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

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2005</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions 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>This module provides an API for the zlib library
      (<url href="http://www.zlib.net">www.zlib.net</url>).
      It is used to compress and decompress data.
      The data format is described by
      <url href="https://www.ietf.org/rfc/rfc1950.txt">RFC 1950</url>,
      <url href="https://www.ietf.org/rfc/rfc1951.txt">RFC 1951</url>, and
      <url href="https://www.ietf.org/rfc/rfc1952.txt">RFC 1952</url>.</p>

    <p>A typical (compress) usage is as follows:</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>,
      can be thrown, where <c>Reason</c> describes the error.</p>

    <p>Typical <c>Reasons</c>s:</p>

    <taglist>
      <tag><c>badarg</c></tag>
      <item>Bad argument.
      </item>
      <tag><c>data_error</c></tag>
      <item>The data contains errors.
      </item>
      <tag><c>stream_error</c></tag>
      <item>Inconsistent stream state.</item>
      <tag><c>einval</c></tag>
      <item>Bad value or wrong function called.</item>
      <tag><c>{need_dictionary,Adler32}</c></tag>
      <item>See <seealso marker="#inflate/2"><c>inflate/2</c></seealso>.
      </item>
    </taglist>
  </description>

  <datatypes>
    <datatype>
      <name name="zstream"/>
      <desc>
        <p>A zlib stream, see <seealso marker="#open/0"><c>open/0</c></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="adler32" arity="2"/>
      <fsummary>Calculate the Adler checksum.</fsummary>
      <desc>
        <p>Calculates 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>Updates 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>
        <p>Example:</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>Combines 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>.</p>
        <p>This function 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="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="compress" arity="1"/>
      <fsummary>Compress data with standard zlib functionality.</fsummary>
      <desc>
        <p>Compresses data with zlib headers and checksum.</p>
      </desc>
    </func>

    <func>
      <name name="crc32" arity="1"/>
      <fsummary>Get current CRC.</fsummary>
      <desc>
        <p>Gets the current calculated CRC checksum.</p>
      </desc>
    </func>

    <func>
      <name name="crc32" arity="2"/>
      <fsummary>Calculate CRC.</fsummary>
      <desc>
        <p>Calculates the CRC checksum for <c><anno>Data</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="crc32" arity="3"/>
      <fsummary>Calculate CRC.</fsummary>
      <desc>
        <p>Updates 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>
        <p>Example:</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 CRCs.</fsummary>
      <desc>
        <p>Combines 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>.</p>
        <p>This function 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="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>Compresses as much data as possible, and
          stops when the input buffer becomes empty. It can introduce
          some output latency (reading input without producing any
          output) except when forced to flush.</p>
        <p>If <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 can degrade compression for some compression algorithms;
          thus, use it 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 <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
          <seealso marker="#deflateReset/1"><c>deflateReset/1</c></seealso> or
          <seealso marker="#deflateEnd/1"><c>deflateEnd/1</c></seealso>.</p>
        <p><c><anno>Flush</anno></c> can be set to <c>finish</c> immediately
          after <seealso marker="#deflateInit/1"><c>deflateInit</c></seealso>
          if all compression is to be done in one step.</p>
        <p>Example:</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="deflateEnd" arity="1"/>
      <fsummary>End deflate session.</fsummary>
      <desc>
        <p>Ends the deflate session and cleans all data used. Notice that this
          function throws a <c>data_error</c> exception if the last call to
          <seealso marker="#deflate/3"><c>deflate/3</c></seealso>
          was not called with <c>Flush</c> set to <c>finish</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>Initializes a zlib stream for compression.</p>
        <p><c><anno>Level</anno></c> decides the compression level to be
          used:</p>
        <list type="bulleted">
          <item>0 (<c>none</c>), gives no compression</item>
          <item>1 (<c>best_speed</c>) gives best speed</item>
          <item>9 (<c>best_compression</c>) gives best compression</item>
        </list>
      </desc>
    </func>

    <func>
      <name name="deflateInit" arity="6"/>
      <fsummary>Initialize a session for compression.</fsummary>
      <desc>
        <p>Initiates a zlib stream for compression.</p>
        <taglist>
          <tag><c><anno>Level</anno></c></tag>
          <item>
            <p>Compression level to use:</p>
            <list type="bulleted">
              <item>0 (<c>none</c>), gives no compression</item>
              <item>1 (<c>best_speed</c>) gives best speed</item>
              <item>9 (<c>best_compression</c>) gives best compression</item>
            </list>
          </item>
          <tag><c><anno>Method</anno></c></tag>
          <item>
            <p>Compression method to use, currently the only supported method
              is <c>deflated</c>.</p>
          </item>
          <tag><c><anno>WindowBits</anno></c></tag>
          <item>
            <p>The base two logarithm of the window size (the size of the
              history buffer). It is to be in the range 8 through 15. Larger
              values result in better compression at the expense of memory
              usage. Defaults to 15 if <seealso marker="#deflateInit/2">
              <c>deflateInit/2</c></seealso> is used. A negative
              <c><anno>WindowBits</anno></c> value suppresses the zlib header
              (and checksum) from the stream. Notice that the zlib source
              mentions this only as a undocumented feature.</p>
	      <warning>
		<p>Due to a known bug in the underlying zlib library, <c>WindowBits</c> values 8 and -8
		do not work as expected. In zlib versions before 1.2.9 values
		8 and -8 are automatically changed to 9 and -9. <em>From zlib version 1.2.9
		value -8 is rejected</em> causing <c>zlib:deflateInit/6</c> to fail
		(8 is still changed to 9). It also seem possible that future versions
		of zlib may fix this bug and start accepting 8 and -8 as is.</p>
		<p>Conclusion: Avoid values 8 and -8 unless you know your zlib version supports them.</p>
	      </warning>
          </item>
          <tag><c><anno>MemLevel</anno></c></tag>
          <item>
            <p>Specifies how much memory is to 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. Defaults to 8.</p>
          </item>
          <tag><c><anno>Strategy</anno></c></tag>
          <item>
            <p>Tunes the compression algorithm. Use the following values:</p>
            <list type="bulleted">
              <item><c>default</c> for normal data</item>
              <item><c>filtered</c> for data produced by a filter (or
                predictor)</item>
              <item><c>huffman_only</c> to force Huffman encoding only
                (no string match)</item>
              <item><c>rle</c> to limit match distances to one (run-length
                encoding)</item>
            </list>
            <p>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 gives better compression for PNG image
              data.</p>
            <p><c><anno>Strategy</anno></c> affects only the compression ratio,
              but not the correctness of the compressed output even if it is not
              set appropriately.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="deflateParams" arity="3"/>
      <fsummary>Dynamicly update deflate parameters.</fsummary>
      <desc>
        <p>Dynamically updates the compression level and compression
          strategy. The interpretation of <c><anno>Level</anno></c> and
          <c><anno>Strategy</anno></c> is as in
          <seealso marker="#deflateInit/6"><c>deflateInit/6</c></seealso>.
          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 can be flushed); the new level takes
          effect only at the next call of
          <seealso marker="#deflate/3"><c>deflate/3</c></seealso>.</p>
        <p>Before the call of <c>deflateParams</c>, the stream state must be
          set as for a call of <c>deflate/3</c>, as the currently available
          input may have to be compressed and flushed.</p>
      </desc>
    </func>

    <func>
      <name name="deflateReset" arity="1"/>
      <fsummary>Reset the deflate session.</fsummary>
      <desc>
        <p>Equivalent to
          <seealso marker="#deflateEnd/1"><c>deflateEnd/1</c></seealso>
          followed by
          <seealso marker="#deflateInit/1"><c>deflateInit/1,2,6</c></seealso>,
          but does not free and reallocate all the internal compression state.
          The stream keeps the same compression level and any other
          attributes.</p>
      </desc>
    </func>

    <func>
      <name name="deflateSetDictionary" arity="2"/>
      <fsummary>Initialize the compression dictionary.</fsummary>
      <desc>
        <p>Initializes the compression dictionary from the specified byte
          sequence without producing any compressed output.</p>
        <p>This function must be called immediately after
          <seealso marker="#deflateInit/1"><c>deflateInit/1,2,6</c></seealso> or
          <seealso marker="#deflateReset/1"><c>deflateReset/1</c></seealso>,
          before any call of
          <seealso marker="#deflate/3"><c>deflate/3</c></seealso>.
          The compressor and decompressor must use the same dictionary (see
          <seealso marker="#inflateSetDictionary/2">
          <c>inflateSetDictionary/2</c></seealso>).</p>
        <p>The Adler checksum of the dictionary is returned.</p>
      </desc>
    </func>

    <func>
      <name name="getBufSize" arity="1"/>
      <fsummary>Get buffer size.</fsummary>
      <desc>
        <p>Gets the size of the intermediate buffer.</p>
      </desc>
    </func>

    <func>
      <name name="gunzip" arity="1"/>
      <fsummary>Uncompress data with gz header.</fsummary>
      <desc>
        <p>Uncompresses data with gz headers and checksum.</p>
      </desc>
    </func>

    <func>
      <name name="gzip" arity="1"/>
      <fsummary>Compress data with gz header.</fsummary>
      <desc>
        <p>Compresses data with gz headers and checksum.</p>
      </desc>
    </func>

    <func>
      <name name="inflate" arity="2"/>
      <fsummary>Decompress data.</fsummary>
      <desc>
        <p>Decompresses as much data as possible.
          It can introduce some output latency (reading
          input without producing any output).</p>
        <p>If a preset dictionary is needed at this point (see
          <seealso marker="#inflateSetDictionary/2">
          <c>inflateSetDictionary/2</c></seealso>), <c>inflate/2</c> throws a
          <c>{need_dictionary,Adler}</c> exception, where <c>Adler</c> is
          the Adler-32 checksum of the dictionary chosen by the compressor.</p>
      </desc>
    </func>

    <func>
      <name name="inflateChunk" arity="1"/>
      <fsummary>Read next uncompressed chunk.</fsummary>
      <desc>
        <p>Reads the next chunk of uncompressed data, initialized by
          <seealso marker="#inflateChunk/2"><c>inflateChunk/2</c></seealso>.</p>
        <p>This function is to be repeatedly called, while it returns
          <c>{more, Decompressed}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="inflateChunk" arity="2"/>
      <fsummary>Decompress data with limited output size.</fsummary>
      <desc>
        <p>Like <seealso marker="#inflate/2"><c>inflate/2</c></seealso>,
          but decompresses no more data than will fit in the buffer configured
          through <seealso marker="#setBufSize/2"><c>setBufSize/2</c></seealso>.
          Is is useful when decompressing a stream with a high compression
          ratio, such that a small amount of compressed input can expand up to
          1000 times.</p>
        <p>This function returns <c>{more, Decompressed}</c>, when there is
          more output available, and
          <seealso marker="#inflateChunk/1"><c>inflateChunk/1</c></seealso>
          is to be used to read it.</p>
        <p>This function can introduce some output latency (reading
          input without producing any output).</p>
        <p>If a preset dictionary is needed at this point (see
          <seealso marker="#inflateSetDictionary/2">
          <c>inflateSetDictionary/2</c></seealso>), this function throws a
          <c>{need_dictionary,Adler}</c> exception, where <c>Adler</c> is
          the Adler-32 checksum of the dictionary chosen by the compressor.</p>
        <p>Example:</p>
        <pre>
walk(Compressed, Handler) ->
    Z = zlib:open(),
    zlib:inflateInit(Z),
    % Limit single uncompressed chunk size to 512kb
    zlib:setBufSize(Z, 512 * 1024),
    loop(Z, Handler, zlib:inflateChunk(Z, Compressed)),
    zlib:inflateEnd(Z),
    zlib:close(Z).

loop(Z, Handler, {more, Uncompressed}) ->
    Handler(Uncompressed),
    loop(Z, Handler, zlib:inflateChunk(Z));
loop(Z, Handler, Uncompressed) ->
    Handler(Uncompressed).</pre>
      </desc>
    </func>

    <func>
      <name name="inflateEnd" arity="1"/>
      <fsummary>End inflate session.</fsummary>
      <desc>
        <p>Ends the inflate session and cleans all data used. Notice
          that this function throws 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="inflateInit" arity="1"/>
      <fsummary>Initialize a session for decompression.</fsummary>
      <desc>
        <p>Initializes a zlib stream for decompression.</p>
      </desc>
    </func>

    <func>
      <name name="inflateInit" arity="2"/>
      <fsummary>Initialize a session for decompression.</fsummary>
      <desc>
        <p>Initializes a decompression session on zlib stream.</p>
        <p><c><anno>WindowBits</anno></c> is the base two logarithm
          of the maximum window size (the size of the history buffer).
          It is to be in the range 8 through 15. Default to 15 if
          <seealso marker="#inflateInit/1"><c>inflateInit/1</c></seealso>
          is used.</p>
        <p>If a compressed stream with a larger window size is specified as
          input, <seealso marker="#inflate/2"><c>inflate/2</c></seealso>
          throws the <c>data_error</c> exception.</p>
        <p>A negative <c><anno>WindowBits</anno></c> value makes zlib
          ignore the zlib header (and checksum) from the stream. Notice that
          the zlib source mentions this only as a undocumented feature.</p>
      </desc>
    </func>

    <func>
      <name name="inflateReset" arity="1"/>
      <fsummary>>Reset the inflate session.</fsummary>
      <desc>
        <p>Equivalent to
          <seealso marker="#inflateEnd/1"><c>inflateEnd/1</c></seealso>
          followed by
          <seealso marker="#inflateInit/1"><c>inflateInit/1</c></seealso>,
          but does not free and reallocate all the internal decompression state.
          The stream will keep attributes that could have been set by
          <c>inflateInit/1,2</c>.</p>
      </desc>
    </func>

    <func>
      <name name="inflateSetDictionary" arity="2"/>
      <fsummary>Initialize the decompression dictionary.</fsummary>
      <desc>
        <p>Initializes the decompression dictionary from the specified
          uncompressed byte sequence. This function must be called
          immediately after a call of
          <seealso marker="#inflate/2"><c>inflate/2</c></seealso>
          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 the same dictionary (see
          <seealso marker="#deflateSetDictionary/2">
          <c>deflateSetDictionary/2</c></seealso>).</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="open" arity="0"/>
      <fsummary>Open a stream and return a stream reference.</fsummary>
      <desc>
        <p>Opens a zlib stream.</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="uncompress" arity="1"/>
      <fsummary>Uncompress data with standard zlib functionality.</fsummary>
      <desc>
        <p>Uncompresses data with zlib headers and checksum.</p>
      </desc>
    </func>

    <func>
      <name name="unzip" arity="1"/>
      <fsummary>Uncompress data without the zlib headers.</fsummary>
      <desc>
        <p>Uncompresses data without zlib headers and checksum.</p>
      </desc>
    </func>

    <func>
      <name name="zip" arity="1"/>
      <fsummary>Compress data without the zlib headers.</fsummary>
      <desc>
        <p>Compresses data without zlib headers and checksum.</p>
      </desc>
    </func>
  </funcs>
</erlref>