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</style><div class="section" id="dynamically-loading-libmpi-at-runtime">
<span id="label-running-dynamic-loading-libmpi"></span><h1><span class="section-number">11.4. </span>Dynamically loading <code class="docutils literal notranslate"><span class="pre">libmpi</span></code> at runtime<a class="headerlink" href="#dynamically-loading-libmpi-at-runtime" title="Permalink to this heading"></a></h1>
<p>If you want to explicitly load Open MPI’s shared library <code class="docutils literal notranslate"><span class="pre">libmpi</span></code> at
runtime by using <code class="docutils literal notranslate"><span class="pre">dlopen()</span></code> from C/C ++ or something like the
<code class="docutils literal notranslate"><span class="pre">ctypes</span></code> package from Python, extra care is required. The
default configuration of Open MPI uses <code class="docutils literal notranslate"><span class="pre">dlopen()</span></code> internally to load
its support components. These components rely on symbols available in
<code class="docutils literal notranslate"><span class="pre">libmpi</span></code>. In order to make the symbols in <code class="docutils literal notranslate"><span class="pre">libmpi</span></code> available to
the components loaded by Open MPI at runtime, your application must load
<code class="docutils literal notranslate"><span class="pre">libmpi</span></code> with the <code class="docutils literal notranslate"><span class="pre">RTLD_GLOBAL</span></code> option.</p>
<p>In C/C++, this option is specified as the second parameter to the
POSIX <code class="docutils literal notranslate"><span class="pre">dlopen(3)</span></code> function.</p>
<p>When using <code class="docutils literal notranslate"><span class="pre">ctypes</span></code> with Python, this can be done with the second
(optional) parameter to <code class="docutils literal notranslate"><span class="pre">CDLL()</span></code>. For example (shown below in Mac OS
X, where Open MPI’s shared library name ends in <code class="docutils literal notranslate"><span class="pre">.dylib</span></code>; other
operating systems use other suffixes, such as <code class="docutils literal notranslate"><span class="pre">.so</span></code>):</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">ctypes</span> <span class="kn">import</span> <span class="o">*</span>
<span class="n">mpi</span> <span class="o">=</span> <span class="n">CDLL</span><span class="p">(</span><span class="s1">'libmpi.0.dylib'</span><span class="p">,</span> <span class="n">RTLD_GLOBAL</span><span class="p">)</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">pythonapi</span><span class="o">.</span><span class="n">Py_GetArgcArgv</span>
<span class="n">argc</span> <span class="o">=</span> <span class="n">c_int</span><span class="p">()</span>
<span class="n">argv</span> <span class="o">=</span> <span class="n">POINTER</span><span class="p">(</span><span class="n">c_char_p</span><span class="p">)()</span>
<span class="n">f</span><span class="p">(</span><span class="n">byref</span><span class="p">(</span><span class="n">argc</span><span class="p">),</span> <span class="n">byref</span><span class="p">(</span><span class="n">argv</span><span class="p">))</span>
<span class="n">mpi</span><span class="o">.</span><span class="n">MPI_Init</span><span class="p">(</span><span class="n">byref</span><span class="p">(</span><span class="n">argc</span><span class="p">),</span> <span class="n">byref</span><span class="p">(</span><span class="n">argv</span><span class="p">))</span>
<span class="c1"># Your MPI program here</span>
<span class="n">mpi</span><span class="o">.</span><span class="n">MPI_Finalize</span><span class="p">()</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The above is just an example showing dynamic loading. If
you want to use MPI in Python, you are much better off using
<a class="reference external" href="https://mpi4py.github.io/">MPI4Py</a>.</p>
</div>
<p>Other scripting languages should have similar options when dynamically
loading shared libraries.</p>
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