Memory Management¶
Aperçu¶
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.
Voir aussi
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 here.
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 the GIL. 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 the GIL held. 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.
Note
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()
.
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.
Ajouté dans la version 3.4.
-
void *PyMem_RawMalloc(size_t n)¶
- Part of the Stable ABI since version 3.13.
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 ifPyMem_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)¶
- Part of the Stable ABI since version 3.13.
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 ifPyMem_RawCalloc(1, 1)
had been called instead.Ajouté dans la version 3.5.
-
void *PyMem_RawRealloc(void *p, size_t n)¶
- Part of the Stable ABI since version 3.13.
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 toPyMem_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 toPyMem_RawMalloc()
,PyMem_RawRealloc()
orPyMem_RawCalloc()
.If the request fails,
PyMem_RawRealloc()
returnsNULL
and p remains a valid pointer to the previous memory area.
-
void PyMem_RawFree(void *p)¶
- Part of the Stable ABI since version 3.13.
Frees the memory block pointed to by p, which must have been returned by a previous call to
PyMem_RawMalloc()
,PyMem_RawRealloc()
orPyMem_RawCalloc()
. Otherwise, or ifPyMem_RawFree(p)
has been called before, undefined behavior occurs.If p is
NULL
, no operation is performed.
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.
The default memory allocator uses the pymalloc memory allocator.
Avertissement
The GIL must be held when using these functions.
Modifié dans la version 3.6: The default allocator is now pymalloc instead of system malloc()
.
-
void *PyMem_Malloc(size_t n)¶
- Part of the Stable ABI.
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 ifPyMem_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)¶
- Part of the Stable ABI since version 3.7.
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 ifPyMem_Calloc(1, 1)
had been called instead.Ajouté dans la version 3.5.
-
void *PyMem_Realloc(void *p, size_t n)¶
- Part of the Stable ABI.
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 toPyMem_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 toPyMem_Malloc()
,PyMem_Realloc()
orPyMem_Calloc()
.If the request fails,
PyMem_Realloc()
returnsNULL
and p remains a valid pointer to the previous memory area.
-
void PyMem_Free(void *p)¶
- Part of the Stable ABI.
Frees the memory block pointed to by p, which must have been returned by a previous call to
PyMem_Malloc()
,PyMem_Realloc()
orPyMem_Calloc()
. Otherwise, or ifPyMem_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 toTYPE*
. 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 toTYPE*
. On return, p will be a pointer to the new memory area, orNULL
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()
.
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)
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 Customize Memory Allocators section.
The default object allocator uses the pymalloc memory allocator.
Avertissement
The GIL must be held when using these functions.
-
void *PyObject_Malloc(size_t n)¶
- Part of the Stable ABI.
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 ifPyObject_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)¶
- Part of the Stable ABI since version 3.7.
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 ifPyObject_Calloc(1, 1)
had been called instead.Ajouté dans la version 3.5.
-
void *PyObject_Realloc(void *p, size_t n)¶
- Part of the Stable ABI.
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 toPyObject_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 toPyObject_Malloc()
,PyObject_Realloc()
orPyObject_Calloc()
.If the request fails,
PyObject_Realloc()
returnsNULL
and p remains a valid pointer to the previous memory area.
-
void PyObject_Free(void *p)¶
- Part of the Stable ABI.
Frees the memory block pointed to by p, which must have been returned by a previous call to
PyObject_Malloc()
,PyObject_Realloc()
orPyObject_Calloc()
. Otherwise, or ifPyObject_Free(p)
has been called before, undefined behavior occurs.If p is
NULL
, no operation is performed.
Default Memory Allocators¶
Default memory allocators:
Configuration |
Nom |
PyMem_RawMalloc |
PyMem_Malloc |
PyObject_Malloc |
---|---|---|---|---|
Release build |
|
|
|
|
Debug build |
|
|
|
|
Release build, without pymalloc |
|
|
|
|
Debug build, without pymalloc |
|
|
|
|
Legend:
Name: value for
PYTHONMALLOC
environment variable.malloc
: system allocators from the standard C library, C functions:malloc()
,calloc()
,realloc()
andfree()
.pymalloc
: pymalloc memory allocator.mimalloc
: mimalloc memory allocator. The pymalloc allocator will be used if mimalloc support isn't available."+ debug": with debug hooks on the Python memory allocators.
"Debug build": Python build in debug mode.
Customize Memory Allocators¶
Ajouté dans la version 3.4.
-
type PyMemAllocatorEx¶
Structure used to describe a memory block allocator. The structure has the following fields:
Champ
Signification
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
Modifié dans la version 3.5: The
PyMemAllocator
structure was renamed toPyMemAllocatorEx
and a newcalloc
field was added.
-
type PyMemAllocatorDomain¶
Enum used to identify an allocator domain. Domains:
-
PYMEM_DOMAIN_RAW¶
Functions:
-
PYMEM_DOMAIN_MEM¶
Functions:
-
PYMEM_DOMAIN_OBJ¶
Functions:
-
PYMEM_DOMAIN_RAW¶
-
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.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
.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.See also
PyPreConfig.allocator
and Preinitialize Python with PyPreConfig.Avertissement
PyMem_SetAllocator()
does have the following contract:It can be called after
Py_PreInitialize()
and beforePy_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 the GIL held). See the section on allocator domains for more information.If called after Python has finish initializing (after
Py_InitializeFromConfig()
has been called) the allocator must wrap the existing allocator. Substituting the current allocator for some other arbitrary one is not supported.
Modifié dans la version 3.12: All allocators must be thread-safe.
-
void PyMem_SetupDebugHooks(void)¶
Setup debug hooks in the Python memory allocators to detect memory errors.
Debug hooks on the Python memory allocators¶
When Python is built in debug mode, the
PyMem_SetupDebugHooks()
function is called at the Python
preinitialization to setup debug hooks on Python memory allocators
to detect memory errors.
The PYTHONMALLOC
environment variable can be used to install debug
hooks on a Python compiled in release mode (ex: PYTHONMALLOC=debug
).
The PyMem_SetupDebugHooks()
function can be used to set debug hooks
after calling 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
PyObject_Free()
is called on a memory block allocated byPyMem_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()
) andPYMEM_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.
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'
forPYMEM_DOMAIN_RAW
.'m'
forPYMEM_DOMAIN_MEM
.'o'
forPYMEM_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
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).
Modifié dans la version 3.6: The PyMem_SetupDebugHooks()
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.
Modifié dans la version 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¶
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 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()
andVirtualFree()
on Windows,mmap()
andmunmap()
if available,malloc()
andfree()
otherwise.
This allocator is disabled if Python is configured with the
--without-pymalloc
option. It can also be disabled at runtime using
the PYTHONMALLOC
environment variable (ex: PYTHONMALLOC=malloc
).
Customize pymalloc Arena Allocator¶
Ajouté dans la version 3.4.
-
type PyObjectArenaAllocator¶
Structure used to describe an arena allocator. The structure has three fields:
Champ
Signification
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.
The mimalloc allocator¶
Ajouté dans la version 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¶
Ajouté dans la version 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 return0
.
Exemples¶
Here is the example from section Aperçu, 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 PyObject_New
,
PyObject_NewVar
and PyObject_Free()
.
These will be explained in the next chapter on defining and implementing new object types in C.