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<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 they regularly manipulate 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: <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code>,
<code class="xref c c-func docutils literal notranslate"><span class="pre">calloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">realloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code>. 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 notranslate"><div class="highlight"><pre><span></span><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">PyBytes_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 bytes 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 class="admonition seealso">
<p class="admonition-title">See also</p>
<p>The <span class="target" id="index-1"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONMALLOC"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONMALLOC</span></code></a> environment variable can be used to configure
the memory allocators used by Python.</p>
<p>The <span class="target" id="index-2"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONMALLOCSTATS"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONMALLOCSTATS</span></code></a> environment variable can be used to print
statistics of the <a class="reference internal" href="#pymalloc"><span class="std std-ref">pymalloc memory allocator</span></a> every time a
new pymalloc object arena is created, and on shutdown.</p>
</div>
</div>
<div class="section" id="raw-memory-interface">
<h2>Raw Memory Interface<a class="headerlink" href="#raw-memory-interface" title="Permalink to this headline">¶</a></h2>
<p>The following function sets are wrappers to the system allocator. These
functions are thread-safe, the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">GIL</span></a> does not
need to be held.</p>
<p>The <a class="reference internal" href="#default-memory-allocators"><span class="std std-ref">default raw memory allocator</span></a> uses
the following functions: <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">calloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">realloc()</span></code>
and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code>; call <code class="docutils literal notranslate"><span class="pre">malloc(1)</span></code> (or <code class="docutils literal notranslate"><span class="pre">calloc(1,</span> <span class="pre">1)</span></code>) when requesting
zero bytes.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
<dl class="function">
<dt id="c.PyMem_RawMalloc">
void* <code class="sig-name descname">PyMem_RawMalloc</code><span class="sig-paren">(</span>size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_RawMalloc" title="Permalink to this definition">¶</a></dt>
<dd><p>Allocates <em>n</em> bytes and returns a pointer of type <code class="xref c c-type docutils literal notranslate"><span class="pre">void*</span></code> to the
allocated memory, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if the request fails.</p>
<p>Requesting zero bytes returns a distinct non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer if possible, as
if <code class="docutils literal notranslate"><span class="pre">PyMem_RawMalloc(1)</span></code> had been called instead. The memory will not have
been initialized in any way.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_RawCalloc">
void* <code class="sig-name descname">PyMem_RawCalloc</code><span class="sig-paren">(</span>size_t<em> nelem</em>, size_t<em> elsize</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_RawCalloc" title="Permalink to this definition">¶</a></dt>
<dd><p>Allocates <em>nelem</em> elements each whose size in bytes is <em>elsize</em> and returns
a pointer of type <code class="xref c c-type docutils literal notranslate"><span class="pre">void*</span></code> to the allocated memory, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if the
request fails. The memory is initialized to zeros.</p>
<p>Requesting zero elements or elements of size zero bytes returns a distinct
non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer if possible, as if <code class="docutils literal notranslate"><span class="pre">PyMem_RawCalloc(1,</span> <span class="pre">1)</span></code> had been
called instead.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.5.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_RawRealloc">
void* <code class="sig-name descname">PyMem_RawRealloc</code><span class="sig-paren">(</span>void<em> *p</em>, size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_RawRealloc" 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.</p>
<p>If <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, the call is equivalent to <code class="docutils literal notranslate"><span class="pre">PyMem_RawMalloc(n)</span></code>; else if
<em>n</em> is equal to zero, the memory block is resized but is not freed, and the
returned pointer is non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<p>Unless <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, it must have been returned by a previous call to
<a class="reference internal" href="#c.PyMem_RawMalloc" title="PyMem_RawMalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawMalloc()</span></code></a>, <a class="reference internal" href="#c.PyMem_RawRealloc" title="PyMem_RawRealloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawRealloc()</span></code></a> or
<a class="reference internal" href="#c.PyMem_RawCalloc" title="PyMem_RawCalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawCalloc()</span></code></a>.</p>
<p>If the request fails, <a class="reference internal" href="#c.PyMem_RawRealloc" title="PyMem_RawRealloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawRealloc()</span></code></a> returns <code class="docutils literal notranslate"><span class="pre">NULL</span></code> and <em>p</em>
remains a valid pointer to the previous memory area.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_RawFree">
void <code class="sig-name descname">PyMem_RawFree</code><span class="sig-paren">(</span>void<em> *p</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_RawFree" 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="#c.PyMem_RawMalloc" title="PyMem_RawMalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawMalloc()</span></code></a>, <a class="reference internal" href="#c.PyMem_RawRealloc" title="PyMem_RawRealloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawRealloc()</span></code></a> or
<a class="reference internal" href="#c.PyMem_RawCalloc" title="PyMem_RawCalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawCalloc()</span></code></a>. Otherwise, or if <code class="docutils literal notranslate"><span class="pre">PyMem_RawFree(p)</span></code> has been
called before, undefined behavior occurs.</p>
<p>If <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, no operation is performed.</p>
</dd></dl>
</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>
<p>The <a class="reference internal" href="#default-memory-allocators"><span class="std std-ref">default memory allocator</span></a> uses the
<a class="reference internal" href="#pymalloc"><span class="std std-ref">pymalloc memory allocator</span></a>.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">GIL</span></a> must be held when using these
functions.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.6: </span>The default allocator is now pymalloc instead of system <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code>.</p>
</div>
<dl class="function">
<dt id="c.PyMem_Malloc">
void* <code class="sig-name descname">PyMem_Malloc</code><span class="sig-paren">(</span>size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_Malloc" title="Permalink to this definition">¶</a></dt>
<dd><p>Allocates <em>n</em> bytes and returns a pointer of type <code class="xref c c-type docutils literal notranslate"><span class="pre">void*</span></code> to the
allocated memory, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if the request fails.</p>
<p>Requesting zero bytes returns a distinct non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer if possible, as
if <code class="docutils literal notranslate"><span class="pre">PyMem_Malloc(1)</span></code> had been called instead. The memory will not have
been initialized in any way.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_Calloc">
void* <code class="sig-name descname">PyMem_Calloc</code><span class="sig-paren">(</span>size_t<em> nelem</em>, size_t<em> elsize</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_Calloc" title="Permalink to this definition">¶</a></dt>
<dd><p>Allocates <em>nelem</em> elements each whose size in bytes is <em>elsize</em> and returns
a pointer of type <code class="xref c c-type docutils literal notranslate"><span class="pre">void*</span></code> to the allocated memory, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if the
request fails. The memory is initialized to zeros.</p>
<p>Requesting zero elements or elements of size zero bytes returns a distinct
non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer if possible, as if <code class="docutils literal notranslate"><span class="pre">PyMem_Calloc(1,</span> <span class="pre">1)</span></code> had been called
instead.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.5.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_Realloc">
void* <code class="sig-name descname">PyMem_Realloc</code><span class="sig-paren">(</span>void<em> *p</em>, size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.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.</p>
<p>If <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, the call is equivalent to <code class="docutils literal notranslate"><span class="pre">PyMem_Malloc(n)</span></code>; else if <em>n</em>
is equal to zero, the memory block is resized but is not freed, and the
returned pointer is non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<p>Unless <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, it must have been returned by a previous call to
<a class="reference internal" href="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>, <a class="reference internal" href="#c.PyMem_Realloc" title="PyMem_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Realloc()</span></code></a> or <a class="reference internal" href="#c.PyMem_Calloc" title="PyMem_Calloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Calloc()</span></code></a>.</p>
<p>If the request fails, <a class="reference internal" href="#c.PyMem_Realloc" title="PyMem_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Realloc()</span></code></a> returns <code class="docutils literal notranslate"><span class="pre">NULL</span></code> and <em>p</em> remains
a valid pointer to the previous memory area.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_Free">
void <code class="sig-name descname">PyMem_Free</code><span class="sig-paren">(</span>void<em> *p</em><span class="sig-paren">)</span><a class="headerlink" href="#c.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="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>, <a class="reference internal" href="#c.PyMem_Realloc" title="PyMem_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Realloc()</span></code></a> or
<a class="reference internal" href="#c.PyMem_Calloc" title="PyMem_Calloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Calloc()</span></code></a>. Otherwise, or if <code class="docutils literal notranslate"><span class="pre">PyMem_Free(p)</span></code> has been called
before, undefined behavior occurs.</p>
<p>If <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, 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="c.PyMem_New">
TYPE* <code class="sig-name descname">PyMem_New</code><span class="sig-paren">(</span>TYPE, size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_New" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <a class="reference internal" href="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>, but allocates <code class="docutils literal notranslate"><span class="pre">(n</span> <span class="pre">*</span> <span class="pre">sizeof(TYPE))</span></code> bytes of
memory. Returns a pointer cast to <code class="xref c c-type docutils literal notranslate"><span class="pre">TYPE*</span></code>. The memory will not have
been initialized in any way.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_Resize">
TYPE* <code class="sig-name descname">PyMem_Resize</code><span class="sig-paren">(</span>void<em> *p</em>, TYPE, size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_Resize" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <a class="reference internal" href="#c.PyMem_Realloc" title="PyMem_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Realloc()</span></code></a>, but the memory block is resized to <code class="docutils literal notranslate"><span class="pre">(n</span> <span class="pre">*</span>
<span class="pre">sizeof(TYPE))</span></code> bytes. Returns a pointer cast to <code class="xref c c-type docutils literal notranslate"><span class="pre">TYPE*</span></code>. On return,
<em>p</em> will be a pointer to the new memory area, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> in the event of
failure.</p>
<p>This is a C preprocessor macro; <em>p</em> is always reassigned. Save the original
value of <em>p</em> to avoid losing memory when handling errors.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_Del">
void <code class="sig-name descname">PyMem_Del</code><span class="sig-paren">(</span>void<em> *p</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_Del" title="Permalink to this definition">¶</a></dt>
<dd><p>Same as <a class="reference internal" href="#c.PyMem_Free" title="PyMem_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Free()</span></code></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>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">PyMem_MALLOC(size)</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PyMem_NEW(type,</span> <span class="pre">size)</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PyMem_REALLOC(ptr,</span> <span class="pre">size)</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PyMem_RESIZE(ptr,</span> <span class="pre">type,</span> <span class="pre">size)</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PyMem_FREE(ptr)</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PyMem_DEL(ptr)</span></code></p></li>
</ul>
</div>
<div class="section" id="object-allocators">
<h2>Object allocators<a class="headerlink" href="#object-allocators" 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>
<p>The <a class="reference internal" href="#default-memory-allocators"><span class="std std-ref">default object allocator</span></a> uses the
<a class="reference internal" href="#pymalloc"><span class="std std-ref">pymalloc memory allocator</span></a>.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">GIL</span></a> must be held when using these
functions.</p>
</div>
<dl class="function">
<dt id="c.PyObject_Malloc">
void* <code class="sig-name descname">PyObject_Malloc</code><span class="sig-paren">(</span>size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_Malloc" title="Permalink to this definition">¶</a></dt>
<dd><p>Allocates <em>n</em> bytes and returns a pointer of type <code class="xref c c-type docutils literal notranslate"><span class="pre">void*</span></code> to the
allocated memory, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if the request fails.</p>
<p>Requesting zero bytes returns a distinct non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer if possible, as
if <code class="docutils literal notranslate"><span class="pre">PyObject_Malloc(1)</span></code> had been called instead. The memory will not have
been initialized in any way.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyObject_Calloc">
void* <code class="sig-name descname">PyObject_Calloc</code><span class="sig-paren">(</span>size_t<em> nelem</em>, size_t<em> elsize</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_Calloc" title="Permalink to this definition">¶</a></dt>
<dd><p>Allocates <em>nelem</em> elements each whose size in bytes is <em>elsize</em> and returns
a pointer of type <code class="xref c c-type docutils literal notranslate"><span class="pre">void*</span></code> to the allocated memory, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> if the
request fails. The memory is initialized to zeros.</p>
<p>Requesting zero elements or elements of size zero bytes returns a distinct
non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer if possible, as if <code class="docutils literal notranslate"><span class="pre">PyObject_Calloc(1,</span> <span class="pre">1)</span></code> had been called
instead.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.5.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="c.PyObject_Realloc">
void* <code class="sig-name descname">PyObject_Realloc</code><span class="sig-paren">(</span>void<em> *p</em>, size_t<em> n</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_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.</p>
<p>If <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, the call is equivalent to <code class="docutils literal notranslate"><span class="pre">PyObject_Malloc(n)</span></code>; else if <em>n</em>
is equal to zero, the memory block is resized but is not freed, and the
returned pointer is non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<p>Unless <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, it must have been returned by a previous call to
<a class="reference internal" href="#c.PyObject_Malloc" title="PyObject_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Malloc()</span></code></a>, <a class="reference internal" href="#c.PyObject_Realloc" title="PyObject_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Realloc()</span></code></a> or <a class="reference internal" href="#c.PyObject_Calloc" title="PyObject_Calloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Calloc()</span></code></a>.</p>
<p>If the request fails, <a class="reference internal" href="#c.PyObject_Realloc" title="PyObject_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Realloc()</span></code></a> returns <code class="docutils literal notranslate"><span class="pre">NULL</span></code> and <em>p</em> remains
a valid pointer to the previous memory area.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyObject_Free">
void <code class="sig-name descname">PyObject_Free</code><span class="sig-paren">(</span>void<em> *p</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_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="#c.PyObject_Malloc" title="PyObject_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Malloc()</span></code></a>, <a class="reference internal" href="#c.PyObject_Realloc" title="PyObject_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Realloc()</span></code></a> or
<a class="reference internal" href="#c.PyObject_Calloc" title="PyObject_Calloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Calloc()</span></code></a>. Otherwise, or if <code class="docutils literal notranslate"><span class="pre">PyObject_Free(p)</span></code> has been called
before, undefined behavior occurs.</p>
<p>If <em>p</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, no operation is performed.</p>
</dd></dl>
</div>
<div class="section" id="default-memory-allocators">
<span id="id1"></span><h2>Default Memory Allocators<a class="headerlink" href="#default-memory-allocators" title="Permalink to this headline">¶</a></h2>
<p>Default memory allocators:</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 28%" />
<col style="width: 18%" />
<col style="width: 16%" />
<col style="width: 19%" />
<col style="width: 18%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Configuration</p></th>
<th class="head"><p>Name</p></th>
<th class="head"><p>PyMem_RawMalloc</p></th>
<th class="head"><p>PyMem_Malloc</p></th>
<th class="head"><p>PyObject_Malloc</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p>Release build</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">"pymalloc"</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">pymalloc</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">pymalloc</span></code></p></td>
</tr>
<tr class="row-odd"><td><p>Debug build</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">"pymalloc_debug"</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code> + debug</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">pymalloc</span></code> + debug</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">pymalloc</span></code> + debug</p></td>
</tr>
<tr class="row-even"><td><p>Release build, without pymalloc</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">"malloc"</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code></p></td>
</tr>
<tr class="row-odd"><td><p>Debug build, without pymalloc</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">"malloc_debug"</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code> + debug</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code> + debug</p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code> + debug</p></td>
</tr>
</tbody>
</table>
<p>Legend:</p>
<ul class="simple">
<li><p>Name: value for <span class="target" id="index-3"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONMALLOC"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONMALLOC</span></code></a> environment variable</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">malloc</span></code>: system allocators from the standard C library, C functions:
<code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">calloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">realloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">pymalloc</span></code>: <a class="reference internal" href="#pymalloc"><span class="std std-ref">pymalloc memory allocator</span></a></p></li>
<li><p>“+ debug”: with debug hooks installed by <a class="reference internal" href="#c.PyMem_SetupDebugHooks" title="PyMem_SetupDebugHooks"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_SetupDebugHooks()</span></code></a></p></li>
</ul>
</div>
<div class="section" id="customize-memory-allocators">
<h2>Customize Memory Allocators<a class="headerlink" href="#customize-memory-allocators" title="Permalink to this headline">¶</a></h2>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
<dl class="type">
<dt id="c.PyMemAllocatorEx">
<code class="sig-name descname">PyMemAllocatorEx</code><a class="headerlink" href="#c.PyMemAllocatorEx" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure used to describe a memory block allocator. The structure has
four fields:</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 60%" />
<col style="width: 40%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Field</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*ctx</span></code></p></td>
<td><p>user context passed as first argument</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">void*</span> <span class="pre">malloc(void</span> <span class="pre">*ctx,</span> <span class="pre">size_t</span> <span class="pre">size)</span></code></p></td>
<td><p>allocate a memory block</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">void*</span> <span class="pre">calloc(void</span> <span class="pre">*ctx,</span> <span class="pre">size_t</span> <span class="pre">nelem,</span> <span class="pre">size_t</span> <span class="pre">elsize)</span></code></p></td>
<td><p>allocate a memory block initialized
with zeros</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">void*</span> <span class="pre">realloc(void</span> <span class="pre">*ctx,</span> <span class="pre">void</span> <span class="pre">*ptr,</span> <span class="pre">size_t</span> <span class="pre">new_size)</span></code></p></td>
<td><p>allocate or resize a memory block</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">free(void</span> <span class="pre">*ctx,</span> <span class="pre">void</span> <span class="pre">*ptr)</span></code></p></td>
<td><p>free a memory block</p></td>
</tr>
</tbody>
</table>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>The <code class="xref c c-type docutils literal notranslate"><span class="pre">PyMemAllocator</span></code> structure was renamed to
<a class="reference internal" href="#c.PyMemAllocatorEx" title="PyMemAllocatorEx"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyMemAllocatorEx</span></code></a> and a new <code class="docutils literal notranslate"><span class="pre">calloc</span></code> field was added.</p>
</div>
</dd></dl>
<dl class="type">
<dt id="c.PyMemAllocatorDomain">
<code class="sig-name descname">PyMemAllocatorDomain</code><a class="headerlink" href="#c.PyMemAllocatorDomain" title="Permalink to this definition">¶</a></dt>
<dd><p>Enum used to identify an allocator domain. Domains:</p>
<dl class="var">
<dt id="c.PYMEM_DOMAIN_RAW">
<code class="sig-name descname">PYMEM_DOMAIN_RAW</code><a class="headerlink" href="#c.PYMEM_DOMAIN_RAW" title="Permalink to this definition">¶</a></dt>
<dd><p>Functions:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#c.PyMem_RawMalloc" title="PyMem_RawMalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawMalloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyMem_RawRealloc" title="PyMem_RawRealloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawRealloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyMem_RawCalloc" title="PyMem_RawCalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawCalloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyMem_RawFree" title="PyMem_RawFree"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawFree()</span></code></a></p></li>
</ul>
</dd></dl>
<dl class="var">
<dt id="c.PYMEM_DOMAIN_MEM">
<code class="sig-name descname">PYMEM_DOMAIN_MEM</code><a class="headerlink" href="#c.PYMEM_DOMAIN_MEM" title="Permalink to this definition">¶</a></dt>
<dd><p>Functions:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>,</p></li>
<li><p><a class="reference internal" href="#c.PyMem_Realloc" title="PyMem_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Realloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyMem_Calloc" title="PyMem_Calloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Calloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyMem_Free" title="PyMem_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Free()</span></code></a></p></li>
</ul>
</dd></dl>
<dl class="var">
<dt id="c.PYMEM_DOMAIN_OBJ">
<code class="sig-name descname">PYMEM_DOMAIN_OBJ</code><a class="headerlink" href="#c.PYMEM_DOMAIN_OBJ" title="Permalink to this definition">¶</a></dt>
<dd><p>Functions:</p>
<ul class="simple">
<li><p><a class="reference internal" href="#c.PyObject_Malloc" title="PyObject_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Malloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyObject_Realloc" title="PyObject_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Realloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyObject_Calloc" title="PyObject_Calloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Calloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="#c.PyObject_Free" title="PyObject_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Free()</span></code></a></p></li>
</ul>
</dd></dl>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_GetAllocator">
void <code class="sig-name descname">PyMem_GetAllocator</code><span class="sig-paren">(</span><a class="reference internal" href="#c.PyMemAllocatorDomain" title="PyMemAllocatorDomain">PyMemAllocatorDomain</a><em> domain</em>, <a class="reference internal" href="#c.PyMemAllocatorEx" title="PyMemAllocatorEx">PyMemAllocatorEx</a><em> *allocator</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_GetAllocator" title="Permalink to this definition">¶</a></dt>
<dd><p>Get the memory block allocator of the specified domain.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_SetAllocator">
void <code class="sig-name descname">PyMem_SetAllocator</code><span class="sig-paren">(</span><a class="reference internal" href="#c.PyMemAllocatorDomain" title="PyMemAllocatorDomain">PyMemAllocatorDomain</a><em> domain</em>, <a class="reference internal" href="#c.PyMemAllocatorEx" title="PyMemAllocatorEx">PyMemAllocatorEx</a><em> *allocator</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_SetAllocator" title="Permalink to this definition">¶</a></dt>
<dd><p>Set the memory block allocator of the specified domain.</p>
<p>The new allocator must return a distinct non-<code class="docutils literal notranslate"><span class="pre">NULL</span></code> pointer when requesting
zero bytes.</p>
<p>For the <a class="reference internal" href="#c.PYMEM_DOMAIN_RAW" title="PYMEM_DOMAIN_RAW"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_RAW</span></code></a> domain, the allocator must be
thread-safe: the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">GIL</span></a> is not held when the
allocator is called.</p>
<p>If the new allocator is not a hook (does not call the previous allocator),
the <a class="reference internal" href="#c.PyMem_SetupDebugHooks" title="PyMem_SetupDebugHooks"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_SetupDebugHooks()</span></code></a> function must be called to reinstall the
debug hooks on top on the new allocator.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyMem_SetupDebugHooks">
void <code class="sig-name descname">PyMem_SetupDebugHooks</code><span class="sig-paren">(</span>void<span class="sig-paren">)</span><a class="headerlink" href="#c.PyMem_SetupDebugHooks" title="Permalink to this definition">¶</a></dt>
<dd><p>Setup hooks to detect bugs in the Python memory allocator functions.</p>
<p>Newly allocated memory is filled with the byte <code class="docutils literal notranslate"><span class="pre">0xCD</span></code> (<code class="docutils literal notranslate"><span class="pre">CLEANBYTE</span></code>),
freed memory is filled with the byte <code class="docutils literal notranslate"><span class="pre">0xDD</span></code> (<code class="docutils literal notranslate"><span class="pre">DEADBYTE</span></code>). Memory blocks
are surrounded by “forbidden bytes” (<code class="docutils literal notranslate"><span class="pre">FORBIDDENBYTE</span></code>: byte <code class="docutils literal notranslate"><span class="pre">0xFD</span></code>).</p>
<p>Runtime checks:</p>
<ul class="simple">
<li><p>Detect API violations, ex: <a class="reference internal" href="#c.PyObject_Free" title="PyObject_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Free()</span></code></a> called on a buffer
allocated by <a class="reference internal" href="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a></p></li>
<li><p>Detect write before the start of the buffer (buffer underflow)</p></li>
<li><p>Detect write after the end of the buffer (buffer overflow)</p></li>
<li><p>Check that the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">GIL</span></a> is held when
allocator functions of <a class="reference internal" href="#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a> (ex:
<a class="reference internal" href="#c.PyObject_Malloc" title="PyObject_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Malloc()</span></code></a>) and <a class="reference internal" href="#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a> (ex:
<a class="reference internal" href="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>) domains are called</p></li>
</ul>
<p>On error, the debug hooks use the <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> module to get the
traceback where a memory block was allocated. The traceback is only
displayed if <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> is tracing Python memory allocations and the
memory block was traced.</p>
<p>These hooks are <a class="reference internal" href="#default-memory-allocators"><span class="std std-ref">installed by default</span></a> if
Python is compiled in debug
mode. The <span class="target" id="index-4"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONMALLOC"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONMALLOC</span></code></a> environment variable can be used to install
debug hooks on a Python compiled in release mode.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.6: </span>This function now also works on Python compiled in release mode.
On error, the debug hooks now use <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> to get the traceback
where a memory block was allocated. The debug hooks now also check
if the GIL is held when functions of <a class="reference internal" href="#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a> and
<a class="reference internal" href="#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a> domains are called.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7.3: </span>Byte patterns <code class="docutils literal notranslate"><span class="pre">0xCB</span></code> (<code class="docutils literal notranslate"><span class="pre">CLEANBYTE</span></code>), <code class="docutils literal notranslate"><span class="pre">0xDB</span></code> (<code class="docutils literal notranslate"><span class="pre">DEADBYTE</span></code>) and
<code class="docutils literal notranslate"><span class="pre">0xFB</span></code> (<code class="docutils literal notranslate"><span class="pre">FORBIDDENBYTE</span></code>) have been replaced with <code class="docutils literal notranslate"><span class="pre">0xCD</span></code>, <code class="docutils literal notranslate"><span class="pre">0xDD</span></code>
and <code class="docutils literal notranslate"><span class="pre">0xFD</span></code> to use the same values than Windows CRT debug <code class="docutils literal notranslate"><span class="pre">malloc()</span></code>
and <code class="docutils literal notranslate"><span class="pre">free()</span></code>.</p>
</div>
</dd></dl>
</div>
<div class="section" id="the-pymalloc-allocator">
<span id="pymalloc"></span><h2>The pymalloc allocator<a class="headerlink" href="#the-pymalloc-allocator" title="Permalink to this headline">¶</a></h2>
<p>Python has a <em>pymalloc</em> allocator optimized for small objects (smaller or equal
to 512 bytes) with a short lifetime. It uses memory mappings called “arenas”
with a fixed size of 256 KiB. It falls back to <a class="reference internal" href="#c.PyMem_RawMalloc" title="PyMem_RawMalloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawMalloc()</span></code></a> and
<a class="reference internal" href="#c.PyMem_RawRealloc" title="PyMem_RawRealloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_RawRealloc()</span></code></a> for allocations larger than 512 bytes.</p>
<p><em>pymalloc</em> is the <a class="reference internal" href="#default-memory-allocators"><span class="std std-ref">default allocator</span></a> of the
<a class="reference internal" href="#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a> (ex: <a class="reference internal" href="#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>) and
<a class="reference internal" href="#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal notranslate"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a> (ex: <a class="reference internal" href="#c.PyObject_Malloc" title="PyObject_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Malloc()</span></code></a>) domains.</p>
<p>The arena allocator uses the following functions:</p>
<ul class="simple">
<li><p><code class="xref c c-func docutils literal notranslate"><span class="pre">VirtualAlloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">VirtualFree()</span></code> on Windows,</p></li>
<li><p><code class="xref c c-func docutils literal notranslate"><span class="pre">mmap()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">munmap()</span></code> if available,</p></li>
<li><p><code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> otherwise.</p></li>
</ul>
<div class="section" id="customize-pymalloc-arena-allocator">
<h3>Customize pymalloc Arena Allocator<a class="headerlink" href="#customize-pymalloc-arena-allocator" title="Permalink to this headline">¶</a></h3>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.4.</span></p>
</div>
<dl class="type">
<dt id="c.PyObjectArenaAllocator">
<code class="sig-name descname">PyObjectArenaAllocator</code><a class="headerlink" href="#c.PyObjectArenaAllocator" title="Permalink to this definition">¶</a></dt>
<dd><p>Structure used to describe an arena allocator. The structure has
three fields:</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 56%" />
<col style="width: 44%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Field</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*ctx</span></code></p></td>
<td><p>user context passed as first argument</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">void*</span> <span class="pre">alloc(void</span> <span class="pre">*ctx,</span> <span class="pre">size_t</span> <span class="pre">size)</span></code></p></td>
<td><p>allocate an arena of size bytes</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">free(void</span> <span class="pre">*ctx,</span> <span class="pre">size_t</span> <span class="pre">size,</span> <span class="pre">void</span> <span class="pre">*ptr)</span></code></p></td>
<td><p>free an arena</p></td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="function">
<dt id="c.PyObject_GetArenaAllocator">
<code class="sig-name descname">PyObject_GetArenaAllocator</code><span class="sig-paren">(</span><a class="reference internal" href="#c.PyObjectArenaAllocator" title="PyObjectArenaAllocator">PyObjectArenaAllocator</a><em> *allocator</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GetArenaAllocator" title="Permalink to this definition">¶</a></dt>
<dd><p>Get the arena allocator.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyObject_SetArenaAllocator">
<code class="sig-name descname">PyObject_SetArenaAllocator</code><span class="sig-paren">(</span><a class="reference internal" href="#c.PyObjectArenaAllocator" title="PyObjectArenaAllocator">PyObjectArenaAllocator</a><em> *allocator</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_SetArenaAllocator" title="Permalink to this definition">¶</a></dt>
<dd><p>Set the arena allocator.</p>
</dd></dl>
</div>
</div>
<div class="section" id="tracemalloc-c-api">
<h2>tracemalloc C API<a class="headerlink" href="#tracemalloc-c-api" title="Permalink to this headline">¶</a></h2>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
<dl class="function">
<dt id="c.PyTraceMalloc_Track">
int <code class="sig-name descname">PyTraceMalloc_Track</code><span class="sig-paren">(</span>unsigned int<em> domain</em>, uintptr_t<em> ptr</em>, size_t<em> size</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyTraceMalloc_Track" title="Permalink to this definition">¶</a></dt>
<dd><p>Track an allocated memory block in the <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> module.</p>
<p>Return <code class="docutils literal notranslate"><span class="pre">0</span></code> on success, return <code class="docutils literal notranslate"><span class="pre">-1</span></code> on error (failed to allocate memory to
store the trace). Return <code class="docutils literal notranslate"><span class="pre">-2</span></code> if tracemalloc is disabled.</p>
<p>If memory block is already tracked, update the existing trace.</p>
</dd></dl>
<dl class="function">
<dt id="c.PyTraceMalloc_Untrack">
int <code class="sig-name descname">PyTraceMalloc_Untrack</code><span class="sig-paren">(</span>unsigned int<em> domain</em>, uintptr_t<em> ptr</em><span class="sig-paren">)</span><a class="headerlink" href="#c.PyTraceMalloc_Untrack" title="Permalink to this definition">¶</a></dt>
<dd><p>Untrack an allocated memory block in the <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tracemalloc</span></code></a> module.
Do nothing if the block was not tracked.</p>
<p>Return <code class="docutils literal notranslate"><span class="pre">-2</span></code> if tracemalloc is disabled, otherwise return <code class="docutils literal notranslate"><span class="pre">0</span></code>.</p>
</dd></dl>
</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"><span class="std std-ref">Overview</span></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 notranslate"><div class="highlight"><pre><span></span><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">PyBytes_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 notranslate"><div class="highlight"><pre><span></span><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">PyBytes_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 notranslate"><div class="highlight"><pre><span></span><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#c.PyObject_New" title="PyObject_New"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_New()</span></code></a>,
<a class="reference internal" href="allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_NewVar()</span></code></a> and <a class="reference internal" href="allocation.html#c.PyObject_Del" title="PyObject_Del"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Del()</span></code></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" role="navigation" aria-label="main navigation">
<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="#raw-memory-interface">Raw Memory Interface</a></li>
<li><a class="reference internal" href="#memory-interface">Memory Interface</a></li>
<li><a class="reference internal" href="#object-allocators">Object allocators</a></li>
<li><a class="reference internal" href="#default-memory-allocators">Default Memory Allocators</a></li>
<li><a class="reference internal" href="#customize-memory-allocators">Customize Memory Allocators</a></li>
<li><a class="reference internal" href="#the-pymalloc-allocator">The pymalloc allocator</a><ul>
<li><a class="reference internal" href="#customize-pymalloc-arena-allocator">Customize pymalloc Arena Allocator</a></li>
</ul>
</li>
<li><a class="reference internal" href="#tracemalloc-c-api">tracemalloc C API</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>
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title="next chapter">Object Implementation Support</a></p>
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