139 lines
6.8 KiB
Text
139 lines
6.8 KiB
Text
Description
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Clzip is a C language version of lzip, fully compatible with lzip 1.4 or
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newer. As clzip is written in C, it may be easier to integrate in
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applications like package managers, embedded devices, or systems lacking a
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C++ compiler.
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Lzip is a lossless data compressor with a user interface similar to the one
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of gzip or bzip2. Lzip uses a simplified form of the 'Lempel-Ziv-Markov
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chain-Algorithm' (LZMA) stream format, chosen to maximize safety and
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interoperability. Lzip can compress about as fast as gzip (lzip -0) or
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compress most files more than bzip2 (lzip -9). Decompression speed is
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intermediate between gzip and bzip2. Lzip is better than gzip and bzip2 from
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a data recovery perspective. Lzip has been designed, written, and tested
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with great care to replace gzip and bzip2 as the standard general-purpose
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compressed format for unix-like systems.
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For compressing/decompressing large files on multiprocessor machines plzip
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can be much faster than lzip at the cost of a slightly reduced compression
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ratio.
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For creation and manipulation of compressed tar archives tarlz can be more
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efficient than using tar and plzip because tarlz is able to keep the
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alignment between tar members and lzip members.
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The lzip file format is designed for data sharing and long-term archiving,
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taking into account both data integrity and decoder availability:
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* The lzip format provides very safe integrity checking and some data
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recovery means. The program lziprecover can repair bit flip errors
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(one of the most common forms of data corruption) in lzip files, and
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provides data recovery capabilities, including error-checked merging
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of damaged copies of a file.
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* The lzip format is as simple as possible (but not simpler). The lzip
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manual provides the source code of a simple decompressor along with a
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detailed explanation of how it works, so that with the only help of the
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lzip manual it would be possible for a digital archaeologist to extract
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the data from a lzip file long after quantum computers eventually
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render LZMA obsolete.
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* Additionally the lzip reference implementation is copylefted, which
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guarantees that it will remain free forever.
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A nice feature of the lzip format is that a corrupt byte is easier to repair
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the nearer it is from the beginning of the file. Therefore, with the help of
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lziprecover, losing an entire archive just because of a corrupt byte near
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the beginning is a thing of the past.
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Clzip uses the same well-defined exit status values used by bzip2, which
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makes it safer than compressors returning ambiguous warning values (like
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gzip) when it is used as a back end for other programs like tar or zutils.
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Clzip will automatically use for each file the largest dictionary size that
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does not exceed neither the file size nor the limit given. Keep in mind that
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the decompression memory requirement is affected at compression time by the
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choice of dictionary size limit.
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The amount of memory required for compression is about 1 or 2 times the
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dictionary size limit (1 if input file size is less than dictionary size
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limit, else 2) plus 9 times the dictionary size really used. The option '-0'
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is special and only requires about 1.5 MiB at most. The amount of memory
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required for decompression is about 46 kB larger than the dictionary size
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really used.
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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 either the
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option '-c' or the option '-o' 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. Clzip will refuse to read compressed data
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from a terminal or write compressed data to a terminal, as this would be
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entirely incomprehensible and might leave the terminal in an abnormal state.
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Clzip will correctly decompress a file which is the concatenation of two or
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more compressed files. The result is the concatenation of the corresponding
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decompressed files. Integrity testing of concatenated compressed files is
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also supported.
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Clzip can produce multimember files, and lziprecover can safely recover the
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undamaged members in case of file damage. Clzip can also split the compressed
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output in volumes of a given size, even when reading from standard input.
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This allows the direct creation of 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 multimember output. The members so created are large,
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about 2 PiB each.
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In spite of its name (Lempel-Ziv-Markov chain-Algorithm), LZMA is not a
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concrete algorithm; it is more like "any algorithm using the LZMA coding
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scheme". For example, the option '-0' of lzip uses the scheme in almost the
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simplest way possible; issuing the longest match it can find, or a literal
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byte if it can't find a match. Inversely, a much more elaborated way of
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finding coding sequences of minimum size than the one currently used by lzip
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could be developed, and the resulting sequence could also be coded using the
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LZMA coding scheme.
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Clzip currently implements two variants of the LZMA algorithm; fast
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(used by option '-0') and normal (used by all other compression levels).
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The high compression of LZMA comes from combining two basic, well-proven
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compression ideas: sliding dictionaries (LZ77/78) and markov models (the
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thing used by every compression algorithm that uses a range encoder or
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similar order-0 entropy coder as its last stage) with segregation of
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contexts according to what the bits 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 the
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definition of Markov chains), G.N.N. Martin (for the definition of range
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encoding), Igor Pavlov (for putting all the above together in LZMA), and
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Julian Seward (for bzip2's CLI).
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LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never have
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been compressed. Decompressed is used to refer to data which have undergone
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the process of decompression.
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Copyright (C) 2010-2021 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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