Error in readMessages from generated ROS message

Hi all,

I have some custom ROS messages that I have loaded into MATLAB utilizing the rosgenmsg routine. Everything went fine and they show up when calling rosmsg list.

When I went to read messages of the custom type from my rosbag I received the following error:

Reference to non-existent field 'Forces_'.

Error in robotics.ros.custom.msggen.sfm_msgs.Forces/reload (line 165)
          Forces_Cell = arrayfun(@(x) feval([obj.SfmMsgsForceClass '.loadobj'], x), strObj.Forces_, 'UniformOutput', false);

Error in robotics.ros.Message/fromStruct (line 172)
              obj.reload(sObj);

Error in robotics.ros.BagSelection/readMessages (line 280)
                  msgs{i}.fromStruct(msgStructs{i});

I did some investigation on how the function works and where the error comes from. In

robotics.ros.Message/fromStruct

the code tries to create a message object from the bag information (which is in struct format). For my custom message of type

sfm_msgs/Forces

the field names for the message and the structs (these are autogenerated by rosgenmsg) differ:

K>> msgs{1}

ans = 

ROS Forces message with properties:

    MessageType: 'sfm_msgs/Forces'
         Header: [1×1 Header]
        Forces_: [0×1 Force]

versus

K>> msgStructs{1}

ans = 

struct with fields:

    MessageType: 'sfm_msgs/Forces'
         Header: [1×1 struct]
         Forces: [1×6 struct]

This leads to the top-most error of my error message. As mentioned, this naming difference is autogenerated by the rosgenmsg routine.

The original .msg message definitions are:

Forces.msg:

std_msgs/Header header
Force[] forces

Force.msg:

uint8 ATTRACTIVE=0
uint8 INT_ROBOT=1
uint8 INT_OBSTACLE=2
uint8 INT_HUMAN=3
uint8 type
string entity_name
geometry_msgs/Vector3 force

So... yeah. I have no idea how to fix this and as far as I am considered this is a bug in the rosgenmsg routine. Hopefully some of you guru's have an idea!

P.S. I am using MATLAB Version: 9.4.0.813654 (R2018a) with Robotics System Toolbox Version 2.0 (R2018a)

Answer 1

[Joep Linssen]

I contacted MATLAB support and they let me know that there are a couple of known bugs with the readMessages function and that I should use the 'DataFormat', 'struct' options. This options is faster, does not require custom message code generation (no rosgenmsg call) and thus does not induce my error. I tried this and it works.

The change I thus made was; from:

readMessages(bagfile)

to:

readMessages(bagfile, 'DataFormat', 'struct')

This circumvents the above mentioned bug.

[Stefan Arnautu]

What version of Matlab are you using?

I use 2017b and I get this error:

Error using robotics.ros.BagSelection/readMessages
Too many input arguments.

Edit:

Just saw you have 2018a, sorry.

The solution doesn't work for 2017b.

[Joep Linssen]

indeed it doesn't. It requires Robotics Toolbox v2.0 which comes with 2018a and later, whereas 2017b still uses v1.0. I recommend switching to 2018a if possible!