ADJTIME(3)                (2017-09-15)                 ADJTIME(3)

     NAME
          adjtime - correct the time to synchronize the system clock

     SYNOPSIS
          #include <sys/time.h>

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

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

          adjtime():
              Since glibc 2.19:
                  _DEFAULT_SOURCE
              Glibc 2.19 and earlier:
                  _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 struc-
          ture 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 pre-
          vious 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.

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     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.

     ATTRIBUTES
          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lb lb lb l l l.
          Interface Attribute Value T{ adjtime() T}   Thread
          safety  MT-Safe

     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 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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