ctypes on Windows on a 64bit Python installation gets a NULL pointer access where one is not expected.

To reproduce the problem, run make.bat then "python ctypes_test.py"


Failure output looks like this:

Three! 1 2 <__main__.LP_Some object at 0x000000000209C4C8>
<__main__.Some object at 0x000000000209C548>
91
Four! 1 2 3 <__main__.LP_Some object at 0x000000000209C4C8>
Traceback (most recent call last):
  File "_ctypes/callbacks.c", line 291, in 'calling callback function'
  File "ctypes_test.py", line 21, in fourprinter
    print(to.contents)
ValueError: NULL pointer access


Otherwise, successful output looks something like:

Three! 1 2 <ctypes.LP_Some object at 0x2acc9482aed0>
<__main__.Some object at 0x2acc9482ae50>
91
Four! 1 2 3 <ctypes.LP_Some object at 0x2acc9482aed0>
<__main__.Some object at 0x2acc9482ae50>
91



Environments exhibiting this problem:

Python 2.7 (r27:82525, Jul  4 2010, 07:43:08) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> platform.architecture()
('64bit', 'WindowsPE')

Python 3.1.2 (r312:79149, Mar 20 2010, 22:55:39) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> platform.architecture()
('64bit', 'WindowsPE')


Environments NOT exhibiting the problem:

Python 2.4.3 (#1, Sep  3 2009, 15:37:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> platform.architecture()
('64bit', 'ELF')

Also Windows on a 32bit architecture seems unaffected.

Ok, this issue is worse, and much easier to reproduce, than I thought. Here is an updated ctypes_test.zip with a repro using only integers and callbacks.

Correct output:
('three', (1, 2, 3))
('four', (1, 2, 3, 4))
('five', (1, 2, 3, 4, 5))

Incorrect output:
('three', (1, 2, 3))
('four', (1, 2, 3, 0))
('five', (1, 2, 3, 0, 5))

I built Python 2.7 from source with MSVC 9. This is not reproducible in Debug mode, or Win32 Release mode. It only happens in x64 Release mode. 

One very interesting discovery is that if I set these options:

Optimization: Disabled (/Od)
Enable Instrinsic Functions: No

the error does not occur!

Probably this issue is duplicate of #9884.

The problem was an uninitialized variable. Attached is the patch that fixes it.