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.
Your libc fork() implementation will lock the stdio mutexes automatically before calling SYS_fork, so the code I wrote in my comment would be fork safe. Your child process would need to spawn the flusher thread again if it's desired though.
> It would take maybe 15 lines of code to implement
This is a very bad reason to justify something. Especially introduce threads. Your response here is like saying "I don't know why people say it's so hard to write multi threaded programs, the thread create API is so simple." It completely misses the point why added complexity can be harmful.
> without rarely occurring bugs.
Except for a glaring thing like "what if fflush gets an I/O error in this background thread"?
> Granted, it would rub me the wrong way if libc did this by default,
This is exactly my point. It needs cooperation from the application layer. It wouldn't make sense to be transparent.
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.