picotimer.c

#define _POSIX_C_SOURCE 200809L

#include <stdatomic.h>
#include <stddef.h>
#include <stdio.h>

/*
#include <windows.h>
*/
#include <pthread.h>
#include <time.h>

static atomic_size_t ticks = 0;

static void *
timer(void *a)
{
	size_t t = 0;

	/* do not use -funroll-all-loops */
	for (;;) {
		atomic_store_explicit(&ticks, t, memory_order_release);
		t++;
		atomic_store_explicit(&ticks, t, memory_order_release);
		t++;
		atomic_store_explicit(&ticks, t, memory_order_release);
		t++;
		atomic_store_explicit(&ticks, t, memory_order_release);
		t++;
	}
}

int
main(void)
{
	pthread_t timer_pid;
	pthread_create(&timer_pid, NULL, timer, NULL);
	/*
	DWORD timer_pid;
	CreateThread(NULL, 0, timer, NULL, 0, &timer_pid);
	*/

	struct timespec old_time;
	clock_gettime(CLOCK_MONOTONIC, &old_time);

	size_t old_tick = 0;

	for (;;) {
		size_t cur_tick = atomic_load_explicit(&ticks, memory_order_acquire);
		if (cur_tick - old_tick < 10000) {
			continue;
		}

		struct timespec cur_time;
		clock_gettime(CLOCK_MONOTONIC, &cur_time);

		double elapsed = (double) (cur_time.tv_sec - old_time.tv_sec)
			+ (cur_time.tv_nsec / 1e9 - old_time.tv_nsec / 1e9);
		double time_unit = elapsed / (cur_tick - old_tick);

		printf("one tick = %.12f seconds\n", time_unit);

		old_time = cur_time;
		old_tick = cur_tick;
	}
}