zlib库WindowBits

/*
ZEXTERN int ZEXPORT deflateInit2 OF((z_streamp strm,
                                     int  level,
                                     int  method,
                                     int  windowBits,
                                     int  memLevel,
                                     int  strategy));

     This is another version of deflateInit with more compression options.  The
   fields next_in, zalloc, zfree and opaque must be initialized before by the
   caller.

     The method parameter is the compression method.  It must be Z_DEFLATED in
   this version of the library.

     The windowBits parameter is the base two logarithm of the window size
   (the size of the history buffer).  It should be in the range 8..15 for this
   version of the library.  Larger values of this parameter result in better
   compression at the expense of memory usage.  The default value is 15 if
   deflateInit is used instead.

     For the current implementation of deflate(), a windowBits value of 8 (a
   window size of 256 bytes) is not supported.  As a result, a request for 8
   will result in 9 (a 512-byte window).  In that case, providing 8 to
   inflateInit2() will result in an error when the zlib header with 9 is
   checked against the initialization of inflate().  The remedy is to not use 8
   with deflateInit2() with this initialization, or at least in that case use 9
   with inflateInit2().

     windowBits can also be -8..-15 for raw deflate.  In this case, -windowBits
   determines the window size.  deflate() will then generate raw deflate data
   with no zlib header or trailer, and will not compute a check value.

     windowBits can also be greater than 15 for optional gzip encoding.  Add
   16 to windowBits to write a simple gzip header and trailer around the
   compressed data instead of a zlib wrapper.  The gzip header will have no
   file name, no extra data, no comment, no modification time (set to zero), no
   header crc, and the operating system will be set to the appropriate value,
   if the operating system was determined at compile time.  If a gzip stream is
   being written, strm->adler is a CRC-32 instead of an Adler-32.

     For raw deflate or gzip encoding, a request for a 256-byte window is
   rejected as invalid, since only the zlib header provides a means of
   transmitting the window size to the decompressor.

     The memLevel parameter specifies how much memory should be allocated
   for the internal compression state.  memLevel=1 uses minimum memory but is
   slow and reduces compression ratio; memLevel=9 uses maximum memory for
   optimal speed.  The default value is 8.  See zconf.h for total memory usage
   as a function of windowBits and memLevel.

     The strategy parameter is used to tune the compression algorithm.  Use the
   value Z_DEFAULT_STRATEGY for normal data, Z_FILTERED for data produced by a
   filter (or predictor), Z_HUFFMAN_ONLY to force Huffman encoding only (no
   string match), or Z_RLE to limit match distances to one (run-length
   encoding).  Filtered data consists mostly of small values with a somewhat
   random distribution.  In this case, the compression algorithm is tuned to
   compress them better.  The effect of Z_FILTERED is to force more Huffman
   coding and less string matching; it is somewhat intermediate between
   Z_DEFAULT_STRATEGY and Z_HUFFMAN_ONLY.  Z_RLE is designed to be almost as
   fast as Z_HUFFMAN_ONLY, but give better compression for PNG image data.  The
   strategy parameter only affects the compression ratio but not the
   correctness of the compressed output even if it is not set appropriately.
   Z_FIXED prevents the use of dynamic Huffman codes, allowing for a simpler
   decoder for special applications.

     deflateInit2 returns Z_OK if success, Z_MEM_ERROR if there was not enough
   memory, Z_STREAM_ERROR if any parameter is invalid (such as an invalid
   method), or Z_VERSION_ERROR if the zlib library version (zlib_version) is
   incompatible with the version assumed by the caller (ZLIB_VERSION).  msg is
   set to null if there is no error message.  deflateInit2 does not perform any
   compression: this will be done by deflate().
*/

/*
ZEXTERN int ZEXPORT inflateInit2 OF((z_streamp strm,
                                     int  windowBits));

     This is another version of inflateInit with an extra parameter.  The
   fields next_in, avail_in, zalloc, zfree and opaque must be initialized
   before by the caller.

     The windowBits parameter is the base two logarithm of the maximum window
   size (the size of the history buffer).  It should be in the range 8..15 for
   this version of the library.  The default value is 15 if inflateInit is used
   instead.  windowBits must be greater than or equal to the windowBits value
   provided to deflateInit2() while compressing, or it must be equal to 15 if
   deflateInit2() was not used.  If a compressed stream with a larger window
   size is given as input, inflate() will return with the error code
   Z_DATA_ERROR instead of trying to allocate a larger window.

     windowBits can also be zero to request that inflate use the window size in
   the zlib header of the compressed stream.

     windowBits can also be -8..-15 for raw inflate.  In this case, -windowBits
   determines the window size.  inflate() will then process raw deflate data,
   not looking for a zlib or gzip header, not generating a check value, and not
   looking for any check values for comparison at the end of the stream.  This
   is for use with other formats that use the deflate compressed data format
   such as zip.  Those formats provide their own check values.  If a custom
   format is developed using the raw deflate format for compressed data, it is
   recommended that a check value such as an Adler-32 or a CRC-32 be applied to
   the uncompressed data as is done in the zlib, gzip, and zip formats.  For
   most applications, the zlib format should be used as is.  Note that comments
   above on the use in deflateInit2() applies to the magnitude of windowBits.

     windowBits can also be greater than 15 for optional gzip decoding.  Add
   32 to windowBits to enable zlib and gzip decoding with automatic header
   detection, or add 16 to decode only the gzip format (the zlib format will
   return a Z_DATA_ERROR).  If a gzip stream is being decoded, strm->adler is a
   CRC-32 instead of an Adler-32.  Unlike the gunzip utility and gzread() (see
   below), inflate() will not automatically decode concatenated gzip streams.
   inflate() will return Z_STREAM_END at the end of the gzip stream.  The state
   would need to be reset to continue decoding a subsequent gzip stream.

     inflateInit2 returns Z_OK if success, Z_MEM_ERROR if there was not enough
   memory, Z_VERSION_ERROR if the zlib library version is incompatible with the
   version assumed by the caller, or Z_STREAM_ERROR if the parameters are
   invalid, such as a null pointer to the structure.  msg is set to null if
   there is no error message.  inflateInit2 does not perform any decompression
   apart from possibly reading the zlib header if present: actual decompression
   will be done by inflate().  (So next_in and avail_in may be modified, but
   next_out and avail_out are unused and unchanged.) The current implementation
   of inflateInit2() does not process any header information -- that is
   deferred until inflate() is called.
*/


ZEXTERN int ZEXPORT inflateReset2 OF((z_streamp strm,
                                      int windowBits));
/*
     This function is the same as inflateReset, but it also permits changing
   the wrap and window size requests.  The windowBits parameter is interpreted
   the same as it is for inflateInit2.  If the window size is changed, then the
   memory allocated for the window is freed, and the window will be reallocated
   by inflate() if needed.

     inflateReset2 returns Z_OK if success, or Z_STREAM_ERROR if the source
   stream state was inconsistent (such as zalloc or state being Z_NULL), or if
   the windowBits parameter is invalid.
*/


/*
ZEXTERN int ZEXPORT inflateBackInit OF((z_streamp strm, int windowBits,
                                        unsigned char FAR *window));

     Initialize the internal stream state for decompression using inflateBack()
   calls.  The fields zalloc, zfree and opaque in strm must be initialized
   before the call.  If zalloc and zfree are Z_NULL, then the default library-
   derived memory allocation routines are used.  windowBits is the base two
   logarithm of the window size, in the range 8..15.  window is a caller
   supplied buffer of that size.  Except for special applications where it is
   assured that deflate was used with small window sizes, windowBits must be 15
   and a 32K byte window must be supplied to be able to decompress general
   deflate streams.

     See inflateBack() for the usage of these routines.

     inflateBackInit will return Z_OK on success, Z_STREAM_ERROR if any of
   the parameters are invalid, Z_MEM_ERROR if the internal state could not be
   allocated, or Z_VERSION_ERROR if the version of the library does not match
   the version of the header file.
*/