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 represents the rest of the elapsed time in nanoseconds.

   A microsecond is equal to one millionth of a second, 1000 nanoseconds, or 1/1000 milliseconds. 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 conditional 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)