[ python-Bugs-1574588 ] ctypes: Pointer-to-pointer unchanged in callback

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Bugs item #1574588, was opened at 2006-10-10 17:16
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Category: Python Library
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Albert Strasheim (albertstrasheim)
Assigned to: Nobody/Anonymous (nobody)
Summary: ctypes: Pointer-to-pointer unchanged in callback

Initial Comment:
This problem is from another post I made to
ctypes-users that didn't show up in the ctypes-users
archive.

C function:

extern CALLBACK_API void foo(void(*callback)(void**)) {
    void* p = 123;
    printf("foo calling callback\n");
    callback(&p);
    printf("callback returned in foo\n");
    printf("p = 0x%p\n", p);
}

I figured that while I try to find out why returning
c_void_p from a callback gives an error, I might as
well return the address via a pointer to a pointer.

In the Python code I have:

import sys
print sys.version
from ctypes import *
x_t = c_int*10
x = x_t()
def callback(ptr):
    print x
    print ptr
    ptr.contents = cast(addressof(x), c_void_p)
    print ptr.contents
#lib = cdll['libcallback.so']
lib = cdll['callback.dll']
lib.foo.argtypes = [CFUNCTYPE(None, POINTER(c_void_p))]
lib.foo(lib.foo.argtypes[0](callback))

Output when I running this script under Python 2.4.3
with ctypes 1.0.0 (I get identical results with Python
2.5 and ctypes 1.0.1):

2.4.3 (#69, Mar 29 2006, 17:35:34) [MSC v.1310 32 bit
(Intel)]
foo calling callback
<__main__.c_long_Array_10 object at 0x00963E90>
<ctypes.LP_c_void_p object at 0x00963EE0>
c_void_p(10048496)
callback returned in foo
p = 0x0000007B

For some reason, the value I assigned to ptr.contents
isn't present when we return to the C code.

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