Other mechanisms have been designed, but they have different qualities. Probably none of them are as performant as signals, which might have been more important

Other mechanisms have been designed, but they have different qualities. Probably none of them are as performant as signals, which might have been more important then than it is today. Creating a new thread for each single signal definitely feels wrong to me. Linux has signalfd() with which you can setup a signal-handling thread, but I think there are problems with that API as well. I think there is some value in being able to run a handler in the context of an existing thread the way traditional signals allow. (While I do vaguely remember that with common signals implementations it's very hard, or impossible, to control which thread is handling a signal).