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diff --git a/doc/clzip.texi b/doc/clzip.texi new file mode 100644 index 0000000..25869a0 --- /dev/null +++ b/doc/clzip.texi @@ -0,0 +1,574 @@ +\input texinfo @c -*-texinfo-*- +@c %**start of header +@setfilename clzip.info +@documentencoding ISO-8859-15 +@settitle Clzip Manual +@finalout +@c %**end of header + +@set UPDATED 30 January 2014 +@set VERSION 1.6-pre1 + +@dircategory Data Compression +@direntry +* Clzip: (clzip). LZMA lossless data compressor +@end direntry + + +@ifnothtml +@titlepage +@title Clzip +@subtitle LZMA lossless data compressor +@subtitle for Clzip version @value{VERSION}, @value{UPDATED} +@author by Antonio Diaz Diaz + +@page +@vskip 0pt plus 1filll +@end titlepage + +@contents +@end ifnothtml + +@node Top +@top + +This manual is for Clzip (version @value{VERSION}, @value{UPDATED}). + +@menu +* Introduction:: Purpose and features of clzip +* Algorithm:: How clzip compresses the data +* Invoking clzip:: Command line interface +* File format:: Detailed format of the compressed file +* Examples:: A small tutorial with examples +* Problems:: Reporting bugs +* Concept index:: Index of concepts +@end menu + +@sp 1 +Copyright @copyright{} 2010, 2011, 2012, 2013, 2014 Antonio Diaz Diaz. + +This manual is free documentation: you have unlimited permission +to copy, distribute and modify it. + + +@node Introduction +@chapter Introduction +@cindex introduction + +Clzip is a lossless data compressor with a user interface similar to the +one of gzip or bzip2. Clzip decompresses almost as fast as gzip, +compresses most files more than bzip2, and is better than both from a +data recovery perspective. Clzip is a clean implementation of the LZMA +algorithm. + +Clzip uses the lzip file format; the files produced by clzip are fully +compatible with lzip-1.4 or newer, and can be rescued with lziprecover. +Clzip is in fact a C language version of lzip, intended for embedded +devices or systems lacking a C++ compiler. + +The lzip file format is designed for long-term data archiving and +provides very safe integrity checking. It is as simple as possible (but +not simpler), so that with the only help of the lzip manual it would be +possible for a digital archaeologist to extract the data from a lzip +file long after quantum computers eventually render LZMA obsolete. +Additionally lzip is copylefted, which guarantees that it will remain +free forever. + +The member trailer stores the 32-bit CRC of the original data, the size +of the original data and the size of the member. These values, together +with the value remaining in the range decoder and the end-of-stream +marker, provide a 4 factor integrity checking which guarantees that the +decompressed version of the data is identical to the original. This +guards against corruption of the compressed data, and against undetected +bugs in clzip (hopefully very unlikely). The chances of data corruption +going undetected are microscopic. Be aware, though, that the check +occurs upon decompression, so it can only tell you that something is +wrong. It can't help you recover the original uncompressed data. + +If you ever need to recover data from a damaged lzip file, try the +lziprecover program. Lziprecover makes lzip files resistant to bit-flip +(one of the most common forms of data corruption), and provides data +recovery capabilities, including error-checked merging of damaged copies +of a file. + +Clzip uses the same well-defined exit status values used by lzip and +bzip2, which makes it safer than compressors returning ambiguous warning +values (like gzip) when it is used as a back end for tar or zutils. + +When compressing, clzip replaces every file given in the command line +with a compressed version of itself, with the name "original_name.lz". +When decompressing, clzip attempts to guess the name for the decompressed +file from that of the compressed file as follows: + +@multitable {anyothername} {becomes} {anyothername.out} +@item filename.lz @tab becomes @tab filename +@item filename.tlz @tab becomes @tab filename.tar +@item anyothername @tab becomes @tab anyothername.out +@end multitable + +(De)compressing a file is much like copying or moving it; therefore clzip +preserves the access and modification dates, permissions, and, when +possible, ownership of the file just as "cp -p" does. (If the user ID or +the group ID can't be duplicated, the file permission bits S_ISUID and +S_ISGID are cleared). + +Clzip is able to read from some types of non regular files if the +@samp{--stdout} option is specified. + +If no file names are specified, clzip compresses (or decompresses) from +standard input to standard output. In this case, clzip will decline to +write compressed output to a terminal, as this would be entirely +incomprehensible and therefore pointless. + +Clzip will correctly decompress a file which is the concatenation of two +or more compressed files. The result is the concatenation of the +corresponding uncompressed files. Integrity testing of concatenated +compressed files is also supported. + +Clzip can produce multi-member files and safely recover, with +lziprecover, the undamaged members in case of file damage. Clzip can +also split the compressed output in volumes of a given size, even when +reading from standard input. This allows the direct creation of +multivolume compressed tar archives. + +Clzip is able to compress and decompress streams of unlimited size by +automatically creating multi-member output. The members so created are +large, about 64 PiB each. + +The amount of memory required for compression is about 1 or 2 times the +dictionary size limit (1 if input file size is less than dictionary size +limit, else 2) plus 9 times the dictionary size really used. The amount +of memory required for decompression is about 46 kB larger than the +dictionary size really used. + +Clzip will automatically use the smallest possible dictionary size +without exceeding the given limit. Keep in mind that the decompression +memory requirement is affected at compression time by the choice of +dictionary size limit. + + +@node Algorithm +@chapter Algorithm +@cindex algorithm + +Clzip implements a simplified version of the LZMA (Lempel-Ziv-Markov +chain-Algorithm) algorithm. The high compression of LZMA comes from +combining two basic, well-proven compression ideas: sliding dictionaries +(LZ77/78) and markov models (the thing used by every compression +algorithm that uses a range encoder or similar order-0 entropy coder as +its last stage) with segregation of contexts according to what the bits +are used for. + +Clzip is a two stage compressor. The first stage is a Lempel-Ziv coder, +which reduces redundancy by translating chunks of data to their +corresponding distance-length pairs. The second stage is a range encoder +that uses a different probability model for each type of data; +distances, lengths, literal bytes, etc. + +The match finder, part of the LZ coder, is the most important piece of +the LZMA algorithm, as it is in many Lempel-Ziv based algorithms. Most +of clzip's execution time is spent in the match finder, and it has the +greatest influence on the compression ratio. + +Here is how it works, step by step: + +1) The member header is written to the output stream. + +2) The first byte is coded literally, because there are no previous +bytes to which the match finder can refer to. + +3) The main encoder advances to the next byte in the input data and +calls the match finder. + +4) The match finder fills an array with the minimum distances before the +current byte where a match of a given length can be found. + +5) Go back to step 3 until a sequence (formed of pairs, repeated +distances and literal bytes) of minimum price has been formed. Where the +price represents the number of output bits produced. + +6) The range encoder encodes the sequence produced by the main encoder +and sends the produced bytes to the output stream. + +7) Go back to step 3 until the input data are finished or until the +member or volume size limits are reached. + +8) The range encoder is flushed. + +9) The member trailer is written to the output stream. + +10) If there are more data to compress, go back to step 1. + +@sp 1 +@noindent +The ideas embodied in clzip are due to (at least) the following people: +Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrey Markov (for +the definition of Markov chains), G.N.N. Martin (for the definition of +range encoding), Igor Pavlov (for putting all the above together in +LZMA), and Julian Seward (for bzip2's CLI). + + +@node Invoking clzip +@chapter Invoking clzip +@cindex invoking +@cindex options +@cindex usage +@cindex version + +The format for running clzip is: + +@example +clzip [@var{options}] [@var{files}] +@end example + +Clzip supports the following options: + +@table @samp +@item -h +@itemx --help +Print an informative help message describing the options and exit. + +@item -V +@itemx --version +Print the version number of clzip on the standard output and exit. + +@item -b @var{bytes} +@itemx --member-size=@var{bytes} +Set the member size limit to @var{bytes}. A small member size may +degrade compression ratio, so use it only when needed. Valid values +range from 100 kB to 64 PiB. Defaults to 64 PiB. + +@item -c +@itemx --stdout +Compress or decompress to standard output. Needed when reading from a +named pipe (fifo) or from a device. Use it to recover as much of the +uncompressed data as possible when decompressing a corrupt file. + +@item -d +@itemx --decompress +Decompress. + +@item -f +@itemx --force +Force overwrite of output files. + +@item -F +@itemx --recompress +Force recompression of files whose name already has the @samp{.lz} or +@samp{.tlz} suffix. + +@item -k +@itemx --keep +Keep (don't delete) input files during compression or decompression. + +@item -m @var{bytes} +@itemx --match-length=@var{bytes} +Set the match length limit in bytes. After a match this long is found, +the search is finished. Valid values range from 5 to 273. Larger values +usually give better compression ratios but longer compression times. + +@item -o @var{file} +@itemx --output=@var{file} +When reading from standard input and @samp{--stdout} has not been +specified, use @samp{@var{file}} as the virtual name of the uncompressed +file. This produces a file named @samp{@var{file}} when decompressing, a +file named @samp{@var{file}.lz} when compressing, and several files +named @samp{@var{file}00001.lz}, @samp{@var{file}00002.lz}, etc, when +compressing and splitting the output in volumes. + +@item -q +@itemx --quiet +Quiet operation. Suppress all messages. + +@item -s @var{bytes} +@itemx --dictionary-size=@var{bytes} +Set the dictionary size limit in bytes. Valid values range from 4 KiB to +512 MiB. Clzip will use the smallest possible dictionary size for each +member without exceeding this limit. Note that dictionary sizes are +quantized. If the specified size does not match one of the valid sizes, +it will be rounded upwards by adding up to (@var{bytes} / 16) to it. + +For maximum compression you should use a dictionary size limit as large +as possible, but keep in mind that the decompression memory requirement +is affected at compression time by the choice of dictionary size limit. + +@item -S @var{bytes} +@itemx --volume-size=@var{bytes} +Split the compressed output into several volume files with names +@samp{original_name00001.lz}, @samp{original_name00002.lz}, etc, and set +the volume size limit to @var{bytes}. Each volume is a complete, maybe +multi-member, lzip file. A small volume size may degrade compression +ratio, so use it only when needed. Valid values range from 100 kB to 4 +EiB. + +@item -t +@itemx --test +Check integrity of the specified file(s), but don't decompress them. +This really performs a trial decompression and throws away the result. +Use it together with @samp{-v} to see information about the file. + +@item -v +@itemx --verbose +Verbose mode.@* +When compressing, show the compression ratio for each file processed. A +second @samp{-v} shows the progress of compression.@* +When decompressing or testing, further -v's (up to 4) increase the +verbosity level, showing status, compression ratio, dictionary size, +and trailer contents (CRC, data size, member size). + +@item -1 .. -9 +Set the compression parameters (dictionary size and match length limit) +as shown in the table below. Note that @samp{-9} can be much slower than +@samp{-1}. These options have no effect when decompressing. + +The bidimensional parameter space of LZMA can't be mapped to a linear +scale optimal for all files. If your files are large, very repetitive, +etc, you may need to use the @samp{--match-length} and +@samp{--dictionary-size} options directly to achieve optimal +performance. For example, @samp{-9m64} usually compresses executables +more (and faster) than @samp{-9}. + +@multitable {Level} {Dictionary size} {Match length limit} +@item Level @tab Dictionary size @tab Match length limit +@item -1 @tab 1 MiB @tab 5 bytes +@item -2 @tab 1.5 MiB @tab 6 bytes +@item -3 @tab 2 MiB @tab 8 bytes +@item -4 @tab 3 MiB @tab 12 bytes +@item -5 @tab 4 MiB @tab 20 bytes +@item -6 @tab 8 MiB @tab 36 bytes +@item -7 @tab 16 MiB @tab 68 bytes +@item -8 @tab 24 MiB @tab 132 bytes +@item -9 @tab 32 MiB @tab 273 bytes +@end multitable + +@item --fast +@itemx --best +Aliases for GNU gzip compatibility. + +@end table + +Numbers given as arguments to options may be followed by a multiplier +and an optional @samp{B} for "byte". + +Table of SI and binary prefixes (unit multipliers): + +@multitable {Prefix} {kilobyte (10^3 = 1000)} {|} {Prefix} {kibibyte (2^10 = 1024)} +@item Prefix @tab Value @tab | @tab Prefix @tab Value +@item k @tab kilobyte (10^3 = 1000) @tab | @tab Ki @tab kibibyte (2^10 = 1024) +@item M @tab megabyte (10^6) @tab | @tab Mi @tab mebibyte (2^20) +@item G @tab gigabyte (10^9) @tab | @tab Gi @tab gibibyte (2^30) +@item T @tab terabyte (10^12) @tab | @tab Ti @tab tebibyte (2^40) +@item P @tab petabyte (10^15) @tab | @tab Pi @tab pebibyte (2^50) +@item E @tab exabyte (10^18) @tab | @tab Ei @tab exbibyte (2^60) +@item Z @tab zettabyte (10^21) @tab | @tab Zi @tab zebibyte (2^70) +@item Y @tab yottabyte (10^24) @tab | @tab Yi @tab yobibyte (2^80) +@end multitable + +@sp 1 +Exit status: 0 for a normal exit, 1 for environmental problems (file not +found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or +invalid input file, 3 for an internal consistency error (eg, bug) which +caused clzip to panic. + + +@node File format +@chapter File format +@cindex file format + +Perfection is reached, not when there is no longer anything to add, but +when there is no longer anything to take away.@* +--- Antoine de Saint-Exupery + +@sp 1 +In the diagram below, a box like this: +@verbatim ++---+ +| | <-- the vertical bars might be missing ++---+ +@end verbatim + +represents one byte; a box like this: +@verbatim ++==============+ +| | ++==============+ +@end verbatim + +represents a variable number of bytes. + +@sp 1 +A lzip file consists of a series of "members" (compressed data sets). +The members simply appear one after another in the file, with no +additional information before, between, or after them. + +Each member has the following structure: +@verbatim ++--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| ID string | VN | DS | Lzma stream | CRC32 | Data size | Member size | ++--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +@end verbatim + +All multibyte values are stored in little endian order. + +@table @samp +@item ID string +A four byte string, identifying the lzip format, with the value "LZIP" +(0x4C, 0x5A, 0x49, 0x50). + +@item VN (version number, 1 byte) +Just in case something needs to be modified in the future. 1 for now. + +@item DS (coded dictionary size, 1 byte) +Lzip divides the distance between any two powers of 2 into 8 equally +spaced intervals, named "wedges". The dictionary size is calculated by +taking a power of 2 (the base size) and substracting from it a number of +wedges between 0 and 7. The size of a wedge is (base_size / 16).@* +Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).@* +Bits 7-5 contain the number of wedges (0 to 7) to substract from the +base size to obtain the dictionary size.@* +Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB@* +Valid values for dictionary size range from 4 KiB to 512 MiB. + +@item Lzma stream +The lzma stream, finished by an end of stream marker. Uses default +values for encoder properties. See the lzip manual for a full +description. + +@item CRC32 (4 bytes) +CRC of the uncompressed original data. + +@item Data size (8 bytes) +Size of the uncompressed original data. + +@item Member size (8 bytes) +Total size of the member, including header and trailer. This field acts +as a distributed index, allows the verification of stream integrity, and +facilitates safe recovery of undamaged members from multi-member files. + +@end table + + +@node Examples +@chapter A small tutorial with examples +@cindex examples + +WARNING! Even if clzip is bug-free, other causes may result in a corrupt +compressed file (bugs in the system libraries, memory errors, etc). +Therefore, if the data you are going to compress are important, give the +@samp{--keep} option to clzip and do not remove the original file until +you verify the compressed file with a command like +@w{@samp{clzip -cd file.lz | cmp file -}}. + +@sp 1 +@noindent +Example 1: Replace a regular file with its compressed version +@samp{file.lz} and show the compression ratio. + +@example +clzip -v file +@end example + +@sp 1 +@noindent +Example 2: Like example 1 but the created @samp{file.lz} is multi-member +with a member size of 1 MiB. The compression ratio is not shown. + +@example +clzip -b 1MiB file +@end example + +@sp 1 +@noindent +Example 3: Restore a regular file from its compressed version +@samp{file.lz}. If the operation is successful, @samp{file.lz} is +removed. + +@example +clzip -d file.lz +@end example + +@sp 1 +@noindent +Example 4: Verify the integrity of the compressed file @samp{file.lz} +and show status. + +@example +clzip -tv file.lz +@end example + +@sp 1 +@noindent +Example 5: Compress a whole floppy in /dev/fd0 and send the output to +@samp{file.lz}. + +@example +clzip -c /dev/fd0 > file.lz +@end example + +@sp 1 +@noindent +Example 6: Decompress @samp{file.lz} partially until 10 KiB of +decompressed data are produced. + +@example +clzip -cd file.lz | dd bs=1024 count=10 +@end example + +@sp 1 +@noindent +Example 7: Decompress @samp{file.lz} partially from decompressed byte +10000 to decompressed byte 15000 (5000 bytes are produced). + +@example +clzip -cd file.lz | dd bs=1000 skip=10 count=5 +@end example + +@sp 1 +@noindent +Example 8: Create a multivolume compressed tar archive with a volume +size of 1440 KiB. + +@example +tar -c some_directory | clzip -S 1440KiB -o volume_name +@end example + +@sp 1 +@noindent +Example 9: Extract a multivolume compressed tar archive. + +@example +clzip -cd volume_name*.lz | tar -xf - +@end example + +@sp 1 +@noindent +Example 10: Create a multivolume compressed backup of a large database +file with a volume size of 650 MB, where each volume is a multi-member +file with a member size of 32 MiB. + +@example +clzip -b 32MiB -S 650MB big_db +@end example + + +@node Problems +@chapter Reporting bugs +@cindex bugs +@cindex getting help + +There are probably bugs in clzip. There are certainly errors and +omissions in this manual. If you report them, they will get fixed. If +you don't, no one will ever know about them and they will remain unfixed +for all eternity, if not longer. + +If you find a bug in clzip, please send electronic mail to +@email{lzip-bug@@nongnu.org}. Include the version number, which you can +find by running @w{@samp{clzip --version}}. + + +@node Concept index +@unnumbered Concept index + +@printindex cp + +@bye |