内存管理 ¶

概述 ¶

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 string 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.

原生内存接口 ¶

The following function sets are wrappers to the system allocator. These functions are thread-safe, the GIL 不需要保持。

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

void* PyMem_RawMalloc ( size_t  n ) ¶

分配 n bytes and returns a pointer of type void* to the allocated memory, or NULL 若请求失败。

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 ) ¶

分配 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.

3.5 版新增。

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.

若 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 .

除非 p is NULL , it must have been returned by a previous call to PyMem_RawMalloc() , PyMem_RawRealloc() or PyMem_RawCalloc() .

若请求失败， PyMem_RawRealloc() 返回 NULL and p remains a valid pointer to the previous memory area.

void PyMem_RawFree ( void  *p ) ¶

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() 。否则，或者若 PyMem_RawFree(p) has been called before, undefined behavior occurs.

若 p is NULL ，没有操作被履行。

内存接口 ¶

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.

默认情况下，这些函数使用 pymalloc 内存分配器 .

void* PyMem_Malloc ( size_t  n ) ¶

分配 n bytes and returns a pointer of type void* to the allocated memory, or NULL 若请求失败。

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 ) ¶

分配 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.

3.5 版新增。

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.

若 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 .

除非 p is NULL , it must have been returned by a previous call to PyMem_Malloc() , PyMem_Realloc() or PyMem_Calloc() .

若请求失败， PyMem_Realloc() 返回 NULL and p remains a valid pointer to the previous memory area.

void PyMem_Free ( void  *p ) ¶

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() 。否则，或者若 PyMem_Free(p) has been called before, undefined behavior occurs.

若 p is NULL ，没有操作被履行。

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

TYPE* PyMem_New ( TYPE, size_t  n ) ¶

如同 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.

TYPE* PyMem_Resize ( void  *p , TYPE, size_t  n ) ¶

如同 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 ) ¶

如同 PyMem_Free() .

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.

• PyMem_MALLOC(size)
• PyMem_NEW(type, size)
• PyMem_REALLOC(ptr, size)
• PyMem_RESIZE(ptr, type, size)
• PyMem_FREE(ptr)
• PyMem_DEL(ptr)

对象分配器 ¶

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.

默认情况下，这些函数使用 pymalloc 内存分配器 .

void* PyObject_Malloc ( size_t  n ) ¶

分配 n bytes and returns a pointer of type void* to the allocated memory, or NULL 若请求失败。

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 ) ¶

分配 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.

3.5 版新增。

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.

若 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 .

除非 p is NULL , it must have been returned by a previous call to PyObject_Malloc() , PyObject_Realloc() or PyObject_Calloc() .

若请求失败， PyObject_Realloc() 返回 NULL and p remains a valid pointer to the previous memory area.

void PyObject_Free ( void  *p ) ¶

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() 。否则，或者若 PyObject_Free(p) has been called before, undefined behavior occurs.

若 p is NULL ，没有操作被履行。

定制内存分配器 ¶

PyMemAllocatorEx ¶

用于描述内存块分配器的结构。结构有 4 个字段：

字段	含义
void *ctx	传递作为第一自变量的用户上下文
void* malloc(void *ctx, size_t size)	分配内存块
void* calloc(void *ctx, size_t nelem, size_t elsize)	分配初始化为 0 的内存块
void* realloc(void *ctx, void *ptr, size_t new_size)	分配或重置内存块大小
void free(void *ctx, void *ptr)	释放内存块

3.5 版改变： PyMemAllocator 结构被重命名为 PyMemAllocatorEx 和新的 calloc 字段被添加。

PyMemAllocatorDomain ¶

用于标识分配器域的枚举。域：

PYMEM_DOMAIN_RAW ¶

函数：

• PyMem_RawMalloc()
• PyMem_RawRealloc()
• PyMem_RawCalloc()
• PyMem_RawFree()

PYMEM_DOMAIN_MEM ¶

函数：

• PyMem_Malloc() ,
• PyMem_Realloc()
• PyMem_Calloc()
• PyMem_Free()

PYMEM_DOMAIN_OBJ ¶

函数：

• PyObject_Malloc()
• PyObject_Realloc()
• PyObject_Calloc()
• PyObject_Free()

void PyMem_GetAllocator ( PyMemAllocatorDomain  domain , PyMemAllocatorEx  *allocator ) ¶

Get the memory block allocator of the specified domain.

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.

对于 PYMEM_DOMAIN_RAW domain, the allocator must be thread-safe: the GIL is not held when the allocator is called.

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

void PyMem_SetupDebugHooks ( void ) ¶

Setup hooks to detect bugs in the Python memory allocator functions.

Newly allocated memory is filled with the byte 0xCB , freed memory is filled with the byte 0xDB .

运行时校验：

• Detect API violations, ex: PyObject_Free() called on a buffer allocated by PyMem_Malloc()
• Detect write before the start of the buffer (buffer underflow)
• Detect write after the end of the buffer (buffer overflow)
• Check that the GIL is held when allocator functions of PYMEM_DOMAIN_OBJ (ex: PyObject_Malloc() ) 和 PYMEM_DOMAIN_MEM (ex: PyMem_Malloc() ) domains are called

当出错时，调试挂钩使用 tracemalloc module to get the traceback where a memory block was allocated. The traceback is only displayed if tracemalloc is tracing Python memory allocations and the memory block was traced.

These hooks are installed by default if Python is compiled in debug mode. The PYTHONMALLOC environment variable can be used to install debug hooks on a Python compiled in release mode.

3.6 版改变： This function now also works on Python compiled in release mode. On error, the debug hooks now use tracemalloc to get the traceback where a memory block was allocated. The debug hooks now also check if the GIL is held when functions of PYMEM_DOMAIN_OBJ and PYMEM_DOMAIN_MEM domains are called.

pymalloc 分配器 ¶

Python 拥有 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 256 KB. It falls back to PyMem_RawMalloc() and PyMem_RawRealloc() for allocations larger than 512 bytes.

pymalloc is the default allocator of the PYMEM_DOMAIN_MEM (ex: PyMem_Malloc() ) 和 PYMEM_DOMAIN_OBJ (ex: PyObject_Malloc() ) 域。

The arena allocator uses the following functions:

• VirtualAlloc() and VirtualFree() 在 Windows，
• mmap() and munmap() 若可用，
• malloc() and free() 否则。

范例 ¶

Here is the example from section 概述 , 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_Del(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_Del()  */
								

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() , PyObject_NewVar() and PyObject_Del() .

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