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 previ-
       ous 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
       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 3.25 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)