P_get for the 'P' pointer type uses PyLong_FromVoidPtr. If the address fits in a platform long, it returns a Python int; otherwise it returns a Python long, which has variable precision. That's fine, but when passing this integer value as an argument, the default behavior is to convert to a C int, which is 32-bit on all supported platforms.

I think the best solution is to set argtypes to properly convert an argument to a pointer type. Another option is to set restype to a subclass of c_void_p. Using a subclass disables the conversion to a Python integer. GetResult checks this by calling _ctypes_simple_instance, which actually returns the opposite of what its name and the source comment suggest. (In 2.5 this function was named IsSimpleSubType, and the source comment was wrong back then too. The "simple" in question was never the metaclass PyCSimpleType, but the base type _SimpleCData.)

POSIX:

# Configure the interpreter to load visible extension-
# module symbols, such as _ctypes_simple_instance, 
# into the global symbol table.
import sys, DLFCN
sys.setdlopenflags((sys.getdlopenflags() & ~DLFCN.RTLD_LOCAL) |
                   DLFCN.RTLD_GLOBAL)
from ctypes import *

_ctypes_simple_instance = PyDLL(None)._ctypes_simple_instance
_ctypes_simple_instance.argtypes = py_object,
malloc = CDLL(None).malloc

class my_void_p(c_void_p): 
    pass

>>> _ctypes_simple_instance(c_void_p)
0
>>> _ctypes_simple_instance(my_void_p)
1

>>> malloc.restype = c_void_p
>>> type(malloc(100))
<type 'int'>
>>> malloc.restype = my_void_p
>>> type(malloc(100))
<class '__main__.my_void_p'>

Windows:

_ctypes_simple_instance isn't exported by _ctypes.pyd.

from ctypes import *

malloc = cdll.msvcrt.malloc

class my_void_p(c_void_p): 
    pass

>>> malloc.restype = c_void_p
>>> type(malloc(100))          
<class 'int'>
>>> malloc.restype = my_void_p
>>> type(malloc(100))         
<class '__main__.my_void_p'>