A kqueue(2) example, listens to VFS mount/umount events.

mount_wait.c

#include <sys/types.h>
#include <sys/event.h>
#include <sys/mount.h>

#include <stdio.h>

int main() {
	int kq = kqueue();
	struct kevent ki[1];

	EV_SET(ki, 0, EVFILT_FS, EV_ADD, 0, 0, 0);
	kevent(kq, ki, 1, NULL, 0, NULL);
	puts("started");

	for (;;) {
		switch (kevent(kq, NULL, 0, ki, 1, NULL)) {
		case -1:
			perror("kevent");
		case 0:
			break;
		default:
			if (ki->fflags & VQ_MOUNT)
				puts("mount");
			else if (ki->fflags & VQ_UNMOUNT)
				puts("umount");
		}
	}

	return 0;
}