# File lib/poll.rb, line 268
 def setCallback( io, callback=nil, *args, &block )
		raise ArgumentError, "Handle #{io.inspect} is not registered" unless
			@masks.has_key?( io )

		rv = nil
		if @callbacks.has_key?( io )
			rv = @callbacks[ io ][:callback]
		end

		if callback || block
			@callbacks[ io ] = { :callback => (callback || block), :args => args }
		else
			@callbacks.delete( io )
		end

		return rv
	end