248 lines
11 KiB
Plaintext
248 lines
11 KiB
Plaintext
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LZMA Utils FAQ
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--------------
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Copyright (C) 2007 Lasse Collin
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Copying and distribution of this file, with or without modification,
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are permitted in any medium without royalty provided the copyright
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notice and this notice are preserved.
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Q: What are LZMA, LZMA Utils, lzma, .lzma, liblzma, LZMA SDK, LZMA_Alone,
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7-Zip and p7zip?
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A: LZMA stands for Lempel-Ziv-Markov chain-Algorithm. LZMA is the name
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of the compression algorithm designed by Igor Pavlov. He is the author
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of 7-Zip, which is a great LGPL'd compression tool for Microsoft
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Windows operating systems. In addition to 7-Zip itself, also LZMA SDK
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is available on the website of 7-Zip. LZMA SDK contains LZMA
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implementations in C++, Java and C#. The C++ version is the original
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implementation which is used also in 7-Zip itself.
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Excluding the unrar plugin, 7-Zip is free software (free as in
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freedom). Thanks to this, it was possible to port it to POSIX
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platforms. The port was done and is maintained by myspace (TODO:
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myspace's real name?). p7zip is a port of 7-Zip's command line version;
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p7zip doesn't include the 7-Zip's GUI.
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In POSIX world, users are used to gzip and bzip2 command line tools.
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Developers know APIs of zlib and libbzip2. LZMA Utils try to ease
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adoption of LZMA on free operating systems by providing a compression
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library and a set of command line tools. The library is called liblzma.
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It provides a zlib-like API making it easy to adapt LZMA compression in
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existing applications. The main command line tool is known as lzma,
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whose command line syntax is very similar to that of gzip and bzip2.
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The original command line tool from LZMA SDK (lzma.exe) was found from
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a directory called LZMA_Alone in the LZMA SDK. It used a simple header
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format in .lzma files. This format was also used by LZMA Utils up to
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and including 4.32.x. In LZMA Utils documentation, LZMA_Alone refers
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to both the file format and the command line tool from LZMA SDK.
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Because of various limitations of the LZMA_Alone file format, a new
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file format was developed. Extending some existing format such as .gz
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used by gzip was considered, but these formats were found to be too
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limited. The filename suffix for the new .lzma format is `.lzma'. The
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same suffix is also used for files in the LZMA_Alone format. To make
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the transition to the new format as transparent as possible, LZMA Utils
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support both the new and old formats transparently.
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7-Zip and LZMA SDK: <http://7-zip.org/>
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p7zip: <http://p7zip.sourceforge.net/>
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LZMA Utils: <http://tukaani.org/lzma/>
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Q: What LZMA implementations there are available?
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A: LZMA SDK contains implementations in C++, Java and C#. The C++ version
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is the original implementation which is part of 7-Zip. LZMA SDK
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contains also a small LZMA decoder in C.
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A port of LZMA SDK to Pascal was made by Alan Birtles
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<http://www.birtles.org.uk/programming/>. It should work with
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multiple Pascal programming language implementations.
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LZMA Utils includes liblzma, which is directly based on LZMA SDK.
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liblzma is written in C (C99, not C89). In contrast to C++ callback
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API used by LZMA SDK, liblzma uses zlib-like stateful C API. I do not
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want to comment whether both/former/latter/neither API(s) are good or
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bad. The only reason to implement a zlib-like API was, that many
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developers are already familiar with zlib, and very many applications
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already use zlib. Having a similar API makes it easier to include LZMA
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support in existing applications.
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See also <http://en.wikipedia.org/wiki/LZMA#External_links>.
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Q: Which file formats are supported by LZMA Utils?
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A: Even when the raw LZMA stream is always the same, it can be wrapped
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in different container formats. The preferred format is the new .lzma
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format. It has magic bytes (the first six bytes: 0xFF 'L' 'Z' 'M'
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'A' 0x00). The format supports chaining up to seven filters, splitting
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data to multiple blocks for easier multi-threading and rough
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random-access reading. The file integrity is verified using CRC32,
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CRC64, or SHA256, and by verifying the uncompressed size of the file.
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LZMA SDK includes a tool called LZMA_Alone. It supports uses a
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primitive header which includes only the mandatory stream information
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required by the LZMA decoder. This format can be both read and
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written by liblzma and the command line tool (use --format=alone to
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create such files).
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.7z is the native archive format used by 7-Zip. This format is not
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supported by liblzma, and probably will never be supported. You
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should use e.g. p7zip to extract .7z files.
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It is possible to implement custom file formats by using raw filter
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mode in liblzma. In this mode the application needs to store the filter
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properties and provide them to liblzma before starting to uncompress
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the data.
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Q: How can I identify files containing LZMA compressed data?
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A: The preferred filename suffix for .lzma files is `.lzma'. `.tar.lzma'
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may be abbreviated to `.tlz'. The same suffixes are used for files in
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LZMA_Alone format. In practice this should be no problem since tools
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included in LZMA Utils support both formats transparently.
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Checking the magic bytes is easy way to detect files in the new .lzma
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format (the first six bytes: 0xFF 'L' 'Z' 'M' 'A' 0x00). The "file"
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command version FIXME contains magic strings for this format.
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The old LZMA_Alone format has no magic bytes. Its header cannot contain
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arbitrary bytes, thus it is possible to make a guess. Unfortunately the
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guessing is usually too hard to be reliable, so don't try it unless you
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are desperate.
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Q: Does the lzma command line tool support sparse files?
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A: Sparse files can (of course) be compressed like normal files, but
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uncompression will not restore sparseness of the file. Use an archiver
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tool to take care of sparseness before compressing the data with lzma.
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The reason for this is that archiver tools handle files, while
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compression tools handle streams or buffers. Being a sparse file is
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a property of the file on the disk, not a property of the stream or
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buffer.
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Q: Can I recover parts of a broken LZMA file (e.g. corrupted CD-R)?
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A: With LZMA_Alone and single-block .lzma files, you can uncompress the
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file until you hit the first broken byte. The data after the broken
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position is lost. LZMA relies on the uncompression history, and if
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bytes are missing in the middle of the file, it is impossible to
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reliably continue after the broken section.
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With multi-block .lzma files it may be possible to locale the next
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block in the file and continue decoding there. A limited recovery
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tool for this kind of situations is planned.
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Q: Is LZMA patented?
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A: No, the authors are not aware of any patents that could affect LZMA.
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However, due to nature of software patents, the authors cannot
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guarantee, that LZMA isn't affected by any third party patent.
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Q: Where can I find documentation about how LZMA works as an algorithm?
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A: Read the source code, Luke. There is no documentation about LZMA
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internals. It is possible that Igor Pavlov is the only person on
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the Earth that completely knows and understands the algorithm.
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You could begin by downloading LZMA SDK, and start reading from
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the LZMA decoder to get some idea about the bitstream format.
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Before you begin, you should know the basics of LZ77 and
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range coding algorithms. LZMA is based on LZ77, but LZMA is
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*a lot* more complex. Range coding is used to compress the
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final bitstream like Huffman coding is used in Deflate.
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Q: What are filters?
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A: In context of .lzma files, a filter means an implementation of a
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compression algorithm. The primary filter is LZMA, which is why
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the names of the tools contain the letters LZMA.
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liblzma and the new .lzma format support also other filters than LZMA.
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There are different types of filters, which are suitable for different
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types of data. Thus, to select the optimal filter and settings, the
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type of the input data being compressed needs to be known.
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Some filters are most useful when combined with another filter like
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LZMA. These filters increase redundancy in the data, without changing
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the size of the data, by taking advantage of properties specific to
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the data being compressed.
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So far, all the filters are always reversible. That is, no matter what
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data you pass to a filter encoder, it can be always defiltered back to
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the original form. Because of this, it is safe to compress for example
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a software package that contains other file types than executables
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using a filter specific to the architechture of the package being
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compressed.
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The old LZMA_Alone format supports only the LZMA filter.
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Q: I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?
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A: BCJ filter is called "x86" in liblzma. BCJ2 is not included,
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because it requires using more than one encoded output stream.
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Q: Can I use LZMA in proprietary, non-free applications?
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A: liblzma is under the GNU LGPL version 2.1 or (at your opinion) any
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later version. To summarise (*NOTE* This summary is not legally
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binding, that is, it doesn't give you any extra permissions compared
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to the LGPL. Read the GNU LGPL carefully for the exact license
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conditions.):
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* All the changes made into the library itself must be published
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under the same license.
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* End users must be able to replace the used liblzma. Easiest way
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to assure this is to link dynamically against liblzma so users
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can replace the shared library file if they want.
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* You must make it clear to your users, that your application uses
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liblzma, and that liblzma is free software under the GNU LGPL.
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A copy of GNU LGPL must be included.
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LZMA SDK contains a special exception which allows linking *unmodified*
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code statically with a non-free application. This exception does *not*
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apply to liblzma.
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As an alternative, you can support the development of LZMA and 7-Zip
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by buying a proprietary license from Igor Pavlov. See homepage of
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LZMA SDK <http://7-zip.org/sdk.html> for more information. Note that
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having a proprietary license from Igor Pavlov doesn't allow you to use
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liblzma in a way that contradicts with the GNU LGPL, because liblzma
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contains code that is not copyrighted by Igor Pavlov. Please contact
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both Lasse Collin and Igor Pavlov if the license conditions of liblzma
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are not suitable for you.
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Q: I would like to help. What can I do?
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A: See the TODO file. Please contact Lasse Collin before starting to do
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anything, because it is possible that someone else is already working
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on the same thing.
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Q: How can I contact the authors?
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A: Lasse Collin is the maintainer of LZMA Utils. You can contact him
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either via IRC (Larhzu on #tukaani at Freenode or IRCnet). Email
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should work too, <lasse.collin@tukaani.org>.
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Igor Pavlov is the father of LZMA. He is the author of 7-Zip
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and LZMA SDK. <http://7-zip.org/>
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NOTE: Please don't bother Igor Pavlov with questions specific
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to LZMA Utils.
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