python-peps/pep-0298.txt

PEP: 298
Title: The Fixed Buffer Interface
Version: $Revision$
Last-Modified: $Date$
Author: Thomas Heller <theller@python.net>
Status: Draft
Type: Standards Track
Created: 26-Jul-2002
Python-Version: 2.3
Post-History: 30-Jul-2002


Abstract

    This PEP proposes an extension to the buffer interface called the
    'fixed buffer interface'.

    The fixed buffer interface fixes the flaws of the 'old' buffer
    interface [1] as defined in Python versions up to and including
    2.2, and has the following semantics:

        The lifetime of the retrieved pointer is clearly defined and
        controlled by the client.

        The buffer size is returned as a 'size_t' data type, which
        allows access to large buffers on platforms where sizeof(int)
        != sizeof(void *).

    (Guido comments: This second sounds like a change we could also
    make to the "old" buffer interface, if we introduce another flag
    bit that's *not* part of the default flags.)


Specification

    The fixed buffer interface exposes new functions which return the
    size and the pointer to the internal memory block of any python
    object which chooses to implement this interface.

    Retrieving a buffer from an object puts this object in a locked
    state during which the buffer may not be freed, resized, or
    reallocated.

    The object must be unlocked again by releasing the buffer if it's
    no longer used by calling another function in the fixed buffer
    interface.  If the object never resizes or reallocates the buffer
    during it's lifetime, this function may be NULL. Failure to call
    this function (if it is != NULL) is a programming error and may
    have unexpected results.

    The fixed buffer interface omits the memory segment model which is
    present in the old buffer interface - only a single memory block
    can be exposed.


Implementation

    Define a new flag in Include/object.h:

        /* PyBufferProcs contains bf_acquirefixedreadbuffer,
           bf_acquirefixedwritebuffer, and bf_releasefixedbuffer */
        #define Py_TPFLAGS_HAVE_FIXEDBUFFER (1L<<15)


    This flag would be included in Py_TPFLAGS_DEFAULT:

        #define Py_TPFLAGS_DEFAULT  ( \
                             ....
                             Py_TPFLAGS_HAVE_FIXEDBUFFER | \
                             ....
                            0)


    Extend the PyBufferProcs structure by new fields in
    Include/object.h:

        typedef size_t (*acquirefixedreadbufferproc)(PyObject *,
                                                     const void **);
        typedef size_t (*acquirefixedwritebufferproc)(PyObject *,
                                                      void **);
        typedef void (*releasefixedbufferproc)(PyObject *);

        typedef struct {
                getreadbufferproc bf_getreadbuffer;
                getwritebufferproc bf_getwritebuffer;
                getsegcountproc bf_getsegcount;
                getcharbufferproc bf_getcharbuffer;
                /* fixed buffer interface functions */
                acquirefixedreadbufferproc bf_acquirefixedreadbuffer;
                acquirefixedwritebufferproc bf_acquirefixedwritebuffer;
                releasefixedbufferproc bf_releasefixedbuffer;
        } PyBufferProcs;


    The new fields are present if the Py_TPFLAGS_HAVE_FIXEDBUFFER
    flag is set in the object's type.

    The Py_TPFLAGS_HAVE_FIXEDBUFFER flag implies the
    Py_TPFLAGS_HAVE_GETCHARBUFFER flag.

    The acquirefixedreadbufferproc and acquirefixedwritebufferproc
    functions return the size in bytes of the memory block on success,
    and fill in the passed void * pointer on success.  If these
    functions fail - either because an error occurs or no memory block
    is exposed - they must set the void * pointer to NULL and raise an
    exception.  The return value is undefined in these cases and
    should not be used.

    If calls to these functions succeed, eventually the buffer must be
    released by a call to the releasefixedbufferproc, supplying the
    original object as argument.  The releasefixedbufferproc cannot
    fail.

    Usually these functions aren't called directly, they are called
    through convenience functions declared in Include/abstract.h:

        int PyObject_AquireFixedReadBuffer(PyObject *obj,
                                           const void **buffer,
                                           size_t *buffer_len);

        int PyObject_AcquireFixedWriteBuffer(PyObject *obj,
                                             void **buffer,
                                             size_t *buffer_len);

        void PyObject_ReleaseFixedBuffer(PyObject *obj);

    The former two functions return 0 on success, set buffer to the
    memory location and buffer_len to the length of the memory block
    in bytes. On failure, or if the fixed buffer interface is not
    implemented by obj, they return -1 and set an exception.

    The latter function doesn't return anything, and cannot fail.


Backward Compatibility

    The size of the PyBufferProcs structure changes if this proposal
    is implemented, but the type's tp_flags slot can be used to
    determine if the additional fields are present.


Reference Implementation

    Will be uploaded to the SourceForge patch manager by the author.


Additional Notes/Comments

    Python strings, Unicode strings, mmap objects, and array objects
    would expose the fixed buffer interface.


Community Feedback

    Greg Ewing doubts the fixed buffer interface is needed at all, he
    thinks the normal buffer interface could be used if the pointer is
    (re)fetched each time it's used.  This seems to be dangerous,
    because even innocent looking calls to the Python API like
    Py_DECREF() may trigger execution of arbitrary Python code.

    The first version of this proposal didn't have the release
    function, but it turned out that this would have been too
    restrictive: mmap and array objects wouldn't have been able to
    implement it, because mmap objects can be closed anytime if not
    locked, and array objects could resize or reallocate the buffer.


Credits

    Scott Gilbert came up with the name 'fixed buffer interface'.


References

    [1] The buffer interface
        http://mail.python.org/pipermail/python-dev/2000-October/009974.html

    [2] The Buffer Problem
        http://www.python.org/peps/pep-0296.html


Copyright

    This document has been placed in the public domain.



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