atexit(3) on macOS

The text 1b18862994d9, as Mac OS X 10.6.8, Mac OS X 10.6.7, Mac OS X 10.6.6, Mac OS X 10.6.5, Mac OS X 10.6.4, Mac OS X 10.6.3, Mac OS X 10.6.2, Mac OS X 10.6.1, Mac OS X 10.6 shipped it.

register a function to be called on exit

atexit(3) on macOS now