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NetBSD 6.1.5 - man page for itimerval (netbsd section 3)

TIMEVAL(3)			   BSD Library Functions Manual 		       TIMEVAL(3)

NAME
     timeval, timespec, itimerval, itimerspec, bintime -- time structures

SYNOPSIS
     #include <sys/time.h>

     void
     TIMEVAL_TO_TIMESPEC(struct timeval *tv, struct timespec *ts);

     void
     TIMESPEC_TO_TIMEVAL(struct timeval *tv, struct timespec *ts);

DESCRIPTION
     The <sys/time.h> header, included by <time.h>, defines various structures related to time
     and timers.

      1.   The following structure is used by gettimeofday(2), among others:

		 struct timeval {
			 time_t 	 tv_sec;
			 suseconds_t	 tv_usec;
		 };

	   The tv_sec member represents the elapsed time, in whole seconds.  The tv_usec member
	   captures rest of the elapsed time, represented as the number of microseconds.

      2.   The following structure is used by nanosleep(2), among others:

		 struct timespec {
			 time_t 	 tv_sec;
			 long		 tv_nsec;
		 };

	   The tv_sec member is again the elapsed time in whole seconds.  The tv_nsec member rep-
	   resents the rest of the elapsed time in nanoseconds.

	   A microsecond is equal to one millionth of a second, 1000 nanoseconds, or 1/1000 mil-
	   liseconds.  To ease the conversions, the macros TIMEVAL_TO_TIMESPEC() and
	   TIMESPEC_TO_TIMEVAL() can be used to convert between struct timeval and struct
	   timespec.

      3.   The following structure is used by setitimer(2), among others:

		 struct itimerval {
			 struct timeval  it_interval;
			 struct timeval  it_value;
		 };

      4.   The following structure is used by timer_settime(2), among others:

		 struct itimerspec {
			 struct timespec it_interval;
			 struct timespec it_value;
		 };

	   Both struct itimerval and struct itimerspec are used to specify when a timer expires.
	   Generally, it_interval specifies the period between successive timer expirations.  A
	   value zero implies that the alarm will fire only once.  If it_value is non-zero, it
	   indicates the time left to the next timer expiration.  A value zero implies that the
	   timer is disabled.

      5.   The following structure is used by bintime(9), among others:

		 struct bintime {
			 time_t 	 sec;
			 uint64_t	 frac;
		 };

	   The sec member specifies the time in seconds and frac represents a 64-bit fraction of
	   seconds.  The struct bintime is meant to be used in the kernel only.  It is further
	   described in timecounter(9).

EXAMPLES
     It can be stressed that the traditional UNIX timeval and timespec structures represent
     elapsed time, measured by the system clock (see hz(9)).  The following sketch implements a
     function suitable for use in a context where the timespec structure is required for a condi-
     tional timeout:

	   static void
	   example(struct timespec *spec, time_t minutes)
	   {
		   struct timeval elapsed;

		   (void)gettimeofday(&elapsed, NULL);

		   _DIAGASSERT(spec != NULL);
		   TIMEVAL_TO_TIMESPEC(&elapsed, spec);

		   /* Add the offset for timeout in minutes. */
		   spec->tv_sec = spec->tv_sec + minutes * 60;
	   }

     A better alternative would use the more precise clock_gettime(2).

SEE ALSO
     timeradd(3), tm(3), bintime_add(9)

BSD					  April 12, 2011				      BSD


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