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

       adjtime - correct the time to synchronize the system clock

       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 gettime-
       ofday(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  com-

       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 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).

       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 ker-
       nel 2.6.26 or later.

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2008-06-22                        ADJTIME(3)

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