libink is a small C library implementing the D-Bus wire protocol, both the server and the client side, without a broker and without any dependency on libdbus, sd-bus, or GIO. It was born inside Finit to let PID 1 be a bus of its own: clients connect straight to the listening socket, authenticate with the standard SASL EXTERNAL handshake, and get kernel-authenticated credentials via SO_PEERCRED.

For what the bus exposes and how to talk to it, see the User Guide, D-Bus Integration. This file covers the library itself.

libink is an internal implementation detail of Finit: built as a libtool convenience library, linked statically into finit and initctl, nothing installed. There is deliberately no ABI promise yet — that comes if/when libink is extracted into a project of its own. External D-Bus clients need none of this; the wire protocol is the compatibility surface, any standard D-Bus library works.

Public API symbols carry the link_* prefix (link.h), internal ones __* (internal.h). Method handlers are registered as vtables of link_method_t/link_property_t; the framework emits variant signatures from the property table so the declared type is the single source of truth.

The boundary to Finit is deliberate: nothing under libink/ includes a Finit header. All glue lives in src/dbus.c — object registration, signal emission from the service/condition/runlevel hook points, and the uev event loop bridge. initctl uses the client half of the same library, so one wire-format implementation serves both ends. If libink is ever spun out, that file is the cut line.

Privileged methods over the system bus are root-only. Finit learns the caller's uid from the broker with GetConnectionUnixUser, but that reply carries no group list, so it cannot honour --with-group membership there the way it does on the local bus (where the kernel hands over the group set via SO_PEERGROUPS). Closing the gap means asking the broker GetConnectionCredentials and reading its UnixGroupIDs, which is a variant holding an au array — the reader (marshal.c) only decodes single-character variant signatures today, so it needs extending first. Finit's own direct users reach it over the local bus, so this has not been pressing.

The test/dbus-*.sh suite exercises the library end to end against a live Finit in a namespace, driven by test/src/dbus-auth-client.c. Wire-format conformance against third-party tools (dbus-send, dbus-monitor) and fuzzing of the parsers are tracked as pre-merge work — this is PID 1's attack surface.