POSIX timers

Why

Traditional Unix interval timers (setitimer(2)) suffer various limitations, including:

Only one timer of each of three types , ITIMER_REAL (wallclock), ITIMER_VIRTUAL (user-mode CPU time), and ITIMER_PROF (user-mode + system-mode CPU time).
Notification of timer expiration is only via signals.
Can't change the notification signals used.
No way to find out about timer overruns.