adjtime(2)

adjtime -- correct the time to allow synchronization of the system clock

Synopsis

   #include <sys/time.h>
   
   int adjtime(struct timeval delta, struct timeval olddelta);

Description

adjtime adjusts the system's notion of the current time, as returned by gettimeofday(2), advancing or retarding it by the amount of time specified in the struct timeval pointed to by delta.

This call may 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 adjustment is effected by speeding up (if the adjustment is positive) or slowing down (if the adjustment is negative) the system's clock by some small percentage, generally a fraction of one percent; the clock may be speeded up at a faster rate for a large positive adjustment. Thus, the time is always a monotonically increasing function.

A time correction from an earlier call to adjtime may not be finished when adjtime is called again. If delta is NULL, then olddelta returns information on the previous adjtime call and there is no effect on the time correction as a result of this call. If olddelta is not NULL, then the structure it points to contains, on return, the time still to be corrected from the earlier call. If olddelta is NULL, the information is not returned.

Only a process with the appropriate privilege (P_SYSOPS) can adjust the time of day.

The adjustment value is silently rounded to the resolution of the system clock.

Return values

On success, adjtime returns 0. On failure, adjtime returns -1 and sets errno to identify the error.

Errors

In the following conditions, adjtime fails and sets errno to:

EFAULT: delta or olddelta points outside the process's allocated address space, or olddelta points to a region of the process's allocated address space that is not writable.
EPERM: The calling process does not have the appropriate privilege (P_SYSOPS) to change the time of day.

References

date(1), gettimeofday(2), stime(2)