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Geoffrey Hunter Author

Pre-defined Messages

Sensor-based messages are contained within the directory:

/opt/ros/<ros version>/share/sensor_msgs/

Defining Custom Messages

ROS messages are defined in .msg files.

An automatic MD5 sum of the .msg file is created and is checked when nodes try and publish/subscribe to topics. Nodes can only communicate when both the message type and MD5 sums match.

The Header

ROS messages may include a special header. This can contain a time stamp and frame ID.

MD5 Checksums

If the message types are not the same, you will normally get a runtime error on launch similar to:

[ERROR] [1481703140.912974056, 0.100000000]: Client [<node_name>] wants topic <topic_name> to have datatype/md5sum [<path>/cd5e73d190d741a2f92e81eda573aca7], but our version has [<path>/2d3a8cd499b9b4a0249fb98fd05cfa48]. Dropping connection.