You need to declare it as a :pointer, or :buffer_in argument, and then allocate some temporary memory yourself.

e.g.

module Foo

...

   attach_function :my_func, [ :pointer ], :void

end

ary = FFI::MemoryPointer.new(:double, 3)

# Fill the array with values

ary.write_array_of_double([ 0.1, 0.2, 0.3 ])

# pass it to the function

Foo.my_func(ary)