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

       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 al-
       ready 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 at-

       |Interface | Attribute     | Value   |
       |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 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.

       adjtime() is intended to be used to make small adjustments to the  sys-
       tem  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 old-
       delta.  (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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Linux                             2017-09-15                        ADJTIME(3)
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