atexit(3) on macOS

The text 0c0a3ac0115a, as Mac OS X 10.1.5, Mac OS X 10.1.4, Mac OS X 10.1.3, Mac OS X 10.1.2, Mac OS X 10.1.1, Mac OS X 10.1, Mac OS X 10.0.4, Mac OS X 10.0 shipped it.

register a function to be called on exit

atexit(3) on macOS now