3.4 KiB
0009 — Parameter processors and bounded LADSPA hosting
Status: accepted, Phases 14–15 (2026-10-27).
Native ABI 1 remains compatible. ABI 2 adds explicitly parameter-only configuration, metadata, validation and sample-offset parameter/MIDI ordering. It is a public C facility; opaque state and automatic native descriptor registration in project files remain unsupported. Native instrument and gain examples exercise the interface before adding a dynamically loaded module lifetime.
Select LADSPA 1.1 for the first external host. Reconsidering the Phase 9 candidates, LV2 needs RDF/features/URIDs/workers, CLAP needs host scheduling/extension negotiation, and VST3 needs a separate C++ SDK adapter and release-specific license review. LADSPA adds a mature Linux C ABI, installed real plugins, straightforward port negotiation and no RDF/SDK runtime library. Its specification is old and stable, not an actively expanding instrument/event ecosystem. This trades MIDI, editors, opaque state and modern automation for a small testable first host. No general multi-format abstraction is introduced.
Use the installed LGPL-2.1-or-later header, system dynamic loader (-ldl), and
user-selected, trusted module files. Neither the header nor plugin binaries are
vendored. Header absence or XTC_DISABLE_LADSPA yields unsupported hosting stubs;
metadata can still be loaded/saved. Plugin licenses are independent of the host
and require review before distribution. See LICENSES.md.
Support zero/one mono audio input and exactly one mono output, at most 64 ports and 16 input controls. Graph stereo consists of independent instances, not a stereo LADSPA layout. Reject incompatible layouts and identities explicitly. Control outputs are connected to private storage, not exposed as graph events. LADSPA has no MIDI or opaque state API: save control values only. Parameter changes rebuild plans on the control thread; sample automation is deferred to Phase 16. Rate-scaled bounds are recalculated at compile. Instance history/tails reset on recompile, not every transport seek (LADSPA has no realtime reset rule). Latency compensation is unsupported: the supported set is zero-latency amp, inverter and sine source, not arbitrary lookahead/delay/routing plugins.
Discovery, dlopen, descriptor calls, instantiate, connect, activate, deactivate, cleanup and dlclose are control-thread operations. Only preallocated copies and run execute in audio; the usual pending/retired plan handoff owns module handles. Partial initialization unwinds on the control thread. The host cannot enforce third-party callback behavior, bound CPU time, catch native crashes/hangs, or undo memory corruption. There is no sandbox or crash recovery inside the audio process. A fault may kill xtc; previously saved documents/recording recovery files remain subject to their ordinary durability guarantees. Test only trusted modules.
Project v5 stores plain identity/layout/control metadata. Parsing never opens modules. Compile failure preserves the source document and active plan. GUI Open rejects missing/incompatible modules, retains its current document, and reports saved module identity/path. Restore the compatible module at that path and retry; a metadata-only C client can preserve/save the document without the module. No silent replacement or deletion occurs. Same-ID/label/ports do not authenticate binaries or guarantee unchanged sound across plugin upgrades.