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       adjtime - correct the time to synchronize the system clock


       #include <sys/time.h>

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

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

           Since glibc 2.19:
           Glibc 2.19 and earlier:


       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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │adjtime() │ Thread safety │ MT-Safe │


       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 kernel 2.6.26 or later.


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


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