| XZ Utils | |
| ======== | |
| 0. Overview | |
| 1. Documentation | |
| 1.1. Overall documentation | |
| 1.2. Documentation for command-line tools | |
| 1.3. Documentation for liblzma | |
| 2. Version numbering | |
| 3. Reporting bugs | |
| 4. Translating the xz tool | |
| 5. Other implementations of the .xz format | |
| 6. Contact information | |
| 0. Overview | |
| ----------- | |
| XZ Utils provide a general-purpose data-compression library plus | |
| command-line tools. The native file format is the .xz format, but | |
| also the legacy .lzma format is supported. The .xz format supports | |
| multiple compression algorithms, which are called "filters" in the | |
| context of XZ Utils. The primary filter is currently LZMA2. With | |
| typical files, XZ Utils create about 30 % smaller files than gzip. | |
| To ease adapting support for the .xz format into existing applications | |
| and scripts, the API of liblzma is somewhat similar to the API of the | |
| popular zlib library. For the same reason, the command-line tool xz | |
| has a command-line syntax similar to that of gzip. | |
| When aiming for the highest compression ratio, the LZMA2 encoder uses | |
| a lot of CPU time and may use, depending on the settings, even | |
| hundreds of megabytes of RAM. However, in fast modes, the LZMA2 encoder | |
| competes with bzip2 in compression speed, RAM usage, and compression | |
| ratio. | |
| LZMA2 is reasonably fast to decompress. It is a little slower than | |
| gzip, but a lot faster than bzip2. Being fast to decompress means | |
| that the .xz format is especially nice when the same file will be | |
| decompressed very many times (usually on different computers), which | |
| is the case e.g. when distributing software packages. In such | |
| situations, it's not too bad if the compression takes some time, | |
| since that needs to be done only once to benefit many people. | |
| With some file types, combining (or "chaining") LZMA2 with an | |
| additional filter can improve the compression ratio. A filter chain may | |
| contain up to four filters, although usually only one or two are used. | |
| For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2 | |
| in the filter chain can improve compression ratio of executable files. | |
| Since the .xz format allows adding new filter IDs, it is possible that | |
| some day there will be a filter that is, for example, much faster to | |
| compress than LZMA2 (but probably with worse compression ratio). | |
| Similarly, it is possible that some day there is a filter that will | |
| compress better than LZMA2. | |
| XZ Utils doesn't support multithreaded compression or decompression | |
| yet. It has been planned though and taken into account when designing | |
| the .xz file format. | |
| 1. Documentation | |
| ---------------- | |
| 1.1. Overall documentation | |
| README This file | |
| INSTALL.generic Generic install instructions for those not familiar | |
| with packages using GNU Autotools | |
| INSTALL Installation instructions specific to XZ Utils | |
| PACKAGERS Information to packagers of XZ Utils | |
| COPYING XZ Utils copyright and license information | |
| COPYING.GPLv2 GNU General Public License version 2 | |
| COPYING.GPLv3 GNU General Public License version 3 | |
| COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1 | |
| AUTHORS The main authors of XZ Utils | |
| THANKS Incomplete list of people who have helped making | |
| this software | |
| NEWS User-visible changes between XZ Utils releases | |
| ChangeLog Detailed list of changes (commit log) | |
| TODO Known bugs and some sort of to-do list | |
| Note that only some of the above files are included in binary | |
| packages. | |
| 1.2. Documentation for command-line tools | |
| The command-line tools are documented as man pages. In source code | |
| releases (and possibly also in some binary packages), the man pages | |
| are also provided in plain text (ASCII only) and PDF formats in the | |
| directory "doc/man" to make the man pages more accessible to those | |
| whose operating system doesn't provide an easy way to view man pages. | |
| 1.3. Documentation for liblzma | |
| The liblzma API headers include short docs about each function | |
| and data type as Doxygen tags. These docs should be quite OK as | |
| a quick reference. | |
| I have planned to write a bunch of very well documented example | |
| programs, which (due to comments) should work as a tutorial to | |
| various features of liblzma. No such example programs have been | |
| written yet. | |
| For now, if you have never used liblzma, libbzip2, or zlib, I | |
| recommend learning the *basics* of the zlib API. Once you know that, | |
| it should be easier to learn liblzma. | |
| http://zlib.net/manual.html | |
| http://zlib.net/zlib_how.html | |
| 2. Version numbering | |
| -------------------- | |
| The version number format of XZ Utils is X.Y.ZS: | |
| - X is the major version. When this is incremented, the library | |
| API and ABI break. | |
| - Y is the minor version. It is incremented when new features | |
| are added without breaking the existing API or ABI. An even Y | |
| indicates a stable release and an odd Y indicates unstable | |
| (alpha or beta version). | |
| - Z is the revision. This has a different meaning for stable and | |
| unstable releases: | |
| * Stable: Z is incremented when bugs get fixed without adding | |
| any new features. This is intended to be convenient for | |
| downstream distributors that want bug fixes but don't want | |
| any new features to minimize the risk of introducing new bugs. | |
| * Unstable: Z is just a counter. API or ABI of features added | |
| in earlier unstable releases having the same X.Y may break. | |
| - S indicates stability of the release. It is missing from the | |
| stable releases, where Y is an even number. When Y is odd, S | |
| is either "alpha" or "beta" to make it very clear that such | |
| versions are not stable releases. The same X.Y.Z combination is | |
| not used for more than one stability level, i.e. after X.Y.Zalpha, | |
| the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta. | |
| 3. Reporting bugs | |
| ----------------- | |
| Naturally it is easiest for me if you already know what causes the | |
| unexpected behavior. Even better if you have a patch to propose. | |
| However, quite often the reason for unexpected behavior is unknown, | |
| so here are a few things to do before sending a bug report: | |
| 1. Try to create a small example how to reproduce the issue. | |
| 2. Compile XZ Utils with debugging code using configure switches | |
| --enable-debug and, if possible, --disable-shared. If you are | |
| using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting | |
| binaries. | |
| 3. Turn on core dumps. The exact command depends on your shell; | |
| for example in GNU bash it is done with "ulimit -c unlimited", | |
| and in tcsh with "limit coredumpsize unlimited". | |
| 4. Try to reproduce the suspected bug. If you get "assertion failed" | |
| message, be sure to include the complete message in your bug | |
| report. If the application leaves a coredump, get a backtrace | |
| using gdb: | |
| $ gdb /path/to/app-binary # Load the app to the debugger. | |
| (gdb) core core # Open the coredump. | |
| (gdb) bt # Print the backtrace. Copy & paste to bug report. | |
| (gdb) quit # Quit gdb. | |
| Report your bug via email or IRC (see Contact information below). | |
| Don't send core dump files or any executables. If you have a small | |
| example file(s) (total size less than 256 KiB), please include | |
| it/them as an attachment. If you have bigger test files, put them | |
| online somewhere and include a URL to the file(s) in the bug report. | |
| Always include the exact version number of XZ Utils in the bug report. | |
| If you are using a snapshot from the git repository, use "git describe" | |
| to get the exact snapshot version. If you are using XZ Utils shipped | |
| in an operating system distribution, mention the distribution name, | |
| distribution version, and exact xz package version; if you cannot | |
| repeat the bug with the code compiled from unpatched source code, | |
| you probably need to report a bug to your distribution's bug tracking | |
| system. | |
| 4. Translating the xz tool | |
| -------------------------- | |
| The messages from the xz tool have been translated into a few | |
| languages. Before starting to translate into a new language, ask | |
| the author whether someone else hasn't already started working on it. | |
| Test your translation. Testing includes comparing the translated | |
| output to the original English version by running the same commands | |
| in both your target locale and with LC_ALL=C. Ask someone to | |
| proof-read and test the translation. | |
| Testing can be done e.g. by installing xz into a temporary directory: | |
| ./configure --disable-shared --prefix=/tmp/xz-test | |
| # <Edit the .po file in the po directory.> | |
| make -C po update-po | |
| make install | |
| bash debug/translations.bash | less | |
| bash debug/translations.bash | less -S # For --list outputs | |
| Repeat the above as needed (no need to re-run configure though). | |
| Note especially the following: | |
| - The output of --help and --long-help must look nice on | |
| an 80-column terminal. It's OK to add extra lines if needed. | |
| - In contrast, don't add extra lines to error messages and such. | |
| They are often preceded with e.g. a filename on the same line, | |
| so you have no way to predict where to put a \n. Let the terminal | |
| do the wrapping even if it looks ugly. Adding new lines will be | |
| even uglier in the generic case even if it looks nice in a few | |
| limited examples. | |
| - Be careful with column alignment in tables and table-like output | |
| (--list, --list --verbose --verbose, --info-memory, --help, and | |
| --long-help): | |
| * All descriptions of options in --help should start in the | |
| same column (but it doesn't need to be the same column as | |
| in the English messages; just be consistent if you change it). | |
| Check that both --help and --long-help look OK, since they | |
| share several strings. | |
| * --list --verbose and --info-memory print lines that have | |
| the format "Description: %s". If you need a longer | |
| description, you can put extra space between the colon | |
| and %s. Then you may need to add extra space to other | |
| strings too so that the result as a whole looks good (all | |
| values start at the same column). | |
| * The columns of the actual tables in --list --verbose --verbose | |
| should be aligned properly. Abbreviate if necessary. It might | |
| be good to keep at least 2 or 3 spaces between column headings | |
| and avoid spaces in the headings so that the columns stand out | |
| better, but this is a matter of opinion. Do what you think | |
| looks best. | |
| - Be careful to put a period at the end of a sentence when the | |
| original version has it, and don't put it when the original | |
| doesn't have it. Similarly, be careful with \n characters | |
| at the beginning and end of the strings. | |
| - Read the TRANSLATORS comments that have been extracted from the | |
| source code and included in xz.pot. If they suggest testing the | |
| translation with some type of command, do it. If testing needs | |
| input files, use e.g. tests/files/good-*.xz. | |
| - When updating the translation, read the fuzzy (modified) strings | |
| carefully, and don't mark them as updated before you actually | |
| have updated them. Reading through the unchanged messages can be | |
| good too; sometimes you may find a better wording for them. | |
| - If you find language problems in the original English strings, | |
| feel free to suggest improvements. Ask if something is unclear. | |
| - The translated messages should be understandable (sometimes this | |
| may be a problem with the original English messages too). Don't | |
| make a direct word-by-word translation from English especially if | |
| the result doesn't sound good in your language. | |
| In short, take your time and pay attention to the details. Making | |
| a good translation is not a quick and trivial thing to do. The | |
| translated xz should look as polished as the English version. | |
| 5. Other implementations of the .xz format | |
| ------------------------------------------ | |
| 7-Zip and the p7zip port of 7-Zip support the .xz format starting | |
| from the version 9.00alpha. | |
| http://7-zip.org/ | |
| http://p7zip.sourceforge.net/ | |
| XZ Embedded is a limited implementation written for use in the Linux | |
| kernel, but it is also suitable for other embedded use. | |
| http://tukaani.org/xz/embedded.html | |
| 6. Contact information | |
| ---------------------- | |
| If you have questions, bug reports, patches etc. related to XZ Utils, | |
| contact Lasse Collin <lasse.collin@tukaani.org> (in Finnish or English). | |
| I'm sometimes slow at replying. If you haven't got a reply within two | |
| weeks, assume that your email has got lost and resend it or use IRC. | |
| You can find me also from #tukaani on Freenode; my nick is Larhzu. | |
| The channel tends to be pretty quiet, so just ask your question and | |
| someone may wake up. | |