| .. 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 <memoryviewobjects>` 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. |
| |
| |
| .. index:: |
| object: memoryview |
| |
| .. _memoryviewobjects: |
| |
| MemoryView objects |
| ================== |
| |
| A :class:`memoryview` object exposes the C level buffer interface as a |
| Python object which can then be passed around like any other object. |
| |
| |
| .. cfunction:: PyObject *PyMemoryView_FromObject(PyObject *obj) |
| |
| Create a memoryview object from an object that defines the buffer interface. |
| |
| |
| .. cfunction:: 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. |
| |
| |
| .. cfunction:: 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. |
| |
| |
| .. cfunction:: 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`. |
| |
| |
| .. cfunction:: 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. |