Arduino Uno - MQTT message using PUBSUB on ESP8266

Question

I'm trying to write code involving MQTT messages to operate an ESP8266 relay ( a Sonoff device ), using PubSub library.

For some, unknown for me, reason - messages larger than 99 chars are not sent. It this a known limitation ? can it be changed ?

void pub_msg(char *inmsg){
        char tmpmsg[255];
        // sprintf(tmpmsg,"[Cl:oc:k] [%s] %s",deviceTopic, inmsg);   ---> wish to add a time stamp
        sprintf(tmpmsg,"[][%s] %s",deviceTopic, inmsg);  ---> using this line just to find that 99 chars sent OK
        Serial.print("published MSG:");
        Serial.println(tmpmsg);
        Serial.print("Total chars:");
        Serial.println(strlen(tmpmsg));
        client.publish(msgTopic, tmpmsg);

}

Answer 1

The maximum MQTT packet size in PubSubClient is 128 bytes. After all the MQTT header information that leaves only 99 bytes for a message.

You can change it by editing the PubSubClient.h file:

#ifndef MQTT_MAX_PACKET_SIZE
#define MQTT_MAX_PACKET_SIZE 128
#endif

As it says in the PubSubClient README.md:

• The maximum message size, including header, is 128 bytes by default. This is configurable via MQTT_MAX_PACKET_SIZE in PubSubClient.h.