Shared memory (shm) - a safe way to pick ids?

[Benson I. Margulies]

I'm on the verge of coding a simple use of the shm system call
family on a system that dosen't supply the more modern mapping 
calls. The question is -- how to pick segment id's that are 
not going to collide with other, unrelated applications? Calling
ftok(3) is fine, so long as anyone does it. But there seems no guarantee
that some other rogue elephant won't go and stir up one of those
bitpatterns. Has anyone else wrestled this item?
-- 
Benson I. Margulies