7 1.1. Overall documentation
8 1.2. Documentation for command-line tools
9 1.3. Documentation for liblzma
13 5. Other implementations of the .xz format
14 6. Contact information
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.
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.
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
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.
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.
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.
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
68 1.1. Overall documentation
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
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 COPYING.CC-BY-SA-4.0 Creative Commons Attribution-ShareAlike 4.0
82 International Public License
84 AUTHORS The main authors of XZ Utils
85 THANKS Incomplete list of people who have helped making
87 NEWS User-visible changes between XZ Utils releases
88 ChangeLog Detailed list of changes (commit log)
89 TODO Known bugs and some sort of to-do list
91 Note that only some of the above files are included in binary
95 1.2. Documentation for command-line tools
97 The command-line tools are documented as man pages. In source code
98 releases (and possibly also in some binary packages), the man pages
99 are also provided in plain text (ASCII only) and PDF formats in the
100 directory "doc/man" to make the man pages more accessible to those
101 whose operating system doesn't provide an easy way to view man pages.
104 1.3. Documentation for liblzma
106 The liblzma API headers include short docs about each function
107 and data type as Doxygen tags. These docs should be quite OK as
110 There are a few example/tutorial programs that should help in
111 getting started with liblzma. In the source package the examples
112 are in "doc/examples" and in binary packages they may be under
113 "examples" in the same directory as this README.
115 Since the liblzma API has similarities to the zlib API, some people
116 may find it useful to read the zlib docs and tutorial too:
118 https://zlib.net/manual.html
119 https://zlib.net/zlib_how.html
125 The version number format of XZ Utils is X.Y.ZS:
127 - X is the major version. When this is incremented, the library
130 - Y is the minor version. It is incremented when new features
131 are added without breaking the existing API or ABI. An even Y
132 indicates a stable release and an odd Y indicates unstable
133 (alpha or beta version).
135 - Z is the revision. This has a different meaning for stable and
138 * Stable: Z is incremented when bugs get fixed without adding
139 any new features. This is intended to be convenient for
140 downstream distributors that want bug fixes but don't want
141 any new features to minimize the risk of introducing new bugs.
143 * Unstable: Z is just a counter. API or ABI of features added
144 in earlier unstable releases having the same X.Y may break.
146 - S indicates stability of the release. It is missing from the
147 stable releases, where Y is an even number. When Y is odd, S
148 is either "alpha" or "beta" to make it very clear that such
149 versions are not stable releases. The same X.Y.Z combination is
150 not used for more than one stability level, i.e. after X.Y.Zalpha,
151 the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
157 Naturally it is easiest for me if you already know what causes the
158 unexpected behavior. Even better if you have a patch to propose.
159 However, quite often the reason for unexpected behavior is unknown,
160 so here are a few things to do before sending a bug report:
162 1. Try to create a small example how to reproduce the issue.
164 2. Compile XZ Utils with debugging code using configure switches
165 --enable-debug and, if possible, --disable-shared. If you are
166 using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting
169 3. Turn on core dumps. The exact command depends on your shell;
170 for example in GNU bash it is done with "ulimit -c unlimited",
171 and in tcsh with "limit coredumpsize unlimited".
173 4. Try to reproduce the suspected bug. If you get "assertion failed"
174 message, be sure to include the complete message in your bug
175 report. If the application leaves a coredump, get a backtrace
177 $ gdb /path/to/app-binary # Load the app to the debugger.
178 (gdb) core core # Open the coredump.
179 (gdb) bt # Print the backtrace. Copy & paste to bug report.
180 (gdb) quit # Quit gdb.
182 Report your bug via email or IRC (see Contact information below).
183 Don't send core dump files or any executables. If you have a small
184 example file(s) (total size less than 256 KiB), please include
185 it/them as an attachment. If you have bigger test files, put them
186 online somewhere and include a URL to the file(s) in the bug report.
188 Always include the exact version number of XZ Utils in the bug report.
189 If you are using a snapshot from the git repository, use "git describe"
190 to get the exact snapshot version. If you are using XZ Utils shipped
191 in an operating system distribution, mention the distribution name,
192 distribution version, and exact xz package version; if you cannot
193 repeat the bug with the code compiled from unpatched source code,
194 you probably need to report a bug to your distribution's bug tracking
201 The xz command line tool and all man pages can be translated.
202 The translations are handled via the Translation Project. If you
203 wish to help translating xz, please join the Translation Project:
205 https://translationproject.org/html/translators.html
207 Below are notes and testing instructions specific to xz
210 Testing can be done by installing xz into a temporary directory:
212 ./configure --disable-shared --prefix=/tmp/xz-test
213 # <Edit the .po file in the po directory.>
216 bash debug/translation.bash | less
217 bash debug/translation.bash | less -S # For --list outputs
219 Repeat the above as needed (no need to re-run configure though).
221 Note especially the following:
223 - The output of --help and --long-help must look nice on
224 an 80-column terminal. It's OK to add extra lines if needed.
226 - In contrast, don't add extra lines to error messages and such.
227 They are often preceded with e.g. a filename on the same line,
228 so you have no way to predict where to put a \n. Let the terminal
229 do the wrapping even if it looks ugly. Adding new lines will be
230 even uglier in the generic case even if it looks nice in a few
233 - Be careful with column alignment in tables and table-like output
234 (--list, --list --verbose --verbose, --info-memory, --help, and
237 * All descriptions of options in --help should start in the
238 same column (but it doesn't need to be the same column as
239 in the English messages; just be consistent if you change it).
240 Check that both --help and --long-help look OK, since they
241 share several strings.
243 * --list --verbose and --info-memory print lines that have
244 the format "Description: %s". If you need a longer
245 description, you can put extra space between the colon
246 and %s. Then you may need to add extra space to other
247 strings too so that the result as a whole looks good (all
248 values start at the same column).
250 * The columns of the actual tables in --list --verbose --verbose
251 should be aligned properly. Abbreviate if necessary. It might
252 be good to keep at least 2 or 3 spaces between column headings
253 and avoid spaces in the headings so that the columns stand out
254 better, but this is a matter of opinion. Do what you think
257 - Be careful to put a period at the end of a sentence when the
258 original version has it, and don't put it when the original
259 doesn't have it. Similarly, be careful with \n characters
260 at the beginning and end of the strings.
262 - Read the TRANSLATORS comments that have been extracted from the
263 source code and included in xz.pot. Some comments suggest
264 testing with a specific command which needs an .xz file. You
265 may use e.g. any tests/files/good-*.xz. However, these test
266 commands are included in translations.bash output, so reading
267 translations.bash output carefully can be enough.
269 - If you find language problems in the original English strings,
270 feel free to suggest improvements. Ask if something is unclear.
272 - The translated messages should be understandable (sometimes this
273 may be a problem with the original English messages too). Don't
274 make a direct word-by-word translation from English especially if
275 the result doesn't sound good in your language.
277 Thanks for your help!
280 5. Other implementations of the .xz format
281 ------------------------------------------
283 7-Zip and the p7zip port of 7-Zip support the .xz format starting
284 from the version 9.00alpha.
287 https://p7zip.sourceforge.net/
289 XZ Embedded is a limited implementation written for use in the Linux
290 kernel, but it is also suitable for other embedded use.
292 https://xz.tukaani.org/xz-embedded/
294 XZ for Java is a complete implementation written in pure Java.
296 https://xz.tukaani.org/xz-for-java/
299 6. Contact information
300 ----------------------
302 If you have questions, bug reports, patches etc. related to XZ Utils,
303 the project maintainers Lasse Collin and Jia Tan can be reached via
306 You might find Lasse also from #tukaani on Libera Chat (IRC).
307 The nick is Larhzu. The channel tends to be pretty quiet,
308 so just ask your question and someone might wake up.