MQTT: Arduino Publish di callback

/*

Publishing in the callback

 - connects to an MQTT server
 - subscribes to the topic "inTopic"
 - when a message is received, republishes it to "outTopic"

 This example shows how to publish messages within the
 callback function. The callback function header needs to
 be declared before the PubSubClient constructor and the
 actual callback defined afterwards.
 This ensures the client reference in the callback function
 is valid.

• /

1. include <SPI.h>
2. include <Ethernet.h>
3. include <PubSubClient.h>

// Update these with values suitable for your network. byte mac[] = { 0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED }; IPAddress ip(192, 168, 0, 4); IPAddress server(192, 168, 0, 100);

// Callback function header void callback(char* topic, byte* payload, unsigned int length);

EthernetClient ethClient; PubSubClient client(server, 1883, callback, ethClient);

// Callback function void callback(char* topic, byte* payload, unsigned int length) {

 // In order to republish this payload, a copy must be made
 // as the orignal payload buffer will be overwritten whilst
 // constructing the PUBLISH packet.

 // Allocate the correct amount of memory for the payload copy
 byte* p = (byte*)malloc(length);
 // Copy the payload to the new buffer
 memcpy(p,payload,length);
 client.publish("outTopic", p, length);
 // Free the memory
 free(p);

}

void setup() {

 Ethernet.begin(mac, ip);
 if (client.connect("arduinoClient")) {
   client.publish("outTopic","hello world");
   client.subscribe("inTopic");
 }

}

void loop() {

 client.loop();

}