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History of LZMA Utils and XZ Utils ================================== Tukaani distribution In 2005, there was a small group working on the Tukaani distribution, which was a Slackware fork. One of the project's goals was to fit the distro on a single 700 MiB ISO-9660 image. Using LZMA instead of gzip helped a lot. Roughly speaking, one could fit data that took 1000 MiB in gzipped form into 700 MiB with LZMA. Naturally, the compression ratio varied across packages, but this was what we got on average. Slackware packages have traditionally had .tgz as the filename suffix, which is an abbreviation of .tar.gz. A logical naming for LZMA compressed packages was .tlz, being an abbreviation of .tar.lzma. At the end of the year 2007, there was no distribution under the Tukaani project anymore, but development of LZMA Utils was kept going. Still, there were .tlz packages around, because at least Vector Linux (a Slackware based distribution) used LZMA for its packages. First versions of the modified pkgtools used the LZMA_Alone tool from Igor Pavlov's LZMA SDK as is. It was fine, because users wouldn't need to interact with LZMA_Alone directly. But people soon wanted to use LZMA for other files too, and the interface of LZMA_Alone wasn't comfortable for those used to gzip and bzip2. First steps of LZMA Utils The first version of LZMA Utils (4.22.0) included a shell script called lzmash. It was a wrapper that had a gzip-like command-line interface. It used the LZMA_Alone tool from LZMA SDK to do all the real work. zgrep, zdiff, and related scripts from gzip were adapted to work with LZMA and were part of the first LZMA Utils release too. LZMA Utils 4.22.0 included also lzmadec, which was a small (less than 10 KiB) decoder-only command-line tool. It was written on top of the decoder-only C code found from the LZMA SDK. lzmadec was convenient in situations where LZMA_Alone (a few hundred KiB) would be too big. lzmash and lzmadec were written by Lasse Collin. Second generation The lzmash script was an ugly and not very secure hack. The last version of LZMA Utils to use lzmash was 4.27.1. LZMA Utils 4.32.0beta1 introduced a new lzma command-line tool written by Ville Koskinen. It was written in C++, and used the encoder and decoder from C++ LZMA SDK with some little modifications. This tool replaced both the lzmash script and the LZMA_Alone command-line tool in LZMA Utils. Introducing this new tool caused some temporary incompatibilities, because the LZMA_Alone executable was simply named lzma like the new command-line tool, but they had a completely different command-line interface. The file format was still the same. Lasse wrote liblzmadec, which was a small decoder-only library based on the C code found from LZMA SDK. liblzmadec had an API similar to zlib, although there were some significant differences, which made it non-trivial to use it in some applications designed for zlib and libbzip2. The lzmadec command-line tool was converted to use liblzmadec. Alexandre Sauvé helped converting the build system to use GNU Autotools. This made it easier to test for certain less portable features needed by the new command-line tool. Since the new command-line tool never got completely finished (for example, it didn't support the LZMA_OPT environment variable), the intent was to not call 4.32.x stable. Similarly, liblzmadec wasn't polished, but appeared to work well enough, so some people started using it too. Because the development of the third generation of LZMA Utils was delayed considerably (3-4 years), the 4.32.x branch had to be kept maintained. It got some bug fixes now and then, and finally it was decided to call it stable, although most of the missing features were never added. File format problems The file format used by LZMA_Alone was primitive. It was designed with embedded systems in mind, and thus provided only a minimal set of features. The two biggest problems for non-embedded use were the lack of magic bytes and an integrity check. Igor and Lasse started developing a new file format with some help from Ville Koskinen. Also Mark Adler, Mikko Pouru, H. Peter Anvin, and Lars Wirzenius helped with some minor things at some point of the development. Designing the new format took quite a long time (actually, too long a time would be a more appropriate expression). It was mostly because Lasse was quite slow at getting things done due to personal reasons. Originally the new format was supposed to use the same .lzma suffix that was already used by the old file format. Switching to the new format wouldn't have caused much trouble when the old format wasn't used by many people. But since the development of the new format took such a long time, the old format got quite popular, and it was decided that the new file format must use a different suffix. It was decided to use .xz as the suffix of the new file format. The first stable .xz file format specification was finally released in December 2008. In addition to fixing the most obvious problems of the old .lzma format, the .xz format added some new features like support for multiple filters (compression algorithms), filter chaining (like piping on the command line), and limited random-access reading. Currently the primary compression algorithm used in .xz is LZMA2. It is an extension on top of the original LZMA to fix some practical problems: LZMA2 adds support for flushing the encoder, uncompressed chunks, eases stateful decoder implementations, and improves support for multithreading. Since LZMA2 is better than the original LZMA, the original LZMA is not supported in .xz. Transition to XZ Utils The early versions of XZ Utils were called LZMA Utils. The first releases were 4.42.0alphas. They dropped the rest of the C++ LZMA SDK. The code was still directly based on LZMA SDK but ported to C and converted from a callback API to a stateful API. Later, Igor Pavlov made a C version of the LZMA encoder too; these ports from C++ to C were independent in LZMA SDK and LZMA Utils. The core of the new LZMA Utils was liblzma, a compression library with a zlib-like API. liblzma supported both the old and new file format. The gzip-like lzma command-line tool was rewritten to use liblzma. The new LZMA Utils code base was renamed to XZ Utils when the name of the new file format had been decided. The liblzma compression library retained its name though, because changing it would have caused unnecessary breakage in applications already using the early liblzma snapshots. The xz command-line tool can emulate the gzip-like lzma tool by creating appropriate symlinks (e.g. lzma -> xz). Thus, practically all scripts using the lzma tool from LZMA Utils will work as is with XZ Utils (and will keep using the old .lzma format). Still, the .lzma format is more or less deprecated. XZ Utils will keep supporting it, but new applications should use the .xz format, and migrating old applications to .xz is often a good idea too.