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 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 adjtime call is restricted to the superuser.

Note

Time is incremented in 3906-microsecond (us) ticks on RISC and 10000 us ticks on VAX.  When adjtime is called with a delta less than 1 second, time is incremented according to the following table until the time is corrected:

Return Values

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. 

See Also

date(1), gettimeofday(2)