mqtt协议数据结构实例
时间: 2023-05-13 21:01:14 浏览: 102
MQTT是互联网上一种轻量级的物联网通信协议,其数据结构较为简单。MQTT消息分为固定头、可变头和消息体三部分。
固定头包含了消息类型(1字节)、标志位(1字节)和剩余长度(1-4字节),其长度不变。
可变头包含了各种不同类型的数据,其具体内容根据消息类型的不同而异。例如,连接消息的可变头包含了协议版本号、客户端标识符、遗嘱消息等信息。
消息体包含了各种不同类型的数据,其具体内容根据消息类型的不同而异。例如,发布消息的消息体包含了主题、负载等信息。
以下是一个发布消息的MQTT数据包实例:
固定头:
1.消息类型(1字节):0x30(十进制为48),表示发布消息。
2.标志位(1字节):0x06,表示消息的QoS等级为1,并且保留位为0。
3.剩余长度(1-4字节):0x0C,表示消息正文的长度为12个字节。
可变头:
1.主题(长度为4字节):0x00 0x04 't' 'e' 's' 't',表示主题为“test”。
2.报文标识符(2字节):0x00 0x01,表示消息的标识符为1。
消息体:
“Hello, world!”(长度为12个字节):0x48 0x65 0x6C 0x6C 0x6F 0x2C 0x20 0x77 0x6F 0x72 0x6C 0x64,表示消息负载为“Hello, world!”。
相关问题
mqtt协议的数据结构
MQTT协议的数据结构主要有以下几个部分:
1. Fixed Header:固定头部,包含了消息类型和标志位等信息,长度为1字节或2字节。
2. 可变头部:根据不同的消息类型而定,长度可变。
3. 消息体:消息体是可变长度的,包含了消息的具体内容。
4. 剩余长度:剩余长度是一个可变长度字段,用于表示整个MQTT消息的长度。
总体结构如下:
```
+-----------------------------------------------+
| Fixed Header |
+-----------------------------------------------+
| Variable Header (optional) |
+-----------------------------------------------+
| Payload (optional) |
+-----------------------------------------------+
| Remaining Length |
+-----------------------------------------------+
```
其中,Fixed Header和Remaining Length是必须包含的部分,Variable Header和Payload是根据消息类型和标志位来决定是否包含的。Fixed Header和Remaining Length的详细解释如下:
1. Fixed Header:
- MQTT控制报文类型(4 bits):用于标识消息类型,包括CONNECT、CONNACK、PUBLISH、PUBACK、PUBREC、PUBREL、PUBCOMP、SUBSCRIBE、SUBACK、UNSUBSCRIBE、UNSUBACK、PINGREQ、PINGRESP、DISCONNECT等消息类型。
- 标志位(4 bits):用于标识消息类型的具体属性,包括QoS级别、是否重发、是否保留等。
2. Remaining Length:
- 可变长度字段,用于表示整个MQTT消息的长度,采用可变长度编码方式,长度为1~4字节不等。
以上就是MQTT协议的数据结构。
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。