
Buffer Protocol
***************

Certain objects available in Python wrap access to an underlying
memory array or *buffer*.  Such objects include the built-in ``bytes``
and ``bytearray``, and some extension types like ``array.array``.
Third-party libraries may define their own types for special purposes,
such as image processing or numeric analysis.

While each of these types have their own semantics, they share the
common characteristic of being backed by a possibly large memory
buffer.  It is then desireable, in some situations, to access that
buffer directly and without intermediate copying.

Python provides such a facility at the C level in the form of the
*buffer protocol*.  This protocol has two sides:

* on the producer side, a type can export a "buffer interface" which
  allows objects of that type to expose information about their
  underlying buffer. This interface is described in the section
  *Buffer Object Structures*;

* on the consumer side, several means are available to obtain a
  pointer to the raw underlying data of an object (for example a
  method parameter).

Simple objects such as ``bytes`` and ``bytearray`` expose their
underlying buffer in byte-oriented form.  Other forms are possible;
for example, the elements exposed by a ``array.array`` can be multi-
byte values.

An example consumer of the buffer interface is the ``write()`` method
of file objects: any object that can export a series of bytes through
the buffer interface can be written to a file.  While ``write()`` only
needs read-only access to the internal contents of the object passed
to it, other methods such as ``readinto()`` need write access to the
contents of their argument.  The buffer interface allows objects to
selectively allow or reject exporting of read-write and read-only
buffers.

There are two ways for a consumer of the buffer interface to acquire a
buffer over a target object:

* call ``PyObject_GetBuffer()`` with the right parameters;

* call ``PyArg_ParseTuple()`` (or one of its siblings) with one of the
  ``y*``, ``w*`` or ``s*`` *format codes*.

In both cases, ``PyBuffer_Release()`` must be called when the buffer
isn't needed anymore.  Failure to do so could lead to various issues
such as resource leaks.


The buffer structure
====================

Buffer structures (or simply "buffers") are useful as a way to expose
the binary data from another object 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.

Contrary to most data types exposed by the Python interpreter, buffers
are not ``PyObject`` pointers but rather simple C structures.  This
allows them to be created and copied very simply.  When a generic
wrapper around a buffer is needed, a *memoryview* object can be
created.

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.  When
   1 is returned, it doesn't guarantee that ``PyObject_GetBuffer()``
   will succeed.

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

   Export a view over some internal data from the target object *obj*.
   *obj* must not be NULL, and *view* must point to an existing
   ``Py_buffer`` structure allocated by the caller (most uses of this
   function will simply declare a local variable of type
   ``Py_buffer``).  The *flags* argument is a bit field indicating
   what kind of buffer is requested.  The buffer interface allows for
   complicated memory layout possibilities; however, some callers
   won't want to handle all the complexity and instead request a
   simple view of the target object (using ``PyBUF_SIMPLE`` for a
   read-only view and ``PyBUF_WRITABLE`` for a read-write view).

   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.

   On success, 0 is returned and the *view* structure is filled with
   useful values.  On error, -1 is returned and an exception is
   raised; the *view* is left in an undefined state.

   The following are the possible values to the *flags* arguments.

   PyBUF_SIMPLE

      This is the default flag.  The returned buffer exposes a read-
      only memory area.  The format of data is assumed to be raw
      unsigned bytes, without any particular structure.  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

      Like ``PyBUF_SIMPLE``, but the returned buffer is writable.  If
      the exporter doesn't support writable buffers, an error is
      raised.

   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
   PyBUF_F_CONTIGUOUS
   PyBUF_ANY_CONTIGUOUS

      These flags indicate that the contiguity returned buffer must be
      respectively, C-contiguous (last 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(Py_buffer *view)

   Release the buffer *view*.  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 ``itemsize`` from the struct-stype ``format``.

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, PyObject *obj, 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.
