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310 lines
17 KiB
ReStructuredText
310 lines
17 KiB
ReStructuredText
.. highlightlang:: c
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.. _bufferobjects:
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Buffer Objects
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--------------
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.. sectionauthor:: Greg Stein <gstein@lyra.org>
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.. sectionauthor:: Benjamin Peterson
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.. index::
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object: buffer
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single: buffer interface
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Python objects implemented in C can export a "buffer interface." These
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functions can be used by an object to expose its data in a raw, byte-oriented
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format. Clients of the object can use the buffer interface to access the object
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data directly, without needing to copy it first.
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Two examples of objects that support the buffer interface are bytes and
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arrays. The bytes object exposes the character contents in the buffer
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interface's byte-oriented form. An array can also expose its contents, but it
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should be noted that array elements may be multi-byte values.
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An example user of the buffer interface is the file object's :meth:`write`
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method. Any object that can export a series of bytes through the buffer
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interface can be written to a file. There are a number of format codes to
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:cfunc:`PyArg_ParseTuple` that operate against an object's buffer interface,
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returning data from the target object.
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.. index:: single: PyBufferProcs
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More information on the buffer interface is provided in the section
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:ref:`buffer-structs`, under the description for :ctype:`PyBufferProcs`.
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Buffer objects are useful as a way to expose the data from another object's
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buffer interface to the Python programmer. They can also be used as a zero-copy
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slicing mechanism. Using their ability to reference a block of memory, it is
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possible to expose any data to the Python programmer quite easily. The memory
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could be a large, constant array in a C extension, it could be a raw block of
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memory for manipulation before passing to an operating system library, or it
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could be used to pass around structured data in its native, in-memory format.
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.. ctype:: Py_buffer
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.. cmember:: void *buf
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A pointer to the start of the memory for the object.
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.. cmember:: Py_ssize_t len
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:noindex:
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The total length of the memory in bytes.
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.. cmember:: int readonly
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An indicator of whether the buffer is read only.
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.. cmember:: const char *format
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:noindex:
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A *NULL* terminated string in :mod:`struct` module style syntax giving the
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contents of the elements available through the buffer. If this is *NULL*,
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``"B"`` (unsigned bytes) is assumed.
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.. cmember:: int ndim
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The number of dimensions the memory represents as a multi-dimensional
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array. If it is 0, :cdata:`strides` and :cdata:`suboffsets` must be
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*NULL*.
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.. cmember:: Py_ssize_t *shape
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An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim` giving the
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shape of the memory as a multi-dimensional array. Note that
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``((*shape)[0] * ... * (*shape)[ndims-1])*itemsize`` should be equal to
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:cdata:`len`.
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.. cmember:: Py_ssize_t *strides
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An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim` giving the
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number of bytes to skip to get to a new element in each dimension.
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.. cmember:: Py_ssize_t *suboffsets
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An array of :ctype:`Py_ssize_t`\s the length of :cdata:`ndim`. If these
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suboffset numbers are greater than or equal to 0, then the value stored
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along the indicated dimension is a pointer and the suboffset value
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dictates how many bytes to add to the pointer after de-referencing. A
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suboffset value that it negative indicates that no de-referencing should
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occur (striding in a contiguous memory block).
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Here is a function that returns a pointer to the element in an N-D array
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pointed to by an N-dimesional index when there are both non-NULL strides
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and suboffsets::
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void *get_item_pointer(int ndim, void *buf, Py_ssize_t *strides,
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Py_ssize_t *suboffsets, Py_ssize_t *indices) {
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char *pointer = (char*)buf;
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int i;
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for (i = 0; i < ndim; i++) {
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pointer += strides[i] * indices[i];
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if (suboffsets[i] >=0 ) {
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pointer = *((char**)pointer) + suboffsets[i];
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}
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}
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return (void*)pointer;
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}
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.. cmember:: Py_ssize_t itemsize
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This is a storage for the itemsize (in bytes) of each element of the
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shared memory. It is technically un-necessary as it can be obtained using
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:cfunc:`PyBuffer_SizeFromFormat`, however an exporter may know this
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information without parsing the format string and it is necessary to know
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the itemsize for proper interpretation of striding. Therefore, storing it
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is more convenient and faster.
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.. cmember:: void *internal
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This is for use internally by the exporting object. For example, this
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might be re-cast as an integer by the exporter and used to store flags
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about whether or not the shape, strides, and suboffsets arrays must be
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freed when the buffer is released. The consumer should never alter this
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value.
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Buffer related functions
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========================
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.. cfunction:: int PyObject_CheckBuffer(PyObject *obj)
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Return 1 if *obj* supports the buffer interface otherwise 0.
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.. cfunction:: int PyObject_GetBuffer(PyObject *obj, PyObject *view, int flags)
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Export *obj* into a :ctype:`Py_buffer`, *view*. These arguments must
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never be *NULL*. The *flags* argument is a bit field indicating what kind
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of buffer the caller is prepared to deal with and therefore what kind of
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buffer the exporter is allowed to return. The buffer interface allows for
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complicated memory sharing possibilities, but some caller may not be able
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to handle all the complexibity but may want to see if the exporter will
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let them take a simpler view to its memory.
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Some exporters may not be able to share memory in every possible way and
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may need to raise errors to signal to some consumers that something is
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just not possible. These errors should be a :exc:`BufferError` unless
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there is another error that is actually causing the problem. The exporter
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can use flags information to simplify how much of the :cdata:`Py_buffer`
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structure is filled in with non-default values and/or raise an error if
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the object can't support a simpler view of its memory.
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0 is returned on success and -1 on error.
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The following table gives possible values to the *flags* arguments.
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+------------------------------+---------------------------------------------------+
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| Flag | Description |
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+==============================+===================================================+
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| :cmacro:`PyBUF_SIMPLE` | This is the default flag state. The returned |
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| | buffer may or may not have writable memory. The |
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| | format of the data will be assumed to be unsigned |
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| | bytes. This is a "stand-alone" flag constant. It |
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| | never needs to be '|'d to the others. The exporter|
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| | will raise an error if it cannot provide such a |
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| | contiguous buffer of bytes. |
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| | |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_WRITABLE` | The returned buffer must be writable. If it is |
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| | not writable, then raise an error. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_STRIDES` | This implies :cmacro:`PyBUF_ND`. The returned |
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| | buffer must provide strides information (i.e. the |
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| | strides cannot be NULL). This would be used when |
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| | the consumer can handle strided, discontiguous |
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| | arrays. Handling strides automatically assumes |
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| | you can handle shape. The exporter can raise an |
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| | error if a strided representation of the data is |
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| | not possible (i.e. without the suboffsets). |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_ND` | The returned buffer must provide shape |
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| | information. The memory will be assumed C-style |
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| | contiguous (last dimension varies the |
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| | fastest). The exporter may raise an error if it |
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| | cannot provide this kind of contiguous buffer. If |
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| | this is not given then shape will be *NULL*. |
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+------------------------------+---------------------------------------------------+
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|:cmacro:`PyBUF_C_CONTIGUOUS` | These flags indicate that the contiguity returned |
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|:cmacro:`PyBUF_F_CONTIGUOUS` | buffer must be respectively, C-contiguous (last |
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|:cmacro:`PyBUF_ANY_CONTIGUOUS`| dimension varies the fastest), Fortran contiguous |
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| | (first dimension varies the fastest) or either |
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| | one. All of these flags imply |
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| | :cmacro:`PyBUF_STRIDES` and guarantee that the |
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| | strides buffer info structure will be filled in |
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| | correctly. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_INDIRECT` | This flag indicates the returned buffer must have |
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| | suboffsets information (which can be NULL if no |
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| | suboffsets are needed). This can be used when |
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| | the consumer can handle indirect array |
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| | referencing implied by these suboffsets. This |
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| | implies :cmacro:`PyBUF_STRIDES`. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_FORMAT` | The returned buffer must have true format |
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| | information if this flag is provided. This would |
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| | be used when the consumer is going to be checking |
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| | for what 'kind' of data is actually stored. An |
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| | exporter should always be able to provide this |
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| | information if requested. If format is not |
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| | explicitly requested then the format must be |
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| | returned as *NULL* (which means ``'B'``, or |
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| | unsigned bytes) |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_STRIDED` | This is equivalent to ``(PyBUF_STRIDES | |
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| | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_STRIDED_RO` | This is equivalent to ``(PyBUF_STRIDES)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_RECORDS` | This is equivalent to ``(PyBUF_STRIDES | |
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| | PyBUF_FORMAT | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_RECORDS_RO` | This is equivalent to ``(PyBUF_STRIDES | |
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| | PyBUF_FORMAT)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_FULL` | This is equivalent to ``(PyBUF_INDIRECT | |
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| | PyBUF_FORMAT | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_FULL_RO`` | This is equivalent to ``(PyBUF_INDIRECT | |
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| | PyBUF_FORMAT)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_CONTIG` | This is equivalent to ``(PyBUF_ND | |
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| | PyBUF_WRITABLE)``. |
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+------------------------------+---------------------------------------------------+
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| :cmacro:`PyBUF_CONTIG_RO` | This is equivalent to ``(PyBUF_ND)``. |
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+------------------------------+---------------------------------------------------+
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.. cfunction:: void PyBuffer_Release(PyObject *obj, Py_buffer *view)
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Release the buffer *view* over *obj*. This shouldd be called when the buffer
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is no longer being used as it may free memory from it.
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.. cfunction:: Py_ssize_t PyBuffer_SizeFromFormat(const char *)
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Return the implied :cdata:`~Py_buffer.itemsize` from the struct-stype
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:cdata:`~Py_buffer.format`.
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.. cfunction:: int PyObject_CopyToObject(PyObject *obj, void *buf, Py_ssize_t len, char fortran)
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Copy *len* bytes of data pointed to by the contiguous chunk of memory pointed
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to by *buf* into the buffer exported by obj. The buffer must of course be
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writable. Return 0 on success and return -1 and raise an error on failure.
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If the object does not have a writable buffer, then an error is raised. If
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*fortran* is ``'F'``, then if the object is multi-dimensional, then the data
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will be copied into the array in Fortran-style (first dimension varies the
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fastest). If *fortran* is ``'C'``, then the data will be copied into the
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array in C-style (last dimension varies the fastest). If *fortran* is
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``'A'``, then it does not matter and the copy will be made in whatever way is
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more efficient.
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.. cfunction:: int PyBuffer_IsContiguous(Py_buffer *view, char fortran)
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Return 1 if the memory defined by the *view* is C-style (*fortran* is
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``'C'``) or Fortran-style (*fortran* is ``'F'``) contiguous or either one
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(*fortran* is ``'A'``). Return 0 otherwise.
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.. cfunction:: void PyBuffer_FillContiguousStrides(int ndim, Py_ssize_t *shape, Py_ssize_t *strides, Py_ssize_t itemsize, char fortran)
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Fill the *strides* array with byte-strides of a contiguous (C-style if
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*fortran* is ``'C'`` or Fortran-style if *fortran* is ``'F'`` array of the
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given shape with the given number of bytes per element.
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.. cfunction:: int PyBuffer_FillInfo(Py_buffer *view, void *buf, Py_ssize_t len, int readonly, int infoflags)
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Fill in a buffer-info structure, *view*, correctly for an exporter that can
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only share a contiguous chunk of memory of "unsigned bytes" of the given
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length. Return 0 on success and -1 (with raising an error) on error.
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MemoryView objects
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==================
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A memoryview object is an extended buffer object that could replace the buffer
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object (but doesn't have to as that could be kept as a simple 1-d memoryview
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object). It, unlike :ctype:`Py_buffer`, is a Python object (exposed as
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:class:`memoryview` in :mod:`builtins`), so it can be used with Python code.
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.. cfunction:: PyObject* PyMemoryView_FromObject(PyObject *obj)
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Return a memoryview object from an object that defines the buffer interface.
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