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9595a3119b
Turns out that this is needed for .lzma files as the spec in LZMA SDK says that end marker may be present even if the size is stored in the header. Such files are rare but exist in the real world. The code in liblzma is so old that the spec didn't exist in LZMA SDK back then and I had understood that such files weren't possible (the lzma tool in LZMA SDK didn't create such files). This modifies the internal API so that LZMA decoder can be told if EOPM is allowed even when the uncompressed size is known. It's allowed with .lzma and not with other uses. Thanks to Karl Beldan for reporting the problem.
173 lines
5.9 KiB
Text
173 lines
5.9 KiB
Text
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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format, which is being or has been replaced by the .xz format.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: 2022-07-13 21:00+0300
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Compared to the previous version (2011-04-12 11:55+0300)
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the section 1.1.3 was modified to allow End of Payload Marker
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with a known Uncompressed Size.
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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 Uncompressed Size is unknown. End of Payload Marker
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is allowed but rarely used if Uncompressed Size is known.
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XZ Utils 5.2.5 and older don't support .lzma files that have
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End of Payload Marker together with a known Uncompressed Size.
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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 .lzma format
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http://tukaani.org/xz/xz-file-format.txt
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