adjtime

ADJTIME(3)                 Linux Programmer's Manual                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 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.

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

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).

       ┌──────────┬───────────────┬─────────┐
       │Interface Attribute     Value   │
       ├──────────┼───────────────┼─────────┤
       │adjtime() │ 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 5.03 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/.



Linux                             2017-09-15                        ADJTIME(3)