Manual Pages  — CLOCK_GETTIME

The clock_id argument can be a value obtained from clock_getcpuclockid(3) or pthread_getcpuclockid(3) as well as the following values:

CLOCK_REALTIME CLOCK_REALTIME_PRECISE CLOCK_REALTIME_FAST
	Increments as a wall clock should.
CLOCK_MONOTONIC CLOCK_MONOTONIC_PRECISE CLOCK_MONOTONIC_FAST
	Increments in SI seconds.
CLOCK_UPTIME CLOCK_UPTIME_PRECISE CLOCK_UPTIME_FAST
	Starts at zero when the kernel boots and increments monotonically in SI seconds while the machine is running.
CLOCK_VIRTUAL
	Increments only when the CPU is running in user mode on behalf of the calling process.
CLOCK_PROF
	Increments when the CPU is running in user or kernel mode.
CLOCK_SECOND
	Returns the current second without performing a full time counter query, using an in-kernel cached value of the current second.
CLOCK_PROCESS_CPUTIME_ID
	Returns the execution time of the calling process.
CLOCK_THREAD_CPUTIME_ID
	Returns the execution time of the calling thread.

The clock IDs CLOCK_REALTIME_FAST, CLOCK_MONOTONIC_FAST, CLOCK_UPTIME_FAST are analogs of corresponding IDs without _FAST suffix but do not perform a full time counter query, so their accuracy is one timer tick. Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at the expense of execution time.

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

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

Only the super-user may set the time of day, using only CLOCK_REALTIME. If the system securelevel is greater than 1 (see init(8)), the time may only be advanced. This limitation is imposed to prevent a malicious super-user from setting arbitrary time stamps on files. The system time can still be adjusted backwards using the adjtime(2) system call even when the system is secure.

The resolution (granularity) of a clock is returned by the clock_getres() system call. This value is placed in a (non-NULL) *tp.