Threading a lengthy C function

Thomas Heller theller at
Thu Nov 20 20:02:44 CET 2003


>>If the original posters intent is simply to call functions in a dll, he
>>should probably try out ctypes - ctypes handles the GIL automatically
>>(release before calling the function, acquire it back after return of
>>the function, and even grabbing the GIL if a callback into Python is
>>done).  And it avoids SWIG completely.


> I'm not sure I understand how the last part is done unless all callback
> functions are known to ctypes and even then I'm not sure how ctypes
> would know how to associate a particular call back event with the owner
> thread. Are ctypes callbacks somehow defined uniquely for each callout?

No.  In Python 2.2, ctypes creates a new threadtstate for each callback
into Python.  In 2.3, the mechanism from PEP 311 is used, which
magically gets the threadstate.

> The other thing I never quite get is what part of the C api I'm allowed
> to use without having the GIL I think it safest to assume I may not call
> any python api without having it.

*Very* few functions, it's mentioned in the docs.

> Wrapping completely independent DLLs seems reasonable, but how does
> ctypes know that a particular extension/dll actually assumes that it has
> the GIL at entry which is mostly what extensions need.

ctypes isn't (or shouldn't be) used to call Python C api functions, only
independent DLLs.  (Hm, didn't I post sick examples calling Python's C
api in the past myself <wink>).


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