記憶體管理¶

總覽¶

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.

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

也參考

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

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

Allocator Domains¶

All allocating functions belong to one of three different "domains" (see also 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 Default Memory Allocators. The APIs used to allocate and free a block of memory must be from the same domain. For example, PyMem_Free() must be used to free memory allocated using 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 attached thread state. The memory is requested directly from the system. See Raw Memory Interface.
"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. The memory is taken from the Python private heap. See Memory Interface.
Object domain: intended for allocating memory for Python objects. The memory is taken from the Python private heap. See Object allocators.

備註

The free-threaded 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(), PyMem_RawMalloc(), or malloc(), but not PyObject_Malloc().

See Memory Allocation APIs.

Raw Memory Interface¶

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

The default raw memory allocator uses the following functions: malloc(), calloc(), realloc() and free(); call malloc(1) (or calloc(1, 1)) when requesting zero bytes.

在 3.4 版被加入.

void *PyMem_RawMalloc(size_t n)¶

為穩定 ABI 的一部分自 3.13 版本開始.

Allocates n bytes and returns a pointer of type 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.

void *PyMem_RawCalloc(size_t nelem, size_t elsize)¶

為穩定 ABI 的一部分自 3.13 版本開始.

Allocates nelem elements each whose size in bytes is elsize and returns a pointer of type 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.

在 3.5 版被加入.

void *PyMem_RawRealloc(void *p, size_t n)¶

為穩定 ABI 的一部分自 3.13 版本開始.

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(), PyMem_RawRealloc() or PyMem_RawCalloc().

If the request fails, PyMem_RawRealloc() returns NULL and p remains a valid pointer to the previous memory area.

void PyMem_RawFree(void *p)¶

為穩定 ABI 的一部分自 3.13 版本開始.

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

If p is NULL, no operation is performed.

記憶體介面¶

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 default memory allocator uses the pymalloc memory allocator, whereas in the free-threaded build, the default is the mimalloc memory allocator instead.

警告

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

在 3.6 版的變更: The default allocator is now pymalloc instead of system malloc().

在 3.13 版的變更: In the free-threaded build, the default allocator is now mimalloc.

void *PyMem_Malloc(size_t n)¶

為穩定 ABI 的一部分.

Allocates n bytes and returns a pointer of type 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.

void *PyMem_Calloc(size_t nelem, size_t elsize)¶

為穩定 ABI 的一部分自 3.7 版本開始.

Allocates nelem elements each whose size in bytes is elsize and returns a pointer of type 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.

在 3.5 版被加入.

void *PyMem_Realloc(void *p, size_t n)¶

為穩定 ABI 的一部分.

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(), PyMem_Realloc() or PyMem_Calloc().

If the request fails, PyMem_Realloc() returns NULL and p remains a valid pointer to the previous memory area.

void PyMem_Free(void *p)¶

為穩定 ABI 的一部分.

Frees the memory block pointed to by p, which must have been returned by a previous call to PyMem_Malloc(), PyMem_Realloc() or 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.

PyMem_New(TYPE, n)¶: Same as 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.

PyMem_Resize(p, TYPE, n)¶

Same as 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.

void PyMem_Del(void *p)¶: 和 PyMem_Free() 相同。

已棄用的別名¶

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

已棄用的別名	對應的函式或巨集
PyMem_MALLOC(size)¶	`PyMem_Malloc()`
PyMem_NEW(type, size)¶	`PyMem_New`
PyMem_REALLOC(ptr, size)¶	`PyMem_Realloc()`
PyMem_RESIZE(ptr, type, size)¶	`PyMem_Resize`
PyMem_FREE(ptr)¶	`PyMem_Free()`
PyMem_DEL(ptr)¶	`PyMem_Free()`

在 3.4 版的變更: 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.

在 2.0 版之後被棄用.

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.

備註

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 section.

The default object allocator uses the pymalloc memory allocator. In the free-threaded build, the default is the mimalloc memory allocator instead.

警告

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

void *PyObject_Malloc(size_t n)¶

為穩定 ABI 的一部分.

Allocates n bytes and returns a pointer of type 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.

void *PyObject_Calloc(size_t nelem, size_t elsize)¶

為穩定 ABI 的一部分自 3.7 版本開始.

Allocates nelem elements each whose size in bytes is elsize and returns a pointer of type 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.

在 3.5 版被加入.

void *PyObject_Realloc(void *p, size_t n)¶

為穩定 ABI 的一部分.

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(), PyObject_Realloc() or PyObject_Calloc().

If the request fails, PyObject_Realloc() returns NULL and p remains a valid pointer to the previous memory area.

void PyObject_Free(void *p)¶

為穩定 ABI 的一部分.

Frees the memory block pointed to by p, which must have been returned by a previous call to PyObject_Malloc(), PyObject_Realloc() or 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 tp_free slot instead.

Do not use this for memory allocated by PyObject_GC_New or PyObject_GC_NewVar; use PyObject_GC_Del() instead.

也參考

PyObject_GC_Del() is the equivalent of this function for memory allocated by types that support garbage collection.
PyObject_Malloc()
PyObject_Realloc()
PyObject_Calloc()
PyObject_New
PyObject_NewVar
PyType_GenericAlloc()
tp_free

Default Memory Allocators¶

Default memory allocators:

配置	名稱	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 PYTHONMALLOC environment variable.
malloc: system allocators from the standard C library, C functions: malloc(), calloc(), realloc() and free().
pymalloc: pymalloc memory allocator.
mimalloc: mimalloc memory allocator.
"+ debug": with debug hooks on the Python memory allocators.
"Debug build": Python build in debug mode.

Customize Memory Allocators¶

在 3.4 版被加入.

type PyMemAllocatorEx¶

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

欄位	意義
`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)`