Onewire temperatures to MQTT broker server - arduino

Question

I am trying to modify the code from the example included in the onewire library for arduino so that no matter how many onewire devices I have plugged it will always find them and publish it to a MQTT using the device ID and the current temperature. I have gotten it to publish the temperature, but am having trouble adding the device ID or ROM which is in HEX to my topic.
So for example i want it to appear like this. Note the topic and msg for MQTT need to be Char* (more info here: http://knolleary.net/arduino-client-for-mqtt/api/#publish1)
topic = Celsius eg 12.09
payload (or msg) = \home[ROM]\temperature\current eg. \home\2894AA6220025\temperature\current
(just an example of the output you normally get when you run the code without my additions, this is the serial output!! notice the ROM and celsius that I want to use)
Have put my full code here, it is just a modification of the included onewire example with the pubsub MQTT part added on.
(see line 155 onwards) https://gist.github.com/matbor/5931466
//publish the temp now to mqtt topic
String strTopic = "/house/285A9282300F1/temperature/current"; // need to replace the 285A9282300F1 with the ROM ID on each LOOP!
char charMsg[10];
String strMsg = dtostrf(celsius, 4, 2, charMsg); //convert celsius to char
char charTopic[strTopic.length() + 1];
//char charMsg[strMsg.length() + 1];
strTopic.toCharArray(charTopic, sizeof(charTopic));
strMsg.toCharArray(charMsg, sizeof(charMsg));
client.publish(charTopic,charMsg);

Answer 1

Add this to the top of your sketch, outside of the loop function:
char hexChars[] = "0123456789ABCDEF";
#define HEX_MSB(v) hexChars[(v & 0xf0) >> 4]
#define HEX_LSB(v) hexChars[v & 0x0f]
This defines a pair of macros that return the most-significant and least-significant bytes of an int as the appropriate HEX character. (There may be more appropriate built-in's for this, but this is what I use out of habit).
The following code will insert the ROM, as a HEX string, into the topic. Note you can create the topic as a char[] directly - you don't need to go via a String object.
char charTopic[] = "/house/XXXXXXXXXXXXXXXX/temperature/current";
for (i = 0; i < 8; i++) {
charTopic[7+i*2] = HEX_MSB(addr[i]);
charTopic[8+i*2] = HEX_LSB(addr[i]);
}
For the payload, I'm not sure if it is 100% necessary, but I always explicitly initialise any char[] to all 0's when using as a buffer. This ensures whatever is written into the buffer will definitely be null-terminated. Again, you don't need to go via String types:
char charMsg[10];
memset(charMsg,'\0',10);
dtostrf(celsius, 4, 2, charMsg);
Finally, publish the message:
client.publish(charTopic,charMsg);