Are there any well-behaved POSIX interval timers?

Question

Inspired by the last leap second, I\'ve been exploring timing (specifically, interval timers) using POSIX calls.

POSIX provides several ways to set up timers, but th

Answer 1

POSIX timers (timer_create) do not require signals; you can also arrange for the timer expiration to be delivered in a thread via the SIGEV_THREAD notification type. Unfortunately glibc's implementation actually creates a new thread for each expiration (which both has a lot of overhead and destroys any hope of realtime-quality robustness) despite the fact that the standard allows reuse of the same thread for each expiration.

Short of that, I would just recommend making your own thread that uses clock_nanosleep with TIMER_ABSTIME and CLOCK_MONOTONIC for an interval timer. Since you mentioned that some broken systems might lack these interfaces, you could simply have a drop-in implementation (based e.g. on pthread_cond_timedwait) on such systems, and figure it might be lower-quality due to lack of monotonic clock, but that this is just a fundamental limitation of using a low-quality implementation like MacOSX.

As for your concern about leap seconds, if ntpd or similar is making your realtime clock jump backwards when a leap second occurs, that's a serious bug in ntpd. POSIX time (seconds since the epoch) are in units of calendar seconds (exactly 1/86400 of a day) per the standard, not SI seconds, and thus the only place leap second logic belongs on a POSIX system (if anywhere) is in mktime/gmtime/localtime when they convert between time_t and broken-down time. I haven't been following the bugs that hit this time, but they seem to have resulted from system software doing a lot of stupid and wrong stuff, not from any fundamental issue.