CLOCK_GETTIME(3) Library Functions Manual CLOCK_GETTIME(3)

clock_gettime, clock_settime, clock_getres, clock_gettime_nsec_np — get/set date and time

#include <time.h>

int
clock_gettime(clockid_t clock_id, struct timespec *tp);

int
clock_settime(clockid_t clock_id, const struct timespec *tp);

int
clock_getres(clockid_t clock_id, struct timespec *tp);

uint64_t
clock_gettime_nsec_np(clockid_t clock_id);

The () and () functions allow the calling process to retrieve or set the value used by a clock which is specified by clock_id.

clock_id can be a value from one of 8 predefined values:

the system's real time (i.e. wall time) clock, expressed as the amount of time since the Epoch. This is the same as the value returned by gettimeofday(2).
clock that increments monotonically, tracking the time since an arbitrary point, and will continue to increment while the system is asleep.
clock that increments monotonically, tracking the time since an arbitrary point like CLOCK_MONOTONIC. However, this clock is unaffected by frequency or time adjustments. It should not be compared to other system time sources.
like CLOCK_MONOTONIC_RAW, but reads a value cached by the system at context switch. This can be read faster, but at a loss of accuracy as it may return values that are milliseconds old.
clock that increments monotonically, in the same manner as CLOCK_MONOTONIC_RAW, but that does not increment while the system is asleep. The returned value is identical to the result of () after the appropriate mach_timebase conversion is applied.
like CLOCK_UPTIME_RAW, but reads a value cached by the system at context switch. This can be read faster, but at a loss of accuracy as it may return values that are milliseconds old.
clock that tracks the amount of CPU (in user- or kernel-mode) used by the calling process.
clock that tracks the amount of CPU (in user- or kernel-mode) used by the calling thread.

The structure pointed to by tp is defined in <sys/time.h> as:

struct timespec {
    time_t  tv_sec;     /* seconds */
    long    tv_nsec;    /* and nanoseconds */
};

Only the CLOCK_REALTIME clock can be set, and only the superuser may do so.

The resolution of a clock is returned by the () call. This value is placed in a (non-null) *tp. This value may be smaller than the actual precision of the underlying clock, but represents a lower bound on the resolution.

As a non-portable extension, the () function will return the clock value in 64-bit nanoseconds.

A 0 return value indicates that the call succeeded. A -1 return value indicates an error occurred, and in this case an error code is stored into the global variable errno. For clock_gettime_nsec_np() a return value of non-0 indicates success. A 0 return value indicates an error occurred and an error code is stored in errno.

clock_gettime(), clock_settime(), clock_getres(), and clock_gettime_nsec_np() will fail if:

[]
clock_id is not a valid value.
[]
The tp argument address referenced invalid memory.

In addition, clock_settime() may return the following errors:

[]
A user other than the superuser attempted to set the time.
[]
clock_id specifies a clock that isn't settable, tp specifies a nanosecond value less than zero or greater than 1000 million, or a value outside the range of the specified clock.

date(1), getitimer(2), gettimeofday(2),

These functions first appeared in Mac OSX 10.12

The clock_gettime(), clock_settime(), and clock_getres() system calls conform to IEEE Std 1003.1b-1993 (“POSIX.1b”). clock_gettime_nsec_np() is a non-portable Darwin extension. The clock IDs CLOCK_MONOTONIC_RAW and CLOCK_UPTIME_RAW are extensions to the POSIX interface.

January 26, 2016 Mac OS X 12