
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. Translating the xz tool
    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 doesn't support multithreaded compression or decompression
    yet. It has been planned though and taken into account when designing
    the .xz file format.


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
    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) 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.

    I have planned to write a bunch of very well documented example
    programs, which (due to comments) should work as a tutorial to
    various features of liblzma. No such example programs have been
    written yet.

    For now, if you have never used liblzma, libbzip2, or zlib, I
    recommend learning the *basics* of the zlib API. Once you know that,
    it should be easier to learn liblzma.

        http://zlib.net/manual.html
        http://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. Translating the xz tool
--------------------------

    The messages from the xz tool have been translated into a few
    languages. Before starting to translate into a new language, ask
    the author whether someone else hasn't already started working on it.

    Test your translation. Testing includes comparing the translated
    output to the original English version by running the same commands
    in both your target locale and with LC_ALL=C. Ask someone to
    proof-read and test the translation.

    Testing can be done e.g. 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
        --long-help):

          * 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. If they suggest testing the
        translation with some type of command, do it. If testing needs
        input files, use e.g. tests/files/good-*.xz.

      - When updating the translation, read the fuzzy (modified) strings
        carefully, and don't mark them as updated before you actually
        have updated them. Reading through the unchanged messages can be
        good too; sometimes you may find a better wording for them.

      - 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.

    In short, take your time and pay attention to the details. Making
    a good translation is not a quick and trivial thing to do. The
    translated xz should look as polished as the English version.


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.

        http://7-zip.org/
        http://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


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.

