linux c mqtt订阅

Linux C MQTT订阅是指在Linux操作系统下使用C语言编写的程序，通过MQTT协议进行消息订阅的过程。MQTT（Message Queuing Telemetry Transport）是一种轻量级的消息传输协议，常用于物联网设备之间的通信。

在Linux C中进行MQTT订阅，需要使用MQTT的C语言库，例如Eclipse Paho或者Mosquitto等。以下是一个简单的Linux C MQTT订阅的示例代码：

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

#define ADDRESS     "tcp://mqtt.eclipse.org:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "example/topic"
#define QOS         1
#define TIMEOUT     10000L

volatile MQTTClient_deliveryToken deliveredtoken;

int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    printf("Message arrived on topic: %s\n", topicName);
    printf("Message: %.*s\n", message->payloadlen, (char*)message->payload);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

void delivered(void *context, MQTTClient_deliveryToken dt) {
    printf("Message with token value %d delivery confirmed\n", dt);
    deliveredtoken = dt;
}

int main(int argc, char* argv[]) {
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    int rc;

    MQTTClient_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    MQTTClient_setCallbacks(client, NULL, NULL, msgarrvd, delivered);

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    MQTTClient_subscribe(client, TOPIC, QOS);

    printf("Subscribed to topic: %s\n", TOPIC);

    while (1) {
        // 这里可以添加其他逻辑代码
    }

    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

上述示例代码使用了Eclipse Paho的C语言库，通过连接到指定的MQTT服务器（例如mqtt.eclipse.org），订阅了名为"example/topic"的主题。当有消息到达时，会调用msgarrvd函数进行处理。