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FFCLOCK(2)                FreeBSD System Calls Manual               FFCLOCK(2)

NAME
     ffclock_getcounter, ffclock_getestimate, ffclock_setestimate - Retrieve
     feed-forward counter, get and set feed-forward clock estimates

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/timeffc.h>

     int
     ffclock_getcounter(ffcounter *ffcount);

     int
     ffclock_getestimate(struct ffclock_estimate *cest);

     int
     ffclock_setestimate(struct ffclock_estimate *cest);

DESCRIPTION
     The ffclock is an alternative method to synchronise the system clock.
     The ffclock implements a feed-forward paradigm and decouples the
     timestamping and timekeeping kernel functions.  This ensures that past
     clock errors do not affect current timekeeping, an approach radically
     different from the feedback alternative implemented by the ntpd daemon
     when adjusting the system clock.  The feed-forward approach has
     demonstrated better performance and higher robustness than a feedback
     approach when synchronising over the network.

     In the feed-forward context, a timestamp is a cumulative value of the
     ticks of the timecounter, which can be converted into seconds by using
     the feed-forward clock estimates.

     The ffclock_getcounter() system call allows the calling process to
     retrieve the current value of the feed-forward counter maintained by the
     kernel.

     The ffclock_getestimate() and ffclock_setestimate() system calls allow
     the caller to get and set the kernel's feed-forward clock parameter
     estimates respectively.  The ffclock_setestimate() system call should be
     invoked by a single instance of a feed-forward synchronisation daemon.
     The ffclock_getestimate() system call can be called by any process to
     retrieve the feed-forward clock estimates.

     The feed-forward approach does not require that the clock estimates be
     retrieved every time a timestamp is to be converted into seconds.  The
     number of system calls can therefore be greatly reduced if the calling
     process retrieves the clock estimates from the clock synchronisation
     daemon instead.  The ffclock_getestimate() must be used when the feed-
     forward synchronisation daemon is not running (see USAGE below).

     The clock parameter estimates structure pointed to by cest is defined in
     <sys/timeffc.h> as:

     struct ffclock_estimate {
             struct bintime update_time;    /* Time of last estimates update. */
             ffcounter      update_ffcount; /* Counter value at last update. */
             ffcounter      leapsec_next;   /* Counter value of next leap second. */
             uint64_t       period;         /* Estimate of counter period. */
             uint32_t       errb_abs;       /* Bound on absolute clock error [ns]. */
             uint32_t       errb_rate;      /* Bound on counter rate error [ps/s]. */
             uint32_t       status;         /* Clock status. */
             int16_t        leapsec_total;  /* All leap seconds seen so far. */
             int8_t         leapsec;        /* Next leap second (in {-1,0,1}). */
     };

     Only the super-user may set the feed-forward clock estimates.

RETURN VALUES
     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The following error codes may be set in errno:

     [EFAULT]           The ffcount or cest pointer referenced invalid memory.

     [EPERM]            A user other than the super-user attempted to set the
                        feed-forward clock parameter estimates.

USAGE
     The feed-forward paradigm enables the definition of specialised clock
     functions.

     In its simplest form, ffclock_getcounter() can be used to establish
     strict order between events or to measure small time intervals very
     accurately with a minimum performance cost.

     Different methods exist to access absolute time (or "wall-clock time")
     tracked by the ffclock. The simplest method uses the ffclock sysctl
     interface kern.ffclock to make the system clock return the ffclock time.
     The clock_gettime(2) system call can then be used to retrieve the current
     time seen by the feed-forward clock.  Note that this setting affects the
     entire system and that a feed-forward synchronisation daemon should be
     running.

     A less automated method consists of retrieving the feed-forward counter
     timestamp from the kernel and using the feed-forward clock parameter
     estimates to convert the timestamp into seconds.  The feed-forward clock
     parameter estimates can be retrieved from the kernel or from the
     synchronisation daemon directly (preferred).  This method allows
     converting timestamps using different clock models as needed by the
     application, while collecting meaningful upper bounds on current clock
     error.

SEE ALSO
     date(1), adjtime(2), clock_gettime(2), ctime(3)

HISTORY
     Feed-forward clock support first appeared in FreeBSD 10.0.

AUTHORS
     The feed-forward clock support was written by Julien Ridoux
     <[email protected]> in collaboration with Darryl Veitch
     <[email protected]> at the University of Melbourne under sponsorship
     from the FreeBSD Foundation.

FreeBSD 11.1-RELEASE-p4        November 21, 2011       FreeBSD 11.1-RELEASE-p4
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