【MQTT编程】Last will and Testament(LWT Payload设置)

比眉伴天荒 2022-11-11 14:59 77阅读 0赞

MQTTClient\_willOptions定义客户端的MQTT “Last Will and Testament” （LWT）设置。 如果客户端意外失去与服务器的连接，服务器将代表客户端将LWT消息发布到LWT主题。 这使其他客户端（已订阅LWT主题）可以知道该客户端已断开连接。 为了为特定客户端启用LWT功能，在将客户端连接到服务器的MQTTClient\_connect（）调用中使用的MQTTClient\_connectOptions结构中传递了指向MQTTClient\_willOptions结构的有效指针。 如果不需要LWT函数，则可以将指向MQTTClient\_willOptions的指针设置为NULL。  
**LWT payload格式**：

struct{
        
    		int len;
    		const void* data;
    }payload;

**Data Fields**：

<table> 
 <thead> 
  <tr> 
   <th>Type</th> 
   <th>Data</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>char</td> 
   <td>struct_id [4]</td> 
   <td>The eyecatcher for this structure. must be MQTW.</td> 
  </tr> 
  <tr> 
   <td>int</td> 
   <td>struct_version</td> 
   <td>The version number of this structure. Must be 0 or 1 0 means there is no binary payload option</td> 
  </tr> 
  <tr> 
   <td>const char *</td> 
   <td>topicName</td> 
   <td>The LWT topic to which the LWT message will be published.</td> 
  </tr> 
  <tr> 
   <td>const char *</td> 
   <td>message</td> 
   <td>The LWT payload in string form.</td> 
  </tr> 
  <tr> 
   <td>int</td> 
   <td>retained</td> 
   <td>The retained flag for the LWT message</td> 
  </tr> 
  <tr> 
   <td>int</td> 
   <td>qos</td> 
   <td>The quality of service setting for the LWT message.</td> 
  </tr> 
 </tbody> 
</table>

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#include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>
    #include "MQTTClient.h"
    #define ADDRESS     "tcp://192.168.1.101:1883"
    #define CLIENTID    "ExampleClientPub"
    #define TOPIC       "MQTT_Examples"
    #define PAYLOAD     "Hello MQTT!"
    #define QOS         1
    #define TIMEOUT     10000L
    
    #define LAST_WILL_TOPIC "Hello_Will"
    #define LAST_WILL_MSG "take care yourself"
    
    int main(int argc, char* argv[])
    {
        
        MQTTClient client;
        MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
        MQTTClient_message pubmsg = MQTTClient_message_initializer;
    		MQTTClient_willOptions will_opts = MQTTClient_willOptions_initializer;
    
        MQTTClient_deliveryToken token;
        int rc;
        MQTTClient_create(&client, ADDRESS, CLIENTID,
            MQTTCLIENT_PERSISTENCE_NONE, NULL);
        conn_opts.keepAliveInterval = 20;
        conn_opts.cleansession = 1;
    
    	will_opts.topicName = LAST_WILL_TOPIC;
    	will_opts.message = LAST_WILL_MSG;
    	conn_opts.will = &will_opts;
    
        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;
        while (1)
        {
        
            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);
            sleep(1);
        }
        
        MQTTClient_disconnect(client, 10000);
        MQTTClient_destroy(&client);
        return rc;
    }

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