Question

This is inconsistent:

from ctypes import *

class S(Structure):
    _fields_ = [("x", POINTER(c_int)), ("y", c_int)]

o = S()
print o.x
print o.y

which returns

<__main__.LP_c_int object at 0x10d3d08c0>
0

So in one case, it returns a ctypes type, in the other case, it returns directly the value.

I have some more generic code where I always need to pass a ctypes type instance (which is also writeable, i.e. writing to it would mean to modify o in the above example). For o.x, this is ok. But not for o.y.

How can I get the c_int instance of o.y?

Answer 1

From the _ctypes source, it seems it doesn’t do this clever automatic conversion for simple ctypes instances when they are wrapped.

So one (ugly) solution is this:

from ctypes import *

def WrapClass(c):
    class C(c): pass
    C.__name__ = "wrapped_" + c.__name__
    return C

class S(Structure):
    _fields_ = [("x", POINTER(c_int)), ("y", WrapClass(c_int))]

o = S()
print o.x
print o.y
print o.y.value