.. highlightlang:: c

.. _memory:

*****************

Memory Management

*****************

.. sectionauthor:: Vladimir Marangozov <Vladimir.Marangozov@inrialpes.fr>

.. _memoryoverview:

Overview

========

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 she regularly manipulates 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()

   single: calloc()

   single: realloc()

   single: free()

To avoid memory corruption, extension writers should never try to operate on

Python objects with the functions exported by the C library: :cfunc:`malloc`,

:cfunc:`calloc`, :cfunc:`realloc` and :cfunc:`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 = PyString_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.

.. _memoryinterface:

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:

.. cfunction:: void* PyMem_Malloc(size_t n)

   Allocates *n* bytes and returns a pointer of type :ctype:`void\*` to the

   allocated memory, or *NULL* if the request fails. Requesting zero bytes returns

   a distinct non-*NULL* pointer if possible, as if :cfunc:`PyMem_Malloc(1)` had

   been called instead. The memory will not have been initialized in any way.

.. cfunction:: 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 :cfunc:`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 :cfunc:`PyMem_Malloc` or :cfunc:`PyMem_Realloc`. If the request fails,

   :cfunc:`PyMem_Realloc` returns *NULL* and *p* remains a valid pointer to the

   previous memory area.

.. cfunction:: void PyMem_Free(void *p)

   Frees the memory block pointed to by *p*, which must have been returned by a

   previous call to :cfunc:`PyMem_Malloc` or :cfunc:`PyMem_Realloc`.  Otherwise, or

   if :cfunc:`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.

.. cfunction:: TYPE* PyMem_New(TYPE, size_t n)

   Same as :cfunc:`PyMem_Malloc`, but allocates ``(n * sizeof(TYPE))`` bytes of

   memory.  Returns a pointer cast to :ctype:`TYPE\*`.  The memory will not have

   been initialized in any way.

.. cfunction:: TYPE* PyMem_Resize(void *p, TYPE, size_t n)

   Same as :cfunc:`PyMem_Realloc`, but the memory block is resized to ``(n *

   sizeof(TYPE))`` bytes.  Returns a pointer cast to :ctype:`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.

.. cfunction:: void PyMem_Del(void *p)

   Same as :cfunc:`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.

:cfunc:`PyMem_MALLOC`, :cfunc:`PyMem_REALLOC`, :cfunc:`PyMem_FREE`.

:cfunc:`PyMem_NEW`, :cfunc:`PyMem_RESIZE`, :cfunc:`PyMem_DEL`.

.. _memoryexamples:

Examples

========

Here is the example from section :ref:`memoryoverview`, 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 = PyString_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 = PyString_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 :cfunc:`PyObject_New`,

:cfunc:`PyObject_NewVar` and :cfunc:`PyObject_Del`.

These will be explained in the next chapter on defining and implementing new

object types in C.