
Buffer Objects
**************

Python objects implemented in C can export a group of functions called
the "buffer interface."  These functions can be used by an object to
expose its data in a raw, byte-oriented format. Clients of the object
can use the buffer interface to access the object data directly,
without needing to copy it first.

Two examples of objects that support the buffer interface are strings
and arrays. The string object exposes the character contents in the
buffer interface's byte-oriented form. An array can also expose its
contents, but it should be noted that array elements may be multi-byte
values.

An example user of the buffer interface is the file object's
``write()`` method. Any object that can export a series of bytes
through the buffer interface can be written to a file. There are a
number of format codes to ``PyArg_ParseTuple`` that operate against an
object's buffer interface, returning data from the target object.

More information on the buffer interface is provided in the section
*Buffer Object Structures*, under the description for
``PyBufferProcs``.

A "buffer object" is defined in the ``bufferobject.h`` header
(included by ``Python.h``). These objects look very similar to string
objects at the Python programming level: they support slicing,
indexing, concatenation, and some other standard string operations.
However, their data can come from one of two sources: from a block of
memory, or from another object which exports the buffer interface.

Buffer objects are useful as a way to expose the data from another
object's buffer interface to the Python programmer. They can also be
used as a zero-copy slicing mechanism. Using their ability to
reference a block of memory, it is possible to expose any data to the
Python programmer quite easily. The memory could be a large, constant
array in a C extension, it could be a raw block of memory for
manipulation before passing to an operating system library, or it
could be used to pass around structured data in its native, in-memory
format.

PyBufferObject

   This subtype of ``PyObject`` represents a buffer object.

PyTypeObject PyBuffer_Type

   The instance of ``PyTypeObject`` which represents the Python buffer
   type; it is the same object as ``buffer`` and  ``types.BufferType``
   in the Python layer. .

int Py_END_OF_BUFFER

   This constant may be passed as the *size* parameter to
   ``PyBuffer_FromObject`` or ``PyBuffer_FromReadWriteObject``.  It
   indicates that the new ``PyBufferObject`` should refer to *base*
   object from the specified *offset* to the end of its exported
   buffer.  Using this enables the caller to avoid querying the *base*
   object for its length.

int PyBuffer_Check(PyObject *p)

   Return true if the argument has type ``PyBuffer_Type``.

PyObject* PyBuffer_FromObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
    Return value: New reference.

   Return a new read-only buffer object.  This raises ``TypeError`` if
   *base* doesn't support the read-only buffer protocol or doesn't
   provide exactly one buffer segment, or it raises ``ValueError`` if
   *offset* is less than zero. The buffer will hold a reference to the
   *base* object, and the buffer's contents will refer to the *base*
   object's buffer interface, starting as position *offset* and
   extending for *size* bytes. If *size* is ``Py_END_OF_BUFFER``, then
   the new buffer's contents extend to the length of the *base*
   object's exported buffer data.

PyObject* PyBuffer_FromReadWriteObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
    Return value: New reference.

   Return a new writable buffer object.  Parameters and exceptions are
   similar to those for ``PyBuffer_FromObject``.  If the *base* object
   does not export the writeable buffer protocol, then ``TypeError``
   is raised.

PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size)
    Return value: New reference.

   Return a new read-only buffer object that reads from a specified
   location in memory, with a specified size.  The caller is
   responsible for ensuring that the memory buffer, passed in as
   *ptr*, is not deallocated while the returned buffer object exists.
   Raises ``ValueError`` if *size* is less than zero.  Note that
   ``Py_END_OF_BUFFER`` may *not* be passed for the *size* parameter;
   ``ValueError`` will be raised in that case.

PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size)
    Return value: New reference.

   Similar to ``PyBuffer_FromMemory``, but the returned buffer is
   writable.

PyObject* PyBuffer_New(Py_ssize_t size)
    Return value: New reference.

   Return a new writable buffer object that maintains its own memory
   buffer of *size* bytes.  ``ValueError`` is returned if *size* is
   not zero or positive. Note that the memory buffer (as returned by
   ``PyObject_AsWriteBuffer``) is not specifically aligned.
