Receiving WM_COPYDATA in Python

Question

I am trying to read from Python the WM_COPYDATA message some applications (I'm trying with Spotify) send to WindowsLiveMessenger to update the "What I'm listening to..." phrase.

From what I have been able to find, WM_COPYDATA messages come in a COPYDATASTRUCT with the following structure:

• dwData in our case 0x547 so that it access the listening now feature
• cbData with the length of the string received
• lpData with a pointer to the string itself, may include Unicode characters

The string should have the following format: \0Music\0status\0format\0song\0artist\0album\0 as stated by ListeningNowTracker

What we receive in a WM_COPYDATA event is a pointer for lParam that contains the COPYDATASTRUCT.

I started tinkering with pywin32 functions and I remembered that they do not accept Unicode characters from past experience, then I switched to ctypes. Despite this being an almost new world in Python for me, I tried with POINTER() and all I got was unknown objects for me or access violations.

I think that the code should create a COPYDATASTRUCT:

class CopyDataStruct(Structure):
    _fields_ = [('dwData', c_int),
                ('cbData', c_int),
                ('lpData', c_void_p)]

Then make the lParam be a pointer to that structure, get the string pointer from lpData and finally get the string with ctypes.string_at(lpData,cbData).

Any tips?

UPDATE 1

The WM_COPYDATA event is received by a hidden window built with win32gui just for this purpose. The copydata event is connected to a function called OnCopyData and this is its header:
def OnCopyData(self, hwnd, msg, wparam, lparam):
The values the function delivers are correct as compared with the ones from the Spy++ messages log.

UPDATE 2

This should be close to what I want, but gives a NULL pointer error.

class CopyDataStruct(ctypes.Structure):
    _fields_ = [('dwData', c_int),
                ('cbData', c_int),
                ('lpData', c_wchar_p)]

PCOPYDATASTRUCT = ctypes.POINTER(CopyDataStruct)
pCDS = ctypes.cast(lparam,  PCOPYDATASTRUCT)
print ctypes.wstring_at(pCDS.contents.lpData)

Answer 1

I wrote the following trivial win32gui app:

import win32con, win32api, win32gui, ctypes, ctypes.wintypes

class COPYDATASTRUCT(ctypes.Structure):
    _fields_ = [
        ('dwData', ctypes.wintypes.LPARAM),
        ('cbData', ctypes.wintypes.DWORD),
        ('lpData', ctypes.c_void_p)
    ]
PCOPYDATASTRUCT = ctypes.POINTER(COPYDATASTRUCT)

class Listener:

    def __init__(self):
        message_map = {
            win32con.WM_COPYDATA: self.OnCopyData
        }
        wc = win32gui.WNDCLASS()
        wc.lpfnWndProc = message_map
        wc.lpszClassName = 'MyWindowClass'
        hinst = wc.hInstance = win32api.GetModuleHandle(None)
        classAtom = win32gui.RegisterClass(wc)
        self.hwnd = win32gui.CreateWindow (
            classAtom,
            "win32gui test",
            0,
            0, 
            0,
            win32con.CW_USEDEFAULT, 
            win32con.CW_USEDEFAULT,
            0, 
            0,
            hinst, 
            None
        )
        print self.hwnd

    def OnCopyData(self, hwnd, msg, wparam, lparam):
        print hwnd
        print msg
        print wparam
        print lparam
        pCDS = ctypes.cast(lparam, PCOPYDATASTRUCT)
        print pCDS.contents.dwData
        print pCDS.contents.cbData
        print ctypes.wstring_at(pCDS.contents.lpData)
        return 1

l = Listener()
win32gui.PumpMessages()

I then sent the window a WM_COPYDATA message from another app (written in Delphi):

Text := 'greetings!';
CopyData.cbData := (Length(Text)+1)*StringElementSize(Text);
CopyData.lpData := PWideChar(Text);
SendMessage(hwnd, WM_COPYDATA, Handle, NativeInt(@CopyData));

The output was:

461584
461584
74
658190
2620592
42
22
greetings!

So it seems that it works trivially, pretty much as you coded it.

The only thing that I can think of is that the text in Spotify's COPYDATASTRUCT is not null-terminated. You should be able to check that quite easily by reading out the data. Make use of the cbData member.