#include <errno.h>
#include <sys/types.h>
#include <machine/atomic.h>
#include <sys/umtx.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <ucontext.h>
#include <sys/thr.h>
#include <sys/mman.h>

struct cond
{
	struct umtx	lock;
	int		count;
};

int main()
{
	int error;
	int pid;
	long tid;
	struct cond *p = mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_ANON, -1, 0);

	if (p == MAP_FAILED)
		err(1, "mmap");

	/* set shared before fork()ing and doing any locking on the area. */
	if (minherit(p, 4096, INHERIT_SHARE) == -1)
		err(2, "minherit");
	
	p->lock.u_owner = 0;
	/*
	 * We will let thread sleep on the address,
	 * however the count value is not used by kernel,
	 * kernel only use the address.
	 */
	p->count = 0;
	thr_self(&tid);
	umtx_lock(&p->lock, tid);
	pid = fork();
	if (pid == -1)
		err(3, "fork");

	if (pid == 0) { /* parent */
		printf("parent hold lock, sleep 3 seconds\n");
		sleep(3);
		printf("parent unlocks lock\n");
		umtx_unlock(&p->lock, tid);
		sleep(3);
		umtx_lock(&p->lock, tid);
		printf("parent is signaling condition variable.\n");
		umtx_wake(&p->count, 0);
		umtx_unlock(&p->lock, tid);
	} else { /* child */
		thr_self(&tid);
 		error = umtx_lock(&p->lock, tid);
		if (error)
			err(3, "umtx_lock returns: %d", error);
		printf("child locked mutex, sleeps on condition variable...\n");
		error = umtx_wait(&p->lock, tid, &p->count);
		if (error)
			err(4, "umtx_wait returns: %d", error);
		printf("child return\n");
	}
	return (0);
}