#include <sys/time.h>int adjtime(struct timeval *delta, struct timeval *olddelta);
The adjtime() function adjusts the system's notion of the current time as returned by gettimeofday(3C), advancing or retarding it by the amount of time specified in the struct timeval pointed to by delta.
The adjustment is effected by speeding up (if that amount of time is positive) or slowing down (if that amount of time is negative) the system's clock by some small percentage, generally a fraction of one percent. 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 0, then olddelta returns the status of the effects of the previous adjtime() call with no effect on the time correction as a result of this call. If olddelta is not a null pointer, then the structure it points to will contain, upon successful return, the number of seconds and/or microseconds still to be corrected from the earlier call. If olddelta is a null pointer, the corresponding information will not be returned.
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.
Only the super-user may adjust the time of day.
The adjustment value will be silently rounded to the resolution of the system clock.
Upon successful completion, adjtime() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.
The adjtime() function will fail if:
The delta or olddelta argument 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.
The tv_usec member of delta is not within valid range (-1000000 to 1000000).
The effective user of the calling process is not super-user.
Additionally, the adjtime() function will fail for 32-bit interfaces if:
The size of the tv_sec member of the timeval structure pointed to by olddelta is too small to contain the correct number of seconds.