Many reading programs have the functionality of saving the place where you left off last time. So when you open some file again in the future, it will continue on the row/page where you last closed/saved it. I want to know what the most common things used as the ID are.

Allegedly, there is some GNOME-specific functionality, but I don’t want that, and want pure C99 POSIX SUSv3 without any special dependencies.

The only other identifier I can think of is the path of the file. One should use realpath() for that, right? Or stat()? (I need last access time anyway. Is there some special thing in the stat struct that one can use as the ID?)