Please don't use arc4random* for cryptographic purposes. RC4 is broken -- there are practical attacks against it. It's fine for non-secure purposes though (eg. monte carlo simulation).
The 'arc4' no longer refers to RC4 on macOS/iOS and OpenBSD. On those systems 'arc4random' is playfully a bacronym for "A Replacement Call for Random". The new arc4random* implementation will now be replaced as cryptographic techniques advance.
It appears the Apple and OpenBSD implementations use the getentropy syscall and then add additional entropy mixing.
