CODEWITHSUNDEEP
pythonctypes
harry
harry

Reputation: 1095

Read void pointer inside a struct in python c_types

I've a struct with void pointer member and trying to print it's value. I'm getting the type as int. How do i retrieve it values.

Below is my python code, How do i read p values.

class MyStruct(Structure):
    _fields_ = [
        ("p", c_void_p)
    ]


def test():
    t = MyStruct()
    val = [1, 2, 3]
    v = (c_int * len(val) )(*val)
    t.p = cast(v, c_void_p)
    print(type(t.p))


if __name__ == '__main__':
    test()

Output:

<class 'int'>

Upvotes: 2

Views: 519

Answers (1)

user2357112
user2357112

Reputation: 281262

For some reason, whoever designed ctypes decided it would be a good idea to transparently convert c_void_p values to Python ints when you retrieve struct members or array elements, or receive c_void_p values from foreign function calls. You can reverse the transformation by constructing a c_void_p again:

pointer_as_c_void_p = c_void_p(t.p)

If you want to convert to pointer-to-int, use cast:

pointer_as_int_pointer = cast(t.p, POINTER(c_int))

You can then index the pointer to retrieve values, or slice it to get a list:

print(pointer_as_int_pointer[1]) # prints 2
print(pointer_as_int_pointer[:3]) # prints [1, 2, 3]

Upvotes: 2

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