The
clock_settime() function sets the clock identified by
clock_id to the absolute time specified by
tp. If the time specified by
tp is not a multiple of the resolution of the clock,
tp is truncated to a multiple of the resolution.
The
clock_gettime() function stores the time of the clock identified by
clock_id into the location specified by
tp.
The
clock_getres() function stores the resolution of the clock identified by
clock_id into the location specified by
res, unless
res is
NULL.
A
clock_id of
CLOCK_REALTIME identifies the realtime clock for the system. For this clock, the values specified by
clock_settime() and obtained by
clock_gettime() represent the amount of time (in seconds and nanoseconds) since 00:00 Universal Coordinated Time, January 1, 1970.
A
clock_id of
CLOCK_MONOTONIC identifies a clock that increases at a steady rate (monotonically). This clock is not affected by calls to
adjtime(2) and
settimeofday(2) and will fail with an
EINVAL error if it's the clock specified in a call to
clock_settime(). The origin of the clock is unspecified.
If the calling user is not the super-user, then the
clock_settime() function in the standard C library will try to use the
clockctl(4) device if present, thus making possible for non privileged users to set the system time. If
clockctl(4) is not present or not accessible, then
clock_settime() reverts to the
clock_settime() system call, which is restricted to the super user.