1 2XZ Embedded 3=========== 4 5 XZ Embedded is a relatively small, limited implementation of the .xz 6 file format. Currently only decoding is implemented. 7 8 XZ Embedded was written for use in the Linux kernel, but the code can 9 be easily used in other environments too, including regular userspace 10 applications. See userspace/xzminidec.c for an example program. 11 12 This README contains information that is useful only when the copy 13 of XZ Embedded isn't part of the Linux kernel tree. You should also 14 read linux/Documentation/xz.txt even if you aren't using XZ Embedded 15 as part of Linux; information in that file is not repeated in this 16 README. 17 18Compiling the Linux kernel module 19 20 The xz_dec module depends on crc32 module, so make sure that you have 21 it enabled (CONFIG_CRC32). 22 23 Building the xz_dec and xz_dec_test modules without support for BCJ 24 filters: 25 26 cd linux/lib/xz 27 make -C /path/to/kernel/source \ 28 KCPPFLAGS=-I"$(pwd)/../../include" M="$(pwd)" \ 29 CONFIG_XZ_DEC=m CONFIG_XZ_DEC_TEST=m 30 31 Building the xz_dec and xz_dec_test modules with support for BCJ 32 filters: 33 34 cd linux/lib/xz 35 make -C /path/to/kernel/source \ 36 KCPPFLAGS=-I"$(pwd)/../../include" M="$(pwd)" \ 37 CONFIG_XZ_DEC=m CONFIG_XZ_DEC_TEST=m CONFIG_XZ_DEC_BCJ=y \ 38 CONFIG_XZ_DEC_X86=y CONFIG_XZ_DEC_POWERPC=y \ 39 CONFIG_XZ_DEC_IA64=y CONFIG_XZ_DEC_ARM=y \ 40 CONFIG_XZ_DEC_ARMTHUMB=y CONFIG_XZ_DEC_SPARC=y 41 42 If you want only one or a few of the BCJ filters, omit the appropriate 43 variables. CONFIG_XZ_DEC_BCJ=y is always required to build the support 44 code shared between all BCJ filters. 45 46 Most people don't need the xz_dec_test module. You can skip building 47 it by omitting CONFIG_XZ_DEC_TEST=m from the make command line. 48 49Compiler requirements 50 51 XZ Embedded should compile as either GNU-C89 (used in the Linux 52 kernel) or with any C99 compiler. Getting the code to compile with 53 non-GNU C89 compiler or a C++ compiler should be quite easy as 54 long as there is a data type for unsigned 64-bit integer (or the 55 code is modified not to support large files, which needs some more 56 care than just using 32-bit integer instead of 64-bit). 57 58 If you use GCC, try to use a recent version. For example, on x86-32, 59 xz_dec_lzma2.c compiled with GCC 3.3.6 is 15-25 % slower than when 60 compiled with GCC 4.3.3. 61 62Embedding into userspace applications 63 64 To embed the XZ decoder, copy the following files into a single 65 directory in your source code tree: 66 67 linux/include/linux/xz.h 68 linux/lib/xz/xz_crc32.c 69 linux/lib/xz/xz_dec_lzma2.c 70 linux/lib/xz/xz_dec_stream.c 71 linux/lib/xz/xz_lzma2.h 72 linux/lib/xz/xz_private.h 73 linux/lib/xz/xz_stream.h 74 userspace/xz_config.h 75 76 Alternatively, xz.h may be placed into a different directory but then 77 that directory must be in the compiler include path when compiling 78 the .c files. 79 80 Your code should use only the functions declared in xz.h. The rest of 81 the .h files are meant only for internal use in XZ Embedded. 82 83 You may want to modify xz_config.h to be more suitable for your build 84 environment. Probably you should at least skim through it even if the 85 default file works as is. 86 87Integrity check support 88 89 XZ Embedded always supports the integrity check types None and 90 CRC32. Support for CRC64 is optional. SHA-256 is currently not 91 supported in XZ Embedded although the .xz format does support it. 92 The xz tool from XZ Utils uses CRC64 by default, but CRC32 is usually 93 enough in embedded systems to keep the code size smaller. 94 95 If you want support for CRC64, you need to copy linux/lib/xz/xz_crc64.c 96 into your application, and #define XZ_USE_CRC64 in xz_config.h or in 97 compiler flags. 98 99 When using the internal CRC32 or CRC64, their lookup tables need to be 100 initialized with xz_crc32_init() and xz_crc64_init(), respectively. 101 See xz.h for details. 102 103 To use external CRC32 or CRC64 code instead of the code from 104 xz_crc32.c or xz_crc64.c, the following #defines may be used 105 in xz_config.h or in compiler flags: 106 107 #define XZ_INTERNAL_CRC32 0 108 #define XZ_INTERNAL_CRC64 0 109 110 Then it is up to you to provide compatible xz_crc32() or xz_crc64() 111 functions. 112 113 If the .xz file being decompressed uses an integrity check type that 114 isn't supported by XZ Embedded, it is treated as an error and the 115 file cannot be decompressed. For multi-call mode, this can be modified 116 by #defining XZ_DEC_ANY_CHECK. Then xz_dec_run() will return 117 XZ_UNSUPPORTED_CHECK when unsupported check type is detected. After 118 that decompression can be continued normally except that the 119 integrity check won't be verified. In single-call mode there's 120 no way to continue decoding, so XZ_DEC_ANY_CHECK is almost useless 121 in single-call mode. 122 123BCJ filter support 124 125 If you want support for one or more BCJ filters, you need to copy also 126 linux/lib/xz/xz_dec_bcj.c into your application, and use appropriate 127 #defines in xz_config.h or in compiler flags. You don't need these 128 #defines in the code that just uses XZ Embedded via xz.h, but having 129 them always #defined doesn't hurt either. 130 131 #define Instruction set BCJ filter endianness 132 XZ_DEC_X86 x86-32 or x86-64 Little endian only 133 XZ_DEC_POWERPC PowerPC Big endian only 134 XZ_DEC_IA64 Itanium (IA-64) Big or little endian 135 XZ_DEC_ARM ARM Little endian only 136 XZ_DEC_ARMTHUMB ARM-Thumb Little endian only 137 XZ_DEC_SPARC SPARC Big or little endian 138 139 While some architectures are (partially) bi-endian, the endianness 140 setting doesn't change the endianness of the instructions on all 141 architectures. That's why Itanium and SPARC filters work for both big 142 and little endian executables (Itanium has little endian instructions 143 and SPARC has big endian instructions). 144 145 There currently is no filter for little endian PowerPC or big endian 146 ARM or ARM-Thumb. Implementing filters for them can be considered if 147 there is a need for such filters in real-world applications. 148 149Notes about shared libraries 150 151 If you are including XZ Embedded into a shared library, you very 152 probably should rename the xz_* functions to prevent symbol 153 conflicts in case your library is linked against some other library 154 or application that also has XZ Embedded in it (which may even be 155 a different version of XZ Embedded). TODO: Provide an easy way 156 to do this. 157 158 Please don't create a shared library of XZ Embedded itself unless 159 it is fine to rebuild everything depending on that shared library 160 everytime you upgrade to a newer version of XZ Embedded. There are 161 no API or ABI stability guarantees between different versions of 162 XZ Embedded. 163 164