CLOCK_SETTIME(2) BSD System Calls Manual CLOCK_SETTIME(2)
clock_settime, clock_gettime, clock_getres -- clock and timer functions
Standard C Library (libc, -lc)
clock_settime(clockid_t clock_id, const struct timespec *tp);
clock_gettime(clockid_t clock_id, struct timespec *tp);
clock_getres(clockid_t clock_id, struct timespec *res);
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,
A clock_id of CLOCK_MONOTONIC identifies a clock that increases at a steady rate (monotoni-
cally). 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
A value of 0 is returned on success. Otherwise, a value of -1 is returned and errno is set
to indicate the error.
The clock_settime(), clock_gettime() and clock_getres() functions will fail if:
[EINVAL] The clock_id argument does not specify a known clock.
[ENOSYS] The function is not supported by this implementation.
The clock_settime() function will fail if:
[EINVAL] The tp argument is outside the range for the specified clock, clock_id.
[EINVAL] The tp argument specified a nanosecond value less than zero of greater
than or equal 1000 million.
[EINVAL] The clock_id argument is a clock that can not be adjusted.
[EPERM] The calling process does not have the appropriate privilege to set the
specified clock, clock_id.
The clock_gettime() function will fail if:
[EFAULT] The tp argument specifies an address that is not a valid part of the
process address space.
ctime(3), time(3), clockctl(4)
The clock_settime(), clock_gettime() and clock_getres() functions conform to IEEE Std
BSD February 11, 1999 BSD