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.\" $NetBSD: ratecheck.9,v 1.11 2008/04/30 13:10:58 martin Exp $
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.\" Copyright (c) 2000 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Christopher G. Demetriou.
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.Dd February 2, 2000
.Dt RATECHECK 9
.Os
.Sh NAME
.Nm ratecheck
.Nd function to help implement rate-limited actions
.Sh SYNOPSIS
.In sys/time.h
.Ft int
.Fn ratecheck "struct timeval *lasttime" "const struct timeval *mininterval"
.Sh DESCRIPTION
The
.Fn ratecheck
function provides a simple time interval check which can be used
when implementing time-based rate-limited actions.
If the difference between the current monotonically-increasing
system time
.Pq Va mono_time
and
.Fa lasttime
is less than the value given by the
.Fa mininterval
argument, zero is returned.
Otherwise,
.Fa lasttime
is set to the current time and a non-zero value is returned.
.Pp
The motivation for implementing
.Fn ratecheck
was to provide a mechanism that could be used to add rate limiting to
diagnostic message output.
If printed too often, diagnostic messages can keep the system from
doing useful work.
If the repeated messages can be caused by deliberate user action
or network events, they can be exploited to cause denial of system service.
.Pp
Note that using a very short time interval (less than a second)
for
.Fa mininterval
defeats the purpose of this function.
(It doesn't take much to flood a 9600 baud serial console with
output, for instance.)
.Sh EXAMPLES
Here is a simple example of use of the
.Fn ratecheck
function:
.Bd -literal
/*
 * The following variables could be global, in a device softc, etc.,
 * depending on the exact usage.
 */
struct timeval drv_lasterr1time;   /* time of last err1 message */
long drv_err1count;                /* # of err1 errs since last msg */
struct timeval drv_lasterr2time;   /* time of last err2 message */
long drv_err2count;                /* # of err2 errs since last msg */

/*
 * The following variable will often be global or shared by all
 * instances of a driver.  It should be initialized, so it can be
 * patched.  Allowing it to be set via an option might be nice,
 * but could lead to an insane proliferation of options.
 */
struct timeval drv_errintvl = { 5, 0 };         /* 5 seconds */

/* error handling/reporting function */
void
drv_errhandler(int err1, int err2)
{

	/*
	 * Note that you should NOT use the same last-event
	 * time variable for dissimilar messages!
	 */
	if (err1) {
		/* handle err1 condition */
		...

		drv_err1count++;
		if (ratecheck(\*[Am]drv_lasterr1notice,
		    \*[Am]drv_errinterval)) {
			printf("drv: %ld err1 errors occurred",
			    drv_err1count);
			drv_err1count = 0;
		}
	}
	if (err2) {
		/* handle err2 condition */
		...

		drv_err2count++;
		if (ratecheck(\*[Am]drv_lasterr2notice,
		    \*[Am]drv_errinterval)) {
			printf("drv: %ld err2 errors occurred",
			    drv_err2count);
			drv_err2count = 0;
		}
	}
}
.Ed
.Sh SEE ALSO
.Xr log 9 ,
.Xr ppsratecheck 9 ,
.Xr printf 9 ,
.Xr time_second 9
.Sh HISTORY
The
.Fn ratecheck
function appeared in
.Nx 1.5 .
.Sh BUGS
.Fn ratecheck
may not work as expected, if
.Fa mininterval
is less than the hardware clock interrupt interval
.Pq Li 1/hz .