paho mqtt c 异步

Paho MQTT C是一个MQTT客户端库，它提供了同步和异步的MQTT客户端实现。异步的实现使用了异步事件循环机制，可以实现非阻塞的MQTT通信。在异步模式下，Paho MQTT C使用了回调函数来处理MQTT消息的收发和连接状态的变化。

要使用Paho MQTT C的异步模式，需要以下步骤：

1. 创建MQTT客户端实例并设置回调函数，如下所示：

MQTTAsync client;
MQTTAsync_create(&client, "tcp://localhost:1883", "example-client", MQTTCLIENT_PERSISTENCE_NONE, NULL);
MQTTAsync_setCallbacks(client, NULL, connlost, msgarrvd, delivered);

2. 设置连接选项和连接参数，如下所示：

MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.onSuccess = onConnect;
conn_opts.onFailure = onConnectFailure;
conn_opts.context = client;
MQTTAsync_connect(client, &conn_opts);

3. 实现回调函数来处理MQTT消息的收发和连接状态的变化，如下所示：

void onConnect(void* context, MQTTAsync_successData* response) {
    printf("Connected\n");
    MQTTAsync_subscribe(client, "topic", 0, NULL);
}

void onConnectFailure(void* context, MQTTAsync_failureData* response) {
    printf("Connect failed, rc %d\n", response ? response->code : 0);
}

void connlost(void* context, char* cause) {
    printf("Connection lost\n");
}

int msgarrvd(void* context, char* topicName, int topicLen, MQTTAsync_message* message) {
    printf("Message arrived\n");
    MQTTAsync_freeMessage(&message);
    MQTTAsync_free(topicName);
    return 1;
}

void delivered(void* context, MQTTAsync_token token) {
    printf("Message delivered\n");
}

4. 在异步事件循环中处理MQTT消息的收发和连接状态的变化，如下所示：

MQTTAsync_enableCallbacks(client);
MQTTAsync_createToken(&token);
MQTTAsync_setTraceCallback(client, trace_callback);
MQTTAsync_connect(client, &conn_opts);
do {
    rc = MQTTAsync_waitForCompletion(client, token, TIMEOUT);
} while (rc == MQTTASYNC_CONTINUE);
MQTTAsync_destroyToken(&token);
MQTTAsync_disconnect(client, NULL);
MQTTAsync_destroy(&client);

通过以上步骤，就可以实现Paho MQTT C的异步模式。