How can I use mach_absolute_time without overflowing?

Question

On Darwin, the POSIX standard clock_gettime(CLOCK_MONOTONIC) timer is not available. Instead, the highest resolution monotonic timer is obtained through the

Answer 1

You're worrying about overflow when multiplying/dividing with values from the mach_timebase_info struct, which is used for conversion to nanoseconds. So, while it may not fit your exact needs, there are easier ways to get a count in nanoseconds or seconds.

All solutions below are using mach_absolute_time internally (and NOT the wall clock).

---

Use double instead of uint64_t

^{(supported in Objective-C and Swift)}

double tbInSeconds = 0;
mach_timebase_info_data_t tb;
kern_return_t kError = mach_timebase_info(&tb);
if (kError == 0) {
    tbInSeconds = 1e-9 * (double)tb.numer / (double)tb.denom;
}

(remove the 1e-9 if you want nanoseconds)

Usage:

uint64_t start = mach_absolute_time();
// do something
uint64_t stop = mach_absolute_time();
double durationInSeconds = tbInSeconds * (stop - start);

---

Use ProcessInfo.processInfo.systemUptime

^{(supported in Objective-C and Swift)}

It does the job in double seconds directly:

CFTimeInterval start = NSProcessInfo.processInfo.systemUptime;
// do something
CFTimeInterval stop = NSProcessInfo.processInfo.systemUptime;
NSTimeInterval durationInSeconds = stop - start;

For reference, source code of systemUptime just does something similar as previous solution:

struct mach_timebase_info info;
mach_timebase_info(&info);
__CFTSRRate = (1.0E9 / (double)info.numer) * (double)info.denom;
__CF1_TSRRate = 1.0 / __CFTSRRate;
uint64_t tsr = mach_absolute_time();
return (CFTimeInterval)((double)tsr * __CF1_TSRRate);

---

Use QuartzCore.CACurrentMediaTime()

^{(supported in Objective-C and Swift)}

Same as systemUptime, but without being open source.

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Use Dispatch.DispatchTime.now()

^{(supported in Swift only)}

Another wrapper around mach_absolute_time(). Base precision is nanoseconds, backed with UInt64.

DispatchTime start = DispatchTime.now()
// do something
DispatchTime stop = DispatchTime.now()
TimeInterval durationInSeconds = Double(end.uptimeNanoseconds - start.uptimeNanoseconds) / 1_000_000_000

For reference, source code of DispatchTime.now() says it basically simply returns a struct DispatchTime(rawValue: mach_absolute_time()). And the calculation for uptimeNanoseconds is:

(result, overflow) = result.multipliedReportingOverflow(by: UInt64(DispatchTime.timebaseInfo.numer))
result = overflow ? UInt64.max : result / UInt64(DispatchTime.timebaseInfo.denom)

So it just discards results if the multiplication can't be stored in an UInt64.