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Linux 2.6 - man page for adjtime (linux section 3)

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

       adjtime - correct the time to synchronize the system clock

       int adjtime(const struct timeval *delta, struct timeval *olddelta);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       adjtime(): _BSD_SOURCE

       The  adjtime()  function  gradually  adjusts  the  system clock (as returned by gettimeof-
       day(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 adjust-
       ment 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.

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

       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.

   Multithreading (see pthreads(7))
       The adjtime() function is thread-safe.

       4.3BSD, System V.

       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  sys-
       tems impose a limit on the adjustment that can be specified in delta.  In the glibc imple-
       mentation, 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).

       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,  adj-
       time()  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.

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

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2013-10-07				       ADJTIME(3)

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