Right way to initialize python embedded in a multi-threaded application

[francis.brosnan at gmail.com]

Hi,

Maybe you already fixed the issue, but for the record, I've got the same
problem and finally it turned out that I was calling PyEval_InitThreads twice
and also after fixing that, I also had to move the call to PyEval_ReleaseLock();  at the end of the entire initialization (not just after PyEval_initThreads).

The key thing there is to follow:

<<at initialization thread>>
Py_Initialize ();
PyEval_InitThreads(); 
/* now call here to initialize all python code by loading external files 
   or internal module loading (i.e. Py_InitModule3) */
/* ..and now, once no more Python C/API call is required, release
   the GIL so other threads can come into play */
PyEval_ReleaseLock ();

<<and now, from other threads, use>>
/* wait til gil acquired */
state  = PyGILState_Ensure();
/* your code */
/* release GIL */
PyGILState_Release (state);

Hope it helps, Cheers!


> I'm embedding python in a multi-threaded C application.
> 
> I've taken care to wrap every call to the Python C API with
> 
> 
> 
> gstate = PyGILState_Ensure();
> 
> // call python code
> 
> PyGILState_Release(gstate);
> 
> 
> 
> But I'm stumped with what to do in the initialization.
> 
> Right after the call to Py_IsInitialized() I've added a call:
> 
> 
> 
> PyEval_InitThreads();
> 
> 
> 
> The docs say that this function leaves the GIL locked when it returns.
> 
> I do some more initializations like importing modules and then I call
> 
> 
> 
> PyEval_ReleaseLock();
> 
> 
> 
> This seems to cause a problem since not long after a call to
> 
> PyGILState_Release(gstate) that's made in a different thread crashes.
> 
> with
> 
> 
> 
> "Fatal Python error: This thread state must be current when releasing"
> 
> 
> 
> If I don't do the call to PyEval_ReleaseLock() in the main thread
> 
> right after initialization, the GIL seems to be released
> 
> after the first PyGILState_Ensure() - PyGILState_Release() pair.
> 
> 
> 
> So what am I doing wrong here?
> 
> What is the correct way of initializing a multi-threaded application?