mqtt发布订阅C语言

在C语言中使用MQTT协议进行发布/订阅操作的步骤如下：

1. 安装MQTT C语言客户端库，例如Eclipse Paho或MQTT-C。
2. 创建MQTT客户端，并设置连接参数，例如服务器地址、端口、客户端ID等。
3. 连接MQTT服务器。
4. 订阅主题，可以使用subscribe函数。
5. 发布消息，可以使用publish函数。
6. 处理接收到的消息，可以使用回调函数on_message。

以下是一个使用MQTT-C库进行发布/订阅操作的简单示例代码：

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "test"
#define QOS         1
#define TIMEOUT     10000L

volatile MQTTClient_deliveryToken deliveredtoken;

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

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

void connlost(void *context, char *cause) {
    printf("\nConnection lost\n");
    printf("  cause: %s\n", cause);
}

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

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

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    }
    printf("Connected to MQTT broker at %s\n", ADDRESS);

    if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to subscribe to topic %s, return code %d\n", TOPIC, rc);
        exit(-1);
    }
    printf("Subscribed to topic %s\n", TOPIC);

    pubmsg.payload = "Hello, world!";
    pubmsg.payloadlen = strlen(pubmsg.payload);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf("Message sent with delivery token %d\n", token);
    MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered);

    while (1) {
        MQTTClient_yield();
    }

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

这个示例代码实现了一个MQTT客户端，它连接到本地的MQTT服务器（地址为tcp://localhost:1883），订阅主题为test，并发布一条消息（内容为“Hello, world!”）。在程序运行过程中，它会不断接收和处理从MQTT服务器上收到的消息。