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.. highlightlang:: c
.. _bufferobjects:
Buffer Protocol
---------------
.. sectionauthor:: Greg Stein <gstein@lyra.org>
.. sectionauthor:: Benjamin Peterson
.. index::
single: buffer interface
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.
Examples of objects that support the buffer interface are :class:`bytes`,
:class:`bytearray` and :class:`array.array`. The bytes and bytearray objects
exposes their bytes contents in the buffer interface's byte-oriented form.
An :class:`array.array` can also expose its contents, but it should be noted
that array elements may be multi-byte values.
An example consumer of the buffer interface is the :meth:`~io.BufferedIOBase.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 :meth:`write` only
needs read-only access to the internal contents of the object passed to it,
other methods such as :meth:`~io.BufferedIOBase.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 :cfunc:`PyObject_GetBuffer` with the right parameters;
* call :cfunc:`PyArg_ParseTuple` (or one of its siblings) with one of the
``y*``, ``w*`` or ``s*`` :ref:`format codes <arg-parsing>`.
In both cases, :cfunc:`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.
.. index:: single: PyBufferProcs
How the buffer interface is exposed by a type object is described in the
section :ref:`buffer-structs`, under the description for :ctype:`PyBufferProcs`.
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 :ctype:`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 :ref:`memoryview <memoryview-objects>` object
can be created.
.. ctype:: Py_buffer
.. cmember:: void *buf
A pointer to the start of the memory for the object.
.. cmember:: Py_ssize_t len
:noindex:
The total length of the memory in bytes.
.. cmember:: int readonly
An indicator of whether the buffer is read only.
.. cmember:: const char *format
:noindex:
A *NULL* terminated string in :mod:`struct` module style syntax giving
the contents of the elements available through the buffer. If this is
*NULL*, ``"B"`` (unsigned bytes) is assumed.
.. cmember:: int ndim
The number of dimensions the memory represents as a multi-dimensional
array. If it is 0, :cdata:`strides` and :cdata:`suboffsets` must be
*NULL*.
.. cmember:: Py_ssize_t *shape
An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim` giving the
shape of the memory as a multi-dimensional array. Note that
``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be equal to
:cdata:`len`.
.. cmember:: Py_ssize_t *strides
An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim` giving the
number of bytes to skip to get to a new element in each dimension.
.. cmember:: Py_ssize_t *suboffsets
An array of :ctype:`Py_ssize_t`\s the length of :cdata:`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;
}
.. cmember:: 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 :cfunc:`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.
.. cmember:: 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
========================
.. cfunction:: int PyObject_CheckBuffer(PyObject *obj)
Return 1 if *obj* supports the buffer interface otherwise 0. When 1 is
returned, it doesn't guarantee that :cfunc:`PyObject_GetBuffer` will
succeed.
.. cfunction:: 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
:ctype:`Py_buffer` structure allocated by the caller (most uses of
this function will simply declare a local variable of type
:ctype:`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 :cmacro:`PyBUF_SIMPLE` for a read-only
view and :cmacro:`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 :exc:`BufferError` unless
there is another error that is actually causing the problem. The
exporter can use flags information to simplify how much of the
:cdata:`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 table gives possible values to the *flags* arguments.
+------------------------------+---------------------------------------------------+
| Flag | Description |
+==============================+===================================================+
| .. cmacro:: 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. |
| | |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_WRITABLE | Like :cmacro:`PyBUF_SIMPLE`, but the returned |
| | buffer is writable. If the exporter doesn't |
| | support |
| | writable buffers, an error is raised. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_STRIDES | This implies :cmacro:`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). |
| | |
+------------------------------+---------------------------------------------------+
| .. cmacro:: 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*. |
| | |
+------------------------------+---------------------------------------------------+
|.. cmacro:: 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 |
| | :cmacro:`PyBUF_STRIDES` and guarantee that the |
| | strides buffer info structure will be filled in |
| | correctly. |
| | |
+------------------------------+---------------------------------------------------+
| .. cmacro:: 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 :cmacro:`PyBUF_STRIDES`. |
| | |
| | |
| | |
+------------------------------+---------------------------------------------------+
| .. cmacro:: 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) |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_STRIDED | This is equivalent to ``(PyBUF_STRIDES | |
| | PyBUF_WRITABLE)``. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_STRIDED_RO | This is equivalent to ``(PyBUF_STRIDES)``. |
| | |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_RECORDS | This is equivalent to ``(PyBUF_STRIDES | |
| | PyBUF_FORMAT | PyBUF_WRITABLE)``. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_RECORDS_RO | This is equivalent to ``(PyBUF_STRIDES | |
| | PyBUF_FORMAT)``. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_FULL | This is equivalent to ``(PyBUF_INDIRECT | |
| | PyBUF_FORMAT | PyBUF_WRITABLE)``. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_FULL_RO | This is equivalent to ``(PyBUF_INDIRECT | |
| | PyBUF_FORMAT)``. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_CONTIG | This is equivalent to ``(PyBUF_ND | |
| | PyBUF_WRITABLE)``. |
+------------------------------+---------------------------------------------------+
| .. cmacro:: PyBUF_CONTIG_RO | This is equivalent to ``(PyBUF_ND)``. |
| | |
+------------------------------+---------------------------------------------------+
.. cfunction:: 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.
.. cfunction:: Py_ssize_t PyBuffer_SizeFromFormat(const char *)
Return the implied :cdata:`~Py_buffer.itemsize` from the struct-stype
:cdata:`~Py_buffer.format`.
.. cfunction:: 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.
.. cfunction:: 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.
.. cfunction:: 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.
.. cfunction:: 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.
|
