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

Python objects implemented in C can export a "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 bytes
and arrays. The bytes 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``.

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.

Py_buffer

   void *buf

      A pointer to the start of the memory for the object.

   Py_ssize_t len

      The total length of the memory in bytes.

   int readonly

      An indicator of whether the buffer is read only.

   const char *format

      A *NULL* terminated string in ``struct`` module style syntax
      giving the contents of the elements available through the
      buffer.  If this is *NULL*, ``"B"`` (unsigned bytes) is assumed.

   int ndim

      The number of dimensions the memory represents as a multi-
      dimensional array.  If it is 0, ``strides`` and ``suboffsets``
      must be *NULL*.

   Py_ssize_t *shape

      An array of ``Py_ssize_t``s the length of ``ndim`` giving the
      shape of the memory as a multi-dimensional array.  Note that
      ``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be
      equal to ``len``.

   Py_ssize_t *strides

      An array of ``Py_ssize_t``s the length of ``ndim`` giving the
      number of bytes to skip to get to a new element in each
      dimension.

   Py_ssize_t *suboffsets

      An array of ``Py_ssize_t``s the length of ``ndim``.  If these
      suboffset numbers are greater than or equal to 0, then the value
      stored along the indicated dimension is a pointer and the
      suboffset value dictates how many bytes to add to the pointer
      after de-referencing. A suboffset value that it negative
      indicates that no de-referencing should occur (striding in a
      contiguous memory block).

      Here is a function that returns a pointer to the element in an
      N-D array pointed to by an N-dimensional index when there are
      both non-NULL strides and suboffsets:

         void *get_item_pointer(int ndim, void *buf, Py_ssize_t *strides,
             Py_ssize_t *suboffsets, Py_ssize_t *indices) {
             char *pointer = (char*)buf;
             int i;
             for (i = 0; i < ndim; i++) {
                 pointer += strides[i] * indices[i];
                 if (suboffsets[i] >=0 ) {
                     pointer = *((char**)pointer) + suboffsets[i];
                 }
             }
             return (void*)pointer;
          }

   Py_ssize_t itemsize

      This is a storage for the itemsize (in bytes) of each element of
      the shared memory. It is technically un-necessary as it can be
      obtained using ``PyBuffer_SizeFromFormat()``, however an
      exporter may know this information without parsing the format
      string and it is necessary to know the itemsize for proper
      interpretation of striding. Therefore, storing it is more
      convenient and faster.

   void *internal

      This is for use internally by the exporting object. For example,
      this might be re-cast as an integer by the exporter and used to
      store flags about whether or not the shape, strides, and
      suboffsets arrays must be freed when the buffer is released. The
      consumer should never alter this value.


Buffer related functions
========================

int PyObject_CheckBuffer(PyObject *obj)

   Return 1 if *obj* supports the buffer interface otherwise 0.

int PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags)

   Export *obj* into a ``Py_buffer``, *view*.  These arguments must
   never be *NULL*.  The *flags* argument is a bit field indicating
   what kind of buffer the caller is prepared to deal with and
   therefore what kind of buffer the exporter is allowed to return.
   The buffer interface allows for complicated memory sharing
   possibilities, but some caller may not be able to handle all the
   complexity but may want to see if the exporter will let them take a
   simpler view to its memory.

   Some exporters may not be able to share memory in every possible
   way and may need to raise errors to signal to some consumers that
   something is just not possible. These errors should be a
   ``BufferError`` unless there is another error that is actually
   causing the problem. The exporter can use flags information to
   simplify how much of the ``Py_buffer`` structure is filled in with
   non-default values and/or raise an error if the object can't
   support a simpler view of its memory.

   0 is returned on success and -1 on error.

   The following table gives possible values to the *flags* arguments.

   +--------------------------------+-----------------------------------------------------+
   | Flag                           | Description                                         |
   +================================+=====================================================+
   | ``PyBUF_SIMPLE``               | This is the default flag state.  The returned       |
   |                                | buffer may or may not have writable memory.  The    |
   |                                | format of the data will be assumed to be unsigned   |
   |                                | bytes.  This is a "stand-alone" flag constant. It   |
   |                                | never needs to be '|'d to the others. The exporter  |
   |                                | will raise an error if it cannot provide such a     |
   |                                | contiguous buffer of bytes.                         |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_WRITABLE``             | The returned buffer must be writable.  If it is not |
   |                                | writable, then raise an error.                      |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_STRIDES``              | This implies ``PyBUF_ND``. The returned buffer must |
   |                                | provide strides information (i.e. the strides       |
   |                                | cannot be NULL). This would be used when the        |
   |                                | consumer can handle strided, discontiguous arrays.  |
   |                                | Handling strides automatically assumes you can      |
   |                                | handle shape.  The exporter can raise an error if a |
   |                                | strided representation of the data is not possible  |
   |                                | (i.e. without the suboffsets).                      |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_ND``                   | The returned buffer must provide shape information. |
   |                                | The memory will be assumed C-style contiguous (last |
   |                                | dimension varies the fastest). The exporter may     |
   |                                | raise an error if it cannot provide this kind of    |
   |                                | contiguous buffer. If this is not given then shape  |
   |                                | will be *NULL*.                                     |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_C_CONTIGUOUS``         | These flags indicate that the contiguity returned   |
   | ``PyBUF_F_CONTIGUOUS``         | buffer must be respectively, C-contiguous (last     |
   | ``PyBUF_ANY_CONTIGUOUS``       | dimension varies the fastest), Fortran contiguous   |
   |                                | (first dimension varies the fastest) or either one. |
   |                                | All of these flags imply ``PyBUF_STRIDES`` and      |
   |                                | guarantee that the strides buffer info structure    |
   |                                | will be filled in correctly.                        |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_INDIRECT``             | This flag indicates the returned buffer must have   |
   |                                | suboffsets information (which can be NULL if no     |
   |                                | suboffsets are needed).  This can be used when the  |
   |                                | consumer can handle indirect array referencing      |
   |                                | implied by these suboffsets. This implies           |
   |                                | ``PyBUF_STRIDES``.                                  |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_FORMAT``               | The returned buffer must have true format           |
   |                                | information if this flag is provided. This would be |
   |                                | used when the consumer is going to be checking for  |
   |                                | what 'kind' of data is actually stored. An exporter |
   |                                | should always be able to provide this information   |
   |                                | if requested. If format is not explicitly requested |
   |                                | then the format must be returned as *NULL* (which   |
   |                                | means ``'B'``, or unsigned bytes)                   |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_STRIDED``              | This is equivalent to ``(PyBUF_STRIDES |            |
   |                                | PyBUF_WRITABLE)``.                                  |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_STRIDED_RO``           | This is equivalent to ``(PyBUF_STRIDES)``.          |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_RECORDS``              | This is equivalent to ``(PyBUF_STRIDES |            |
   |                                | PyBUF_FORMAT | PyBUF_WRITABLE)``.                   |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_RECORDS_RO``           | This is equivalent to ``(PyBUF_STRIDES |            |
   |                                | PyBUF_FORMAT)``.                                    |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_FULL``                 | This is equivalent to ``(PyBUF_INDIRECT |           |
   |                                | PyBUF_FORMAT | PyBUF_WRITABLE)``.                   |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_FULL_RO``              | This is equivalent to ``(PyBUF_INDIRECT |           |
   |                                | PyBUF_FORMAT)``.                                    |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_CONTIG``               | This is equivalent to ``(PyBUF_ND |                 |
   |                                | PyBUF_WRITABLE)``.                                  |
   +--------------------------------+-----------------------------------------------------+
   | ``PyBUF_CONTIG_RO``            | This is equivalent to ``(PyBUF_ND)``.               |
   +--------------------------------+-----------------------------------------------------+

void PyBuffer_Release(PyObject *obj, Py_buffer *view)

   Release the buffer *view* over *obj*.  This should be called when
   the buffer is no longer being used as it may free memory from it.

Py_ssize_t PyBuffer_SizeFromFormat(const char *)

   Return the implied ``~Py_buffer.itemsize`` from the struct-stype
   ``~Py_buffer.format``.

int PyObject_CopyToObject(PyObject *obj, void *buf, Py_ssize_t len, char fortran)

   Copy *len* bytes of data pointed to by the contiguous chunk of
   memory pointed to by *buf* into the buffer exported by obj.  The
   buffer must of course be writable.  Return 0 on success and return
   -1 and raise an error on failure.  If the object does not have a
   writable buffer, then an error is raised.  If *fortran* is ``'F'``,
   then if the object is multi-dimensional, then the data will be
   copied into the array in Fortran-style (first dimension varies the
   fastest).  If *fortran* is ``'C'``, then the data will be copied
   into the array in C-style (last dimension varies the fastest).  If
   *fortran* is ``'A'``, then it does not matter and the copy will be
   made in whatever way is more efficient.

int PyBuffer_IsContiguous(Py_buffer *view, char fortran)

   Return 1 if the memory defined by the *view* is C-style (*fortran*
   is ``'C'``) or Fortran-style (*fortran* is ``'F'``) contiguous or
   either one (*fortran* is ``'A'``).  Return 0 otherwise.

void PyBuffer_FillContiguousStrides(int ndim, Py_ssize_t *shape, Py_ssize_t *strides, Py_ssize_t itemsize, char fortran)

   Fill the *strides* array with byte-strides of a contiguous (C-style
   if *fortran* is ``'C'`` or Fortran-style if *fortran* is ``'F'``
   array of the given shape with the given number of bytes per
   element.

int PyBuffer_FillInfo(Py_buffer *view, void *buf, Py_ssize_t len, int readonly, int infoflags)

   Fill in a buffer-info structure, *view*, correctly for an exporter
   that can only share a contiguous chunk of memory of "unsigned
   bytes" of the given length.  Return 0 on success and -1 (with
   raising an error) on error.


MemoryView objects
==================

A memoryview object is an extended buffer object that could replace
the buffer object (but doesn't have to as that could be kept as a
simple 1-d memoryview object).  It, unlike ``Py_buffer``, is a Python
object (exposed as ``memoryview`` in ``builtins``), so it can be used
with Python code.

PyObject* PyMemoryView_FromObject(PyObject *obj)

   Return a memoryview object from an object that defines the buffer
   interface.
