[issue18017] ctypes.PyDLL documentation
New submission from Marc Brünink:
The documentation for is not very clear regarding the usage of CDLL and PyDLL. Especially it is not obvious that you should use PyDLL whenever you call any function of the Python/C API. Since calling a Python/C API function without owning the GIL will most likely cause latent segfaults, I think it warrants a warning box in section 16.17.2.2 and maybe also somewhere prominent in http://docs.python.org/dev/c-api/intro.html.
For section 16.17.2.2 I would put a box next the decription of CDLL saying something like: "The Python GIL is released before a function call. It is not safe to call any Pyhon/C API function within the loaded library without acquiring the GIL first. Thus, if the loaded library calls functions of the Python/C API, for example in case it creates and returns a new Python object, and does not manually acquire and release the GIL, use PyDLL instead."
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assignee: docs@python
components: Documentation
messages: 189629
nosy: Marc.Brünink, docs@python
priority: normal
severity: normal
status: open
title: ctypes.PyDLL documentation
versions: Python 3.5
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Python tracker
Mark Lawrence added the comment:
@Marc can you prepare a patch for this issue?
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nosy: +BreamoreBoy
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Python tracker
eryksun added the comment:
16.17.2.2 already has a warning after it introduces CDLL, OleDLL, and WinDLL:
The Python global interpreter lock is released before
calling any function exported by these libraries, and
reacquired afterwards.
It links to the glossary entry:
https://docs.python.org/3/glossary.html#term-global-interpreter-lock
It wouldn't hurt to elaborate and box this warning.
Subsequently, PyDLL is documented to *not* release the GIL. Also, section 16.17.2.4 documents that CFUNCTYPE and WINFUNCTYPE functions release the GIL and PYFUNCTYPE functions will *not* release the GIL.
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nosy: +eryksun
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Python tracker
marc added the comment:
I think the problem was a combination of two issues.
First, the warning in 16.17.2.2 is a bit hidden. Kind of squeezed in
between CDLL, etc and PyDLL. In contrast, 16.17.2.4 does a much better
job, because it restates the fact that the GIL is released at each and
every item. I think this is much better, because chances are high that
you actually read the entry completely. In contrast, in case you
are in a hurry you might miss the warning in 16.17.2.2
I think the second problem was that back then I was not aware of the
fact that you need the GIL to call the Python/C API. That was a bit
stupid indeed. But as a Python beginner it was just not obvious. Maybe
the Introduction of the Python/C API needs some work, too. No GIL
mentioned at https://docs.python.org/3/c-api/intro.html.
Furthermore, the examples at
https://docs.python.org/3/extending/extending.html also do not talk
about the GIL.
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nosy: +marc@bruenink.de
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Python tracker
Change by Mark Lawrence
Eryk Sun
participants (5)
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Eryk Sun -
eryksun
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marc
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Marc Brünink
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Mark Lawrence