path: root/erts/doc/src/zlib.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2005</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>zlib</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>zlib.xml</file>
  </header>
  <module>zlib</module>
  <modulesummary>Zlib Compression interface.</modulesummary>
  <description>
    <p>The zlib module provides an API for the zlib library
      (http://www.zlib.org).
      It is used to compress and decompress data. The
      data format is described by RFCs 1950 to 1952.</p>
    <p>A typical (compress) usage looks like:</p>
    <pre>
Z = zlib:open(),
ok = zlib:deflateInit(Z,default),

Compress = fun(end_of_data, _Cont) -> [];
              (Data, Cont) ->
                 [zlib:deflate(Z, Data)|Cont(Read(),Cont)]
           end,
Compressed = Compress(Read(),Compress),
Last = zlib:deflate(Z, [], finish),
ok = zlib:deflateEnd(Z),
zlib:close(Z),
list_to_binary([Compressed|Last])</pre>
    <p>In all functions errors, <c>{'EXIT',{Reason,Backtrace}}</c>,
      might be thrown, where <c>Reason</c> describes the
      error. Typical reasons are:</p>
    <taglist>
      <tag><c>badarg</c></tag>
      <item>
        <p>Bad argument</p>
      </item>
      <tag><c>data_error</c></tag>
      <item>
        <p>The data contains errors</p>
      </item>
      <tag><c>stream_error</c></tag>
      <item>
        <p>Inconsistent stream state</p>
      </item>
      <tag><c>einval</c></tag>
      <item>
        <p>Bad value or wrong function called</p>
      </item>
      <tag><c>{need_dictionary,Adler32}</c></tag>
      <item>
        <p>See <c>inflate/2</c></p>
      </item>
    </taglist>
  </description>

  <datatypes>
    <datatype>
      <name name="zstream"/>
      <desc>
        <p>A zlib stream, see <seealso marker="#open/0">open/0</seealso>.
        </p>
       </desc>
    </datatype>
    <datatype>
      <name name="zlevel"/>
    </datatype>
    <datatype>
      <name name="zmemlevel"/>
    </datatype>
    <datatype>
      <name name="zmethod"/>
    </datatype>
    <datatype>
      <name name="zstrategy"/>
    </datatype>
    <datatype>
      <name name="zwindowbits"/>
      <desc>
        <p>Normally in the range <c>-15..-8 | 8..15</c>.</p>
      </desc>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="open" arity="0"/>
      <fsummary>Open a stream and return a stream reference</fsummary>
      <desc>
        <p>Open a zlib stream.</p>
      </desc>
    </func>
    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a stream</fsummary>
      <desc>
        <p>Closes the stream referenced by <c><anno>Z</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="deflateInit" arity="1"/>
      <fsummary>Initialize a session for compression</fsummary>
      <desc>
        <p>Same as <c>zlib:deflateInit(<anno>Z</anno>, default)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="deflateInit" arity="2"/>
      <fsummary>Initialize a session for compression</fsummary>
      <desc>
        <p>Initialize a zlib stream for compression.</p>
        <p><c><anno>Level</anno></c> decides the compression level to be used, 0
          (<c>none</c>), gives no compression at all, 1
          (<c>best_speed</c>) gives best speed and 9
          (<c>best_compression</c>) gives best compression.</p>
      </desc>
    </func>
    <func>
      <name name="deflateInit" arity="6"/>
      <fsummary>Initialize a session for compression</fsummary>
      <desc>
        <p>Initiates a zlib stream for compression.</p>
        <p>The <c><anno>Level</anno></c> parameter decides the compression level to be
          used, 0 (<c>none</c>), gives no compression at all, 1
          (<c>best_speed</c>) gives best speed and 9
          (<c>best_compression</c>) gives best compression.</p>
        <p>The <c><anno>Method</anno></c> parameter decides which compression method to use,
          currently the only supported method is <c>deflated</c>.</p>
        <p>The <c><anno>WindowBits</anno></c> parameter is the base two logarithm
          of the window size (the size of the history buffer).  It
          should be in the range 8 through 15. Larger values
          of this parameter result in better compression at the
          expense of memory usage. The default value is 15 if
          <c>deflateInit/2</c>. A negative <c><anno>WindowBits</anno></c>
          value suppresses the zlib header (and checksum) from the
          stream. Note that the zlib source mentions this only as a
          undocumented feature.</p>
        <p>The <c><anno>MemLevel</anno></c> parameter specifies how much memory
          should be allocated for the internal compression
          state. <c><anno>MemLevel</anno></c>=1 uses minimum memory but is slow and
          reduces compression ratio; <c><anno>MemLevel</anno></c>=9 uses maximum
          memory for optimal speed. The default value is 8.</p>
        <p>The <c><anno>Strategy</anno></c> parameter is used to tune
        the compression algorithm. Use the value <c>default</c> for
        normal data, <c>filtered</c> for data produced by a filter (or
        predictor), <c>huffman_only</c> to force Huffman encoding
        only (no string match), or <c>rle</c> 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 <c>filtered</c>is to
        force more Huffman coding and less string matching; it is
        somewhat intermediate between <c>default</c> and
        <c>huffman_only</c>. <c>rle</c> is designed to be almost as
        fast as <c>huffman_only</c>, but give better compression for PNG
        image data. The <c><anno>Strategy</anno></c> parameter only
        affects the compression ratio but not the correctness of the
        compressed output even if it is not set appropriately.</p>
      </desc>
    </func>
    <func>
      <name name="deflate" arity="2"/>
      <fsummary>Compress data</fsummary>
      <desc>
        <p>Same as <c>deflate(<anno>Z</anno>, <anno>Data</anno>, none)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="deflate" arity="3"/>
      <fsummary>Compress data</fsummary>
      <desc>
        <p><c>deflate/3</c> compresses as much data as possible, and
          stops when the input buffer becomes empty. It may introduce
          some output latency (reading input without producing any
          output) except when forced to flush.</p>
        <p>If the parameter <c><anno>Flush</anno></c> is set to <c>sync</c>, all
          pending output is flushed to the output buffer and the
          output is aligned on a byte boundary, so that the
          decompressor can get all input data available so far. 
          Flushing may degrade compression for some compression algorithms and so
          it should be used only when necessary.</p>
        <p>If <c><anno>Flush</anno></c> is set to <c>full</c>, all output is flushed as with
          <c>sync</c>, and the compression state is reset so that decompression can
          restart from this point if previous compressed data has been damaged or if
          random access is desired. Using <c>full</c> too often can seriously degrade
          the compression.</p>
        <p>If the parameter <c><anno>Flush</anno></c> is set to <c>finish</c>,
          pending input is processed, pending output is flushed and
          <c>deflate/3</c> returns. Afterwards the only possible
          operations on the stream are <c>deflateReset/1</c> or <c>deflateEnd/1</c>.</p>
        <p><c><anno>Flush</anno></c> can be set to <c>finish</c> immediately after
          <c>deflateInit</c> if all compression is to be done in one step.</p>
        <pre>
 
zlib:deflateInit(Z),
B1 = zlib:deflate(Z,Data),
B2 = zlib:deflate(Z,&lt;&lt; &gt;&gt;,finish),
zlib:deflateEnd(Z),
list_to_binary([B1,B2])</pre>
      </desc>
    </func>
    <func>
      <name name="deflateSetDictionary" arity="2"/>
      <fsummary>Initialize the compression dictionary</fsummary>
      <desc>
        <p>Initializes the compression dictionary from the given byte
          sequence without producing any compressed output. This
          function must be called immediately after
          <c>deflateInit/[1|2|6]</c> or <c>deflateReset/1</c>, before
          any call of <c>deflate/3</c>. The compressor and
          decompressor must use exactly the same dictionary (see
          <c>inflateSetDictionary/2</c>). The adler checksum of the
          dictionary is returned.</p>
      </desc>
    </func>
    <func>
      <name name="deflateReset" arity="1"/>
      <fsummary>Reset the deflate session</fsummary>
      <desc>
        <p>This function is equivalent to <c>deflateEnd/1</c>
          followed by <c>deflateInit/[1|2|6]</c>, but does not free
          and reallocate all the internal compression state.  The
          stream will keep the same compression level and any other
          attributes.</p>
      </desc>
    </func>
    <func>
      <name name="deflateParams" arity="3"/>
      <fsummary>Dynamicly update deflate parameters</fsummary>
      <desc>
        <p>Dynamically update the compression level and compression
          strategy.  The interpretation of <c><anno>Level</anno></c> and
          <c><anno>Strategy</anno></c> is as in <c>deflateInit/6</c>.  This can be
          used to switch between compression and straight copy of the
          input data, or to switch to a different kind of input data
          requiring a different strategy. If the compression level is
          changed, the input available so far is compressed with the
          old level (and may be flushed); the new level will take
          effect only at the next call of <c>deflate/3</c>.</p>
        <p>Before the call of <c>deflateParams</c>, the stream state must be set as for
          a call of <c>deflate/3</c>, since the currently available input may have to
          be compressed and flushed.</p>
      </desc>
    </func>
    <func>
      <name name="deflateEnd" arity="1"/>
      <fsummary>End deflate session</fsummary>
      <desc>
        <p>End the deflate session and cleans all data used.
          Note that this function will throw an <c>data_error</c>
          exception if the last call to
          <c>deflate/3</c> was not called with <c>Flush</c> set to
          <c>finish</c>.</p>
      </desc>
    </func>
    <func>
      <name name="inflateInit" arity="1"/>
      <fsummary>Initialize a session for decompression</fsummary>
      <desc>
        <p>Initialize a zlib stream for decompression.</p>
      </desc>
    </func>
    <func>
      <name name="inflateInit" arity="2"/>
      <fsummary>Initialize a session for decompression</fsummary>
      <desc>
        <p>Initialize decompression session on zlib stream.</p>
        <p>The <c><anno>WindowBits</anno></c> parameter is the base two logarithm
          of the maximum window size (the size of the history buffer).
          It should be in the range 8 through 15.
          The default value is 15 if <c>inflateInit/1</c> is used.
          If a compressed stream with a larger window size is
          given as input, inflate() will throw the <c>data_error</c>
          exception. A negative <c><anno>WindowBits</anno></c> value makes zlib ignore the
          zlib header (and checksum) from the stream.  Note that the zlib
          source mentions this only as a undocumented feature.</p>
      </desc>
    </func>
    <func>
      <name name="inflate" arity="2"/>
      <fsummary>Decompress data</fsummary>
      <desc>
        <p><c>inflate/2</c> decompresses as much data as possible.
          It may introduce some output latency (reading
          input without producing any output).</p>
        <p>If a preset dictionary is needed at this point (see
          <c>inflateSetDictionary</c> below), <c>inflate/2</c> throws a
          <c>{need_dictionary,Adler}</c> exception where <c>Adler</c> is 
          the adler32 checksum of the dictionary chosen by the
          compressor.</p>
      </desc>
    </func>
    <func>
      <name name="inflateSetDictionary" arity="2"/>
      <fsummary>Initialize the decompression dictionary</fsummary>
      <desc>
        <p>Initializes the decompression dictionary from the given
          uncompressed byte sequence. This function must be called
          immediately after a call of <c>inflate/2</c> if this call
          threw a <c>{need_dictionary,Adler}</c> exception.
          The dictionary chosen by the
          compressor can be determined from the Adler value thrown
          by the call to <c>inflate/2</c>. The compressor and decompressor
          must use exactly the same dictionary (see <c>deflateSetDictionary/2</c>).</p>
        <p>Example:</p>
        <pre>
unpack(Z, Compressed, Dict) ->
     case catch zlib:inflate(Z, Compressed) of
          {'EXIT',{{need_dictionary,DictID},_}} ->
                   zlib:inflateSetDictionary(Z, Dict),
                 Uncompressed = zlib:inflate(Z, []);
          Uncompressed ->
                 Uncompressed
     end.</pre>
      </desc>
    </func>
    <func>
      <name name="inflateReset" arity="1"/>
      <fsummary>>Reset the inflate session</fsummary>
      <desc>
        <p>This function is equivalent to <c>inflateEnd/1</c> followed
          by <c>inflateInit/1</c>, but does not free and reallocate all
          the internal decompression state.  The stream will keep
          attributes that may have been set by <c>inflateInit/[1|2]</c>.</p>
      </desc>
    </func>
    <func>
      <name name="inflateEnd" arity="1"/>
      <fsummary>End inflate session</fsummary>
      <desc>
        <p>End the inflate session and cleans all data used. Note
          that this function will throw a <c>data_error</c> exception
          if no end of stream was found (meaning that not all data
          has been uncompressed).</p>
      </desc>
    </func>
    <func>
      <name name="setBufSize" arity="2"/>
      <fsummary>Set buffer size</fsummary>
      <desc>
        <p>Sets the intermediate buffer size.</p>
      </desc>
    </func>
    <func>
      <name name="getBufSize" arity="1"/>
      <fsummary>Get buffer size</fsummary>
      <desc>
        <p>Get the size of intermediate buffer.</p>
      </desc>
    </func>
    <func>
      <name name="crc32" arity="1"/>
      <fsummary>Get current CRC</fsummary>
      <desc>
        <p>Get the current calculated CRC checksum.</p>
      </desc>
    </func>
    <func>
      <name name="crc32" arity="2"/>
      <fsummary>Calculate CRC</fsummary>
      <desc>
        <p>Calculate the CRC checksum for <c><anno>Data</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="crc32" arity="3"/>
      <fsummary>Calculate CRC</fsummary>
      <desc>
        <p>Update a running CRC checksum for <c><anno>Data</anno></c>.
          If <c><anno>Data</anno></c> is the empty binary or the empty iolist, this function returns
          the required initial value for the crc.</p>
        <pre>
Crc = lists:foldl(fun(Data,Crc0) ->
                      zlib:crc32(Z, Crc0, Data),
                  end, zlib:crc32(Z,&lt;&lt; &gt;&gt;), Datas)</pre>
      </desc>
    </func>
    <func>
      <name name="crc32_combine" arity="4"/>
      <fsummary>Combine two CRC's</fsummary>
      <desc>
	<p>Combine two CRC checksums into one.  For two binaries or iolists,
	  <c>Data1</c> and <c>Data2</c> with sizes of <c>Size1</c> and
	  <c><anno>Size2</anno></c>, with CRC checksums <c><anno>CRC1</anno></c> and
	  <c><anno>CRC2</anno></c>.  <c>crc32_combine/4</c> returns the <c><anno>CRC</anno></c>
	  checksum of <c>[Data1,Data2]</c>, requiring
	  only <c><anno>CRC1</anno></c>, <c><anno>CRC2</anno></c>, and <c><anno>Size2</anno></c>.
	</p>
      </desc>
    </func>
    <func>
      <name name="adler32" arity="2"/>
      <fsummary>Calculate the adler checksum</fsummary>
      <desc>
        <p>Calculate the Adler-32 checksum for <c><anno>Data</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="adler32" arity="3"/>
      <fsummary>Calculate the adler checksum</fsummary>
      <desc>
        <p>Update a running Adler-32 checksum for <c><anno>Data</anno></c>.
          If <c><anno>Data</anno></c> is the empty binary or the empty iolist, this function returns
          the required initial value for the checksum.</p>
        <pre>
Crc = lists:foldl(fun(Data,Crc0) ->
                      zlib:adler32(Z, Crc0, Data),
                  end, zlib:adler32(Z,&lt;&lt; &gt;&gt;), Datas)</pre>
      </desc>
    </func>
    <func>
      <name name="adler32_combine" arity="4"/>
      <fsummary>Combine two Adler-32 checksums</fsummary>
      <desc>
	<p>Combine two Adler-32 checksums into one.  For two binaries or iolists,
	  <c>Data1</c> and <c>Data2</c> with sizes of <c>Size1</c> and
	  <c><anno>Size2</anno></c>, with Adler-32 checksums <c><anno>Adler1</anno></c> and
	  <c><anno>Adler2</anno></c>.  <c>adler32_combine/4</c> returns the <c><anno>Adler</anno></c>
	  checksum of <c>[Data1,Data2]</c>, requiring
	  only <c><anno>Adler1</anno></c>, <c><anno>Adler2</anno></c>, and <c><anno>Size2</anno></c>.
	</p>
      </desc>
    </func>
    <func>
      <name name="compress" arity="1"/>
      <fsummary>Compress data with standard zlib functionality</fsummary>
      <desc>
        <p>Compress data (with zlib headers and checksum).</p>
      </desc>
    </func>
    <func>
      <name name="uncompress" arity="1"/>
      <fsummary>Uncompress data with standard zlib functionality</fsummary>
      <desc>
        <p>Uncompress data (with zlib headers and checksum).</p>
      </desc>
    </func>
    <func>
      <name name="zip" arity="1"/>
      <fsummary>Compress data without the zlib headers</fsummary>
      <desc>
        <p>Compress data (without zlib headers and checksum).</p>
      </desc>
    </func>
    <func>
      <name name="unzip" arity="1"/>
      <fsummary>Uncompress data without the zlib headers</fsummary>
      <desc>
        <p>Uncompress data (without zlib headers and checksum).</p>
      </desc>
    </func>
    <func>
      <name name="gzip" arity="1"/>
      <fsummary>Compress data with gz header</fsummary>
      <desc>
        <p>Compress data (with gz headers and checksum).</p>
      </desc>
    </func>
    <func>
      <name name="gunzip" arity="1"/>
      <fsummary>Uncompress data with gz header</fsummary>
      <desc>
        <p>Uncompress data (with gz headers and checksum).</p>
      </desc>
    </func>
  </funcs>
</erlref>