busybox/archival/libarchive/unxz

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. See userspace/xzminidec.c for an example program.

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 x86-32,
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 x86-32 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.