[kwdc]

Are Apple using C for this level of subsystem? I thought a lot was in Objective-C or is that just higher level stuff?

[w0mbat]

Apple uses C for some bits.

Also there are Objective C APIs which take format strings, like +[NSString stringWithFormat:] and the system function NSLog().

Even Swift uses format strings in places. https://developer.apple.com/documentation/swift/string/31267...