10.7 Buffer Object Structures
The buffer interface exports a model where an object can expose its internal data as a set of chunks of data, where each chunk is specified as a pointer/length pair. These chunks are called segments and are presumed to be non-contiguous in memory.
If an object does not export the buffer interface, then its tp_as_buffer member in the PyTypeObject structure should be NULL. Otherwise, the tp_as_buffer will point to a PyBufferProcs structure.
Note:
It is very important that your PyTypeObject structure
uses Py_TPFLAGS_DEFAULT for the value of the
tp_flags member rather than 0. This tells the Python
runtime that your PyBufferProcs structure contains the
bf_getcharbuffer slot. Older versions of Python did not have
this member, so a new Python interpreter using an old extension needs
to be able to test for its presence before using it.
-
Structure used to hold the function pointers which define an
implementation of the buffer protocol.
The first slot is bf_getreadbuffer, of type getreadbufferproc. If this slot is NULL, then the object does not support reading from the internal data. This is non-sensical, so implementors should fill this in, but callers should test that the slot contains a non-NULL value.
The next slot is bf_getwritebuffer having type getwritebufferproc. This slot may be NULL if the object does not allow writing into its returned buffers.
The third slot is bf_getsegcount, with type getsegcountproc. This slot must not be NULL and is used to inform the caller how many segments the object contains. Simple objects such as PyString_Type and PyBuffer_Type objects contain a single segment.
The last slot is bf_getcharbuffer, of type getcharbufferproc. This slot will only be present if the Py_TPFLAGS_HAVE_GETCHARBUFFER flag is present in the tp_flags field of the object's PyTypeObject. Before using this slot, the caller should test whether it is present by using the PyType_HasFeature() function. If present, it may be NULL, indicating that the object's contents cannot be used as 8-bit characters. The slot function may also raise an error if the object's contents cannot be interpreted as 8-bit characters. For example, if the object is an array which is configured to hold floating point values, an exception may be raised if a caller attempts to use bf_getcharbuffer to fetch a sequence of 8-bit characters. This notion of exporting the internal buffers as ``text'' is used to distinguish between objects that are binary in nature, and those which have character-based content.
Note: The current policy seems to state that these characters may be multi-byte characters. This implies that a buffer size of N does not mean there are N characters present.
- Flag bit set in the type structure to indicate that the bf_getcharbuffer slot is known. This being set does not indicate that the object supports the buffer interface or that the bf_getcharbuffer slot is non-NULL.
-
Return a pointer to a readable segment of the buffer. This function
is allowed to raise an exception, in which case it must return
-1. The segment which is passed must be zero or positive, and strictly less than the number of segments returned by the bf_getsegcount slot function. On success, it returns the length of the buffer memory, and sets*ptrptrto a pointer to that memory.
-
Return a pointer to a writable memory buffer in
*ptrptr, and the length of that segment as the function return value. The memory buffer must correspond to buffer segment segment. Must return-1and set an exception on error. TypeError should be raised if the object only supports read-only buffers, and SystemError should be raised when segment specifies a segment that doesn't exist.
-
Return the number of memory segments which comprise the buffer. If
lenp is not NULL, the implementation must report the sum of
the sizes (in bytes) of all segments in
*lenp. The function cannot fail.
-
Return the size of the memory buffer in ptrptr for segment
segment.
*ptrptris set to the memory buffer.
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Python/C API Reference Manual |  |
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Release 2.4, documentation updated on 29 November 2004.