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/translations.bash | less
 bash debug/translations.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.
 http://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.