The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/contrib/xz-embedded/

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Folder linux/ 2019-04-22 08:11:37
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File COPYING 322 bytes 2019-04-22 08:11:37
File README 5408 bytes 2019-04-22 08:11:38

    1 
    2 XZ 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.
   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 
   18 Compiling 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 
   49 Compiler 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 
   62 Embedding 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 
   87 BCJ filter support
   88 
   89     If you want support for one or more BCJ filters, you need to copy also
   90     linux/lib/xz/xz_dec_bcj.c into your application, and use appropriate
   91     #defines in xz_config.h or in compiler flags. You don't need these
   92     #defines in the code that just uses XZ Embedded via xz.h, but having
   93     them always #defined doesn't hurt either.
   94 
   95         #define             Instruction set     BCJ filter endianness
   96         XZ_DEC_X86          x86-32 or x86-64    Little endian only
   97         XZ_DEC_POWERPC      PowerPC             Big endian only
   98         XZ_DEC_IA64         Itanium (IA-64)     Big or little endian
   99         XZ_DEC_ARM          ARM                 Little endian only
  100         XZ_DEC_ARMTHUMB     ARM-Thumb           Little endian only
  101         XZ_DEC_SPARC        SPARC               Big or little endian
  102 
  103     While some architectures are (partially) bi-endian, the endianness
  104     setting doesn't change the endianness of the instructions on all
  105     architectures. That's why Itanium and SPARC filters work for both big
  106     and little endian executables (Itanium has little endian instructions
  107     and SPARC has big endian instructions).
  108 
  109     There currently is no filter for little endian PowerPC or big endian
  110     ARM or ARM-Thumb. Implementing filters for them can be considered if
  111     there is a need for such filters in real-world applications.
  112 
  113 Notes about shared libraries
  114 
  115     If you are including XZ Embedded into a shared library, you very
  116     probably should rename the xz_* functions to prevent symbol
  117     conflicts in case your library is linked against some other library
  118     or application that also has XZ Embedded in it (which may even be
  119     a different version of XZ Embedded). TODO: Provide an easy way
  120     to do this.
  121 
  122     Please don't create a shared library of XZ Embedded itself unless
  123     it is fine to rebuild everything depending on that shared library
  124     everytime you upgrade to a newer version of XZ Embedded. There are
  125     no API or ABI stability guarantees between different versions of
  126     XZ Embedded.
  127 

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