7 1.1. Overall documentation
8 1.2. Documentation for command line tools
9 1.3. Documentation for liblzma
11 3. Other implementations of the .xz format
12 4. Contact information
18 XZ Utils provide a general purporse data compression library and
19 command line tools. The native file format is the .xz format, but
20 also the legacy .lzma format is supported. The .xz format supports
21 multiple compression algorithms, which are called "filters" in
22 context of XZ Utils. The primary filter is currently LZMA2. With
23 typical files, XZ Utils create about 30 % smaller files than gzip.
25 To ease adapting support for the .xz format into existing applications
26 and scripts, the API of liblzma is somewhat similar to the API of the
27 popular zlib library. For the same reason, the command line tool xz
28 has similar command line syntax than that of gzip.
30 When aiming for the highest compression ratio, LZMA2 encoder uses
31 a lot of CPU time and may use, depending on the settings, even
32 hundreds of megabytes of RAM. However, in fast modes, LZMA2 encoder
33 competes with bzip2 in compression speed, RAM usage, and compression
36 LZMA2 is reasonably fast to decompress. It is a little slower than
37 gzip, but a lot faster than bzip2. Being fast to decompress means
38 that the .xz format is especially nice when the same file will be
39 decompressed very many times (usually on different computers), which
40 is the case e.g. when distributing software packages. In such
41 situations, it's not too bad if the compression takes some time,
42 since that needs to be done only once to benefit many people.
44 With some file types, combining (or "chaining") LZMA2 with an
45 additional filter can improve compression ratio. A filter chain may
46 contain up to four filters, although usually only one two is used.
47 For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2
48 in the filter chain can improve compression ratio of executable files.
50 Since the .xz format allows adding new filter IDs, it is possible that
51 some day there will be a filter that is, for example, much faster to
52 compress than LZMA2 (but probably with worse compression ratio).
53 Similarly, it is possible that some day there is a filter that will
54 compress better than LZMA2.
56 XZ Utils doesn't support multithreaded compression or decompression
57 yet. It has been planned though and taken into account when designing
64 1.1. Overall documentation
68 INSTALL.generic Generic install instructions for those not familiar
69 with packages using GNU Autotools
70 INSTALL Installation instructions specific to XZ Utils
71 PACKAGERS Information to packagers of XZ Utils
73 COPYING XZ Utils copyright and license information
74 COPYING.GPLv2 GNU General Public License version 2
75 COPYING.GPLv3 GNU General Public License version 3
76 COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1
78 AUTHORS The main authors of XZ Utils
79 THANKS Incomplete list of people who have helped making
81 NEWS User-visible changes between XZ Utils releases
82 ChangeLog Detailed list of changes (commit log)
84 Note that only some of the above files are included in binary
88 1.2. Documentation for command line tools
90 The command line tools are documented as man pages. In source code
91 releases (and possibly also in some binary packages), the man pages
92 are also provided in plain text (ASCII only) and PDF formats in the
93 directory "doc/man" to make the man pages more accessible to those
94 whose operating system doesn't provide an easy way to view man pages.
97 1.3. Documentation for liblzma
99 The liblzma API headers include short docs about each function
100 and data type as Doxygen tags. These docs should be quite OK as
103 I have planned to write a bunch of very well documented example
104 programs, which (due to comments) should work as a tutorial to
105 various features of liblzma. No such example programs have been
108 For now, if you have never used liblzma, libbzip2, or zlib, I
109 recommend learning *basics* of zlib API. Once you know that, it
110 should be easier to learn liblzma.
112 http://zlib.net/manual.html
113 http://zlib.net/zlib_how.html
119 The version number format of XZ Utils is X.Y.ZS:
121 - X is the major version. When this is incremented, the library
124 - Y is the minor version. It is incremented when new features are
125 added without breaking existing API or ABI. Even Y indicates
126 stable release and odd Y indicates unstable (alpha or beta
129 - Z is the revision. This has different meaning for stable and
131 * Stable: Z is incremented when bugs get fixed without adding
133 * Unstable: Z is just a counter. API or ABI of features added
134 in earlier unstable releases having the same X.Y may break.
136 - S indicates stability of the release. It is missing from the
137 stable releases where Y is an even number. When Y is odd, S
138 is either "alpha" or "beta" to make it very clear that such
139 versions are not stable releases. The same X.Y.Z combination is
140 not used for more than one stability level i.e. after X.Y.Zalpha,
141 the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
144 3. Other implementations of the .xz format
145 ------------------------------------------
147 7-Zip and the p7zip port of 7-Zip support the .xz format starting
148 from the version 9.00alpha.
151 http://p7zip.sourceforge.net/
153 XZ Embedded is a limited implementation written for use in the Linux
154 kernel, but it is also suitable for other embedded use.
156 http://tukaani.org/xz-embedded/
159 4. Contact information
160 ----------------------
162 If you have questions, bug reports, patches etc. related to XZ Utils,
163 contact Lasse Collin <lasse.collin@tukaani.org>. tukaani.org uses
164 greylisting to reduce spam, thus when you send your first email, it
165 may get delayed by a few hours. In addition to that, I'm sometimes
166 slow at replying. If you haven't got a reply within two weeks, assume
167 that your email has got lost and resend it or use IRC.
169 You can find me also from #tukaani on Freenode; my nick is Larhzu.
170 The channel tends to be pretty quiet, so just ask your question and