Bugs item #1574593, was opened at 2006-10-10 17:18
Message generated for change (Comment added) made by theller
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Category: Python Library
Group: Python 2.5
>Status: Closed
>Resolution: Works For Me
Priority: 5
Private: No
Submitted By: Albert Strasheim (albertstrasheim)
Assigned to: Thomas Heller (theller)
Summary: ctypes: Returning c_void_p from callback doesn\'t work
Initial Comment:
C code:
extern CALLBACK_API void* foo(void*(*callback)()) {
printf("foo calling callback\n");
callback();
printf("callback returned in foo\n");
}
callback.py contains:
import sys
print sys.version
from ctypes import *
def callback(*args):
return c_void_p()
#lib = cdll['libcallback.so']
lib = cdll['callback.dll']
lib.foo.argtypes = [CFUNCTYPE(c_void_p)]
lib.foo(lib.foo.argtypes[0](callback))
With Python 2.4.3 and ctypes 1.0.0 + Thomas Heller's
patch for another issue (which doesn't seem to affect
this situation, but anyway) I get the following error:
2.4.3 (#69, Mar 29 2006, 17:35:34) [MSC v.1310 32 bit
(Intel)]
foo calling callback
Traceback (most recent call last):
File "source/callbacks.c", line 216, in 'converting
callback result'
TypeError: cannot be converted to pointer
Exception in None ignored
callback returned in foo
With Python 2.5 and ctypes 1.0.1 I get:
2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32
bit (Intel)]
foo calling callback
Traceback (most recent call last):
File "\loewis\25\python\Modules\_ctypes\callbacks.c",
line 216, in
'converting callback result'
TypeError: cannot be converted to pointer
Exception in <function callback at 0x009C3CF0> ignored
callback returned in foo
Returning a Python integer from callback() doesn't
cause an error to be raised.
----------------------------------------------------------------------
>Comment By: Thomas Heller (theller)
Date: 2007-01-09 21:50
Message:
Logged In: YES
user_id=11105
Originator: NO
Sorry for the late reply, and I hope you are still interested in this.
Basically, when you return something from the callback, ctypes does the
same as if you would do this "c_void_p(something)". Now, you cannot
construct a c_void_p instance from a c_void_p instance, you get exactly
the same error as you mention above:
>>> c_void_p(c_void_p(42))
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: cannot be converted to pointer
>>>
I'm not sure if this should be considered a bug or not, anyway there is an
easy workaround: Return an integer from the callback, or, if you have a
c_void_p instance, the .value attribute thereof.
I think this should not be fixed.
----------------------------------------------------------------------
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