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README

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. Translating the xz tool
13    5. Other implementations of the .xz format
14    6. Contact information
15
16
170. Overview
18-----------
19
20    XZ Utils provide a general-purpose data-compression library plus
21    command-line tools. The native file format is the .xz format, but
22    also the legacy .lzma format is supported. The .xz format supports
23    multiple compression algorithms, which are called "filters" in the
24    context of XZ Utils. The primary filter is currently LZMA2. With
25    typical files, XZ Utils create about 30 % smaller files than gzip.
26
27    To ease adapting support for the .xz format into existing applications
28    and scripts, the API of liblzma is somewhat similar to the API of the
29    popular zlib library. For the same reason, the command-line tool xz
30    has a command-line syntax similar to that of gzip.
31
32    When aiming for the highest compression ratio, the LZMA2 encoder uses
33    a lot of CPU time and may use, depending on the settings, even
34    hundreds of megabytes of RAM. However, in fast modes, the LZMA2 encoder
35    competes with bzip2 in compression speed, RAM usage, and compression
36    ratio.
37
38    LZMA2 is reasonably fast to decompress. It is a little slower than
39    gzip, but a lot faster than bzip2. Being fast to decompress means
40    that the .xz format is especially nice when the same file will be
41    decompressed very many times (usually on different computers), which
42    is the case e.g. when distributing software packages. In such
43    situations, it's not too bad if the compression takes some time,
44    since that needs to be done only once to benefit many people.
45
46    With some file types, combining (or "chaining") LZMA2 with an
47    additional filter can improve the compression ratio. A filter chain may
48    contain up to four filters, although usually only one or two are used.
49    For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2
50    in the filter chain can improve compression ratio of executable files.
51
52    Since the .xz format allows adding new filter IDs, it is possible that
53    some day there will be a filter that is, for example, much faster to
54    compress than LZMA2 (but probably with worse compression ratio).
55    Similarly, it is possible that some day there is a filter that will
56    compress better than LZMA2.
57
58    XZ Utils doesn't support multithreaded compression or decompression
59    yet. It has been planned though and taken into account when designing
60    the .xz file format.
61
62
631. Documentation
64----------------
65
661.1. Overall documentation
67
68    README              This file
69
70    INSTALL.generic     Generic install instructions for those not familiar
71                        with packages using GNU Autotools
72    INSTALL             Installation instructions specific to XZ Utils
73    PACKAGERS           Information to packagers of XZ Utils
74
75    COPYING             XZ Utils copyright and license information
76    COPYING.GPLv2       GNU General Public License version 2
77    COPYING.GPLv3       GNU General Public License version 3
78    COPYING.LGPLv2.1    GNU Lesser General Public License version 2.1
79
80    AUTHORS             The main authors of XZ Utils
81    THANKS              Incomplete list of people who have helped making
82                        this software
83    NEWS                User-visible changes between XZ Utils releases
84    ChangeLog           Detailed list of changes (commit log)
85    TODO                Known bugs and some sort of to-do list
86
87    Note that only some of the above files are included in binary
88    packages.
89
90
911.2. Documentation for command-line tools
92
93    The command-line tools are documented as man pages. In source code
94    releases (and possibly also in some binary packages), the man pages
95    are also provided in plain text (ASCII only) and PDF formats in the
96    directory "doc/man" to make the man pages more accessible to those
97    whose operating system doesn't provide an easy way to view man pages.
98
99
1001.3. Documentation for liblzma
101
102    The liblzma API headers include short docs about each function
103    and data type as Doxygen tags. These docs should be quite OK as
104    a quick reference.
105
106    I have planned to write a bunch of very well documented example
107    programs, which (due to comments) should work as a tutorial to
108    various features of liblzma. No such example programs have been
109    written yet.
110
111    For now, if you have never used liblzma, libbzip2, or zlib, I
112    recommend learning the *basics* of the zlib API. Once you know that,
113    it should be easier to learn liblzma.
114
115        http://zlib.net/manual.html
116        http://zlib.net/zlib_how.html
117
118
1192. Version numbering
120--------------------
121
122    The version number format of XZ Utils is X.Y.ZS:
123
124      - X is the major version. When this is incremented, the library
125        API and ABI break.
126
127      - Y is the minor version. It is incremented when new features
128        are added without breaking the existing API or ABI. An even Y
129        indicates a stable release and an odd Y indicates unstable
130        (alpha or beta version).
131
132      - Z is the revision. This has a different meaning for stable and
133        unstable releases:
134
135          * Stable: Z is incremented when bugs get fixed without adding
136            any new features. This is intended to be convenient for
137            downstream distributors that want bug fixes but don't want
138            any new features to minimize the risk of introducing new bugs.
139
140          * Unstable: Z is just a counter. API or ABI of features added
141            in earlier unstable releases having the same X.Y may break.
142
143      - S indicates stability of the release. It is missing from the
144        stable releases, where Y is an even number. When Y is odd, S
145        is either "alpha" or "beta" to make it very clear that such
146        versions are not stable releases. The same X.Y.Z combination is
147        not used for more than one stability level, i.e. after X.Y.Zalpha,
148        the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
149
150
1513. Reporting bugs
152-----------------
153
154    Naturally it is easiest for me if you already know what causes the
155    unexpected behavior. Even better if you have a patch to propose.
156    However, quite often the reason for unexpected behavior is unknown,
157    so here are a few things to do before sending a bug report:
158
159      1. Try to create a small example how to reproduce the issue.
160
161      2. Compile XZ Utils with debugging code using configure switches
162         --enable-debug and, if possible, --disable-shared. If you are
163         using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting
164         binaries.
165
166      3. Turn on core dumps. The exact command depends on your shell;
167         for example in GNU bash it is done with "ulimit -c unlimited",
168         and in tcsh with "limit coredumpsize unlimited".
169
170      4. Try to reproduce the suspected bug. If you get "assertion failed"
171         message, be sure to include the complete message in your bug
172         report. If the application leaves a coredump, get a backtrace
173         using gdb:
174           $ gdb /path/to/app-binary   # Load the app to the debugger.
175           (gdb) core core   # Open the coredump.
176           (gdb) bt   # Print the backtrace. Copy & paste to bug report.
177           (gdb) quit   # Quit gdb.
178
179    Report your bug via email or IRC (see Contact information below).
180    Don't send core dump files or any executables. If you have a small
181    example file(s) (total size less than 256 KiB), please include
182    it/them as an attachment. If you have bigger test files, put them
183    online somewhere and include a URL to the file(s) in the bug report.
184
185    Always include the exact version number of XZ Utils in the bug report.
186    If you are using a snapshot from the git repository, use "git describe"
187    to get the exact snapshot version. If you are using XZ Utils shipped
188    in an operating system distribution, mention the distribution name,
189    distribution version, and exact xz package version; if you cannot
190    repeat the bug with the code compiled from unpatched source code,
191    you probably need to report a bug to your distribution's bug tracking
192    system.
193
194
1954. Translating the xz tool
196--------------------------
197
198    The messages from the xz tool have been translated into a few
199    languages. Before starting to translate into a new language, ask
200    the author whether someone else hasn't already started working on it.
201
202    Test your translation. Testing includes comparing the translated
203    output to the original English version by running the same commands
204    in both your target locale and with LC_ALL=C. Ask someone to
205    proof-read and test the translation.
206
207    Testing can be done e.g. by installing xz into a temporary directory:
208
209        ./configure --disable-shared --prefix=/tmp/xz-test
210        # <Edit the .po file in the po directory.>
211        make -C po update-po
212        make install
213        bash debug/translation.bash | less
214        bash debug/translation.bash | less -S  # For --list outputs
215
216    Repeat the above as needed (no need to re-run configure though).
217
218    Note especially the following:
219
220      - The output of --help and --long-help must look nice on
221        an 80-column terminal. It's OK to add extra lines if needed.
222
223      - In contrast, don't add extra lines to error messages and such.
224        They are often preceded with e.g. a filename on the same line,
225        so you have no way to predict where to put a \n. Let the terminal
226        do the wrapping even if it looks ugly. Adding new lines will be
227        even uglier in the generic case even if it looks nice in a few
228        limited examples.
229
230      - Be careful with column alignment in tables and table-like output
231        (--list, --list --verbose --verbose, --info-memory, --help, and
232        --long-help):
233
234          * All descriptions of options in --help should start in the
235            same column (but it doesn't need to be the same column as
236            in the English messages; just be consistent if you change it).
237            Check that both --help and --long-help look OK, since they
238            share several strings.
239
240          * --list --verbose and --info-memory print lines that have
241            the format "Description:   %s". If you need a longer
242            description, you can put extra space between the colon
243            and %s. Then you may need to add extra space to other
244            strings too so that the result as a whole looks good (all
245            values start at the same column).
246
247          * The columns of the actual tables in --list --verbose --verbose
248            should be aligned properly. Abbreviate if necessary. It might
249            be good to keep at least 2 or 3 spaces between column headings
250            and avoid spaces in the headings so that the columns stand out
251            better, but this is a matter of opinion. Do what you think
252            looks best.
253
254      - Be careful to put a period at the end of a sentence when the
255        original version has it, and don't put it when the original
256        doesn't have it. Similarly, be careful with \n characters
257        at the beginning and end of the strings.
258
259      - Read the TRANSLATORS comments that have been extracted from the
260        source code and included in xz.pot. If they suggest testing the
261        translation with some type of command, do it. If testing needs
262        input files, use e.g. tests/files/good-*.xz.
263
264      - When updating the translation, read the fuzzy (modified) strings
265        carefully, and don't mark them as updated before you actually
266        have updated them. Reading through the unchanged messages can be
267        good too; sometimes you may find a better wording for them.
268
269      - If you find language problems in the original English strings,
270        feel free to suggest improvements. Ask if something is unclear.
271
272      - The translated messages should be understandable (sometimes this
273        may be a problem with the original English messages too). Don't
274        make a direct word-by-word translation from English especially if
275        the result doesn't sound good in your language.
276
277    In short, take your time and pay attention to the details. Making
278    a good translation is not a quick and trivial thing to do. The
279    translated xz should look as polished as the English version.
280
281
2825. Other implementations of the .xz format
283------------------------------------------
284
285    7-Zip and the p7zip port of 7-Zip support the .xz format starting
286    from the version 9.00alpha.
287
288        http://7-zip.org/
289        http://p7zip.sourceforge.net/
290
291    XZ Embedded is a limited implementation written for use in the Linux
292    kernel, but it is also suitable for other embedded use.
293
294        http://tukaani.org/xz/embedded.html
295
296
2976. Contact information
298----------------------
299
300    If you have questions, bug reports, patches etc. related to XZ Utils,
301    contact Lasse Collin <lasse.collin@tukaani.org> (in Finnish or English).
302    I'm sometimes slow at replying. If you haven't got a reply within two
303    weeks, assume that your email has got lost and resend it or use IRC.
304
305    You can find me also from #tukaani on Freenode; my nick is Larhzu.
306    The channel tends to be pretty quiet, so just ask your question and
307    someone may wake up.
308
309