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. Other implementations of the .xz format
5. Contact information
0. Overview
-----------
XZ Utils provide a general-purpose data compression library and
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
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 similar command line syntax than that of gzip.
When aiming for the highest compression ratio, 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, 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 compression ratio. A filter chain may
contain up to four filters, although usually only one two is 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 *basics* of 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 existing API or ABI. Even Y indicates
stable release and odd Y indicates unstable (alpha or beta
version).
- Z is the revision. This has different meaning for stable and
unstable releases:
* Stable: Z is incremented when bugs get fixed without adding
any new features.
* 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 an 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. 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
5. 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).
tukaani.org uses greylisting to reduce spam, thus when you send your
first email, it may get delayed by a few hours. In addition to that,
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.
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