How do I allocate a buffer with Ruby's Fiddle for use with Win32 functions such as SendMessage?

Question

The function signature for SendMessageW as per MSDN:

LRESULT SendMessageW(
  [in] HWND   hWnd,
  [in] UINT   Msg,
  [in] WPARAM wParam,
  [in] LPARAM lParam
);

I wish to get the text in a window title/caption using the WM_GETTEXT Msg. The problem I am having is that LPARAM is a long int type, and using standard C, the char buffer could be cast to LPARAM, but using Ruby and Fiddle the calling convention is not so clear. I have tried:

captionText = String.new
User32::SendMessageW(hWnd, User32::WM_GETTEXT, bufLen, captionText) 

which gives an implicit conversion from string to int error, and:

User32::SendMessageW(hWnd, User32::WM_GETTEXT, bufLen, captionText.to_i)

Which appears to do nothing.

I already have the window handle and length of the text from earlier calls, e.g:

hWnd = User32::FindWindow(<class name of window>, <window title>)
bufLen = User32::SendMessageW(hWnd, User32::WM_GETTEXTLENGTH, 0, 0) + 1

I have defined the WinAPI functions in a manner similar to the examples here, and they work:

https://coder-question.com/cq-blog/432387?force_isolation=true

I am aware of the GetWindowText and GetWindowTextLength functions, however GetWindowTextLength is not working for child windows, so I've decided to try SendMessage instead.