diff options
Diffstat (limited to 'man5/tzfile.5')
| -rw-r--r-- | man5/tzfile.5 | 144 |
1 files changed, 78 insertions, 66 deletions
diff --git a/man5/tzfile.5 b/man5/tzfile.5 index 45afecc10..867348d67 100644 --- a/man5/tzfile.5 +++ b/man5/tzfile.5 @@ -26,23 +26,24 @@ a signed binary integer is represented using two's complement, and a boolean is represented by a one-byte binary integer that is either 0 (false) or 1 (true). The format begins with a 44-byte header containing the following fields: -.IP * 2 +.RS "\w' 'u" +.IP \(bu "\w'\(bu 'u" The magic four-byte ASCII sequence .q "TZif" identifies the file as a timezone information file. -.IP * +.IP \(bu A byte identifying the version of the file's format (as of 2021, either an ASCII NUL, .q "2", .q "3", or .q "4" ). -.IP * +.IP \(bu Fifteen bytes containing zeros reserved for future use. -.IP * +.IP \(bu Six four-byte integer values, in the following order: -.RS -.TP +.RS "\w' \(bu 'u" +.TP "\w' 'u" .B tzh_ttisutcnt The number of UT/local indicators stored in the file. (UT is Universal Time.) @@ -65,17 +66,18 @@ in the file (must not be zero). The number of bytes of time zone abbreviation strings stored in the file. .RE -.P +.PP The above header is followed by the following fields, whose lengths depend on the contents of the header: -.IP * 2 +.RS "\w' 'u" +.IP \(bu "\w'\(bu 'u" .B tzh_timecnt four-byte signed integer values sorted in ascending order. These values are written in network byte order. Each is used as a transition time (as returned by .BR time (2)) at which the rules for computing local time change. -.IP * +.IP \(bu .B tzh_timecnt one-byte unsigned integer values; each one but the last tells which of the different types of local time types @@ -83,22 +85,22 @@ described in the file is associated with the time period starting with the same-indexed transition time and continuing up to but not including the next transition time. (The last time type is present only for consistency checking with the -POSIX-style TZ string described below.) +POSIX.1-2017-style TZ string described below.) These values serve as indices into the next field. -.IP * +.IP \(bu .B tzh_typecnt .B ttinfo entries, each defined as follows: -.in +.5i +.in +2 .sp .nf -.ta .5i +\w'unsigned char\0\0'u +.ta \w'\0\0\0\0'u +\w'unsigned char\0'u struct ttinfo { int32_t tt_utoff; unsigned char tt_isdst; unsigned char tt_desigidx; }; -.in -.5i +.in .fi .sp Each structure is written as a four-byte signed integer value for @@ -132,7 +134,8 @@ Also, in realistic applications is in the range [\-89999, 93599] (i.e., more than \-25 hours and less than 26 hours); this allows easy support by implementations that already support the POSIX-required range [\-24:59:59, 25:59:59]. -.IP * +.RS "\w' 'u" +.IP \(bu "\w'\(bu 'u" .B tzh_charcnt bytes that represent time zone designations, which are null-terminated byte strings, each indexed by the @@ -140,7 +143,7 @@ which are null-terminated byte strings, each indexed by the values mentioned above. The byte strings can overlap if one is a suffix of the other. The encoding of these strings is not specified. -.IP * +.IP \(bu .B tzh_leapcnt pairs of four-byte values, written in network byte order; the first value of each pair gives the nonnegative time @@ -167,22 +170,24 @@ otherwise, for timestamps before the first occurrence time, the leap-second correction is zero if the first pair's correction is 1 or \-1, and is unspecified otherwise (which can happen only in files truncated at the start). -.IP * +.IP \(bu .B tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte boolean; they tell whether the transition times associated with local time types were specified as standard time or local (wall clock) time. -.IP * +.IP \(bu .B tzh_ttisutcnt UT/local indicators, each stored as a one-byte boolean; they tell whether the transition times associated with local time types were specified as UT or local time. If a UT/local indicator is set, the corresponding standard/wall indicator must also be set. -.P +.RE +.PP The standard/wall and UT/local indicators were designed for transforming a TZif file's transition times into transitions appropriate -for another time zone specified via a POSIX-style TZ string that lacks rules. +for another time zone specified via +a POSIX.1-2017-style TZ string that lacks rules. For example, when TZ="EET\*-2EEST" and there is no TZif file "EET\*-2EEST", the idea was to adapt the transition times from a TZif file with the well-known name "posixrules" that is present only for this purpose and @@ -194,7 +199,7 @@ so users desiring (say) Greek time should instead specify TZ="Europe/Athens" for better historical coverage, falling back on TZ="EET\*-2EEST,M3.5.0/3,M10.5.0/4" if POSIX conformance is required and older timestamps need not be handled accurately. -.P +.PP The .BR localtime (3) function @@ -211,13 +216,14 @@ 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 second header and data comes a newline-enclosed string +in the style of the contents of a POSIX.1-2017 TZ environment variable, +for use in handling instants after the last transition time stored in the file or for all instants if the file has no transitions. -The POSIX-style TZ string is empty (i.e., nothing between the newlines) -if there is no POSIX-style representation for such instants. -If nonempty, the POSIX-style TZ string must agree with the local time +The TZ string is empty (i.e., nothing between the newlines) +if there is no POSIX.1-2017-style representation for such instants. +If nonempty, the TZ string must agree with the local time type after the last transition time if present in the eight-byte data; for example, given the string .q "WET0WEST,M3.5.0/1,M10.5.0" @@ -229,8 +235,8 @@ Also, if there is at least one transition, time type 0 is associated with the time period from the indefinite past up to but not including the earliest transition time. .SS Version 3 format -For version-3-format timezone files, the POSIX-TZ-style string may -use two minor extensions to the POSIX TZ format, as described in +For version-3-format timezone files, the TZ string may +use two minor extensions to the POSIX.1-2017 TZ format, as described in .BR 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 @@ -250,14 +256,14 @@ releases will likely add leap second entries after the expiration, and the added leap seconds will change how post-expiration timestamps are treated. .SS Interoperability considerations Future changes to the format may append more data. -.P +.PP Version 1 files are considered a legacy format and should not be generated, as they do not support transition times after the year 2038. Readers that understand only Version 1 must ignore any data that extends beyond the calculated end of the version 1 data block. -.P +.PP Other than version 1, writers should generate the lowest version number needed by a file's data. For example, a writer should generate a version 4 file @@ -266,7 +272,7 @@ Likewise, a writer not generating a version 4 file should generate a version 3 file only if TZ string extensions are necessary to accurately model transition times. -.P +.PP The sequence of time changes defined by the version 1 header and data block should be a contiguous sub-sequence of the time changes defined by the version 2+ header and data @@ -278,12 +284,12 @@ supporting obsolescent readers use a .B tzh_timecnt of zero in the version 1 data block to save space. -.P +.PP When a TZif file contains a leap second table expiration time, TZif readers should either refuse to process post-expiration timestamps, or process them as if the expiration time did not exist (possibly with an error indication). -.P +.PP Time zone designations should consist of at least three (3) and no more than six (6) ASCII characters from the set of alphanumerics, @@ -292,15 +298,15 @@ and .q "+". This is for compatibility with POSIX requirements for time zone abbreviations. -.P +.PP When reading a version 2 or higher file, readers should ignore the version 1 header and data block except for the purpose of skipping over them. -.P +.PP Readers should calculate the total lengths of the headers and data blocks and check that they all fit within the actual file size, as part of a validity check for the file. -.P +.PP When a positive leap second occurs, readers should append an extra second to the local minute containing the second just before the leap second. If this occurs when the UTC offset is not a multiple of 60 @@ -312,16 +318,18 @@ This section documents common problems in reading or writing TZif files. Most of these are problems in generating TZif files for use by older readers. The goals of this section are: -.IP * 2 +.RS "\w' 'u" +.IP \(bu "\w'\(bu 'u" to help TZif writers output files that avoid common pitfalls in older or buggy TZif readers, -.IP * +.IP \(bu to help TZif readers avoid common pitfalls when reading files generated by future TZif writers, and -.IP * +.IP \(bu to help any future specification authors see what sort of problems arise when the TZif format is changed. -.P +.RE +.PP When new versions of the TZif format have been defined, a design goal has been that a reader can successfully use a TZif file even if the file is of a later TZif version than what the @@ -333,23 +341,24 @@ new-version data useful even for older-version readers. This section attempts to document these compatibility issues and workarounds, as well as to document other common bugs in readers. -.P +.PP Interoperability problems with TZif include the following: -.IP * 2 +.RS "\w' 'u" +.IP \(bu "\w'\(bu 'u" Some readers examine only version 1 data. As a partial workaround, a writer can output as much version 1 data as possible. However, a reader should ignore version 1 data, and should use version 2+ data even if the reader's native timestamps have only 32 bits. -.IP * +.IP \(bu Some readers designed for version 2 might mishandle timestamps after a version 3 or higher file's last transition, because -they cannot parse extensions to POSIX in the TZ-like string. +they cannot parse extensions to POSIX.1-2017 in the TZ-like string. As a partial workaround, a writer can output more transitions than necessary, so that only far-future timestamps are mishandled by version 2 readers. -.IP * +.IP \(bu Some readers designed for version 2 do not support permanent daylight saving time with transitions after 24:00 \(en e.g., a TZ string @@ -367,22 +376,22 @@ for the next time zone east \(en e.g., .q "AST4" for permanent Atlantic Standard Time (\-04). -.IP * +.IP \(bu Some readers designed for version 2 or 3, and that require strict conformance to RFC 8536, reject version 4 files whose leap second tables are truncated at the start or that end in expiration times. -.IP * +.IP \(bu Some readers ignore the footer, and instead predict future timestamps from the time type of the last transition. As a partial workaround, a writer can output more transitions than necessary. -.IP * +.IP \(bu Some readers do not use time type 0 for timestamps before the first transition, in that they infer a time type using a heuristic that does not always select time type 0. As a partial workaround, a writer can output a dummy (no-op) first transition at an early time. -.IP * +.IP \(bu Some readers mishandle timestamps before the first transition that has a timestamp not less than \-2**31. Readers that support only 32-bit timestamps are likely to be @@ -391,12 +400,12 @@ more prone to this problem, for example, when they process bits. As a partial workaround, a writer can output a dummy transition at timestamp \-2**31. -.IP * +.IP \(bu Some readers mishandle a transition if its timestamp has the minimum possible signed 64-bit value. Timestamps less than \-2**59 are not recommended. -.IP * -Some readers mishandle POSIX-style TZ strings that +.IP \(bu +Some readers mishandle TZ strings that contain .q "<" or @@ -407,11 +416,11 @@ or .q ">" for time zone abbreviations containing only alphabetic characters. -.IP * +.IP \(bu Many readers mishandle time zone abbreviations that contain non-ASCII characters. These characters are not recommended. -.IP * +.IP \(bu Some readers may mishandle time zone abbreviations that contain fewer than 3 or more than 6 characters, or that contain ASCII characters other than alphanumerics, @@ -419,23 +428,23 @@ contain ASCII characters other than alphanumerics, and .q "+". These abbreviations are not recommended. -.IP * +.IP \(bu Some readers mishandle TZif files that specify daylight-saving time UT offsets that are less than the UT offsets for the corresponding standard time. These readers do not support locations like Ireland, which -uses the equivalent of the POSIX TZ string +uses the equivalent of the TZ string .q "IST\*-1GMT0,M10.5.0,M3.5.0/1", observing standard time (IST, +01) in summer and daylight saving time (GMT, +00) in winter. As a partial workaround, a writer can output data for the -equivalent of the POSIX TZ string +equivalent of the TZ string .q "GMT0IST,M3.5.0/1,M10.5.0", thus swapping standard and daylight saving time. Although this workaround misidentifies which part of the year uses daylight saving time, it records UT offsets and time zone abbreviations correctly. -.IP * +.IP \(bu Some readers generate ambiguous timestamps for positive leap seconds that occur when the UTC offset is not a multiple of 60 seconds. For example, in a timezone with UTC offset +01:23:45 and with @@ -446,38 +455,41 @@ instead of mapping the latter to 01:23:46, and they will map 78796815 to This has not yet been a practical problem, since no civil authority has observed such UTC offsets since leap seconds were introduced in 1972. -.P +.RE +.PP Some interoperability problems are reader bugs that are listed here mostly as warnings to developers of readers. -.IP * 2 +.RS "\w' 'u" +.IP \(bu "\w'\(bu 'u" Some readers do not support negative timestamps. Developers of distributed applications should keep this in mind if they need to deal with pre-1970 data. -.IP * +.IP \(bu Some readers mishandle timestamps before the first transition that has a nonnegative timestamp. Readers that do not support negative timestamps are likely to be more prone to this problem. -.IP * +.IP \(bu Some readers mishandle time zone abbreviations like .q "\*-08" that contain .q "+", .q "\*-", or digits. -.IP * +.IP \(bu Some readers mishandle UT offsets that are out of the traditional range of \-12 through +12 hours, and so do not support locations like Kiritimati that are outside this range. -.IP * +.IP \(bu Some readers mishandle UT offsets in the range [\-3599, \-1] seconds from UT, because they integer-divide the offset by 3600 to get 0 and then display the hour part as .q "+00". -.IP * +.IP \(bu Some readers mishandle UT offsets that are not a multiple of one hour, or of 15 minutes, or of 1 minute. +.RE .SH SEE ALSO .BR time (2), .BR localtime (3), @@ -485,7 +497,7 @@ of one hour, or of 15 minutes, or of 1 minute. .BR tzselect (8), .BR zdump (8), .BR zic (8). -.P +.PP Olson A, Eggert P, Murchison K. The Time Zone Information Format (TZif). 2019 Feb. .UR https://\:datatracker.ietf.org/\:doc/\:html/\:rfc8536 |