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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. */
};
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
<jridoux@unimelb.edu.au> in collaboration with Darryl Veitch
<dveitch@unimelb.edu.au> at the University of Melbourne under sponsorship
from the FreeBSD Foundation.
FreeBSD 14.0-RELEASE-p11 November 21, 2011 FreeBSD 14.0-RELEASE-p11