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<div class="section" id="module-dl">
<h1>36.6. <tt class="xref docutils literal"><span class="pre">dl</span></tt> — Call C functions in shared objects<a class="headerlink" href="#module-dl" title="Permalink to this headline">¶</a></h1>
<p><em>Platforms: </em>Unix</p>
<p class="deprecated">
<span class="versionmodified">Deprecated since version 2.6: </span>The <tt class="xref docutils literal"><span class="pre">dl</span></tt> module has been removed in Python 3.0. Use the <a title="A foreign function library for Python." class="reference external" href="ctypes.html#module-ctypes"><tt class="xref docutils literal"><span class="pre">ctypes</span></tt></a>
module instead.</p>
<p>The <tt class="xref docutils literal"><span class="pre">dl</span></tt> module defines an interface to the <tt class="xref docutils literal"><span class="pre">dlopen()</span></tt> function, which
is the most common interface on Unix platforms for handling dynamically linked
libraries. It allows the program to call arbitrary functions in such a library.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">The <tt class="xref docutils literal"><span class="pre">dl</span></tt> module bypasses the Python type system and error handling. If
used incorrectly it may cause segmentation faults, crashes or other incorrect
behaviour.</p>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This module will not work unless <tt class="docutils literal"><span class="pre">sizeof(int)</span> <span class="pre">==</span> <span class="pre">sizeof(long)</span> <span class="pre">==</span> <span class="pre">sizeof(char</span>
<span class="pre">*)</span></tt> If this is not the case, <a title="exceptions.SystemError" class="reference external" href="exceptions.html#exceptions.SystemError"><tt class="xref docutils literal"><span class="pre">SystemError</span></tt></a> will be raised on import.</p>
</div>
<p>The <tt class="xref docutils literal"><span class="pre">dl</span></tt> module defines the following function:</p>
<dl class="function">
<dt id="dl.open">
<tt class="descclassname">dl.</tt><tt class="descname">open</tt><big>(</big><em>name</em><span class="optional">[</span>, <em>mode=RTLD_LAZY</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#dl.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open a shared object file, and return a handle. Mode signifies late binding
(<a title="dl.RTLD_LAZY" class="reference internal" href="#dl.RTLD_LAZY"><tt class="xref docutils literal"><span class="pre">RTLD_LAZY</span></tt></a>) or immediate binding (<a title="dl.RTLD_NOW" class="reference internal" href="#dl.RTLD_NOW"><tt class="xref docutils literal"><span class="pre">RTLD_NOW</span></tt></a>). Default is
<a title="dl.RTLD_LAZY" class="reference internal" href="#dl.RTLD_LAZY"><tt class="xref docutils literal"><span class="pre">RTLD_LAZY</span></tt></a>. Note that some systems do not support <a title="dl.RTLD_NOW" class="reference internal" href="#dl.RTLD_NOW"><tt class="xref docutils literal"><span class="pre">RTLD_NOW</span></tt></a>.</p>
<p>Return value is a <tt class="xref docutils literal"><span class="pre">dlobject</span></tt>.</p>
</dd></dl>
<p>The <tt class="xref docutils literal"><span class="pre">dl</span></tt> module defines the following constants:</p>
<dl class="data">
<dt id="dl.RTLD_LAZY">
<tt class="descclassname">dl.</tt><tt class="descname">RTLD_LAZY</tt><a class="headerlink" href="#dl.RTLD_LAZY" title="Permalink to this definition">¶</a></dt>
<dd>Useful as an argument to <a title="dl.open" class="reference internal" href="#dl.open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a>.</dd></dl>
<dl class="data">
<dt id="dl.RTLD_NOW">
<tt class="descclassname">dl.</tt><tt class="descname">RTLD_NOW</tt><a class="headerlink" href="#dl.RTLD_NOW" title="Permalink to this definition">¶</a></dt>
<dd>Useful as an argument to <a title="dl.open" class="reference internal" href="#dl.open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a>. Note that on systems which do not
support immediate binding, this constant will not appear in the module. For
maximum portability, use <a title="hasattr" class="reference external" href="functions.html#hasattr"><tt class="xref docutils literal"><span class="pre">hasattr()</span></tt></a> to determine if the system supports
immediate binding.</dd></dl>
<p>The <tt class="xref docutils literal"><span class="pre">dl</span></tt> module defines the following exception:</p>
<dl class="exception">
<dt id="dl.error">
<em class="property">exception </em><tt class="descclassname">dl.</tt><tt class="descname">error</tt><a class="headerlink" href="#dl.error" title="Permalink to this definition">¶</a></dt>
<dd>Exception raised when an error has occurred inside the dynamic loading and
linking routines.</dd></dl>
<p>Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">dl</span><span class="o">,</span> <span class="nn">time</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">=</span><span class="n">dl</span><span class="o">.</span><span class="n">open</span><span class="p">(</span><span class="s">'/lib/libc.so.6'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">call</span><span class="p">(</span><span class="s">'time'</span><span class="p">),</span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span>
<span class="go">(929723914, 929723914.498)</span>
</pre></div>
</div>
<p>This example was tried on a Debian GNU/Linux system, and is a good example of
the fact that using this module is usually a bad alternative.</p>
<div class="section" id="dl-objects">
<span id="id1"></span><h2>36.6.1. Dl Objects<a class="headerlink" href="#dl-objects" title="Permalink to this headline">¶</a></h2>
<p>Dl objects, as returned by <a title="dl.open" class="reference internal" href="#dl.open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a> above, have the following methods:</p>
<dl class="method">
<dt id="dl.dl.close">
<tt class="descclassname">dl.</tt><tt class="descname">close</tt><big>(</big><big>)</big><a class="headerlink" href="#dl.dl.close" title="Permalink to this definition">¶</a></dt>
<dd>Free all resources, except the memory.</dd></dl>
<dl class="method">
<dt id="dl.dl.sym">
<tt class="descclassname">dl.</tt><tt class="descname">sym</tt><big>(</big><em>name</em><big>)</big><a class="headerlink" href="#dl.dl.sym" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the pointer for the function named <em>name</em>, as a number, if it exists in
the referenced shared object, otherwise <tt class="xref docutils literal"><span class="pre">None</span></tt>. This is useful in code like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">if</span> <span class="n">a</span><span class="o">.</span><span class="n">sym</span><span class="p">(</span><span class="s">'time'</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">a</span><span class="o">.</span><span class="n">call</span><span class="p">(</span><span class="s">'time'</span><span class="p">)</span>
<span class="gp">... </span><span class="k">else</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span>
</pre></div>
</div>
<p>(Note that this function will return a non-zero number, as zero is the <em>NULL</em>
pointer)</p>
</dd></dl>
<dl class="method">
<dt id="dl.dl.call">
<tt class="descclassname">dl.</tt><tt class="descname">call</tt><big>(</big><em>name</em><span class="optional">[</span>, <em>arg1</em><span class="optional">[</span>, <em>arg2...</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#dl.dl.call" title="Permalink to this definition">¶</a></dt>
<dd><p>Call the function named <em>name</em> in the referenced shared object. The arguments
must be either Python integers, which will be passed as is, Python strings, to
which a pointer will be passed, or <tt class="xref docutils literal"><span class="pre">None</span></tt>, which will be passed as <em>NULL</em>.
Note that strings should only be passed to functions as <tt class="xref docutils literal"><span class="pre">const</span> <span class="pre">char*</span></tt>,
as Python will not like its string mutated.</p>
<p>There must be at most 10 arguments, and arguments not given will be treated as
<tt class="xref docutils literal"><span class="pre">None</span></tt>. The function’s return value must be a C <tt class="xref docutils literal"><span class="pre">long</span></tt>, which is a
Python integer.</p>
</dd></dl>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">36.6. <tt class="docutils literal"><span class="pre">dl</span></tt> — Call C functions in shared objects</a><ul>
<li><a class="reference external" href="#dl-objects">36.6.1. Dl Objects</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="crypt.html"
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