README
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