I have been using a custom Qt4 reactor that derives from PosixReactorBase. As a result it creates a _Win32Waker to allow threads and signals to wake up the IO thread. It seems though that the current implementation only works about half of the time. The other half it exists with : File "...\Lib\python2.6\site-packages\twisted\internet\posixbase.py", line 170, in __init__ ReactorBase.__init__(self) File "...\Lib\python2.6\site-packages\twisted\internet\base.py", line 424, in __init__ self._initThreads() File "...\Lib\python2.6\site-packages\twisted\internet\base.py", line 813, in _initThreads self.installWaker() File "...\Lib\python2.6\site-packages\twisted\internet\posixbase.py", line 206, in installWaker self.waker = _Waker(self) File "...\Lib\python2.6\site-packages\twisted\internet\posixbase.py", line 77, in __init__ client.connect(server.getsockname()) File "<string>", line 1, in connect socket.error: [Errno 10049] The requested address is not valid in its context I have attached a simple test that shows that the following code does not always return "127.0.0.1", but sometimes returns "0.0.0.0" as the IP address. # Following select_trigger (from asyncore)'s example; server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) client = socket.socket(socket.AF_INET, socket.SOCK_STREAM) client.setsockopt(socket.IPPROTO_TCP, 1, 1) server.bind(('127.0.0.1', 0)) server.listen(1) client.connect(server.getsockname()) My current workaround just calls the following instead: client.connect(('127.0.0.1', server.getsockname()[1])) Any ideas on what is really causing the error? If there is not a better solution can this be added to trunk for future releases? Thanks, Aron