Function kqueue_register

freebsd/kern/kern_event.c:1376–1636

* A ref to kq (obtained via kqueue_acquire) must be held. */

 * A ref to kq (obtained via kqueue_acquire) must be held.
 */
static int
kqueue_register(struct kqueue *kq, struct kevent *kev, struct thread *td,
    int mflag)
{
	struct filterops *fops;
	struct file *fp;
	struct knote *kn, *tkn;
	struct knlist *knl;
	int error, filt, event;
	int haskqglobal, filedesc_unlock;

	if ((kev->flags & (EV_ENABLE | EV_DISABLE)) == (EV_ENABLE | EV_DISABLE))
		return (EINVAL);

	fp = NULL;
	kn = NULL;
	knl = NULL;
	error = 0;
	haskqglobal = 0;
	filedesc_unlock = 0;

	filt = kev->filter;
	fops = kqueue_fo_find(filt);
	if (fops == NULL)
		return EINVAL;

	if (kev->flags & EV_ADD) {
		/*
		 * Prevent waiting with locks.  Non-sleepable
		 * allocation failures are handled in the loop, only
		 * if the spare knote appears to be actually required.
		 */
		tkn = knote_alloc(mflag);
	} else {
		tkn = NULL;
	}

findkn:
	if (fops->f_isfd) {
		KASSERT(td != NULL, ("td is NULL"));
		if (kev->ident > INT_MAX)
			error = EBADF;
		else
			error = fget(td, kev->ident, &cap_event_rights, &fp);
		if (error)
			goto done;

		if ((kev->flags & EV_ADD) == EV_ADD && kqueue_expand(kq, fops,
		    kev->ident, M_NOWAIT) != 0) {
			/* try again */
			fdrop(fp, td);
			fp = NULL;
			error = kqueue_expand(kq, fops, kev->ident, mflag);
			if (error)
				goto done;
			goto findkn;
		}