Uptime iOS Objective-C - millisecond precision

Question

I'm trying to get uptime for iOS. I was using mach_absolute_time - but I found that it paused during sleep.

I found this snippet:

- (time_t)uptime
{
    struct timeval boottime;
    int mib[2] = {CTL_KERN, KERN_BOOTTIME};
    size_t size = sizeof(boottime);
    time_t now;
    time_t uptime = -1;

    (void)time(&now);

    if (sysctl(mib, 2, &boottime, &size, NULL, 0) != -1 && boottime.tv_sec != 0)
    {
        uptime = now - boottime.tv_sec;
    }
    return uptime;
}

It does the trick. BUT, it's returning whole seconds. Any way to get milliseconds out of this?

Answer 1

If you want something pure Objective-C, try

NSTimeInterval uptime = [[NSProcessInfo processInfo] systemUptime];

(NSTimeInterval is a typedef for double, representing seconds.)

Answer 2

The kernel does not (apparently) store a higher-resolution timestamp of its boot time.

KERN_BOOTTIME is implemented by the sysctl_boottime function in bsd/kern/kern_sysctl.c. It calls boottime_sec.

boottime_sec is implemented in bsd/kern/kern_time.c. It calls clock_get_boottime_nanotime, which has a promising name.

clock_get_boottime_nanotime is implemented in osfmk/kern/clock.c. It is hard-coded to always return 0 in its nanosecs argument.