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Name

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(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. 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 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.

Return Values

A 0 return value indicates that the call succeeded. A -1 return value indicates an error occurred, and in this case an error code is stored into the global variable errno.

Errors

The following error codes may be set in errno:
EFAULT
delta or olddelta points outside the process’s allocated address space, or olddelta points to a region of the process’ allocated address space that is not writable.
EINVAL
tv_usec field in delta is not within valid range (-1000000 to 1000000).
EPERM
The effective user of the calling process is not super-user.

See Also

date(1) , gettimeofday(3C)

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