Ctypes DLL call ArgumentError with c_char_Array

Question

i'm trying to write a Python DLL Wrapper for a C Project

• https://github.com/OpenEtherCATsociety/SOEM -> C Project

• The Python DLL Wrapper can be found here: https://github.com/GitHubStefan13/SOEM-for-Python

Original C Code that is important:

1. char IOmap[4096];
2. int ec_config_overlap(uint8 usetable, void *pIOmap);

Python Wrapper

1. IOMap = ctypes.POINTER(ctypes.c_char * 4096)
2. c_ec_config_overlap = ethercat.ec_config_overlap c_ec_config_overlap.argtypes = [ctypes.c_unit8, IOMap] c_ec_config_overlap.restype = ctypes.c_int

When im trying to define a function in Python

def ec_config_overlap(usetable, PIOMap): return c_ec_config_overlap(usetable, PIOMap

and call it. I receive the error

ctypes.ArgumentError: argument 2: : expected LPc_char_Array_4096 instance instead of _ctypes.PyPointerType.

I understand the Error but how do i go around to actually make the ctype a Array[4096] instead of the PyPointerType?

Answer 1

This syntax creates array instances:

>>> import ctypes
>>> (ctypes.c_char*4096)()
<__main__.c_char_Array_4096 object at 0x0000024D84E2D7C8>

Since it is a char array, you can also use:

>>> create_string_buffer(4096)
<__main__.c_char_Array_4096 object at 0x0000025AE48FE948>

The type of your function should be:

c_ec_config_overlap.argtypes = [ctypes.c_uint8, ctypes.c_void_p]

But for better type checking you can also use:

c_ec_config_overlap.argtypes = [ctypes.c_uint8, ctypes.c_char_p]