Publishing

The C SDK does neither capture nor encoding. The complete publishing pipeline is: you capture → you encode → feed raw data with rtc_write_sample → the SDK packetizes and sends. Standard flow:

rtc_create_local_track  →  rtc_set_keyframe_request_callback  →  rtc_publish_local_track
        →  rtc_write_sample in a loop  →  rtc_unpublish_local_track  →  rtc_destroy_local_track

rtc_create_local_track

void* rtc_create_local_track(int codec);

Creates a local track and returns the track handle; returns NULL on failure.

Parameter	Description
`codec`	Codec constant: `RTC_CODEC_H264` / `RTC_CODEC_H265` / `RTC_CODEC_VP8` / `RTC_CODEC_VP9` / `RTC_CODEC_AV1` / `RTC_CODEC_OPUS` / `RTC_CODEC_AAC`

Whether the track is audio or video is determined by the codec; you don’t need to specify it separately.

rtc_destroy_local_track

void rtc_destroy_local_track(void* track_handle);

Destroys a local track. A published track should be unpublished with rtc_unpublish_local_track before it’s destroyed. Starting with 0.0.11, it first removes the keyframe request callback and waits for any running keyframe callback to return before returning; after that you can safely free the context passed to rtc_set_keyframe_request_callback. In 0.0.10 and earlier, the keyframe callback may still fire after the track is destroyed.

rtc_publish_local_track

int rtc_publish_local_track(void* handle, void* track_handle, rtc_publish_options_t* options);

Publishes a local track to the channel.

Parameter	Description
`handle`	Instance handle
`track_handle`	Track handle returned by `rtc_create_local_track`
`options`	Publish options, see below

Publish options rtc_publish_options_t

Field	Type	Description
`desc`	`char[64]`	Track description, required and can’t be an empty string, such as `"camera"` / `"screen"` / `"microphone"`
`width`	`int`	Video width, required for video tracks
`height`	`int`	Video height, required for video tracks
`fps`	`int`	Frame rate, required for video tracks
`sample_rate`	`int`	Sample rate, required for audio tracks
`channel_count`	`int`	Number of audio channels, required for audio tracks
`angle`	`int`	Video angle, optional
`bitrate`	`int`	Bitrate (bps), optional
`props`	`const char*`	Custom business properties as a JSON object string, optional

If props fails to parse, the SDK only logs a warning and continues publishing as if there were no custom properties; it doesn’t fail the publish.

Returns

Return value	Meaning
`RTC_OK`	Published successfully
`RTC_INVALID_PARAM`	Invalid handle, invalid track handle, or `options` is `NULL`
`RTC_NOT_CONNECTED`	Not yet joined to the channel
`RTC_ERROR`	Publish failed (missing required parameters, rejected by the engine, negotiation failed, etc.)

rtc_unpublish_local_track

int rtc_unpublish_local_track(void* handle, void* track_handle);

Unpublishes. Return values are the same as above.

rtc_publish_mcu_video_track / rtc_publish_mcu_audio_track

Publishes the channel-level composite stream; see Composite stream (program).

rtc_write_sample

int rtc_write_sample(void* track_handle, uint8_t* data, int length, uint32_t samples);

Pushes one frame of encoded data to a published track.

Parameter	Description
`data`	Encoded raw data. For video, one complete frame (Annex-B NALUs); for audio, one encoded packet
`length`	Data length in bytes
`samples`	RTP timestamp increment—neither a byte count nor milliseconds

Compute samples from each codec’s clock rate:

Codec	Clock rate	Common values
H.264 / H.265 / VP8 / VP9 / AV1	90000	30 fps → `90000/30 = 3000`; 25 fps → `3600`
Opus	48000	20 ms per packet → `48000 * 0.02 = 960`

Returns

Return value	Meaning
`RTC_OK`	Written successfully
`RTC_INVALID_PARAM`	Invalid track handle, or `data` is `NULL` / `length <= 0`
`RTC_ERROR`	Write failed (track not published, underlying send error, etc.)

The SDK copies the data you pass in, so you can reuse or free the data buffer as soon as the call returns.

The SDK doesn’t control send pacing—each frame is sent as soon as it’s written. When your data source is faster than real time (reading a file, TTS synthesis), you must write at the pace of the frame duration (for example, one audio packet every 20 ms). Otherwise several seconds of data go out in an instant, the remote jitter buffer can’t hold it and drops some, and it sounds like swallowed words or sped-up audio.

rtc_set_keyframe_request_callback

typedef void (*rtc_keyframe_request_callback)(void* context, const char* track_id);
void rtc_set_keyframe_request_callback(void* track_handle,
                                       rtc_keyframe_request_callback callback,
                                       void* context);

Sets the keyframe request callback. It fires when the SFU uses RTCP (PLI / FIR) to ask this track to produce a keyframe immediately; in the callback, have the encoder produce an IDR frame right away and send it with rtc_write_sample.

You must set it on the track before publishing. Without it, newly joined viewers don’t see video until the encoder’s next natural GOP—with a long GOP, that’s several seconds of black screen.The callback fires on a separate thread, so synchronize with your encoding thread. Pass NULL to clear the callback.

static void on_keyframe_request(void* ctx, const char* track_id) {
    // Don't do heavy work here; set a flag so the encoding thread encodes an IDR on the next frame
    encoder_request_idr(track_id);
}

void* video = rtc_create_local_track(RTC_CODEC_H264);
rtc_set_keyframe_request_callback(video, on_keyframe_request, NULL);
rtc_publish_local_track(rtc, video, &vopts);   // Publish after setting the callback

Complete example

// 1. Create tracks
void* video = rtc_create_local_track(RTC_CODEC_H264);
void* audio = rtc_create_local_track(RTC_CODEC_OPUS);

// 2. Register the keyframe request callback on the video track (before publishing)
rtc_set_keyframe_request_callback(video, on_keyframe_request, NULL);

// 3. Configure publish options
rtc_publish_options_t vopts = {0};
strcpy(vopts.desc, "camera");
vopts.width = 1280; vopts.height = 720; vopts.fps = 30;

rtc_publish_options_t aopts = {0};
strcpy(aopts.desc, "microphone");
aopts.sample_rate = 48000; aopts.channel_count = 2;

// 4. Publish
rtc_publish_local_track(rtc, video, &vopts);
rtc_publish_local_track(rtc, audio, &aopts);

// 5. Push data
rtc_write_sample(video, h264_frame, h264_len, 3000);  // 30fps
rtc_write_sample(audio, opus_frame, opus_len, 960);   // 20ms

// 6. Clean up
rtc_unpublish_local_track(rtc, video);
rtc_unpublish_local_track(rtc, audio);
rtc_destroy_local_track(video);
rtc_destroy_local_track(audio);

The C interface doesn’t support publishing multi-layer simulcast streams—server-side scenarios (recording / MCU / AI agents) almost never need to publish multiple layers. Multi-layer switching on the subscriber side is done automatically by the SDK with no configuration. If you do need to publish multiple layers, contact us.

Overview

Web SDK

Android SDK

Swift SDK (iOS/macOS)

C SDK (server/embedded)

Python SDK (server-side AI)

Server API

rtc_create_local_track

rtc_destroy_local_track

rtc_publish_local_track

rtc_unpublish_local_track

rtc_publish_mcu_video_track / rtc_publish_mcu_audio_track

rtc_write_sample

rtc_set_keyframe_request_callback

Complete example

​rtc_create_local_track

​rtc_destroy_local_track

​rtc_publish_local_track

​rtc_unpublish_local_track

​rtc_publish_mcu_video_track / rtc_publish_mcu_audio_track

​rtc_write_sample

​rtc_set_keyframe_request_callback

​Complete example

rtc_create_local_track

rtc_destroy_local_track

rtc_publish_local_track

rtc_unpublish_local_track

rtc_publish_mcu_video_track / rtc_publish_mcu_audio_track

rtc_write_sample

rtc_set_keyframe_request_callback

Complete example