xz-archive/doc/faq.txt

XZ Utils FAQ
============

Q:  What are LZMA, LZMA Utils, lzma, .lzma, liblzma, LZMA SDK, LZMA_Alone,
    7-Zip and p7zip?

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/>


Q:  What LZMA implementations there are available?

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 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>.


Q:  Which file formats are supported by LZMA Utils?

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.

    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).

    .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.

    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.


Q:  How can I identify files containing LZMA compressed data?

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.


Q:  Does the lzma command line tool support sparse files?

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.


Q:  Can I recover parts of a broken LZMA file (e.g. corrupted CD-R)?

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.

    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.


Q:  Is LZMA patented?

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.


Q:  Where can I find documentation about how LZMA works as an algorithm?

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.


Q:  What are filters?

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.

    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.


Q:  I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?

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?

A:  Yes. See the file COPYING for details.


Q:  I would like to help. What can I do?

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.


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.