Re: [Libevent-users] a dead looping bug when changing system time backward

[William Ahern <william@xxxxxxxxxxxxxxxxxx>]

On Tue, Apr 10, 2012 at 11:44:39AM +0800, ååé wrote:
> Hi all,
> 
> I found an issue that it will trigger the timer callback dramatically
> like a dead looping when moving the system time backward for a year
> and then moving it forward to the accurate time.

AFAICT, the libevent 2.x code uses clock_gettime(CLOCK_MONOTONIC) if
available.

Does your system provide CLOCK_MONOTONIC? Most do (including the ones I use
the most, Linux and OpenBSD). One notable exception is OS X. But OS X _does_
provide a monotonic counter. If you're using OS X, try this compat wrapper:

/*
 * clock_gettime()
 *
 * OS X doesn't implement the clock_gettime() POSIX interface, but does
 * provide a monotonic clock through mach_absolute_time(). On i386 and
 * x86_64 architectures this clock is in nanosecond units, but not so on
 * other devices. mach_timebase_info() provides the conversion parameters.
 *
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
#if __APPLE__

#include <time.h>            /* struct timespec */

#include <errno.h>           /* errno EINVAL */

#include <sys/time.h>        /* TIMEVAL_TO_TIMESPEC struct timeval gettimeofday(3) */

#include <mach/mach_time.h>  /* mach_timebase_info_data_t mach_timebase_info() mach_absolute_time() */


#define CLOCK_REALTIME  0
#define CLOCK_VIRTUAL   1
#define CLOCK_PROF      2
#define CLOCK_MONOTONIC 3

static mach_timebase_info_data_t clock_timebase = {
	.numer = 1, .denom = 1,
}; /* clock_timebase */

void clock_gettime_init(void) __attribute__((constructor));

void clock_gettime_init(void) {
	if (mach_timebase_info(&clock_timebase) != KERN_SUCCESS)
		__builtin_abort();
} /* clock_gettime_init() */

static int clock_gettime(int clockid, struct timespec *ts) {
	switch (clockid) {
	case CLOCK_REALTIME: {
		struct timeval tv;

		if (0 != gettimeofday(&tv, 0))
			return -1;

		TIMEVAL_TO_TIMESPEC(&tv, ts);

		return 0;
	}
	case CLOCK_MONOTONIC: {
		unsigned long long abt;

		abt = mach_absolute_time();
		abt = abt * clock_timebase.numer / clock_timebase.denom;

		ts->tv_sec = abt / 1000000000UL;
		ts->tv_nsec = abt % 1000000000UL;

		return 0;
	}
	default:
		errno = EINVAL;

		return -1;
	} /* switch() */
} /* clock_gettime() */

#endif /* __APPLE__ */

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