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
   4.1. Testing 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
ratio.

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.0BSD          BSD Zero Clause License
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
packages.

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) format 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:

    https://zlib.net/manual.html
    https://zlib.net/zlib_how.html

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
     binaries.

  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
system.

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:

    https://translationproject.org/html/translators.html

Updates to translations won't be accepted by methods that bypass
the Translation Project because there is a risk of duplicate work:
translation updates made in the xz repository aren't seen by the
translators in the Translation Project. If you have found bugs in
a translation, please report them to the Language-Team address
which can be found near the beginning of the PO file.

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

4.1. Testing translations

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

If building from Git repository (not tarball), generate the
Autotools files:

    ./autogen.sh

Create a subdirectory for the build files. The tmp-build directory
can be deleted after testing.

    mkdir tmp-build
    cd tmp-build
    ../configure --disable-shared --enable-debug --prefix=$PWD/inst

Edit the .po file in the po directory. Then build and install to
the "tmp-build/inst" directory, and use translations.bash to see
how some of the messages look. Repeat these  steps if needed:

    make -C po update-po
    make -j"$(nproc)" install
    bash ../debug/translation.bash | less
    bash ../debug/translation.bash | less -S  # For --list outputs

To test other languages, set the LANGUAGE environment variable
before running translations.bash. The value should match the PO file
name without the .po suffix. Example:

    export LANGUAGE=fi

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.

    https://7-zip.org/
    https://p7zip.sourceforge.net/

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

    https://tukaani.org/xz/embedded.html

XZ for Java is a complete implementation written in pure Java.

    https://tukaani.org/xz/java.html

6. Contact information

XZ Utils in general:
  - Home page: https://tukaani.org/xz/
  - Email to maintainer(s): [email protected]
  - IRC: #tukaani on Libera Chat
  - GitHub: https://github.com/tukaani-project/xz

Lead maintainer:
  - Email: Lasse Collin <[email protected]>
  - IRC: Larhzu on Libera Chat