How do I pass a class object to a ctypes.windll.LoadLibrary(DLL) function?

Question

I’m working with Python and trying to talk to this simple USB device using WinUSB (from Microsoft). I’m having a problem since in order to find the device, calling the setupAPI function SetupDiEnumDeviceInterfaces takes a struct object, which I defined as a class:

class _SP_DEVINFO_DATA:
    def __init__(self, ClassGUID, DevInst = ""):        
        '''flags = SPINT_DEFAULT, SPINT_REMOVED, or SPINT_ACTIVE'''
        self._ClassGUID = None
        self._DevInst = None
        self._Reserved = None
        self._cbSize = None

    ###Getters:
    def getClassGUID(self):
        return self._ClassGUID
    def getDevInst(self):
        return self._DevInst
    def getReserved(self):
        return self._Reserved
    def getcbSize(self):
        return self._cbSize

    ###Setters:
    def setClassGUID(self, value):
        self._ClassGUID = value
    def setDevInst(self, value):
        self._DevInst = value
    def setReserved(self, value):
        self._Reserved = value
    def setcbSize(self):
        self._cbSize = sys.getsizeinfo(self)       

    ClassGUID = property(getClassGUID, setClassGUID, None, "Class GUID")
    DevInst = property(getDevInst, setDevInst, None, "Device Instance")
    Reserved = property(getReserved, setReserved, None, "RESERVED: DO NOT USE")
    cbSize = property(getcbSize, setcbSize, None, "CB Size. Set automatically")

I tried using the property because it gave me the error :

<type 'exceptions.TypeError'>: Don't know how to convert parameter

otherwise, and I’d read that defining parameters like this would resolve the problem, but it doesn’t, and I’m not certain what to do here.

I want to use WinUSB because I only need to read from the device and write to the device, and that’s it, and WinUSB seems to have what I need, but until I can get past this problem, I’m kind of stuck

Any suggestions? How do I pass a class object to a DLL function loaded with ctypes.windll.LoadLibrary(DLL)?

And if there’s an easier way to do this, I’m all for that too.

Thanks.

Asked By: Will

||

Source

Answer 1

Your class should be derived from ctypes.Structure.

import ctypes

class _SP_DEVINFO_DATA(ctypes.Structure):

    _fields_ = [("cbSize", ctypes.c_ulong),
                ("ClassGuid", ctypes.c_ubyte * 12),
                ("DevInst", ctypes.c_ulong),
                ("Reserved", ctypes.c_voidp)]

    def __init__(self, guid, inst):
        self.cbSize = 24
        # GUID doesn't work like this... Needs proper conversion
        #self.ClassGuid = (ctypes.c_ubyte * 12)(bytearray(guid))
        self.DevInst = (ctypes.c_ulong)(inst)
        self.Reserved = None

Answered By: Roland Smith

Answer 2

As @Roland said, you must derive from ctypes.Structure. Here’s a working version:

import ctypes
from ctypes import wintypes
import uuid

class _SP_DEVINFO_DATA(ctypes.Structure):
    _fields_ = [("cbSize", wintypes.DWORD),
                ("ClassGuid", ctypes.c_char * 16),
                ("DevInst", wintypes.DWORD),
                ("Reserved", wintypes.LPVOID)]

    def __init__(self, guid, inst):
        self.cbSize = ctypes.sizeof(_SP_DEVINFO_DATA)
        self.ClassGuid = uuid.UUID(guid).get_bytes()
        self.DevInst = (ctypes.c_ulong)(inst)
        self.Reserved = None

    def __repr__(self):
        return "_SP_DEV_INFO_DATA(cbsize={},ClassGuid={},DevInst={})".format(
            self.cbSize,uuid.UUID(bytes=self.ClassGuid),hex(self.DevInst))

sp = _SP_DEVINFO_DATA('08751880-13bb-11e2-96f0-402cf4ca5e51',0x12345678)
print sp

Output:

_SP_DEV_INFO_DATA(cbsize=28,ClassGuid=08751880-13bb-11e2-96f0-402cf4ca5e51,DevInst=0x12345678L)

Answered By: Mark Tolonen

How do I pass a class object to a ctypes.windll.LoadLibrary(DLL) function?

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