Python PyGILState_{Ensure/Release} causes segfault while returning to C++ from Python code

Question

UPDATE Well, it looks like adding PyEval_InitThreads() before the call to PyGILState_Ensure() does the trick. In my haste to figure things out I incorrectly

Answer 1

I ran into this exact problem as well. The documentation for anything relating to threads in CPython is unfortunately patchy at best.

Essentially, you need to do the following:

In your main thread, BEFORE any other threads are spawned, you need to call PyEval_InitThreads(). A good place to do this is right after you call PyInitialize().

Now, PyEval_InitThreads() not only initializes the Python interpreter thread-state, it also implicitly acquires the Global Interpreter Lock. This means, you need to release the lock before you call PyGILEnsure_State() in some other thread, otherwise your program will hang. You can do this with the function PyEval_ReleaseLock().

So basically, in your main thread, before any other threads are launched, you want to say:

PyInitialize();
PyEval_InitThreads();
PyEval_ReleaseLock();

Then, in any additional thread, anytime you use the Python API you need to say:

PyGILState_STATE gstate;
gstate = PyGILState_Ensure();

/* ... some code that does things with Python ... */

PyGILState_Release(gstate);