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<section id="building-c-and-c-extensions">
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<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="Link to this heading">¶</a></h1>
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<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,
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<code class="docutils literal notranslate"><span class="pre">.pyd</span></code> on Windows), which exports an <em>initialization function</em>.</p>
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<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>,
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and must be named after the module name, with an appropriate extension.
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When using setuptools, the correct filename is generated automatically.</p>
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<p>The initialization function has the signature:</p>
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<dl class="c function">
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<dt class="sig sig-object c" id="c.PyInit_modulename">
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<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="Link to this definition">¶</a><br /></dt>
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<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>
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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>
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<p>For modules with ASCII-only names, the function must be named
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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
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are allowed. In this case, the initialization function name is
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<code class="docutils literal notranslate"><span class="pre">PyInitU_<modulename></span></code>, with <code class="docutils literal notranslate"><span class="pre"><modulename></span></code> encoded using Python’s
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<em>punycode</em> encoding with hyphens replaced by underscores. In Python:</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">initfunc_name</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
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<span class="k">try</span><span class="p">:</span>
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<span class="n">suffix</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">'_'</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">'ascii'</span><span class="p">)</span>
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<span class="k">except</span> <span class="ne">UnicodeEncodeError</span><span class="p">:</span>
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<span class="n">suffix</span> <span class="o">=</span> <span class="sa">b</span><span class="s1">'U_'</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">'punycode'</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">'-'</span><span class="p">,</span> <span class="sa">b</span><span class="s1">'_'</span><span class="p">)</span>
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<span class="k">return</span> <span class="sa">b</span><span class="s1">'PyInit'</span> <span class="o">+</span> <span class="n">suffix</span>
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</pre></div>
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<p>It is possible to export multiple modules from a single shared library by
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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>
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<section id="building-c-and-c-extensions-with-setuptools">
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<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="Link to this heading">¶</a></h2>
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<p>Python 3.12 and newer no longer come with distutils. Please refer to the
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<code class="docutils literal notranslate"><span class="pre">setuptools</span></code> documentation at
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to learn more about how build and distribute C/C++ extensions with setuptools.</p>
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<li><a class="reference internal" href="#building-c-and-c-extensions-with-setuptools">4.1. Building C and C++ Extensions with setuptools</a></li>
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Last updated on Apr 08, 2025 (14:33 UTC).
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<a href="/bugs.html">Found a bug</a>?
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Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 8.2.3.
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