OK its been awhile since this was first brought up. I since have printed a copy of International Code of Signaling provided in the above link and have been doing some brushing up. Thank you all for the information.
I want to make sure I have this right, while I am anchored and I see another vessel approaching which I believe is coming too close to my anchored vessel and is possibly putting us both in a dangerous situation, I should signal either by flashlight or horn "U", dot,dot,dash, "you are running into danger", correct?
M or L are also options, correct?
I get this all from the original post and just want to make sure I have it correct. Thanks everybody.