adjtime(2)adjtime(2)NAMEadjtime() - correct the time to synchronize the system clock
SYNOPSISDESCRIPTION
The function adjusts the current time of the system. The time is
either advanced or retarded by the amount of time specified in the
pointed to by delta.
The adjustment is made by applying small correctional adjustments to
the value of current time that the system keeps. The time is always
increasing monotonically, but at a rate slightly slower or faster than
normal.
A time correction for an earlier call to may not be complete when is
called. The second call to stops the first call to if delta is non-
NULL, but does not undo the effects of the previous call. If delta is
NULL, then no time correction will be done.
If olddelta is not a NULL pointer, then the structure it points to will
contain, upon 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.
The call to returns immediately, though its effect will continue until
the whole correction is made or until modified by another call to
either with a non-NULL delta or to change the system time (see
Only a user with appropriate privileges can call successfully with a
non-NULL delta. Any user can call with a NULL delta to report the cor‐
rection left from the previous call.
Limits
is defined in as having at least 2 members, tv_sec (seconds) and
tv_usec (microseconds).
When is called, if the delta.tv_sec field is greater than 31536000
(approx. 365 days), or less than −31536000, then fails with an errno of
The tv_usec field is not used in the calculations to determine the lim‐
its, and so the actual limits on adjustments are seconds.
Note that the desired seconds may be negative. Since the type of the
tv_sec field is (unsigned long), any negative values for tv_sec need to
be cast.
Any olddelta value returned by the function will be returned such that
the signs of non-zero members are the same.
Interaction with Other System Calls
A call to change the system time terminates the correction currently in
effect. A subsequent call to will return {0, 0} for the olddelta
parameter. This includes system calls such as and
Security Restrictions
This system call requires the privilege in order to successfully usa a
non-NULL delta. Processes owned by the superuser have this privilege.
Processes owned by other users may have this privilege, depending on
system configuration. See privileges(5) for more information about
privileged access on systems that support fine-grained privileges.
RETURN VALUE
Upon successful completion, returns a value of 0; otherwise, it returns
a value −1 and sets to indicate the error.
ERRORS
fails if one or more of the following is true:
if the process does not have the appropriate privilege.
The address specified for
delta (or olddelta) is invalid.
If delta.tv_sec is greater than 31536000 (approx.
365 days) or less than −31536000. The
delta.tv_usec field is not used in calculation of
these limits. If the user wants to adjust time
greater than these limits, an appropriate alter‐
native interface should be used.
EXAMPLES
The following code snippet will take the time forward 20 minutes.
The following code fragment will repeatedly call a user-defined func‐
tion until the adjustment requested in a previous call to (called from
either the same process or another process) is completed.
AUTHOR
was developed by the University of California, Berkeley and AT&T.
SEE ALSOdate(1), gettimeofday(2), stime(2), clock_settime(2), clocks(2),
getitimer(2), setitimer(2), alarm(2), privileges(5).
adjtime(2)
