C

moq-c exposes MoQ to C, C++, and any language with a C FFI through a stable ABI: a generated moq.h, a static libmoq.a that links the whole Rust runtime in, and a pkg-config file for its native link dependencies. The OBS plugin is built on it. It was published as libmoq through 0.6; the header and library file names did not change.

Install

Each moq-c-v* release ships moq-c-<version>-<target>.tar.gz with include/moq.h, lib/libmoq.a, and a pkg-config file listing the system libraries the archive needs. They are a matched pair: compile against the header that came with the archive you link. Targets: Linux x86_64 and aarch64, macOS arm64, Windows x64.

export PKG_CONFIG_PATH="moq-c-$ver-$target/lib/pkgconfig"
cc app.c $(pkg-config --cflags --libs --static moq-c) -o app

With CMake, point CMAKE_PREFIX_PATH at the extracted archive, then find_package(moq-c) and link moq::c.

From source, add_subdirectory(rs/moq-c) in CMake gives the same moq::c target. A bare cargo build --release -p moq-c writes target/release/libmoq.a, and moq.h lands in the build script's OUT_DIR under include/, a hashed path that --message-format=json reports as out_dir. It writes no moq-c.pc; nix build .#moq-c produces the same install layout as the release tarball, pkg-config file included.

Shape of the API

• Handles and callbacks. Every object is an integer handle; every async result arrives on a required moq_status_callback with a void *user_data. A status > 0 is a live result, 0 a clean close, < 0 an error, and the last two are terminal: moq-c never touches user_data again, so free it there. *_cancel only requests shutdown; the terminal callback still fires.
• Errors. Negative return codes named by MOQ_ERROR_* in moq.h, with moq_error() giving the reason for the last failure on the calling thread. Auth rejections (401, 403) have their own codes so you don't retry them. A protocol failure also fills moq_error_protocol() with the session or stream scope, the verbatim wire code, and a known kind; do not parse moq_error() for that.
• Threading. Any function from any thread. Raw publish calls block until the codec takes the frame, which paces a publisher.
• Connection health. moq_session_stats() reports available metrics with per-field validity flags. moq_session_snapshot() samples those metrics and the negotiated draft name together from the same connection. Its protocol string is backed by static storage. Both return an offline error between reconnects and leave the destination untouched. moq_session_bandwidth() mints an allocator over the send estimate; moq_bandwidth_reserve claims a share for an app-owned track, and moq_encode_video / moq_encode_audio take the same handle so the built-in video encoder follows the grant.
• Raw playback. Raw audio and video consumers start at the newest cached group when opened, so rebuilding a live decoder skips the retained backlog.
• Raw audio encode. moq_audio_encoder_output.codec names the codec: "opus", "pcm", or "aac". frame_duration_us sets the Opus frame length: 2500, 5000, 10000, 20000, 40000, or 60000, with 0 meaning the codec's default (20 ms for Opus, 1024 samples for AAC). AAC-LC encodes through the platform's encoder, so a host without one refuses it.
• Audio channel layouts. A channels count also names the speaker layout, by the WAVE convention: 1 is mono, 2 stereo, 3 2.1, 4 quad, 5 5.0, 6 5.1, 7 6.1, and 8 7.1, interleaved front left, front right, center, LFE, back, then side. moq_decode_audio remixes to the count you ask for; past 8 channels the samples pass through but can't be remixed.
• Raw decode output. moq_video_decoder_output selects the decoded CPU pixel format (MOQ_VIDEO_PIXEL_FORMAT_I420 or _RGBA) and target size (width/height, both zero for native; otherwise even and non-zero). Unknown formats and invalid sizes fail moq_decode_video before subscribing.
• Decoded frames. Each frame id owns its decoded picture until moq_decode_video_frame_free, including after moq_decode_video_cancel and the terminal callback. The first moq_decode_video_frame call converts it to the requested layout and size; its data pointer then stays put until the id is freed. A conversion failure returns a negative code for that frame only. Free frames promptly: held frames hold decoder buffers, and a decoder whose buffers are all held stalls.
• Encoded video metadata. moq_video_init.hint is a zero-initialized moq_video_hint with has_* flags for coded dimensions, bitrate (bits per second), frame rate, and latency preference. Hints seed a video codec track's catalog; detected dimensions take precedence.
• Client config. A zeroed moq_client_config means the defaults for every knob, which is what lets a new one be appended without disturbing callers. Fields cover protocol (versions), TLS (tls_fingerprints, tls_roots, tls_cert/_key, tls_host_name), transport (bind, connect_timeout_us, failover_delay_us/resolution_delay_us for Happy Eyeballs, websocket_enabled/_delay_us), and tuning (reconnect backoff, quic_*). Every duration is in microseconds. A knob whose default isn't zero carries a has_* flag, so setting backoff_timeout_us = 0 needs has_backoff_timeout = true to mean "retry forever" rather than "use the default". moq_client_defaults() reports what a NULL config dials with.
• Server. moq_server_listen binds before it returns (a bad address or certificate fails there) and hands each incoming session to on_request as a request handle. Read moq_session_request_path and _query to route and authenticate, then moq_session_request_accept (a session handle, with origins like moq_session_connect) or moq_session_request_reject with an HTTP-style code (401 and 403 become the protocol's unauthorized close). An accepted session reports 1 once SETUP completes and never reconnects. moq_server_addr reports an ephemeral port and moq_server_fingerprints the hashes a client pins for a tls_generate certificate. moq_server_close stops listening; its terminal callback fires once the sockets are released.
• Demand. A watcher on a published track (moq_publish_track_demand, moq_publish_media_demand, moq_encode_video_demand, moq_encode_audio_demand) calls on_demand with MOQ_DEMAND_USED or MOQ_DEMAND_UNUSED right away and again on every change, so an encoder on a battery-powered device runs only while someone is watching. The first call is the current state, so a track that went unused before the watcher existed still reports it. moq_publish_demand_cancel stops it; the terminal callback still fires. A container has no single demand and is refused. Demand follows the last real subscriber: an origin that served the track drops its source copy on the unused edge, so a cache linger never delays it. Only a relay keeps what it already delivered warm for 30 seconds, and a returning subscriber is served that cache only once the publisher confirms it is still current.
• Requests. moq_publish_dynamic serves subscriptions to tracks the broadcast never declared: each arrives as a request handle, read its name with moq_track_request_name, then moq_track_request_accept (a raw track handle), moq_track_request_video / _audio (the media handle moq_publish_video / _audio return), or moq_track_request_abort with an application code the subscriber sees. Without a live handler an unknown name is refused. moq_publish_track_dynamic does the same for fetches of groups a track no longer has cached, delivered as moq_group_request_* (sequence, priority, frame_start); moq_group_request_accept starts the producer at frame_start so written frames keep their group indices. Register it with moq_track_request_dynamic before accepting a track that was itself requested by a fetch, so that pending group survives the transition. Both handlers stop with moq_publish_dynamic_cancel.
• Everything the bindings can do (list): media publish and consume with the catalog managed for you, raw pixels and PCM with the codec inside (moq_encode_video, moq_encode_audio, and the moq_decode_* mirrors), raw tracks with timestamps and datagrams, JSON and binary data tracks (snapshot or stream, each advertised in the catalog for as long as it lives), group fetch, catalog sections, shared video properties, and stalled hints. The three advertising operations are moq_origin_create_broadcast (unannounced producer, invisible to everyone), moq_publish_announce / moq_publish_unannounce (exact-path advertisement), moq_publish_close (ends the broadcast for good and releases its handle), and moq_origin_dynamic (a claim over a path prefix and everything beneath it; "" for everything). A route is a capability, not an inventory. moq_origin_announced takes a moq_announce_config (NULL or zeroed for everything): a literal prefix and an optional relative pattern filter; each moq_announce_event has a kind (MOQ_ANNOUNCE_KIND_START, _UPDATE, _END, or _LIVE once every route live when the listener started has been delivered, with no prefix or captures); prefix stays relative to the origin, while captures reports what each wildcard matched when has_captures is true. Paths with a .-prefixed segment below the prefix are hidden; set hidden or name the dot segment in prefix to list them.

moq_client_config config;
memset(&config, 0, sizeof(config));   // zeroed means "the defaults", every field

moq_string fp = { hex_sha256, strlen(hex_sha256) };
config.tls_fingerprints = &fp;        // trust one self-signed relay
config.tls_fingerprints_len = 1;

int session = moq_session_connect(url, url_len, &config, origin, 0, on_status, user_data);
if (session < 0)
    return fail(moq_error());

For a locally encoded media track, call moq_publish_media_flush(media, timestamp_us) after moq_publish_media_frame with the same broadcast-clock PTS. The monotonic handoff time is sampled inside moq-c. Do not call it for file, pipe, or network imports; those remain clock-free. Invalid handles and unrepresentable timestamps return a negative error code.

Call moq_publish_media_discontinuity(media) when the source seeks, pauses, or changes its time base. It publishes a timeline marker and restarts handoff measurement without lowering advertised jitter. Resume with timestamps that continue forward on the broadcast media clock; this does not permit timestamp rewinds. On a video track, resume with a keyframe: a delta frame before it fails.

Connection stats

Every field in moq_connection_stats carries a matching <field>_valid flag, false when the transport backend does not report it (a false flag is not the same as zero). Native QUIC reports every metric; the browser WebTransport reports few or none.

Field	Unit	Meaning
rtt_us	microseconds	Smoothed round-trip time.
estimated_send_rate_bps	bits per second	Send bandwidth from the congestion controller.
estimated_recv_rate_bps	bits per second	Receive bandwidth from MoQ PROBE.
bytes_sent	bytes	Total sent, including retransmissions and overhead.
bytes_received	bytes	Total received, including duplicates and overhead.
bytes_lost	bytes	Total lost, detected via retransmission or acknowledgement.
packets_sent	datagrams	Total datagrams sent.
packets_received	datagrams	Total datagrams received.
packets_lost	datagrams	Total datagrams detected as lost.

The header is the reference; each function carries a doc comment. Source and a worked example: rs/moq-c, API docs on docs.rs/moq-c.

Raw track publisher metadata has an optional maximum age. Omitting it imposes no publisher age limit; zero keeps the live edge. Local cache limits still apply, and media imports explicitly retain 30 seconds. See publisher retention.