Signals are indeed a nightmare. Your example of adding tons of syscalls to make up for lack of safety shows that you understand that to be true.

And no, creating threads to solve this fringe problem in a spin loop with a sleep is not what I'd call "smart". It's unnecessary complexity and in most cases, totally wasted work.

The smartest thing to do is still probably not buffering. What's wrong with the thread? It would take maybe 15 lines of code to implement. It would be correct without rarely occurring bugs. It doesn't need signals or timers. It wouldn't add overhead to stdio calls. It's a generalized abstraction. You won't need to change your program's event loop code. Create the thread with a tiny 64kb stack and what's not to like? Granted, it would rub me the wrong way if libc did this by default, since I wouldn't want mystery threads appearing in htop for my hello world programs. But for an app developer, this is a sure fire solution.