Question

I am trying to write a wrapper to a native library in Linux. Problem is this:

definition in c:

int mymethod(mystruct* ptr)

in python:

_lib.mymethod.argtypes = (ctypes.POINTER(mystruct),)                                                     
_lib.mymethod.restype = ctypes.c_int
s = mystruct()

_lib.mymethod(ctypes.byref(s))
# raises: expected LP_mystruct instance instead of pointer to mystruct

_lib.mymethod(ctypes.pointer(s))
# raises expected LP_mystruct instance instead of LP_mystruct

errors. How to pass a structure as a pointer to a native method ?

Thanks.

Mete

Answer 1

The problem is that the higher level “POINTER” from ctypes is, in Python, a different object than “a generic pointer” (ctypes.CArgObject by ctypes.byref)which is returned or a single number representing a memory address (which is what is returned by ctype’s adrresof) – you can either annotate your function to receive a `ctypes.c_voidp and call it with _lib.mymethod(ctypes.addressof(a)) instead –

Or if you want to work on the stronged-typed side to avoid errors that would crash Python (a type error raises a Python exception instead – a wrong parameter passed to a C unction would cause a segmentation fault on the Python interpreter itself), you have to create a variable to hold the new “type” which is a POINTER to your structure – and then create an instance of this type with the address of your structure:

mystruct_pointer = ctypes.POINTER(mystruct)
_lib.mymethod.argtypes = (mystruct_pointer,)
_lib.mymethod.restype = ctypes.c_int

s = mystruct()

_lib.mymethod(mystruct_pointer.from_address(ctypes.addressof(s)))