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 IRC: Larhzu on #tukaani on Libera Chat GitHub: https://github.com/tukaani-project/xz-embedded