#include <time.h> int adjtime(const struct timeval *delta, struct timeval *olddelta);
The adjtime function 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 incrementing 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 correction from an earlier call to adjtime() may not be finished when adjtime() is called again. If olddelta is non-nil, the structure pointed to will contain, upon return, the number of microseconds still to be corrected from the earlier call.
This call may be used by 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 adjtime() is restricted to the super-user.
An argument points outside the process's allocated address space.
The process's effective user ID is not that of the super-user.