File content as of revision 2:f706efb3ea13:

// In this project an MQTT client is created.
// It is publishing parameters measured by a DHT11 sensor.
// Ethernet connection is assured via an ENC28J60 module.

#include <mbed.h>
#include <UIPEthernet.h>
#include <UIPClient.h>
#include <MQTTClient.h>
#include <DHT11.h>

// UIPEthernet is the name of a global instance of UIPEthernetClass.
// Do not change the name! It is used within the UIPEthernet library.
// Adapt the SPI pin names to your mbed platform/board if not present yet.
#if defined(TARGET_LPC1768)
UIPEthernetClass    UIPEthernet(p11, p12, p13, p8);         // mosi, miso, sck, cs
DHT11               dht11(p6);
#elif defined(TARGET_NUCLEO_F103RB)
UIPEthernetClass    UIPEthernet(PB_5, PB_4, PB_3, PB_6);    // mosi, miso, sck, cs
DHT11               dht11(PC_14);
#endif

// Make sure that the MAC number is unique within the connected network.
const uint8_t       MY_MAC[6] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05 };
// This client is using a static IP address. Make sure DHCP is disabled on your router/modem.
// IP address must be unique and compatible with your network too. Change as appropriate.
const IPAddress     MY_IP(192, 168, 1, 181);
char                message_buff[100];
void                onMessage(char* topic, uint8_t* payload, unsigned int length);
// In case your MQTT broker has different IP adress modify the following definition accordingly.
IPAddress           serverIP(192, 168, 1, 30);  // MQTT broker (e.g. 'Mosquitto' running on a Raspberry Pi or Linux device)
// The MQTT broker is like a post office distributing messages received from publishing clients to all subscribers (clients).  
// So the messages published by this client will be sent via the broker to all clients which subscribed to such messages.
// This way also this client will receive all subscribed messages from other clients, but via the broker.
EthernetClient      ethernetClient;
void                onMqttMessage(char* topic, uint8_t* payload, unsigned int length);
MQTTClient          mqttClient(serverIP, 1883, onMqttMessage, ethernetClient);
const int           PERIOD = 10;    // period for publishing the messages (in seconds)
Serial              pc(USBTX, USBRX);

/**
 * @brief   Main
 * @note
 * @param
 * @retval
 */
int main(void) {
    const int   MAX_COUNT = 5;
    int         i = 0;
    bool        connected = false;
    time_t      t = 0;
    time_t      lastTime = 0;

    // initialize the ethernet device
    UIPEthernet.begin(MY_MAC, MY_IP);
    pc.printf("Connecting to MQTT broker ..\r\n");
    do
    {
        wait(1.0);
        connected = mqttClient.connect("myMqttClient");
    } while(!connected && (i < MAX_COUNT));

    if(connected) {
        pc.printf("MQTT broker connected.\r\n");
        // The client can subscribe to as many MQTT messages as you like.
        mqttClient.subscribe("outdoor/gasmeter"); 
        mqttClient.subscribe("livingroom/temperature");
        mqttClient.subscribe("boiler/outlet/temperature");
    }
    else {
        pc.printf("Failed to connect to MQTT broker.\r\n");
    }

    while(1) {
        t = time(NULL);
        if(t >= (lastTime + PERIOD)) {
            lastTime = t;
            if(connected) {
                pc.printf("---------------------\r\n");
                pc.printf("%ds:\r\n", t);

                int state = dht11.readData();
                if(state == DHT11::OK) {
                    float   hum = dht11.readHumidity();
                    sprintf(message_buff, "%4.1f", hum);
                    pc.printf("  hum = %s%%\r\n", message_buff);
                    mqttClient.publish("outdoor/humidity", message_buff);

                    float   temp = dht11.readTemperature();
                    sprintf(message_buff, "%5.1f", temp);
                    pc.printf("  temp = %s'C\r\n", message_buff);
                    mqttClient.publish("outdoor/temperature", message_buff);

                    float   dewPoint = temp - (100 - hum) / 5.0;
                    sprintf(message_buff, "%5.1f", dewPoint);
                    pc.printf("  dew point = %s'C\r\n", message_buff);
                    mqttClient.publish("outdoor/dewpoint", message_buff);
                }
                else
                    pc.printf("  DHT11 error: %d\r\n", state);
            }
        }

        mqttClient.loop();  // MQTT client loop processing
    }
}

/**
 * @brief   Called on new MQTT message arrival
 * @note    
 * @param   topic:      The topic of the new message
 *          payload:    The payload of the new message
 *          length:     Payload's length
 * @retval
 */
void onMqttMessage(char* topic, uint8_t* payload, unsigned int length) {
    int i = 0;

    pc.printf("Message arrived:\r\n");
    pc.printf("  Topic: %s\r\n", topic);
    pc.printf("  Length: %d\r\n", length);

    // create character buffer with ending null terminator (string)
    for(i = 0; i < length; i++) {
        message_buff[i] = payload[i];
    }

    message_buff[i] = '\0';
    pc.printf("  Payload: %s\r\n", message_buff);
}