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diff --git a/bzlib-src/bzip2.txt b/bzlib-src/bzip2.txt new file mode 100644 index 0000000..d2deb39 --- /dev/null +++ b/bzlib-src/bzip2.txt @@ -0,0 +1,391 @@ + +NAME + bzip2, bunzip2 - a block-sorting file compressor, v1.0.6 + bzcat - decompresses files to stdout + bzip2recover - recovers data from damaged bzip2 files + + +SYNOPSIS + bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ] + bunzip2 [ -fkvsVL ] [ filenames ... ] + bzcat [ -s ] [ filenames ... ] + bzip2recover filename + + +DESCRIPTION + 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 sta- + tistical compressors. + + The command-line options are deliberately very similar to + those of GNU gzip, but they are not identical. + + bzip2 expects a list of file names to accompany the com- + mand-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, per- + missions, and, when possible, ownership as the correspond- + ing 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 preserv- + ing original file names, permissions, ownerships or dates + in filesystems which lack these concepts, or have serious + file name length restrictions, such as MS-DOS. + + bzip2 and bunzip2 will by default not overwrite existing + files. If you want this to happen, specify the -f flag. + + If no file names are specified, bzip2 compresses from + standard input to standard output. In this case, bzip2 + will decline to write compressed output to a terminal, as + this would be entirely incomprehensible and therefore + pointless. + + bunzip2 (or bzip2 -d) decompresses all specified files. + Files which were not created by bzip2 will be detected and + ignored, and a warning issued. 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, + .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it cannot + guess the name of the original file, and uses the original + name with .out appended. + + As with compression, supplying no filenames causes decom- + pression from standard input to standard output. + + bunzip2 will correctly decompress a file which is the con- + catenation 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 com- + pressed and decompressed like this. The resulting outputs + are fed sequentially to stdout. Compression of multiple + files in this manner generates a stream containing multi- + ple compressed file representations. Such a stream can be + decompressed correctly only by bzip2 version 0.9.0 or + later. Earlier versions of bzip2 will stop after decom- + pressing the first file in the stream. + + bzcat (or bzip2 -dc) decompresses all specified files to + the standard output. + + bzip2 will read arguments from the environment variables + BZIP2 and 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, bzip2 uses 32-bit + CRCs to make sure that the decompressed version of a file + is identical to the original. This guards against corrup- + tion of the compressed data, and against undetected bugs + in 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 + 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 bzip2 to panic. + + +OPTIONS + -c --stdout + Compress or decompress to standard output. + + -d --decompress + Force decompression. bzip2, bunzip2 and 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 bzip2 to decompress. + + -z --compress + The complement to -d: forces compression, + regardless of the invocation name. + + -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. + + -f --force + Force overwrite of output files. Normally, bzip2 + will not overwrite existing output files. Also + forces bzip2 to break hard links to files, which it + otherwise wouldn't do. + + bzip2 normally declines to decompress files which + don't have the correct magic header bytes. If + forced (-f), however, it will pass such files + through unmodified. This is how GNU gzip behaves. + + -k --keep + Keep (don't delete) input files during compression + or decompression. + + -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. + + -q --quiet + Suppress non-essential warning messages. Messages + pertaining to I/O errors and other critical events + will not be suppressed. + + -v --verbose + Verbose mode -- show the compression ratio for each + file processed. Further -v's increase the ver- + bosity level, spewing out lots of information which + is primarily of interest for diagnostic purposes. + + -L --license -V --version + Display the software version, license terms and + conditions. + + -1 (or --fast) to -9 (or --best) + Set the block size to 100 k, 200 k .. 900 k when + compressing. Has no effect when decompressing. + See MEMORY MANAGEMENT below. The --fast and --best + aliases are primarily for GNU gzip compatibility. + In particular, --fast doesn't make things signifi- + cantly faster. And --best merely selects the + default behaviour. + + -- Treats all subsequent arguments as file names, even + if they start with a dash. This is so you can han- + dle files with names beginning with a dash, for + example: bzip2 -- -myfilename. + + --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 ver- + sions, which was sometimes useful. 0.9.5 and above + have an improved algorithm which renders these + flags irrelevant. + + +MEMORY MANAGEMENT + 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) respec- + tively. At decompression time, the block size used for + compression is read from the header of the compressed + file, and 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 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, + bunzip2 will require about 3700 kbytes to decompress. To + support decompression of any file on a 4 megabyte machine, + bunzip2 has an option to decompress using approximately + half this amount of memory, about 2300 kbytes. Decompres- + sion 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 con- + straints allow, since that maximises the compression + achieved. Compression and decompression speed are virtu- + ally 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 Compres- + sion 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 domi- + nated 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 + + +RECOVERING DATA FROM DAMAGED FILES + bzip2 compresses files in blocks, usually 900kbytes long. + Each block is handled independently. If a media or trans- + mission 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. + + 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 bzip2 -t to test + the integrity of the resulting files, and decompress those + which are undamaged. + + bzip2recover takes a single argument, the name of the dam- + aged file, and writes a number of files + "rec00001file.bz2", "rec00002file.bz2", etc, containing + the extracted blocks. The output filenames are + designed so that the use of wildcards in subsequent pro- + cessing -- for example, "bzip2 -dc rec*file.bz2 > recov- + ered_data" -- processes the files in the correct order. + + 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 min- + imise any potential data loss through media or transmis- + sion errors, you might consider compressing with a smaller + block size. + + +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 mon- + itor progress in great detail, if you want. + + Decompression speed is unaffected by these phenomena. + + bzip2 usually allocates several megabytes of memory to + operate in, and then charges all over it in a fairly ran- + dom fashion. This means that performance, both for com- + pressing 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 bzip2 will per- + form best on machines with very large caches. + + +CAVEATS + I/O error messages are not as helpful as they could be. + 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 1.0.6 of bzip2. Com- + pressed data created by this version is entirely forwards + and backwards compatible with the previous public + releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1, + 1.0.2 and above, 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. + + bzip2recover versions prior to 1.0.2 used 32-bit integers + to represent bit positions in compressed files, so they + could not handle compressed files more than 512 megabytes + long. Versions 1.0.2 and above use 64-bit ints on some + platforms which support them (GNU supported targets, and + Windows). To establish whether or not bzip2recover was + built with such a limitation, run it without arguments. + In any event you can build yourself an unlimited version + if you can recompile it with MaybeUInt64 set to be an + unsigned 64-bit integer. + + +AUTHOR + Julian Seward, jsewardbzip.org. + + http://www.bzip.org + + The ideas embodied in bzip2 are due to (at least) the fol- + lowing people: Michael Burrows and David Wheeler (for the + block sorting transformation), David Wheeler (again, for + the Huffman coder), Peter Fenwick (for the structured cod- + ing model in the original bzip, and many refinements), and + Alistair Moffat, Radford Neal and Ian Witten (for the + arithmetic coder in the original bzip). I am much + indebted for their help, support and advice. See the man- + ual 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 compres- + sion. Bela Lubkin encouraged me to improve the worst-case + compression performance. Donna Robinson XMLised the docu- + mentation. The bz* scripts are derived from those of GNU + gzip. Many people sent patches, helped with portability + problems, lent machines, gave advice and were generally + helpful. + |