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 4.09 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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