extracting null pointer address from PyCObject with ctypes

Gordon Allott gordallott at gmail.com
Sat Oct 11 02:59:41 CEST 2008


Aaron "Castironpi" Brady wrote:

> Yes, well said.  But no, not true, not necessarily.  You can choose/
> change return types with your code.  If the call is defined already
> and you can't change the return, just define a new one that returns
> long.
> --
> http://mail.python.org/mailman/listinfo/python-list

the problem is that the pointer or long or whatever it is thats returned
won't be the data I am after. the code for PyCObject_FromVoidPtr is as
follows:

PyObject *
PyCObject_FromVoidPtr(void *cobj, void (*destr)(void *))
{
    PyCObject *self;

    self = PyObject_NEW(PyCObject, &PyCObject_Type);
    if (self == NULL)
        return NULL;
    self->cobject=cobj;
    self->destructor=destr;
    self->desc=NULL;

    return (PyObject *)self;
}


it obviously creates a new PyObject and returns that, which has already
happened once (the address I am after is passed to python via
PyCObject_FromVoidPtr(adress_i_am_after, NULL), doing that puts the
address I am after into the .cobject attribute of a new pyobject
structure and passes that to the python script via the 'display' key in
a dictionary.

If I were to then take the pycobject in this display key and pass it via
ctypes into PyCObject_FromVoidPtr it would simply create a new pycobject
 and put a pointer to the old pycobject in the new pycobject's .cobject
attribute. it just means that I am getting further and further away from
where I want to be surely? if I were to take the current pointer at this
stage, to get to the address I actually want in C it would have to
follow something along the lines of
long address_i_want = (long)(new_pycobj->cobject->cobject);

What would be great is if there is some easy simple way of accessing the
.cobject attribute of the first pycobject thats passed via the
dictionary to python.

-- 
Gord Allott (gordallott at gmail.com)

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