| The .lzma File Format | |
| ===================== | |
| 0. Preface | |
| 0.1. Notices and Acknowledgements | |
| 0.2. Changes | |
| 1. File Format | |
| 1.1. Header | |
| 1.1.1. Properties | |
| 1.1.2. Dictionary Size | |
| 1.1.3. Uncompressed Size | |
| 1.2. LZMA Compressed Data | |
| 2. References | |
| 0. Preface | |
| This document describes the .lzma file format, which is | |
| sometimes also called LZMA_Alone format. It is a legacy file | |
| format, which is being or has been replaced by the .xz format. | |
| The MIME type of the .lzma format is `application/x-lzma'. | |
| The most commonly used software to handle .lzma files are | |
| LZMA SDK, LZMA Utils, 7-Zip, and XZ Utils. This document | |
| describes some of the differences between these implementations | |
| and gives hints what subset of the .lzma format is the most | |
| portable. | |
| 0.1. Notices and Acknowledgements | |
| This file format was designed by Igor Pavlov for use in | |
| LZMA SDK. This document was written by Lasse Collin | |
| <lasse.collin@tukaani.org> using the documentation found | |
| from the LZMA SDK. | |
| This document has been put into the public domain. | |
| 0.2. Changes | |
| Last modified: 2024-01-16 18:00+0800 | |
| Compared to the previous version (2022-07-13 21:00+0300) | |
| the section 2 was modified to change links from http to | |
| https and to update XZ links. | |
| 1. File Format | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+==========================+ | |
| | Header | LZMA Compressed Data | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+==========================+ | |
| The .lzma format file consist of 13-byte Header followed by | |
| the LZMA Compressed Data. | |
| Unlike the .gz, .bz2, and .xz formats, it is not possible to | |
| concatenate multiple .lzma files as is and expect the | |
| decompression tool to decode the resulting file as if it were | |
| a single .lzma file. | |
| For example, the command line tools from LZMA Utils and | |
| LZMA SDK silently ignore all the data after the first .lzma | |
| stream. In contrast, the command line tool from XZ Utils | |
| considers the .lzma file to be corrupt if there is data after | |
| the first .lzma stream. | |
| 1.1. Header | |
| +------------+----+----+----+----+--+--+--+--+--+--+--+--+ | |
| | Properties | Dictionary Size | Uncompressed Size | | |
| +------------+----+----+----+----+--+--+--+--+--+--+--+--+ | |
| 1.1.1. Properties | |
| The Properties field contains three properties. An abbreviation | |
| is given in parentheses, followed by the value range of the | |
| property. The field consists of | |
| 1) the number of literal context bits (lc, [0, 8]); | |
| 2) the number of literal position bits (lp, [0, 4]); and | |
| 3) the number of position bits (pb, [0, 4]). | |
| The properties are encoded using the following formula: | |
| Properties = (pb * 5 + lp) * 9 + lc | |
| The following C code illustrates a straightforward way to | |
| decode the Properties field: | |
| uint8_t lc, lp, pb; | |
| uint8_t prop = get_lzma_properties(); | |
| if (prop > (4 * 5 + 4) * 9 + 8) | |
| return LZMA_PROPERTIES_ERROR; | |
| pb = prop / (9 * 5); | |
| prop -= pb * 9 * 5; | |
| lp = prop / 9; | |
| lc = prop - lp * 9; | |
| XZ Utils has an additional requirement: lc + lp <= 4. Files | |
| which don't follow this requirement cannot be decompressed | |
| with XZ Utils. Usually this isn't a problem since the most | |
| common lc/lp/pb values are 3/0/2. It is the only lc/lp/pb | |
| combination that the files created by LZMA Utils can have, | |
| but LZMA Utils can decompress files with any lc/lp/pb. | |
| 1.1.2. Dictionary Size | |
| Dictionary Size is stored as an unsigned 32-bit little endian | |
| integer. Any 32-bit value is possible, but for maximum | |
| portability, only sizes of 2^n and 2^n + 2^(n-1) should be | |
| used. | |
| LZMA Utils creates only files with dictionary size 2^n, | |
| 16 <= n <= 25. LZMA Utils can decompress files with any | |
| dictionary size. | |
| XZ Utils creates and decompresses .lzma files only with | |
| dictionary sizes 2^n and 2^n + 2^(n-1). If some other | |
| dictionary size is specified when compressing, the value | |
| stored in the Dictionary Size field is a rounded up, but the | |
| specified value is still used in the actual compression code. | |
| 1.1.3. Uncompressed Size | |
| Uncompressed Size is stored as unsigned 64-bit little endian | |
| integer. A special value of 0xFFFF_FFFF_FFFF_FFFF indicates | |
| that Uncompressed Size is unknown. End of Payload Marker (*) | |
| is used if Uncompressed Size is unknown. End of Payload Marker | |
| is allowed but rarely used if Uncompressed Size is known. | |
| XZ Utils 5.2.5 and older don't support .lzma files that have | |
| End of Payload Marker together with a known Uncompressed Size. | |
| XZ Utils rejects files whose Uncompressed Size field specifies | |
| a known size that is 256 GiB or more. This is to reject false | |
| positives when trying to guess if the input file is in the | |
| .lzma format. When Uncompressed Size is unknown, there is no | |
| limit for the uncompressed size of the file. | |
| (*) Some tools use the term End of Stream (EOS) marker | |
| instead of End of Payload Marker. | |
| 1.2. LZMA Compressed Data | |
| Detailed description of the format of this field is out of | |
| scope of this document. | |
| 2. References | |
| LZMA SDK - The original LZMA implementation | |
| https://7-zip.org/sdk.html | |
| 7-Zip | |
| https://7-zip.org/ | |
| LZMA Utils - LZMA adapted to POSIX-like systems | |
| https://tukaani.org/lzma/ | |
| XZ Utils - The next generation of LZMA Utils | |
| https://xz.tukaani.org/xz-utils/ | |
| The .xz file format - The successor of the .lzma format | |
| https://xz.tukaani.org/format/xz-file-format.txt | |