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python/vendor/Python-2.7.6/Doc/c-api/buffer.rst
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Buffers and Memoryview Objects
.. highlightlang:: c
.. sectionauthor:: Greg Stein <gstein@lyra.org>
.. sectionauthor:: Benjamin Peterson
.. index:: object: buffer single: buffer interface
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 :meth:`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 :c:func:`PyArg_ParseTuple` that operate against an object's buffer interface, returning data from the target object.
Starting from version 1.6, Python has been providing Python-level buffer objects and a C-level buffer API so that any built-in or used-defined type can expose its characteristics. Both, however, have been deprecated because of various shortcomings, and have been officially removed in Python 3 in favour of a new C-level buffer API and a new Python-level object named :class:`memoryview`.
The new buffer API has been backported to Python 2.6, and the :class:`memoryview` object has been backported to Python 2.7. It is strongly advised to use them rather than the old APIs, unless you are blocked from doing so for compatibility reasons.
The new-style Py_buffer struct
.. c:type:: Py_buffer
.. c:member:: void *buf
A pointer to the start of the memory for the object.
.. c:member:: Py_ssize_t len
:noindex:
The total length of the memory in bytes.
.. c:member:: int readonly
An indicator of whether the buffer is read only.
.. c:member:: 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.
.. c:member:: int ndim
The number of dimensions the memory represents as a multi-dimensional
array. If it is 0, :c:data:`strides` and :c:data:`suboffsets` must be
*NULL*.
.. c:member:: Py_ssize_t *shape
An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim` giving the
shape of the memory as a multi-dimensional array. Note that
``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be equal to
:c:data:`len`.
.. c:member:: Py_ssize_t *strides
An array of :c:type:`Py_ssize_t`\s the length of :c:data:`ndim` giving the
number of bytes to skip to get to a new element in each dimension.
.. c:member:: Py_ssize_t *suboffsets
An array of :c:type:`Py_ssize_t`\s the length of :c:data:`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-dimesional 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;
}
.. c:member:: 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 :c:func:`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.
.. c:member:: 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.
MemoryView objects
.. versionadded:: 2.7
A :class:`memoryview` object exposes the new C level buffer interface as a Python object which can then be passed around like any other object.
.. c:function:: PyObject *PyMemoryView_FromObject(PyObject *obj) Create a memoryview object from an object that defines the new buffer interface.
.. c:function:: PyObject *PyMemoryView_FromBuffer(Py_buffer *view) Create a memoryview object wrapping the given buffer-info structure *view*. The memoryview object then owns the buffer, which means you shouldn't try to release it yourself: it will be released on deallocation of the memoryview object.
.. c:function:: PyObject *PyMemoryView_GetContiguous(PyObject *obj, int buffertype, char order) Create a memoryview object to a contiguous chunk of memory (in either 'C' or 'F'ortran *order*) from an object that defines the buffer interface. If memory is contiguous, the memoryview object points to the original memory. Otherwise copy is made and the memoryview points to a new bytes object.
.. c:function:: int PyMemoryView_Check(PyObject *obj) Return true if the object *obj* is a memoryview object. It is not currently allowed to create subclasses of :class:`memoryview`.
.. c:function:: Py_buffer *PyMemoryView_GET_BUFFER(PyObject *obj) Return a pointer to the buffer-info structure wrapped by the given object. The object **must** be a memoryview instance; this macro doesn't check its type, you must do it yourself or you will risk crashes.
Old-style buffer objects
.. index:: single: PyBufferProcs
More information on the old buffer interface is provided in the section :ref:`buffer-structs`, under the description for :c:type:`PyBufferProcs`.
A "buffer object" is defined in the :file:`bufferobject.h` header (included by :file:`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.
.. c:type:: PyBufferObject This subtype of :c:type:`PyObject` represents a buffer object.
.. c:var:: PyTypeObject PyBuffer_Type .. index:: single: BufferType (in module types) The instance of :c:type:`PyTypeObject` which represents the Python buffer type; it is the same object as ``buffer`` and ``types.BufferType`` in the Python layer. .
.. c:var:: int Py_END_OF_BUFFER This constant may be passed as the *size* parameter to :c:func:`PyBuffer_FromObject` or :c:func:`PyBuffer_FromReadWriteObject`. It indicates that the new :c:type:`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.
.. c:function:: int PyBuffer_Check(PyObject *p) Return true if the argument has type :c:data:`PyBuffer_Type`.
.. c:function:: PyObject* PyBuffer_FromObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
Return a new read-only buffer object. This raises :exc:`TypeError` if
*base* doesn't support the read-only buffer protocol or doesn't provide
exactly one buffer segment, or it raises :exc:`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
:const:`Py_END_OF_BUFFER`, then the new buffer's contents extend to the
length of the *base* object's exported buffer data.
.. versionchanged:: 2.5
This function used an :c:type:`int` type for *offset* and *size*. This
might require changes in your code for properly supporting 64-bit
systems.
.. c:function:: PyObject* PyBuffer_FromReadWriteObject(PyObject *base, Py_ssize_t offset, Py_ssize_t size)
Return a new writable buffer object. Parameters and exceptions are similar
to those for :c:func:`PyBuffer_FromObject`. If the *base* object does not
export the writeable buffer protocol, then :exc:`TypeError` is raised.
.. versionchanged:: 2.5
This function used an :c:type:`int` type for *offset* and *size*. This
might require changes in your code for properly supporting 64-bit
systems.
.. c:function:: PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size)
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 :exc:`ValueError` if *size* is less
than zero. Note that :const:`Py_END_OF_BUFFER` may *not* be passed for the
*size* parameter; :exc:`ValueError` will be raised in that case.
.. versionchanged:: 2.5
This function used an :c:type:`int` type for *size*. This might require
changes in your code for properly supporting 64-bit systems.
.. c:function:: PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size)
Similar to :c:func:`PyBuffer_FromMemory`, but the returned buffer is
writable.
.. versionchanged:: 2.5
This function used an :c:type:`int` type for *size*. This might require
changes in your code for properly supporting 64-bit systems.
.. c:function:: PyObject* PyBuffer_New(Py_ssize_t size)
Return a new writable buffer object that maintains its own memory buffer of
*size* bytes. :exc:`ValueError` is returned if *size* is not zero or
positive. Note that the memory buffer (as returned by
:c:func:`PyObject_AsWriteBuffer`) is not specifically aligned.
.. versionchanged:: 2.5
This function used an :c:type:`int` type for *size*. This might require
changes in your code for properly supporting 64-bit systems.
