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Comment by audidude | Hacker News Reader

CLOCK_MONOTONIC is not available on OS X. Also, isn't CLOCK_MONOTONIC_RAW fetching the counter from rdtsc instruction on x86? (And therefore can go backwards in time if your process is migrated to another CPU).

Keep in mind, also, that the CLOCK_MONOTONIC is not part of the core POSIX standard, so OS X is technically "compliant" even though they didn't implement it. Add this to a list of annoyances, such as no pthread_cond_timed_wait().

You can, however, get a monotonic clock by getting access to the clock service on mach.

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No CLOCK_MONOTONIC in OS X?

In a David Tennant voice: Wait, what?? What?!

  ~$ grep CLOCK_MONOTONIC /usr/include -rn
  ~$

Ahh, it's in the POSIX realtime extension... Dammit.

Apparently [1] one solution would be to #IFDEF DARWIN the following, as on Darwin SYSTEM_CLOCK is supposed to be a monotonic boot clock.

  #include <mach/clock.h>
  #include <mach/mach.h>
  
     ...
  
  clock_serv_t cclock;
  mach_timespec_t mts;

  host_get_clock_service(mach_host_self(), SYSTEM_CLOCK,
  &cclock);
  clock_get_time(cclock, &mts);
  mach_port_deallocate(mach_task_self(), cclock);

Not sure if processor affinity would affect that code (don't have time right now to do the in depth reading on it), but you could always set affinity to CPU0. Though then you'd have to make sure to run the sudo'd command as a child process and appropriately reset affinity to default before a call to exec...

On OS X, use mach_absolute_time() to get a monotonic clock. A lot of APIs in OS X use the monotonic clock, e.g. CoreAnimation wraps mach_absolute_time() as CACurrentMediaTime() and uses that to calculate animation start times and delays.

pthread_cond_timedwait has existed on OS X since Tiger. Look in /usr/include/pthread.h.

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