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<div class="section" id="module-crcmod">
<h1><tt class="xref docutils literal"><span class="pre">crcmod</span></tt> – CRC calculation<a class="headerlink" href="#module-crcmod" title="Permalink to this headline">¶</a></h1>
<p>This module provides a function factory <a title="crcmod.mkCrcFun" class="reference internal" href="#crcmod.mkCrcFun"><tt class="xref docutils literal"><span class="pre">mkCrcFun()</span></tt></a> and a class <a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a>
for calculating CRCs of byte strings using common CRC algorithms.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>This documentation normally shows Python 2.x usage. Python 3.x usage is very similar,
with the main difference that input strings must be explicitly defined as
<tt class="xref docutils literal"><span class="pre">bytes</span></tt> type, or an object that supports the buffer protocol. E.g.:</p>
<div class="last highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">crc_value</span> <span class="o">=</span> <span class="n">crc_function</span><span class="p">(</span><span class="n">b</span><span class="s">'123456789'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">crc_value</span> <span class="o">=</span> <span class="n">crc_function</span><span class="p">(</span><span class="nb">bytearray</span><span class="p">((</span><span class="mi">49</span><span class="p">,</span> <span class="mi">50</span><span class="p">,</span> <span class="mi">51</span><span class="p">,</span> <span class="mi">52</span><span class="p">,</span> <span class="mi">53</span><span class="p">,</span> <span class="mi">54</span><span class="p">,</span> <span class="mi">55</span><span class="p">,</span> <span class="mi">56</span><span class="p">,</span> <span class="mi">57</span><span class="p">)))</span>
</pre></div>
</div>
</div>
<div class="section" id="mkcrcfun-crc-function-factory">
<h2><a title="crcmod.mkCrcFun" class="reference internal" href="#crcmod.mkCrcFun"><tt class="xref docutils literal"><span class="pre">mkCrcFun()</span></tt></a> – CRC function factory<a class="headerlink" href="#mkcrcfun-crc-function-factory" title="Permalink to this headline">¶</a></h2>
<p>The function factory provides a simple interface for CRC calculation.</p>
<dl class="function">
<dt id="crcmod.mkCrcFun">
<tt class="descclassname">crcmod.</tt><tt class="descname">mkCrcFun</tt><big>(</big><em>poly</em><span class="optional">[</span>, <em>initCrc</em>, <em>rev</em>, <em>xorOut</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#crcmod.mkCrcFun" title="Permalink to this definition">¶</a></dt>
<dd><p>Function factory that returns a new function for calculating CRCs
using a specified CRC algorithm.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><em>poly</em> – The generator polynomial to use in calculating the CRC. The value
is specified as a Python integer or long integer. The bits in this integer
are the coefficients of the polynomial. The only polynomials allowed are
those that generate 8, 16, 24, 32, or 64 bit CRCs.</li>
<li><em>initCrc</em> – Initial value used to start the CRC calculation. This initial
value should be the initial shift register value, reversed if it uses a
reversed algorithm, and then XORed with the final XOR value. That is
equivalent to the CRC result the algorithm should return for a
zero-length string. Defaults to all bits set because that starting value
will take leading zero bytes into account. Starting with zero will ignore
all leading zero bytes.</li>
<li><em>rev</em> – A flag that selects a bit reversed algorithm when <tt class="xref xref docutils literal"><span class="pre">True</span></tt>. Defaults to
<tt class="xref xref docutils literal"><span class="pre">True</span></tt> because the bit reversed algorithms are more efficient.</li>
<li><em>xorOut</em> – Final value to XOR with the calculated CRC value. Used by some
CRC algorithms. Defaults to zero.</li>
</ul>
</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">CRC calculation function</p>
</td>
</tr>
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first last">function</p>
</td>
</tr>
</tbody>
</table>
<p>The function that is returned is as follows:</p>
<dl class="function">
<dt id="crcmod..crc_function">
<tt class="descclassname">.</tt><tt class="descname">crc_function</tt><big>(</big><em>data</em><span class="optional">[</span>, <em>crc=initCrc</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#crcmod..crc_function" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><em>data</em> (byte string) – Data for which to calculate the CRC.</li>
<li><em>crc</em> – Initial CRC value.</li>
</ul>
</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">Calculated CRC value.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first last">integer</p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<div class="section" id="examples">
<h3>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h3>
<p><strong>CRC-32</strong> Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">crcmod</span>
<span class="gp">>>> </span><span class="n">crc32_func</span> <span class="o">=</span> <span class="n">crcmod</span><span class="o">.</span><span class="n">mkCrcFun</span><span class="p">(</span><span class="mh">0x104c11db7</span><span class="p">,</span> <span class="n">initCrc</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">xorOut</span><span class="o">=</span><span class="mh">0xFFFFFFFF</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">crc32_func</span><span class="p">(</span><span class="s">'123456789'</span><span class="p">))</span>
<span class="go">'0xcbf43926L'</span>
</pre></div>
</div>
<p>The CRC-32 uses a “reversed” algorithm, used for many common CRC algorithms.
Less common is the non-reversed algorithm, as used by the 16-bit <strong>XMODEM</strong> CRC:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">xmodem_crc_func</span> <span class="o">=</span> <span class="n">crcmod</span><span class="o">.</span><span class="n">mkCrcFun</span><span class="p">(</span><span class="mh">0x11021</span><span class="p">,</span> <span class="n">rev</span><span class="o">=</span><span class="bp">False</span><span class="p">,</span> <span class="n">initCrc</span><span class="o">=</span><span class="mh">0x0000</span><span class="p">,</span> <span class="n">xorOut</span><span class="o">=</span><span class="mh">0x0000</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">xmodem_crc_func</span><span class="p">(</span><span class="s">'123456789'</span><span class="p">))</span>
<span class="go">'0x31c3'</span>
</pre></div>
</div>
<p>The CRC function can be called multiple times. On subsequent calls, pass the
CRC value previously calculated as a second parameter:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">crc_value</span> <span class="o">=</span> <span class="n">crc32_func</span><span class="p">(</span><span class="s">'1234'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">crc_value</span> <span class="o">=</span> <span class="n">crc32_func</span><span class="p">(</span><span class="s">'56789'</span><span class="p">,</span> <span class="n">crc_value</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">crc_value</span><span class="p">)</span>
<span class="go">'0xcbf43926L'</span>
</pre></div>
</div>
<p>Python 3.x example: Unicode strings are not accepted as input. Byte strings are acceptable.
You may calculate a CRC for an object that implements the buffer protocol:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">crcmod</span>
<span class="gp">>>> </span><span class="n">crc32_func</span> <span class="o">=</span> <span class="n">crcmod</span><span class="o">.</span><span class="n">mkCrcFun</span><span class="p">(</span><span class="mh">0x104c11db7</span><span class="p">,</span> <span class="n">initCrc</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">xorOut</span><span class="o">=</span><span class="mh">0xFFFFFFFF</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">crc32_func</span><span class="p">(</span><span class="s">'123456789'</span><span class="p">))</span>
<span class="gp">...</span>
<span class="go">TypeError: Unicode-objects must be encoded before calculating a CRC</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">crc32_func</span><span class="p">(</span><span class="n">b</span><span class="s">'123456789'</span><span class="p">))</span>
<span class="go">'0xcbf43926'</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">crc32_func</span><span class="p">(</span><span class="nb">bytearray</span><span class="p">((</span><span class="mi">49</span><span class="p">,</span> <span class="mi">50</span><span class="p">,</span> <span class="mi">51</span><span class="p">,</span> <span class="mi">52</span><span class="p">,</span> <span class="mi">53</span><span class="p">,</span> <span class="mi">54</span><span class="p">,</span> <span class="mi">55</span><span class="p">,</span> <span class="mi">56</span><span class="p">,</span> <span class="mi">57</span><span class="p">))))</span>
<span class="go">'0xcbf43926'</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="class-crc">
<h2>Class <a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a><a class="headerlink" href="#class-crc" title="Permalink to this headline">¶</a></h2>
<p>The class provides an interface similar to the Python <a title="(in Python v2.6)" class="reference external" href="http://docs.python.org/library/hashlib.html#module-hashlib"><tt class="xref docutils literal"><span class="pre">hashlib</span></tt></a>, <a title="(in Python v2.6)" class="reference external" href="http://docs.python.org/library/md5.html#module-md5"><tt class="xref docutils literal"><span class="pre">md5</span></tt></a> and <a title="(in Python v2.6)" class="reference external" href="http://docs.python.org/library/sha.html#module-sha"><tt class="xref docutils literal"><span class="pre">sha</span></tt></a> modules.</p>
<dl class="class">
<dt id="crcmod.Crc">
<em class="property">class </em><tt class="descclassname">crcmod.</tt><tt class="descname">Crc</tt><big>(</big><em>poly</em><span class="optional">[</span>, <em>initCrc</em>, <em>rev</em>, <em>xorOut</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#crcmod.Crc" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns a new <a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a> object for calculating CRCs using a specified CRC algorithm.</p>
<p>The parameters are the same as those for the factory function <a title="crcmod.mkCrcFun" class="reference internal" href="#crcmod.mkCrcFun"><tt class="xref docutils literal"><span class="pre">mkCrcFun()</span></tt></a>.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><em>poly</em> – The generator polynomial to use in calculating the CRC. The value
is specified as a Python integer or long integer. The bits in this integer
are the coefficients of the polynomial. The only polynomials allowed are
those that generate 8, 16, 24, 32, or 64 bit CRCs.</li>
<li><em>initCrc</em> – Initial value used to start the CRC calculation. This initial
value should be the initial shift register value, reversed if it uses a
reversed algorithm, and then XORed with the final XOR value. That is
equivalent to the CRC result the algorithm should return for a
zero-length string. Defaults to all bits set because that starting value
will take leading zero bytes into account. Starting with zero will ignore
all leading zero bytes.</li>
<li><em>rev</em> – A flag that selects a bit reversed algorithm when <tt class="xref xref docutils literal"><span class="pre">True</span></tt>. Defaults to
<tt class="xref xref docutils literal"><span class="pre">True</span></tt> because the bit reversed algorithms are more efficient.</li>
<li><em>xorOut</em> – Final value to XOR with the calculated CRC value. Used by some
CRC algorithms. Defaults to zero.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<p><a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a> objects contain the following constant values:</p>
<dl class="attribute">
<dt id="crcmod.Crc.digest_size">
<tt class="descname">digest_size</tt><a class="headerlink" href="#crcmod.Crc.digest_size" title="Permalink to this definition">¶</a></dt>
<dd>The size of the resulting digest in bytes. This depends on the width of the CRC polynomial.
E.g. for a 32-bit CRC, <a title="crcmod.Crc.digest_size" class="reference internal" href="#crcmod.Crc.digest_size"><tt class="xref docutils literal"><span class="pre">digest_size</span></tt></a> will be <tt class="docutils literal"><span class="pre">4</span></tt>.</dd></dl>
<dl class="attribute">
<dt id="crcmod.Crc.crcValue">
<tt class="descname">crcValue</tt><a class="headerlink" href="#crcmod.Crc.crcValue" title="Permalink to this definition">¶</a></dt>
<dd>The calculated CRC value, as an integer, for the data that has been input
using <a title="crcmod.Crc.update" class="reference internal" href="#crcmod.Crc.update"><tt class="xref docutils literal"><span class="pre">update()</span></tt></a>. This value is updated after each call to <a title="crcmod.Crc.update" class="reference internal" href="#crcmod.Crc.update"><tt class="xref docutils literal"><span class="pre">update()</span></tt></a>.</dd></dl>
<p><a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a> objects support the following methods:</p>
<dl class="method">
<dt id="crcmod.Crc.new">
<tt class="descname">new</tt><big>(</big><span class="optional">[</span><em>arg</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#crcmod.Crc.new" title="Permalink to this definition">¶</a></dt>
<dd>Create a new instance of the <a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a> class initialized to the same
values as the original instance. The CRC value is set to the initial
value. If a string is provided in the optional <tt class="docutils literal"><span class="pre">arg</span></tt> parameter, it is
passed to the <a title="crcmod.Crc.update" class="reference internal" href="#crcmod.Crc.update"><tt class="xref docutils literal"><span class="pre">update()</span></tt></a> method.</dd></dl>
<dl class="method">
<dt id="crcmod.Crc.copy">
<tt class="descname">copy</tt><big>(</big><big>)</big><a class="headerlink" href="#crcmod.Crc.copy" title="Permalink to this definition">¶</a></dt>
<dd>Create a new instance of the <a title="crcmod.Crc" class="reference internal" href="#crcmod.Crc"><tt class="xref docutils literal"><span class="pre">Crc</span></tt></a> class initialized to the same
values as the original instance. The CRC value is copied from the current
value. This allows multiple CRC calculations using a common initial
string.</dd></dl>
<dl class="method">
<dt id="crcmod.Crc.update">
<tt class="descname">update</tt><big>(</big><em>data</em><big>)</big><a class="headerlink" href="#crcmod.Crc.update" title="Permalink to this definition">¶</a></dt>
<dd><table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameter:</th><td class="field-body"><em>data</em> (byte string) – Data for which to calculate the CRC</td>
</tr>
</tbody>
</table>
<p>Update the calculated CRC value for the specified input data.</p>
</dd></dl>
<dl class="method">
<dt id="crcmod.Crc.digest">
<tt class="descname">digest</tt><big>(</big><big>)</big><a class="headerlink" href="#crcmod.Crc.digest" title="Permalink to this definition">¶</a></dt>
<dd>Return the current CRC value as a string of bytes. The length of
this string is specified in the <a title="crcmod.Crc.digest_size" class="reference internal" href="#crcmod.Crc.digest_size"><tt class="xref docutils literal"><span class="pre">digest_size</span></tt></a> attribute.</dd></dl>
<dl class="method">
<dt id="crcmod.Crc.hexdigest">
<tt class="descname">hexdigest</tt><big>(</big><big>)</big><a class="headerlink" href="#crcmod.Crc.hexdigest" title="Permalink to this definition">¶</a></dt>
<dd>Return the current CRC value as a string of hex digits. The length
of this string is twice the <a title="crcmod.Crc.digest_size" class="reference internal" href="#crcmod.Crc.digest_size"><tt class="xref docutils literal"><span class="pre">digest_size</span></tt></a> attribute.</dd></dl>
<dl class="method">
<dt id="crcmod.Crc.generateCode">
<tt class="descname">generateCode</tt><big>(</big><em>functionName</em>, <em>out</em><span class="optional">[</span>, <em>dataType</em>, <em>crcType</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#crcmod.Crc.generateCode" title="Permalink to this definition">¶</a></dt>
<dd><p>Generate a C/C++ function.</p>
<table class="docutils field-list" frame="void" rules="none">
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<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><em>functionName</em> – String specifying the name of the function.</li>
<li><em>out</em> – An open file-like object with a write method.
This specifies where the generated code is written.</li>
<li><em>dataType</em> – An optional parameter specifying the data type of the input
data to the function. Defaults to <tt class="docutils literal"><span class="pre">UINT8</span></tt>.</li>
<li><em>crcType</em> – An optional parameter specifying the data type of the CRC value.
Defaults to one of <tt class="docutils literal"><span class="pre">UINT8</span></tt>, <tt class="docutils literal"><span class="pre">UINT16</span></tt>, <tt class="docutils literal"><span class="pre">UINT32</span></tt>, or <tt class="docutils literal"><span class="pre">UINT64</span></tt> depending
on the size of the CRC value.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
</dd></dl>
<div class="section" id="id1">
<h3>Examples<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h3>
<p><strong>CRC-32</strong> Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">crcmod</span>
<span class="gp">>>> </span><span class="n">crc32</span> <span class="o">=</span> <span class="n">crcmod</span><span class="o">.</span><span class="n">Crc</span><span class="p">(</span><span class="mh">0x104c11db7</span><span class="p">,</span> <span class="n">initCrc</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">xorOut</span><span class="o">=</span><span class="mh">0xFFFFFFFF</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">crc32</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="s">'123456789'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">hex</span><span class="p">(</span><span class="n">crc32</span><span class="o">.</span><span class="n">crcValue</span><span class="p">)</span>
<span class="go">'0xcbf43926L'</span>
<span class="gp">>>> </span><span class="n">crc32</span><span class="o">.</span><span class="n">hexdigest</span><span class="p">()</span>
<span class="go">'CBF43926'</span>
</pre></div>
</div>
<p>The <a title="crcmod.Crc.update" class="reference internal" href="#crcmod.Crc.update"><tt class="xref docutils literal"><span class="pre">Crc.update()</span></tt></a> method can be called multiple times, and the CRC value is updated with each call:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">crc32new</span> <span class="o">=</span> <span class="n">crc32</span><span class="o">.</span><span class="n">new</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">crc32new</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="s">'1234'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">crc32new</span><span class="o">.</span><span class="n">hexdigest</span><span class="p">()</span>
<span class="go">'9BE3E0A3'</span>
<span class="gp">>>> </span><span class="n">crc32new</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="s">'56789'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">crc32new</span><span class="o">.</span><span class="n">hexdigest</span><span class="p">()</span>
<span class="go">'CBF43926'</span>
</pre></div>
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<li><a class="reference external" href="#"><tt class="docutils literal"><span class="pre">crcmod</span></tt> – CRC calculation</a><ul>
<li><a class="reference external" href="#mkcrcfun-crc-function-factory"><tt class="docutils literal"><span class="pre">mkCrcFun()</span></tt> – CRC function factory</a><ul>
<li><a class="reference external" href="#examples">Examples</a></li>
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<li><a class="reference external" href="#class-crc">Class <tt class="docutils literal"><span class="pre">Crc</span></tt></a><ul>
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