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.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.TH ADJTIME 3 2008-06-22 "Linux" "Linux Programmer's Manual"
.SH NAME
adjtime \- correct the time to synchronize the system clock
.SH SYNOPSIS
.nf
.BI "int adjtime(const struct timeval *" delta ", struct timeval *" olddelta );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR adjtime ():
_BSD_SOURCE
.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:
.in +4n
.nf
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
.fi
.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.
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.
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 CONFORMING TO
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.
.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)
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