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 =['GetActiveWindow'],DL::TYPE_VOID,

which has wrong number of arguments, needs an array passed into it.[]) returns type nil

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

GetActiveWindow -