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date Tue, 18 Mar 2025 16:23:26 -0400
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1
2 History of LZMA Utils and XZ Utils
3 ==================================
4
5 Tukaani distribution
6
7 In 2005, there was a small group working on the Tukaani distribution,
8 which was a Slackware fork. One of the project's goals was to fit the
9 distro on a single 700 MiB ISO-9660 image. Using LZMA instead of gzip
10 helped a lot. Roughly speaking, one could fit data that took 1000 MiB
11 in gzipped form into 700 MiB with LZMA. Naturally, the compression
12 ratio varied across packages, but this was what we got on average.
13
14 Slackware packages have traditionally had .tgz as the filename suffix,
15 which is an abbreviation of .tar.gz. A logical naming for LZMA
16 compressed packages was .tlz, being an abbreviation of .tar.lzma.
17
18 At the end of the year 2007, there was no distribution under the
19 Tukaani project anymore, but development of LZMA Utils was kept going.
20 Still, there were .tlz packages around, because at least Vector Linux
21 (a Slackware based distribution) used LZMA for its packages.
22
23 First versions of the modified pkgtools used the LZMA_Alone tool from
24 Igor Pavlov's LZMA SDK as is. It was fine, because users wouldn't need
25 to interact with LZMA_Alone directly. But people soon wanted to use
26 LZMA for other files too, and the interface of LZMA_Alone wasn't
27 comfortable for those used to gzip and bzip2.
28
29
30 First steps of LZMA Utils
31
32 The first version of LZMA Utils (4.22.0) included a shell script called
33 lzmash. It was a wrapper that had a gzip-like command-line interface. It
34 used the LZMA_Alone tool from LZMA SDK to do all the real work. zgrep,
35 zdiff, and related scripts from gzip were adapted to work with LZMA and
36 were part of the first LZMA Utils release too.
37
38 LZMA Utils 4.22.0 included also lzmadec, which was a small (less than
39 10 KiB) decoder-only command-line tool. It was written on top of the
40 decoder-only C code found from the LZMA SDK. lzmadec was convenient in
41 situations where LZMA_Alone (a few hundred KiB) would be too big.
42
43 lzmash and lzmadec were written by Lasse Collin.
44
45
46 Second generation
47
48 The lzmash script was an ugly and not very secure hack. The last
49 version of LZMA Utils to use lzmash was 4.27.1.
50
51 LZMA Utils 4.32.0beta1 introduced a new lzma command-line tool written
52 by Ville Koskinen. It was written in C++, and used the encoder and
53 decoder from C++ LZMA SDK with some little modifications. This tool
54 replaced both the lzmash script and the LZMA_Alone command-line tool
55 in LZMA Utils.
56
57 Introducing this new tool caused some temporary incompatibilities,
58 because the LZMA_Alone executable was simply named lzma like the new
59 command-line tool, but they had a completely different command-line
60 interface. The file format was still the same.
61
62 Lasse wrote liblzmadec, which was a small decoder-only library based
63 on the C code found from LZMA SDK. liblzmadec had an API similar to
64 zlib, although there were some significant differences, which made it
65 non-trivial to use it in some applications designed for zlib and
66 libbzip2.
67
68 The lzmadec command-line tool was converted to use liblzmadec.
69
70 Alexandre Sauvé helped converting the build system to use GNU
71 Autotools. This made it easier to test for certain less portable
72 features needed by the new command-line tool.
73
74 Since the new command-line tool never got completely finished (for
75 example, it didn't support the LZMA_OPT environment variable), the
76 intent was to not call 4.32.x stable. Similarly, liblzmadec wasn't
77 polished, but appeared to work well enough, so some people started
78 using it too.
79
80 Because the development of the third generation of LZMA Utils was
81 delayed considerably (3-4 years), the 4.32.x branch had to be kept
82 maintained. It got some bug fixes now and then, and finally it was
83 decided to call it stable, although most of the missing features were
84 never added.
85
86
87 File format problems
88
89 The file format used by LZMA_Alone was primitive. It was designed with
90 embedded systems in mind, and thus provided only a minimal set of
91 features. The two biggest problems for non-embedded use were the lack
92 of magic bytes and an integrity check.
93
94 Igor and Lasse started developing a new file format with some help
95 from Ville Koskinen. Also Mark Adler, Mikko Pouru, H. Peter Anvin,
96 and Lars Wirzenius helped with some minor things at some point of the
97 development. Designing the new format took quite a long time (actually,
98 too long a time would be a more appropriate expression). It was mostly
99 because Lasse was quite slow at getting things done due to personal
100 reasons.
101
102 Originally the new format was supposed to use the same .lzma suffix
103 that was already used by the old file format. Switching to the new
104 format wouldn't have caused much trouble when the old format wasn't
105 used by many people. But since the development of the new format took
106 such a long time, the old format got quite popular, and it was decided
107 that the new file format must use a different suffix.
108
109 It was decided to use .xz as the suffix of the new file format. The
110 first stable .xz file format specification was finally released in
111 December 2008. In addition to fixing the most obvious problems of
112 the old .lzma format, the .xz format added some new features like
113 support for multiple filters (compression algorithms), filter chaining
114 (like piping on the command line), and limited random-access reading.
115
116 Currently the primary compression algorithm used in .xz is LZMA2.
117 It is an extension on top of the original LZMA to fix some practical
118 problems: LZMA2 adds support for flushing the encoder, uncompressed
119 chunks, eases stateful decoder implementations, and improves support
120 for multithreading. Since LZMA2 is better than the original LZMA, the
121 original LZMA is not supported in .xz.
122
123
124 Transition to XZ Utils
125
126 The early versions of XZ Utils were called LZMA Utils. The first
127 releases were 4.42.0alphas. They dropped the rest of the C++ LZMA SDK.
128 The code was still directly based on LZMA SDK but ported to C and
129 converted from a callback API to a stateful API. Later, Igor Pavlov
130 made a C version of the LZMA encoder too; these ports from C++ to C
131 were independent in LZMA SDK and LZMA Utils.
132
133 The core of the new LZMA Utils was liblzma, a compression library with
134 a zlib-like API. liblzma supported both the old and new file format.
135 The gzip-like lzma command-line tool was rewritten to use liblzma.
136
137 The new LZMA Utils code base was renamed to XZ Utils when the name
138 of the new file format had been decided. The liblzma compression
139 library retained its name though, because changing it would have
140 caused unnecessary breakage in applications already using the early
141 liblzma snapshots.
142
143 The xz command-line tool can emulate the gzip-like lzma tool by
144 creating appropriate symlinks (e.g. lzma -> xz). Thus, practically
145 all scripts using the lzma tool from LZMA Utils will work as is with
146 XZ Utils (and will keep using the old .lzma format). Still, the .lzma
147 format is more or less deprecated. XZ Utils will keep supporting it,
148 but new applications should use the .xz format, and migrating old
149 applications to .xz is often a good idea too.
150