help with DL and user32.dll

Very intermediate into programming and never done anything with visual basic, although if anyone could help me better understand the following I would be very greatful!

In my ruby script I have

require "dl" user32 = DL.dlopen('user32')

actWindow = DL::CFunc.new(user32['GetActiveWindow'],DL::TYPE_VOID, 'GetActiveWindow') actWindow.call()

which has wrong number of arguments, needs an array passed into it.

actWindow.call() returns type nil

I'm not sure what I should be passing into call method as the function is of DL::TYPE_VOID?

GetActiveWindow - GetActiveWindow function (winuser.h) - Win32 apps | Microsoft Docs