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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>Memory Management — 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="Python/C API Reference Manual" href="index.html" /> <link rel="next" title="Object Implementation Support" href="objimpl.html" /> <link rel="prev" title="Initialization, Finalization, and Threads" href="init.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="objimpl.html" title="Object Implementation Support" accesskey="N">next</a> |</li> <li class="right" > <a href="init.html" title="Initialization, Finalization, and Threads" 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" accesskey="U">Python/C API Reference Manual</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="memory-management"> <span id="memory"></span><h1>Memory Management<a class="headerlink" href="#memory-management" title="Permalink to this headline">¶</a></h1> <div class="section" id="overview"> <span id="memoryoverview"></span><h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h2> <p>Memory management in Python involves a private heap containing all Python objects and data structures. The management of this private heap is ensured internally by the <em>Python memory manager</em>. The Python memory manager has different components which deal with various dynamic storage management aspects, like sharing, segmentation, preallocation or caching.</p> <p>At the lowest level, a raw memory allocator ensures that there is enough room in the private heap for storing all Python-related data by interacting with the memory manager of the operating system. On top of the raw memory allocator, several object-specific allocators operate on the same heap and implement distinct memory management policies adapted to the peculiarities of every object type. For example, integer objects are managed differently within the heap than strings, tuples or dictionaries because integers imply different storage requirements and speed/space tradeoffs. The Python memory manager thus delegates some of the work to the object-specific allocators, but ensures that the latter operate within the bounds of the private heap.</p> <p>It is important to understand that the management of the Python heap is performed by the interpreter itself and that the user has no control over it, even if she regularly manipulates object pointers to memory blocks inside that heap. The allocation of heap space for Python objects and other internal buffers is performed on demand by the Python memory manager through the Python/C API functions listed in this document.</p> <p id="index-0">To avoid memory corruption, extension writers should never try to operate on Python objects with the functions exported by the C library: <tt class="xref c c-func docutils literal"><span class="pre">malloc()</span></tt>, <tt class="xref c c-func docutils literal"><span class="pre">calloc()</span></tt>, <tt class="xref c c-func docutils literal"><span class="pre">realloc()</span></tt> and <tt class="xref c c-func docutils literal"><span class="pre">free()</span></tt>. This will result in mixed calls between the C allocator and the Python memory manager with fatal consequences, because they implement different algorithms and operate on different heaps. However, one may safely allocate and release memory blocks with the C library allocator for individual purposes, as shown in the following example:</p> <div class="highlight-c"><div class="highlight"><pre><span class="n">PyObject</span> <span class="o">*</span><span class="n">res</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">malloc</span><span class="p">(</span><span class="n">BUFSIZ</span><span class="p">);</span> <span class="cm">/* for I/O */</span> <span class="k">if</span> <span class="p">(</span><span class="n">buf</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span> <span class="n">PyErr_NoMemory</span><span class="p">();</span> <span class="p">...</span><span class="n">Do</span> <span class="n">some</span> <span class="n">I</span><span class="o">/</span><span class="n">O</span> <span class="n">operation</span> <span class="n">involving</span> <span class="n">buf</span><span class="p">...</span> <span class="n">res</span> <span class="o">=</span> <span class="n">PyString_FromString</span><span class="p">(</span><span class="n">buf</span><span class="p">);</span> <span class="n">free</span><span class="p">(</span><span class="n">buf</span><span class="p">);</span> <span class="cm">/* malloc'ed */</span> <span class="k">return</span> <span class="n">res</span><span class="p">;</span> </pre></div> </div> <p>In this example, the memory request for the I/O buffer is handled by the C library allocator. The Python memory manager is involved only in the allocation of the string object returned as a result.</p> <p>In most situations, however, it is recommended to allocate memory from the Python heap specifically because the latter is under control of the Python memory manager. For example, this is required when the interpreter is extended with new object types written in C. Another reason for using the Python heap is the desire to <em>inform</em> the Python memory manager about the memory needs of the extension module. Even when the requested memory is used exclusively for internal, highly-specific purposes, delegating all memory requests to the Python memory manager causes the interpreter to have a more accurate image of its memory footprint as a whole. Consequently, under certain circumstances, the Python memory manager may or may not trigger appropriate actions, like garbage collection, memory compaction or other preventive procedures. Note that by using the C library allocator as shown in the previous example, the allocated memory for the I/O buffer escapes completely the Python memory manager.</p> </div> <div class="section" id="memory-interface"> <span id="memoryinterface"></span><h2>Memory Interface<a class="headerlink" href="#memory-interface" title="Permalink to this headline">¶</a></h2> <p>The following function sets, modeled after the ANSI C standard, but specifying behavior when requesting zero bytes, are available for allocating and releasing memory from the Python heap:</p> <dl class="function"> <dt id="PyMem_Malloc"> void* <tt class="descname">PyMem_Malloc</tt><big>(</big>size_t<em> n</em><big>)</big><a class="headerlink" href="#PyMem_Malloc" title="Permalink to this definition">¶</a></dt> <dd><p>Allocates <em>n</em> bytes and returns a pointer of type <tt class="xref c c-type docutils literal"><span class="pre">void*</span></tt> to the allocated memory, or <em>NULL</em> if the request fails. Requesting zero bytes returns a distinct non-<em>NULL</em> pointer if possible, as if <tt class="docutils literal"><span class="pre">PyMem_Malloc(1)</span></tt> had been called instead. The memory will not have been initialized in any way.</p> </dd></dl> <dl class="function"> <dt id="PyMem_Realloc"> void* <tt class="descname">PyMem_Realloc</tt><big>(</big>void<em> *p</em>, size_t<em> n</em><big>)</big><a class="headerlink" href="#PyMem_Realloc" title="Permalink to this definition">¶</a></dt> <dd><p>Resizes the memory block pointed to by <em>p</em> to <em>n</em> bytes. The contents will be unchanged to the minimum of the old and the new sizes. If <em>p</em> is <em>NULL</em>, the call is equivalent to <tt class="docutils literal"><span class="pre">PyMem_Malloc(n)</span></tt>; else if <em>n</em> is equal to zero, the memory block is resized but is not freed, and the returned pointer is non-<em>NULL</em>. Unless <em>p</em> is <em>NULL</em>, it must have been returned by a previous call to <a class="reference internal" href="#PyMem_Malloc" title="PyMem_Malloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Malloc()</span></tt></a> or <a class="reference internal" href="#PyMem_Realloc" title="PyMem_Realloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Realloc()</span></tt></a>. If the request fails, <a class="reference internal" href="#PyMem_Realloc" title="PyMem_Realloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Realloc()</span></tt></a> returns <em>NULL</em> and <em>p</em> remains a valid pointer to the previous memory area.</p> </dd></dl> <dl class="function"> <dt id="PyMem_Free"> void <tt class="descname">PyMem_Free</tt><big>(</big>void<em> *p</em><big>)</big><a class="headerlink" href="#PyMem_Free" title="Permalink to this definition">¶</a></dt> <dd><p>Frees the memory block pointed to by <em>p</em>, which must have been returned by a previous call to <a class="reference internal" href="#PyMem_Malloc" title="PyMem_Malloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Malloc()</span></tt></a> or <a class="reference internal" href="#PyMem_Realloc" title="PyMem_Realloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Realloc()</span></tt></a>. Otherwise, or if <tt class="docutils literal"><span class="pre">PyMem_Free(p)</span></tt> has been called before, undefined behavior occurs. If <em>p</em> is <em>NULL</em>, no operation is performed.</p> </dd></dl> <p>The following type-oriented macros are provided for convenience. Note that <em>TYPE</em> refers to any C type.</p> <dl class="function"> <dt id="PyMem_New"> TYPE* <tt class="descname">PyMem_New</tt><big>(</big>TYPE, size_t<em> n</em><big>)</big><a class="headerlink" href="#PyMem_New" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <a class="reference internal" href="#PyMem_Malloc" title="PyMem_Malloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Malloc()</span></tt></a>, but allocates <tt class="docutils literal"><span class="pre">(n</span> <span class="pre">*</span> <span class="pre">sizeof(TYPE))</span></tt> bytes of memory. Returns a pointer cast to <tt class="xref c c-type docutils literal"><span class="pre">TYPE*</span></tt>. The memory will not have been initialized in any way.</p> </dd></dl> <dl class="function"> <dt id="PyMem_Resize"> TYPE* <tt class="descname">PyMem_Resize</tt><big>(</big>void<em> *p</em>, TYPE, size_t<em> n</em><big>)</big><a class="headerlink" href="#PyMem_Resize" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <a class="reference internal" href="#PyMem_Realloc" title="PyMem_Realloc"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Realloc()</span></tt></a>, but the memory block is resized to <tt class="docutils literal"><span class="pre">(n</span> <span class="pre">*</span> <span class="pre">sizeof(TYPE))</span></tt> bytes. Returns a pointer cast to <tt class="xref c c-type docutils literal"><span class="pre">TYPE*</span></tt>. On return, <em>p</em> will be a pointer to the new memory area, or <em>NULL</em> in the event of failure. This is a C preprocessor macro; p is always reassigned. Save the original value of p to avoid losing memory when handling errors.</p> </dd></dl> <dl class="function"> <dt id="PyMem_Del"> void <tt class="descname">PyMem_Del</tt><big>(</big>void<em> *p</em><big>)</big><a class="headerlink" href="#PyMem_Del" title="Permalink to this definition">¶</a></dt> <dd><p>Same as <a class="reference internal" href="#PyMem_Free" title="PyMem_Free"><tt class="xref c c-func docutils literal"><span class="pre">PyMem_Free()</span></tt></a>.</p> </dd></dl> <p>In addition, the following macro sets are provided for calling the Python memory allocator directly, without involving the C API functions listed above. However, note that their use does not preserve binary compatibility across Python versions and is therefore deprecated in extension modules.</p> <p><tt class="xref c c-func docutils literal"><span class="pre">PyMem_MALLOC()</span></tt>, <tt class="xref c c-func docutils literal"><span class="pre">PyMem_REALLOC()</span></tt>, <tt class="xref c c-func docutils literal"><span class="pre">PyMem_FREE()</span></tt>.</p> <p><tt class="xref c c-func docutils literal"><span class="pre">PyMem_NEW()</span></tt>, <tt class="xref c c-func docutils literal"><span class="pre">PyMem_RESIZE()</span></tt>, <tt class="xref c c-func docutils literal"><span class="pre">PyMem_DEL()</span></tt>.</p> </div> <div class="section" id="examples"> <span id="memoryexamples"></span><h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2> <p>Here is the example from section <a class="reference internal" href="#memoryoverview"><em>Overview</em></a>, rewritten so that the I/O buffer is allocated from the Python heap by using the first function set:</p> <div class="highlight-c"><div class="highlight"><pre><span class="n">PyObject</span> <span class="o">*</span><span class="n">res</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">PyMem_Malloc</span><span class="p">(</span><span class="n">BUFSIZ</span><span class="p">);</span> <span class="cm">/* for I/O */</span> <span class="k">if</span> <span class="p">(</span><span class="n">buf</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span> <span class="n">PyErr_NoMemory</span><span class="p">();</span> <span class="cm">/* ...Do some I/O operation involving buf... */</span> <span class="n">res</span> <span class="o">=</span> <span class="n">PyString_FromString</span><span class="p">(</span><span class="n">buf</span><span class="p">);</span> <span class="n">PyMem_Free</span><span class="p">(</span><span class="n">buf</span><span class="p">);</span> <span class="cm">/* allocated with PyMem_Malloc */</span> <span class="k">return</span> <span class="n">res</span><span class="p">;</span> </pre></div> </div> <p>The same code using the type-oriented function set:</p> <div class="highlight-c"><div class="highlight"><pre><span class="n">PyObject</span> <span class="o">*</span><span class="n">res</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf</span> <span class="o">=</span> <span class="n">PyMem_New</span><span class="p">(</span><span class="kt">char</span><span class="p">,</span> <span class="n">BUFSIZ</span><span class="p">);</span> <span class="cm">/* for I/O */</span> <span class="k">if</span> <span class="p">(</span><span class="n">buf</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="k">return</span> <span class="n">PyErr_NoMemory</span><span class="p">();</span> <span class="cm">/* ...Do some I/O operation involving buf... */</span> <span class="n">res</span> <span class="o">=</span> <span class="n">PyString_FromString</span><span class="p">(</span><span class="n">buf</span><span class="p">);</span> <span class="n">PyMem_Del</span><span class="p">(</span><span class="n">buf</span><span class="p">);</span> <span class="cm">/* allocated with PyMem_New */</span> <span class="k">return</span> <span class="n">res</span><span class="p">;</span> </pre></div> </div> <p>Note that in the two examples above, the buffer is always manipulated via functions belonging to the same set. Indeed, it is required to use the same memory API family for a given memory block, so that the risk of mixing different allocators is reduced to a minimum. The following code sequence contains two errors, one of which is labeled as <em>fatal</em> because it mixes two different allocators operating on different heaps.</p> <div class="highlight-c"><div class="highlight"><pre><span class="kt">char</span> <span class="o">*</span><span class="n">buf1</span> <span class="o">=</span> <span class="n">PyMem_New</span><span class="p">(</span><span class="kt">char</span><span class="p">,</span> <span class="n">BUFSIZ</span><span class="p">);</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf2</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">malloc</span><span class="p">(</span><span class="n">BUFSIZ</span><span class="p">);</span> <span class="kt">char</span> <span class="o">*</span><span class="n">buf3</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">PyMem_Malloc</span><span class="p">(</span><span class="n">BUFSIZ</span><span class="p">);</span> <span class="p">...</span> <span class="n">PyMem_Del</span><span class="p">(</span><span class="n">buf3</span><span class="p">);</span> <span class="cm">/* Wrong -- should be PyMem_Free() */</span> <span class="n">free</span><span class="p">(</span><span class="n">buf2</span><span class="p">);</span> <span class="cm">/* Right -- allocated via malloc() */</span> <span class="n">free</span><span class="p">(</span><span class="n">buf1</span><span class="p">);</span> <span class="cm">/* Fatal -- should be PyMem_Del() */</span> </pre></div> </div> <p>In addition to the functions aimed at handling raw memory blocks from the Python heap, objects in Python are allocated and released with <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>, <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> and <a class="reference internal" href="allocation.html#PyObject_Del" title="PyObject_Del"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_Del()</span></tt></a>.</p> <p>These will be explained in the next chapter on defining and implementing new object types in C.</p> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Memory Management</a><ul> <li><a class="reference internal" href="#overview">Overview</a></li> <li><a class="reference internal" href="#memory-interface">Memory Interface</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="init.html" title="previous chapter">Initialization, Finalization, and Threads</a></p> <h4>Next topic</h4> <p class="topless"><a href="objimpl.html" title="next chapter">Object Implementation Support</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/memory.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="objimpl.html" title="Object Implementation Support" >next</a> |</li> <li class="right" > <a href="init.html" title="Initialization, Finalization, and Threads" >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> </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>? 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