C语言实现MQTT发布和订阅接口

MQTT是一种轻量级的发布/订阅协议，让我们来看看如何使用C语言实现MQTT发布和订阅接口。

首先，需要使用MQTT客户端库，例如Eclipse Paho，这是一款流行的跨平台MQTT客户端库，支持C语言。可以通过以下代码行安装Eclipse Paho：

git clone https://github.com/eclipse/paho.mqtt.c.git
cd paho.mqtt.c
make

接下来，让我们看看如何实现MQTT发布和订阅接口：

### MQTT发布接口

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "MQTT Examples"
#define PAYLOAD     "Hello World!"
#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 = PAYLOAD;
    pubmsg.payloadlen = strlen(PAYLOAD);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);

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

这段代码使用Eclipse Paho库实现了MQTT发布接口。首先，通过MQTTClient_create函数创建MQTT客户端，然后使用MQTTClient_connect函数连接到MQTT代理。接下来，设置MQTT消息的主题、负载、QoS和保留。最后，使用MQTTClient_publishMessage函数将消息发布到MQTT代理，使用MQTTClient_waitForCompletion函数等待发布完成，并使用MQTTClient_disconnect函数断开连接。

### MQTT订阅接口

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "MQTT Examples"
#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_token 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);
    }

    MQTTClient_subscribe(client, TOPIC, QOS);
    MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered);

    while (1)
    {
        // do something
    }

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

这段代码使用Eclipse Paho库实现了MQTT订阅接口。首先，通过MQTTClient_create函数创建MQTT客户端，然后使用MQTTClient_connect函数连接到MQTT代理。接下来，使用MQTTClient_subscribe函数订阅MQTT消息，并设置MQTTClient_setCallbacks回调函数处理MQTT消息。最后，使用while循环等待MQTT消息的到来。

以上就是使用C语言实现MQTT发布和订阅接口的方法。