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diff --git a/winsup/bz2lib/bzip2.1 b/winsup/bz2lib/bzip2.1 new file mode 100644 index 0000000..7de54a0 --- /dev/null +++ b/winsup/bz2lib/bzip2.1 @@ -0,0 +1,439 @@ +.PU +.TH bzip2 1 +.SH NAME +bzip2, bunzip2 \- a block-sorting file compressor, v1.0 +.br +bzcat \- decompresses files to stdout +.br +bzip2recover \- recovers data from damaged bzip2 files + +.SH SYNOPSIS +.ll +8 +.B bzip2 +.RB [ " \-cdfkqstvzVL123456789 " ] +[ +.I "filenames \&..." +] +.ll -8 +.br +.B bunzip2 +.RB [ " \-fkvsVL " ] +[ +.I "filenames \&..." +] +.br +.B bzcat +.RB [ " \-s " ] +[ +.I "filenames \&..." +] +.br +.B bzip2recover +.I "filename" + +.SH DESCRIPTION +.I bzip2 +compresses files using the Burrows-Wheeler block sorting +text compression algorithm, and Huffman coding. Compression is +generally considerably better than that achieved by more conventional +LZ77/LZ78-based compressors, and approaches the performance of the PPM +family of statistical compressors. + +The command-line options are deliberately very similar to +those of +.I GNU gzip, +but they are not identical. + +.I bzip2 +expects a list of file names to accompany the +command-line flags. Each file is replaced by a compressed version of +itself, with the name "original_name.bz2". +Each compressed file +has the same modification date, permissions, and, when possible, +ownership as the corresponding original, so that these properties can +be correctly restored at decompression time. File name handling is +naive in the sense that there is no mechanism for preserving original +file names, permissions, ownerships or dates in filesystems which lack +these concepts, or have serious file name length restrictions, such as +MS-DOS. + +.I bzip2 +and +.I bunzip2 +will by default not overwrite existing +files. If you want this to happen, specify the \-f flag. + +If no file names are specified, +.I bzip2 +compresses from standard +input to standard output. In this case, +.I bzip2 +will decline to +write compressed output to a terminal, as this would be entirely +incomprehensible and therefore pointless. + +.I bunzip2 +(or +.I bzip2 \-d) +decompresses all +specified files. Files which were not created by +.I bzip2 +will be detected and ignored, and a warning issued. +.I bzip2 +attempts to guess the filename for the decompressed file +from that of the compressed file as follows: + + filename.bz2 becomes filename + filename.bz becomes filename + filename.tbz2 becomes filename.tar + filename.tbz becomes filename.tar + anyothername becomes anyothername.out + +If the file does not end in one of the recognised endings, +.I .bz2, +.I .bz, +.I .tbz2 +or +.I .tbz, +.I bzip2 +complains that it cannot +guess the name of the original file, and uses the original name +with +.I .out +appended. + +As with compression, supplying no +filenames causes decompression from +standard input to standard output. + +.I bunzip2 +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 (\-t) +of concatenated +compressed files is also supported. + +You can also compress or decompress files to the standard output by +giving the \-c flag. Multiple files may be compressed and +decompressed like this. The resulting outputs are fed sequentially to +stdout. Compression of multiple files +in this manner generates a stream +containing multiple compressed file representations. Such a stream +can be decompressed correctly only by +.I bzip2 +version 0.9.0 or +later. Earlier versions of +.I bzip2 +will stop after decompressing +the first file in the stream. + +.I bzcat +(or +.I bzip2 -dc) +decompresses all specified files to +the standard output. + +.I bzip2 +will read arguments from the environment variables +.I BZIP2 +and +.I BZIP, +in that order, and will process them +before any arguments read from the command line. This gives a +convenient way to supply default arguments. + +Compression is always performed, even if the compressed +file is slightly +larger than the original. Files of less than about one hundred bytes +tend to get larger, since the compression mechanism has a constant +overhead in the region of 50 bytes. Random data (including the output +of most file compressors) is coded at about 8.05 bits per byte, giving +an expansion of around 0.5%. + +As a self-check for your protection, +.I +bzip2 +uses 32-bit CRCs to +make sure that the decompressed version of a file is identical to the +original. This guards against corruption of the compressed data, and +against undetected bugs in +.I bzip2 +(hopefully very unlikely). The +chances of data corruption going undetected is microscopic, about one +chance in four billion for each file processed. 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. You can use +.I bzip2recover +to try to recover data from +damaged files. + +Return values: 0 for a normal exit, 1 for environmental problems (file +not found, invalid flags, I/O errors, &c), 2 to indicate a corrupt +compressed file, 3 for an internal consistency error (eg, bug) which +caused +.I bzip2 +to panic. + +.SH OPTIONS +.TP +.B \-c --stdout +Compress or decompress to standard output. +.TP +.B \-d --decompress +Force decompression. +.I bzip2, +.I bunzip2 +and +.I bzcat +are +really the same program, and the decision about what actions to take is +done on the basis of which name is used. This flag overrides that +mechanism, and forces +.I bzip2 +to decompress. +.TP +.B \-z --compress +The complement to \-d: forces compression, regardless of the +invokation name. +.TP +.B \-t --test +Check integrity of the specified file(s), but don't decompress them. +This really performs a trial decompression and throws away the result. +.TP +.B \-f --force +Force overwrite of output files. Normally, +.I bzip2 +will not overwrite +existing output files. Also forces +.I bzip2 +to break hard links +to files, which it otherwise wouldn't do. +.TP +.B \-k --keep +Keep (don't delete) input files during compression +or decompression. +.TP +.B \-s --small +Reduce memory usage, for compression, decompression and testing. Files +are decompressed and tested using a modified algorithm which only +requires 2.5 bytes per block byte. This means any file can be +decompressed in 2300k of memory, albeit at about half the normal speed. + +During compression, \-s selects a block size of 200k, which limits +memory use to around the same figure, at the expense of your compression +ratio. In short, if your machine is low on memory (8 megabytes or +less), use \-s for everything. See MEMORY MANAGEMENT below. +.TP +.B \-q --quiet +Suppress non-essential warning messages. Messages pertaining to +I/O errors and other critical events will not be suppressed. +.TP +.B \-v --verbose +Verbose mode -- show the compression ratio for each file processed. +Further \-v's increase the verbosity level, spewing out lots of +information which is primarily of interest for diagnostic purposes. +.TP +.B \-L --license -V --version +Display the software version, license terms and conditions. +.TP +.B \-1 to \-9 +Set the block size to 100 k, 200 k .. 900 k when compressing. Has no +effect when decompressing. See MEMORY MANAGEMENT below. +.TP +.B \-- +Treats all subsequent arguments as file names, even if they start +with a dash. This is so you can handle files with names beginning +with a dash, for example: bzip2 \-- \-myfilename. +.TP +.B \--repetitive-fast --repetitive-best +These flags are redundant in versions 0.9.5 and above. They provided +some coarse control over the behaviour of the sorting algorithm in +earlier versions, which was sometimes useful. 0.9.5 and above have an +improved algorithm which renders these flags irrelevant. + +.SH MEMORY MANAGEMENT +.I bzip2 +compresses large files in blocks. The block size affects +both the compression ratio achieved, and the amount of memory needed for +compression and decompression. The flags \-1 through \-9 +specify the block size to be 100,000 bytes through 900,000 bytes (the +default) respectively. At decompression time, the block size used for +compression is read from the header of the compressed file, and +.I bunzip2 +then allocates itself just enough memory to decompress +the file. Since block sizes are stored in compressed files, it follows +that the flags \-1 to \-9 are irrelevant to and so ignored +during decompression. + +Compression and decompression requirements, +in bytes, can be estimated as: + + Compression: 400k + ( 8 x block size ) + + Decompression: 100k + ( 4 x block size ), or + 100k + ( 2.5 x block size ) + +Larger block sizes give rapidly diminishing marginal returns. Most of +the compression comes from the first two or three hundred k of block +size, a fact worth bearing in mind when using +.I bzip2 +on small machines. +It is also important to appreciate that the decompression memory +requirement is set at compression time by the choice of block size. + +For files compressed with the default 900k block size, +.I bunzip2 +will require about 3700 kbytes to decompress. To support decompression +of any file on a 4 megabyte machine, +.I bunzip2 +has an option to +decompress using approximately half this amount of memory, about 2300 +kbytes. Decompression speed is also halved, so you should use this +option only where necessary. The relevant flag is -s. + +In general, try and use the largest block size memory constraints allow, +since that maximises the compression achieved. Compression and +decompression speed are virtually unaffected by block size. + +Another significant point applies to files which fit in a single block +-- that means most files you'd encounter using a large block size. The +amount of real memory touched is proportional to the size of the file, +since the file is smaller than a block. For example, compressing a file +20,000 bytes long with the flag -9 will cause the compressor to +allocate around 7600k of memory, but only touch 400k + 20000 * 8 = 560 +kbytes of it. Similarly, the decompressor will allocate 3700k but only +touch 100k + 20000 * 4 = 180 kbytes. + +Here is a table which summarises the maximum memory usage for different +block sizes. Also recorded is the total compressed size for 14 files of +the Calgary Text Compression Corpus totalling 3,141,622 bytes. This +column gives some feel for how compression varies with block size. +These figures tend to understate the advantage of larger block sizes for +larger files, since the Corpus is dominated by smaller files. + + Compress Decompress Decompress Corpus + Flag usage usage -s usage Size + + -1 1200k 500k 350k 914704 + -2 2000k 900k 600k 877703 + -3 2800k 1300k 850k 860338 + -4 3600k 1700k 1100k 846899 + -5 4400k 2100k 1350k 845160 + -6 5200k 2500k 1600k 838626 + -7 6100k 2900k 1850k 834096 + -8 6800k 3300k 2100k 828642 + -9 7600k 3700k 2350k 828642 + +.SH RECOVERING DATA FROM DAMAGED FILES +.I bzip2 +compresses files in blocks, usually 900kbytes long. Each +block is handled independently. If a media or transmission error causes +a multi-block .bz2 +file to become damaged, it may be possible to +recover data from the undamaged blocks in the file. + +The compressed representation of each block is delimited by a 48-bit +pattern, which makes it possible to find the block boundaries with +reasonable certainty. Each block also carries its own 32-bit CRC, so +damaged blocks can be distinguished from undamaged ones. + +.I bzip2recover +is a simple program whose purpose is to search for +blocks in .bz2 files, and write each block out into its own .bz2 +file. You can then use +.I bzip2 +\-t +to test the +integrity of the resulting files, and decompress those which are +undamaged. + +.I bzip2recover +takes a single argument, the name of the damaged file, +and writes a number of files "rec0001file.bz2", +"rec0002file.bz2", etc, containing the extracted blocks. +The output filenames are designed so that the use of +wildcards in subsequent processing -- for example, +"bzip2 -dc rec*file.bz2 > recovered_data" -- lists the files in +the correct order. + +.I bzip2recover +should be of most use dealing with large .bz2 +files, as these will contain many blocks. It is clearly +futile to use it on damaged single-block files, since a +damaged block cannot be recovered. If you wish to minimise +any potential data loss through media or transmission errors, +you might consider compressing with a smaller +block size. + +.SH PERFORMANCE NOTES +The sorting phase of compression gathers together similar strings in the +file. Because of this, files containing very long runs of repeated +symbols, like "aabaabaabaab ..." (repeated several hundred times) may +compress more slowly than normal. Versions 0.9.5 and above fare much +better than previous versions in this respect. The ratio between +worst-case and average-case compression time is in the region of 10:1. +For previous versions, this figure was more like 100:1. You can use the +\-vvvv option to monitor progress in great detail, if you want. + +Decompression speed is unaffected by these phenomena. + +.I bzip2 +usually allocates several megabytes of memory to operate +in, and then charges all over it in a fairly random fashion. This means +that performance, both for compressing and decompressing, is largely +determined by the speed at which your machine can service cache misses. +Because of this, small changes to the code to reduce the miss rate have +been observed to give disproportionately large performance improvements. +I imagine +.I bzip2 +will perform best on machines with very large caches. + +.SH CAVEATS +I/O error messages are not as helpful as they could be. +.I bzip2 +tries hard to detect I/O errors and exit cleanly, but the details of +what the problem is sometimes seem rather misleading. + +This manual page pertains to version 1.0 of +.I bzip2. +Compressed +data created by this version is entirely forwards and backwards +compatible with the previous public releases, versions 0.1pl2, 0.9.0 +and 0.9.5, +but with the following exception: 0.9.0 and above can correctly +decompress multiple concatenated compressed files. 0.1pl2 cannot do +this; it will stop after decompressing just the first file in the +stream. + +.I bzip2recover +uses 32-bit integers to represent bit positions in +compressed files, so it cannot handle compressed files more than 512 +megabytes long. This could easily be fixed. + +.SH AUTHOR +Julian Seward, jseward@acm.org. + +http://sourceware.cygnus.com/bzip2 +http://www.muraroa.demon.co.uk + +The ideas embodied in +.I bzip2 +are due to (at least) the following +people: Michael Burrows and David Wheeler (for the block sorting +transformation), David Wheeler (again, for the Huffman coder), Peter +Fenwick (for the structured coding model in the original +.I bzip, +and many refinements), and Alistair Moffat, Radford Neal and Ian Witten +(for the arithmetic coder in the original +.I bzip). +I am much +indebted for their help, support and advice. See the manual in the +source distribution for pointers to sources of documentation. Christian +von Roques encouraged me to look for faster sorting algorithms, so as to +speed up compression. Bela Lubkin encouraged me to improve the +worst-case compression performance. Many people sent patches, helped +with portability problems, lent machines, gave advice and were generally +helpful. |