c++ extension, problem passing argument

Aaron Brady castironpi at gmail.com
Sat Mar 14 02:08:47 CET 2009

On Mar 13, 5:42 pm, Matteo <tadweles... at gmail.com> wrote:
> On 13 Mar, 22:35, Aaron Brady <castiro... at gmail.com> wrote:
> > On Mar 13, 1:34 pm, Matteo <tadweles... at gmail.com> wrote:
> > > hmmm... looks like SWIG has a problem with double pointers. I googled
> > > around a bit and found:
> > >http://osdir.com/ml/programming.swig/2003-02/msg00029.html
> > > anyone knows how to write a small wrapper to do the appropriate
> > > dereferencing?
> > 'ctypes' may be able to do it.  I've done something like this in the
> > past:
> > double_ptr= ctypes._cast( PyObject, sing_ptr )
> > Up your alley?
> Thanks for your suggestions, but still no luck here.
> ctypes appears to work only with its own types, I tried its cast,
> byref and pointer functions, but only received TypeError exceptions
> such as:
> TypeError: byref() argument must be a ctypes instance, not 'list'
> TypeError: _type_ must have storage info
> This is getting really annoying :(
> The following link may contain useful info, but I find it somewhat
> obscurehttp://embedded.eecs.berkeley.edu/Alumni/pinhong/scriptEDA/pyTypemapF...

I'm on my last one (suggestion).  To get a pointer to the actual
address of an mmap instance, (and access it's 'data' attribute), I
used the following:

from _ctypes import _cast_addr
from ctypes import *
_mmap_cast= _data_cast= PYFUNCTYPE(py_object, py_object, py_object,
_offset_cast= _record_cast = PYFUNCTYPE(py_object, c_void_p,
py_object, py_object)(_cast_addr)

class MMAP(Structure):
    _fields_= [

def pdata( map ):
    a= _data_cast( map, None, POINTER( MMAP ) )
    return a.contents.data

It worked on my machine in 2.5, no promises.  It was a workaround for
the '_type_ must have storage info' error.  Lo and behold,
'a.contents' was an MMAP, so '.data' was a 'c_long' interpretation of
the 'char* data' member of the C structure, PyMmapObject or
something.  I found it by studying the 'ctypes.cast' code.  Well good
luck and sorry for the suggestions.

More information about the Python-list mailing list