Truncated MQTT Messages

Firmware Libraries
1

In my testing with the MQTT library, I’ve noticed that messages greater than 223 bytes get truncated. I wrote the following program to demonstrate the problem.

The program sends three MQTT messages and subscribers receive the output shown below the code. I include the output that subscribers see when I send the 300-byte message from MQTTlens. This proves that the broker and the subscriber ends are working properly.

So, I believe this isolated the problem to the “publish” end of things, but I don’t know if it is a issue with the MQTT library or “more likely” how I am attempting to use it. Please help if you can.

#include "Particle.h"
#include "MQTT.h"


	MQTT* mqtt;
	char clientName[] = "testClient1";
	byte brokerIP[] = {192,168,86,53};
	char topic[] = "garage_door_state_refresh";

	char m200bytes[] = "12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890";

	char m223bytes[] = "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123";

	char m300bytes[] = "123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890";
	 
void setup() {
	mqtt = new MQTT(brokerIP, 1883, 15, callback);
	mqtt->connect(clientName);
	mqtt->publish(topic, m200bytes);
	mqtt->publish(topic, m223bytes);
	mqtt->publish(topic, m300bytes);
}

void loop() {
}

void callback(char* topic, byte* payload, unsigned int length) {
}

MQTT MESSAGES RECEIVED FROM THE ABOVE PROGRAM
Note: Subscriber received a trucated 300 byte message.

12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890

1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123

1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123

300 BYTE MESSAGE RECEIVED FROM MQTT_LENS
Note: Subscriber received a complete 300 byte message

123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
2

The maximum message size can be changed by changing the #define MQTT_MAX_MESSAGE_SIZE in MQTT.h. I think this is, why your messages get cut off.

regards,

Christoph

1 Like
3

That was exactly what I needed … I upped that setting, and my code now works as intended.

Thank you !!!