NAME

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

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#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);

DESCRIPTION

The clock_gettime() and clock_settime() 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 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
CLOCK_REALTIME_COARSE: Increments in SI seconds like a wall clock. It uses a 1970 epoch and implements the UTC timescale. The count of physical SI seconds since 1970, adjusted by subtracting the number of positive leap seconds and adding the number of negative leap seconds. Behavior during a leap second is not defined by and POSIX standard.
CLOCK_MONOTONIC
CLOCK_MONOTONIC_PRECISE
CLOCK_MONOTONIC_FAST
CLOCK_MONOTONIC_COARSE
CLOCK_BOOTTIME: Increments in SI seconds, even while the system is suspended. Its epoch is unspecified. The count is not adjusted by leap seconds. FreeBSD implements
CLOCK_UPTIME
CLOCK_UPTIME_PRECISE
CLOCK_UPTIME_FAST: Increments monotonically in SI seconds while the machine is running. The count is not adjusted by leap seconds. The epoch is unspecified.
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.
CLOCK_TAI: Increments in SI seconds like a wall clock. It uses a 1970 epoch and implements the TAI timescale. Similar to CLOCK_REALTIME, but without leap seconds. It will increase monotonically during a leap second. Will return EINVAL if the current offset between TAI and UTC is not known, which may be the case early in boot before NTP or other time daemon has synchronized.

The clock IDs CLOCK_BOOTTIME, CLOCK_REALTIME, CLOCK_TAI, CLOCK_MONOTONIC, and CLOCK_UPTIME perform a full time counter query. The clock IDs with the _FAST suffix, i.e., CLOCK_REALTIME_FAST, CLOCK_MONOTONIC_FAST, and CLOCK_UPTIME_FAST, do not perform a full time counter query, so their accuracy is one timer tick. Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE, and CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at the expense of execution time. The clock IDs CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE are aliases of corresponding IDs with _FAST suffix for compatibility with other systems. Finally, CLOCK_BOOTTIME is an alias for CLOCK_MONOTONIC for compatibility with other systems and is unrelated to the kern.boottime sysctl(8).

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(7) 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.

RETURN VALUES

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.

ERRORS

The following error codes may be set in errno:

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

STANDARDS

The clock_gettime(), clock_settime(), and clock_getres() system calls conform to IEEE Std 1003.1-2008 (“POSIX.1”). The clock IDs CLOCK_BOOTTIME, CLOCK_MONOTONIC_FAST, CLOCK_MONOTONIC_PRECISE, CLOCK_REALTIME_FAST, CLOCK_REALTIME_PRECISE, CLOCK_SECOND, CLOCK_TAI, CLOCK_UPTIME, CLOCK_UPTIME_FAST, and CLOCK_UPTIME_PRECISE are FreeBSD extensions to the POSIX interface.

UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal emissions. However, the time_t type is a simple count that does not provide a unique encoding for leap seconds, nor a specification for what values should be used to encode a leap second.

HISTORY

The clock_gettime(), clock_settime(), and clock_getres() system calls first appeared in FreeBSD 3.0.