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 as a wall clock should.

CLOCK_MONOTONIC

 

CLOCK_MONOTONIC_PRECISE

 

CLOCK_MONOTONIC_FAST

 

CLOCK_MONOTONIC_COARSE

Increments in SI seconds.

CLOCK_UPTIME

 

CLOCK_UPTIME_PRECISE

 

CLOCK_UPTIME_FAST

 

CLOCK_BOOTTIME

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, 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_UPTIME for compatibility with other systems.

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.

SEE ALSO

date(1), adjtime(2), clock_getcpuclockid(3), ctime(3), pthread_getcpuclockid(3)

STANDARDS

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.

HISTORY

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