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ADJTIME(3) Linux Programmer's Manual ADJTIME(3)
adjtime - correct the time to synchronize the system clock
int adjtime(const struct timeval *delta, struct timeval *olddelta);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
adjtime(): _BSD_SOURCE
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.
On success, adjtime() returns 0. On failure, -1 is returned, and errno is set
to indicate the error.
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.
4.3BSD, System V.
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).
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.
adjtimex(2), gettimeofday(2), time(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-06-22 ADJTIME(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface