XZ Utils is free general-purpose data compression software with a high compression ratio https://tukaani.org/xz/
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XZ Utils

0. Overview
1. Documentation
1.1. Overall documentation
1.2. Documentation for command-line tools
1.3. Documentation for liblzma
2. Version numbering
3. Reporting bugs
4. Translations
5. Other implementations of the .xz format
6. Contact information

0. Overview

XZ Utils provide a general-purpose data-compression library plus
command-line tools. The native file format is the .xz format, but
also the legacy .lzma format is supported. The .xz format supports
multiple compression algorithms, which are called "filters" in the
context of XZ Utils. The primary filter is currently LZMA2. With
typical files, XZ Utils create about 30 % smaller files than gzip.

To ease adapting support for the .xz format into existing applications
and scripts, the API of liblzma is somewhat similar to the API of the
popular zlib library. For the same reason, the command-line tool xz
has a command-line syntax similar to that of gzip.

When aiming for the highest compression ratio, the LZMA2 encoder uses
a lot of CPU time and may use, depending on the settings, even
hundreds of megabytes of RAM. However, in fast modes, the LZMA2 encoder
competes with bzip2 in compression speed, RAM usage, and compression

LZMA2 is reasonably fast to decompress. It is a little slower than
gzip, but a lot faster than bzip2. Being fast to decompress means
that the .xz format is especially nice when the same file will be
decompressed very many times (usually on different computers), which
is the case e.g. when distributing software packages. In such
situations, it's not too bad if the compression takes some time,
since that needs to be done only once to benefit many people.

With some file types, combining (or "chaining") LZMA2 with an
additional filter can improve the compression ratio. A filter chain may
contain up to four filters, although usually only one or two are used.
For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2
in the filter chain can improve compression ratio of executable files.

Since the .xz format allows adding new filter IDs, it is possible that
some day there will be a filter that is, for example, much faster to
compress than LZMA2 (but probably with worse compression ratio).
Similarly, it is possible that some day there is a filter that will
compress better than LZMA2.

XZ Utils supports multithreaded compression. XZ Utils doesn't support
multithreaded decompression yet. It has been planned though and taken
into account when designing the .xz file format. In the future, files
that were created in threaded mode can be decompressed in threaded
mode too.

1. Documentation

1.1. Overall documentation

README This file

INSTALL.generic Generic install instructions for those not familiar
with packages using GNU Autotools
INSTALL Installation instructions specific to XZ Utils
PACKAGERS Information to packagers of XZ Utils

COPYING XZ Utils copyright and license information
COPYING.GPLv2 GNU General Public License version 2
COPYING.GPLv3 GNU General Public License version 3
COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1

AUTHORS The main authors of XZ Utils
THANKS Incomplete list of people who have helped making
this software
NEWS User-visible changes between XZ Utils releases
ChangeLog Detailed list of changes (commit log)
TODO Known bugs and some sort of to-do list

Note that only some of the above files are included in binary

1.2. Documentation for command-line tools

The command-line tools are documented as man pages. In source code
releases (and possibly also in some binary packages), the man pages
are also provided in plain text (ASCII only) and PDF formats in the
directory "doc/man" to make the man pages more accessible to those
whose operating system doesn't provide an easy way to view man pages.

1.3. Documentation for liblzma

The liblzma API headers include short docs about each function
and data type as Doxygen tags. These docs should be quite OK as
a quick reference.

There are a few example/tutorial programs that should help in
getting started with liblzma. In the source package the examples
are in "doc/examples" and in binary packages they may be under
"examples" in the same directory as this README.

Since the liblzma API has similarities to the zlib API, some people
may find it useful to read the zlib docs and tutorial too:


2. Version numbering

The version number format of XZ Utils is X.Y.ZS:

- X is the major version. When this is incremented, the library
API and ABI break.

- Y is the minor version. It is incremented when new features
are added without breaking the existing API or ABI. An even Y
indicates a stable release and an odd Y indicates unstable
(alpha or beta version).

- Z is the revision. This has a different meaning for stable and
unstable releases:

* Stable: Z is incremented when bugs get fixed without adding
any new features. This is intended to be convenient for
downstream distributors that want bug fixes but don't want
any new features to minimize the risk of introducing new bugs.

* Unstable: Z is just a counter. API or ABI of features added
in earlier unstable releases having the same X.Y may break.

- S indicates stability of the release. It is missing from the
stable releases, where Y is an even number. When Y is odd, S
is either "alpha" or "beta" to make it very clear that such
versions are not stable releases. The same X.Y.Z combination is
not used for more than one stability level, i.e. after X.Y.Zalpha,
the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.

3. Reporting bugs

Naturally it is easiest for me if you already know what causes the
unexpected behavior. Even better if you have a patch to propose.
However, quite often the reason for unexpected behavior is unknown,
so here are a few things to do before sending a bug report:

1. Try to create a small example how to reproduce the issue.

2. Compile XZ Utils with debugging code using configure switches
--enable-debug and, if possible, --disable-shared. If you are
using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting

3. Turn on core dumps. The exact command depends on your shell;
for example in GNU bash it is done with "ulimit -c unlimited",
and in tcsh with "limit coredumpsize unlimited".

4. Try to reproduce the suspected bug. If you get "assertion failed"
message, be sure to include the complete message in your bug
report. If the application leaves a coredump, get a backtrace
using gdb:
$ gdb /path/to/app-binary # Load the app to the debugger.
(gdb) core core # Open the coredump.
(gdb) bt # Print the backtrace. Copy & paste to bug report.
(gdb) quit # Quit gdb.

Report your bug via email or IRC (see Contact information below).
Don't send core dump files or any executables. If you have a small
example file(s) (total size less than 256 KiB), please include
it/them as an attachment. If you have bigger test files, put them
online somewhere and include a URL to the file(s) in the bug report.

Always include the exact version number of XZ Utils in the bug report.
If you are using a snapshot from the git repository, use "git describe"
to get the exact snapshot version. If you are using XZ Utils shipped
in an operating system distribution, mention the distribution name,
distribution version, and exact xz package version; if you cannot
repeat the bug with the code compiled from unpatched source code,
you probably need to report a bug to your distribution's bug tracking

4. Translations

The xz command line tool and all man pages can be translated.
The translations are handled via the Translation Project. If you
wish to help translating xz, please join the Translation Project:


Below are notes and testing instructions specific to xz

Testing can be done by installing xz into a temporary directory:

./configure --disable-shared --prefix=/tmp/xz-test
# <Edit the .po file in the po directory.>
make -C po update-po
make install
bash debug/translation.bash | less
bash debug/translation.bash | less -S # For --list outputs

Repeat the above as needed (no need to re-run configure though).

Note especially the following:

- The output of --help and --long-help must look nice on
an 80-column terminal. It's OK to add extra lines if needed.

- In contrast, don't add extra lines to error messages and such.
They are often preceded with e.g. a filename on the same line,
so you have no way to predict where to put a \n. Let the terminal
do the wrapping even if it looks ugly. Adding new lines will be
even uglier in the generic case even if it looks nice in a few
limited examples.

- Be careful with column alignment in tables and table-like output
(--list, --list --verbose --verbose, --info-memory, --help, and

* All descriptions of options in --help should start in the
same column (but it doesn't need to be the same column as
in the English messages; just be consistent if you change it).
Check that both --help and --long-help look OK, since they
share several strings.

* --list --verbose and --info-memory print lines that have
the format "Description: %s". If you need a longer
description, you can put extra space between the colon
and %s. Then you may need to add extra space to other
strings too so that the result as a whole looks good (all
values start at the same column).

* The columns of the actual tables in --list --verbose --verbose
should be aligned properly. Abbreviate if necessary. It might
be good to keep at least 2 or 3 spaces between column headings
and avoid spaces in the headings so that the columns stand out
better, but this is a matter of opinion. Do what you think
looks best.

- Be careful to put a period at the end of a sentence when the
original version has it, and don't put it when the original
doesn't have it. Similarly, be careful with \n characters
at the beginning and end of the strings.

- Read the TRANSLATORS comments that have been extracted from the
source code and included in xz.pot. Some comments suggest
testing with a specific command which needs an .xz file. You
may use e.g. any tests/files/good-*.xz. However, these test
commands are included in translations.bash output, so reading
translations.bash output carefully can be enough.

- If you find language problems in the original English strings,
feel free to suggest improvements. Ask if something is unclear.

- The translated messages should be understandable (sometimes this
may be a problem with the original English messages too). Don't
make a direct word-by-word translation from English especially if
the result doesn't sound good in your language.

Thanks for your help!

5. Other implementations of the .xz format

7-Zip and the p7zip port of 7-Zip support the .xz format starting
from the version 9.00alpha.


XZ Embedded is a limited implementation written for use in the Linux
kernel, but it is also suitable for other embedded use.


6. Contact information

If you have questions, bug reports, patches etc. related to XZ Utils,
contact Lasse Collin <lasse.collin@tukaani.org> (in Finnish or English).
I'm sometimes slow at replying. If you haven't got a reply within two
weeks, assume that your email has got lost and resend it or use IRC.

You can find me also from #tukaani on Freenode; my nick is Larhzu.
The channel tends to be pretty quiet, so just ask your question and
someone may wake up.