Things UNIX can do atomically

This is a catalog of things UNIX-like/POSIX-compliant operating systems can do atomically, making them useful as building blocks for thread-safe and multi-process-safe programs without mutexes or read/write locks.  The list is by no means exhaustive and I expect it to be updated frequently for the foreseeable future.

The philosophy here is to let the kernel do as much work as possible.  At my most pessimistic, I trust the kernel developers more than a trust myself.  More practically, it’s stupid to spend CPU time locking around an operation that’s already atomic.  Added 2010-01-07.