adjtime(3) — Linux manual page

adjtime(3)              Library Functions Manual              adjtime(3)

NAME

       adjtime - correct the time to synchronize the system clock

LIBRARY

       Standard C library (libc, -lc)

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 │
       └─────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       None.

HISTORY

       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 could 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

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Linux man-pages 6.9.1          2024-05-02                     adjtime(3)

Pages that refer to this page: adjtimex(2), clock_getres(2), timeval(3type), time(7)

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