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# 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.

## 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 not further 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.

## 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.

## 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, some pseudo-random variations are inserted into the block, and compression is restarted. Lower values of `workFactor` reduce the tolerance of compression to repetitive data. You should set this parameter carefully; too low, and compression ratio suffers, 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 randomisation process is entirely transparent. If the library decides to randomise and restart compression on a block, it does so without comment. Randomised blocks are automatically de-randomised during decompression, so data integrity is never compromised.

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
             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.

## High-level interface

This interface provides functions for reading and writing `bzip2` format files. First, some general points.

- All of the functions take an `int*` first argument, `bzerror`. After each call, `bzerror` should be consulted first to determine the outcome of the call. If `bzerror` is `BZ_OK`, the call completed successfully, and only then should the return value of the function (if any) be consulted. If `bzerror` is `BZ_IO_ERROR`, there was an error reading/writing the underlying compressed file, and you should then consult `errno`/`perror` to determine the cause of the difficulty. `bzerror` may also be set to various other values; precise details are given on a per-function basis below.
- If `bzerror` indicates an error (ie, anything except `BZ_OK` and `BZ_STREAM_END`), you should immediately call `bzReadClose` (or `bzWriteClose`, depending on whether you are attempting to read or to write) to free up all resources associated with the stream. Once an error has been indicated, behaviour of all calls except `bzReadClose` (`bzWriteClose`) is undefined. The implication is that (1) `bzerror` should be checked after each call, and (2) if `bzerror` indicates an error, `bzReadClose` (`bzWriteClose`) should then be called to clean up.
- The `FILE*` arguments passed to `bzReadOpen`/`bzWriteOpen` should be set to binary mode. Most Unix systems will do this by default, but other platforms, including Windows and Mac, will not. If you omit this, you may encounter problems when moving code to new platforms.
- Memory allocation requests are handled by `malloc`/`free`. At present there is no facility for user-defined memory allocators in the file I/O functions (could easily be added, though).

### `bzReadOpen`

       typedef void BZFILE;

       BZFILE *bzReadOpen ( int *bzerror, FILE *f,
                            int small, int verbosity,
                            void *unused, int nUnused );

Prepare to read compressed data from file handle `f`. `f` should refer to a file which has been opened for reading, and for which the error indicator (`ferror(f)`)is not set. If `small` is 1, the library will try to decompress using less memory, at the expense of speed.

For reasons explained below, `bzRead` will decompress the `nUnused` bytes starting at `unused`, before starting to read from the file `f`. At most `BZ_MAX_UNUSED` bytes may be supplied like this. If this facility is not required, you should pass `NULL` and `0` for `unused` and n`Unused` respectively.

For the meaning of parameters `small` and `verbosity`, see `bzDecompressInit`.

The amount of memory needed to decompress a file cannot be determined until the file's header has been read. So it is possible that `bzReadOpen` returns `BZ_OK` but a subsequent call of `bzRead` will return `BZ_MEM_ERROR`.

Possible assignments to `bzerror`:

          BZ_PARAM_ERROR
             if f is NULL
             or small is neither 0 nor 1
             or (unused == NULL && nUnused != 0)
             or (unused != NULL && !(0 <= nUnused <= BZ_MAX_UNUSED))
          BZ_IO_ERROR
             if ferror(f) is nonzero
          BZ_MEM_ERROR
             if insufficient memory is available
          BZ_OK
             otherwise.

Possible return values:

          Pointer to an abstract BZFILE
             if bzerror is BZ_OK
          NULL
             otherwise

Allowable next actions:

          bzRead
             if bzerror is BZ_OK
          bzClose
             otherwise

### `bzRead`

       int bzRead ( int *bzerror, BZFILE *b, void *buf, int len );

Reads up to `len` (uncompressed) bytes from the compressed file `b` into the buffer `buf`. If the read was successful, `bzerror` is set to `BZ_OK` and the number of bytes read is returned. If the logical end-of-stream was detected, `bzerror` will be set to `BZ_STREAM_END`, and the number of bytes read is returned. All other `bzerror` values denote an error.

`bzRead` will supply `len` bytes, unless the logical stream end is detected or an error occurs. Because of this, it is possible to detect the stream end by observing when the number of bytes returned is less than the number requested. Nevertheless, this is regarded as inadvisable; you should instead check `bzerror` after every call and watch out for `BZ_STREAM_END`.

Internally, `bzRead` copies data from the compressed file in chunks of size `BZ_MAX_UNUSED` bytes before decompressing it. If the file contains more bytes than strictly needed to reach the logical end-of-stream, `bzRead` will almost certainly read some of the trailing data before signalling `BZ_SEQUENCE_END`. To collect the read but unused data once `BZ_SEQUENCE_END` has appeared, call `bzReadGetUnused` immediately before `bzReadClose`.

Possible assignments to `bzerror`:

          BZ_PARAM_ERROR
             if b is NULL or buf is NULL or len < 0
          BZ_SEQUENCE_ERROR
             if b was opened with bzWriteOpen
          BZ_IO_ERROR
             if there is an error reading from the compressed file
          BZ_UNEXPECTED_EOF
             if the compressed file ended before the logical end-of-stream was detected
          BZ_DATA_ERROR
             if a data integrity error was detected in the compressed stream
          BZ_DATA_ERROR_MAGIC
             if the stream does not begin with the requisite header bytes (ie, is not
             a bzip2 data file).  This is really a special case of BZ_DATA_ERROR.
          BZ_MEM_ERROR
             if insufficient memory was available
          BZ_STREAM_END
             if the logical end of stream was detected.
          BZ_OK
             otherwise.

Possible return values:

          number of bytes read
             if bzerror is BZ_OK or BZ_STREAM_END
          undefined
             otherwise

Allowable next actions:

          collect data from buf, then bzRead or bzReadClose
             if bzerror is BZ_OK
          collect data from buf, then bzReadClose or bzReadGetUnused
             if bzerror is BZ_SEQUENCE_END
          bzReadClose
             otherwise

### `bzReadGetUnused`

       void bzReadGetUnused ( int* bzerror, BZFILE *b,
                              void** unused, int* nUnused );

Returns data which was read from the compressed file but was not needed to get to the logical end-of-stream. `*unused` is set to the address of the data, and `*nUnused` to the number of bytes. `*nUnused` will be set to a value between `0` and `BZ_MAX_UNUSED` inclusive.

This function may only be called once `bzRead` has signalled `BZ_STREAM_END` but before `bzReadClose`.

Possible assignments to `bzerror`:

          BZ_PARAM_ERROR
             if b is NULL
             or unused is NULL or nUnused is NULL
          BZ_SEQUENCE_ERROR
             if BZ_STREAM_END has not been signalled
             or if b was opened with bzWriteOpen
         BZ_OK
             otherwise

Allowable next actions:

          bzReadClose

### `bzReadClose`

       void bzReadClose ( int *bzerror, BZFILE *b );

Releases all memory pertaining to the compressed file `b`. `bzReadClose` does not call `fclose` on the underlying file handle, so you should do that yourself if appropriate. `bzReadClose` should be called to clean up after all error situations.

Possible assignments to `bzerror`:

          BZ_SEQUENCE_ERROR
             if b was opened with bzOpenWrite
          BZ_OK
             otherwise

Allowable next actions:

          none

### `bzWriteOpen`

       BZFILE *bzWriteOpen ( int *bzerror, FILE *f,
                             int blockSize100k, int verbosity,
                             int workFactor );

Prepare to write compressed data to file handle `f`. `f` should refer to a file which has been opened for writing, and for which the error indicator (`ferror(f)`)is not set.

For the meaning of parameters `blockSize100k`, `verbosity` and `workFactor`, see
`bzCompressInit`.

All required memory is allocated at this stage, so if the call completes successfully, `BZ_MEM_ERROR` cannot be signalled by a subsequent call to `bzWrite`.

Possible assignments to `bzerror`:

          BZ_PARAM_ERROR
             if f is NULL
             or blockSize100k < 1 or blockSize100k > 9
          BZ_IO_ERROR
             if ferror(f) is nonzero
          BZ_MEM_ERROR
             if insufficient memory is available
          BZ_OK
             otherwise

Possible return values:

          Pointer to an abstract BZFILE
             if bzerror is BZ_OK
          NULL
             otherwise

Allowable next actions:

          bzWrite
             if bzerror is BZ_OK
             (you could go directly to bzWriteClose, but this would be pretty pointless)
          bzWriteClose
             otherwise

### `bzWrite`

       void bzWrite ( int *bzerror, BZFILE *b, void *buf, int len );

Absorbs `len` bytes from the buffer `buf`, eventually to be compressed and written to the file.

Possible assignments to `bzerror`:

          BZ_PARAM_ERROR
             if b is NULL or buf is NULL or len < 0
          BZ_SEQUENCE_ERROR
             if b was opened with bzReadOpen
          BZ_IO_ERROR
             if there is an error writing the compressed file.
          BZ_OK
             otherwise

### `bzWriteClose`

       int bzWriteClose ( int *bzerror, BZFILE* f,
                          int abandon,
                          unsigned int* nbytes_in,
                          unsigned int* nbytes_out );

Compresses and flushes to the compressed file all data so far supplied by `bzWrite`. The logical end-of-stream markers are also written, so subsequent calls to `bzWrite` are illegal. All memory associated with the compressed file `b` is released. `fflush` is called on the compressed file, but it is not `fclose`'d.

If `bzWriteClose` is called to clean up after an error, the only action is to release the memory. The library records the error codes issued by previous calls, so this situation will be detected automatically. There is no attempt to complete the compression operation, nor to `fflush` the compressed file. You can force this behaviour to happen even in the case of no error, by passing a nonzero value to `abandon`.

If `nbytes_in` is non-null, `*nbytes_in` will be set to be the total volume of uncompressed data handled. Similarly, `nbytes_out` will be set to the total volume of compressed data written.

Possible assignments to `bzerror`:

          BZ_SEQUENCE_ERROR
             if b was opened with bzReadOpen
          BZ_IO_ERROR
             if there is an error writing the compressed file
          BZ_OK
             otherwise

### Handling embedded compressed data streams

The high-level library facilitates use of `bzip2` data streams which form some part of a surrounding, larger data stream.

- For writing, the library takes an open file handle, writes compressed data to it, `fflush`es it but does not `fclose` it. The calling application can write its own data before and after the compressed data stream, using that same file handle.
- Reading is more complex, and the facilities are not as general as they could be since generality is hard to reconcile with efficiency. `bzRead` reads from the compressed file in blocks of size `BZ_MAX_UNUSED` bytes, and in doing so probably will overshoot the logical end of compressed stream. To recover this data once decompression has ended, call `bzReadGetUnused` after the last call of `bzRead` (the one returning `BZ_STREAM_END`) but before calling `bzReadClose`.

This mechanism makes it easy to decompress multiple `bzip2` streams placed end-to-end. As the end of one stream, when `bzRead` returns `BZ_STREAM_END`, call `bzReadGetUnused` to collect the unused data (copy it into your own buffer somewhere). That data forms the start of the next compressed stream. To start uncompressing that next stream, call `bzReadOpen` again, feeding in the unused data via the `unused`/`nUnused` parameters. Keep doing this until `BZ_STREAM_END` return coincides with the physical end of file (`feof(f)`). In this situation `bzReadGetUnused` will of course return no data.

This should give some feel for how the high-level interface can be used. If you require extra flexibility, you'll have to bite the bullet and get to grips with the low-level interface.

### Standard file-reading/writing code

Here's how you'd write data to a compressed file:

    FILE*   f;
    BZFILE* b;
    int     nBuf;
    char    buf[ /* whatever size you like */ ];
    int     bzerror;
    int     nWritten;

    f = fopen ( "myfile.bz2", "w" );
    if (!f) {
       /* handle error */
    }
    b = bzWriteOpen ( &bzerror, f, 9 );
    if (bzerror != BZ_OK) {
       bzWriteClose ( b );
       /* handle error */
    }

    while ( /* condition */ ) {
       /* get data to write into buf, and set nBuf appropriately */
       nWritten = bzWrite ( &bzerror, b, buf, nBuf );
       if (bzerror == BZ_IO_ERROR) {
          bzWriteClose ( &bzerror, b );
          /* handle error */
       }
    }

    bzWriteClose ( &bzerror, b );
    if (bzerror == BZ_IO_ERROR) {
       /* handle error */
    }

And to read from a compressed file:

    FILE*   f;
    BZFILE* b;
    int     nBuf;
    char    buf[ /* whatever size you like */ ];
    int     bzerror;
    int     nWritten;

    f = fopen ( "myfile.bz2", "r" );
    if (!f) {
       /* handle error */
    }
    b = bzReadOpen ( &bzerror, f, 0, NULL, 0 );
    if (bzerror != BZ_OK) {
       bzReadClose ( &bzerror, b );
       /* handle error */
    }

    bzerror = BZ_OK;
    while (bzerror == BZ_OK && /* arbitrary other conditions */) {
       nBuf = bzRead ( &bzerror, b, buf, /* size of buf */ );
       if (bzerror == BZ_OK) {
          /* do something with buf[0 .. nBuf-1] */
       }
    }
    if (bzerror != BZ_STREAM_END) {
       bzReadClose ( &bzerror, b );
       /* handle error */
    } else {
       bzReadClose ( &bzerror );
    }

## Utility functions

### `bzBuffToBuffCompress`

       int bzBuffToBuffCompress( char*         dest,
                                 unsigned int* destLen,
                                 char*         source,
                                 unsigned int  sourceLen,
                                 int           blockSize100k,
                                 int           verbosity,
                                 int           workFactor );

Attempts to compress the data in `source[0 .. sourceLen-1]` into the destination buffer, `dest[0 .. *destLen-1]`. If the destination buffer is big enough, `*destLen` is set to the size of the compressed data, and `BZ_OK` is returned. If the compressed data won't fit, `*destLen` is unchanged, and `BZ_OUTBUFF_FULL` is returned.

Compression in this manner is a one-shot event, done with a single call to this function. The resulting compressed data is a complete `bzip2` format data stream. There is no mechanism for making additional calls to provide extra input data. If you want that kind of mechanism, use the low-level interface.

For the meaning of parameters `blockSize100k`, `verbosity` and `workFactor`,
see `bzCompressInit`.

To guarantee that the compressed data will fit in its buffer, allocate an output buffer of size 1% larger than the uncompressed data, plus fifty bytes.

`bzBuffToBuffDecompress` will not write data at or beyond `dest[*destLen]`, even in case of buffer overflow.

Possible return values:

          BZ_PARAM_ERROR
             if dest is NULL or destLen is NULL
             or blockSize100k < 1 or blockSize100k > 9
             or verbosity < 0 or verbosity > 4
             or workFactor < 0 or workFactor > 250
          BZ_MEM_ERROR
             if insufficient memory is available
          BZ_OUTBUFF_FULL
             if the size of the compressed data exceeds *destLen
          BZ_OK
             otherwise

### `bzBuffToBuffDecompress`

       int bzBuffToBuffDecompress ( char*         dest,
                                    unsigned int* destLen,
                                    char*         source,
                                    unsigned int  sourceLen,
                                    int           small,
                                    int           verbosity );

Attempts to decompress the data in `source[0 .. sourceLen-1]` into the destination buffer, `dest[0 .. *destLen-1]`. If the destination buffer is big enough, `*destLen` is set to the size of the uncompressed data, and `BZ_OK` is returned. If the compressed data won't fit, `*destLen` is unchanged, and `BZ_OUTBUFF_FULL` is returned.

`source` is assumed to hold a complete `bzip2` format data stream. `bzBuffToBuffDecompress` tries to decompress the entirety of the stream into the output buffer.

For the meaning of parameters `small` and `verbosity`, see `bzDecompressInit`.

Because the compression ratio of the compressed data cannot be known in advance, there is no easy way to guarantee that the output buffer will be big enough. You may of course make arrangements in your code to record the size of the uncompressed data, but such a mechanism is beyond the scope of this library.

`bzBuffToBuffDecompress` will not write data at or beyond `dest[*destLen]`, even in case of buffer overflow.

Possible return values:

          BZ_PARAM_ERROR
             if dest is NULL or destLen is NULL
             or small != 0 && small != 1
             or verbosity < 0 or verbosity > 4
          BZ_MEM_ERROR
             if insufficient memory is available
          BZ_OUTBUFF_FULL
             if the size of the compressed data exceeds *destLen
          BZ_DATA_ERROR
             if a data integrity error was detected in the compressed data
          BZ_DATA_ERROR_MAGIC
             if the compressed data doesn't begin with the right magic bytes
          BZ_UNEXPECTED_EOF
             if the compressed data ends unexpectedly
          BZ_OK
             otherwise

## Using the library in a `stdio`-free environment

### Getting rid of `stdio`

In a deeply embedded application, you might want to use just the memory-to-memory functions. You can do this conveniently by compiling the library with preprocessor symbol `BZ_NO_STDIO` defined. Doing this gives you a library containing only the following eight functions:

`bzCompressInit`, `bzCompress`, `bzCompressEnd`
`bzDecompressInit`, `bzDecompress`, `bzDecompressEnd`
`bzBuffToBuffCompress`, `bzBuffToBuffDecompress`

When compiled like this, all functions will ignore `verbosity` settings.

### Critical error handling

`libbzip2` contains a number of internal assertion checks which should, needless to say, never be activated. Nevertheless, if an assertion should fail, behaviour depends on whether or not the library was compiled with `BZ_NO_STDIO` set.

For a normal compile, an assertion failure yields the message

       bzip2/libbzip2, v0.9.0: internal error number N.
       This is a bug in bzip2/libbzip2, v0.9.0.  Please report
       it to me at: jseward@acm.org.  If this happened when
       you were using some program which uses libbzip2 as a
       component, you should also report this bug to the author(s)
       of that program.  Please make an effort to report this bug;
       timely and accurate bug reports eventually lead to higher
       quality software.  Thx.  Julian Seward, 27 June 1998.

where `N` is some error code number. `exit(3)` is then called.

For a `stdio`-free library, assertion failures result in a call to a function declared as:

       extern void bz_internal_error ( int errcode );

The relevant code is passed as a parameter. You should supply such a function.

In either case, once an assertion failure has occurred, any `bz_stream` records involved can be regarded as invalid. You should not attempt to resume normal operation with them.

You may, of course, change critical error handling to suit your needs. As I said above, critical errors indicate bugs in the library and should not occur. All "normal" error situations are indicated via error return codes from functions, and can be recovered from.

## Making a Windows DLL

Everything related to Windows has been contributed by Yoshioka Tsuneo
(`QWF00133@niftyserve.or.jp` / `tsuneo-y@is.aist-nara.ac.jp`), so you should send your queries to him (but perhaps Cc: me, `jseward@acm.org`).

My vague understanding of what to do is: using Visual C++ 5.0, open the project file `libbz2.dsp`, and build. That's all.

If you can't open the project file for some reason, make a new one, naming these files: `blocksort.c`, `bzlib.c`, `compress.c`, `crctable.c`, `decompress.c`, `huffman.c`,
`randtable.c` and `libbz2.def`. You might also need to name the header files `bzlib.h` and `bzlib_private.h`.

If you don't use VC++, you may need to define the proprocessor symbol `_WIN32`.

Finally, `dlltest.c` is a sample program using the DLL. It has a project file, `dlltest.dsp`.

I haven't tried any of this stuff myself, but it all looks plausible.

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