Question

Foreword: I’m pretty familiar with Python but had never touched C until a week ago… now I’m trying to speak to a motor controller using ctypes.

So i’ve been playing around with this particular function (VCS_GetProtocolStackSettings), and was unable to get it to work until just recently. My problem is that, although it’s returning a ‘1’ (ie is sucessful per the spec below), I seem unable to access the return parameters.

Here is the my code. You can see that I am storing the return parameters in pointers to uint32 objects, I just have no idea how to get at them.

lib=ctypes.WinDLL('C:\\Program Files (x86)\\maxon motor ag\\EPOS Positioning Controller\\EPOS2\\04 Programming\\Windows DLL\\EposCmd64.dll')

typeDict={  'char': ctypes.c_char,
        'char*': ctypes.c_char_p,
        '__int8': ctypes.c_int8,
        'BYTE': ctypes.c_uint8,
        'short': ctypes.c_int16,
        'WORD': ctypes.c_uint16,
        'long': ctypes.c_int32,
        'DWORD': ctypes.c_uint32,
        'BOOL': ctypes.c_int32,
        'HANDLE': ctypes.POINTER(ctypes.c_uint32)
        }

def VCS_GetProtocolStackSettings(KeyHandle):
    '''Returns the communication parameters 'baudrate and 
        'timeout'
    '''

    func=lib['VCS_GetProtocolStackSettings']
    func.argtypes
    func.restype=typeDict['BOOL']   

    pBaudrate=ctypes.pointer(typeDict['DWORD']())
    pTimeout=ctypes.pointer(typeDict['DWORD']())
    pErrorCode=ctypes.pointer(typeDict['DWORD']())

    cKeyHandle=typeDict['HANDLE'](KeyHandle)

    return func(KeyHandle,pBaudrate,pTimeout,pErrorCode)

Answer 1

pBaudrate.contents.value will access the value in the pointer, but you only really need a c_uint() and pass it by reference to the function. Below is an example. It is also an example of making a function more “Pythonic” by throwing an exception on failure.

import ctypes
from ctypes import wintypes

lib = ctypes.WinDLL(dllnamehere)
lib.VCS_GetProtocolStackSettings.argtypes = [wintypes.HANDLE,wintypes.PDWORD,wintypes.PDWORD,wintypes.PDWORD]
lib.VCS_GetProtocolStackSettings.restype = wintypes.BOOL

def VCS_GetProtocolStackSettings(KeyHandle):
    baudrate = wintypes.DWORD()
    timeout = wintypes.DWORD()
    errorcode = wintypes.DWORD()
    result = lib.VCS_GetProtocolStackSettings(KeyHandle,ctypes.byref(baudrate),ctypes.byref(timeout),ctypes.byref(errorcode))
    if not result:
        raise RuntimeError('error code = {}'.format(errorcode.value))
    return baudrate.value,timeout.value

P.S. Double check that WinDLL is appropriate. Windows system DLLs use the __stdcall calling convention, but many DLLs use __cdecl calling convention and you would use CDLL instead.