c语言实现mqtt发布订阅

你好！要使用C语言实现MQTT发布订阅功能，你可以使用一些开源的MQTT库，如Paho MQTT C库。下面是一个简单的示例代码：

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

#define MQTT_ADDRESS     "tcp://mqtt broker的地址:mqtt端口号"
#define MQTT_CLIENT_ID   "your_client_id"
#define MQTT_TOPIC       "your_topic"

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

int main() {
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    MQTTClient_deliveryToken token;
    int rc;

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

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("连接到MQTT Broker失败，返回代码：%d\n", rc);
        exit(-1);
    }

    MQTTClient_subscribe(client, MQTT_TOPIC, 1);

    while (1) {
        // 发布消息
        char payload[100];
        sprintf(payload, "Hello, World! %d", rand());
        pubmsg.payload = payload;
        pubmsg.payloadlen = strlen(payload);
        pubmsg.qos = 1;
        pubmsg.retained = 0;
        MQTTClient_publishMessage(client, MQTT_TOPIC, &pubmsg, &token);
        rc = MQTTClient_waitForCompletion(client, token, 1000);
        printf("已发布消息：%s\n", payload);

        // 每隔1秒发布一次消息
        sleep(1);
    }

    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);

    return 0;
}

你需要替换代码中的MQTT_ADDRESS、MQTT_CLIENT_ID和MQTT_TOPIC为你实际的MQTT Broker地址、客户端ID和订阅主题。运行这段代码后，它将以每秒一次的频率发布一条带有随机数的消息，并在收到订阅的消息时打印出来。

这只是一个简单的示例，你可以根据需要进行修改和扩展。希望对你有帮助！