adjtime man page on FreeBSD

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ADJTIME(2)		    BSD System Calls Manual		    ADJTIME(2)

     adjtime — correct the time to allow synchronization of the system clock

     Standard C Library (libc, -lc)

     #include <sys/time.h>

     adjtime(const struct timeval *delta, struct timeval *olddelta);

     The adjtime() system call makes small adjustments to the system time, as
     returned by gettimeofday(2), advancing or retarding it by the time speci‐
     fied 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 not a null pointer, the
     structure pointed to will contain, upon return, the number of microsec‐
     onds still to be corrected from the earlier call.

     This call may be used by time servers that synchronize the clocks of com‐
     puters 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() system call is restricted to the super-user.

     The adjtime() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     The adjtime() system call 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

     date(1), gettimeofday(2), timed(8), timedc(8)

     R. Gusella and S. Zatti, TSP: The Time Synchronization Protocol for UNIX

     The adjtime() system call appeared in 4.3BSD.

BSD				 June 4, 1993				   BSD

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