NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       adjtime - correct the time to synchronize the system clock

SYNOPSIS         top

       #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         top

       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         top

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

ERRORS         top

       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         top

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

       ┌──────────┬───────────────┬─────────┐
       │Interface Attribute     Value   │
       ├──────────┼───────────────┼─────────┤
       │adjtime() │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘

CONFORMING TO         top

       4.3BSD, System V.

NOTES         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 4.07 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                            2016-03-15                       ADJTIME(3)