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1
2 XZ Utils
3 ========
4
5 0. Overview
6 1. Documentation
7 1.1. Overall documentation
8 1.2. Documentation for command-line tools
9 1.3. Documentation for liblzma
10 2. Version numbering
11 3. Reporting bugs
12 4. Translations
13 5. Other implementations of the .xz format
14 6. Contact information
15
16
17 0. Overview
18 -----------
19
20 XZ Utils provide a general-purpose data-compression library plus
21 command-line tools. The native file format is the .xz format, but
22 also the legacy .lzma format is supported. The .xz format supports
23 multiple compression algorithms, which are called "filters" in the
24 context of XZ Utils. The primary filter is currently LZMA2. With
25 typical files, XZ Utils create about 30 % smaller files than gzip.
26
27 To ease adapting support for the .xz format into existing applications
28 and scripts, the API of liblzma is somewhat similar to the API of the
29 popular zlib library. For the same reason, the command-line tool xz
30 has a command-line syntax similar to that of gzip.
31
32 When aiming for the highest compression ratio, the LZMA2 encoder uses
33 a lot of CPU time and may use, depending on the settings, even
34 hundreds of megabytes of RAM. However, in fast modes, the LZMA2 encoder
35 competes with bzip2 in compression speed, RAM usage, and compression
36 ratio.
37
38 LZMA2 is reasonably fast to decompress. It is a little slower than
39 gzip, but a lot faster than bzip2. Being fast to decompress means
40 that the .xz format is especially nice when the same file will be
41 decompressed very many times (usually on different computers), which
42 is the case e.g. when distributing software packages. In such
43 situations, it's not too bad if the compression takes some time,
44 since that needs to be done only once to benefit many people.
45
46 With some file types, combining (or "chaining") LZMA2 with an
47 additional filter can improve the compression ratio. A filter chain may
48 contain up to four filters, although usually only one or two are used.
49 For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2
50 in the filter chain can improve compression ratio of executable files.
51
52 Since the .xz format allows adding new filter IDs, it is possible that
53 some day there will be a filter that is, for example, much faster to
54 compress than LZMA2 (but probably with worse compression ratio).
55 Similarly, it is possible that some day there is a filter that will
56 compress better than LZMA2.
57
58 XZ Utils supports multithreaded compression. XZ Utils doesn't support
59 multithreaded decompression yet. It has been planned though and taken
60 into account when designing the .xz file format. In the future, files
61 that were created in threaded mode can be decompressed in threaded
62 mode too.
63
64
65 1. Documentation
66 ----------------
67
68 1.1. Overall documentation
69
70 README This file
71
72 INSTALL.generic Generic install instructions for those not
73 familiar with packages using GNU Autotools
74 INSTALL Installation instructions specific to XZ Utils
75 PACKAGERS Information to packagers of XZ Utils
76
77 COPYING XZ Utils copyright and license information
78 COPYING.0BSD BSD Zero Clause License
79 COPYING.GPLv2 GNU General Public License version 2
80 COPYING.GPLv3 GNU General Public License version 3
81 COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1
82
83 AUTHORS The main authors of XZ Utils
84 THANKS Incomplete list of people who have helped making
85 this software
86 NEWS User-visible changes between XZ Utils releases
87 ChangeLog Detailed list of changes (commit log)
88 TODO Known bugs and some sort of to-do list
89
90 Note that only some of the above files are included in binary
91 packages.
92
93
94 1.2. Documentation for command-line tools
95
96 The command-line tools are documented as man pages. In source code
97 releases (and possibly also in some binary packages), the man pages
98 are also provided in plain text (ASCII only) format in the directory
99 "doc/man" to make the man pages more accessible to those whose
100 operating system doesn't provide an easy way to view man pages.
101
102
103 1.3. Documentation for liblzma
104
105 The liblzma API headers include short docs about each function
106 and data type as Doxygen tags. These docs should be quite OK as
107 a quick reference.
108
109 There are a few example/tutorial programs that should help in
110 getting started with liblzma. In the source package the examples
111 are in "doc/examples" and in binary packages they may be under
112 "examples" in the same directory as this README.
113
114 Since the liblzma API has similarities to the zlib API, some people
115 may find it useful to read the zlib docs and tutorial too:
116
117 https://zlib.net/manual.html
118 https://zlib.net/zlib_how.html
119
120
121 2. Version numbering
122 --------------------
123
124 The version number format of XZ Utils is X.Y.ZS:
125
126 - X is the major version. When this is incremented, the library
127 API and ABI break.
128
129 - Y is the minor version. It is incremented when new features
130 are added without breaking the existing API or ABI. An even Y
131 indicates a stable release and an odd Y indicates unstable
132 (alpha or beta version).
133
134 - Z is the revision. This has a different meaning for stable and
135 unstable releases:
136
137 * Stable: Z is incremented when bugs get fixed without adding
138 any new features. This is intended to be convenient for
139 downstream distributors that want bug fixes but don't want
140 any new features to minimize the risk of introducing new bugs.
141
142 * Unstable: Z is just a counter. API or ABI of features added
143 in earlier unstable releases having the same X.Y may break.
144
145 - S indicates stability of the release. It is missing from the
146 stable releases, where Y is an even number. When Y is odd, S
147 is either "alpha" or "beta" to make it very clear that such
148 versions are not stable releases. The same X.Y.Z combination is
149 not used for more than one stability level, i.e. after X.Y.Zalpha,
150 the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
151
152
153 3. Reporting bugs
154 -----------------
155
156 Naturally it is easiest for me if you already know what causes the
157 unexpected behavior. Even better if you have a patch to propose.
158 However, quite often the reason for unexpected behavior is unknown,
159 so here are a few things to do before sending a bug report:
160
161 1. Try to create a small example how to reproduce the issue.
162
163 2. Compile XZ Utils with debugging code using configure switches
164 --enable-debug and, if possible, --disable-shared. If you are
165 using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting
166 binaries.
167
168 3. Turn on core dumps. The exact command depends on your shell;
169 for example in GNU bash it is done with "ulimit -c unlimited",
170 and in tcsh with "limit coredumpsize unlimited".
171
172 4. Try to reproduce the suspected bug. If you get "assertion failed"
173 message, be sure to include the complete message in your bug
174 report. If the application leaves a coredump, get a backtrace
175 using gdb:
176 $ gdb /path/to/app-binary # Load the app to the debugger.
177 (gdb) core core # Open the coredump.
178 (gdb) bt # Print the backtrace. Copy & paste to bug report.
179 (gdb) quit # Quit gdb.
180
181 Report your bug via email or IRC (see Contact information below).
182 Don't send core dump files or any executables. If you have a small
183 example file(s) (total size less than 256 KiB), please include
184 it/them as an attachment. If you have bigger test files, put them
185 online somewhere and include a URL to the file(s) in the bug report.
186
187 Always include the exact version number of XZ Utils in the bug report.
188 If you are using a snapshot from the git repository, use "git describe"
189 to get the exact snapshot version. If you are using XZ Utils shipped
190 in an operating system distribution, mention the distribution name,
191 distribution version, and exact xz package version; if you cannot
192 repeat the bug with the code compiled from unpatched source code,
193 you probably need to report a bug to your distribution's bug tracking
194 system.
195
196
197 4. Translations
198 ---------------
199
200 The xz command line tool and all man pages can be translated.
201 The translations are handled via the Translation Project. If you
202 wish to help translating xz, please join the Translation Project:
203
204 https://translationproject.org/html/translators.html
205
206 Below are notes and testing instructions specific to xz
207 translations.
208
209 Testing can be done by installing xz into a temporary directory:
210
211 ./configure --disable-shared --prefix=/tmp/xz-test
212 # <Edit the .po file in the po directory.>
213 make -C po update-po
214 make install
215 bash debug/translation.bash | less
216 bash debug/translation.bash | less -S # For --list outputs
217
218 Repeat the above as needed (no need to re-run configure though).
219
220 Note especially the following:
221
222 - The output of --help and --long-help must look nice on
223 an 80-column terminal. It's OK to add extra lines if needed.
224
225 - In contrast, don't add extra lines to error messages and such.
226 They are often preceded with e.g. a filename on the same line,
227 so you have no way to predict where to put a \n. Let the terminal
228 do the wrapping even if it looks ugly. Adding new lines will be
229 even uglier in the generic case even if it looks nice in a few
230 limited examples.
231
232 - Be careful with column alignment in tables and table-like output
233 (--list, --list --verbose --verbose, --info-memory, --help, and
234 --long-help):
235
236 * All descriptions of options in --help should start in the
237 same column (but it doesn't need to be the same column as
238 in the English messages; just be consistent if you change it).
239 Check that both --help and --long-help look OK, since they
240 share several strings.
241
242 * --list --verbose and --info-memory print lines that have
243 the format "Description: %s". If you need a longer
244 description, you can put extra space between the colon
245 and %s. Then you may need to add extra space to other
246 strings too so that the result as a whole looks good (all
247 values start at the same column).
248
249 * The columns of the actual tables in --list --verbose --verbose
250 should be aligned properly. Abbreviate if necessary. It might
251 be good to keep at least 2 or 3 spaces between column headings
252 and avoid spaces in the headings so that the columns stand out
253 better, but this is a matter of opinion. Do what you think
254 looks best.
255
256 - Be careful to put a period at the end of a sentence when the
257 original version has it, and don't put it when the original
258 doesn't have it. Similarly, be careful with \n characters
259 at the beginning and end of the strings.
260
261 - Read the TRANSLATORS comments that have been extracted from the
262 source code and included in xz.pot. Some comments suggest
263 testing with a specific command which needs an .xz file. You
264 may use e.g. any tests/files/good-*.xz. However, these test
265 commands are included in translations.bash output, so reading
266 translations.bash output carefully can be enough.
267
268 - If you find language problems in the original English strings,
269 feel free to suggest improvements. Ask if something is unclear.
270
271 - The translated messages should be understandable (sometimes this
272 may be a problem with the original English messages too). Don't
273 make a direct word-by-word translation from English especially if
274 the result doesn't sound good in your language.
275
276 Thanks for your help!
277
278
279 5. Other implementations of the .xz format
280 ------------------------------------------
281
282 7-Zip and the p7zip port of 7-Zip support the .xz format starting
283 from the version 9.00alpha.
284
285 https://7-zip.org/
286 https://p7zip.sourceforge.net/
287
288 XZ Embedded is a limited implementation written for use in the Linux
289 kernel, but it is also suitable for other embedded use.
290
291 https://tukaani.org/xz/embedded.html
292
293 XZ for Java is a complete implementation written in pure Java.
294
295 https://tukaani.org/xz/java.html
296
297
298 6. Contact information
299 ----------------------
300
301 XZ Utils in general:
302 - Home page: https://tukaani.org/xz/
303 - Email to maintainer(s): xz@tukaani.org
304 - IRC: #tukaani on Libera Chat
305 - GitHub: https://github.com/tukaani-project/xz
306
307 Lead maintainer:
308 - Email: Lasse Collin <lasse.collin@tukaani.org>
309 - IRC: Larhzu on Libera Chat
310