From f6ffb4b264ae2ac3e9086abb8c825b195b207bff Mon Sep 17 00:00:00 2001 From: Benjamin Peterson Date: Mon, 20 Jan 2014 00:09:53 -0500 Subject: [PATCH] document that a new Python thread context is created in ctypes callbacks (closes #6627) Patch by Nikolaus Rath. --- Doc/library/ctypes.rst | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst index ae10b71d4a..f001165320 100644 --- a/Doc/library/ctypes.rst +++ b/Doc/library/ctypes.rst @@ -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: -- 2.40.0