C语言代码封装MQTT协议报文，了解MQTT协议通信过程

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

// 定义MQTT报文类型
#define MQTT_CONNECT    0x10
#define MQTT_CONNACK    0x20
#define MQTT_PUBLISH    0x30
#define MQTT_PUBACK     0x40
#define MQTT_SUBSCRIBE  0x80
#define MQTT_SUBACK     0x90
#define MQTT_UNSUBSCRIBE    0xA0
#define MQTT_UNSUBACK   0xB0
#define MQTT_PINGREQ    0xC0
#define MQTT_PINGRESP   0xD0
#define MQTT_DISCONNECT    0xE0

// 定义MQTT连接标志
#define MQTT_CONNECT_FLAG_CLEAN     0x02
#define MQTT_CONNECT_FLAG_WILL      0x04
#define MQTT_CONNECT_FLAG_WILL_QOS0 0x00
#define MQTT_CONNECT_FLAG_WILL_QOS1 0x08
#define MQTT_CONNECT_FLAG_WILL_QOS2 0x10
#define MQTT_CONNECT_FLAG_WILL_RETAIN   0x20
#define MQTT_CONNECT_FLAG_PASSWORD  0x40
#define MQTT_CONNECT_FLAG_USERNAME  0x80

// 定义MQTT报文结构体
typedef struct mqtt_packet 
{
	unsigned char *data;
	unsigned int length;
}
mqtt_packet_t;

// 建立socket连接并返回socket文件描述符
int socket_connect(char *address, int port) 
{
	struct sockaddr_in server_address;
	int socket_fd = socket(AF_INET, SOCK_STREAM, 0);
	if (socket_fd == -1) 
	{
		printf("Failed to create socket!\n");
		return -1;
	}
	server_address.sin_family = AF_INET;
	server_address.sin_port = htons(port);
	if ((inet_pton(AF_INET, address, &server_address.sin_addr)) <= 0) 
	{
		printf("Invalid address/ Address not supported\n");
		return -1;
	}
	if (connect(socket_fd, (struct sockaddr *)&server_address, sizeof(server_address)) < 0) 
	{
		printf("Connection Failed!\n");
		return -1;
	}
	return socket_fd;
}
// 打包MQTT连接报文 
mqtt_packet_t *mqtt_connect(char *client_id, char *username, char *password) 
{
	mqtt_packet_t *packet = (mqtt_packet_t *)malloc(sizeof(mqtt_packet_t));
	unsigned char *data = (unsigned char *)malloc(256);
	unsigned int length = 0;
	// 固定报头 
	data[length++] = MQTT_CONNECT;
	// 可变报头 
	data[length++] = 0x0C;
	// 清理会话标志和协议版本号
	data[length++] = 'M';
	data[length++] = 'Q';
	data[length++] = 'T';
	data[length++] = 'T';
	data[length++] = 0x04;
	// 协议版本号 // 连接标志 
	unsigned char flags = MQTT_CONNECT_FLAG_CLEAN;
	if (username != NULL) 
	{
		flags |= MQTT_CONNECT_FLAG_USERNAME;
	}
	if (password != NULL) 
	{
		flags |= MQTT_CONNECT_FLAG_PASSWORD;
	}
	data[length++] = flags;
	data[length++] = 0xFF;
	// 保持连接时间低8位 
	data[length++] = 0xFF;
	// 保持连接时间高8位 // 剩余长度 
	unsigned char remaining_length = length - 1;
	data[remaining_length++] = (unsigned char)(length - 2);
	packet->data = data;
	packet->length = length;
	return packet;
}
// 发送MQTT报文 
void mqtt_send(int socket_fd, mqtt_packet_t *packet) 
{
	if (send(socket_fd, packet->data, packet->length, 0) < 0) 
	{
		printf("Failed to send message!\n");
	}
}
// 接收MQTT报文
int mqtt_recv(int socket_fd, mqtt_packet_t *packet) 
{
	unsigned char header[2];
	if (recv(socket_fd, header, 2, 0) != 2) 
	{
		printf("Failed to receive message header!\n");
		return -1
	}
	unsigned int remaining_length = 0;
	unsigned int multiplier = 1;
	int i = 1;
	do 
	{
		if (recv(socket_fd, &header[i], 1, 0) != 1) 
		{
			printf("Failed to receive remaining_length byte %d!\n", i);
			return -1;
		}
		remaining_length += (header[i] & 127) * multiplier;
		multiplier *= 128;
		i++;
	}
	while ((header[i - 1] & 128) != 0);
	packet->length = remaining_length + i;
	packet->data = (unsigned char *)malloc(packet->length);
	memcpy(packet->data, header, 2);
	if (recv(socket_fd, packet->data + 2, packet->length - 2, 0) != packet->length - 2) 
	{
		printf("Failed to receive full message!\n");
		return -1;
	}
	return 0;
}


int main(int argc, char *argv[]) 
{
	// 建立 TCP 连接 
	int socket_fd = socket_connect("test.mosquitto.org", 1883);
	if (socket_fd == -1) 
	{
		printf("Failed to connect to MQTT server!\n");
		return -1;
	}
	printf("Connected to MQTT server!\n");
	// 打包并发送 MQTT 连接报文
	mqtt_packet_t *connect_packet = mqtt_connect("test_client", NULL, NULL);
	mqtt_send(socket_fd, connect_packet);
	printf("Sent MQTT CONNECT packet!\n");
	free(connect_packet->data);
	free(connect_packet);
	// 接收 MQTT CONNACK 报文
	mqtt_packet_t *connack_packet = (mqtt_packet_t *)malloc(sizeof(mqtt_packet_t));
	if (mqtt_recv(socket_fd, connack_packet) != 0) 
	{
		printf("Failed to receive MQTT CONNACK packet!\n");
		return -1;
	}
	if (connack_packet->data[1] != 0x00) 
	{
		printf("MQTT server rejected connection!\n");
		return -1;
	}
	printf("Received MQTT CONNACK packet!\n");
	free(connack_packet->data);
	free(connack_packet);
	// 断开 TCP 连接 close(socket_fd); return 0; 
}