Incorrect output, length error when using a String object about pubsubclient

[knolleary]

Hi,

the client library has a maximum packet size of 128 bytes - which your code is exceeding. This isn't a scenario the code guards against (it does for incoming messages, but not for outgoing).

The length value is written to the header and then that number of bytes are copied to the internal payload buffer. If length exceeds the internal buffer, it carries on writing beyond the end of the array, which could cause memory corruption.

I'll put a fix in that guards against this, but in the meantime, if you want to increase the max packet size, change MAX_PACKET_SIZE in PubSubClient.h - this will obviously increase the memory usage of the code.

Nick

[DougieLawson]

Thanks Nick. I'll change the length to 140 (you can guess why).

[knolleary]

Actually, looking at it after a cup of tea, the write method only supports a single byte of length - which limits you to a 128 byte message. So you cannot simply change the #define.

When I get a chance later today, I'll put a fix in that enables multi-byte length encoding.

Once that is in place you'll be able to increase the message size.

You'll need to allow for the message headers when determining your message size. The sum is:
1 byte fixed header
2 bytes length (for messages 128 < length <16383 )
2 bytes for the topic length
topic bytes
payload bytes

which gives you 5+topic+payload bytes.

This assumes a qos 0 message (which is all the arduino lib currently supports), otherwise you need another 2 bytes for message ID in there.

[DougieLawson]

I've found that forked copy of your library. I'll take a look at that.

[knolleary]

Which forked copy? I haven't kept track of the handful of forks others have done, but I'm not aware of anyone making these changes already.

[DougieLawson]

https://github.com/karlp/pubsubclient

Or I'll put my S/370 (aka zSeries now) Assembler knowledge back in the cupboard, sharpen my C/C++ coding pencil and hack at your code.

I've been doing messaging and queuing for 30 years (along with lots of DL/I and DB2 databases), This MQTT stuff is like a little IMS thing. It's lots of fun. Look me up in BluePages.

[knolleary]

Reviewing the open issues. The library now handles larger packets (when the MAX packet size constant is changed in the .h file). Nothing more to do here.