clock_gettime,
clock_settime,
clock_getres —
get/set/calibrate 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);
The
clock_gettime() and
clock_settime() 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
clock_getcpuclockid(3)
or
pthread_getcpuclockid(3),
or one of the following predefined values:
-
-
CLOCK_REALTIME- time that increments as a wall clock should
-
-
CLOCK_PROCESS_CPUTIME_ID- time that increments when the CPU is running in user or kernel mode on
behalf of the calling process
-
-
CLOCK_THREAD_CPUTIME_ID- time that increments when the CPU is running in user or kernel mode on
behalf of the calling thread
-
-
CLOCK_MONOTONIC- time that increments as a wall clock should but whose absolute value is
meaningless and cannot jump, providing accurate realtime interval
measurement, even across suspend and resume
-
-
CLOCK_BOOTTIME- time whose absolute value is the time that has elapsed since the system
was booted
-
-
CLOCK_UPTIME- time whose absolute value is the time the system has been running and not
suspended, providing accurate uptime measurement, both absolute and
interval
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. 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 superuser 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() 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.
clock_gettime(),
clock_settime(), and
clock_getres() will fail if:
-
-
- [
EINVAL]
- clock_id is not a valid value.
-
-
- [
EFAULT]
- The tp argument address referenced
invalid memory.
In addition,
clock_settime() may return the
following errors:
-
-
- [
EPERM]
- A user other than the superuser attempted to set the time.
-
-
- [
EINVAL]
- 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),
adjtime(2),
getitimer(2),
gettimeofday(2),
clock_getcpuclockid(3),
ctime(3),
pthread_getcpuclockid(3)
The
clock_getres(),
clock_gettime(), and
clock_settime() functions conform to
IEEE Std 1003.1-2008
(“POSIX.1”).
The
CLOCK_BOOTTIME and
CLOCK_UPTIME clocks are extensions to that.
The
CLOCK_PROCESS_CPUTIME_ID and
CLOCK_THREAD_CPUTIME_ID clocks appeared in
OpenBSD 5.4. The
CLOCK_UPTIME clock first appeared in
FreeBSD 7.0 and was added to
OpenBSD in
OpenBSD 5.5. The
CLOCK_BOOTTIME clock first appeared in
Linux 2.6.39 and was added to
OpenBSD in
OpenBSD 6.3.
![[OpenBSD]](/openbsd.gif){kind=small}