ADJTIME(2) BSD System Calls Manual ADJTIME(2)
adjtime -- correct the time to allow synchronization of the system clock
adjtime(const struct timeval *delta, struct timeval *olddelta);
adjtime() makes small adjustments to the system time, as returned by
gettimeofday(2), advancing or retarding it by the time specified by the
timeval delta. If delta is negative, the clock is slowed down by incre-
menting it more slowly than normal until the correction is complete. If
delta is positive, a larger increment than normal is used. The skew used
to perform the correction is generally a fraction of one percent. Thus,
the time is always a monotonically increasing function. A time correc-
tion from an earlier call to adjtime() may not be finished when adjtime()
is called again. If delta is null, no adjustment is done. If olddelta
is non-null, the structure pointed to will contain, upon return, the num-
ber of microseconds still to be corrected from the earlier call. Setting
the time with settimeofday(2) will cancel any in-progress time adjust-
This call may be used by time servers that synchronize the clocks of com-
puters in a local area network. Such time servers would slow down the
clocks of some machines and speed up the clocks of others to bring them
to the average network time.
Only the superuser may adjust the time using the adjtime() function.
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
adjtime() will fail if:
[EFAULT] Either of the arguments point outside the process's
allocated address space.
[EPERM] The delta() argument is non-null and the process's
effective user ID is not that of the superuser.
date(1), adjfreq(2), gettimeofday(2), ntpd(8)
The adjtime() function call appeared in 4.3BSD.
Other operating systems restrict calling adjtime to the superuser and
might not allow requesting the current correction without specifying a
BSD December 10, 2014 BSD