Some platforms' audio APIs are under a non-disclosure agreement, a console's for instance, so their backends cannot live in this repository. Such a backend lives in the developer's own repository and joins the library when it is configured, without a fork.
Point MAUL_AUDIO_PRIVATE_BACKEND at a directory that holds a backend.cmake:
cmake -B build -DMAUL_AUDIO_PRIVATE_BACKEND=/path/to/console-backend
The library's build includes that file once its target exists, so the file adds the backend's sources to maul-audio and links what the platform needs:
target_sources(maul-audio PRIVATE "${CMAKE_CURRENT_LIST_DIR}/console_backend.c")
target_include_directories(maul-audio PRIVATE "${PROJECT_SOURCE_DIR}/src")
target_link_libraries(maul-audio PRIVATE ConsoleAudio::ConsoleAudio)
The sources define one function, maudGetPrivateBackend, which returns the backend's table (maudBackend in src/backend.h) with its kind set to maud_backendPrivate.
With a private backend built in, maud_backendPrivate asks for it by name and maud_backendNative tries it before the platform's own backends. Without one, asking for maud_backendPrivate returns maud_errorUnsupported, as for any backend a build lacks.
src/backend.h is the contract, the same one the backends in this repository follow: each hook's comment says what it must do and which hooks may be NULL, and src/device.h says how a backend reports devices coming and going. It is not a stable interface. A private backend is built against one release of the library and moves with it.
The library's real-time rules bind a private backend as they bind every other: nothing on the audio thread allocates, locks or waits, and the backend starts no thread of its own except, where the platform runs no audio thread, the stream thread src/worker.h provides.
test/private_backend/ is a minimal backend kept outside src/ (one output device, rendered on the caller's thread); CI builds the library with it to prove the slot works from outside the tree.