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clock_gettime, clock_settime, clock_getresget/set/calibrate date and time


Standard C Library (libc, -lc)


#include <time.h>

clock_gettime(clockid_t clock_id, struct timespec *tp);

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

clock_getres(clockid_t clock_id, struct timespec *tp);


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

The clock_id argument can be one of the following values: CLOCK_REALTIME, CLOCK_REALTIME_PRECISE, CLOCK_REALTIME_FAST for time that increments as a wall clock should; CLOCK_MONOTONIC, CLOCK_MONOTONIC_PRECISE, CLOCK_MONOTONIC_FAST which increments in SI seconds; CLOCK_UPTIME, CLOCK_UPTIME_PRECISE, CLOCK_UPTIME_FAST which starts at zero when the kernel boots and increments monotonically in SI seconds while the machine is running; CLOCK_VIRTUAL for time that increments only when the CPU is running in user mode on behalf of the calling process; CLOCK_PROF for time that increments when the CPU is running in user or kernel mode; or CLOCK_SECOND which returns the current second without performing a full time counter query, using in-kernel cached value of current second.

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 () system call. This value is placed in a (non-NULL) *tp.


Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The following error codes may be set in errno:

The clock_id or timespec argument was not a valid value.
A user other than the super-user attempted to set the time.


date(1), adjtime(2), ctime(3), timed(8)


The clock_gettime(), clock_settime(), and clock_getres() system calls conform to IEEE Std 1003.1b-1993 (“POSIX.1b”). The clock IDs CLOCK_REALTIME_FAST, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_FAST, CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME, CLOCK_UPTIME_FAST, CLOCK_UPTIME_PRECISE, CLOCK_SECOND are FreeBSD extensions to the POSIX interface.

December 27, 2015 Debian