MQTT 使用中的问题汇总

  版本：eclipse-paho-mqtt-c-1.3.12 

 

  开发工具：QT

 

 

  问题1、其他都正常，唯独无法收到订阅消息

  原因：

MQTTClient_setCallbacks函数必须在MQTTClient_connect前调用


问题2、MQTTClient_messageArrived回调函数异常，多次收到重复内容。
原因：

MQTTClient_messageArrived回调函数返回了0
函数返回值说明：
1：成功
2：失败，会再次回调

return This function must return 0 or 1 indicating whether or not
 * the message has been safely received by the client application. <br>
 * Returning 1 indicates that the message has been successfully handled.
 * To free the message storage, ::MQTTClient_freeMessage must be called.
 * To free the topic name storage, ::MQTTClient_free must be called.<br>
 * Returning 0 indicates that there was a problem. In this
 * case, the client library will reinvoke MQTTClient_messageArrived() to
 * attempt to deliver the message to the application again.
 * Do not free the message and topic storage when returning 0, otherwise
 * the redelivery will fail.

 

问题3、MQTTClient_publishMessage阻塞
原因：在同一个线程进行消息订阅和消息推送，导致MQTT程序内部造成死循环