mirror of https://git.tukaani.org/xz.git
XZ Utils is free general-purpose data compression software with a high compression ratio
https://tukaani.org/xz/
31d80c6b26
RHEL/CentOS 7 shipped with 5.1.2alpha, including the threaded
encoder that is behind #ifdef LZMA_UNSTABLE in the API headers.
In 5.1.2alpha these symbols are under XZ_5.1.2alpha in liblzma.map.
API/ABI compatibility tracking isn't done between development
releases so newer releases didn't have XZ_5.1.2alpha anymore.
Later RHEL/CentOS 7 updated xz to 5.2.2 but they wanted to keep
the exported symbols compatible with 5.1.2alpha. After checking
the ABI changes it turned out that >= 5.2.0 ABI is backward
compatible with the threaded encoder functions from 5.1.2alpha
(but not vice versa as fixes and extensions to these functions
were made between 5.1.2alpha and 5.2.0).
In RHEL/CentOS 7, XZ Utils 5.2.2 was patched with
xz-5.2.2-compat-libs.patch to modify liblzma.map:
- XZ_5.1.2alpha was added with lzma_stream_encoder_mt and
lzma_stream_encoder_mt_memusage. This matched XZ Utils 5.1.2alpha.
- XZ_5.2 was replaced with XZ_5.2.2. It is clear that this was
an error; the intention was to keep using XZ_5.2 (XZ_5.2.2
has never been used in XZ Utils). So XZ_5.2.2 lists all
symbols that were listed under XZ_5.2 before the patch.
lzma_stream_encoder_mt and _mt_memusage are included too so
they are listed both here and under XZ_5.1.2alpha.
The patch didn't add any __asm__(".symver ...") lines to the .c
files. Thus the resulting liblzma.so exports the threaded encoder
functions under XZ_5.1.2alpha only. Listing the two functions
also under XZ_5.2.2 in liblzma.map has no effect without
matching .symver lines.
The lack of XZ_5.2 in RHEL/CentOS 7 means that binaries linked
against unpatched XZ Utils 5.2.x won't run on RHEL/CentOS 7.
This is unfortunate but this alone isn't too bad as the problem
is contained within RHEL/CentOS 7 and doesn't affect users
of other distributions. It could also be fixed internally in
RHEL/CentOS 7.
The second problem is more serious: In XZ Utils 5.2.2 the API
headers don't have #ifdef LZMA_UNSTABLE for obvious reasons.
This is true in RHEL/CentOS 7 version too. Thus now programs
using new APIs can be compiled without an extra #define. However,
the programs end up depending on symbol version XZ_5.1.2alpha
(and possibly also XZ_5.2.2) instead of XZ_5.2 as they would
with an unpatched XZ Utils 5.2.2. This means that such binaries
won't run on other distributions shipping XZ Utils >= 5.2.0 as
they don't provide XZ_5.1.2alpha or XZ_5.2.2; they only provide
XZ_5.2 (and XZ_5.0). (This includes RHEL/CentOS 8 as the patch
luckily isn't included there anymore with XZ Utils 5.2.4.)
Binaries built by RHEL/CentOS 7 users get distributed and then
people wonder why they don't run on some other distribution.
Seems that people have found out about the patch and been copying
it to some build scripts, seemingly curing the symptoms but
actually spreading the illness further and outside RHEL/CentOS 7.
The ill patch seems to be from late 2016 (RHEL 7.3) and in 2017 it
had spread at least to EasyBuild. I heard about the events only
recently. :-(
This commit splits liblzma.map into two versions: one for
GNU/Linux and another for other OSes that can use symbol versioning
(FreeBSD, Solaris, maybe others). The Linux-specific file and the
matching additions to .c files add full compatibility with binaries
that have been built against a RHEL/CentOS-patched liblzma. Builds
for OSes other than GNU/Linux won't get the vaccine as they should
be immune to the problem (I really hope that no build script uses
the RHEL/CentOS 7 patch outside GNU/Linux).
The RHEL/CentOS compatibility symbols XZ_5.1.2alpha and XZ_5.2.2
are intentionally put *after* XZ_5.2 in liblzma_linux.map. This way
if one forgets to #define HAVE_SYMBOL_VERSIONS_LINUX when building,
the resulting liblzma.so.5 will have lzma_stream_encoder_mt@@XZ_5.2
since XZ_5.2 {...} is the first one that lists that function.
Without HAVE_SYMBOL_VERSIONS_LINUX @XZ_5.1.2alpha and @XZ_5.2.2
will be missing but that's still a minor problem compared to
only having lzma_stream_encoder_mt@@XZ_5.1.2alpha!
The "local: *;" line was moved to XZ_5.0 so that it doesn't need
to be moved around. It doesn't matter where it is put.
Having two similar liblzma_*.map files is a bit silly as it is,
at least for now, easily possible to generate the generic one
from the Linux-specific file. But that adds extra steps and
increases the risk of mistakes when supporting more than one
build system. So I rather maintain two files in parallel and let
validate_map.sh check that they are in sync when "make mydist"
is run.
This adds .symver lines for lzma_stream_encoder_mt@XZ_5.2.2 and
lzma_stream_encoder_mt_memusage@XZ_5.2.2 even though these
weren't exported by RHEL/CentOS 7 (only @@XZ_5.1.2alpha was
for these two). I added these anyway because someone might
misunderstand the RHEL/CentOS 7 patch and think that @XZ_5.2.2
(@@XZ_5.2.2) versions were exported too.
At glance one could suggest using __typeof__ to copy the function
prototypes when making aliases. However, this doesn't work trivially
because __typeof__ won't copy attributes (lzma_nothrow, lzma_pure)
and it won't change symbol visibility from hidden to default (done
by LZMA_API()). Attributes could be copied with __copy__ attribute
but that needs GCC 9 and a fallback method would be needed anyway.
This uses __symver__ attribute with GCC >= 10 and
__asm__(".symver ...") with everything else. The attribute method
is required for LTO (-flto) support with GCC. Using -flto with
GCC older than 10 is now broken on GNU/Linux and will not be fixed
(can silently result in a broken liblzma build that has dangerously
incorrect symbol versions). LTO builds with Clang seem to work
with the traditional __asm__(".symver ...") method.
Thanks to Boud Roukema for reporting the problem and discussing
the details and testing the fix.
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| build-aux | ||
| cmake | ||
| debug | ||
| doc | ||
| dos | ||
| extra | ||
| lib | ||
| m4 | ||
| macosx | ||
| po | ||
| po4a | ||
| src | ||
| tests | ||
| windows | ||
| .gitignore | ||
| AUTHORS | ||
| CMakeLists.txt | ||
| COPYING | ||
| COPYING.GPLv2 | ||
| COPYING.GPLv3 | ||
| COPYING.LGPLv2.1 | ||
| ChangeLog | ||
| Doxyfile.in | ||
| INSTALL | ||
| INSTALL.generic | ||
| Makefile.am | ||
| NEWS | ||
| PACKAGERS | ||
| README | ||
| THANKS | ||
| TODO | ||
| autogen.sh | ||
| configure.ac | ||
README
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. Translations
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 supports multithreaded compression. XZ Utils doesn't support
multithreaded decompression yet. It has been planned though and taken
into account when designing the .xz file format. In the future, files
that were created in threaded mode can be decompressed in threaded
mode too.
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.
There are a few example/tutorial programs that should help in
getting started with liblzma. In the source package the examples
are in "doc/examples" and in binary packages they may be under
"examples" in the same directory as this README.
Since the liblzma API has similarities to the zlib API, some people
may find it useful to read the zlib docs and tutorial too:
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. Translations
---------------
The xz command line tool and all man pages can be translated.
The translations are handled via the Translation Project. If you
wish to help translating xz, please join the Translation Project:
https://translationproject.org/html/translators.html
Several strings will change in a future version of xz so if you
wish to start a new translation, look at the code in the xz git
repository instead of a 5.2.x release.
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.
https://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.