> ## Documentation Index
> Fetch the complete documentation index at: https://docs.stmlink.com/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> 对外开放的服务端接口有两组前缀，都用同一套鉴权：`/server/v1/...`（SRTC 与 SMeeting 的主接口）和 `/stm/srvapi/v1/...`（SMeeting 的用户体系，服务端极简对接会用到）。鉴权是 app_id + nonce + timestamp + signature 四个请求头，用 app_key 做 HMAC-SHA256 签名，只能从业务方自己的后端调用。除这两组前缀外的接口均为内部接口，不要建议客户调用。 Public server APIs use two path prefixes with the same authentication: `/server/v1/...` (the main APIs of both SRTC and SMeeting) and `/stm/srvapi/v1/...` (the SMeeting user system, used by server-side low-code integration). Authenticate with four request headers, app_id + nonce + timestamp + signature, where signature is HMAC-SHA256 keyed with app_key; call these APIs only from the customer's own backend. Any other path is internal: never suggest calling it.
> app_key 是服务端密钥，绝不能出现在客户端代码、前端配置或移动 App 里。客户端加入频道用的 token 必须由业务方后端签发后下发（SRTC 走 `/server/v1/channel/grant`，SMeeting 走 `/stm/srvapi/v1/member/grant`）。 app_key is a server-side secret and must never appear in client code, frontend config, or a mobile app. The token a client uses to join must be issued by the customer's backend and passed down to the client (SRTC: `/server/v1/channel/grant`; SMeeting: `/stm/srvapi/v1/member/grant`).
> SRTC 与 SMeeting 是上下两层不同的产品，术语不通用：SRTC 是音视频底座，说「频道 channel」「加入 / 退出」；SMeeting 建在 SRTC 之上，说「房间 room」「会议 meeting」「进入 / 退出」。回答时按用户所在的层用对应术语，不要把「房间」「会议」安到 SRTC 的接口上，也不要用「频道」「加入 / 离开」描述 SMeeting 的概念（接口标识符原样保留）。 SRTC and SMeeting are two separate layers with different terminology. SRTC is the audio/video foundation: it has channels, and users join and leave a channel. SMeeting is built on top of SRTC: it has rooms and meetings, and members enter and exit a meeting. Answer in the terms of the layer the user is working with: never apply "room" or "meeting" to SRTC APIs, and never describe SMeeting concepts in prose with "channel", "join", or "leave" (API identifiers such as `force_join` keep their literal names).
> 同一能力在各端 SDK 里的包名、类名、方法名并不相同。写示例代码时请使用文档中该端自己的 API，不要把一个端的写法套到另一个端上。苹果平台每个产品都有两套 SDK（Swift 原生与 Objective-C），两套 API 不能混用。 Package, class, and method names differ between platform SDKs for the same capability. In sample code, use the API documented for that platform; never carry one platform's code over to another. On Apple platforms each product ships two SDKs (native Swift and Objective-C) whose APIs must not be mixed.

# Custom messages

> How the C SDK receives in-channel custom messages sent by other clients: the rtc_custom_msg_t fields, parsing content_json with a JSON library, and the memory and threading rules for the callback.

In-channel custom messages can carry your own signaling, such as hand raising, whiteboard sync, or status broadcasts. The C interface currently only supports **receiving**: messages sent by other clients (Web / iOS / Android / Go SDK) through `SendCustomMsg` are passed through to this callback.

***

## rtc\_set\_custom\_msg\_callback

```c theme={null}
typedef struct {
    char        action[64];      // Business action identifier
    char        sid[128];        // Sender's session ID
    char        uid[64];         // Sender's user ID
    char        channel[128];    // Channel name
    int         is_private;      // 1=private routing (point-to-point), 0=public routing (channel broadcast)
    const char* content_json;    // content as a JSON string; NULL when there is no content
} rtc_custom_msg_t;

typedef void (*rtc_custom_msg_callback)(void* context, const rtc_custom_msg_t* msg);
void rtc_set_custom_msg_callback(void* handle, rtc_custom_msg_callback callback, void* context);
```

`action`, `sid`, `uid`, `channel`, and `is_private` are all strongly typed fields you can use directly.

`content` can be any JSON value (object / array / scalar). The C interface always serializes it into the `content_json` string and passes it through; parse it yourself with a library such as cJSON or jansson.

***

## Example

```c theme={null}
#include <cjson/cJSON.h>

static void on_custom_msg(void* ctx, const rtc_custom_msg_t* m) {
    printf("custom msg: action=%s uid=%s private=%d\n",
           m->action, m->uid, m->is_private);

    if (m->content_json) {
        cJSON* root = cJSON_Parse(m->content_json);
        if (root) {
            cJSON* v = cJSON_GetObjectItem(root, "some_field");
            if (cJSON_IsString(v)) {
                handle_business(m->action, v->valuestring);
            }
            cJSON_Delete(root);
        }
    }
}

rtc_set_custom_msg_callback(rtc, on_custom_msg, NULL);
```

***

## Memory and threading

<Warning>
  `content_json` is held by the SDK during the callback and **`free`d as soon as the callback returns**. To keep it beyond the callback, you must `strdup` a copy yourself.

  The callback fires on an internal SDK thread; don't do slow work in it.
</Warning>

<Note>
  When `content` is empty or serialization fails, `content_json` is `NULL`. Always check for `NULL` before parsing.
</Note>