test_time.c

#include <errno.h>
#include <error.h>
#include <time.h>
#include <stdio.h>
#include <unistd.h>

/*
 * lhs < rhs:  return <0
 * lhs == rhs: return 0
 * lhs > rhs:  return >0
 */
static inline int timespec_compare(const struct timespec *lhs, const struct timespec *rhs)
{
        if (lhs->tv_sec < rhs->tv_sec)
                return -1;
        if (lhs->tv_sec > rhs->tv_sec)
                return 1;
        return lhs->tv_nsec - rhs->tv_nsec;
}

#define test_time(clock) do { \
	struct timespec new, old = { 0 }; \
	while (1) { \
		int err = clock_gettime(clock, &new); \
		if (err) { \
			error(0, errno, #clock); \
		} else if (timespec_compare(&new, &old) < 0) { \
			printf(#clock ": new %lu %lu < old %lu %lu\n", \
				   new.tv_sec, new.tv_nsec, old.tv_sec, old.tv_nsec); \
		} \
		old = new; \
	} \
} while (0)

int main()
{
	if (fork()) {
		if (fork())
			test_time(CLOCK_MONOTONIC_RAW);
		else
			test_time(CLOCK_MONOTONIC);
	} else {
		test_time(CLOCK_REALTIME);
	}
	return 0;
}