adjtime(2)
NAME
adjtime − correct the time to allow synchronization of the system clock
SYNTAX
#include <sys/time.h>
adjtime(delta, olddelta)
struct timeval *delta;
struct timeval *olddelta;
DESCRIPTION
The adjtime system call changes the system time, as returned by gettimeofday(,), moving it backward or forward by the number of microseconds corresponding to the timeval delta.
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 adjtime may not be finished when adjtime is called again. If olddelta is non-zero, 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 adjtime call is restricted to the superuser.
NOTE
Time is incremented in 10ms ticks. When adjtime is called with an argument other than zero, ticks of 9ms or 11ms are used until the time is corrected. A delta of less than 1ms would have no effect.
RETURN VALUE
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.
DIAGNOSTICS
The following error codes may be set in errno:
[EFAULT]
An argument points outside the process’s allocated address space.
[EPERM]
The process’s effective user ID is not that of the super-user.