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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Supporting Cyclic Garbage Collection — Python 2.7.5 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.7.5', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.5 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python 2.7.5 documentation" href="../index.html" /> <link rel="up" title="Object Implementation Support" href="objimpl.html" /> <link rel="next" title="Distributing Python Modules" href="../distutils/index.html" /> <link rel="prev" title="Type Objects" href="typeobj.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="../distutils/index.html" title="Distributing Python Modules" accesskey="N">next</a> |</li> <li class="right" > <a href="typeobj.html" title="Type Objects" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >Python/C API Reference Manual</a> »</li> <li><a href="objimpl.html" accesskey="U">Object Implementation Support</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="supporting-cyclic-garbage-collection"> <span id="supporting-cycle-detection"></span><h1>Supporting Cyclic Garbage Collection<a class="headerlink" href="#supporting-cyclic-garbage-collection" title="Permalink to this headline">¶</a></h1> <p>Python’s support for detecting and collecting garbage which involves circular references requires support from object types which are “containers” for other objects which may also be containers. Types which do not store references to other objects, or which only store references to atomic types (such as numbers or strings), do not need to provide any explicit support for garbage collection.</p> <p>To create a container type, the <tt class="xref py py-attr docutils literal"><span class="pre">tp_flags</span></tt> field of the type object must include the <a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><tt class="xref py py-const docutils literal"><span class="pre">Py_TPFLAGS_HAVE_GC</span></tt></a> and provide an implementation of the <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler. If instances of the type are mutable, a <tt class="xref py py-attr docutils literal"><span class="pre">tp_clear</span></tt> implementation must also be provided.</p> <dl class="data"> <dt> <tt class="descname">Py_TPFLAGS_HAVE_GC</tt></dt> <dd><p>Objects with a type with this flag set must conform with the rules documented here. For convenience these objects will be referred to as container objects.</p> </dd></dl> <p>Constructors for container types must conform to two rules:</p> <ol class="arabic simple"> <li>The memory for the object must be allocated using <a class="reference internal" href="#PyObject_GC_New" title="PyObject_GC_New"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_New()</span></tt></a> or <a class="reference internal" href="#PyObject_GC_NewVar" title="PyObject_GC_NewVar"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_NewVar()</span></tt></a>.</li> <li>Once all the fields which may contain references to other containers are initialized, it must call <a class="reference internal" href="#PyObject_GC_Track" title="PyObject_GC_Track"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_Track()</span></tt></a>.</li> </ol> <dl class="function"> <dt id="PyObject_GC_New"> TYPE* <tt class="descname">PyObject_GC_New</tt><big>(</big>TYPE, <a class="reference internal" href="type.html#PyTypeObject" title="PyTypeObject">PyTypeObject</a><em> *type</em><big>)</big><a class="headerlink" href="#PyObject_GC_New" title="Permalink to this definition">¶</a></dt> <dd><p>Analogous to <a class="reference internal" href="allocation.html#PyObject_New" title="PyObject_New"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_New()</span></tt></a> but for container objects with the <a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><tt class="xref py py-const docutils literal"><span class="pre">Py_TPFLAGS_HAVE_GC</span></tt></a> flag set.</p> </dd></dl> <dl class="function"> <dt id="PyObject_GC_NewVar"> TYPE* <tt class="descname">PyObject_GC_NewVar</tt><big>(</big>TYPE, <a class="reference internal" href="type.html#PyTypeObject" title="PyTypeObject">PyTypeObject</a><em> *type</em>, Py_ssize_t<em> size</em><big>)</big><a class="headerlink" href="#PyObject_GC_NewVar" title="Permalink to this definition">¶</a></dt> <dd><p>Analogous to <a class="reference internal" href="allocation.html#PyObject_NewVar" title="PyObject_NewVar"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_NewVar()</span></tt></a> but for container objects with the <a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><tt class="xref py py-const docutils literal"><span class="pre">Py_TPFLAGS_HAVE_GC</span></tt></a> flag set.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref c c-type docutils literal"><span class="pre">int</span></tt> type for <em>size</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> <dl class="function"> <dt id="PyObject_GC_Resize"> TYPE* <tt class="descname">PyObject_GC_Resize</tt><big>(</big>TYPE, <a class="reference internal" href="structures.html#PyVarObject" title="PyVarObject">PyVarObject</a><em> *op</em>, Py_ssize_t<em> newsize</em><big>)</big><a class="headerlink" href="#PyObject_GC_Resize" title="Permalink to this definition">¶</a></dt> <dd><p>Resize an object allocated by <a class="reference internal" href="allocation.html#PyObject_NewVar" title="PyObject_NewVar"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_NewVar()</span></tt></a>. Returns the resized object or <em>NULL</em> on failure.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref c c-type docutils literal"><span class="pre">int</span></tt> type for <em>newsize</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> <dl class="function"> <dt id="PyObject_GC_Track"> void <tt class="descname">PyObject_GC_Track</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *op</em><big>)</big><a class="headerlink" href="#PyObject_GC_Track" title="Permalink to this definition">¶</a></dt> <dd><p>Adds the object <em>op</em> to the set of container objects tracked by the collector. The collector can run at unexpected times so objects must be valid while being tracked. This should be called once all the fields followed by the <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler become valid, usually near the end of the constructor.</p> </dd></dl> <dl class="function"> <dt id="_PyObject_GC_TRACK"> void <tt class="descname">_PyObject_GC_TRACK</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *op</em><big>)</big><a class="headerlink" href="#_PyObject_GC_TRACK" title="Permalink to this definition">¶</a></dt> <dd><p>A macro version of <a class="reference internal" href="#PyObject_GC_Track" title="PyObject_GC_Track"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_Track()</span></tt></a>. It should not be used for extension modules.</p> </dd></dl> <p>Similarly, the deallocator for the object must conform to a similar pair of rules:</p> <ol class="arabic simple"> <li>Before fields which refer to other containers are invalidated, <a class="reference internal" href="#PyObject_GC_UnTrack" title="PyObject_GC_UnTrack"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_UnTrack()</span></tt></a> must be called.</li> <li>The object’s memory must be deallocated using <a class="reference internal" href="#PyObject_GC_Del" title="PyObject_GC_Del"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_Del()</span></tt></a>.</li> </ol> <dl class="function"> <dt id="PyObject_GC_Del"> void <tt class="descname">PyObject_GC_Del</tt><big>(</big>void<em> *op</em><big>)</big><a class="headerlink" href="#PyObject_GC_Del" title="Permalink to this definition">¶</a></dt> <dd><p>Releases memory allocated to an object using <a class="reference internal" href="#PyObject_GC_New" title="PyObject_GC_New"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_New()</span></tt></a> or <a class="reference internal" href="#PyObject_GC_NewVar" title="PyObject_GC_NewVar"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_NewVar()</span></tt></a>.</p> </dd></dl> <dl class="function"> <dt id="PyObject_GC_UnTrack"> void <tt class="descname">PyObject_GC_UnTrack</tt><big>(</big>void<em> *op</em><big>)</big><a class="headerlink" href="#PyObject_GC_UnTrack" title="Permalink to this definition">¶</a></dt> <dd><p>Remove the object <em>op</em> from the set of container objects tracked by the collector. Note that <a class="reference internal" href="#PyObject_GC_Track" title="PyObject_GC_Track"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_Track()</span></tt></a> can be called again on this object to add it back to the set of tracked objects. The deallocator (<tt class="xref py py-attr docutils literal"><span class="pre">tp_dealloc</span></tt> handler) should call this for the object before any of the fields used by the <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler become invalid.</p> </dd></dl> <dl class="function"> <dt id="_PyObject_GC_UNTRACK"> void <tt class="descname">_PyObject_GC_UNTRACK</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *op</em><big>)</big><a class="headerlink" href="#_PyObject_GC_UNTRACK" title="Permalink to this definition">¶</a></dt> <dd><p>A macro version of <a class="reference internal" href="#PyObject_GC_UnTrack" title="PyObject_GC_UnTrack"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_GC_UnTrack()</span></tt></a>. It should not be used for extension modules.</p> </dd></dl> <p>The <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler accepts a function parameter of this type:</p> <dl class="type"> <dt id="visitproc"> int <tt class="descname">(*visitproc)</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *object</em>, void<em> *arg</em><big>)</big><a class="headerlink" href="#visitproc" title="Permalink to this definition">¶</a></dt> <dd><p>Type of the visitor function passed to the <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler. The function should be called with an object to traverse as <em>object</em> and the third parameter to the <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler as <em>arg</em>. The Python core uses several visitor functions to implement cyclic garbage detection; it’s not expected that users will need to write their own visitor functions.</p> </dd></dl> <p>The <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handler must have the following type:</p> <dl class="type"> <dt id="traverseproc"> int <tt class="descname">(*traverseproc)</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *self</em>, <a class="reference internal" href="#visitproc" title="visitproc">visitproc</a><em> visit</em>, void<em> *arg</em><big>)</big><a class="headerlink" href="#traverseproc" title="Permalink to this definition">¶</a></dt> <dd><p>Traversal function for a container object. Implementations must call the <em>visit</em> function for each object directly contained by <em>self</em>, with the parameters to <em>visit</em> being the contained object and the <em>arg</em> value passed to the handler. The <em>visit</em> function must not be called with a <em>NULL</em> object argument. If <em>visit</em> returns a non-zero value that value should be returned immediately.</p> </dd></dl> <p>To simplify writing <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handlers, a <a class="reference internal" href="#Py_VISIT" title="Py_VISIT"><tt class="xref c c-func docutils literal"><span class="pre">Py_VISIT()</span></tt></a> macro is provided. In order to use this macro, the <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> implementation must name its arguments exactly <em>visit</em> and <em>arg</em>:</p> <dl class="function"> <dt id="Py_VISIT"> void <tt class="descname">Py_VISIT</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *o</em><big>)</big><a class="headerlink" href="#Py_VISIT" title="Permalink to this definition">¶</a></dt> <dd><p>Call the <em>visit</em> callback, with arguments <em>o</em> and <em>arg</em>. If <em>visit</em> returns a non-zero value, then return it. Using this macro, <tt class="xref py py-attr docutils literal"><span class="pre">tp_traverse</span></tt> handlers look like:</p> <div class="highlight-c"><div class="highlight"><pre><span class="k">static</span> <span class="kt">int</span> <span class="nf">my_traverse</span><span class="p">(</span><span class="n">Noddy</span> <span class="o">*</span><span class="n">self</span><span class="p">,</span> <span class="n">visitproc</span> <span class="n">visit</span><span class="p">,</span> <span class="kt">void</span> <span class="o">*</span><span class="n">arg</span><span class="p">)</span> <span class="p">{</span> <span class="n">Py_VISIT</span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">foo</span><span class="p">);</span> <span class="n">Py_VISIT</span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">bar</span><span class="p">);</span> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p class="versionadded"> <span class="versionmodified">New in version 2.4.</span></p> </dd></dl> <p>The <tt class="xref py py-attr docutils literal"><span class="pre">tp_clear</span></tt> handler must be of the <a class="reference internal" href="#inquiry" title="inquiry"><tt class="xref c c-type docutils literal"><span class="pre">inquiry</span></tt></a> type, or <em>NULL</em> if the object is immutable.</p> <dl class="type"> <dt id="inquiry"> int <tt class="descname">(*inquiry)</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em> *self</em><big>)</big><a class="headerlink" href="#inquiry" title="Permalink to this definition">¶</a></dt> <dd><p>Drop references that may have created reference cycles. Immutable objects do not have to define this method since they can never directly create reference cycles. Note that the object must still be valid after calling this method (don’t just call <a class="reference internal" href="refcounting.html#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></a> on a reference). The collector will call this method if it detects that this object is involved in a reference cycle.</p> </dd></dl> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="typeobj.html" title="previous chapter">Type Objects</a></p> <h4>Next topic</h4> <p class="topless"><a href="../distutils/index.html" title="next chapter">Distributing Python Modules</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/c-api/gcsupport.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="../distutils/index.html" title="Distributing Python Modules" >next</a> |</li> <li class="right" > <a href="typeobj.html" title="Type Objects" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >Python/C API Reference Manual</a> »</li> <li><a href="objimpl.html" >Object Implementation Support</a> »</li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 1990-2020, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="http://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Oct 13, 2020. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.1.3. </div> </body> </html>