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NetBSD 6.1.5 - man page for ratecheck (netbsd section 9)

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RATECHECK(9)			  BSD Kernel Developer's Manual 		     RATECHECK(9)

NAME
     ratecheck -- function to help implement rate-limited actions

SYNOPSIS
     #include <sys/time.h>

     int
     ratecheck(struct timeval *lasttime, const struct timeval *mininterval);

DESCRIPTION
     The ratecheck() function provides a simple time interval check which can be used when imple-
     menting time-based rate-limited actions.  If the difference between the current monotoni-
     cally-increasing system time (mono_time) and lasttime is less than the value given by the
     mininterval argument, zero is returned.  Otherwise, lasttime is set to the current time and
     a non-zero value is returned.

     The motivation for implementing 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.

     Note that using a very short time interval (less than a second) for mininterval defeats the
     purpose of this function.	(It doesn't take much to flood a 9600 baud serial console with
     output, for instance.)

EXAMPLES
     Here is a simple example of use of the ratecheck() function:

     /*
      * 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(&drv_lasterr1notice,
			 &drv_errinterval)) {
			     printf("drv: %ld err1 errors occurred",
				 drv_err1count);
			     drv_err1count = 0;
		     }
	     }
	     if (err2) {
		     /* handle err2 condition */
		     ...

		     drv_err2count++;
		     if (ratecheck(&drv_lasterr2notice,
			 &drv_errinterval)) {
			     printf("drv: %ld err2 errors occurred",
				 drv_err2count);
			     drv_err2count = 0;
		     }
	     }
     }

SEE ALSO
     log(9), ppsratecheck(9), printf(9), time_second(9)

HISTORY
     The ratecheck() function appeared in NetBSD 1.5.

BUGS
     ratecheck() may not work as expected, if mininterval is less than the hardware clock inter-
     rupt interval (1/hz).

BSD					 February 2, 2000				      BSD
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