Roll dice for a while in C

dice.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

unsigned long long counts[6] = {0};
unsigned long long trials = 0;

void
roll_dice(void)
{
	/* Does not check for overflows. If you think you're going to do more
	 * than ULLONG_MAX trials, it's your responsibility to check! */
	int rmax = RAND_MAX >> 3;
	int r = rand();
	do {
		counts[r & 7] += !((r & 6) == 6);
		trials += !((r & 6) == 6);
		r >>= 3;
		rmax >>= 3;
	} while (rmax);
}

void
print_table(void)
{
	int i;
	printf("%llu trials:\n", trials);
	for (i = 0; i < 6; ++i) {
		printf("%d: %llu (%0.4f)\n",
		       i, counts[i], (float) counts[i] / trials);
	}
}

void
trial_time(int runtime)
{
	int trials_per_grain;
	int i;

	unsigned long long start_trials = trials;
	clock_t start_clock = clock();

	/* Run initially for 10ms, then approximate time target time at 10ms
	 * granularity. This is much faster than checking the clock every
	 * iteration. */
	while (clock() - start_clock < CLOCKS_PER_SEC / 100)
		roll_dice();
	trials_per_grain = trials - start_trials;

	while (clock() - start_clock < CLOCKS_PER_SEC * runtime) {
		for (i = 0; i < trials_per_grain; ++i)
			roll_dice();
	}
}

int
main(void)
{
	trial_time(1);
	print_table();
}