man5/tzfile.5

   1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
   2 .\" This file is in the public domain, so clarified as of
   3 .\" 1996-06-05 by Arthur David Olson <arthur_david_olson@nih.gov>.
   4 .\" %%%LICENSE_END
   5 .\"
   6 .\" @(#)tzfile.5        7.11
   7 .\"
   8 .TH TZFILE 5 2015-05-07 "" "Linux Programmer's Manual"
   9 .SH NAME
  10 tzfile \- timezone information
  11 .SH DESCRIPTION
  12 This page describes the structure of the timezone files used by
  13 .BR tzset (3).
  14 These files are typically found under one of the directories
  15 .IR /usr/lib/zoneinfo
  16 or
  17 .IR /usr/share/zoneinfo .
  18
  19 Timezone information files begin with a 44-byte header structured as follows:
  20 .IP * 3
  21 The magic four-byte sequence
  22 "TZif" identifying this as a
  23 timezone information file.
  24 .IP *
  25 A single character identifying the version of the file's format:
  26 either an ASCII NUL (\(aq\\0\(aq) or a \(aq2\(aq (\fB0x32\fP).
  27 .IP *
  28 Fifteen bytes containing zeros reserved for future use.
  29 .IP *
  30 Six four-byte values of type
  31 .IR long ,
  32 written in a "standard" byte order
  33 (the high-order byte of the value is written first).
  34 These values are,
  35 in order:
  36 .RS
  37 .TP
  38 .I tzh_ttisgmtcnt
  39 The number of UTC/local indicators stored in the file.
  40 .TP
  41 .I tzh_ttisstdcnt
  42 The number of standard/wall indicators stored in the file.
  43 .TP
  44 .I tzh_leapcnt
  45 The number of leap seconds for which data is stored in the file.
  46 .TP
  47 .I tzh_timecnt
  48 The number of "transition times" for which data is stored
  49 in the file.
  50 .TP
  51 .I tzh_typecnt
  52 The number of "local time types" for which data is stored
  53 in the file (must not be zero).
  54 .TP
  55 .I tzh_charcnt
  56 The number of characters of "timezone abbreviation strings"
  57 stored in the file.
  58 .RE
  59 .PP
  60 The above header is followed by
  61 .I tzh_timecnt
  62 four-byte values of type
  63 .IR long ,
  64 sorted in ascending order.
  65 These values are written in "standard" byte order.
  66 Each is used as a transition time (as returned by
  67 .BR time (2))
  68 at which the rules for computing local time change.
  69 Next come
  70 .I tzh_timecnt
  71 one-byte values of type
  72 .IR "unsigned char" ;
  73 each one tells which of the different types of "local time" types
  74 described in the file is associated with the same-indexed transition time.
  75 These values serve as indices into an array of
  76 .I ttinfo
  77 structures (with
  78 .I tzh_typecnt
  79 entries) that appear next in the file;
  80 these structures are defined as follows:
  81 .in +4n
  82 .sp
  83 .nf
  84 struct ttinfo {
  85     long         tt_gmtoff;
  86     int          tt_isdst;
  87     unsigned int tt_abbrind;
  88 };
  89 .in
  90 .fi
  91 .sp
  92 Each structure is written as a four-byte value for
  93 .I tt_gmtoff
  94 of type
  95 .IR long ,
  96 in a standard byte order, followed by a one-byte value for
  97 .I tt_isdst
  98 and a one-byte value for
  99 .IR tt_abbrind .
 100 In each structure,
 101 .I tt_gmtoff
 102 gives the number of seconds to be added to UTC,
 103 .I tt_isdst
 104 tells whether
 105 .I tm_isdst
 106 should be set by
 107 .BR localtime (3),
 108 and
 109 .I tt_abbrind
 110 serves as an index into the array of timezone abbreviation characters
 111 that follow the
 112 .I ttinfo
 113 structure(s) in the file.
 114 .PP
 115 Then there are
 116 .I tzh_leapcnt
 117 pairs of four-byte values, written in standard byte order;
 118 the first value of each pair gives the time
 119 (as returned by
 120 .BR time (2))
 121 at which a leap second occurs;
 122 the second gives the
 123 .I total
 124 number of leap seconds to be applied after the given time.
 125 The pairs of values are sorted in ascending order by time.
 126 .PP
 127 Then there are
 128 .I tzh_ttisstdcnt
 129 standard/wall indicators, each stored as a one-byte value;
 130 they tell whether the transition times associated with local time types
 131 were specified as standard time or wall clock time,
 132 and are used when a timezone file is used in handling POSIX-style
 133 timezone environment variables.
 134 .PP
 135 Finally, there are
 136 .I tzh_ttisgmtcnt
 137 UTC/local indicators, each stored as a one-byte value;
 138 they tell whether the transition times associated with local time types
 139 were specified as UTC or local time,
 140 and are used when a timezone file is used in handling POSIX-style
 141 timezone environment variables.
 142 .PP
 143 .BR localtime (3)
 144 uses the first standard-time
 145 .I ttinfo
 146 structure in the file
 147 (or simply the first
 148 .I ttinfo
 149 structure in the absence of a standard-time structure)
 150 if either
 151 .I tzh_timecnt
 152 is zero or the time argument is less than the first transition time recorded
 153 in the file.
 154 .SS Version 2 format
 155 For version-2-format timezone files,
 156 the above header and data is followed by a second header and data,
 157 identical in format except that
 158 eight bytes are used for each transition time or leap-second time
 159 (and that the version byte in the header record is
 160 \fB0x32\fP rather than \fB0x00\fP).
 161 After the second header and data comes a newline-enclosed,
 162 POSIX-TZ-environment-variable-style string for use in handling instants
 163 after the last transition time stored in the file
 164 (with nothing between the newlines if there is no POSIX representation for
 165 such instants).
 166 .PP
 167 The second section of the timezone file consists of another 44-byte header
 168 record, identical in structure to the one at the beginning of the file,
 169 except that it applies to the data that follows,
 170 which is also identical in structure
 171 to the first section of the timezone file, with the following differences:
 172 .IP * 3
 173 The transition time values, after the header, are eight-byte values.
 174 .IP *
 175 In each leap second record, the leap second value is an eight-byte value.
 176 The accumulated leap second count is still a four-byte value.
 177 .PP
 178 In all cases, the eight-byte time values are given in
 179 the "standard" byte order,
 180 the high-order byte first.
 181 .SS POSIX timezone string
 182 The second eight-byte time value section is followed by an optional
 183 third section:
 184 a single ASCII newline character (\(aq\\n\(aq),
 185 then a text string followed by a second
 186 newline character.
 187 The text string is a POSIX timezone string, whose format is described in the
 188 .BR tzset (3)
 189 manual page.
 190 .PP
 191 The POSIX timezone string defines a rule for computing transition times
 192 that follow the last transition time explicitly specified in the timezone
 193 information file.
 194 .SS Summary of the timezone information file format
 195 \&
 196 .sp 2
 197 .RS
 198 .nf
 199 Four-byte value section
 200 (header version \fB0x00\fP or \fB0x32\fP)
 201         Header record
 202         Four-byte transition times
 203         Transition time index
 204         \fBttinfo\fP structures
 205         Timezone abbreviation array
 206         Leap second records
 207         Standard/Wall array
 208         UTC/Local array
 209
 210 Eight-byte value section
 211 (only if first header version is \fB0x32\fP,
 212 the second header's version is also \fB0x32\fP)
 213         Header record
 214         Eight-byte transition times
 215         Transition time index
 216         \fBttinfo\fP structures
 217         Timezone abbreviation array
 218         Leap second records
 219         Standard/Wall array
 220         UTC/Local array
 221
 222 Third section
 223 (optional, only in \fB0x32\fP version files)
 224         Newline character
 225         Timezone string
 226         Newline character
 227 .fi
 228 .RE
 229
 230 .SH SEE ALSO
 231 .BR ctime (3),
 232 .BR tzset (3),
 233 .BR tzselect (8),
 234
 235 .I timezone/tzfile.h
 236 in the glibc source tree