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+
+XZ Embedded
+===========
+
+ XZ Embedded is a relatively small, limited implementation of the .xz
+ file format. Currently only decoding is implemented.
+
+ XZ Embedded was written for use in the Linux kernel, but the code can
+ be easily used in other environments too, including regular userspace
+ applications.
+
+ This README contains information that is useful only when the copy
+ of XZ Embedded isn't part of the Linux kernel tree. You should also
+ read linux/Documentation/xz.txt even if you aren't using XZ Embedded
+ as part of Linux; information in that file is not repeated in this
+ README.
+
+Compiling the Linux kernel module
+
+ The xz_dec module depends on crc32 module, so make sure that you have
+ it enabled (CONFIG_CRC32).
+
+ Building the xz_dec and xz_dec_test modules without support for BCJ
+ filters:
+
+ cd linux/lib/xz
+ make -C /path/to/kernel/source \
+ KCPPFLAGS=-I"$(pwd)/../../include" M="$(pwd)" \
+ CONFIG_XZ_DEC=m CONFIG_XZ_DEC_TEST=m
+
+ Building the xz_dec and xz_dec_test modules with support for BCJ
+ filters:
+
+ cd linux/lib/xz
+ make -C /path/to/kernel/source \
+ KCPPFLAGS=-I"$(pwd)/../../include" M="$(pwd)" \
+ CONFIG_XZ_DEC=m CONFIG_XZ_DEC_TEST=m CONFIG_XZ_DEC_BCJ=y \
+ CONFIG_XZ_DEC_X86=y CONFIG_XZ_DEC_POWERPC=y \
+ CONFIG_XZ_DEC_IA64=y CONFIG_XZ_DEC_ARM=y \
+ CONFIG_XZ_DEC_ARMTHUMB=y CONFIG_XZ_DEC_SPARC=y
+
+ If you want only one or a few of the BCJ filters, omit the appropriate
+ variables. CONFIG_XZ_DEC_BCJ=y is always required to build the support
+ code shared between all BCJ filters.
+
+ Most people don't need the xz_dec_test module. You can skip building
+ it by omitting CONFIG_XZ_DEC_TEST=m from the make command line.
+
+Compiler requirements
+
+ XZ Embedded should compile as either GNU-C89 (used in the Linux
+ kernel) or with any C99 compiler. Getting the code to compile with
+ non-GNU C89 compiler or a C++ compiler should be quite easy as
+ long as there is a data type for unsigned 64-bit integer (or the
+ code is modified not to support large files, which needs some more
+ care than just using 32-bit integer instead of 64-bit).
+
+ If you use GCC, try to use a recent version. For example, on x86,
+ xz_dec_lzma2.c compiled with GCC 3.3.6 is 15-25 % slower than when
+ compiled with GCC 4.3.3.
+
+Embedding into userspace applications
+
+ To embed the XZ decoder, copy the following files into a single
+ directory in your source code tree:
+
+ linux/include/linux/xz.h
+ linux/lib/xz/xz_crc32.c
+ linux/lib/xz/xz_dec_lzma2.c
+ linux/lib/xz/xz_dec_stream.c
+ linux/lib/xz/xz_lzma2.h
+ linux/lib/xz/xz_private.h
+ linux/lib/xz/xz_stream.h
+ userspace/xz_config.h
+
+ Alternatively, xz.h may be placed into a different directory but then
+ that directory must be in the compiler include path when compiling
+ the .c files.
+
+ Your code should use only the functions declared in xz.h. The rest of
+ the .h files are meant only for internal use in XZ Embedded.
+
+ You may want to modify xz_config.h to be more suitable for your build
+ environment. Probably you should at least skim through it even if the
+ default file works as is.
+
+BCJ filter support
+
+ If you want support for one or more BCJ filters, you need to copy also
+ linux/lib/xz/xz_dec_bcj.c into your application, and use appropriate
+ #defines in xz_config.h or in compiler flags. You don't need these
+ #defines in the code that just uses XZ Embedded via xz.h, but having
+ them always #defined doesn't hurt either.
+
+ #define Instruction set BCJ filter endianness
+ XZ_DEC_X86 x86 or x86-64 Little endian only
+ XZ_DEC_POWERPC PowerPC Big endian only
+ XZ_DEC_IA64 Itanium (IA-64) Big or little endian
+ XZ_DEC_ARM ARM Little endian only
+ XZ_DEC_ARMTHUMB ARM-Thumb Little endian only
+ XZ_DEC_SPARC SPARC Big or little endian
+
+ While some architectures are (partially) bi-endian, the endianness
+ setting doesn't change the endianness of the instructions on all
+ architectures. That's why Itanium and SPARC filters work for both big
+ and little endian executables (Itanium has little endian instructions
+ and SPARC has big endian instructions).
+
+ There currently is no filter for little endian PowerPC or big endian
+ ARM or ARM-Thumb. Implementing filters for them can be considered if
+ there is a need for such filters in real-world applications.
+
+Notes about shared libraries
+
+ If you are including XZ Embedded into a shared library, you very
+ probably should rename the xz_* functions to prevent symbol
+ conflicts in case your library is linked against some other library
+ or application that also has XZ Embedded in it (which may even be
+ a different version of XZ Embedded). TODO: Provide an easy way
+ to do this.
+
+ Please don't create a shared library of XZ Embedded itself unless
+ it is fine to rebuild everything depending on that shared library
+ everytime you upgrade to a newer version of XZ Embedded. There are
+ no API or ABI stability guarantees between different versions of
+ XZ Embedded.
+
+Specifying the calling convention
+
+ XZ_FUNC macro was included to support declaring functions with __init
+ in Linux. Outside Linux, it can be used to specify the calling
+ convention on systems that support multiple calling conventions.
+ For example, on Windows, you may make all functions use the stdcall
+ calling convention by defining XZ_FUNC=__stdcall when building and
+ using the functions from XZ Embedded.
+