ADJTIME(3) Linux Programmer's Manual ADJTIME(3)
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 al-
ready 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 at-
tributes(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 adj-
time() 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 sys-
tem 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 old-
delta. (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 5.07 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/.
Linux 2017-09-15 ADJTIME(3)