Keep getting extra data in message

[cihankurt98]

Let's say im sending the following message:

rostopic pub /velocity std_msgs/String "hello"

I get the following in my MQTT client: ��data�hello

As you can see, I never said something about the unknown character and data part. Where is it coming from? From messagepack? How can I remove it?

[cihankurt98]

I could remove the strange characters by switching from msgpack to json serialization. However, I don't want the "data" part. I can't do a string remove aswell, somehow it doesn't recognize it.

[cihankurt98]

It seems like when I go from MQTT to ROS I always get a deserialization error. No matter what serialization package I use. I tried msgpack and cbor2. There is always more data expected then there actually is. It happens when the mqtt_msg is being converted to a ROS message.

msg_dict = self._deserialize(mqtt_msg.payload) The line above gives me the following errors:

msgpack: unpack(b) received extra data.

cbor2: premature end of stream (expected to read 2478212847434349172 bytes, got 4 instead)

[mayershoc]

Hi cihankurt98, are you using python 2 or 3? If you are using python3, this might be related to a deserialization issue I used to get. Feel free to give my fork a try.

[ledmonster]

Sorry for replying too late. This is a specification. ROS primitive data type like Bool or String has its message on data attribute.

• http://docs.ros.org/en/melodic/api/std_msgs/html/msg/String.html
• http://docs.ros.org/en/melodic/api/std_msgs/html/msg/Bool.html

So these would be serialized to like {"data": True}.