From d724864095abee771ba6f603744ece811f164af9 Mon Sep 17 00:00:00 2001
From: Edwin vd Oetelaar PA2LVD <edwin@oetelaar.com>
Date: Sun, 26 Jun 2016 20:53:07 +0200
Subject: [PATCH] Reduce stack usage in loop(). No need to duplicate topic
 string onto stack before giving it to callback() Just move it one byte in
 buffer to add space for 'C' string end \0x00

---
 src/PubSubClient.cpp | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/src/PubSubClient.cpp b/src/PubSubClient.cpp
index 9658c4a..5932bdb 100755
--- a/src/PubSubClient.cpp
+++ b/src/PubSubClient.cpp
@@ -306,12 +306,10 @@ boolean PubSubClient::loop() {
                 uint8_t type = buffer[0]&0xF0;
                 if (type == MQTTPUBLISH) {
                     if (callback) {
-                        uint16_t tl = (buffer[llen+1]<<8)+buffer[llen+2];
-                        char topic[tl+1];
-                        for (uint16_t i=0;i<tl;i++) {
-                            topic[i] = buffer[llen+3+i];
-                        }
-                        topic[tl] = 0;
+                        uint16_t tl = (buffer[llen+1]<<8)+buffer[llen+2]; /* topic length in bytes */
+                        memmove(buffer+llen+2,buffer+llen+3,tl); /* move topic inside buffer 1 byte to front */
+                        buffer[llen+2+tl] = 0; /* end the topic as a 'C' string with \x00 */
+                        char *topic = (char*) buffer+llen+2;
                         // msgId only present for QOS>0
                         if ((buffer[0]&0x06) == MQTTQOS1) {
                             msgId = (buffer[llen+3+tl]<<8)+buffer[llen+3+tl+1];