[Python-Dev] Buffer interface in abstract.c?

[M.-A. Lemburg]

Greg Stein wrote:
> 
> Mark Hammond wrote:
> > ...
> > Therefore, I would like to propose these functions to be added to
> > abstract.c:
> >
> > int PyObject_GetBufferSize();
> > void *PyObject_GetReadWriteBuffer(); /* or "char *"?  */
> > const void *PyObject_GetReadOnlyBuffer();
> >
> > Although equivalent functions exist for the buffer object, I can't see the
> > equivalent abstract implementations - ie, that work with any object
> > supporting the protocol.
> >
> > Im willing to provide a patch if there is agreement a) the general idea is
> > good, and b) my specific spelling of the idea is OK (less likely -
> > PyBuffer_* seems better, but loses any implication of being abstract?).
> 
> Marc-Andre proposed exactly the same thing back at the end of March (to
> me and Guido). The two of us hashed out some of the stuff and M.A. came
> up with a full patch for the stuff. Guido was relatively non-committal
> at the point one way or another, but said they seemed fine. It appears
> the stuff never made it into source control.
> 
> If Marc-Andre can resurface the final proposal/patch, then we'd be set.

Below is the code I currently use. I don't really remember if this
is what Greg and I discussed a while back, but I'm sure he'll
correct me ;-) Note that you the buffer length is implicitly
returned by these APIs.

/* Takes an arbitrary object which must support the character (single
   segment) buffer interface and returns a pointer to a read-only
   memory location useable as character based input for subsequent
   processing.

   buffer and buffer_len are only set in case no error
   occurrs. Otherwise, -1 is returned and an exception set.

*/

static
int PyObject_AsCharBuffer(PyObject *obj,
			  const char **buffer,
			  int *buffer_len)
{
    PyBufferProcs *pb = obj->ob_type->tp_as_buffer;
    const char *pp;
    int len;

    if ( pb == NULL ||
	 pb->bf_getcharbuffer == NULL ||
	 pb->bf_getsegcount == NULL ) {
	PyErr_SetString(PyExc_TypeError,
			"expected a character buffer object");
	goto onError;
    }
    if ( (*pb->bf_getsegcount)(obj,NULL) != 1 ) {
	PyErr_SetString(PyExc_TypeError,
			"expected a single-segment buffer object");
	goto onError;
    }
    len = (*pb->bf_getcharbuffer)(obj,0,&pp);
    if (len < 0)
	goto onError;
    *buffer = pp;
    *buffer_len = len;
    return 0;

 onError:
    return -1;
}

/* Same as PyObject_AsCharBuffer() except that this API expects a
   readable (single segment) buffer interface and returns a pointer
   to a read-only memory location which can contain arbitrary data.

   buffer and buffer_len are only set in case no error
   occurrs. Otherwise, -1 is returned and an exception set.

*/

static
int PyObject_AsReadBuffer(PyObject *obj,
			  const void **buffer,
			  int *buffer_len)
{
    PyBufferProcs *pb = obj->ob_type->tp_as_buffer;
    void *pp;
    int len;

    if ( pb == NULL ||
	 pb->bf_getreadbuffer == NULL ||
	 pb->bf_getsegcount == NULL ) {
	PyErr_SetString(PyExc_TypeError,
			"expected a readable buffer object");
	goto onError;
    }
    if ( (*pb->bf_getsegcount)(obj,NULL) != 1 ) {
	PyErr_SetString(PyExc_TypeError,
			"expected a single-segment buffer object");
	goto onError;
    }
    len = (*pb->bf_getreadbuffer)(obj,0,&pp);
    if (len < 0)
	goto onError;
    *buffer = pp;
    *buffer_len = len;
    return 0;

 onError:
    return -1;
}

/* Takes an arbitrary object which must support the writeable (single
   segment) buffer interface and returns a pointer to a writeable
   memory location in buffer of size buffer_len.

   buffer and buffer_len are only set in case no error
   occurrs. Otherwise, -1 is returned and an exception set.

*/

static
int PyObject_AsWriteBuffer(PyObject *obj,
			   void **buffer,
			   int *buffer_len)
{
    PyBufferProcs *pb = obj->ob_type->tp_as_buffer;
    void*pp;
    int len;

    if ( pb == NULL ||
	 pb->bf_getwritebuffer == NULL ||
	 pb->bf_getsegcount == NULL ) {
	PyErr_SetString(PyExc_TypeError,
			"expected a writeable buffer object");
	goto onError;
    }
    if ( (*pb->bf_getsegcount)(obj,NULL) != 1 ) {
	PyErr_SetString(PyExc_TypeError,
			"expected a single-segment buffer object");
	goto onError;
    }
    len = (*pb->bf_getwritebuffer)(obj,0,&pp);
    if (len < 0)
	goto onError;
    *buffer = pp;
    *buffer_len = len;
    return 0;

 onError:
    return -1;
}


-- 
Marc-Andre Lemburg
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