
Memory Management
*****************


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.

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 = 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.


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:

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_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`` or ``PyMem_Realloc``. 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`` or
   ``PyMem_Realloc``.  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``.

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``, ``PyMem_REALLOC``, ``PyMem_FREE``.

``PyMem_NEW``, ``PyMem_RESIZE``, ``PyMem_DEL``.


Examples
========

Here is the example from section *Overview*, 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
``PyObject_New``, ``PyObject_NewVar`` and ``PyObject_Del``.

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