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Updated faq.txt.

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Some questions worth answering were removed, because I
currently don't have good up to date answers to them.
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@ -2,185 +2,96 @@
XZ Utils FAQ
============
Q: What are LZMA, LZMA Utils, lzma, .lzma, liblzma, LZMA SDK, LZMA_Alone,
7-Zip and p7zip?
Q: What do the letters XZ mean?
A: LZMA stands for Lempel-Ziv-Markov chain-Algorithm. LZMA is the name
of the compression algorithm designed by Igor Pavlov. He is the author
of 7-Zip, which is a great LGPL'd compression tool for Microsoft
Windows operating systems. In addition to 7-Zip itself, also LZMA SDK
is available on the website of 7-Zip. LZMA SDK contains LZMA
implementations in C++, Java and C#. The C++ version is the original
implementation which is used also in 7-Zip itself.
Excluding the unrar plugin, 7-Zip is free software (free as in
freedom). Thanks to this, it was possible to port it to POSIX
platforms. The port was done and is maintained by myspace (TODO:
myspace's real name?). p7zip is a port of 7-Zip's command line version;
p7zip doesn't include the 7-Zip's GUI.
In POSIX world, users are used to gzip and bzip2 command line tools.
Developers know APIs of zlib and libbzip2. LZMA Utils try to ease
adoption of LZMA on free operating systems by providing a compression
library and a set of command line tools. The library is called liblzma.
It provides a zlib-like API making it easy to adapt LZMA compression in
existing applications. The main command line tool is known as lzma,
whose command line syntax is very similar to that of gzip and bzip2.
The original command line tool from LZMA SDK (lzma.exe) was found from
a directory called LZMA_Alone in the LZMA SDK. It used a simple header
format in .lzma files. This format was also used by LZMA Utils up to
and including 4.32.x. In LZMA Utils documentation, LZMA_Alone refers
to both the file format and the command line tool from LZMA SDK.
Because of various limitations of the LZMA_Alone file format, a new
file format was developed. Extending some existing format such as .gz
used by gzip was considered, but these formats were found to be too
limited. The filename suffix for the new .lzma format is `.lzma'. The
same suffix is also used for files in the LZMA_Alone format. To make
the transition to the new format as transparent as possible, LZMA Utils
support both the new and old formats transparently.
7-Zip and LZMA SDK: <http://7-zip.org/>
p7zip: <http://p7zip.sourceforge.net/>
LZMA Utils: <http://tukaani.org/lzma/>
A: Nothing. They are just two letters, which come from the file format
suffix .xz. The .xz suffix was selected, because it seemed to be
pretty much unused. It is no deeper meaning.
Q: What LZMA implementations there are available?
Q: What are LZMA and LZMA2?
A: LZMA SDK contains implementations in C++, Java and C#. The C++ version
is the original implementation which is part of 7-Zip. LZMA SDK
contains also a small LZMA decoder in C.
A: LZMA stands for Lempel-Ziv-Markov chain-Algorithm. It is the name
of the compression algorithm designed by Igor Pavlov for 7-Zip.
LZMA is based on LZ77 and range encoding.
A port of LZMA SDK to Pascal was made by Alan Birtles
<http://www.birtles.org.uk/programming/>. It should work with
multiple Pascal programming language implementations.
LZMA Utils includes liblzma, which is directly based on LZMA SDK.
liblzma is written in C (C99, not C89). In contrast to C++ callback
API used by LZMA SDK, liblzma uses zlib-like stateful C API. I do not
want to comment whether both/former/latter/neither API(s) are good or
bad. The only reason to implement a zlib-like API was, that many
developers are already familiar with zlib, and very many applications
already use zlib. Having a similar API makes it easier to include LZMA
support in existing applications.
See also <http://en.wikipedia.org/wiki/LZMA#External_links>.
LZMA2 is an updated version of the original LZMA to fix a couple of
practical issues. In context of XZ Utils, LZMA is called LZMA1 to
emphasize that LZMA is not the same thing as LZMA2. LZMA2 is the
primary compression algorithm in the .xz file format.
Q: Which file formats are supported by LZMA Utils?
Q: There are many LZMA related projects. How does XZ Utils relate to them?
A: Even when the raw LZMA stream is always the same, it can be wrapped
in different container formats. The preferred format is the new .lzma
format. It has magic bytes (the first six bytes: 0xFF 'L' 'Z' 'M'
'A' 0x00). The format supports chaining up to seven filters, splitting
data to multiple blocks for easier multi-threading and rough
random-access reading. The file integrity is verified using CRC32,
CRC64, or SHA256, and by verifying the uncompressed size of the file.
A: 7-Zip and LZMA SDK are the original projects. LZMA SDK is roughly
a subset of the 7-Zip source tree.
LZMA SDK includes a tool called LZMA_Alone. It supports uses a
primitive header which includes only the mandatory stream information
required by the LZMA decoder. This format can be both read and
written by liblzma and the command line tool (use --format=alone to
create such files).
p7zip is 7-Zip's command line tools ported to POSIX-like systems.
.7z is the native archive format used by 7-Zip. This format is not
supported by liblzma, and probably will never be supported. You
should use e.g. p7zip to extract .7z files.
LZMA Utils provide a gzip-like lzma tool for POSIX-like systems.
LZMA Utils are based on LZMA SDK. XZ Utils are the successor to
LZMA Utils.
It is possible to implement custom file formats by using raw filter
mode in liblzma. In this mode the application needs to store the filter
properties and provide them to liblzma before starting to uncompress
the data.
There are several other projects using LZMA. Most are more or less
based on LZMA SDK.
Q: How can I identify files containing LZMA compressed data?
Q: Do XZ Utils support the .7z format?
A: The preferred filename suffix for .lzma files is `.lzma'. `.tar.lzma'
may be abbreviated to `.tlz'. The same suffixes are used for files in
LZMA_Alone format. In practice this should be no problem since tools
included in LZMA Utils support both formats transparently.
Checking the magic bytes is easy way to detect files in the new .lzma
format (the first six bytes: 0xFF 'L' 'Z' 'M' 'A' 0x00). The "file"
command version FIXME contains magic strings for this format.
The old LZMA_Alone format has no magic bytes. Its header cannot contain
arbitrary bytes, thus it is possible to make a guess. Unfortunately the
guessing is usually too hard to be reliable, so don't try it unless you
are desperate.
A: No. Use 7-Zip (Windows) or p7zip (POSIX-like systems) to handle .7z
files.
Q: Does the lzma command line tool support sparse files?
Q: I have many .tar.7z files. Can I convert them to .tar.xz without
spending hours recompressing the data?
A: Sparse files can (of course) be compressed like normal files, but
uncompression will not restore sparseness of the file. Use an archiver
tool to take care of sparseness before compressing the data with lzma.
The reason for this is that archiver tools handle files, while
compression tools handle streams or buffers. Being a sparse file is
a property of the file on the disk, not a property of the stream or
buffer.
A: In the "extra" directory, there is a script named 7z2lzma.bash which
is able to convert some .7z files to the .lzma format (not .xz). It
needs the 7za (or 7z) command from p7zip. The script may silently
produce corrupt output if certain assumptions are not met, so
decompress the resulting .lzma file and compare it against the
original before deleting the original file!
Q: Can I recover parts of a broken LZMA file (e.g. corrupted CD-R)?
Q: I have many .lzma files. Can I quickly convert them to the .xz format?
A: With LZMA_Alone and single-block .lzma files, you can uncompress the
file until you hit the first broken byte. The data after the broken
position is lost. LZMA relies on the uncompression history, and if
bytes are missing in the middle of the file, it is impossible to
reliably continue after the broken section.
A: For now, no. Since XZ Utils supports the .lzma format, it's usually
not too bad to keep the old files in the old format. If you want to
do the conversion anyway, you need to decompress the .lzma files and
then recompress to the .xz format.
With multi-block .lzma files it may be possible to locale the next
block in the file and continue decoding there. A limited recovery
tool for this kind of situations is planned.
Technically, there is a way to make the conversion relatively fast
(roughly twice the time that normal decompression takes). Writing
such a tool would take quite a bit time though, and would probably
be useful to only a few people. If you really want such a conversion
tool, contact Lasse Collin and offer some money.
Q: Is LZMA patented?
Q: Can I recover parts of a broken .xz file (e.g. corrupted CD-R)?
A: No, the authors are not aware of any patents that could affect LZMA.
However, due to nature of software patents, the authors cannot
guarantee, that LZMA isn't affected by any third party patent.
A: It may be possible if the file consist of multiple blocks, which
typically is not the case if the file was created in single-threaded
mode. There is no recovery program yet.
Q: Where can I find documentation about how LZMA works as an algorithm?
Q: Is (some part of) XZ Utils patented?
A: Read the source code, Luke. There is no documentation about LZMA
internals. It is possible that Igor Pavlov is the only person on
the Earth that completely knows and understands the algorithm.
You could begin by downloading LZMA SDK, and start reading from
the LZMA decoder to get some idea about the bitstream format.
Before you begin, you should know the basics of LZ77 and
range coding algorithms. LZMA is based on LZ77, but LZMA is
*a lot* more complex. Range coding is used to compress the
final bitstream like Huffman coding is used in Deflate.
A: Lasse Collin is not aware of any patents that could affect XZ Utils.
However, due to nature of software patents, it's not possible to
guarantee that XZ Utils isn't affected by any third party patent(s).
Q: What are filters?
Q: Where can I find documentation about the file format and algorithms?
A: In context of .lzma files, a filter means an implementation of a
compression algorithm. The primary filter is LZMA, which is why
the names of the tools contain the letters LZMA.
A: The .xz format is documented in xz-file-format.txt. It is a container
format only, and doesn't include descriptions of any non-trivial
filters.
liblzma and the new .lzma format support also other filters than LZMA.
There are different types of filters, which are suitable for different
types of data. Thus, to select the optimal filter and settings, the
type of the input data being compressed needs to be known.
Some filters are most useful when combined with another filter like
LZMA. These filters increase redundancy in the data, without changing
the size of the data, by taking advantage of properties specific to
the data being compressed.
So far, all the filters are always reversible. That is, no matter what
data you pass to a filter encoder, it can be always defiltered back to
the original form. Because of this, it is safe to compress for example
a software package that contains other file types than executables
using a filter specific to the architechture of the package being
compressed.
The old LZMA_Alone format supports only the LZMA filter.
Documenting LZMA and LZMA2 is planned, but for now, there is no other
documentation that the source code. Before you begin, you should know
the basics of LZ77 and range coding algorithms. LZMA is based on LZ77,
but LZMA is *a lot* more complex. Range coding is used to compress
the final bitstream like Huffman coding is used in Deflate.
Q: I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?
@ -189,27 +100,23 @@ A: BCJ filter is called "x86" in liblzma. BCJ2 is not included,
because it requires using more than one encoded output stream.
Q: Can I use LZMA in proprietary, non-free applications?
Q: How do I build a program that needs liblzmadec (lzmadec.h)?
A: Yes. See the file COPYING for details.
A: liblzmadec is part of LZMA Utils. XZ Utils has liblzma, but no
liblzmadec. The code using liblzmadec should be ported to use
liblzma instead. If you cannot or don't want to do that, download
LZMA Utils from <http://tukaani.org/lzma/>.
Q: I would like to help. What can I do?
Q: The default build of liblzma is too big. How can I make it smaller?
A: See the TODO file. Please contact Lasse Collin before starting to do
anything, because it is possible that someone else is already working
on the same thing.
A: Give --enable-small to the configure script. Use also appropriate
--enable or --disable options to include only those filter encoders
and decoders and integrity checks that you actually need. Use
CFLAGS=-Os (with GCC) or equivalent to tell your compiler to optimize
for size. See INSTALL for information about configure options.
Q: How can I contact the authors?
A: Lasse Collin is the maintainer of LZMA Utils. You can contact him
either via IRC (Larhzu on #tukaani at Freenode or IRCnet). Email
should work too, <lasse.collin@tukaani.org>.
Igor Pavlov is the father of LZMA. He is the author of 7-Zip
and LZMA SDK. <http://7-zip.org/>
NOTE: Please don't bother Igor Pavlov with questions specific
to LZMA Utils.
If the result is still too big, take a look at XZ Embedded. It is
a separate project, which provides a limited but signinificantly
smaller XZ decoder implementation than XZ Utils.