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  <section id="building-c-and-c-extensions">
<span id="building"></span><h1><span class="section-number">4. </span>Building C and C++ Extensions<a class="headerlink" href="#building-c-and-c-extensions" title="Permalink to this headline">¶</a></h1>
<p>A C extension for CPython is a shared library (e.g. a <code class="docutils literal notranslate"><span class="pre">.so</span></code> file on Linux,
<code class="docutils literal notranslate"><span class="pre">.pyd</span></code> on Windows), which exports an <em>initialization function</em>.</p>
<p>To be importable, the shared library must be available on <span class="target" id="index-0"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONPATH</span></code></a>,
and must be named after the module name, with an appropriate extension.
When using setuptools, the correct filename is generated automatically.</p>
<p>The initialization function has the signature:</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyInit_modulename">
<a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyInit_modulename</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyInit_modulename" title="Permalink to this definition">¶</a><br /></dt>
<dd></dd></dl>

<p>It returns either a fully initialized module, or a <a class="reference internal" href="../c-api/module.html#c.PyModuleDef" title="PyModuleDef"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyModuleDef</span></code></a>
instance. See <a class="reference internal" href="../c-api/module.html#initializing-modules"><span class="std std-ref">Initializing C modules</span></a> for details.</p>
<p>For modules with ASCII-only names, the function must be named
<code class="docutils literal notranslate"><span class="pre">PyInit_&lt;modulename&gt;</span></code>, with <code class="docutils literal notranslate"><span class="pre">&lt;modulename&gt;</span></code> replaced by the name of the
module. When using <a class="reference internal" href="../c-api/module.html#multi-phase-initialization"><span class="std std-ref">Multi-phase initialization</span></a>, non-ASCII module names
are allowed. In this case, the initialization function name is
<code class="docutils literal notranslate"><span class="pre">PyInitU_&lt;modulename&gt;</span></code>, with <code class="docutils literal notranslate"><span class="pre">&lt;modulename&gt;</span></code> encoded using Python’s
<em>punycode</em> encoding with hyphens replaced by underscores. In Python:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">initfunc_name</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
    <span class="k">try</span><span class="p">:</span>
        <span class="n">suffix</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">&#39;_&#39;</span> <span class="o">+</span> <span class="n">name</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">&#39;ascii&#39;</span><span class="p">)</span>
    <span class="k">except</span> <span class="ne">UnicodeEncodeError</span><span class="p">:</span>
        <span class="n">suffix</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">&#39;U_&#39;</span> <span class="o">+</span> <span class="n">name</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">&#39;punycode&#39;</span><span class="p">)</span><span class="o">.</span><span class="n">replace</span><span class="p">(</span><span class="sa">b</span><span class="s1">&#39;-&#39;</span><span class="p">,</span> <span class="sa">b</span><span class="s1">&#39;_&#39;</span><span class="p">)</span>
    <span class="k">return</span> <span class="sa">b</span><span class="s1">&#39;PyInit&#39;</span> <span class="o">+</span> <span class="n">suffix</span>
</pre></div>
</div>
<p>It is possible to export multiple modules from a single shared library by
defining multiple initialization functions. However, importing them requires
using symbolic links or a custom importer, because by default only the
function corresponding to the filename is found.
See the <em>“Multiple modules in one library”</em> section in <span class="target" id="index-1"></span><a class="pep reference external" href="https://peps.python.org/pep-0489/"><strong>PEP 489</strong></a> for details.</p>
<section id="building-c-and-c-extensions-with-setuptools">
<span id="setuptools-index"></span><span id="install-index"></span><h2><span class="section-number">4.1. </span>Building C and C++ Extensions with setuptools<a class="headerlink" href="#building-c-and-c-extensions-with-setuptools" title="Permalink to this headline">¶</a></h2>
<p>Python 3.12 and newer no longer come with distutils. Please refer to the
<code class="docutils literal notranslate"><span class="pre">setuptools</span></code> documentation at
<a class="reference external" href="https://setuptools.readthedocs.io/en/latest/setuptools.html">https://setuptools.readthedocs.io/en/latest/setuptools.html</a>
to learn more about how build and distribute C/C++ extensions with setuptools.</p>
</section>
</section>


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