#include <sys/atomic.h>
#include <sys/wait.h>

#include <assert.h>
#include <err.h>
#include <errno.h>
#include <pthread.h>
#include <unistd.h>

volatile unsigned lock = 0;
__thread unsigned locks = 0;

static void *
start(void *cookie)
{
	pid_t pid;

	for (;;) {
		while (atomic_cas_uint(&lock, 0, 1) != 0)
			continue;
		locks++;
		pid = fork();
		if (--locks)
			errx(1, "lock count");
		if (atomic_cas_uint(&lock, 1, 0) != 1)
			errx(1, "lost lock");
		switch (pid) {
		case -1:	/* error */
			err(1, "fork");
		case 0:		/* child */
			_exit(0);
		default:	/* parent */
			break;
		}
		if (wait4(pid, NULL, 0, NULL) == -1)
			err(1, "wait4");
	}
}

int
main(void)
{
	pthread_t t;
	unsigned n = 100;
	int error;

	while (n --> 0) {
		error = pthread_create(&t, NULL, &start, NULL);
		if (error)
			errc(1, error, "pthread_create");
	}

	(void)pthread_join(t, NULL);
	return 0;
}