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This is Info file bzip2.info, produced by Makeinfo version 1.68 from the input file manual.texi. START-INFO-DIR-ENTRY * Bzip2: (bzip2). A program and library for data compression. END-INFO-DIR-ENTRY File: bzip2.info, Node: Top, Next: Overview, Prev: (dir), Up: (dir) This program, `bzip2', and associated library `libbzip2', are Copyright (C) 1996-1999 Julian R Seward. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. * The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required. * Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software. * The name of the author may not be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE AUTHOR "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. Julian Seward, Cambridge, UK. `jseward@acm.org' `http://www.muraroa.demon.co.uk' `bzip2'/`libbzip2' version 0.9.5 of 24 May 1999. PATENTS: To the best of my knowledge, `bzip2' does not use any patented algorithms. However, I do not have the resources available to carry out a full patent search. Therefore I cannot give any guarantee of the above statement. * Menu: * Overview:: An overview of bzip2 * How to use bzip2:: Invoking bzip2 * Programming with libbzip2:: Using the libbz library * Miscellanea:: Random thoughts -- The Detailed Node Listing -- Programming with `libbzip2' * Top-level structure:: * Error handling:: * Low-level interface:: * High-level interface:: * Utility functions:: * zlib compatibility functions:: * Using the library in a stdio-free environment:: * Making a Windows DLL:: Miscellanea * Limitations of the compressed file format:: * Portability issues:: * Reporting bugs:: * Did you get the right package?:: * Testing:: * Further reading:: File: bzip2.info, Node: Overview, Next: How to use bzip2, Prev: Top, Up: Top Introduction ************ `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. `bzip2' is built on top of `libbzip2', a flexible library for handling compressed data in the `bzip2' format. This manual describes both how to use the program and how to work with the library interface. Most of the manual is devoted to this library, not the program, which is good news if your interest is only in the program. Chapter 2 describes how to use `bzip2'; this is the only part you need to read if you just want to know how to operate the program. Chapter 3 describes the programming interfaces in detail, and Chapter 4 records some miscellaneous notes which I thought ought to be recorded somewhere. * Menu: * How to use bzip2:: * Programming with libbzip2:: * Miscellanea:: File: bzip2.info, Node: How to use bzip2, Next: Programming with libbzip2, Prev: Overview, Up: Top How to use `bzip2' ****************** This chapter contains a copy of the `bzip2' man page, and nothing else. NAME * `bzip2', `bunzip2' - a block-sorting file compressor, v0.9.5 * `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 statistical 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 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. `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 decompression from standard input to standard output. `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 `bzip2' version 0.9.0 or later. Earlier versions of `bzip2' will stop after decompressing 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 corruption 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 invokation 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. `-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 verbosity 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 to -9' Set the block size to 100 k, 200 k .. 900 k when compressing. Has no effect when decompressing. See MEMORY MANAGEMENT below. `--' 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'. `--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. 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) respectively. 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. 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 RECOVERING DATA FROM DAMAGED FILES .................................. `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. `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 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. `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. 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. `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 `bzip2' will perform 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 0.9.5 of `bzip2'. Compressed data created by this version is entirely forwards and backwards compatible with the previous public releases, versions 0.1pl2 and 0.9.0, 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' 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. AUTHOR ...... Julian Seward, `jseward@acm.org'. The ideas embodied in `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 `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 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. File: bzip2.info, Node: Programming with libbzip2, Next: Miscellanea, Prev: How to use bzip2, Up: Top Programming with `libbzip2' *************************** This chapter describes the programming interface to `libbzip2'. For general background information, particularly about memory use and performance aspects, you'd be well advised to read Chapter 2 as well. * Menu: * Top-level structure:: Functions and data structures * Error handling:: * Low-level interface:: * High-level interface:: * Utility functions:: * zlib compatibility functions:: * Using the library in a stdio-free environment:: * Making a Windows DLL:: File: bzip2.info, Node: Top-level structure, Next: Error handling, Prev: Programming with libbzip2, Up: Programming with libbzip2 Top-level structure =================== `libbzip2' is a flexible library for compressing and decompressing data in the `bzip2' data format. Although packaged as a single entity, it helps to regard the library as three separate parts: the low level interface, and the high level interface, and some utility functions. The structure of `libbzip2''s interfaces is similar to that of Jean-loup Gailly's and Mark Adler's excellent `zlib' library. Low-level summary ----------------- This interface provides services for compressing and decompressing data in memory. There's no provision for dealing with files, streams or any other I/O mechanisms, just straight memory-to-memory work. In fact, this part of the library can be compiled without inclusion of `stdio.h', which may be helpful for embedded applications. The low-level part of the library has no global variables and is therefore thread-safe. Six routines make up the low level interface: `bzCompressInit', `bzCompress', and `bzCompressEnd' for compression, and a corresponding trio `bzDecompressInit', `bzDecompress' and `bzDecompressEnd' for decompression. The `*Init' functions allocate memory for compression/decompression and do other initialisations, whilst the `*End' functions close down operations and release memory. The real work is done by `bzCompress' and `bzDecompress'. These compress/decompress data from a user-supplied input buffer to a user-supplied output buffer. These buffers can be any size; arbitrary quantities of data are handled by making repeated calls to these functions. This is a flexible mechanism allowing a consumer-pull style of activity, or producer-push, or a mixture of both. High-level summary ------------------ This interface provides some handy wrappers around the low-level interface to facilitate reading and writing `bzip2' format files (`.bz2' files). The routines provide hooks to facilitate reading files in which the `bzip2' data stream is embedded within some larger-scale file structure, or where there are multiple `bzip2' data streams concatenated end-to-end. For reading files, `bzReadOpen', `bzRead', `bzReadClose' and `bzReadGetUnused' are supplied. For writing files, `bzWriteOpen', `bzWrite' and `bzWriteFinish' are available. As with the low-level library, no global variables are used so the library is per se thread-safe. However, if I/O errors occur whilst reading or writing the underlying compressed files, you may have to consult `errno' to determine the cause of the error. In that case, you'd need a C library which correctly supports `errno' in a multithreaded environment. To make the library a little simpler and more portable, `bzReadOpen' and `bzWriteOpen' require you to pass them file handles (`FILE*'s) which have previously been opened for reading or writing respectively. That avoids portability problems associated with file operations and file attributes, whilst not being much of an imposition on the programmer. Utility functions summary ------------------------- For very simple needs, `bzBuffToBuffCompress' and `bzBuffToBuffDecompress' are provided. These compress data in memory from one buffer to another buffer in a single function call. You should assess whether these functions fulfill your memory-to-memory compression/decompression requirements before investing effort in understanding the more general but more complex low-level interface. Yoshioka Tsuneo (`QWF00133@niftyserve.or.jp' / `tsuneo-y@is.aist-nara.ac.jp') has contributed some functions to give better `zlib' compatibility. These functions are `bzopen', `bzread', `bzwrite', `bzflush', `bzclose', `bzerror' and `bzlibVersion'. You may find these functions more convenient for simple file reading and writing, than those in the high-level interface. These functions are not (yet) officially part of the library, and are minimally documented here. If they break, you get to keep all the pieces. I hope to document them properly when time permits. Yoshioka also contributed modifications to allow the library to be built as a Windows DLL. File: bzip2.info, Node: Error handling, Next: Low-level interface, Prev: Top-level structure, Up: Programming with libbzip2 Error handling ============== The library is designed to recover cleanly in all situations, including the worst-case situation of decompressing random data. I'm not 100% sure that it can always do this, so you might want to add a signal handler to catch segmentation violations during decompression if you are feeling especially paranoid. I would be interested in hearing more about the robustness of the library to corrupted compressed data. The file `bzlib.h' contains all definitions needed to use the library. In particular, you should definitely not include `bzlib_private.h'. In `bzlib.h', the various return values are defined. The following list is not intended as an exhaustive description of the circumstances in which a given value may be returned - those descriptions are given later. Rather, it is intended to convey the rough meaning of each return value. The first five actions are normal and not intended to denote an error situation. `BZ_OK' The requested action was completed successfully. `BZ_RUN_OK' `BZ_FLUSH_OK' `BZ_FINISH_OK' In `bzCompress', the requested flush/finish/nothing-special action was completed successfully. `BZ_STREAM_END' Compression of data was completed, or the logical stream end was detected during decompression. The following return values indicate an error of some kind. `BZ_SEQUENCE_ERROR' When using the library, it is important to call the functions in the correct sequence and with data structures (buffers etc) in the correct states. `libbzip2' checks as much as it can to ensure this is happening, and returns `BZ_SEQUENCE_ERROR' if not. Code which complies precisely with the function semantics, as detailed below, should never receive this value; such an event denotes buggy code which you should investigate. `BZ_PARAM_ERROR' Returned when a parameter to a function call is out of range or otherwise manifestly incorrect. As with `BZ_SEQUENCE_ERROR', this denotes a bug in the client code. The distinction between `BZ_PARAM_ERROR' and `BZ_SEQUENCE_ERROR' is a bit hazy, but still worth making. `BZ_MEM_ERROR' Returned when a request to allocate memory failed. Note that the quantity of memory needed to decompress a stream cannot be determined until the stream's header has been read. So `bzDecompress' and `bzRead' may return `BZ_MEM_ERROR' even though some of the compressed data has been read. The same is not true for compression; once `bzCompressInit' or `bzWriteOpen' have successfully completed, `BZ_MEM_ERROR' cannot occur. `BZ_DATA_ERROR' Returned when a data integrity error is detected during decompression. Most importantly, this means when stored and computed CRCs for the data do not match. This value is also returned upon detection of any other anomaly in the compressed data. `BZ_DATA_ERROR_MAGIC' As a special case of `BZ_DATA_ERROR', it is sometimes useful to know when the compressed stream does not start with the correct magic bytes (`'B' 'Z' 'h''). `BZ_IO_ERROR' Returned by `bzRead' and `bzRead' when there is an error reading or writing in the compressed file, and by `bzReadOpen' and `bzWriteOpen' for attempts to use a file for which the error indicator (viz, `ferror(f)') is set. On receipt of `BZ_IO_ERROR', the caller should consult `errno' and/or `perror' to acquire operating-system specific information about the problem. `BZ_UNEXPECTED_EOF' Returned by `bzRead' when the compressed file finishes before the logical end of stream is detected. `BZ_OUTBUFF_FULL' Returned by `bzBuffToBuffCompress' and `bzBuffToBuffDecompress' to indicate that the output data will not fit into the output buffer provided. File: bzip2.info, Node: Low-level interface, Next: High-level interface, Prev: Error handling, Up: Programming with libbzip2 Low-level interface =================== `bzCompressInit' ---------------- typedef struct { char *next_in; unsigned int avail_in; unsigned int total_in; char *next_out; unsigned int avail_out; unsigned int total_out; void *state; void *(*bzalloc)(void *,int,int); void (*bzfree)(void *,void *); void *opaque; } bz_stream; int bzCompressInit ( bz_stream *strm, int blockSize100k, int verbosity, int workFactor ); Prepares for compression. The `bz_stream' structure holds all data pertaining to the compression activity. A `bz_stream' structure should be allocated and initialised prior to the call. The fields of `bz_stream' comprise the entirety of the user-visible data. `state' is a pointer to the private data structures required for compression. Custom memory allocators are supported, via fields `bzalloc', `bzfree', and `opaque'. The value `opaque' is passed to as the first argument to all calls to `bzalloc' and `bzfree', but is otherwise ignored by the library. The call `bzalloc ( opaque, n, m )' is expected to return a pointer `p' to `n * m' bytes of memory, and `bzfree ( opaque, p )' should free that memory. If you don't want to use a custom memory allocator, set `bzalloc', `bzfree' and `opaque' to `NULL', and the library will then use the standard `malloc'/`free' routines. Before calling `bzCompressInit', fields `bzalloc', `bzfree' and `opaque' should be filled appropriately, as just described. Upon return, the internal state will have been allocated and initialised, and `total_in' and `total_out' will have been set to zero. These last two fields are used by the library to inform the caller of the total amount of data passed into and out of the library, respectively. You should not try to change them. Parameter `blockSize100k' specifies the block size to be used for compression. It should be a value between 1 and 9 inclusive, and the actual block size used is 100000 x this figure. 9 gives the best compression but takes most memory. Parameter `verbosity' should be set to a number between 0 and 4 inclusive. 0 is silent, and greater numbers give increasingly verbose monitoring/debugging output. If the library has been compiled with `-DBZ_NO_STDIO', no such output will appear for any verbosity setting. Parameter `workFactor' controls how the compression phase behaves when presented with worst case, highly repetitive, input data. If compression runs into difficulties caused by repetitive data, the library switches from the standard sorting algorithm to a fallback algorithm. The fallback is slower than the standard algorithm by perhaps a factor of three, but always behaves reasonably, no matter how bad the input. Lower values of `workFactor' reduce the amount of effort the standard algorithm will expend before resorting to the fallback. You should set this parameter carefully; too low, and many inputs will be handled by the fallback algorithm and so compress rather slowly, too high, and your average-to-worst case compression times can become very large. The default value of 30 gives reasonable behaviour over a wide range of circumstances. Allowable values range from 0 to 250 inclusive. 0 is a special case, equivalent to using the default value of 30. Note that the compressed output generated is the same regardless of whether or not the fallback algorithm is used. Be aware also that this parameter may disappear entirely in future versions of the library. In principle it should be possible to devise a good way to automatically choose which algorithm to use. Such a mechanism would render the parameter obsolete. Possible return values: `BZ_PARAM_ERROR' if `strm' is `NULL' or `blockSize' < 1 or `blockSize' > 9 or `verbosity' < 0 or `verbosity' > 4 or `workFactor' < 0 or `workFactor' > 250 `BZ_MEM_ERROR' if not enough memory is available `BZ_OK' otherwise Allowable next actions: `bzCompress' if `BZ_OK' is returned no specific action needed in case of error `bzCompress' ------------ int bzCompress ( bz_stream *strm, int action ); Provides more input and/or output buffer space for the library. The caller maintains input and output buffers, and calls `bzCompress' to transfer data between them. Before each call to `bzCompress', `next_in' should point at the data to be compressed, and `avail_in' should indicate how many bytes the library may read. `bzCompress' updates `next_in', `avail_in' and `total_in' to reflect the number of bytes it has read. Similarly, `next_out' should point to a buffer in which the compressed data is to be placed, with `avail_out' indicating how much output space is available. `bzCompress' updates `next_out', `avail_out' and `total_out' to reflect the number of bytes output. You may provide and remove as little or as much data as you like on each call of `bzCompress'. In the limit, it is acceptable to supply and remove data one byte at a time, although this would be terribly inefficient. You should always ensure that at least one byte of output space is available at each call. A second purpose of `bzCompress' is to request a change of mode of the compressed stream. Conceptually, a compressed stream can be in one of four states: IDLE, RUNNING, FLUSHING and FINISHING. Before initialisation (`bzCompressInit') and after termination (`bzCompressEnd'), a stream is regarded as IDLE. Upon initialisation (`bzCompressInit'), the stream is placed in the RUNNING state. Subsequent calls to `bzCompress' should pass `BZ_RUN' as the requested action; other actions are illegal and will result in `BZ_SEQUENCE_ERROR'. At some point, the calling program will have provided all the input data it wants to. It will then want to finish up - in effect, asking the library to process any data it might have buffered internally. In this state, `bzCompress' will no longer attempt to read data from `next_in', but it will want to write data to `next_out'. Because the output buffer supplied by the user can be arbitrarily small, the finishing-up operation cannot necessarily be done with a single call of `bzCompress'. Instead, the calling program passes `BZ_FINISH' as an action to `bzCompress'. This changes the stream's state to FINISHING. Any remaining input (ie, `next_in[0 .. avail_in-1]') is compressed and transferred to the output buffer. To do this, `bzCompress' must be called repeatedly until all the output has been consumed. At that point, `bzCompress' returns `BZ_STREAM_END', and the stream's state is set back to IDLE. `bzCompressEnd' should then be called. Just to make sure the calling program does not cheat, the library makes a note of `avail_in' at the time of the first call to `bzCompress' which has `BZ_FINISH' as an action (ie, at the time the program has announced its intention to not supply any more input). By comparing this value with that of `avail_in' over subsequent calls to `bzCompress', the library can detect any attempts to slip in more data to compress. Any calls for which this is detected will return `BZ_SEQUENCE_ERROR'. This indicates a programming mistake which should be corrected. Instead of asking to finish, the calling program may ask `bzCompress' to take all the remaining input, compress it and terminate the current (Burrows-Wheeler) compression block. This could be useful for error control purposes. The mechanism is analogous to that for finishing: call `bzCompress' with an action of `BZ_FLUSH', remove output data, and persist with the `BZ_FLUSH' action until the value `BZ_RUN' is returned. As with finishing, `bzCompress' detects any attempt to provide more input data once the flush has begun. Once the flush is complete, the stream returns to the normal RUNNING state. This all sounds pretty complex, but isn't really. Here's a table which shows which actions are allowable in each state, what action will be taken, what the next state is, and what the non-error return values are. Note that you can't explicitly ask what state the stream is in, but nor do you need to - it can be inferred from the values returned by `bzCompress'. IDLE/`any' Illegal. IDLE state only exists after `bzCompressEnd' or before `bzCompressInit'. Return value = `BZ_SEQUENCE_ERROR' RUNNING/`BZ_RUN' Compress from `next_in' to `next_out' as much as possible. Next state = RUNNING Return value = `BZ_RUN_OK' RUNNING/`BZ_FLUSH' Remember current value of `next_in'. Compress from `next_in' to `next_out' as much as possible, but do not accept any more input. Next state = FLUSHING Return value = `BZ_FLUSH_OK' RUNNING/`BZ_FINISH' Remember current value of `next_in'. Compress from `next_in' to `next_out' as much as possible, but do not accept any more input. Next state = FINISHING Return value = `BZ_FINISH_OK' FLUSHING/`BZ_FLUSH' Compress from `next_in' to `next_out' as much as possible, but do not accept any more input. If all the existing input has been used up and all compressed output has been removed Next state = RUNNING; Return value = `BZ_RUN_OK' else Next state = FLUSHING; Return value = `BZ_FLUSH_OK' FLUSHING/other Illegal. Return value = `BZ_SEQUENCE_ERROR' FINISHING/`BZ_FINISH' Compress from `next_in' to `next_out' as much as possible, but to not accept any more input. If all the existing input has been used up and all compressed output has been removed Next state = IDLE; Return value = `BZ_STREAM_END' else Next state = FINISHING; Return value = `BZ_FINISHING' FINISHING/other Illegal. Return value = `BZ_SEQUENCE_ERROR' That still looks complicated? Well, fair enough. The usual sequence of calls for compressing a load of data is: * Get started with `bzCompressInit'. * Shovel data in and shlurp out its compressed form using zero or more calls of `bzCompress' with action = `BZ_RUN'. * Finish up. Repeatedly call `bzCompress' with action = `BZ_FINISH', copying out the compressed output, until `BZ_STREAM_END' is returned. * Close up and go home. Call `bzCompressEnd'. If the data you want to compress fits into your input buffer all at once, you can skip the calls of `bzCompress ( ..., BZ_RUN )' and just do the `bzCompress ( ..., BZ_FINISH )' calls. All required memory is allocated by `bzCompressInit'. The compression library can accept any data at all (obviously). So you shouldn't get any error return values from the `bzCompress' calls. If you do, they will be `BZ_SEQUENCE_ERROR', and indicate a bug in your programming. Trivial other possible return values: `BZ_PARAM_ERROR' if `strm' is `NULL', or `strm->s' is `NULL' `bzCompressEnd' --------------- int bzCompressEnd ( bz_stream *strm ); Releases all memory associated with a compression stream. Possible return values: `BZ_PARAM_ERROR' if `strm' is `NULL' or `strm->s' is `NULL' `BZ_OK' otherwise `bzDecompressInit' ------------------ int bzDecompressInit ( bz_stream *strm, int verbosity, int small ); Prepares for decompression. As with `bzCompressInit', a `bz_stream' record should be allocated and initialised before the call. Fields `bzalloc', `bzfree' and `opaque' should be set if a custom memory allocator is required, or made `NULL' for the normal `malloc'/`free' routines. Upon return, the internal state will have been initialised, and `total_in' and `total_out' will be zero. For the meaning of parameter `verbosity', see `bzCompressInit'. If `small' is nonzero, the library will use an alternative decompression algorithm which uses less memory but at the cost of decompressing more slowly (roughly speaking, half the speed, but the maximum memory requirement drops to around 2300k). See Chapter 2 for more information on memory management. Note that the amount of memory needed to decompress a stream cannot be determined until the stream's header has been read, so even if `bzDecompressInit' succeeds, a subsequent `bzDecompress' could fail with `BZ_MEM_ERROR'. Possible return values: `BZ_PARAM_ERROR' if `(small != 0 && small != 1)' or `(verbosity < 0 || verbosity > 4)' `BZ_MEM_ERROR' if insufficient memory is available Allowable next actions: `bzDecompress' if `BZ_OK' was returned no specific action required in case of error `bzDecompress' -------------- int bzDecompress ( bz_stream *strm ); Provides more input and/out output buffer space for the library. The caller maintains input and output buffers, and uses `bzDecompress' to transfer data between them. Before each call to `bzDecompress', `next_in' should point at the compressed data, and `avail_in' should indicate how many bytes the library may read. `bzDecompress' updates `next_in', `avail_in' and `total_in' to reflect the number of bytes it has read. Similarly, `next_out' should point to a buffer in which the uncompressed output is to be placed, with `avail_out' indicating how much output space is available. `bzCompress' updates `next_out', `avail_out' and `total_out' to reflect the number of bytes output. You may provide and remove as little or as much data as you like on each call of `bzDecompress'. In the limit, it is acceptable to supply and remove data one byte at a time, although this would be terribly inefficient. You should always ensure that at least one byte of output space is available at each call. Use of `bzDecompress' is simpler than `bzCompress'. You should provide input and remove output as described above, and repeatedly call `bzDecompress' until `BZ_STREAM_END' is returned. Appearance of `BZ_STREAM_END' denotes that `bzDecompress' has detected the logical end of the compressed stream. `bzDecompress' will not produce `BZ_STREAM_END' until all output data has been placed into the output buffer, so once `BZ_STREAM_END' appears, you are guaranteed to have available all the decompressed output, and `bzDecompressEnd' can safely be called. If case of an error return value, you should call `bzDecompressEnd' to clean up and release memory. Possible return values: `BZ_PARAM_ERROR' if `strm' is `NULL' or `strm->s' is `NULL' or `strm->avail_out < 1' `BZ_DATA_ERROR' if a data integrity error is detected in the compressed stream `BZ_DATA_ERROR_MAGIC' if the compressed stream doesn't begin with the right magic bytes `BZ_MEM_ERROR' if there wasn't enough memory available `BZ_STREAM_END' if the logical end of the data stream was detected and all output in has been consumed, eg `s->avail_out > 0' `BZ_OK' otherwise Allowable next actions: `bzDecompress' if `BZ_OK' was returned `bzDecompressEnd' otherwise `bzDecompressEnd' ----------------- int bzDecompressEnd ( bz_stream *strm ); Releases all memory associated with a decompression stream. Possible return values: `BZ_PARAM_ERROR' if `strm' is `NULL' or `strm->s' is `NULL' `BZ_OK' otherwise Allowable next actions: None.