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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 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 3.24 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/.