document that a new Python thread context is created in ctypes callbacks (closes...

Patch by Nikolaus Rath.

diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
index 2c36e64b98b1945e8352a871ce30faa8efbf6862..2f9727679b58d7878cc233ee25878fda892ee89c 100644 (file)
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -1080,12 +1080,18 @@ 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 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: