.. highlight:: c


.. _memory:

*****************
Memory Management
*****************

.. sectionauthor:: Vladimir Marangozov <Vladimir.Marangozov@inrialpes.fr>



.. _memoryoverview:

Overview
========

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 *Python memory manager*.  The Python memory manager has
different components which deal with various dynamic storage management aspects,
like sharing, segmentation, preallocation or caching.

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.

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.

.. index::
   single: malloc (C function)
   single: calloc (C function)
   single: realloc (C function)
   single: free (C function)

To avoid memory corruption, extension writers should never try to operate on
Python objects with the functions exported by the C library: :c:func:`malloc`,
:c:func:`calloc`, :c:func:`realloc` and :c:func:`free`.  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::

   PyObject *res;
   char *buf = (char *) malloc(BUFSIZ); /* for I/O */

   if (buf == NULL)
       return PyErr_NoMemory();
   ...Do some I/O operation involving buf...
   res = PyBytes_FromString(buf);
   free(buf); /* malloc'ed */
   return res;

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.

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 *inform* 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.

.. seealso::

   The :envvar:`PYTHONMALLOC` environment variable can be used to configure
   the memory allocators used by Python.

   The :envvar:`PYTHONMALLOCSTATS` environment variable can be used to print
   statistics of the :ref:`pymalloc memory allocator <pymalloc>` every time a
   new pymalloc object arena is created, and on shutdown.

Allocator Domains
=================

.. _allocator-domains:

All allocating functions belong to one of three different "domains" (see also
:c:type:`PyMemAllocatorDomain`). These domains represent different allocation
strategies and are optimized for different purposes. The specific details on
how every domain allocates memory or what internal functions each domain calls
is considered an implementation detail, but for debugging purposes a simplified
table can be found at :ref:`default-memory-allocators`.
The APIs used to allocate and free a block of memory must be from the same domain.
For example, :c:func:`PyMem_Free` must be used to free memory allocated using :c:func:`PyMem_Malloc`.

The three allocation domains are:

* Raw domain: intended for allocating memory for general-purpose memory
  buffers where the allocation *must* go to the system allocator or where the
  allocator can operate without an :term:`attached thread state`. The memory
  is requested directly from the system. See :ref:`Raw Memory Interface <raw-memoryinterface>`.

* "Mem" domain: intended for allocating memory for Python buffers and
  general-purpose memory buffers where the allocation must be performed with
  an :term:`attached thread state`. The memory is taken from the Python private heap.
  See :ref:`Memory Interface <memoryinterface>`.

* Object domain: intended for allocating memory for Python objects. The
  memory is taken from the Python private heap. See :ref:`Object allocators <objectinterface>`.

.. note::

  The :term:`free-threaded <free threading>` build requires that only Python objects are allocated using the "object" domain
  and that all Python objects are allocated using that domain. This differs from the prior Python versions,
  where this was only a best practice and not a hard requirement.

  For example, buffers (non-Python objects) should be allocated using :c:func:`PyMem_Malloc`,
  :c:func:`PyMem_RawMalloc`, or :c:func:`malloc`, but not :c:func:`PyObject_Malloc`.

  See :ref:`Memory Allocation APIs <free-threaded-memory-allocation>`.


.. _raw-memoryinterface:

Raw Memory Interface
====================

The following function sets are wrappers to the system allocator. These
functions are thread-safe, so a :term:`thread state` does not
need to be :term:`attached <attached thread state>`.

The :ref:`default raw memory allocator <default-memory-allocators>` uses
the following functions: :c:func:`malloc`, :c:func:`calloc`, :c:func:`realloc`
and :c:func:`!free`; call ``malloc(1)`` (or ``calloc(1, 1)``) when requesting
zero bytes.

.. versionadded:: 3.4

.. c:function:: void* PyMem_RawMalloc(size_t n)

   Allocates *n* bytes and returns a pointer of type :c:expr:`void*` to the
   allocated memory, or ``NULL`` if the request fails.

   Requesting zero bytes returns a distinct non-``NULL`` pointer if possible, as
   if ``PyMem_RawMalloc(1)`` had been called instead. The memory will not have
   been initialized in any way.


.. c:function:: void* PyMem_RawCalloc(size_t nelem, size_t elsize)

   Allocates *nelem* elements each whose size in bytes is *elsize* and returns
   a pointer of type :c:expr:`void*` to the allocated memory, or ``NULL`` if the
   request fails. The memory is initialized to zeros.

   Requesting zero elements or elements of size zero bytes returns a distinct
   non-``NULL`` pointer if possible, as if ``PyMem_RawCalloc(1, 1)`` had been
   called instead.

   .. versionadded:: 3.5


.. c:function:: void* PyMem_RawRealloc(void *p, size_t n)

   Resizes the memory block pointed to by *p* to *n* bytes. The contents will
   be unchanged to the minimum of the old and the new sizes.

   If *p* is ``NULL``, the call is equivalent to ``PyMem_RawMalloc(n)``; else if
   *n* is equal to zero, the memory block is resized but is not freed, and the
   returned pointer is non-``NULL``.

   Unless *p* is ``NULL``, it must have been returned by a previous call to
   :c:func:`PyMem_RawMalloc`, :c:func:`PyMem_RawRealloc` or
   :c:func:`PyMem_RawCalloc`.

   If the request fails, :c:func:`PyMem_RawRealloc` returns ``NULL`` and *p*
   remains a valid pointer to the previous memory area.


.. c:function:: void PyMem_RawFree(void *p)

   Frees the memory block pointed to by *p*, which must have been returned by a
   previous call to :c:func:`PyMem_RawMalloc`, :c:func:`PyMem_RawRealloc` or
   :c:func:`PyMem_RawCalloc`.  Otherwise, or if ``PyMem_RawFree(p)`` has been
   called before, undefined behavior occurs.

   If *p* is ``NULL``, no operation is performed.


.. _memoryinterface:

Memory Interface
================

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.

In the GIL-enabled build (default build) the
:ref:`default memory allocator <default-memory-allocators>` uses the
:ref:`pymalloc memory allocator <pymalloc>`, whereas in the
:term:`free-threaded build`, the default is the
:ref:`mimalloc memory allocator <mimalloc>` instead.

.. warning::

   There must be an :term:`attached thread state` when using these functions.

.. versionchanged:: 3.6

   The default allocator is now pymalloc instead of system :c:func:`malloc`.

.. versionchanged:: 3.13

   In the :term:`free-threaded <free threading>` build, the default allocator
   is now :ref:`mimalloc <mimalloc>`.

.. c:function:: void* PyMem_Malloc(size_t n)

   Allocates *n* bytes and returns a pointer of type :c:expr:`void*` to the
   allocated memory, or ``NULL`` if the request fails.

   Requesting zero bytes returns a distinct non-``NULL`` pointer if possible, as
   if ``PyMem_Malloc(1)`` had been called instead. The memory will not have
   been initialized in any way.


.. c:function:: void* PyMem_Calloc(size_t nelem, size_t elsize)

   Allocates *nelem* elements each whose size in bytes is *elsize* and returns
   a pointer of type :c:expr:`void*` to the allocated memory, or ``NULL`` if the
   request fails. The memory is initialized to zeros.

   Requesting zero elements or elements of size zero bytes returns a distinct
   non-``NULL`` pointer if possible, as if ``PyMem_Calloc(1, 1)`` had been called
   instead.

   .. versionadded:: 3.5


.. c:function:: void* PyMem_Realloc(void *p, size_t n)

   Resizes the memory block pointed to by *p* to *n* bytes. The contents will be
   unchanged to the minimum of the old and the new sizes.

   If *p* is ``NULL``, the call is equivalent to ``PyMem_Malloc(n)``; else if *n*
   is equal to zero, the memory block is resized but is not freed, and the
   returned pointer is non-``NULL``.

   Unless *p* is ``NULL``, it must have been returned by a previous call to
   :c:func:`PyMem_Malloc`, :c:func:`PyMem_Realloc` or :c:func:`PyMem_Calloc`.

   If the request fails, :c:func:`PyMem_Realloc` returns ``NULL`` and *p* remains
   a valid pointer to the previous memory area.


.. c:function:: void PyMem_Free(void *p)

   Frees the memory block pointed to by *p*, which must have been returned by a
   previous call to :c:func:`PyMem_Malloc`, :c:func:`PyMem_Realloc` or
   :c:func:`PyMem_Calloc`.  Otherwise, or if ``PyMem_Free(p)`` has been called
   before, undefined behavior occurs.

   If *p* is ``NULL``, no operation is performed.

The following type-oriented macros are provided for convenience.  Note  that
*TYPE* refers to any C type.


.. c:macro:: PyMem_New(TYPE, n)

   Same as :c:func:`PyMem_Malloc`, but allocates ``(n * sizeof(TYPE))`` bytes of
   memory.  Returns a pointer cast to ``TYPE*``.  The memory will not have
   been initialized in any way.


.. c:macro:: PyMem_Resize(p, TYPE, n)

   Same as :c:func:`PyMem_Realloc`, but the memory block is resized to ``(n *
   sizeof(TYPE))`` bytes.  Returns a pointer cast to ``TYPE*``. On return,
   *p* will be a pointer to the new memory area, or ``NULL`` 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.


.. c:function:: void PyMem_Del(void *p)

   Same as :c:func:`PyMem_Free`.


Deprecated aliases
------------------

These are :term:`soft deprecated` aliases to existing functions and macros.
They exist solely for backwards compatibility.

.. list-table::
   :widths: auto
   :header-rows: 1

   * * Deprecated alias
     * Corresponding function or macro
   * * .. c:macro:: PyMem_MALLOC(size)
     * :c:func:`PyMem_Malloc`
   * * .. c:macro:: PyMem_NEW(type, size)
     * :c:macro:`PyMem_New`
   * * .. c:macro:: PyMem_REALLOC(ptr, size)
     * :c:func:`PyMem_Realloc`
   * * .. c:macro:: PyMem_RESIZE(ptr, type, size)
     * :c:macro:`PyMem_Resize`
   * * .. c:macro:: PyMem_FREE(ptr)
     * :c:func:`PyMem_Free`
   * * .. c:macro:: PyMem_DEL(ptr)
     * :c:func:`PyMem_Free`

.. versionchanged:: 3.4

   The macros are now aliases of the corresponding functions and macros.
   Previously, their behavior was the same, but their use did not necessarily
   preserve binary compatibility across Python versions.

.. deprecated:: 2.0


.. _objectinterface:

Object allocators
=================

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.

.. note::
    There is no guarantee that the memory returned by these allocators can be
    successfully cast to a Python object when intercepting the allocating
    functions in this domain by the methods described in
    the :ref:`Customize Memory Allocators <customize-memory-allocators>` section.

The :ref:`default object allocator <default-memory-allocators>` uses the
:ref:`pymalloc memory allocator <pymalloc>`.  In the
:term:`free-threaded <free threading>` build, the default is the
:ref:`mimalloc memory allocator <mimalloc>` instead.

.. warning::

   There must be an :term:`attached thread state` when using these functions.

.. c:function:: void* PyObject_Malloc(size_t n)

   Allocates *n* bytes and returns a pointer of type :c:expr:`void*` to the
   allocated memory, or ``NULL`` if the request fails.

   Requesting zero bytes returns a distinct non-``NULL`` pointer if possible, as
   if ``PyObject_Malloc(1)`` had been called instead. The memory will not have
   been initialized in any way.


.. c:function:: void* PyObject_Calloc(size_t nelem, size_t elsize)

   Allocates *nelem* elements each whose size in bytes is *elsize* and returns
   a pointer of type :c:expr:`void*` to the allocated memory, or ``NULL`` if the
   request fails. The memory is initialized to zeros.

   Requesting zero elements or elements of size zero bytes returns a distinct
   non-``NULL`` pointer if possible, as if ``PyObject_Calloc(1, 1)`` had been called
   instead.

   .. versionadded:: 3.5


.. c:function:: void* PyObject_Realloc(void *p, size_t n)

   Resizes the memory block pointed to by *p* to *n* bytes. The contents will be
   unchanged to the minimum of the old and the new sizes.

   If *p* is ``NULL``, the call is equivalent to ``PyObject_Malloc(n)``; else if *n*
   is equal to zero, the memory block is resized but is not freed, and the
   returned pointer is non-``NULL``.

   Unless *p* is ``NULL``, it must have been returned by a previous call to
   :c:func:`PyObject_Malloc`, :c:func:`PyObject_Realloc` or :c:func:`PyObject_Calloc`.

   If the request fails, :c:func:`PyObject_Realloc` returns ``NULL`` and *p* remains
   a valid pointer to the previous memory area.


.. c:function:: void PyObject_Free(void *p)

   Frees the memory block pointed to by *p*, which must have been returned by a
   previous call to :c:func:`PyObject_Malloc`, :c:func:`PyObject_Realloc` or
   :c:func:`PyObject_Calloc`.  Otherwise, or if ``PyObject_Free(p)`` has been called
   before, undefined behavior occurs.

   If *p* is ``NULL``, no operation is performed.

   Do not call this directly to free an object's memory; call the type's
   :c:member:`~PyTypeObject.tp_free` slot instead.

   Do not use this for memory allocated by :c:macro:`PyObject_GC_New` or
   :c:macro:`PyObject_GC_NewVar`; use :c:func:`PyObject_GC_Del` instead.

   .. seealso::

      * :c:func:`PyObject_GC_Del` is the equivalent of this function for memory
        allocated by types that support garbage collection.
      * :c:func:`PyObject_Malloc`
      * :c:func:`PyObject_Realloc`
      * :c:func:`PyObject_Calloc`
      * :c:macro:`PyObject_New`
      * :c:macro:`PyObject_NewVar`
      * :c:func:`PyType_GenericAlloc`
      * :c:member:`~PyTypeObject.tp_free`


.. _default-memory-allocators:

Default Memory Allocators
=========================

Default memory allocators:

===================================  =======================  ====================  ======================  ======================
Configuration                        Name                     PyMem_RawMalloc       PyMem_Malloc            PyObject_Malloc
===================================  =======================  ====================  ======================  ======================
Release build                        ``"pymalloc"``           ``malloc``            ``pymalloc``            ``pymalloc``
Debug build                          ``"pymalloc_debug"``     ``malloc`` + debug    ``pymalloc`` + debug    ``pymalloc`` + debug
Release build, without pymalloc      ``"malloc"``             ``malloc``            ``malloc``              ``malloc``
Debug build, without pymalloc        ``"malloc_debug"``       ``malloc`` + debug    ``malloc`` + debug      ``malloc`` + debug
Free-threaded build                  ``"mimalloc"``           ``mimalloc``          ``mimalloc``            ``mimalloc``
Free-threaded debug build            ``"mimalloc_debug"``     ``mimalloc`` + debug  ``mimalloc`` + debug    ``mimalloc`` + debug
===================================  =======================  ====================  ======================  ======================

Legend:

* Name: value for :envvar:`PYTHONMALLOC` environment variable.
* ``malloc``: system allocators from the standard C library, C functions:
  :c:func:`malloc`, :c:func:`calloc`, :c:func:`realloc` and :c:func:`free`.
* ``pymalloc``: :ref:`pymalloc memory allocator <pymalloc>`.
* ``mimalloc``: :ref:`mimalloc memory allocator <mimalloc>`.
* "+ debug": with :ref:`debug hooks on the Python memory allocators
  <pymem-debug-hooks>`.
* "Debug build": :ref:`Python build in debug mode <debug-build>`.

.. _customize-memory-allocators:

Customize Memory Allocators
===========================

.. versionadded:: 3.4

.. c:type:: PyMemAllocatorEx

   Structure used to describe a memory block allocator. The structure has
   the following fields:

   +----------------------------------------------------------+---------------------------------------+
   | Field                                                    | Meaning                               |
   +==========================================================+=======================================+
   | ``void *ctx``                                            | user context passed as first argument |
   +----------------------------------------------------------+---------------------------------------+
   | ``void* malloc(void *ctx, size_t size)``                 | allocate a memory block               |
   +----------------------------------------------------------+---------------------------------------+
   | ``void* calloc(void *ctx, size_t nelem, size_t elsize)`` | allocate a memory block initialized   |
   |                                                          | with zeros                            |
   +----------------------------------------------------------+---------------------------------------+
   | ``void* realloc(void *ctx, void *ptr, size_t new_size)`` | allocate or resize a memory block     |
   +----------------------------------------------------------+---------------------------------------+
   | ``void free(void *ctx, void *ptr)``                      | free a memory block                   |
   +----------------------------------------------------------+---------------------------------------+

   .. versionchanged:: 3.5
      The :c:type:`!PyMemAllocator` structure was renamed to
      :c:type:`PyMemAllocatorEx` and a new ``calloc`` field was added.


.. c:type:: PyMemAllocatorDomain

   Enum used to identify an allocator domain. Domains:

   .. c:namespace:: NULL

   .. c:macro:: PYMEM_DOMAIN_RAW

      Functions:

      * :c:func:`PyMem_RawMalloc`
      * :c:func:`PyMem_RawRealloc`
      * :c:func:`PyMem_RawCalloc`
      * :c:func:`PyMem_RawFree`

   .. c:macro:: PYMEM_DOMAIN_MEM

      Functions:

      * :c:func:`PyMem_Malloc`,
      * :c:func:`PyMem_Realloc`
      * :c:func:`PyMem_Calloc`
      * :c:func:`PyMem_Free`

   .. c:macro:: PYMEM_DOMAIN_OBJ

      Functions:

      * :c:func:`PyObject_Malloc`
      * :c:func:`PyObject_Realloc`
      * :c:func:`PyObject_Calloc`
      * :c:func:`PyObject_Free`

.. c:function:: void PyMem_GetAllocator(PyMemAllocatorDomain domain, PyMemAllocatorEx *allocator)

   Get the memory block allocator of the specified domain.


.. c:function:: void PyMem_SetAllocator(PyMemAllocatorDomain domain, PyMemAllocatorEx *allocator)

   Set the memory block allocator of the specified domain.

   The new allocator must return a distinct non-``NULL`` pointer when requesting
   zero bytes.

   For the :c:macro:`PYMEM_DOMAIN_RAW` domain, the allocator must be
   thread-safe: a :term:`thread state` is not :term:`attached <attached thread state>`
   when the allocator is called.

   For the remaining domains, the allocator must also be thread-safe:
   the allocator may be called in different interpreters that do not
   share a :term:`GIL`.

   If the new allocator is not a hook (does not call the previous allocator),
   the :c:func:`PyMem_SetupDebugHooks` function must be called to reinstall the
   debug hooks on top on the new allocator.

   See also :c:member:`PyPreConfig.allocator` and :ref:`Preinitialize Python
   with PyPreConfig <c-preinit>`.

   .. warning::

       :c:func:`PyMem_SetAllocator` does have the following contract:

       * It can be called after :c:func:`Py_PreInitialize` and before
         :c:func:`Py_InitializeFromConfig` to install a custom memory
         allocator. There are no restrictions over the installed allocator
         other than the ones imposed by the domain (for instance, the Raw
         Domain allows the allocator to be called without an :term:`attached thread state`).
         See :ref:`the section on allocator domains <allocator-domains>` for more
         information.

       * If called after Python has finish initializing (after
         :c:func:`Py_InitializeFromConfig` has been called) the allocator
         **must** wrap the existing allocator. Substituting the current
         allocator for some other arbitrary one is **not supported**.

   .. versionchanged:: 3.12
      All allocators must be thread-safe.


.. c:function:: void PyMem_SetupDebugHooks(void)

   Setup :ref:`debug hooks in the Python memory allocators <pymem-debug-hooks>`
   to detect memory errors.


.. _pymem-debug-hooks:

Debug hooks on the Python memory allocators
===========================================

When :ref:`Python is built in debug mode <debug-build>`, the
:c:func:`PyMem_SetupDebugHooks` function is called at the :ref:`Python
preinitialization <c-preinit>` to setup debug hooks on Python memory allocators
to detect memory errors.

The :envvar:`PYTHONMALLOC` environment variable can be used to install debug
hooks on a Python compiled in release mode (ex: ``PYTHONMALLOC=debug``).

The :c:func:`PyMem_SetupDebugHooks` function can be used to set debug hooks
after calling :c:func:`PyMem_SetAllocator`.

These debug hooks fill dynamically allocated memory blocks with special,
recognizable bit patterns. Newly allocated memory is filled with the byte
``0xCD`` (``PYMEM_CLEANBYTE``), freed memory is filled with the byte ``0xDD``
(``PYMEM_DEADBYTE``). Memory blocks are surrounded by "forbidden bytes"
filled with the byte ``0xFD`` (``PYMEM_FORBIDDENBYTE``). Strings of these bytes
are unlikely to be valid addresses, floats, or ASCII strings.

Runtime checks:

- Detect API violations. For example, detect if :c:func:`PyObject_Free` is
  called on a memory block allocated by :c:func:`PyMem_Malloc`.
- Detect write before the start of the buffer (buffer underflow).
- Detect write after the end of the buffer (buffer overflow).
- Check that there is an :term:`attached thread state` when
  allocator functions of :c:macro:`PYMEM_DOMAIN_OBJ` (ex:
  :c:func:`PyObject_Malloc`) and :c:macro:`PYMEM_DOMAIN_MEM` (ex:
  :c:func:`PyMem_Malloc`) domains are called.

On error, the debug hooks use the :mod:`tracemalloc` module to get the
traceback where a memory block was allocated. The traceback is only displayed
if :mod:`tracemalloc` is tracing Python memory allocations and the memory block
was traced.

Let *S* = ``sizeof(size_t)``. ``2*S`` bytes are added at each end of each block
of *N* bytes requested.  The memory layout is like so, where p represents the
address returned by a malloc-like or realloc-like function (``p[i:j]`` means
the slice of bytes from ``*(p+i)`` inclusive up to ``*(p+j)`` exclusive; note
that the treatment of negative indices differs from a Python slice):

``p[-2*S:-S]``
    Number of bytes originally asked for.  This is a size_t, big-endian (easier
    to read in a memory dump).
``p[-S]``
    API identifier (ASCII character):

    * ``'r'`` for :c:macro:`PYMEM_DOMAIN_RAW`.
    * ``'m'`` for :c:macro:`PYMEM_DOMAIN_MEM`.
    * ``'o'`` for :c:macro:`PYMEM_DOMAIN_OBJ`.

``p[-S+1:0]``
    Copies of PYMEM_FORBIDDENBYTE.  Used to catch under- writes and reads.

``p[0:N]``
    The requested memory, filled with copies of PYMEM_CLEANBYTE, used to catch
    reference to uninitialized memory.  When a realloc-like function is called
    requesting a larger memory block, the new excess bytes are also filled with
    PYMEM_CLEANBYTE.  When a free-like function is called, these are
    overwritten with PYMEM_DEADBYTE, to catch reference to freed memory.  When
    a realloc- like function is called requesting a smaller memory block, the
    excess old bytes are also filled with PYMEM_DEADBYTE.

``p[N:N+S]``
    Copies of PYMEM_FORBIDDENBYTE.  Used to catch over- writes and reads.

``p[N+S:N+2*S]``
    Only used if the ``PYMEM_DEBUG_SERIALNO`` macro is defined (not defined by
    default).

    A serial number, incremented by 1 on each call to a malloc-like or
    realloc-like function.  Big-endian :c:type:`size_t`.  If "bad memory" is detected
    later, the serial number gives an excellent way to set a breakpoint on the
    next run, to capture the instant at which this block was passed out.  The
    static function bumpserialno() in obmalloc.c is the only place the serial
    number is incremented, and exists so you can set such a breakpoint easily.

A realloc-like or free-like function first checks that the PYMEM_FORBIDDENBYTE
bytes at each end are intact.  If they've been altered, diagnostic output is
written to stderr, and the program is aborted via Py_FatalError().  The other
main failure mode is provoking a memory error when a program reads up one of
the special bit patterns and tries to use it as an address.  If you get in a
debugger then and look at the object, you're likely to see that it's entirely
filled with PYMEM_DEADBYTE (meaning freed memory is getting used) or
PYMEM_CLEANBYTE (meaning uninitialized memory is getting used).

.. versionchanged:: 3.6
   The :c:func:`PyMem_SetupDebugHooks` function now also works on Python
   compiled in release mode.  On error, the debug hooks now use
   :mod:`tracemalloc` to get the traceback where a memory block was allocated.
   The debug hooks now also check if there is an :term:`attached thread state` when
   functions of :c:macro:`PYMEM_DOMAIN_OBJ` and :c:macro:`PYMEM_DOMAIN_MEM` domains are
   called.

.. versionchanged:: 3.8
   Byte patterns ``0xCB`` (``PYMEM_CLEANBYTE``), ``0xDB`` (``PYMEM_DEADBYTE``)
   and ``0xFB`` (``PYMEM_FORBIDDENBYTE``) have been replaced with ``0xCD``,
   ``0xDD`` and ``0xFD`` to use the same values than Windows CRT debug
   ``malloc()`` and ``free()``.


.. _pymalloc:

The pymalloc allocator
======================

Python has a *pymalloc* 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 either 256 KiB on 32-bit platforms or 1 MiB on 64-bit
platforms. It falls back to :c:func:`PyMem_RawMalloc` and
:c:func:`PyMem_RawRealloc` for allocations larger than 512 bytes.

*pymalloc* is the :ref:`default allocator <default-memory-allocators>` of the
:c:macro:`PYMEM_DOMAIN_MEM` (ex: :c:func:`PyMem_Malloc`) and
:c:macro:`PYMEM_DOMAIN_OBJ` (ex: :c:func:`PyObject_Malloc`) domains.

The arena allocator uses the following functions:

* :c:func:`!VirtualAlloc` and :c:func:`!VirtualFree` on Windows,
* :c:func:`!mmap` and :c:func:`!munmap` if available,
* :c:func:`malloc` and :c:func:`free` otherwise.

This allocator is disabled if Python is configured with the
:option:`--without-pymalloc` option. It can also be disabled at runtime using
the :envvar:`PYTHONMALLOC` environment variable (ex: ``PYTHONMALLOC=malloc``).

Typically, it makes sense to disable the pymalloc allocator when building
Python with AddressSanitizer (:option:`--with-address-sanitizer`) which helps
uncover low level bugs within the C code.

Customize pymalloc Arena Allocator
----------------------------------

.. versionadded:: 3.4

.. c:type:: PyObjectArenaAllocator

   Structure used to describe an arena allocator. The structure has
   three fields:

   +--------------------------------------------------+---------------------------------------+
   | Field                                            | Meaning                               |
   +==================================================+=======================================+
   | ``void *ctx``                                    | user context passed as first argument |
   +--------------------------------------------------+---------------------------------------+
   | ``void* alloc(void *ctx, size_t size)``          | allocate an arena of size bytes       |
   +--------------------------------------------------+---------------------------------------+
   | ``void free(void *ctx, void *ptr, size_t size)`` | free an arena                         |
   +--------------------------------------------------+---------------------------------------+

.. c:function:: void PyObject_GetArenaAllocator(PyObjectArenaAllocator *allocator)

   Get the arena allocator.

.. c:function:: void PyObject_SetArenaAllocator(PyObjectArenaAllocator *allocator)

   Set the arena allocator.

.. _mimalloc:

The mimalloc allocator
======================

.. versionadded:: 3.13

Python supports the `mimalloc <https://github.com/microsoft/mimalloc/>`__
allocator when the underlying platform support is available.
mimalloc is a general purpose allocator with excellent performance
characteristics, initially developed by Daan Leijen for the runtime systems
of the Koka and Lean languages.

Unlike :ref:`pymalloc <pymalloc>`, which is optimized for small objects (512
bytes or fewer), mimalloc handles allocations of any size.

In the :term:`free-threaded <free threading>` build, mimalloc is the default
and **required** allocator for the :c:macro:`PYMEM_DOMAIN_MEM` and
:c:macro:`PYMEM_DOMAIN_OBJ` domains.  It cannot be disabled in free-threaded
builds.  The free-threaded build uses per-thread mimalloc heaps, which allows
allocation and deallocation to proceed without locking in most cases.

In the default (non-free-threaded) build, mimalloc is available but not the
default allocator.  It can be selected at runtime using
:envvar:`PYTHONMALLOC`\ ``=mimalloc`` (or ``mimalloc_debug`` to include
:ref:`debug hooks <pymem-debug-hooks>`).  It can be disabled at build time
using the :option:`--without-mimalloc` configure option, but this option
cannot be combined with :option:`--disable-gil`.

tracemalloc C API
=================

.. versionadded:: 3.7

.. c:function:: int PyTraceMalloc_Track(unsigned int domain, uintptr_t ptr, size_t size)

   Track an allocated memory block in the :mod:`tracemalloc` module.

   Return ``0`` on success, return ``-1`` on error (failed to allocate memory to
   store the trace). Return ``-2`` if tracemalloc is disabled.

   If memory block is already tracked, update the existing trace.

.. c:function:: int PyTraceMalloc_Untrack(unsigned int domain, uintptr_t ptr)

   Untrack an allocated memory block in the :mod:`tracemalloc` module.
   Do nothing if the block was not tracked.

   Return ``-2`` if tracemalloc is disabled, otherwise return ``0``.


.. _memoryexamples:

Examples
========

Here is the example from section :ref:`memoryoverview`, rewritten so that the
I/O buffer is allocated from the Python heap by using the first function set::

   PyObject *res;
   char *buf = (char *) PyMem_Malloc(BUFSIZ); /* for I/O */

   if (buf == NULL)
       return PyErr_NoMemory();
   /* ...Do some I/O operation involving buf... */
   res = PyBytes_FromString(buf);
   PyMem_Free(buf); /* allocated with PyMem_Malloc */
   return res;

The same code using the type-oriented function set::

   PyObject *res;
   char *buf = PyMem_New(char, BUFSIZ); /* for I/O */

   if (buf == NULL)
       return PyErr_NoMemory();
   /* ...Do some I/O operation involving buf... */
   res = PyBytes_FromString(buf);
   PyMem_Free(buf); /* allocated with PyMem_New */
   return res;

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 *fatal* because it mixes two different
allocators operating on different heaps. ::

   char *buf1 = PyMem_New(char, BUFSIZ);
   char *buf2 = (char *) malloc(BUFSIZ);
   char *buf3 = (char *) PyMem_Malloc(BUFSIZ);
   ...
   PyMem_Del(buf3);  /* Wrong -- should be PyMem_Free() */
   free(buf2);       /* Right -- allocated via malloc() */
   free(buf1);       /* Fatal -- should be PyMem_Free()  */

In addition to the functions aimed at handling raw memory blocks from the Python
heap, objects in Python are allocated and released with :c:macro:`PyObject_New`,
:c:macro:`PyObject_NewVar` and :c:func:`PyObject_Free`.

These will be explained in the next chapter on defining and implementing new
object types in C.
