adjtime(2)adjtime(2)Nameadjtime - correct the time to allow synchronization of the system clock
struct timeval *delta;
struct timeval *olddelta;
The system call changes the system time, as returned by moving it back‐
ward or forward by the number of microseconds corresponding to the
The time is maintained by incrementing it with a machine-dependent tick
every clock interrupt. If delta is negative, the clock is slowed down
by incrementing it in smaller ticks until the correction is made. If
delta is positive, a larger tick is used. Thus, the time is always a
monotonically increasing function. A time correction from an earlier
call to may not be finished when is called again. If olddelta is
nonzero, then the structure pointed to will contain, upon return, the
number of microseconds still to be corrected from the earlier call.
This call can be used in time servers that synchronize the clocks of
computers 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.
The call is restricted to the superuser.
Time is incremented in 3906-microsecond (us) ticks on RISC and 10000 us
ticks on VAX. When is called with a delta less than 1 second, time is
incremented according to the following table until the time is cor‐
Default Fast Slow
System Increment Increment Increment
RISC 3906 us 3921 us 3891 us
VAX 10000 us 10001 us 9999 us
A return value of 0 indicates that the call succeeded. A return value
of -1 indicates that an error occurred, and in this case an error code
is stored in the global variable errno.
The following error codes may be set in errno:
[EFAULT] An argument points outside the process's allocated
[EPERM] The process's effective user ID is not that of the
See Alsodate(1), gettimeofday(2)adjtime(2)