MQTT協議C語言異步處理

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。在C語言中實現MQTT的異步處理，通常需要使用異步I/O和多線程技術。以下是一個簡化的示例，展示了如何使用C語言和libmosquitto庫實現MQTT的異步處理。

1. 首先，確保已經安裝了libmosquitto庫。在Ubuntu系統中，可以使用以下命令安裝：

sudo apt-get install libmosquitto-dev

2. 創建一個C文件，例如mqtt_async.c，并編寫以下代碼：

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

#define CLIENT_ID "C_MQTT_ASYNC_EXAMPLE"
#define TOPIC "test/topic"
#define BUFFER_SIZE 2048

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

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

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

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

void on_disconnect(struct mosquitto *mosq, int rc) {
    printf("Disconnected with result code %d\n", rc);
}

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

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

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

    mosquitto_connect_async(mosq, argv[1], 1883, 60);
    mosquitto_loop_start(mosq);

    while (1) {
        rc = mosquitto_poll(mosq, 1000, 5, NULL);
        if (rc == MOSQ_POLL_ERR_connACK) {
            on_connect(mosq, rc);
        } else if (rc == MOSQ_POLL_ERR_MSG_ARRIVED) {
            on_message(mosq, NULL);
        } else if (rc == MOSQ_POLL_ERR_PUBLISH) {
            on_publish(mosq, 0);
        } else if (rc == MOSQ_POLL_ERR_SUBSCRIBE) {
            on_subscribe(mosq, 0, 0);
        } else if (rc == MOSQ_POLL_ERR_DISCONNECT) {
            on_disconnect(mosq, rc);
            break;
        }
    }

    mosquitto_destroy(mosq);
    return 0;
}

3. 編譯并運行程序：

gcc mqtt_async.c -o mqtt_async -lmosquitto
./mqtt_async <broker_ip>

將<broker_ip>替換為MQTT代理的IP地址。程序將連接到MQTT代理，訂閱test/topic主題，并在接收到消息時打印消息內容。

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。在實際項目中，你可能還需要考慮使用多線程來進一步提高性能和響應能力。