MQTT（Message Queuing Telemetry Transport）协议（二）

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下面为你详细介绍如何基于 TCP 协议对 MQTT 进行封装，包括实现思路、代码示例及代码解释。

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 1883

// 封装 MQTT CONNECT 报文
void mqtt_connect_packet(unsigned char *packet, const char *client_id) {
    int index = 0;
    // 固定报头，CONNECT 报文类型为 0x10
    packet[index++] = 0x10; 
    // 剩余长度，这里简单假设为固定值，实际需要根据具体内容计算
    packet[index++] = 12 + strlen(client_id); 
    // 协议名，MQTT 3.1.1
    packet[index++] = 0x00;
    packet[index++] = 0x04;
    packet[index++] = 'M';
    packet[index++] = 'Q';
    packet[index++] = 'T';
    packet[index++] = 'T';
    // 协议级别
    packet[index++] = 0x04; 
    // 连接标志
    packet[index++] = 0x02; 
    // 保持活动时间
    packet[index++] = 0x00;
    packet[index++] = 0x3C; 
    // 客户端 ID 长度
    packet[index++] = strlen(client_id); 
    // 客户端 ID
    strcpy((char *)&packet[index], client_id);
}

int main() {
    int sockfd;
    struct sockaddr_in server_addr;
    unsigned char packet[1024];

    // 创建 TCP 套接字
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket creation error");
        return -1;
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERVER_PORT);

    // 将 IP 地址从点分十进制转换为二进制形式
    if (inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr) <= 0) {
        perror("Invalid address/ Address not supported");
        return -1;
    }

    // 连接到服务器
    if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Connection Failed");
        return -1;
    }

    // 封装 MQTT CONNECT 报文
    mqtt_connect_packet(packet, "test_client");
    int packet_length = 12 + strlen("test_client");

    // 发送 MQTT CONNECT 报文
    if (send(sockfd, packet, packet_length, 0) != packet_length) {
        perror("send failed");
        return -1;
    }

    // 接收服务器响应
    unsigned char response[1024];
    ssize_t recv_len = recv(sockfd, response, sizeof(response), 0);
    if (recv_len > 0) {
        printf("Received response from server:\n");
        for (int i = 0; i < recv_len; i++) {
            printf("%02X ", response[i]);
        }
        printf("\n");
    }

    // 关闭套接字
    close(sockfd);
    return 0;
}

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订阅者代码（mqtt_subscriber.c）

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

#define ADDRESS     "tcp://test.mosquitto.org:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "test/topic"
#define QOS         1
#define TIMEOUT     10000L

// 连接丢失回调函数
void connlost(void *context, char *cause) {
    printf("\nConnection lost\n");
    printf("  cause: %s\n", cause);
}

// 消息到达回调函数
void message_arrived(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(*(message->payload + i));
    }
    putchar('\n');
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
}

// 消息交付完成回调函数
void delivered(void *context, MQTTClient_deliveryToken dt) {
    printf("Message with token value %d delivery confirmed\n", dt);
}

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

    // 创建 MQTT 客户端实例
    MQTTClient_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    // 设置回调函数
    MQTTClient_setCallbacks(client, NULL, connlost, message_arrived, delivered);

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

    // 订阅主题
    if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to subscribe, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
    printf("Subscribed to topic: %s\n", TOPIC);

    // 持续处理消息
    while(1) {
        // 可在此添加更多逻辑
    }

    // 断开连接
    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

发布者代码（mqtt_publisher.c）

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

#define ADDRESS     "tcp://test.mosquitto.org:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "test/topic"
#define PAYLOAD     "Hello, MQTT!"
#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;

    // 创建 MQTT 客户端实例
    MQTTClient_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

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

    // 设置要发布的消息
    pubmsg.payload = (void*)PAYLOAD;
    pubmsg.payloadlen = strlen(PAYLOAD);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;

    // 发布消息
    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf("Waiting for up to %d seconds for publication of %s\n"
            "on topic %s for client with ClientID: %s\n",
            (int)(TIMEOUT/1000), PAYLOAD, TOPIC, CLIENTID);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);

    // 断开连接
    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}