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

[Andrew MacIntyre]

> From: Glynn Clements
> Andrew MacIntyre wrote:
> 
> > > That was what I tried first, but it results in Rast_get_c_row()
> > > segfaulting.
> >
> > Then it seems to me that there's some problem with the SWIG wrapper
for
> > Rast_get_c_row(), if it won't accept a valid SWIG pointer object
> > returned from another wrapped function.
> 
> That's what I suspect, namely that the typemap for pointers is getting
> in the way.
> 
> Removing the typemap would mean that you couldn't pass a Python
> string, array, buffer, sequence, etc directly to a GRASS function, but
> would have to explicitly allocate and populate a block of memory.

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)...

> > > There is a typemap for pointers, which converts a CObject, string,
> > > buffer or sequence. I have no idea how to test for or convert a
> > > SWIG-wrapped pointer.
> >
> > In the SWIG wrapper?  I see a note about SWIG_ConvertPtr() in
> > http://www.swig.org/Doc1.3/Python.html#Python_nn64 ?
> 
> That might help, although I still don't know how to test whether a
> given "PyObject *" is a SWIG-wrapped pointer.

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 (if the
exception flag is used; returning NULL will then propagate the
exception).  As far as SWIGged pointers go, this seems to be the closest
you can get to Python's *Check* APIs.

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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