[Cython] Accessing C attributes from Python functions in the interpreter

[Robert Bradshaw]

On Jun 7, 2008, at 4:24 PM, Johannes Wienke wrote:

> Hi,
>
> maybe I've just got a lack of understanding or I don't know...
>
> I've got a class that in general looks like this:
>
> cpdef class Foo:
> 	cdef char* myData
>
> 	cdef void setData(Foo self, char *data)
> 		self.myData = data
>
> 	cpdef doSomething(Foo self)
> 		print self.myData
>
> The problem is that working with myData directly from Cython is no
> problem but calling doSomething from the python interpreter causes a
> segfault because self.myData then points to NULL. I've also observed
> that self in setData points to a different address than self in
> doSomething if it is called from the interpreter.
>
> What's the problem with this approach?


Where are you setting your myData? If you do

     Foo().doSomething()

then it will segfault because the data isn't set yet. If you have

     def set_data_from_python(self, py_string):
         self.myData = py_string

Then the conversion will happen, but myData is set to point to the  
inside of py_string (i.e. no copying is actually done) and so the  
instant py_string gets deallocated the actual char* gets reclaimed.

Hopefully this helps, though the code snippet above doesn't crash by  
itself.

- Robert
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