[GRASS-user] Question with Python-SWIG example [SEC=UNCLASSIFIED]

[Andrew MacIntyre]

> From: Glynn Clements
> 
> Andrew MacIntyre wrote:
> 
> > > > In the case where SWIG is using its own objects to contain
returned
> > > > pointers, then I expect pyobj_to_ptr() needs to be changed to
use
> > > > SWIG_ConvertPtr() rather than PyCObject_AsVoidPtr() to retrieve
the
> > > > actual pointer, or the output typemaps for returned pointers
need to be
> > > > changed to use Python's CObjects rather than native SWIG wrapper
> > > > objects.
> > >
> > > So, what do you suggest? Remove the PyCObject_Check() call,
explicitly
> > > check for string, read buffer, write buffer, and sequence, then
call
> > > SWIG_ConvertPtr() on anything which is left over?
> >
> > That's what I would try.
> 
> Except ... SWIG_ConvertPtr() seems to require that you have access to
> the type descriptor. The documentation seems to suggest (although, as
> usual, it's unclear) that you need to know the actual type.

From the SWIG examples I inferred the following type references for 
the pointers in question:

CELL*  =>  SWIGTYPE_p_CELL
FCELL*  =>  SWIGTYPE_p_FCELL
DCELL*  =>  SWIGTYPE_p_DCELL

provided that CELL*/FCELL*/DCELL* are typedef'ed.

So I expected that in pyobj_to_ptr() you could do something like

	int rc;
	void *ptr;
	...
	switch (type_parm) {
		case CELL_PTR:
			rc = SWIG_ConvertPtr(PyObj, &ptr,
SWIGTYPE_p_CELL, SWIG_POINTER_EXCEPTION);
			break;
		case FCELL_PTR:
			rc = SWIG_ConvertPtr(PyObj, &ptr,
SWIGTYPE_p_FCELL, SWIG_POINTER_EXCEPTION);
			break;
		case DCELL_PTR:
			rc = SWIG_ConvertPtr(PyObj, &ptr,
SWIGTYPE_p_DCELL, SWIG_POINTER_EXCEPTION);
			break;
	}
	if (rc == -1)
		return NULL;
	...

"type_parm" is the type code parameter passed in to pyobj_to_ptr() - I
don't 
have a link to the swig file to check the actual parameter name.

> > I don't think you need to get rid of the typemap - I suspect you
might
> > have to expand it to distinguish between the different types of
pointers
> > though (see below)...
> 
> > I get the impression from the above linked documentation that, for
> > example, there should be an explicit typemap reference for CELL
pointers
> > (as returned by Rast_allocate_c_buf()) and that the typemap for CELL
> > pointers as an input argument should use SWIG_ConvertPtr(), which
will
> > set an exception if the inbound object is not a suitable input
> 
> GRASS uses "void *" to refer to any of CELL*/FCELL*/DCELL* quite
> extensively.
> 
> I think that I'm not going to put any more effort into the SWIG
> wrapper unless usable documentation materialises. In the meantime, I'm
> going to look for alternatives.

If you decide to investigate using ctypes, the following links might be
of interest:

- http://starship.python.net/crew/theller/ctypes/old/codegen.html

- http://www.language-binding.net/pygccxml/pygccxml.html

The latter seems more focussed on C++.  I haven't used either so can't
give 
any guidance.

Cheers,
Andrew.

-------------------------> "These thoughts are mine alone!" <---------
Andrew MacIntyre           Operations Branch
tel:   +61 2 6219 5356     Communications Infrastructure Division
fax:   +61 2 6253 3277     Australian Communications & Media Authority
email: andrew.macintyre at acma.gov.au            http://www.acma.gov.au/


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