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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  com-
       pleted 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 capabil-
              ity 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 mono-
       tonically 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-06-22                        ADJTIME(3)