Converting Python instance to ctypes.c_void_p

Question

I'm trying to convert a Python object into C void pointer so that it would be callable from C API's, however the following code doesn't seem to work from ctypes import *

class myclass:
    def __init__(self):
        self.val = 6
        return


foo = myclass()

data = c_void_p(foo)

Output:

Traceback (most recent call last):
  File "test.py", line 26, in <module>
    data = c_void_p(foo)
TypeError: cannot be converted to pointer

I've also tried pointer() and byref(), but none of them works. How can I achieve what I'm trying to do? Thanks in advance.

I think we need to see the use case. Are you wanting to pass the pointer to a 'C' API. Do you have an example of using it? — quamrana
– quamrana, Commented Feb 22, 2018 at 19:28
@quamrana Let's say I have this code in C: void bar(void* obj) { printf("%d\n", (int*)obj) }, then I would need to cast data to c_void_p in order to pass it down to this function. — John Liu
– John Liu, Commented Feb 22, 2018 at 19:34
Perhaps you mean data = c_void_p(foo.val) if the C function itself is going to recover an int. — quamrana
– quamrana, Commented Feb 22, 2018 at 19:38
@quamrana I'm trying to pass in the whole object with multiple attributes, so this method would only work with primitive types. — John Liu
– John Liu, Commented Feb 22, 2018 at 20:10

Michael Butscher · Accepted Answer · 2018-02-22 19:35:20Z

5

Use data = ctypes.py_object(foo).

ctypes.py_object represents the PyObject * type in C.

answered Feb 22, 2018 at 19:35

Michael Butscher

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