ADJTIME(2) BSD System Calls Manual ADJTIME(2)
adjtime -- correct the time to allow synchronization of the system clock
Standard C Library (libc, -lc)
adjtime(const struct timeval *delta, struct timeval *olddelta);
adjtime() 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.
If the calling user is not the super user, then the adjtime() function in the standard C
library will try to use the clockctl(4) device if present, thus making possible for non
privileged users to adjust the system time. If clockctl(4) is not present or not accessi-
ble, then adjtime() reverts to the adjtime() system call, which is restricted to the super
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.
adjtime() will fail if:
[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.
date(1), gettimeofday(2), clockctl(4), timed(8), timedc(8)
R. Gusella and S. Zatti, TSP: The Time Synchronization Protocol for UNIX 4.3BSD.
The adjtime() function call appeared in 4.3BSD.
BSD June 4, 1993 BSD