MQTT協議C語言實現與多主題訂閱

MQTT（Message Queuing Telemetry Transport）是一種輕量級的發布/訂閱消息傳輸協議，廣泛應用于物聯網場景。下面是一個簡單的C語言實現MQTT客戶端，該客戶端可以訂閱多個主題。

首先，你需要安裝一個MQTT庫，例如libmosquitto。在Ubuntu系統中，可以使用以下命令安裝：

sudo apt-get install libmosquitto-dev

接下來，創建一個C文件，例如mqtt_subscribe.c，并編寫以下代碼：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mosquitto.h>

#define TOPIC1 "/topic1"
#define TOPIC2 "/topic2"
#define BUFFER_SIZE 256

void on_connect(struct mosquitto *mosq, void *userdata, int rc) {
    printf("Connected with result code %d\n", rc);
    mosquitto_subscribe(mosq, TOPIC1, 0);
    mosquitto_subscribe(mosq, TOPIC2, 0);
}

void on_message(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *msg) {
    printf("Received message on topic: %s\n", msg->topic);
    printf("Message: %s\n", msg->payload);
}

void on_publish(struct mosquitto *mosq, void *userdata, int mid) {
    printf("Message %d published.\n", mid);
}

void on_subscribe(struct mosquitto *mosq, void *userdata, int mid, int granted_qos) {
    printf("Subscribed: %d\n", mid);
}

void on_disconnect(struct mosquitto *mosq, void *userdata, int rc) {
    printf("Disconnected.\n");
}

int main(int argc, char *argv[]) {
    struct mosquitto *mosq;
    int rc;

    if (argc != 2) {
        printf("Usage: %s <broker address>\n", argv[0]);
        return 1;
    }

    mosquitto_lib_init();

    mosq = mosquitto_new(NULL, true);
    if (!mosq) {
        printf("Failed to create mosquitto client\n");
        return 1;
    }

    mosquitto_username_pw_set(mosq, "your_username", "your_password");
    mosquitto_connect(mosq, argv[1], 1883, 60);
    mosquitto_set_callback(mosq, on_connect, on_message, on_publish, on_subscribe, on_disconnect);

    rc = mosquitto_loop_start(mosq);
    if (rc != MOSQ_ERR_SUCCESS) {
        printf("Failed to start loop\n");
        return 1;
    }

    mosquitto_destroy(mosq);
    mosquitto_lib_cleanup();

    return 0;
}

在這個示例中，我們定義了兩個主題TOPIC1和TOPIC2，并在連接成功后訂閱這兩個主題。當收到消息時，會打印出消息的主題和內容。

要編譯這個程序，使用以下命令：

gcc mqtt_subscribe.c -o mqtt_subscribe -lmosquitto

然后，運行編譯后的程序，并提供MQTT代理的地址：

./mqtt_subscribe your_mqtt_broker_address

這樣，你就可以訂閱多個主題并接收消息了。注意，這個示例僅用于演示目的，實際應用中可能需要處理更多的錯誤情況和邊緣情況。