In printf, output n first, then \n means line feed, and then output lock, \b means that the cursor moves back one space, and there are two \ b \ bs, so return lo, ve to cover ck, and then output you, line feed;

The second time printf outputs how, \r means that the cursor returns to the first line to output foreign (how has been covered), and then the cursor moves back four squares continuously to output ever (eign has been covered).

So the end result is

tin

I

love

you

forever

That is to say, Sn, I love you forever!