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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       adjtime(): _BSD_SOURCE

       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.

       On success, adjtime() returns 0.  On failure, -1 is returned, and errno
       is set to indicate the error.

       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.

       4.3BSD, System V.

       The adjustment that adjtime() makes to the clock is carried out in such
       a manner that the clock is always monotonically increasing.  Using adj-
       time()  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.

       adjtimex(2), gettimeofday(2), time(7)

       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux                             2008-06-22                        ADJTIME(3)
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