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_ts 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_ts 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_ts 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.