1
2XZ 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
170. 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
651. Documentation
66----------------
67
681.1. Overall documentation
69
70 README This file
71
72 INSTALL.generic Generic install instructions for those not familiar
73 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.GPLv2 GNU General Public License version 2
79 COPYING.GPLv3 GNU General Public License version 3
80 COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1
81
82 AUTHORS The main authors of XZ Utils
83 THANKS Incomplete list of people who have helped making
84 this software
85 NEWS User-visible changes between XZ Utils releases
86 ChangeLog Detailed list of changes (commit log)
87 TODO Known bugs and some sort of to-do list
88
89 Note that only some of the above files are included in binary
90 packages.
91
92
931.2. Documentation for command-line tools
94
95 The command-line tools are documented as man pages. In source code
96 releases (and possibly also in some binary packages), the man pages
97 are also provided in plain text (ASCII only) and PDF formats in the
98 directory "doc/man" to make the man pages more accessible to those
99 whose operating system doesn't provide an easy way to view man pages.
100
101
1021.3. Documentation for liblzma
103
104 The liblzma API headers include short docs about each function
105 and data type as Doxygen tags. These docs should be quite OK as
106 a quick reference.
107
108 There are a few example/tutorial programs that should help in
109 getting started with liblzma. In the source package the examples
110 are in "doc/examples" and in binary packages they may be under
111 "examples" in the same directory as this README.
112
113 Since the liblzma API has similarities to the zlib API, some people
114 may find it useful to read the zlib docs and tutorial too:
115
116 https://zlib.net/manual.html
117 https://zlib.net/zlib_how.html
118
119
1202. Version numbering
121--------------------
122
123 The version number format of XZ Utils is X.Y.ZS:
124
125 - X is the major version. When this is incremented, the library
126 API and ABI break.
127
128 - Y is the minor version. It is incremented when new features
129 are added without breaking the existing API or ABI. An even Y
130 indicates a stable release and an odd Y indicates unstable
131 (alpha or beta version).
132
133 - Z is the revision. This has a different meaning for stable and
134 unstable releases:
135
136 * Stable: Z is incremented when bugs get fixed without adding
137 any new features. This is intended to be convenient for
138 downstream distributors that want bug fixes but don't want
139 any new features to minimize the risk of introducing new bugs.
140
141 * Unstable: Z is just a counter. API or ABI of features added
142 in earlier unstable releases having the same X.Y may break.
143
144 - S indicates stability of the release. It is missing from the
145 stable releases, where Y is an even number. When Y is odd, S
146 is either "alpha" or "beta" to make it very clear that such
147 versions are not stable releases. The same X.Y.Z combination is
148 not used for more than one stability level, i.e. after X.Y.Zalpha,
149 the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
150
151
1523. Reporting bugs
153-----------------
154
155 Naturally it is easiest for me if you already know what causes the
156 unexpected behavior. Even better if you have a patch to propose.
157 However, quite often the reason for unexpected behavior is unknown,
158 so here are a few things to do before sending a bug report:
159
160 1. Try to create a small example how to reproduce the issue.
161
162 2. Compile XZ Utils with debugging code using configure switches
163 --enable-debug and, if possible, --disable-shared. If you are
164 using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting
165 binaries.
166
167 3. Turn on core dumps. The exact command depends on your shell;
168 for example in GNU bash it is done with "ulimit -c unlimited",
169 and in tcsh with "limit coredumpsize unlimited".
170
171 4. Try to reproduce the suspected bug. If you get "assertion failed"
172 message, be sure to include the complete message in your bug
173 report. If the application leaves a coredump, get a backtrace
174 using gdb:
175 $ gdb /path/to/app-binary # Load the app to the debugger.
176 (gdb) core core # Open the coredump.
177 (gdb) bt # Print the backtrace. Copy & paste to bug report.
178 (gdb) quit # Quit gdb.
179
180 Report your bug via email or IRC (see Contact information below).
181 Don't send core dump files or any executables. If you have a small
182 example file(s) (total size less than 256 KiB), please include
183 it/them as an attachment. If you have bigger test files, put them
184 online somewhere and include a URL to the file(s) in the bug report.
185
186 Always include the exact version number of XZ Utils in the bug report.
187 If you are using a snapshot from the git repository, use "git describe"
188 to get the exact snapshot version. If you are using XZ Utils shipped
189 in an operating system distribution, mention the distribution name,
190 distribution version, and exact xz package version; if you cannot
191 repeat the bug with the code compiled from unpatched source code,
192 you probably need to report a bug to your distribution's bug tracking
193 system.
194
195
1964. Translations
197---------------
198
199 The xz command line tool and all man pages can be translated.
200 The translations are handled via the Translation Project. If you
201 wish to help translating xz, please join the Translation Project:
202
203 https://translationproject.org/html/translators.html
204
205 Below are notes and testing instructions specific to xz
206 translations.
207
208 Testing can be done by installing xz into a temporary directory:
209
210 ./configure --disable-shared --prefix=/tmp/xz-test
211 # <Edit the .po file in the po directory.>
212 make -C po update-po
213 make install
214 bash debug/translation.bash | less
215 bash debug/translation.bash | less -S # For --list outputs
216
217 Repeat the above as needed (no need to re-run configure though).
218
219 Note especially the following:
220
221 - The output of --help and --long-help must look nice on
222 an 80-column terminal. It's OK to add extra lines if needed.
223
224 - In contrast, don't add extra lines to error messages and such.
225 They are often preceded with e.g. a filename on the same line,
226 so you have no way to predict where to put a \n. Let the terminal
227 do the wrapping even if it looks ugly. Adding new lines will be
228 even uglier in the generic case even if it looks nice in a few
229 limited examples.
230
231 - Be careful with column alignment in tables and table-like output
232 (--list, --list --verbose --verbose, --info-memory, --help, and
233 --long-help):
234
235 * All descriptions of options in --help should start in the
236 same column (but it doesn't need to be the same column as
237 in the English messages; just be consistent if you change it).
238 Check that both --help and --long-help look OK, since they
239 share several strings.
240
241 * --list --verbose and --info-memory print lines that have
242 the format "Description: %s". If you need a longer
243 description, you can put extra space between the colon
244 and %s. Then you may need to add extra space to other
245 strings too so that the result as a whole looks good (all
246 values start at the same column).
247
248 * The columns of the actual tables in --list --verbose --verbose
249 should be aligned properly. Abbreviate if necessary. It might
250 be good to keep at least 2 or 3 spaces between column headings
251 and avoid spaces in the headings so that the columns stand out
252 better, but this is a matter of opinion. Do what you think
253 looks best.
254
255 - Be careful to put a period at the end of a sentence when the
256 original version has it, and don't put it when the original
257 doesn't have it. Similarly, be careful with \n characters
258 at the beginning and end of the strings.
259
260 - Read the TRANSLATORS comments that have been extracted from the
261 source code and included in xz.pot. Some comments suggest
262 testing with a specific command which needs an .xz file. You
263 may use e.g. any tests/files/good-*.xz. However, these test
264 commands are included in translations.bash output, so reading
265 translations.bash output carefully can be enough.
266
267 - If you find language problems in the original English strings,
268 feel free to suggest improvements. Ask if something is unclear.
269
270 - The translated messages should be understandable (sometimes this
271 may be a problem with the original English messages too). Don't
272 make a direct word-by-word translation from English especially if
273 the result doesn't sound good in your language.
274
275 Thanks for your help!
276
277
2785. Other implementations of the .xz format
279------------------------------------------
280
281 7-Zip and the p7zip port of 7-Zip support the .xz format starting
282 from the version 9.00alpha.
283
284 https://7-zip.org/
285 https://p7zip.sourceforge.net/
286
287 XZ Embedded is a limited implementation written for use in the Linux
288 kernel, but it is also suitable for other embedded use.
289
290 https://tukaani.org/xz/embedded.html
291
292 XZ for Java is a complete implementation written in pure Java.
293
294 https://tukaani.org/xz/java.html
295
296
2976. Contact information
298----------------------
299
300 If you have questions, bug reports, patches etc. related to XZ Utils,
301 the project maintainers Lasse Collin and Jia Tan can be reached via
302 <xz@tukaani.org>.
303
304 You might find Lasse also from #tukaani on Libera Chat (IRC).
305 The nick is Larhzu. The channel tends to be pretty quiet,
306 so just ask your question and someone might wake up.
307
308