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(python2.1-api.info)Buffer Objects

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Buffer Objects
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This manual section was written by Greg Stein <gstein@lyra.org>.
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 `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 `PyArgs_ParseTuple()' that operate against an object's buffer
interface, returning data from the target object.

More information on the buffer interface is provided in the section
"Buffer Object Structures" (section Note: Buffer Object Structures),
under the description for `PyBufferProcs' .

A "buffer object" is defined in the `bufferobject.h' header (included
by `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.

`PyBufferObject'
     This subtype of `PyObject' represents a buffer object.

`PyTypeObject PyBuffer_Type'
     The instance of `PyTypeObject' which represents the Python buffer
     type; it is the same object as `types.BufferType' in the Python
     layer.  .

`int Py_END_OF_BUFFER'
     This constant may be passed as the SIZE parameter to
     `PyBuffer_FromObject()' or `PyBuffer_FromReadWriteObject()'. It
     indicates that the new `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.

`int PyBuffer_Check(PyObject *p)'
     Return true if the argument has type `PyBuffer_Type'.

`PyObject* PyBuffer_FromObject(PyObject *base, int offset, int size)'
     Return a new read-only buffer object.  This raises `TypeError' if
     BASE doesn't support the read-only buffer protocol or doesn't
     provide exactly one buffer segment, or it raises `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 `Py_END_OF_BUFFER', then the
     new buffer's contents extend to the length of the BASE object's
     exported buffer data.

`PyObject* PyBuffer_FromReadWriteObject(PyObject *base, int offset, int size)'
     Return a new writable buffer object.  Parameters and exceptions are
     similar to those for `PyBuffer_FromObject()'.  If the BASE object
     does not export the writeable buffer protocol, then `TypeError' is
     raised.

`PyObject* PyBuffer_FromMemory(void *ptr, int 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 `ValueError' if SIZE is less than zero.  Note that
     `Py_END_OF_BUFFER' may _not_ be passed for the SIZE parameter;
     `ValueError' will be raised in that case.

`PyObject* PyBuffer_FromReadWriteMemory(void *ptr, int size)'
     Similar to `PyBuffer_FromMemory()', but the returned buffer is
     writable.

`PyObject* PyBuffer_New(int size)'
     Returns a new writable buffer object that maintains its own memory
     buffer of SIZE bytes.  `ValueError' is returned if SIZE is not
     zero or positive.