mqtt客户端c语言代码(paho库及二次封装)

### 回答1：
MQTT(消息队列遥测传输)是一种轻量级的通信协议，常用于物联网设备之间的通信。

以下是使用C语言编写的MQTT客户端代码，使用了Paho库进行二次封装：

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

#define MQTT_SERVER "tcp://localhost:1883"
#define MQTT_TOPIC "topic/test"
#define MQTT_QOS 0

void messageArrived(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char* payload = malloc(message->payloadlen + 1);
    strncpy(payload, message->payload, message->payloadlen);
    payload[message->payloadlen] = '\0';
    printf("Message received: %s\n", payload);
    free(payload);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
}

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

    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    MQTTClient_create(&client, MQTT_SERVER, "ClientID", MQTTCLIENT_PERSISTENCE_NONE, NULL);
    MQTTClient_setCallbacks(client, NULL, NULL, messageArrived, NULL);

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

    MQTTClient_subscribe(client, MQTT_TOPIC, MQTT_QOS);

    char* payload = "Hello, MQTT!";
    pubmsg.payload = payload;
    pubmsg.payloadlen = strlen(payload);
    pubmsg.qos = MQTT_QOS;
    pubmsg.retained = 0;

    MQTTClient_publishMessage(client, MQTT_TOPIC, &pubmsg, &token);
    MQTTClient_waitForCompletion(client, token, 1000);

    sleep(1);
    MQTTClient_disconnect(client, 1000);
    MQTTClient_destroy(&client);

    return 0;
}

以上代码创建了一个MQTT客户端，连接到本地的MQTT服务器。订阅了"topic/test"主题，接收到消息时会调用`messageArrived`函数进行处理。

在主函数中，先建立连接并订阅主题，然后发出一条消息"Hello, MQTT!"发布到指定主题。

最后，等待1秒后断开连接并销毁客户端。

这个示例代码演示了如何使用Paho库进行MQTT客户端开发，并进行了一次消息发布和订阅的操作。

### 回答2：
MQTT是一种轻量级的通信协议，常用于物联网设备之间的通信。paho库是一个开源的MQTT实现库，可以在各种编程语言中使用。下面是使用C语言进行MQTT客户端编程的示例代码：

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

#define ADDRESS     "tcp://mqtt.eclipse.org:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "MQTT Examples"
#define QOS         1
#define TIMEOUT     10000L

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

int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    printf("Message arrived\n");
    printf("     topic: %s\n", topicName);
    printf("   message: ");
    for (int i = 0; i < message->payloadlen; i++) {
        putchar(((char *)message->payload)[i]);
    }
    putchar('\n');
    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() {
    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, connlost, msgarrvd, delivered);

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

    MQTTClient_subscribe(client, TOPIC, QOS);

    char payload[100];
    sprintf(payload, "Hello MQTT!");
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    pubmsg.payload = payload;
    pubmsg.payloadlen = strlen(payload);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;

    MQTTClient_deliveryToken token;
    if ((rc = MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to publish message, return code %d\n", rc);
        exit(-1);
    }

    printf("Waiting for publication to be acknowledged...\n");
    if ((rc = MQTTClient_waitForCompletion(client, token, TIMEOUT)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to receive acknowledgement, return code %d\n", rc);
        exit(-1);
    }
    printf("Message published\n");

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

    return 0;
}

上述代码使用了paho库来连接到`mqtt.eclipse.org`的公共MQTT服务器，并发送和接收消息。在`main`函数中，首先创建了一个`MQTTClient`对象，然后设置了连接选项并建立了连接。接着订阅了一个主题，用于接收消息。然后构造了一个要发布的消息，然后使用`MQTTClient_publishMessage`方法将消息发布到指定的主题上，并监听发布结果是否被确认。最后，断开了与服务器的连接并销毁了`MQTTClient`对象。

这是一个非常简单的MQTT客户端示例，可以根据具体需求进行二次封装和扩展。

### 回答3：
以下是一个使用paho库和二次封装的MQTT客户端C语言代码示例：

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

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

typedef void (*message_callback_t)(char *topic, char *payload);

void messageArrived(MessageData *data) {
    char topic[64];
    char payload[64];

    strncpy(topic, data->topicName->lenstring.data, data->topicName->lenstring.len);
    topic[data->topicName->lenstring.len] = '\0';

    strncpy(payload, data->message->payload, data->message->payloadlen);
    payload[data->message->payloadlen] = '\0';

    message_callback_t cb = (message_callback_t)data->message->mqtt->userData;
    cb(topic, payload);
}

void sendMessage(MQTTClient client, char *topic, char *payload) {
    MQTTMessage message = MQTTMessage_initializer;
    message.qos = QOS;
    message.retained = 0;
    message.payload = payload;
    message.payloadlen = strlen(payload);

    MQTTClient_publishMessage(client, topic, &message, NULL);
}

void subscribeToTopic(MQTTClient client, char *topic, message_callback_t cb) {
    MQTTClient_subscribe(client, topic, QOS);
    MQTTClient_setCallbacks(client, cb, NULL, messageArrived, NULL);
}

void disconnectClient(MQTTClient client) {
    MQTTClient_disconnect(client, TIMEOUT);
    MQTTClient_destroy(&client);
}

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

    rc = MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL);
    if (rc != MQTTCLIENT_SUCCESS) {
        printf("Failed to create MQTT client, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

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

    printf("Connected to MQTT broker\n");

    // 订阅主题并指定消息回调函数
    subscribeToTopic(client, TOPIC, messageCallback);

    // 发送消息
    sendMessage(client, TOPIC, "Hello, MQTT!");

    // 断开连接
    disconnectClient(client);

    return rc;
}

这个例子使用paho库创建一个MQTT客户端，连接到指定的MQTT代理服务器。它订阅一个主题并指定一个消息回调函数来处理接收到的消息。然后，它发送一条消息到指定的主题上，并最终断开连接。你可以根据自己的需求修改其中的主题、服务器地址、客户端ID等参数，并在`message_callback_t`函数中处理接收到的消息。