README - jrn/xz - Git at Google


 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 purporse 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)

     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 reprocude 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.

	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 purporse 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)

	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 reprocude 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.