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205 lines
8.5 KiB
Text
205 lines
8.5 KiB
Text
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XZ Utils
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========
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Important
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This is a beta version. The .xz file format is now stable though,
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which means that files created with the beta version will be
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decompressible with all future XZ Utils versions too (assuming
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that there are no catastrophic bugs).
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liblzma API is pretty stable now, although minor tweaks may still
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be done if really needed. The ABI is not stable yet. The major
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soname will be bumped right before the first stable release.
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Probably it will be bumped to something like .so.5.0.0 because
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some distributions using the alpha versions already had to use
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other versions than .so.0.0.0.
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Excluding the Doxygen style docs in libilzma API headers, the
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documentation in this package (including the rest of this
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README) is not very up to date, and may contain incorrect or
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misleading information.
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Overview
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LZMA is a general purporse compression algorithm designed by
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Igor Pavlov as part of 7-Zip. It provides high compression ratio
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while keeping the decompression speed fast.
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XZ Utils are an attempt to make LZMA compression easy to use
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on free (as in freedom) operating systems. This is achieved by
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providing tools and libraries which are similar to use than the
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equivalents of the most popular existing compression algorithms.
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XZ Utils consist of a few relatively separate parts:
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* liblzma is an encoder/decoder library with support for several
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filters (algorithm implementations). The primary filter is LZMA.
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* libzfile (or whatever the name will be) enables reading from and
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writing to gzip, bzip2 and LZMA compressed and uncompressed files
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with an API similar to the standard ANSI-C file I/O.
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[ NOTE: libzfile is not implemented yet. ]
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* xz command line tool has almost identical syntax than gzip
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and bzip2. It makes LZMA easy for average users, but also
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provides advanced options to finetune the compression settings.
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* A few shell scripts make diffing and grepping LZMA compressed
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files easy. The scripts were adapted from gzip and bzip2.
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Supported platforms
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XZ Utils are developed on GNU+Linux, but they should work at
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least on *BSDs and Solaris. They probably work on some other
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POSIX-like operating systems too.
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If you use GCC to compile XZ Utils, you need at least version
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3.x.x. GCC version 2.xx.x doesn't support some C99 features used
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in XZ Utils source code, thus GCC 2 won't compile XZ Utils.
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If you have written patches to make XZ Utils to work on previously
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unsupported platform, please send the patches to me! I will consider
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including them to the official version. It's nice to minimize the
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need of third-party patching.
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One exception: Don't request or send patches to change the whole
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source package to C89. I find C99 substantially nicer to write and
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maintain. However, the public library headers must be in C89 to
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avoid frustrating those who maintain programs, which are strictly
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in C89 or C++.
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Platform-specific notes
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On some Tru64 systems using the native C99 compiler, the configure
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script may reject the compiler as non-C99 compiler. This may happen
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if there is no stdbool.h available. You can still compile XZ Utils
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on such a system by passing ac_cv_prog_cc_c99= to configure script.
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Fixing this bug seems to be non-trivial since if the configure
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doesn't check for stdbool.h, it runs into problems at least on
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Solaris.
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Version numbering
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The version number of XZ Utils has absolutely nothing to do with
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the version number of LZMA SDK or 7-Zip. The new version number
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format of XZ Utils is X.Y.ZS:
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- X is the major version. When this is incremented, the library
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API and ABI break.
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- Y is the minor version. It is incremented when new features are
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added without breaking existing API or ABI. Even Y indicates
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stable release and odd Y indicates unstable (alpha or beta
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version).
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- Z is the revision. This has different meaning for stable and
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unstable releases:
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* Stable: Z is incremented when bugs get fixed without adding
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any new features.
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* Unstable: Z is just a counter. API or ABI of features added
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in earlier unstable releases having the same X.Y may break.
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- S indicates stability of the release. It is missing from the
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stable releases where Y is an even number. When Y is odd, S
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is either "alpha" or "beta" to make it very clear that such
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versions are not stable releases. The same X.Y.Z combination is
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not used for more than one stability level i.e. after X.Y.Zalpha,
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the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
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configure options
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If you are not familiar with `configure' scripts, read the file
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INSTALL first.
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In most cases, the default --enable/--disable/--with/--without options
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are what you want. Don't touch them if you are unsure.
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--disable-encoder
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Do not compile the encoder component of liblzma. This
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implies --disable-match-finders. If you need only
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the decoder, you can decrease the library size
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dramatically with this option.
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The default is to build the encoder.
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--disable-decoder
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Do not compile the decoder component of liblzma.
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The default is to build the decoder.
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--enable-filters=
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liblzma supports several filters. See liblzma-intro.txt
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for a little more information about these.
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The default is to build all the filters.
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--enable-match-finders=
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liblzma includes two categories of match finders:
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hash chains and binary trees. Hash chains (hc3 and hc4)
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are quite fast but they don't provide the best compression
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ratio. Binary trees (bt2, bt3 and bt4) give excellent
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compression ratio, but they are slower and need more
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memory than hash chains.
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You need to enable at least one match finder to build the
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LZMA filter encoder. Usually hash chains are used only in
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the fast mode, while binary trees are used to when the best
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compression ratio is wanted.
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The default is to build all the match finders.
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--enable-checks=
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liblzma support multiple integrity checks. CRC32 is
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mandatory, and cannot be omitted. See liblzma-intro.txt
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for more information about usage of the integrity checks.
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--disable-assembler
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liblzma includes some assembler optimizations. Currently
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there is only assembler code for CRC32 and CRC64 for
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32-bit x86.
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All the assembler code in liblzma is position-independent
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code, which is suitable for use in shared libraries and
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position-independent executables. So far only i386
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instructions are used, but the code is optimized for i686
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class CPUs. If you are compiling liblzma exclusively for
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pre-i686 systems, you may want to disable the assembler
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code.
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--enable-small
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Omits precomputed tables. This makes liblzma a few KiB
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smaller. Startup time increases, because the tables need
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to be computed first.
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--enable-debug
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This enables the assert() macro and possibly some other
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run-time consistency checks. It slows down things somewhat,
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so you normally don't want to have this enabled.
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--enable-werror
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Makes all compiler warnings an error, that abort the
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compilation. This may help catching bugs, and should work
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on most systems. This has no effect on the resulting
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binaries.
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Static vs. dynamic linking of the command line tools
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By default, the command line tools are linked statically against
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liblzma. There a are a few reasons:
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- The executable(s) can be in /bin while the shared liblzma can still
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be in /usr/lib (if the distro uses such file system hierachy).
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- It's easier to copy the executables to other systems, since they
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depend only on libc.
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- It's slightly faster on some architectures like x86.
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If you don't like this, you can get the command line tools linked
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against the shared liblzma by specifying --disable-static to configure.
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This disables building static liblzma completely.
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