pp
 121 .Nm localtime
 122 uses the first standard-time
 123 .Li ttinfo
 124 structure in the file
 125 (or simply the first
 126 .Li ttinfo
 127 structure in the absence of a standard-time structure)
 128 if either
 129 .Li tzh_timecnt
 130 is zero or the time argument is less than the first transition time recorded
 131 in the file.
 132 .Sh SEE ALSO
 133 .Xr ctime 3 ,
 134 .Xr time2posix 3 ,
 135 .Xr zic 8
 136 .\" @(#)tzfile.5        7.2
 137 .\" This file is in the public domain, so clarified as of
 138 .\" 1996-06-05 by Arthur David Olson (arthur_david_olson@nih.gov).