Update CHANGES.txt for 1.9

Added new example for publishing in callback.

CHANGES.txt

@@ -1,22 +0,0 @@
-1.4
-    * Fixed connection lost handling
-
-1.3
-    * Fixed packet reading bug in PubSubClient.readPacket
-
-1.2
-    * Fixed compile error when used with arduino-0016 or later
-
-
-1.1
-    * Reduced size of library
-    * Added support for Will messages
-    * Clarified licensing - see LICENSE.txt
-
-
-1.0
-    * Only Quality of Service (QOS) 0 messaging is supported
-    * The maximum message size, including header, is 128 bytes
-    * The keepalive interval is set to 30 seconds
-    * No support for Will messages
-

PubSubClient.cpp

@@ -1,196 +0,0 @@
-/*
- PubSubClient.cpp - A simple client for MQTT.
-  Nicholas O'Leary
-  http://knolleary.net
-*/
-
-#include "WConstants.h"
-#include "PubSubClient.h"
-#include "Client.h"
-#include "string.h"
-
-#define MQTTCONNECT 1<<4
-#define MQTTPUBLISH 3<<4
-#define MQTTSUBSCRIBE 8<<4
-
-PubSubClient::PubSubClient(uint8_t *ip, uint16_t port, void (*callback)(char*,uint8_t*,int)) : _client(ip,port) {
-   this->callback = callback;
-}
-int PubSubClient::connect(char *id) {
-   return connect(id,0,0,0,0);
-}
-
-int PubSubClient::connect(char *id, char* willTopic, uint8_t willQos, uint8_t willRetain, char* willMessage) {
-   if (!connected()) {
-      if (_client.connect()) {
-         nextMsgId = 1;
-         uint8_t d[9] = {0x00,0x06,0x4d,0x51,0x49,0x73,0x64,0x70,0x03};
-         uint8_t length = 0;
-         int j;
-         for (j = 0;j<9;j++) {
-            buffer[length++] = d[j];
-         }
-         if (willTopic) {
-            buffer[length++] = 0x06|(willQos<<3)|(willRetain<<5);
-         } else {
-            buffer[length++] = 0x02;
-         }
-         buffer[length++] = 0;
-         buffer[length++] = (KEEPALIVE/1000);
-         length = writeString(id,buffer,length);
-         if (willTopic) {
-            length = writeString(willTopic,buffer,length);
-            length = writeString(willMessage,buffer,length);
-         }
-         write(MQTTCONNECT,buffer,length);
-         while (!_client.available()) {}
-         uint8_t len = readPacket();
-         
-         if (len == 4 && buffer[3] == 0) {
-            lastActivity = millis();
-            return 1;
-         }
-      }
-      _client.stop();
-   }
-   return 0;
-}
-
-uint8_t PubSubClient::readByte() {
-   while(!_client.available()) {}
-   return _client.read();
-}
-
-uint8_t PubSubClient::readPacket() {
-   uint8_t len = 0;
-   buffer[len++] = readByte();
-   uint8_t multiplier = 1;
-   uint8_t length = 0;
-   uint8_t digit = 0;
-   do {
-      digit = readByte();
-      buffer[len++] = digit;
-      length += (digit & 127) * multiplier;
-      multiplier *= 128;
-   } while ((digit & 128) != 0);
-   
-   for (int i = 0;i<length;i++)
-   {
-      if (len < MAX_PACKET_SIZE) {
-         buffer[len++] = readByte();
-      } else {
-         readByte();
-         len = 0; // This will cause the packet to be ignored.
-      }
-   }
-
-   return len;
-}
-
-int PubSubClient::loop() {
-   if (connected()) {
-      long t = millis();
-      if (t - lastActivity > KEEPALIVE) {
-         _client.write(192);
-         _client.write((uint8_t)0);
-         lastActivity = t;
-      }
-      if (_client.available()) {
-         uint8_t len = readPacket();
-         if (len > 0) {
-            uint8_t type = buffer[0]>>4;
-            if (type == 3) { // PUBLISH
-               if (callback) {
-                  uint8_t tl = (buffer[2]<<3)+buffer[3];
-                  char topic[tl+1];
-                  for (int i=0;i<tl;i++) {
-                     topic[i] = buffer[4+i];
-                  }
-                  topic[tl] = 0;
-                  // ignore msgID - only support QoS 0 subs
-                  uint8_t *payload = buffer+4+tl;
-                  callback(topic,payload,len-4-tl);
-               }
-            } else if (type == 12) { // PINGREG
-               _client.write(208);
-               _client.write((uint8_t)0);
-               lastActivity = t;
-            }
-         }
-      }
-      return 1;
-   }
-   return 0;
-}
-
-int PubSubClient::publish(char* topic, char* payload) {
-   return publish(topic,(uint8_t*)payload,strlen(payload));
-}
-
-
-int PubSubClient::publish(char* topic, uint8_t* payload, uint8_t plength) {
-   if (connected()) {
-      uint8_t length = writeString(topic,buffer,0);
-      int i;
-      for (i=0;i<plength;i++) {
-         buffer[length++] = payload[i];
-      }
-      //header |= 1; retain
-      write(MQTTPUBLISH,buffer,length);
-      return 1;
-   }
-   return 0;
-}
-
-
-int PubSubClient::write(uint8_t header, uint8_t* buf, uint8_t length) {
-   _client.write(header);
-   _client.write(length);
-   for (int i=0;i<length;i++) {
-      _client.write(buf[i]);
-   }
-   return 0;
-}
-
-
-void PubSubClient::subscribe(char* topic) {
-   if (connected()) {
-      uint8_t length = 2;
-      nextMsgId++;
-      buffer[0] = nextMsgId >> 8;
-      buffer[1] = nextMsgId - (buffer[0]<<8);
-      length = writeString(topic, buffer,length);
-      buffer[length++] = 0; // Only do QoS 0 subs
-      write(MQTTSUBSCRIBE,buffer,length);
-   }
-}
-
-void PubSubClient::disconnect() {
-   _client.write(224);
-   _client.write((uint8_t)0);
-   _client.stop();
-   lastActivity = millis();
-}
-
-uint8_t PubSubClient::writeString(char* string, uint8_t* buf, uint8_t pos) {
-   char* idp = string;
-   uint8_t i = 0;
-   pos += 2;
-   while (*idp) {
-      buf[pos++] = *idp++;
-      i++;
-   }
-   buf[pos-i-2] = 0;
-   buf[pos-i-1] = i;
-   return pos;
-}
-
-
-int PubSubClient::connected() {
-   int rc = (int)_client.connected();
-   if (!rc) _client.stop();
-   return rc;
-}
-
-
-

PubSubClient.h

@@ -1,40 +0,0 @@
-/*
- PubSubClient.h - A simple client for MQTT.
-  Nicholas O'Leary
-  http://knolleary.net
-*/
-
-#ifndef PubSubClient_h
-#define PubSubClient_h
-
-#include "Client.h"
-
-#define MAX_PACKET_SIZE 128
-#define KEEPALIVE 15000 // max value = 255000
-
-
-class PubSubClient {
-private:
-   Client _client;
-   uint8_t buffer[MAX_PACKET_SIZE];
-   uint8_t nextMsgId;
-   long lastActivity;
-   void (*callback)(char*,uint8_t*,int);
-   uint8_t readPacket();
-   uint8_t readByte();
-   int write(uint8_t header, uint8_t* buf, uint8_t length);
-   uint8_t writeString(char* string, uint8_t* buf, uint8_t pos);
-public:
-   PubSubClient(uint8_t *, uint16_t, void(*)(char*,uint8_t*,int));
-   int connect(char *);
-   int connect(char*, char*, uint8_t, uint8_t, char*);
-   void disconnect();
-   int publish(char *, char *);
-   int publish(char *, uint8_t *, uint8_t);
-   void subscribe(char *);
-   int loop();
-   int connected();
-};
-
-
-#endif

PubSubClient/CHANGES.txt

@@ -0,0 +1,47 @@
+1.9
+   * Do not split MQTT packets over multiple calls to _client->write()
+   * API change: All constructors now require an instance of Client
+      to be passed in. 
+   * Fixed example to match 1.8 api changes - dpslwk
+   * Added username/password support - WilHall
+   * Added publish_P - publishes messages from PROGMEM - jobytaffey
+
+1.8
+    * KeepAlive interval is configurable in PubSubClient.h
+    * Maximum packet size is configurable in PubSubClient.h
+    * API change: Return boolean rather than int from various functions
+    * API change: Length parameter in message callback changed 
+       from int to unsigned int
+    * Various internal tidy-ups around types
+1.7
+    * Improved keepalive handling
+    * Updated to the Arduino-1.0 API
+1.6
+    * Added the ability to publish a retained message
+
+1.5
+    * Added default constructor
+    * Fixed compile error when used with arduino-0021 or later
+
+1.4
+    * Fixed connection lost handling
+
+1.3
+    * Fixed packet reading bug in PubSubClient.readPacket
+
+1.2
+    * Fixed compile error when used with arduino-0016 or later
+
+
+1.1
+    * Reduced size of library
+    * Added support for Will messages
+    * Clarified licensing - see LICENSE.txt
+
+
+1.0
+    * Only Quality of Service (QOS) 0 messaging is supported
+    * The maximum message size, including header, is 128 bytes
+    * The keepalive interval is set to 30 seconds
+    * No support for Will messages
+

PubSubClient/LICENSE.txt

@@ -1,4 +1,4 @@
-Copyright (c) 2008-2009 Nicholas O'Leary
+Copyright (c) 2008-2012 Nicholas O'Leary
 
 Permission is hereby granted, free of charge, to any person obtaining
 a copy of this software and associated documentation files (the

PubSubClient/PubSubClient.cpp

@@ -0,0 +1,337 @@
+/*
+ PubSubClient.cpp - A simple client for MQTT.
+  Nicholas O'Leary
+  http://knolleary.net
+*/
+
+#include "PubSubClient.h"
+#include <string.h>
+
+PubSubClient::PubSubClient(Client& client) {
+   this->_client = &client;
+}
+
+PubSubClient::PubSubClient(uint8_t *ip, uint16_t port, void (*callback)(char*,uint8_t*,unsigned int), Client& client) {
+   this->_client = &client;
+   this->callback = callback;
+   this->ip = ip;
+   this->port = port;
+}
+
+PubSubClient::PubSubClient(char* domain, uint16_t port, void (*callback)(char*,uint8_t*,unsigned int), Client& client) {
+   this->_client = &client;
+   this->callback = callback;
+   this->domain = domain;
+   this->port = port;
+}
+
+boolean PubSubClient::connect(char *id) {
+   return connect(id,NULL,NULL,0,0,0,0);
+}
+
+boolean PubSubClient::connect(char *id, char *user, char *pass) {
+   return connect(id,user,pass,0,0,0,0);
+}
+
+boolean PubSubClient::connect(char *id, char* willTopic, uint8_t willQos, uint8_t willRetain, char* willMessage)
+{
+   return connect(id,NULL,NULL,willTopic,willQos,willRetain,willMessage);
+}
+
+boolean PubSubClient::connect(char *id, char *user, char *pass, char* willTopic, uint8_t willQos, uint8_t willRetain, char* willMessage) {
+   if (!connected()) {
+      int result = 0;
+      
+      if (domain != NULL) {
+        result = _client->connect(this->domain, this->port);
+      } else {
+        result = _client->connect(this->ip, this->port);
+      }
+      
+      if (result) {
+         nextMsgId = 1;
+         uint8_t d[9] = {0x00,0x06,'M','Q','I','s','d','p',MQTTPROTOCOLVERSION};
+         // Leave room in the buffer for header and variable length field
+         uint16_t length = 5;
+         unsigned int j;
+         for (j = 0;j<9;j++) {
+            buffer[length++] = d[j];
+         }
+
+         uint8_t v;
+         if (willTopic) {
+            v = 0x06|(willQos<<3)|(willRetain<<5);
+         } else {
+            v = 0x02;
+         }
+
+         if(user != NULL) {
+            v = v|0x80;
+
+            if(pass != NULL) {
+               v = v|(0x80>>1);
+            }
+         }
+
+         buffer[length++] = v;
+
+         buffer[length++] = ((MQTT_KEEPALIVE) >> 8);
+         buffer[length++] = ((MQTT_KEEPALIVE) & 0xFF);
+         length = writeString(id,buffer,length);
+         if (willTopic) {
+            length = writeString(willTopic,buffer,length);
+            length = writeString(willMessage,buffer,length);
+         }
+
+         if(user != NULL) {
+            length = writeString(user,buffer,length);
+            if(pass != NULL) {
+               length = writeString(pass,buffer,length);
+            }
+         }
+         
+         write(MQTTCONNECT,buffer,length-5);
+         
+         lastInActivity = lastOutActivity = millis();
+         
+         while (!_client->available()) {
+            unsigned long t = millis();
+            if (t-lastInActivity > MQTT_KEEPALIVE*1000UL) {
+               _client->stop();
+               return false;
+            }
+         }
+         uint16_t len = readPacket();
+         
+         if (len == 4 && buffer[3] == 0) {
+            lastInActivity = millis();
+            pingOutstanding = false;
+            return true;
+         }
+      }
+      _client->stop();
+   }
+   return false;
+}
+
+uint8_t PubSubClient::readByte() {
+   while(!_client->available()) {}
+   return _client->read();
+}
+
+uint16_t PubSubClient::readPacket() {
+   uint16_t len = 0;
+   buffer[len++] = readByte();
+   uint8_t multiplier = 1;
+   uint16_t length = 0;
+   uint8_t digit = 0;
+   do {
+      digit = readByte();
+      buffer[len++] = digit;
+      length += (digit & 127) * multiplier;
+      multiplier *= 128;
+   } while ((digit & 128) != 0);
+   
+   for (uint16_t i = 0;i<length;i++)
+   {
+      if (len < MQTT_MAX_PACKET_SIZE) {
+         buffer[len++] = readByte();
+      } else {
+         readByte();
+         len = 0; // This will cause the packet to be ignored.
+      }
+   }
+
+   return len;
+}
+
+boolean PubSubClient::loop() {
+   if (connected()) {
+      unsigned long t = millis();
+      if ((t - lastInActivity > MQTT_KEEPALIVE*1000UL) || (t - lastOutActivity > MQTT_KEEPALIVE*1000UL)) {
+         if (pingOutstanding) {
+            _client->stop();
+            return false;
+         } else {
+            buffer[0] = MQTTPINGREQ;
+            buffer[1] = 0;
+            _client->write(buffer,2);
+            lastOutActivity = t;
+            lastInActivity = t;
+            pingOutstanding = true;
+         }
+      }
+      if (_client->available()) {
+         uint16_t len = readPacket();
+         if (len > 0) {
+            lastInActivity = t;
+            uint8_t type = buffer[0]&0xF0;
+            if (type == MQTTPUBLISH) {
+               if (callback) {
+                  uint16_t tl = (buffer[2]<<8)+buffer[3];
+                  char topic[tl+1];
+                  for (uint16_t i=0;i<tl;i++) {
+                     topic[i] = buffer[4+i];
+                  }
+                  topic[tl] = 0;
+                  // ignore msgID - only support QoS 0 subs
+                  uint8_t *payload = buffer+4+tl;
+                  callback(topic,payload,len-4-tl);
+               }
+            } else if (type == MQTTPINGREQ) {
+               buffer[0] = MQTTPINGRESP;
+               buffer[1] = 0;
+               _client->write(buffer,2);
+            } else if (type == MQTTPINGRESP) {
+               pingOutstanding = false;
+            }
+         }
+      }
+      return true;
+   }
+   return false;
+}
+
+boolean PubSubClient::publish(char* topic, char* payload) {
+   return publish(topic,(uint8_t*)payload,strlen(payload),false);
+}
+
+boolean PubSubClient::publish(char* topic, uint8_t* payload, unsigned int plength) {
+   return publish(topic, payload, plength, false);
+}
+
+boolean PubSubClient::publish(char* topic, uint8_t* payload, unsigned int plength, boolean retained) {
+   if (connected()) {
+      // Leave room in the buffer for header and variable length field
+      uint16_t length = 5;
+      length = writeString(topic,buffer,length);
+      uint16_t i;
+      for (i=0;i<plength;i++) {
+         buffer[length++] = payload[i];
+      }
+      uint8_t header = MQTTPUBLISH;
+      if (retained) {
+         header |= 1;
+      }
+      return write(header,buffer,length-5);
+   }
+   return false;
+}
+
+boolean PubSubClient::publish_P(char* topic, uint8_t* PROGMEM payload, unsigned int plength, boolean retained) {
+   uint8_t llen = 0;
+   uint8_t digit;
+   int rc;
+   uint16_t tlen;
+   int pos = 0;
+   int i;
+   uint8_t header;
+   unsigned int len;
+   
+   if (!connected()) {
+      return false;
+   }
+   
+   tlen = strlen(topic);
+   
+   header = MQTTPUBLISH;
+   if (retained) {
+      header |= 1;
+   }
+   buffer[pos++] = header;
+   len = plength + 2 + tlen;
+   do {
+      digit = len % 128;
+      len = len / 128;
+      if (len > 0) {
+         digit |= 0x80;
+      }
+      buffer[pos++] = digit;
+      llen++;
+   } while(len>0);
+   
+   pos = writeString(topic,buffer,pos);
+   
+   rc += _client->write(buffer,pos);
+   
+   for (i=0;i<plength;i++) {
+      rc += _client->write((char)pgm_read_byte_near(payload + i));
+   }
+   
+   lastOutActivity = millis();
+   return rc == len + 1 + plength;
+}
+
+boolean PubSubClient::write(uint8_t header, uint8_t* buf, uint16_t length) {
+   uint8_t lenBuf[4];
+   uint8_t llen = 0;
+   uint8_t digit;
+   uint8_t pos = 0;
+   uint8_t rc;
+   uint8_t len = length;
+   do {
+      digit = len % 128;
+      len = len / 128;
+      if (len > 0) {
+         digit |= 0x80;
+      }
+      lenBuf[pos++] = digit;
+      llen++;
+   } while(len>0);
+
+   buf[4-llen] = header;
+   for (int i=0;i<llen;i++) {
+      buf[5-llen+i] = lenBuf[i];
+   }
+   rc = _client->write(buf+(4-llen),length+1+llen);
+   
+   lastOutActivity = millis();
+   return (rc == 1+llen+length);
+}
+
+
+boolean PubSubClient::subscribe(char* topic) {
+   if (connected()) {
+      // Leave room in the buffer for header and variable length field
+      uint16_t length = 7;
+      nextMsgId++;
+      if (nextMsgId == 0) {
+         nextMsgId = 1;
+      }
+      buffer[0] = (nextMsgId >> 8);
+      buffer[1] = (nextMsgId & 0xFF);
+      length = writeString(topic, buffer,length);
+      buffer[length++] = 0; // Only do QoS 0 subs
+      return write(MQTTSUBSCRIBE|MQTTQOS1,buffer,length-5);
+   }
+   return false;
+}
+
+void PubSubClient::disconnect() {
+   buffer[0] = MQTTDISCONNECT;
+   buffer[1] = 0;
+   _client->write(buffer,2);
+   _client->stop();
+   lastInActivity = lastOutActivity = millis();
+}
+
+uint16_t PubSubClient::writeString(char* string, uint8_t* buf, uint16_t pos) {
+   char* idp = string;
+   uint16_t i = 0;
+   pos += 2;
+   while (*idp) {
+      buf[pos++] = *idp++;
+      i++;
+   }
+   buf[pos-i-2] = (i >> 8);
+   buf[pos-i-1] = (i & 0xFF);
+   return pos;
+}
+
+
+boolean PubSubClient::connected() {
+   int rc = (int)_client->connected();
+   if (!rc) _client->stop();
+   return rc;
+}
+

PubSubClient/PubSubClient.h

@@ -0,0 +1,75 @@
+/*
+ PubSubClient.h - A simple client for MQTT.
+  Nicholas O'Leary
+  http://knolleary.net
+*/
+
+#ifndef PubSubClient_h
+#define PubSubClient_h
+
+#include <Arduino.h>
+#include "Client.h"
+
+// MQTT_MAX_PACKET_SIZE : Maximum packet size
+#define MQTT_MAX_PACKET_SIZE 128
+
+// MQTT_KEEPALIVE : keepAlive interval in Seconds
+#define MQTT_KEEPALIVE 15
+
+#define MQTTPROTOCOLVERSION 3
+#define MQTTCONNECT     1 << 4  // Client request to connect to Server
+#define MQTTCONNACK     2 << 4  // Connect Acknowledgment
+#define MQTTPUBLISH     3 << 4  // Publish message
+#define MQTTPUBACK      4 << 4  // Publish Acknowledgment
+#define MQTTPUBREC      5 << 4  // Publish Received (assured delivery part 1)
+#define MQTTPUBREL      6 << 4  // Publish Release (assured delivery part 2)
+#define MQTTPUBCOMP     7 << 4  // Publish Complete (assured delivery part 3)
+#define MQTTSUBSCRIBE   8 << 4  // Client Subscribe request
+#define MQTTSUBACK      9 << 4  // Subscribe Acknowledgment
+#define MQTTUNSUBSCRIBE 10 << 4 // Client Unsubscribe request
+#define MQTTUNSUBACK    11 << 4 // Unsubscribe Acknowledgment
+#define MQTTPINGREQ     12 << 4 // PING Request
+#define MQTTPINGRESP    13 << 4 // PING Response
+#define MQTTDISCONNECT  14 << 4 // Client is Disconnecting
+#define MQTTReserved    15 << 4 // Reserved
+
+#define MQTTQOS0        (0 << 1)
+#define MQTTQOS1        (1 << 1)
+#define MQTTQOS2        (2 << 1)
+
+class PubSubClient {
+private:
+   Client* _client;
+   uint8_t buffer[MQTT_MAX_PACKET_SIZE];
+   uint16_t nextMsgId;
+   unsigned long lastOutActivity;
+   unsigned long lastInActivity;
+   bool pingOutstanding;
+   void (*callback)(char*,uint8_t*,unsigned int);
+   uint16_t readPacket();
+   uint8_t readByte();
+   boolean write(uint8_t header, uint8_t* buf, uint16_t length);
+   uint16_t writeString(char* string, uint8_t* buf, uint16_t pos);
+   uint8_t *ip;
+   char* domain;
+   uint16_t port;
+public:
+   PubSubClient(Client& client);
+   PubSubClient(uint8_t *, uint16_t, void(*)(char*,uint8_t*,unsigned int),Client& client);
+   PubSubClient(char*, uint16_t, void(*)(char*,uint8_t*,unsigned int),Client& client);
+   boolean connect(char *);
+   boolean connect(char *, char *, char *);
+   boolean connect(char *, char *, uint8_t, uint8_t, char *);
+   boolean connect(char *, char *, char *, char *, uint8_t, uint8_t, char*);
+   void disconnect();
+   boolean publish(char *, char *);
+   boolean publish(char *, uint8_t *, unsigned int);
+   boolean publish(char *, uint8_t *, unsigned int, boolean);
+   boolean publish_P(char *, uint8_t PROGMEM *, unsigned int, boolean);
+   boolean subscribe(char *);
+   boolean loop();
+   boolean connected();
+};
+
+
+#endif

PubSubClient/examples/mqtt_auth/mqtt_auth.ino

@@ -0,0 +1,39 @@
+/*
+ Basic MQTT example with Authentication
+ 
+  - connects to an MQTT server, providing username
+    and password
+  - publishes "hello world" to the topic "outTopic"
+  - subscribes to the topic "inTopic"
+*/
+
+#include <SPI.h>
+#include <Ethernet.h>
+#include <PubSubClient.h>
+
+// Update these with values suitable for your network.
+byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
+byte server[] = { 172, 16, 0, 2 };
+byte ip[]     = { 172, 16, 0, 100 };
+
+void callback(char* topic, byte* payload, unsigned int length) {
+  // handle message arrived
+}
+
+EthernetClient ethClient;
+PubSubClient client(server, 1883, callback, ethClient);
+
+void setup()
+{
+  Ethernet.begin(mac, ip);
+  if (client.connect("arduinoClient", "testuser", "testpass")) {
+    client.publish("outTopic","hello world");
+    client.subscribe("inTopic");
+  }
+}
+
+void loop()
+{
+  client.loop();
+}
+

PubSubClient/examples/mqtt_basic/mqtt_basic.ino

@@ -0,0 +1,38 @@
+/*
+ Basic MQTT example 
+ 
+  - connects to an MQTT server
+  - publishes "hello world" to the topic "outTopic"
+  - subscribes to the topic "inTopic"
+*/
+
+#include <SPI.h>
+#include <Ethernet.h>
+#include <PubSubClient.h>
+
+// Update these with values suitable for your network.
+byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
+byte server[] = { 172, 16, 0, 2 };
+byte ip[]     = { 172, 16, 0, 100 };
+
+void callback(char* topic, byte* payload, unsigned int length) {
+  // handle message arrived
+}
+
+EthernetClient ethClient;
+PubSubClient client(server, 1883, callback, ethClient);
+
+void setup()
+{
+  Ethernet.begin(mac, ip);
+  if (client.connect("arduinoClient")) {
+    client.publish("outTopic","hello world");
+    client.subscribe("inTopic");
+  }
+}
+
+void loop()
+{
+  client.loop();
+}
+

PubSubClient/examples/mqtt_publish_in_callback/mqtt_publish_in_callback.ino

@@ -0,0 +1,51 @@
+/*
+ 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.
+  
+*/
+
+#include <SPI.h>
+#include <Ethernet.h>
+#include <PubSubClient.h>
+
+// Update these with values suitable for your network.
+byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
+byte server[] = { 172, 16, 0, 2 };
+byte ip[]     = { 172, 16, 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) {
+  client.publish("outTopic", payload, length);
+}
+
+void setup()
+{
+  
+  Ethernet.begin(mac, ip);
+  if (client.connect("arduinoClient")) {
+    client.publish("outTopic","hello world");
+    client.subscribe("inTopic");
+  }
+}
+
+void loop()
+{
+  client.loop();
+}
+