xref: /external/xz-embedded

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.

Integrity check support

    XZ Embedded always supports the integrity check types None and
    CRC32. Support for CRC64 is optional. SHA-256 is currently not
    supported in XZ Embedded although the .xz format does support it.
    The xz tool from XZ Utils uses CRC64 by default, but CRC32 is usually
    enough in embedded systems to keep the code size smaller.

    If you want support for CRC64, you need to copy linux/lib/xz/xz_crc64.c
    into your application, and #define XZ_USE_CRC64 in xz_config.h or in
    compiler flags.

    When using the internal CRC32 or CRC64, their lookup tables need to be
    initialized with xz_crc32_init() and xz_crc64_init(), respectively.
    See xz.h for details.

    To use external CRC32 or CRC64 code instead of the code from
    xz_crc32.c or xz_crc64.c, the following #defines may be used
    in xz_config.h or in compiler flags:

        #define XZ_INTERNAL_CRC32 0
        #define XZ_INTERNAL_CRC64 0

    Then it is up to you to provide compatible xz_crc32() or xz_crc64()
    functions.

    If the .xz file being decompressed uses an integrity check type that
    isn't supported by XZ Embedded, it is treated as an error and the
    file cannot be decompressed. For multi-call mode, this can be modified
    by #defining XZ_DEC_ANY_CHECK. Then xz_dec_run() will return
    XZ_UNSUPPORTED_CHECK when unsupported check type is detected. After
    that decompression can be continued normally except that the
    integrity check won't be verified. In single-call mode there's
    no way to continue decoding, so XZ_DEC_ANY_CHECK is almost useless
    in single-call mode.

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.