c语言实现mqtt协议 代码实例

MQTT是一种轻量级的消息传输协议，通常被用于物联网设备之间的通信。下面是一个简单的C语言实现MQTT协议的代码示例，仅供参考：

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

#define MAX_BUFFER_SIZE 1024

// MQTT消息类型
typedef enum mqtt_msg_type {
    CONNECT = 1,
    CONNACK,
    PUBLISH,
    PUBACK,
    PUBREC,
    PUBREL,
    PUBCOMP,
    SUBSCRIBE,
    SUBACK,
    UNSUBSCRIBE,
    UNSUBACK,
    PINGREQ,
    PINGRESP,
    DISCONNECT
} mqtt_msg_type_t;

// MQTT连接标志
typedef struct mqtt_connect_flags {
    unsigned int username_flag : 1;
    unsigned int password_flag : 1;
    unsigned int will_retain : 1;
    unsigned int will_qos : 2;
    unsigned int will_flag : 1;
    unsigned int clean_session : 1;
    unsigned int reserved : 1;
} mqtt_connect_flags_t;

// MQTT连接负载
typedef struct mqtt_connect_payload {
    char *protocol_name;
    unsigned char protocol_version;
    mqtt_connect_flags_t flags;
    unsigned short keep_alive;
    char *client_id;
    char *will_topic;
    char *will_payload;
    char *username;
    char *password;
} mqtt_connect_payload_t;

// MQTT消息头
typedef struct mqtt_header {
    unsigned int message_type : 4;
    unsigned int dup_flag : 1;
    unsigned int qos_level : 2;
    unsigned int retain_flag : 1;
    unsigned int remaining_length : 7;
} mqtt_header_t;

// MQTT消息
typedef struct mqtt_message {
    mqtt_header_t header;
    unsigned char *payload;
} mqtt_message_t;

// MQTT连接函数
void mqtt_connect(int sockfd, mqtt_connect_payload_t *payload) {
    mqtt_message_t msg = {0};
    msg.header.message_type = CONNECT;
    msg.header.remaining_length = 10 + strlen(payload->protocol_name) + 2 + 1 + 1 + strlen(payload->client_id) + 2;
    if (payload->will_flag) {
        msg.header.remaining_length += strlen(payload->will_topic) + 2 + strlen(payload->will_payload) + 2;
    }
    if (payload->username_flag) {
        msg.header.remaining_length += strlen(payload->username) + 2;
    }
    if (payload->password_flag) {
        msg.header.remaining_length += strlen(payload->password) + 2;
    }
    msg.payload = (unsigned char *)malloc(msg.header.remaining_length);
    unsigned char *p = msg.payload;
    p[0] = 0x00;
    p[1] = strlen(payload->protocol_name);
    memcpy(p+2, payload->protocol_name, strlen(payload->protocol_name));
    p += strlen(payload->protocol_name) + 2;
    p[0] = payload->protocol_version;
    p += 1;
    p[0] = payload->flags.username_flag << 7 |
            payload->flags.password_flag << 6 |
            payload->flags.will_retain << 5 |
            payload->flags.will_qos << 3 |
            payload->flags.will_flag << 2 |
            payload->flags.clean_session << 1;
    p += 1;
    p[0] = payload->keep_alive >> 8;
    p[1] = payload->keep_alive & 0xFF;
    p += 2;
    p[0] = strlen(payload->client_id) >> 8;
    p[1] = strlen(payload->client_id) & 0xFF;
    memcpy(p+2, payload->client_id, strlen(payload->client_id));
    p += strlen(payload->client_id) + 2;
    if (payload->will_flag) {
        p[0] = strlen(payload->will_topic) >> 8;
        p[1] = strlen(payload->will_topic) & 0xFF;
        memcpy(p+2, payload->will_topic, strlen(payload->will_topic));
        p += strlen(payload->will_topic) + 2;
        p[0] = strlen(payload->will_payload) >> 8;
        p[1] = strlen(payload->will_payload) & 0xFF;
        memcpy(p+2, payload->will_payload, strlen(payload->will_payload));
        p += strlen(payload->will_payload) + 2;
    }
    if (payload->username_flag) {
        p[0] = strlen(payload->username) >> 8;
        p[1] = strlen(payload->username) & 0xFF;
        memcpy(p+2, payload->username, strlen(payload->username));
        p += strlen(payload->username) + 2;
    }
    if (payload->password_flag) {
        p[0] = strlen(payload->password) >> 8;
        p[1] = strlen(payload->password) & 0xFF;
        memcpy(p+2, payload->password, strlen(payload->password));
        p += strlen(payload->password) + 2;
    }
    write(sockfd, &msg.header, sizeof(mqtt_header_t) + msg.header.remaining_length);
    free(msg.payload);
}

// MQTT订阅函数
void mqtt_subscribe(int sockfd, char *topic, unsigned short packet_id) {
    mqtt_message_t msg = {0};
    msg.header.message_type = SUBSCRIBE;
    msg.header.qos_level = 1;
    msg.header.remaining_length = 2 + strlen(topic) + 1;
    msg.payload = (unsigned char *)malloc(msg.header.remaining_length);
    unsigned char *p = msg.payload;
    p[0] = packet_id >> 8;
    p[1] = packet_id & 0xFF;
    memcpy(p+2, topic, strlen(topic));
    p += strlen(topic);
    p[0] = 0x01;
    write(sockfd, &msg.header, sizeof(mqtt_header_t) + msg.header.remaining_length);
    free(msg.payload);
}

int main() {
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    server_addr.sin_port = htons(1883);
    connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr));

    mqtt_connect_payload_t connect_payload = {0};
    connect_payload.protocol_name = "MQTT";
    connect_payload.protocol_version = 4;
    connect_payload.flags.clean_session = 1;
    connect_payload.keep_alive = 60;
    connect_payload.client_id = "test";
    mqtt_connect(sockfd, &connect_payload);

    mqtt_subscribe(sockfd, "test/topic", 1);

    char buffer[MAX_BUFFER_SIZE];
    while (1) {
        int len = read(sockfd, buffer, MAX_BUFFER_SIZE);
        if (len <= 0) {
            break;
        }
        printf("Received message: %s\n", buffer);
    }

    close(sockfd);
    return 0;
}

需要注意的是，这只是一个示例代码，实际应用中还需要进行更多的处理和错误处理。