MQTT用C语言实现

MQTT（Message Queuing Telemetry Transport）是一种轻量级的消息传输协议，通常用于物联网应用中的设备间通信。在C语言中实现MQTT协议，可以使用MQTT客户端库，例如Paho MQTT C client，该库提供了C语言实现的MQTT客户端。

接下来，我将介绍如何使用Paho MQTT C client实现MQTT协议的发布和订阅功能。

1. 安装Paho MQTT C client库

在Ubuntu系统中，可以使用以下命令安装：

sudo apt-get install libssl-dev
git clone https://github.com/eclipse/paho.mqtt.c.git
cd paho.mqtt.c
make
sudo make install

2. MQTT发布消息

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

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

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(EXIT_FAILURE);
    }

    pubmsg.payload = "Hello world!";
    pubmsg.payloadlen = strlen(pubmsg.payload);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;

    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf("Waiting for publication of %s\n"
            "on topic %s for client with ClientID: %s\n",
            pubmsg.payload, TOPIC, CLIENTID);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);
    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

3. MQTT订阅消息

#include <stdio.h>
#include <stdlib.h>
#include <unistd.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;
    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(EXIT_FAILURE);
    }

    MQTTClient_subscribe(client, TOPIC, QOS);
    printf("Subscribed to topic %s for client %s\n", TOPIC, CLIENTID);

    while(1)
    {
        sleep(1);
    }

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

以上是基于Paho MQTT C client库实现MQTT协议发布和订阅的示例代码。在实际应用中，需要根据具体的需求进行相应的修改。