Embedding Python in a C extension

[Carl Banks]

On Jun 2, 1:46 pm, Paul Grunau wrote:
> I have a problem with embedding Python into a C extension in Windows
> Vista. I have implemented a timer routine in C as an extension, which
> I can import into Python 2.6.5 and run. Each timer interval, the
> extension calls a C CALLBACK function. I want to be able to have this
> CALLBACK function call a Python function, so that I can write the
> timer handler in Python.
>
> I can write Python functions and call them from a C extension function
> with no trouble in all cases EXCEPT when the function that calls the
> Python code is the timer's CALLBACK function. That is, if I call
> PyRun_SimpleString or PyRun_SimpleFile, or PyImport_ImportModule, or
> basically any Python-y function in the CALLBACK function, Python
> crashes and I get a window saying "Python has stopped working. Windows
> is searching for a solution to the problem."
>
> The Python code runs fine if I call it from a function in the
> extension that is not the CALLBACK function. And regular C code runs
> in the CALLBACK function properly. It only crashes if I try to run the
> Python code from the timer CALLBACK function (or any C function I call
> from the CALLBACK function).
>
> Does anybody know how to fix this? Is there a workaround of some sort?

See PEP 311.  When an external/uncertain thread calls into Python, it
has to be sure a Python thread state exists for that thread, and that
the thread holds the global interpreter lock.  This is done by
surrounding all Python code with the following calls:

PyGILState_STATE state = PyGILState_Ensure();

and

PyGILState_Release(state);

Normally, code in extension modules can rely on the current thread
having the global lock at all entry points, so it doesn't have worry
about the thread state.  But a callback from a timer can't assume
that.


Carl Banks