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format, which is being or has been replaced by the .xz format.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: 2011-04-12 11:55+0300
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 and only if Uncompressed Size is unknown.
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
http://7-zip.org/sdk.html
7-Zip
http://7-zip.org/
LZMA Utils - LZMA adapted to POSIX-like systems
http://tukaani.org/lzma/
XZ Utils - The next generation of LZMA Utils
http://tukaani.org/xz/
The .xz file format - The successor of the .lzma format
http://tukaani.org/xz/xz-file-format.txt
