'\" t
.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH adjtime 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
adjtime \- correct the time to synchronize the system clock
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/time.h>
.PP
.BI "int adjtime(const struct timeval *" delta ", struct timeval *" olddelta );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR adjtime ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE
.fi
.SH DESCRIPTION
The
.BR adjtime ()
function gradually adjusts the system clock (as returned by
.BR gettimeofday (2)).
The amount of time by which the clock is to be adjusted is specified
in the structure pointed to by
.IR delta .
This structure has the following form:
.PP
.in +4n
.EX
struct timeval {
    time_t      tv_sec;     /* seconds */
    suseconds_t tv_usec;    /* microseconds */
};
.EE
.in
.PP
If the adjustment in
.I delta
is positive, then the system clock is speeded up by some
small percentage (i.e., by adding a small
amount of time to the clock value in each second) until the adjustment
has been completed.
If the adjustment in
.I delta
is negative, then the clock is slowed down in a similar fashion.
.PP
If a clock adjustment from an earlier
.BR adjtime ()
call is already in progress
at the time of a later
.BR adjtime ()
call, and
.I delta
is not NULL for the later call, then the earlier adjustment is stopped,
but any already completed part of that adjustment is not undone.
.PP
If
.I olddelta
is not NULL, then the buffer that it points to is used to return
the amount of time remaining from any previous adjustment that
has not yet been completed.
.SH RETURN VALUE
On success,
.BR adjtime ()
returns 0.
On failure, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
The adjustment in
.I delta
is outside the permitted range.
.TP
.B EPERM
The caller does not have sufficient privilege to adjust the time.
Under Linux, the
.B CAP_SYS_TIME
capability is required.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR adjtime ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
None.
.SH HISTORY
4.3BSD, System V.
.SH NOTES
The adjustment that
.BR adjtime ()
makes to the clock is carried out in such a manner that the clock
is always monotonically increasing.
Using
.BR adjtime ()
to adjust the time prevents the problems that can be caused for certain
applications (e.g.,
.BR make (1))
by abrupt positive or negative jumps in the system time.
.PP
.BR adjtime ()
is intended to be used to make small adjustments to the system time.
Most systems impose a limit on the adjustment that can be specified in
.IR delta .
In the glibc implementation,
.I delta
must be less than or equal to (INT_MAX / 1000000 \- 2)
and greater than or equal to (INT_MIN / 1000000 + 2)
(respectively 2145 and \-2145 seconds on i386).
.SH BUGS
A longstanding bug
.\" http://sourceware.org/bugzilla/show_bug?id=2449
.\" http://bugzilla.kernel.org/show_bug.cgi?id=6761
meant that if
.I delta
was specified as NULL,
no valid information about the outstanding clock adjustment was returned in
.IR olddelta .
(In this circumstance,
.BR adjtime ()
should return the outstanding clock adjustment, without changing it.)
This bug is fixed
.\" Thanks to the new adjtimex() ADJ_OFFSET_SS_READ flag
on systems with glibc 2.8 or later and
Linux kernel 2.6.26 or later.
.SH SEE ALSO
.BR adjtimex (2),
.BR gettimeofday (2),
.BR time (7)