Reference index

10. XSUB

An extended subscriber with subscription forwarding upstream. XSUB exposes exactly the same entry points as SUB — zlink_set_sub_option/zlink_get_sub_option, zlink_set_subscription/zlink_unset_subscription, zlink_subscribe_part, and zlink_subscription_at — with no XSUB-specific addition. This category exists only so XSUB has a place in the taxonomy; see the SUB category for every entry, and the XSUB specification for the exact contract (identical in substance to the SUB specification, restated for the XSUB subject).

The one distinction from SUB is architectural rather than a different entry point: XSUB forwards subscribe/unsubscribe messages upstream (to a paired XPUB, typically through a proxy), where SUB does not expose that forwarding. No additional function reflects this — it is a wire-level behavior of the socket type, not part of its public API.