There is no solution, much less one that is portable across different Unixen.

The core problem is that, because of the ability of filesystems to effectively contain other filesystems within them, the number of bits to uniquely identify a file within a filesystem is not a constant number across different filesystem types. It's a harder problem on Linux because Linux is also full of filesystems that aren't really filesystems, where trying to come up with a persistent, unique identifier for people to use is a lot more bother than it's really worth.