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Added documentation about the legacy .lzma file format.
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Lasse Collin
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The .lzma File Format
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=====================
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0. Preface
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0.1. Notices and Acknowledgements
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0.2. Changes
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1. File Format
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1.1. Header
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1.1.1. Properties
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1.1.2. Dictionary Size
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1.1.3. Uncompressed Size
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1.2. LZMA Compressed Data
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2. References
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0. Preface
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This document describes the .lzma file format, which is
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sometimes also called LZMA_Alone format. It is a legacy file
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format, which is being or has been replaced by the .xz format.
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The MIME type of the .lzma format is `application/x-lzma'.
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The most commonly used software to handle .lzma files are
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LZMA SDK, LZMA Utils, 7-Zip, and XZ Utils. This document
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describes some of the differences between these implementations
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and gives hints what subset of the .lzma format is the most
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portable.
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0.1. Notices and Acknowledgements
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This file format was designed by Igor Pavlov for use in
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LZMA SDK. This document was written by Lasse Collin
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<lasse.collin@tukaani.org> using the documentation found
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from the LZMA SDK.
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This document has been put into the public domain.
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0.2. Changes
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Last modified: 2009-05-01 11:15+0300
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1. File Format
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+-+-+-+-+-+-+-+-+-+-+-+-+-+==========================+
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| Header | LZMA Compressed Data |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+==========================+
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The .lzma format file consist of 13-byte Header followed by
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the LZMA Compressed Data.
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Unlike the .gz, .bz2, and .xz formats, it is not possible to
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concatenate multiple .lzma files as is and expect the
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decompression tool to decode the resulting file as if it were
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a single .lzma file.
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For example, the command line tools from LZMA Utils and
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LZMA SDK silently ignore all the data after the first .lzma
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stream. In contrast, the command line tool from XZ Utils
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considers the .lzma file to be corrupt if there is data after
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the first .lzma stream.
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1.1. Header
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+------------+----+----+----+----+--+--+--+--+--+--+--+--+
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| Properties | Dictionary Size | Uncompressed Size |
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+------------+----+----+----+----+--+--+--+--+--+--+--+--+
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1.1.1. Properties
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The Properties field contains three properties. An abbreviation
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is given in parentheses, followed by the value range of the
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property. The field consists of
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1) the number of literal context bits (lc, [0, 8]);
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2) the number of literal position bits (lp, [0, 4]); and
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3) the number of position bits (pb, [0, 4]).
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The properties are encoded using the following formula:
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Properties = (pb * 5 + lp) * 9 + lc
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The following C code illustrates a straightforward way to
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decode the Properties field:
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uint8_t lc, lp, pb;
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uint8_t prop = get_lzma_properties();
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if (prop > (4 * 5 + 4) * 9 + 8)
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return LZMA_PROPERTIES_ERROR;
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pb = prop / (9 * 5);
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prop -= pb * 9 * 5;
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lp = prop / 9;
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lc = prop - lp * 9;
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XZ Utils has an additional requirement: lc + lp <= 4. Files
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which don't follow this requirement cannot be decompressed
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with XZ Utils. Usually this isn't a problem since the most
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common lc/lp/pb values are 3/0/2. It is the only lc/lp/pb
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combination that the files created by LZMA Utils can have,
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but LZMA Utils can decompress files with any lc/lp/pb.
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1.1.2. Dictionary Size
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Dictionary Size is stored as an unsigned 32-bit little endian
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integer. Any 32-bit value is possible, but for maximum
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portability, only sizes of 2^n and 2^n + 2^(n-1) should be
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used.
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LZMA Utils creates only files with dictionary size 2^n,
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16 <= n <= 25. LZMA Utils can decompress files with any
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dictionary size.
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XZ Utils creates and decompresses .lzma files only with
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dictionary sizes 2^n and 2^n + 2^(n-1). If some other
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dictionary size is specified when compressing, the value
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stored in the Dictionary Size field is a rounded up, but the
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specified value is still used in the actual compression code.
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1.1.3. Uncompressed Size
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Uncompressed Size is stored as unsigned 64-bit little endian
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integer. A special value of 0xFFFF_FFFF_FFFF_FFFF indicates
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that Uncompressed Size is unknown. End of Payload Marker (*)
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is used if and only if Uncompressed Size is unknown.
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XZ Utils rejects files whose Uncompressed Size field specifies
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a known size that is 256 GiB or more. This is to reject false
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positives when trying to guess if the input file is in the
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.lzma format. When Uncompressed Size is unknown, there is no
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limit for the uncompressed size of the file.
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(*) Some tools use the term End of Stream (EOS) marker
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instead of End of Payload Marker.
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1.2. LZMA Compressed Data
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Detailed description of the format of this field is out of
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scope of this document.
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2. References
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LZMA SDK - The original LZMA implementation
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http://7-zip.org/sdk.html
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7-Zip
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http://7-zip.org/
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LZMA Utils - LZMA adapted to POSIX-like systems
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http://tukaani.org/lzma/
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XZ Utils - The next generation of LZMA Utils
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http://tukaani.org/xz/
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The .xz file format - The successor of the the .lzma format
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http://tukaani.org/xz/xz-file-format.txt
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