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document that a new Python thread context is created in ctypes callbacks (closes #6627) 

Patch by Nikolaus Rath.
This commit is contained in:
Benjamin Peterson 2014-01-20 00:09:53 -05:00
parent e83ed43281
commit f6ffb4b264
1 changed files with 7 additions and 1 deletions
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Doc/library/ctypes.rst
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@ -1022,12 +1022,18 @@ As we can easily check, our array is sorted now::
1 5 7 33 99
>>>
**Important note for callback functions:**
**Important notes for callback functions:**
Make sure you keep references to :func:`CFUNCTYPE` objects as long as they are
used from C code. :mod:`ctypes` doesn't, and if you don't, they may be garbage
collected, crashing your program when a callback is made.
Also, note that if the callback function is called in a thread created outside
of Python's control (e.g. by the foreign code that calls the callback), ctypes
creates a new dummy Python thread on every invocation. This behavior is correct
for most purposes, but it means that values stored with `threading.local` will
*not* survive across different callbacks, even when those calls are made from
the same C thread.
.. _ctypes-accessing-values-exported-from-dlls: