ftok system call

In the system call msgget(key,mode) for the argument key i want to use ftok call . I am really confused with the argument pathname in ftok call. I want to know the pathname of which file I want to specify over there. I am just a beginer to advanced unix plz help me out.

If you are starting out - beej's guide is remarkably good

This explains ftok() for beginners:
Message Queues