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

[Andrew MacIntyre]

> From: Glynn Clements
> 
> Andrew MacIntyre wrote:
> 
> > > > After some googling, it seems that the pointer address should be
> > > > obtainable by using int() on the pointer object returned by the
SWIG
> > > > wrapper (i.e. int(p) in the above example), with the resulting
value
> > > > usable in ctypes' from_address() method.  I might be wrong, but
I
> > > > think that e.g. ctypes.c_void_p(int(p)) might also work to
create a
> > > > ctypes void pointer instance.
> > >
> > > That doesn't seem to work either:
> > >
> > > 	> import grass.lib.grass as gis
> > > 	> import grass.lib.raster as rast
> > > 	> import ctypes
> > > 	> gis.G_gisinit("")
> > > 	> p = rast.Rast_allocate_c_buf()
> > > 	> a = ctypes.cast(int(p), ctypes.POINTER(ctypes.c_int))
> > > 	> infd = rast.Rast_open_old("elevation.dem", "")
> > > 	> rast.Rast_get_c_row(infd, a, 10)
> >
> > I think the above line should read:
> >
> >  	rast. Rast_get_c_row(infd, p, 10)
> 
> 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.

> 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 ?
 
> The fact that passing "a" doesn't segfault appears to indicate that it
> was successfully converted to a pointer. The fact that accessing a[0]
> afterwards leads me to suspect that it may be getting converted to a
> pointer to the "container" rather than to the array proper.

Well, "a" is a ctypes object, so  I wouldn't have expected a useful
coercion.

Sadly, I doubt I can contribute much more to this matter :(

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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