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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 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).

BUGS

     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.