Understanding PyEval_InitThreads

[Gernot Hillier]

Hi!

Thomas Heller wrote:
> When porting an extension from Windows to Linux, I got crashes when
> C code calls back into Python.
> 
> Here's what I'm doing:
> 
>     Py_BEGIN_ALLOW_THREAD
>     call_some_c_code()
>     Py_END_ALLOW_THREAD
> 
> The call_some_c_code() function calls back into Python, so I have to
> acquire the interpreter lock again, and create a new threadstate in
> the callback. Note that g_interp is initialized when the extension
> module is loaded:
> 
> static PyInterpreterState *g_interp;
> 
> void call_back_into_python(void)
> {
>     PyThreadState *pts;
> 
>     PyEval_AcquireLock();
>     pts = PyThreadState_New(g_interp);


I can't answer your question exactly - but I personally would suggest to not 
create a new thread state but doing it this way:

    Py_BEGIN_ALLOW_THREAD
    call_some_c_code(_save)
    Py_END_ALLOW_THREAD


If you don't understand this, please note that Py_BEGIN_ALLOW_THREADS 
expands to "{ PyThreadState *_save; _save = PyEval_SaveThread();".

call_some_c_code() has to pass the PyThreadState* to 
call_back_into_python(). There you should do s.th. like this:

void call_back_into_python(PyThreadState *s)
{
    PyEval_PyEval_RestoreThread(s)
    // do your work
    PyEval_SaveThread();
}
    
HTH...

-- 
Ciao,

Gernot