
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.

        NOTE: The version of XZ Embedded in the Linux kernel lacks a few
        build-time-selectable optional features that are present in the
        upstream XZ Embedded project: support for concatated .xz files,
        CRC64, and ignoring unsupported check. These aren't in Linux
        because they don't seem useful there but they would add to the
        code size.

    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/staging/xz.rst even if you aren't using
    XZ Embedded as part of Linux; information in that file is not
    repeated in this README.

Conformance to the .xz file format specification

    As of the .xz file format specification version 1.2.0, this
    decompressor implementation has the following limitations:

      - SHA-256 isn't supported. It can be ignored as an unsupported
        checked type if that feature is enabled at build time.

      - Delta filter is not included.

      - BCJ filters don't support non-default start offset.

      - LZMA2 supports at most 3 GiB dictionary.

    There are a couple of corner cases where things have been simplified
    at expense of detecting errors as early as possible. These should not
    matter in practice at all since they don't cause security issues. But
    it is good to know this if testing the code with the test files from
    XZ Utils.

Compiler requirements

    XZ Embedded should compile with any C99 or C11 compiler. The code
    should also be GNU-C89 compatible still. GNU-C89 was used in the
    Linux kernel until 2022. GNU-C89 support likely will be dropped
    at some point.

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.

Supporting concatenated .xz files

    Regular .xz files can be concatenated as is and the xz command line
    tool will decompress all streams from a concatenated file (a few
    other popular formats and tools support this too). This kind of .xz
    files are more common than one might think because pxz, an early
    threaded XZ compressor, created this kind of .xz files.

    The xz_dec_run() function will stop after decompressing one stream.
    This is good when XZ data is stored inside some other file format.
    However, if one is decompressing regular standalone .xz files, one
    will want to decompress all streams in the file. This is easy with
    xz_dec_catrun(). To include support for xz_dec_catrun(), you need
    to #define XZ_DEC_CONCATENATED in xz_config.h or in compiler flags.

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
    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 instructions
        XZ_DEC_ARMTHUMB     ARM-Thumb           Big or little endian
        XZ_DEC_ARM64        ARM64               Big or little endian
        XZ_DEC_SPARC        SPARC               Big or little endian
        XZ_DEC_RISCV        RISC-V              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 many filters work for both big and little
    endian executables (Itanium and ARM based architectures have little
    endian instructions and SPARC has big endian instructions).

Notes about shared libraries

    If you are including XZ Embedded into a shared library, you 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).

    Please don't create a shared library of XZ Embedded itself unless
    it is fine to rebuild everything depending on that shared library
    every time you upgrade to a newer version of XZ Embedded. There are
    no API or ABI stability guarantees between different versions of
    XZ Embedded.

Contact information

    Email: Lasse Collin <lasse.collin@tukaani.org>
    IRC: Larhzu on #tukaani on Libera Chat
    GitHub: https://github.com/tukaani-project/xz-embedded