[Python-3000] threading, part 2 --- + a bit of ctypes FFI worry (original) (raw)

Tim Peters tim.peters at gmail.com
Sat Aug 12 12:29:07 CEST 2006

[Josiah Carlson]

... Python 2.3.5 (#62, Feb 8 2005, 16:23:02) [MSC v.1200 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> import ctypes >>> import threading >>> import time >>> def foo(): ... try: ... while 1: ... time.sleep(.01) ... finally: ... print "I quit!" ... >>> x = threading.Thread(target=foo) >>> x.start() >>> for i,j in threading.active.items(): ... if j is x: ... break ... >>> ctypes.pythonapi.PyThreadStateSetAsyncExc(i, ctypes.pyobject(Exception))

As I discovered to my chagrin when I added a similar test to the test suite a few days ago, that's got a subtle error on most 64-bit boxes. When the ctypes docs talk about passing and returning integers, they never explain what "integers" /means/, but it seems the docs implicitly have a 32-bit-only view of the world here. In reality "integer" seems to mean the native C int type. But a Python thread id is a native C long (== a Python short integer), and the code above fails in a baffling way on most 64-bit boxes: the call returns 0 instead; i.e. the thread id isn't found, and no exception gets set. So I believe that needs to be:

ctypes.pythonapi.PyThreadState_SetAsyncExc(
    ctypes.c_long(i),
    ctypes.py_object(Exception))

to make it portable.

It's unclear to me how to write portable ctypes code in the presence of a gazillion integer typedefs and #defines, such as for Py_ssize_t. That doesn't map to a fixed C integral type cross-platform, so what can you do? You're not required to answer that ;-)

Thread ids may bite us someday too. Python casts the platform's notion of a thread id to C long, but there's no guarantee this won't lose information (or is even legal) on all platforms. We'd probably be safer casting to, e.g., Py_uintptr_t (some thread implementions return an index into a kernel or library thread-info table, but at least some in my lifetime returned a pointer to a thread-info struct, and that's definitely fatter than C long on some boxes).

1 >>> I quit! Exception in thread Thread-2:Traceback (most recent call last): _File "C:\python23\lib\threading.py", line 442, in bootstrap self.run() File "C:\python23\lib\threading.py", line 422, in run _self._target(*self._args, **self.kwargs) File "", line 4, in foo Exception

It's really cool that you can do this from ctypes, eh? That's exactly the right level of abstraction for this attractive nuisance too ;-)

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