Gerenciamento de Memória

Visão Geral

O gerenciamento de memória em Python envolve um heap privado contendo todos os objetos e estruturas de dados Python. O gerenciamento desse heap privado é garantido internamente pelo gerenciador de memória do Python. O gerenciador de memória do Python possui diferentes componentes que lidam com diversos aspectos dinâmicos do gerenciamento de armazenamento, como compartilhamento, segmentação, pré-alocação ou cache.

No nível mais baixo, um alocador de memória bruta garante que haja espaço suficiente no heap privado para armazenar todos os dados relacionados ao Python, interagindo com o gerenciador de memória do sistema operacional. Além do alocador de memória bruta, vários alocadores específicos de objeto operam no mesmo heap e implementam políticas distintas de gerenciamento de memória, adaptadas às peculiaridades de cada tipo de objeto. Por exemplo, objetos inteiros são gerenciados dentro do heap de forma diferente de strings, tuplas ou dicionários, pois inteiros implicam diferentes requisitos de armazenamento e compensações entre velocidade e espaço. O gerenciador de memória do Python, portanto, delega parte do trabalho aos alocadores específicos de objeto, mas garante que estes operem dentro dos limites do heap privado.

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.

Ver também

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.

Raw Memory Interface

The following function sets are wrappers to the system allocator. These functions are thread-safe, the GIL does not need to be held.

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.

Novo na versão 3.4.

void* PyMem_RawMalloc(size_t n)

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)

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.

Novo na versão 3.5.

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 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)

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.

Interface da Memória

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 uses the pymalloc memory allocator.

Aviso

The GIL must be held when using these functions.

Alterado na versão 3.6: The default allocator is now pymalloc instead of system malloc().

void* PyMem_Malloc(size_t n)

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)

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.

Novo na versão 3.5.

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 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)

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.

TYPE* PyMem_New(TYPE, size_t 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.

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

Same as PyMem_Free().

Além disso, os seguintes conjuntos de macros são fornecidos para chamar o alocador de memória do Python diretamente, sem envolver as funções da API C listadas acima. No entanto, observe que seu uso não preserva a compatibilidade binária entre as versões do Python e, portanto, está descontinuado em módulos de extensão.

• PyMem_MALLOC(size)

• PyMem_NEW(type, size)

• PyMem_REALLOC(ptr, size)

• PyMem_RESIZE(ptr, type, size)

• PyMem_FREE(ptr)

• PyMem_DEL(ptr)

Alocadores de objeto

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 object allocator uses the pymalloc memory allocator.

Aviso

The GIL must be held when using these functions.

void* PyObject_Malloc(size_t n)

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)

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.

Novo na versão 3.5.

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 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)

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.

Alocadores de memória padrão

Alocadores de memória padrão:

Configuração	Nome	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

Legenda:

• 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

• “+ debug”: with debug hooks installed by PyMem_SetupDebugHooks()

Alocadores de memória

Novo na versão 3.4.

PyMemAllocatorEx

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

Campo	Significado
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

Alterado na versão 3.5: A estrutura PyMemAllocator foi renomeada para PyMemAllocatorEx e um novo campo calloc foi adicionado.

PyMemAllocatorDomain

Enum used to identify an allocator domain. Domains:

PYMEM_DOMAIN_RAW

Funções:

• PyMem_RawMalloc()

• PyMem_RawRealloc()

• PyMem_RawCalloc()

• PyMem_RawFree()

PYMEM_DOMAIN_MEM

Funções:

• PyMem_Malloc(),

• PyMem_Realloc()

• PyMem_Calloc()

• PyMem_Free()

PYMEM_DOMAIN_OBJ

Funções:

• 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.

For the 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 0xCD (CLEANBYTE), freed memory is filled with the byte 0xDD (DEADBYTE). Memory blocks are surrounded by “forbidden bytes” (FORBIDDENBYTE: byte 0xFD).

Checagens em Tempo de Execução:

• 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()) and PYMEM_DOMAIN_MEM (ex: PyMem_Malloc()) domains are called

On error, the debug hooks use the 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.

Alterado na versão 3.6: 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.

Alterado na versão 3.8: Byte patterns 0xCB (CLEANBYTE), 0xDB (DEADBYTE) and 0xFB (FORBIDDENBYTE) have been replaced with 0xCD, 0xDD and 0xFD to use the same values than Windows CRT debug malloc() and free().

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 256 KiB. 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()) and PYMEM_DOMAIN_OBJ (ex: PyObject_Malloc()) domains.

The arena allocator uses the following functions:

• VirtualAlloc() e VirtualFree() no Windows,

• mmap() e munmap() se disponível,

• malloc() e free() do contrário.

Customize pymalloc Arena Allocator

Novo na versão 3.4.

PyObjectArenaAllocator

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

Campo	Significado
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

void PyObject_GetArenaAllocator(PyObjectArenaAllocator *allocator)

Get the arena allocator.

void PyObject_SetArenaAllocator(PyObjectArenaAllocator *allocator)

Set the arena allocator.

tracemalloc C API

Novo na versão 3.7.

int PyTraceMalloc_Track(unsigned int domain, uintptr_t ptr, size_t size)

Track an allocated memory block in the 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.

int PyTraceMalloc_Untrack(unsigned int domain, uintptr_t ptr)

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

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

Exemplos

Here is the example from section Visão Geral, 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.