This is MQTT client for arduino connect to broker via ESP8266 AT command, port from MQTT client library for Contiki

You can find the Native MQTT client library for ESP8266 work well here: esp_mqtt

• Support subscribing, publishing, authentication, will messages, keep alive pings and all 3 QoS levels (it should be a fully functional client).
• Easy to setup and use

You will need V0.2 AT+ commands from Espressif, if you do not have you can download and flash the following: ESP8266 At Command 0.2: https://github.com/espressif/esp8266_at

You will also need the ESPDUINO library for Arduino that can be found here: https://github.com/tuanpmt/espduino

you have to change the SERIALBUFFERSIZE minimum is 64 bytes in

C:\Program Files (x86)\Arduino\hardware\arduino\cores\arduino\HardwareSerial.cpp

WORKING

#include <espduino.h>
#include <SoftwareSerial.h>
 
SoftwareSerial debugPort(2, 3); // RX, TX
 
ESP esp(&Serial, &debugPort, 13);
 
 
void wifiCb(uint8_t status)
{
  debugPort.println("WIFI: Connected");
  esp.mqttConnect("mqtt.domain.io", 1880);
}
 
void mqttConnected(uint32_t* args)
{
  debugPort.println("MQTT:Connected");
  esp.subscribe("/topic");
}
 
void mqttDisconnected(uint32_t* args)
{
  debugPort.println("MQTT:Disconnected");
}
 
void mqttPublished(uint32_t* args)
{
  debugPort.println("MQTT:Published");
}
 
void mqttData(uint32_t* args)
{
  mqtt_event_data_t *event_data = (mqtt_event_data_t *)args;
  char topic[16];
  char data[32];
 
  memcpy(topic, event_data->topic, event_data->topic_length);
  memcpy(data, event_data->data, event_data->data_length);
  topic[event_data->topic_length] = 0;
  data[event_data->data_length] = 0;
  debugPort.print("Received, topic:");
  debugPort.print(topic);
  debugPort.print(", data:");
  debugPort.println(data);
}
 
const char clientId[] = "clientId";
const char user[] = "user";
const char pass[] = "pass";
 
 
void setup() {
  delay(200);
  Serial.begin(115200);
  debugPort.begin(9600);
 
  /* setup event */
  esp.wifiCb.attach(&wifiCb);
  esp.mqttConnected.attach(&mqttConnected);
  esp.mqttDisconnected.attach(&mqttDisconnected);
  esp.mqttPublished.attach(&mqttPublished);
  esp.mqttData.attach(&mqttData);
 
  /* Init data */
  esp.initMqttClient(clientId, user, pass, 30);
 
  /* wifi connect */
  esp.wifiConnect("DVES_HOME", "yourpassword");
}
 
void loop() {
  esp.process();
}