ADJTIME(3)						     Linux Programmer's Manual							ADJTIME(3)

NAME

       adjtime - correct the time to synchronize the system clock

SYNOPSIS

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

   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 nega-
       tive 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 3.27 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)