adjtime(3) — Linux manual pages

adjtime
Prev	Section 3	Next

Name

adjtime — correct the time to synchronize the system clock

Synopsis

`int adjtime(`	const struct timeval *	`delta`,
	struct timeval *	`olddelta)`;

Note

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
`adjtime`():
`_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.

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

Currently, if delta is specified as NULL, no valid information about the outstanding clock adjustment is returned in olddelta. (In this circumstance, adjtime() should return the outstanding clock adjustment, without changing it.) This is the result of a kernel limitation.

COLOPHON

This page is part of release 2.79 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/.

Copyright (c) 2006 by Michael Kerrisk <mtk.manpagesgmail.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.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Prev	Up	Next
addseverity	Home	aio_cancel

struct	timeval {
	time_t	`tv_sec`;	/* seconds */
	suseconds_t	`tv_usec`;	/* microseconds */
};