File content as of revision 2:6cd2390302ac:

// In this example an MQTT client is created.
// It is publishing a simple 'example/hello' message with payload 'Hello World.'
// and subscribes to 'outdoor/temperature' messages assumably published by other client(s).
// Ethernet connection is assured via an ENC28J60 module.
#include "mbed.h"
#include <UIPEthernet.h>
#include <UIPClient.h>
#include <MQTTClient.h>
#include <string>

using namespace     std;

Serial              pc(USBTX, USBRX);

// 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
#elif defined(TARGET_LPC1114)
UIPEthernetClass    UIPEthernet(dp2, dp1, dp6, dp25);       // mosi, miso, sck, cs
#elif defined(TARGET_LPC11U68)
UIPEthernetClass    UIPEthernet(P0_9, P0_8, P1_29, P0_2);   // mosi, miso, sck, cs
#elif defined(TARGET_NUCLEO_F103RB)
UIPEthernetClass    UIPEthernet(PB_5, PB_4, PB_3, PB_6);    // mosi, miso, sck, cs
#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 };
// 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];
IPAddress           serverIP(192, 168, 1, 30);  // MQTT broker (e.g. 'Mosquitto' running on a Raspberry Pi or Linux machine)
EthernetClient      ethernetClient;
void                onMqttMessage(char* topic, uint8_t* payload, unsigned int length);
MQTTClient          mqttClient(serverIP, 1883, onMqttMessage, ethernetClient);

/**
 * @brief
 * @note
 * @param
 * @retval
 */

int main(void) {
    const int   MAX_COUNT = 5;
    int         i = 0;
    bool        connected = false;
    char*       payload = "Hello World.";
    time_t      t = 0;
    time_t      lastTime = t;

    // initialize the ethernet device

    UIPEthernet.begin(MY_MAC, MY_IP);

    // The MQTT broker is like a post office distributing messages published by clients to all subscribers (clients).
    // So the 'example/hello' 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 (in this case 'outdoor/temperature' messages)
    // published by other clients (however not directly but via the broker).
    pc.printf("Connecting to MQTT broker ..\r\n");
    do
    {
        wait(1.0);
        connected = mqttClient.connect("myMQTTHelloClient");
    } while(!connected && (i < MAX_COUNT));

    if(connected) {
        pc.printf("MQTT broker connected.\r\n");
        mqttClient.subscribe("outdoor/temperature");
        pc.printf("Subscribed to: outdoor/temperature\r\n");
    }
    else {
        pc.printf("Failed to connect to MQTT broker.\r\n");
    }

    while(1) {
        t = time(NULL);
        if(t > (lastTime + 5)) {
            lastTime = t;
            if(connected) {
                mqttClient.publish("example/hello", payload);
                pc.printf("Published: example/hello\r\n");
            }
        }

        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);
}