ADJTIME
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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NAME
adjtime - correct the time to synchronize the system clock
SYNOPSIS
#include <sys/time.h>
int adjtime(const struct timeval *delta, struct timeval *olddelta);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
adjtime():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
DESCRIPTION
The
adjtime()
function gradually adjusts the system clock (as returned by
gettimeofday(2)).
The amount of time by which the clock is to be adjusted is specified
in the structure pointed to by
delta.
This structure has the following form:
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
If the adjustment in
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
delta
is negative, then the clock is slowed down in a similar fashion.
If a clock adjustment from an earlier
adjtime()
call is already in progress
at the time of a later
adjtime()
call, and
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
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.
RETURN VALUE
On success,
adjtime()
returns 0.
On failure, -1 is returned, and
errno
is set to indicate the error.
ERRORS
- EINVAL
-
The adjustment in
delta
is outside the permitted range.
- EPERM
-
The caller does not have sufficient privilege to adjust the time.
Under Linux, the
CAP_SYS_TIME
capability is required.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
adjtime()
| Thread safety | MT-Safe
|
CONFORMING TO
4.3BSD, System V.
NOTES
The adjustment that
adjtime()
makes to the clock is carried out in such a manner that the clock
is always monotonically increasing.
Using
adjtime()
to adjust the time prevents the problems that can be caused for certain
applications (e.g.,
make(1))
by abrupt positive or negative jumps in the system time.
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
delta.
In the glibc implementation,
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).
BUGS
A longstanding bug
meant that if
delta
was specified as NULL,
no valid information about the outstanding clock adjustment was returned in
olddelta.
(In this circumstance,
adjtime()
should return the outstanding clock adjustment, without changing it.)
This bug is fixed
on systems with glibc 2.8 or later and
Linux kernel 2.6.26 or later.
SEE ALSO
adjtimex(2),
gettimeofday(2),
time(7)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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