That's actually accept_filter(9). The man page for freebsd has a interesting info: The accept filter concept was pioneered by David Filo at Yahoo! and

That's actually accept_filter(9). The man page for freebsd has a interesting info: The accept filter concept was pioneered by David Filo at Yahoo! and refined to be a loadable module system by Alfred Perlstein. The closest you can get by using kqueue is to set a low water mark, so that a read event is only returned when there's enough data ready.