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 - http://msdn.microsoft.com/en-us/library/windows/desktop/ms646292(v=vs.85).aspx