Python calling DLL calling Python, "WindowsError: exception: access violation reading 0x00000004"

Question

I have an application written in Python.

The application calls some functions in the dll (using ctypes) that calls some functions from the python C API to load and run some functions in a (different) python module. This causes WindowsError: exception: access violation reading 0x00000004 Some cout debugging tells me that the access violation happens on a call to the Python C API.

I know the DLL successfully loads the python module, running to completion when I test it as a stand alone .exe but when it is run from the python application it gives this access violation error.

I first though that maybe I should use Py_NewInterpreter in the DLL to create a new separate space for these python module functions to run. This would seem convienent as there is no need for the application and the modules runs by the DLL to share any data. http://docs.python.org/2/c-api/init.html#Py_NewInterpreter

However my initial tests with Py_NewInterpreter have proved unsuccessful simply making further errors and crashing.

So my question is how should I implement this/what do I need to do make the DLL load of the python modules work?

Ulrich Eckhardt · Accepted Answer · 2013-01-25 19:36:35Z

1

This looks like dereferencing a null pointer, i.e. a pointer to a structure/class is null and you are reading a subobject at offset 4. Sprinkle a few assert()s throughout your code to make sure you don't miss any null pointer. Note also that the Python C API regularly returns null in case of an exception.

That said, you have a Python program importing a DLL via ctypes. This DLL in turn embeds a Python interpreter via Python's C API. Right? If I'm not mistaken, that effectively gives you two interpreter instances in one process. The problem there is that any global objects are shared between the two and that this might not be coordinated.

What I would try is to write a proper Python (wrapper) module instead of using ctypes to import the DLL. However, there's another thing that I would do in parallel and that is to ask on the Python user mailinglist, whether this setup can work or not and why. There is also a news gateway in case you prefer that.

answered Jan 25, 2013 at 19:36

Ulrich Eckhardt

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ceorron Over a year ago

I finally resolved this today, yes you cannot reliably "double load" python. Instead of doing as you suggested and adding "a proper python wrapper module" I resolved to build another application .exe that the dll now calls using an ipc layer, this integrates python, this way I can have two python instances.

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