README revision 207753
1 2XZ Utils 3======== 4 5 0. Overview 6 1. Documentation 7 1.1. Overall documentation 8 1.2. Documentation for command line tools 9 1.3. Documentation for liblzma 10 2. Version numbering 11 3. Reporting bugs 12 4. Other implementations of the .xz format 13 5. Contact information 14 15 160. Overview 17----------- 18 19 XZ Utils provide a general-purpose data compression library and 20 command line tools. The native file format is the .xz format, but 21 also the legacy .lzma format is supported. The .xz format supports 22 multiple compression algorithms, which are called "filters" in 23 context of XZ Utils. The primary filter is currently LZMA2. With 24 typical files, XZ Utils create about 30 % smaller files than gzip. 25 26 To ease adapting support for the .xz format into existing applications 27 and scripts, the API of liblzma is somewhat similar to the API of the 28 popular zlib library. For the same reason, the command line tool xz 29 has similar command line syntax than that of gzip. 30 31 When aiming for the highest compression ratio, LZMA2 encoder uses 32 a lot of CPU time and may use, depending on the settings, even 33 hundreds of megabytes of RAM. However, in fast modes, LZMA2 encoder 34 competes with bzip2 in compression speed, RAM usage, and compression 35 ratio. 36 37 LZMA2 is reasonably fast to decompress. It is a little slower than 38 gzip, but a lot faster than bzip2. Being fast to decompress means 39 that the .xz format is especially nice when the same file will be 40 decompressed very many times (usually on different computers), which 41 is the case e.g. when distributing software packages. In such 42 situations, it's not too bad if the compression takes some time, 43 since that needs to be done only once to benefit many people. 44 45 With some file types, combining (or "chaining") LZMA2 with an 46 additional filter can improve compression ratio. A filter chain may 47 contain up to four filters, although usually only one two is used. 48 For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2 49 in the filter chain can improve compression ratio of executable files. 50 51 Since the .xz format allows adding new filter IDs, it is possible that 52 some day there will be a filter that is, for example, much faster to 53 compress than LZMA2 (but probably with worse compression ratio). 54 Similarly, it is possible that some day there is a filter that will 55 compress better than LZMA2. 56 57 XZ Utils doesn't support multithreaded compression or decompression 58 yet. It has been planned though and taken into account when designing 59 the .xz file format. 60 61 621. Documentation 63---------------- 64 651.1. Overall documentation 66 67 README This file 68 69 INSTALL.generic Generic install instructions for those not familiar 70 with packages using GNU Autotools 71 INSTALL Installation instructions specific to XZ Utils 72 PACKAGERS Information to packagers of XZ Utils 73 74 COPYING XZ Utils copyright and license information 75 COPYING.GPLv2 GNU General Public License version 2 76 COPYING.GPLv3 GNU General Public License version 3 77 COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1 78 79 AUTHORS The main authors of XZ Utils 80 THANKS Incomplete list of people who have helped making 81 this software 82 NEWS User-visible changes between XZ Utils releases 83 ChangeLog Detailed list of changes (commit log) 84 TODO Known bugs and some sort of to-do list 85 86 Note that only some of the above files are included in binary 87 packages. 88 89 901.2. Documentation for command line tools 91 92 The command line tools are documented as man pages. In source code 93 releases (and possibly also in some binary packages), the man pages 94 are also provided in plain text (ASCII only) and PDF formats in the 95 directory "doc/man" to make the man pages more accessible to those 96 whose operating system doesn't provide an easy way to view man pages. 97 98 991.3. Documentation for liblzma 100 101 The liblzma API headers include short docs about each function 102 and data type as Doxygen tags. These docs should be quite OK as 103 a quick reference. 104 105 I have planned to write a bunch of very well documented example 106 programs, which (due to comments) should work as a tutorial to 107 various features of liblzma. No such example programs have been 108 written yet. 109 110 For now, if you have never used liblzma, libbzip2, or zlib, I 111 recommend learning *basics* of zlib API. Once you know that, it 112 should be easier to learn liblzma. 113 114 http://zlib.net/manual.html 115 http://zlib.net/zlib_how.html 116 117 1182. Version numbering 119-------------------- 120 121 The version number format of XZ Utils is X.Y.ZS: 122 123 - X is the major version. When this is incremented, the library 124 API and ABI break. 125 126 - Y is the minor version. It is incremented when new features are 127 added without breaking existing API or ABI. Even Y indicates 128 stable release and odd Y indicates unstable (alpha or beta 129 version). 130 131 - Z is the revision. This has different meaning for stable and 132 unstable releases: 133 * Stable: Z is incremented when bugs get fixed without adding 134 any new features. 135 * Unstable: Z is just a counter. API or ABI of features added 136 in earlier unstable releases having the same X.Y may break. 137 138 - S indicates stability of the release. It is missing from the 139 stable releases where Y is an even number. When Y is odd, S 140 is either "alpha" or "beta" to make it very clear that such 141 versions are not stable releases. The same X.Y.Z combination is 142 not used for more than one stability level i.e. after X.Y.Zalpha, 143 the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta. 144 145 1463. Reporting bugs 147----------------- 148 149 Naturally it is easiest for me if you already know what causes the 150 unexpected behavior. Even better if you have a patch to propose. 151 However, quite often the reason for unexpected behavior is unknown, 152 so here are a few things to do before sending a bug report: 153 154 1. Try to create a small example how to reproduce the issue. 155 156 2. Compile XZ Utils with debugging code using configure switches 157 --enable-debug and, if possible, --disable-shared. If you are 158 using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting 159 binaries. 160 161 3. Turn on core dumps. The exact command depends on your shell; 162 for example in GNU bash it is done with "ulimit -c unlimited", 163 and in tcsh with "limit coredumpsize unlimited". 164 165 4. Try to reproduce the suspected bug. If you get "assertion failed" 166 message, be sure to include the complete message in your bug 167 report. If the application leaves a coredump, get a backtrace 168 using gdb: 169 $ gdb /path/to/app-binary # Load the app to the debugger. 170 (gdb) core core # Open the coredump. 171 (gdb) bt # Print the backtrace. Copy & paste to bug report. 172 (gdb) quit # Quit gdb. 173 174 Report your bug via email or IRC (see Contact information below). 175 Don't send core dump files or any executables. If you have a small 176 example file(s) (total size less than 256 KiB), please include 177 it/them as an attachment. If you have bigger test files, put them 178 online somewhere and include an URL to the file(s) in the bug report. 179 180 Always include the exact version number of XZ Utils in the bug report. 181 If you are using a snapshot from the git repository, use "git describe" 182 to get the exact snapshot version. If you are using XZ Utils shipped 183 in an operating system distribution, mention the distribution name, 184 distribution version, and exact xz package version; if you cannot 185 repeat the bug with the code compiled from unpatched source code, 186 you probably need to report a bug to your distribution's bug tracking 187 system. 188 189 1904. Other implementations of the .xz format 191------------------------------------------ 192 193 7-Zip and the p7zip port of 7-Zip support the .xz format starting 194 from the version 9.00alpha. 195 196 http://7-zip.org/ 197 http://p7zip.sourceforge.net/ 198 199 XZ Embedded is a limited implementation written for use in the Linux 200 kernel, but it is also suitable for other embedded use. 201 202 http://tukaani.org/xz/embedded.html 203 204 2055. Contact information 206---------------------- 207 208 If you have questions, bug reports, patches etc. related to XZ Utils, 209 contact Lasse Collin <lasse.collin@tukaani.org> (in Finnish or English). 210 tukaani.org uses greylisting to reduce spam, thus when you send your 211 first email, it may get delayed by a few hours. In addition to that, 212 I'm sometimes slow at replying. If you haven't got a reply within two 213 weeks, assume that your email has got lost and resend it or use IRC. 214 215 You can find me also from #tukaani on Freenode; my nick is Larhzu. 216 The channel tends to be pretty quiet, so just ask your question and 217 someone may wake up. 218 219