108 lines
5.1 KiB
Text
108 lines
5.1 KiB
Text
Description
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Clzip is a lossless data compressor with a user interface similar to the
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one of gzip or bzip2. Clzip decompresses almost as fast as gzip,
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compresses most files more than bzip2, and is better than both from a
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data recovery perspective. Clzip is a clean implementation of the LZMA
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algorithm.
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Clzip uses the lzip file format; the files produced by clzip are fully
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compatible with lzip-1.4 or newer, and can be rescued with lziprecover.
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Clzip is in fact a C language version of lzip, intended for embedded
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devices or systems lacking a C++ compiler.
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The lzip file format is designed for long-term data archiving and
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provides very safe integrity checking. It is as simple as possible (but
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not simpler), so that with the only help of the lzip manual it would be
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possible for a digital archaeologist to extract the data from a lzip
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file long after quantum computers eventually render LZMA obsolete.
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Additionally lzip is copylefted, which guarantees that it will remain
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free forever.
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The member trailer stores the 32-bit CRC of the original data, the size
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of the original data and the size of the member. These values, together
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with the value remaining in the range decoder and the end-of-stream
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marker, provide a 4 factor integrity checking which guarantees that the
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decompressed version of the data is identical to the original. This
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guards against corruption of the compressed data, and against undetected
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bugs in clzip (hopefully very unlikely). The chances of data corruption
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going undetected are microscopic. Be aware, though, that the check
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occurs upon decompression, so it can only tell you that something is
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wrong. It can't help you recover the original uncompressed data.
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If you ever need to recover data from a damaged lzip file, try the
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lziprecover program. Lziprecover makes lzip files resistant to bit-flip
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(one of the most common forms of data corruption), and provides data
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recovery capabilities, including error-checked merging of damaged copies
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of a file.
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Clzip uses the same well-defined exit status values used by lzip and
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bzip2, which makes it safer than compressors returning ambiguous warning
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values (like gzip) when it is used as a back end for tar or zutils.
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When compressing, clzip replaces every file given in the command line
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with a compressed version of itself, with the name "original_name.lz".
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When decompressing, clzip attempts to guess the name for the decompressed
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file from that of the compressed file as follows:
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filename.lz becomes filename
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filename.tlz becomes filename.tar
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anyothername becomes anyothername.out
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(De)compressing a file is much like copying or moving it; therefore clzip
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preserves the access and modification dates, permissions, and, when
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possible, ownership of the file just as "cp -p" does. (If the user ID or
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the group ID can't be duplicated, the file permission bits S_ISUID and
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S_ISGID are cleared).
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Clzip is able to read from some types of non regular files if the
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"--stdout" option is specified.
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If no file names are specified, clzip compresses (or decompresses) from
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standard input to standard output. In this case, clzip will decline to
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write compressed output to a terminal, as this would be entirely
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incomprehensible and therefore pointless.
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Clzip will correctly decompress a file which is the concatenation of two
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or more compressed files. The result is the concatenation of the
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corresponding uncompressed files. Integrity testing of concatenated
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compressed files is also supported.
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Clzip can produce multi-member files and safely recover, with
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lziprecover, the undamaged members in case of file damage. Clzip can
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also split the compressed output in volumes of a given size, even when
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reading from standard input. This allows the direct creation of
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multivolume compressed tar archives.
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Clzip is able to compress and decompress streams of unlimited size by
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automatically creating multi-member output. The members so created are
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large, about 64 PiB each.
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Clzip will automatically use the smallest possible dictionary size
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without exceeding the given limit. Keep in mind that the decompression
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memory requirement is affected at compression time by the choice of
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dictionary size limit.
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Clzip implements a simplified version of the LZMA (Lempel-Ziv-Markov
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chain-Algorithm) algorithm. The high compression of LZMA comes from
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combining two basic, well-proven compression ideas: sliding dictionaries
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(LZ77/78) and markov models (the thing used by every compression
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algorithm that uses a range encoder or similar order-0 entropy coder as
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its last stage) with segregation of contexts according to what the bits
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are used for.
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The ideas embodied in clzip are due to (at least) the following people:
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Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrey Markov (for
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the definition of Markov chains), G.N.N. Martin (for the definition of
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range encoding), Igor Pavlov (for putting all the above together in
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LZMA), and Julian Seward (for bzip2's CLI).
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Copyright (C) 2010, 2011, 2012, 2013, 2014 Antonio Diaz Diaz.
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This file is free documentation: you have unlimited permission to copy,
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distribute and modify it.
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The file Makefile.in is a data file used by configure to produce the
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Makefile. It has the same copyright owner and permissions that configure
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itself.
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