python_doc_md/Advanced/c-api/memory.md

# Memory Management {#memory}

## Overview {#memoryoverview}

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: `malloc`{.interpreted-text role="c:func"}, `calloc`{.interpreted-text role="c:func"}, `realloc`{.interpreted-text role="c:func"} and `free`{.interpreted-text role="c:func"}. 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:

``` c
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 `PYTHONMALLOC`{.interpreted-text role="envvar"} environment variable can be used to configure the memory allocators used by Python.

The `PYTHONMALLOCSTATS`{.interpreted-text role="envvar"} environment variable can be used to print statistics of the `pymalloc memory allocator <pymalloc>`{.interpreted-text role="ref"} 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 `PyMemAllocatorDomain`{.interpreted-text role="c:type"}). 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 `default-memory-allocators`{.interpreted-text role="ref"}. The APIs used to allocate and free a block of memory must be from the same domain. For example, `PyMem_Free`{.interpreted-text role="c:func"} must be used to free memory allocated using `PyMem_Malloc`{.interpreted-text role="c:func"}.
:::

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 `attached thread state`{.interpreted-text role="term"}. The memory is requested directly from the system. See `Raw Memory Interface <raw-memoryinterface>`{.interpreted-text role="ref"}.
- \"Mem\" domain: intended for allocating memory for Python buffers and general-purpose memory buffers where the allocation must be performed with an `attached thread state`{.interpreted-text role="term"}. The memory is taken from the Python private heap. See `Memory Interface <memoryinterface>`{.interpreted-text role="ref"}.
- Object domain: intended for allocating memory for Python objects. The memory is taken from the Python private heap. See `Object allocators <objectinterface>`{.interpreted-text role="ref"}.

:::: note
::: title
Note
:::

The `free-threaded <free threading>`{.interpreted-text role="term"} 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 `PyMem_Malloc`{.interpreted-text role="c:func"}, `PyMem_RawMalloc`{.interpreted-text role="c:func"}, or `malloc`{.interpreted-text role="c:func"}, but not `PyObject_Malloc`{.interpreted-text role="c:func"}.

See `Memory Allocation APIs <free-threaded-memory-allocation>`{.interpreted-text role="ref"}.
::::

## Raw Memory Interface {#raw-memoryinterface}

The following function sets are wrappers to the system allocator. These functions are thread-safe, so a `thread state`{.interpreted-text role="term"} does not need to be `attached <attached thread state>`{.interpreted-text role="term"}.

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

::: versionadded
3.4
:::

> Allocates *n* bytes and returns a pointer of type `void*`{.interpreted-text role="c:expr"} 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.

> Allocates *nelem* elements each whose size in bytes is *elsize* and returns a pointer of type `void*`{.interpreted-text role="c:expr"} 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
> :::

> 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 `PyMem_RawMalloc`{.interpreted-text role="c:func"}, `PyMem_RawRealloc`{.interpreted-text role="c:func"} or `PyMem_RawCalloc`{.interpreted-text role="c:func"}.
>
> If the request fails, `PyMem_RawRealloc`{.interpreted-text role="c:func"} returns `NULL` and *p* remains a valid pointer to the previous memory area.

> Frees the memory block pointed to by *p*, which must have been returned by a previous call to `PyMem_RawMalloc`{.interpreted-text role="c:func"}, `PyMem_RawRealloc`{.interpreted-text role="c:func"} or `PyMem_RawCalloc`{.interpreted-text role="c:func"}. Otherwise, or if `PyMem_RawFree(p)` has been called before, undefined behavior occurs.
>
> If *p* is `NULL`, no operation is performed.

## Memory Interface {#memoryinterface}

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.

The `default memory allocator <default-memory-allocators>`{.interpreted-text role="ref"} uses the `pymalloc memory allocator <pymalloc>`{.interpreted-text role="ref"}.

:::: warning
::: title
Warning
:::

There must be an `attached thread state`{.interpreted-text role="term"} when using these functions.
::::

::: versionchanged
3.6

The default allocator is now pymalloc instead of system `malloc`{.interpreted-text role="c:func"}.
:::

> Allocates *n* bytes and returns a pointer of type `void*`{.interpreted-text role="c:expr"} 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.

> Allocates *nelem* elements each whose size in bytes is *elsize* and returns a pointer of type `void*`{.interpreted-text role="c:expr"} 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
> :::

> 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 `PyMem_Malloc`{.interpreted-text role="c:func"}, `PyMem_Realloc`{.interpreted-text role="c:func"} or `PyMem_Calloc`{.interpreted-text role="c:func"}.
>
> If the request fails, `PyMem_Realloc`{.interpreted-text role="c:func"} returns `NULL` and *p* remains a valid pointer to the previous memory area.

> Frees the memory block pointed to by *p*, which must have been returned by a previous call to `PyMem_Malloc`{.interpreted-text role="c:func"}, `PyMem_Realloc`{.interpreted-text role="c:func"} or `PyMem_Calloc`{.interpreted-text role="c:func"}. 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.

> Same as `PyMem_Malloc`{.interpreted-text role="c:func"}, but allocates `(n * sizeof(TYPE))` bytes of memory. Returns a pointer cast to `TYPE*`. The memory will not have been initialized in any way.

> Same as `PyMem_Realloc`{.interpreted-text role="c:func"}, 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.

> Same as `PyMem_Free`{.interpreted-text role="c:func"}.

### Deprecated aliases

These are `soft deprecated`{.interpreted-text role="term"} aliases to existing functions and macros. They exist solely for backwards compatibility.

  Deprecated alias   Corresponding function or macro
  ------------------ --------------------------------------------------
                     `PyMem_Malloc`{.interpreted-text role="c:func"}
                     `PyMem_New`{.interpreted-text role="c:macro"}
                     `PyMem_Realloc`{.interpreted-text role="c:func"}
                     `PyMem_Resize`{.interpreted-text role="c:macro"}
                     `PyMem_Free`{.interpreted-text role="c:func"}
                     `PyMem_Free`{.interpreted-text role="c:func"}

::: 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
:::

## Object allocators {#objectinterface}

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
::: title
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 `Customize Memory Allocators <customize-memory-allocators>`{.interpreted-text role="ref"} section.
::::

The `default object allocator <default-memory-allocators>`{.interpreted-text role="ref"} uses the `pymalloc memory allocator <pymalloc>`{.interpreted-text role="ref"}.

:::: warning
::: title
Warning
:::

There must be an `attached thread state`{.interpreted-text role="term"} when using these functions.
::::

> Allocates *n* bytes and returns a pointer of type `void*`{.interpreted-text role="c:expr"} 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.

> Allocates *nelem* elements each whose size in bytes is *elsize* and returns a pointer of type `void*`{.interpreted-text role="c:expr"} 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
> :::

> 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 `PyObject_Malloc`{.interpreted-text role="c:func"}, `PyObject_Realloc`{.interpreted-text role="c:func"} or `PyObject_Calloc`{.interpreted-text role="c:func"}.
>
> If the request fails, `PyObject_Realloc`{.interpreted-text role="c:func"} returns `NULL` and *p* remains a valid pointer to the previous memory area.

> Frees the memory block pointed to by *p*, which must have been returned by a previous call to `PyObject_Malloc`{.interpreted-text role="c:func"}, `PyObject_Realloc`{.interpreted-text role="c:func"} or `PyObject_Calloc`{.interpreted-text role="c:func"}. 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 `~PyTypeObject.tp_free`{.interpreted-text role="c:member"} slot instead.
>
> Do not use this for memory allocated by `PyObject_GC_New`{.interpreted-text role="c:macro"} or `PyObject_GC_NewVar`{.interpreted-text role="c:macro"}; use `PyObject_GC_Del`{.interpreted-text role="c:func"} instead.
>
> ::: seealso
> - `PyObject_GC_Del`{.interpreted-text role="c:func"} is the equivalent of this function for memory allocated by types that support garbage collection.
> - `PyObject_Malloc`{.interpreted-text role="c:func"}
> - `PyObject_Realloc`{.interpreted-text role="c:func"}
> - `PyObject_Calloc`{.interpreted-text role="c:func"}
> - `PyObject_New`{.interpreted-text role="c:macro"}
> - `PyObject_NewVar`{.interpreted-text role="c:macro"}
> - `PyType_GenericAlloc`{.interpreted-text role="c:func"}
> - `~PyTypeObject.tp_free`{.interpreted-text role="c:member"}
> :::

## 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

Legend:

- Name: value for `PYTHONMALLOC`{.interpreted-text role="envvar"} environment variable.
- `malloc`: system allocators from the standard C library, C functions: `malloc`{.interpreted-text role="c:func"}, `calloc`{.interpreted-text role="c:func"}, `realloc`{.interpreted-text role="c:func"} and `free`{.interpreted-text role="c:func"}.
- `pymalloc`: `pymalloc memory allocator <pymalloc>`{.interpreted-text role="ref"}.
- `mimalloc`: `mimalloc memory allocator <mimalloc>`{.interpreted-text role="ref"}. The pymalloc allocator will be used if mimalloc support isn\'t available.
- \"+ debug\": with `debug hooks on the Python memory allocators
  <pymem-debug-hooks>`{.interpreted-text role="ref"}.
- \"Debug build\": `Python build in debug mode <debug-build>`{.interpreted-text role="ref"}.

## Customize Memory Allocators

::: versionadded
3.4
:::

> 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 `!PyMemAllocator`{.interpreted-text role="c:type"} structure was renamed to `PyMemAllocatorEx`{.interpreted-text role="c:type"} and a new `calloc` field was added.
> :::

> Enum used to identify an allocator domain. Domains:
>
> > Functions:
> >
> > - `PyMem_RawMalloc`{.interpreted-text role="c:func"}
> > - `PyMem_RawRealloc`{.interpreted-text role="c:func"}
> > - `PyMem_RawCalloc`{.interpreted-text role="c:func"}
> > - `PyMem_RawFree`{.interpreted-text role="c:func"}
>
> > Functions:
> >
> > - `PyMem_Malloc`{.interpreted-text role="c:func"},
> > - `PyMem_Realloc`{.interpreted-text role="c:func"}
> > - `PyMem_Calloc`{.interpreted-text role="c:func"}
> > - `PyMem_Free`{.interpreted-text role="c:func"}
>
> > Functions:
> >
> > - `PyObject_Malloc`{.interpreted-text role="c:func"}
> > - `PyObject_Realloc`{.interpreted-text role="c:func"}
> > - `PyObject_Calloc`{.interpreted-text role="c:func"}
> > - `PyObject_Free`{.interpreted-text role="c:func"}

> Get the memory block allocator of the specified domain.

> 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 `PYMEM_DOMAIN_RAW`{.interpreted-text role="c:macro"} domain, the allocator must be thread-safe: a `thread state`{.interpreted-text role="term"} is not `attached <attached thread state>`{.interpreted-text role="term"} 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 `GIL`{.interpreted-text role="term"}.
>
> If the new allocator is not a hook (does not call the previous allocator), the `PyMem_SetupDebugHooks`{.interpreted-text role="c:func"} function must be called to reinstall the debug hooks on top on the new allocator.
>
> See also `PyPreConfig.allocator`{.interpreted-text role="c:member"} and `Preinitialize Python
> with PyPreConfig <c-preinit>`{.interpreted-text role="ref"}.
>
> :::: warning
> ::: title
> Warning
> :::
>
> `PyMem_SetAllocator`{.interpreted-text role="c:func"} does have the following contract:
>
> - It can be called after `Py_PreInitialize`{.interpreted-text role="c:func"} and before `Py_InitializeFromConfig`{.interpreted-text role="c:func"} 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 `attached thread state`{.interpreted-text role="term"}). See `the section on allocator domains <allocator-domains>`{.interpreted-text role="ref"} for more information.
> - If called after Python has finish initializing (after `Py_InitializeFromConfig`{.interpreted-text role="c:func"} 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.
> :::

> Setup `debug hooks in the Python memory allocators <pymem-debug-hooks>`{.interpreted-text role="ref"} to detect memory errors.

## Debug hooks on the Python memory allocators {#pymem-debug-hooks}

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

The `PYTHONMALLOC`{.interpreted-text role="envvar"} environment variable can be used to install debug hooks on a Python compiled in release mode (ex: `PYTHONMALLOC=debug`).

The `PyMem_SetupDebugHooks`{.interpreted-text role="c:func"} function can be used to set debug hooks after calling `PyMem_SetAllocator`{.interpreted-text role="c:func"}.

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 `PyObject_Free`{.interpreted-text role="c:func"} is called on a memory block allocated by `PyMem_Malloc`{.interpreted-text role="c:func"}.
- 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 `attached thread state`{.interpreted-text role="term"} when allocator functions of `PYMEM_DOMAIN_OBJ`{.interpreted-text role="c:macro"} (ex: `PyObject_Malloc`{.interpreted-text role="c:func"}) and `PYMEM_DOMAIN_MEM`{.interpreted-text role="c:macro"} (ex: `PyMem_Malloc`{.interpreted-text role="c:func"}) domains are called.

On error, the debug hooks use the `tracemalloc`{.interpreted-text role="mod"} module to get the traceback where a memory block was allocated. The traceback is only displayed if `tracemalloc`{.interpreted-text role="mod"} 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 `PYMEM_DOMAIN_RAW`{.interpreted-text role="c:macro"}.
    - `'m'` for `PYMEM_DOMAIN_MEM`{.interpreted-text role="c:macro"}.
    - `'o'` for `PYMEM_DOMAIN_OBJ`{.interpreted-text role="c:macro"}.

`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 `size_t`{.interpreted-text role="c:type"}. 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 `PyMem_SetupDebugHooks`{.interpreted-text role="c:func"} function now also works on Python compiled in release mode. On error, the debug hooks now use `tracemalloc`{.interpreted-text role="mod"} to get the traceback where a memory block was allocated. The debug hooks now also check if there is an `attached thread state`{.interpreted-text role="term"} when functions of `PYMEM_DOMAIN_OBJ`{.interpreted-text role="c:macro"} and `PYMEM_DOMAIN_MEM`{.interpreted-text role="c:macro"} 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()`.
:::

## The pymalloc allocator {#pymalloc}

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. When Python is configured with `--with-pymalloc-hugepages`{.interpreted-text role="option"}, the arena size on 64-bit platforms is increased to 2 MiB to match the huge page size, and arena allocation will attempt to use huge pages (`MAP_HUGETLB` on Linux, `MEM_LARGE_PAGES` on Windows) with automatic fallback to regular pages. It falls back to `PyMem_RawMalloc`{.interpreted-text role="c:func"} and `PyMem_RawRealloc`{.interpreted-text role="c:func"} for allocations larger than 512 bytes.

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

The arena allocator uses the following functions:

- `!VirtualAlloc`{.interpreted-text role="c:func"} and `!VirtualFree`{.interpreted-text role="c:func"} on Windows,
- `!mmap`{.interpreted-text role="c:func"} and `!munmap`{.interpreted-text role="c:func"} if available,
- `malloc`{.interpreted-text role="c:func"} and `free`{.interpreted-text role="c:func"} otherwise.

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

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

### Customize pymalloc Arena Allocator

::: versionadded
3.4
:::

> 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
>   ----------------------------------------------------------------------------------------

> Get the arena allocator.

> Set the arena allocator.

## The mimalloc allocator {#mimalloc}

::: versionadded
3.13
:::

Python supports the 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.\"

## tracemalloc C API

::: versionadded
3.7
:::

> Track an allocated memory block in the `tracemalloc`{.interpreted-text role="mod"} 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.

> Untrack an allocated memory block in the `tracemalloc`{.interpreted-text role="mod"} module. Do nothing if the block was not tracked.
>
> Return `-2` if tracemalloc is disabled, otherwise return `0`.

## Examples {#memoryexamples}

Here is the example from section `memoryoverview`{.interpreted-text role="ref"}, rewritten so that the I/O buffer is allocated from the Python heap by using the first function set:

``` c
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:

``` c
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. :

``` c
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 `PyObject_New`{.interpreted-text role="c:macro"}, `PyObject_NewVar`{.interpreted-text role="c:macro"} and `PyObject_Free`{.interpreted-text role="c:func"}.

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