How can I send empty mqtt message to broker to delete some retained message?

[Zeni241]

How can I send empty mqtt message to broker to delete some retained message?
I tried following but it is not working:

Code: Select all

esp_mqtt_client_publish(mqttclient, "mytopic"," ",0, 0, 0);
esp_mqtt_client_publish(mqttclient, "mytopic",' ',0, 0, 0);
esp_mqtt_client_publish(mqttclient, "mytopic",NULL,0, 0, 0);

[Zeni241]

I was missing setting retained flag!
Now following deleted retained message on broker:
esp_mqtt_client_publish(mqttclient, "mytopic"," ",0, 0, 1)

[regan.xavier]

I am facing an issue while trying to delete a retained message from an MQTT topic. I use the following code to publish an empty message with the retained flag to clear the retained message on the topic:

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1. esp_mqtt_client_publish(mqtt_client, topic, " ", 0, 0, 1);

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However, after calling this function, I encounter the following error:

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1. E (28226) task_wdt: Task watchdog got triggered. The following tasks/users did not reset the watchdog in time:
2. E (28226) task_wdt:  - IDLE1 (CPU 1)
3. E (28226) task_wdt: Tasks currently running:
4. E (28226) task_wdt: CPU 0: IDLE0
5. E (28226) task_wdt: CPU 1: mqtt_task
6. E (28226) task_wdt: Print CPU 1 backtrace

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Could anyone suggest what might be causing the Task WDT to get triggered in this case? Any insights or suggestions on how to resolve this issue would be highly appreciated!

[ESP_Sprite]

You could select the menuconfig option to make the task wdt panic; that should give you a backtrace that may give us more info.

[chegewara]

Code: Select all

    /* Acceptable publish messages:
        data == NULL, len == 0: publish null message
        data valid,   len == 0: publish all data, payload len is determined from string length
        data valid,   len >  0: publish data with defined length
     */
    if (len <= 0 && data != NULL) {
        len = strlen(data);
    }
    

Did you try to send NULL msg instead?

BTW, your message seems to have 1 space in string, it is not empty string

[regan.xavier]

Thanks chegewara for the response. I removed the single space in the payload.

And in the AWS after giving the access to receive zero-payloads, it got fixed.