union.rb

require 'ffi'

module Union
  extend ::FFI::Library
  ffi_lib 'union'

  class U < FFI::Union
    layout(
      :d, :double,
      :i, :int
    )
  end

  attach_function :dump_union, [U.by_value], :void
end

u = Union::U.new
u[:d] = 1.0

Union.dump_union u