man/man2/adjtime.2

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 ADJTIME 2 "Sep 19, 2022"
 NAME
adjtime - correct the time to allow synchronization of the system clock
 SYNOPSIS
#include <sys/time.h>

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


 DESCRIPTION
The adjtime() function 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. 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 delta is 0, then olddelta returns the status of the effects of
the previous adjtime() call with no effect on the time correction as a
result of this call. 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 a processes with appropriate privileges can adjust the time of day.

The adjustment value will be silently rounded to the resolution of the system
clock.
 RETURN VALUES
Upon successful completion, adjtime() returns 0. Otherwise, it
returns -1 and sets errno to indicate the error.
 ERRORS
The adjtime() function will fail if:
EFAULT

The delta or olddelta argument points outside the process's
allocated address space, or olddelta points to a region of the process's
allocated address space that is not writable.


EINVAL

The tv_usec member of delta is not within valid range (-1000000
to 1000000).


EPERM

The {PRIV_SYS_TIME} privilege is not asserted in the effective set of the
calling process.


Additionally, the adjtime() function will fail for 32-bit interfaces if:
EOVERFLOW

The size of the tv_sec member of the timeval structure pointed to
by olddelta is too small to contain the correct number of seconds.


 SEE ALSO
 date (1),  gettimeofday (3C),  privileges (7)