static PyObject *
thread_get_ident(PyObject *self, PyObject *Py_UNUSED(ignored))
{
    PyThread_ident_t ident = PyThread_get_thread_ident_ex();  // <-- ULL
    if (ident == PYTHREAD_INVALID_THREAD_ID) {
        PyErr_SetString(ThreadError, "no current thread ident");
        return NULL;
    }
    return PyLong_FromUnsignedLongLong(ident);
}

class _MainThread(Thread):
    def __init__(self):
        Thread.__init__(self, name="MainThread", daemon=False)
        self._started.set()
        self._ident = _get_main_thread_ident()
        self._handle = _make_thread_handle(self._ident)
        if _HAVE_THREAD_NATIVE_ID:
            self._set_native_id()
        with _active_limbo_lock:
            _active[self._ident] = self

static PyObject *
thread__get_main_thread_ident(PyObject *module, PyObject *Py_UNUSED(ignored))
{
    return PyLong_FromUnsignedLongLong(_PyRuntime.main_thread);
}

    /* Platform-specific identifier and PyThreadState, respectively, for the
       main thread in the main interpreter. */
    unsigned long main_thread;

    // Set it to the ID of the main thread of the main interpreter.
    runtime->main_thread = PyThread_get_thread_ident();

>>> import threading
>>> ct = threading.current_thread()
>>> ct
<_DummyThread(Dummy-1, started daemon 18446744072483979068)>
>>> hex(ct.ident)
'0xffffffffb6f33f3c'
>>> main = threading.main_thread()
>>> hex(main.ident)
'0xb6f33f3c'
>>> main._set_ident()
>>> hex(main.ident)
'0xffffffffb6f33f3c'

def current_thread():
    """Return the current Thread object, corresponding to the caller's thread of control.
    If the caller's thread of control was not created through the threading
    module, a dummy thread object with limited functionality is returned.
    """
    try:
        return _active[get_ident()]
    except KeyError:
        return _DummyThread()