1 2History of LZMA Utils and XZ Utils 3================================== 4 5Tukaani distribution 6 7 In 2005, there was a small group working on Tukaani distribution, which 8 was a Slackware fork. One of the project goals was to fit the distro on 9 a single 700 MiB ISO-9660 image. Using LZMA instead of gzip helped a 10 lot. Roughly speaking, one could fit data that took 1000 MiB in gzipped 11 form into 700 MiB with LZMA. Naturally compression ratio varied across 12 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 30First steps of LZMA Utils 31 32 The first version of LZMA Utils (4.22.0) included a shell script called 33 lzmash. It was wrapper that had 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 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 46Second 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 little modifications. This tool replaced 54 both the lzmash script and the LZMA_Alone command line tool in LZMA 55 Utils. 56 57 Introducing this new tool caused some temporary incompatibilities, 58 because LZMA_Alone executable was simply named lzma like the new 59 command line tool, but they had 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 API similar to zlib, 64 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 build system to use GNU Autotools. 71 This made is easier to test for certain less portable features needed 72 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 LZMA_OPT environment variable), the intent 76 was to not call 4.32.x stable. Similarly, liblzmadec wasn't polished, 77 but appeared to work well enough, so some people started using it too. 78 79 Because the development of the third generation of LZMA Utils was 80 delayed considerably (3-4 years), the 4.32.x branch had to be kept 81 maintained. It got some bug fixes now and then, and finally it was 82 decided to call it stable, although most of the missing features were 83 never added. 84 85 86File format problems 87 88 The file format used by LZMA_Alone was primitive. It was designed for 89 embedded systems in mind, and thus provided only minimal set of 90 features. The two biggest problems for non-embedded use were lack of 91 magic bytes and integrity check. 92 93 Igor and Lasse started developing a new file format with some help 94 from Ville Koskinen. Also Mark Adler, Mikko Pouru, H. Peter Anvin, 95 and Lars Wirzenius helped with some minor things at some point of the 96 development. Designing the new format took quite a long time (actually, 97 too long time would be more appropriate expression). It was mostly 98 because Lasse was quite slow at getting things done due to personal 99 reasons. 100 101 Originally the new format was supposed to use the same .lzma suffix 102 that was already used by the old file format. Switching to the new 103 format wouldn't have caused much trouble when the old format wasn't 104 used by many people. But since the development of the new format took 105 so long time, the old format got quite popular, and it was decided 106 that the new file format must use a different suffix. 107 108 It was decided to use .xz as the suffix of the new file format. The 109 first stable .xz file format specification was finally released in 110 December 2008. In addition to fixing the most obvious problems of 111 the old .lzma format, the .xz format added some new features like 112 support for multiple filters (compression algorithms), filter chaining 113 (like piping on the command line), and limited random-access reading. 114 115 Currently the primary compression algorithm used in .xz is LZMA2. 116 It is an extension on top of the original LZMA to fix some practical 117 problems: LZMA2 adds support for flushing the encoder, uncompressed 118 chunks, eases stateful decoder implementations, and improves support 119 for multithreading. Since LZMA2 is better than the original LZMA, the 120 original LZMA is not supported in .xz. 121 122 123Transition to XZ Utils 124 125 The early versions of XZ Utils were called LZMA Utils. The first 126 releases were 4.42.0alphas. They dropped the rest of the C++ LZMA SDK. 127 The code was still directly based on LZMA SDK but ported to C and 128 converted from callback API to stateful API. Later, Igor Pavlov made 129 C version of the LZMA encoder too; these ports from C++ to C were 130 independent in LZMA SDK and LZMA Utils. 131 132 The core of the new LZMA Utils was liblzma, a compression library with 133 zlib-like API. liblzma supported both the old and new file format. The 134 gzip-like lzma command line tool was rewritten to use liblzma. 135 136 The new LZMA Utils code base was renamed to XZ Utils when the name 137 of the new file format had been decided. The liblzma compression 138 library retained its name though, because changing it would have 139 caused unnecessary breakage in applications already using the early 140 liblzma snapshots. 141 142 The xz command line tool can emulate the gzip-like lzma tool by 143 creating appropriate symlinks (e.g. lzma -> xz). Thus, practically 144 all scripts using the lzma tool from LZMA Utils will work as is with 145 XZ Utils (and will keep using the old .lzma format). Still, the .lzma 146 format is more or less deprecated. XZ Utils will keep supporting it, 147 but new applications should use the .xz format, and migrating old 148 applications to .xz is often a good idea too. 149 150