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Generate custom ROS messages for Matlab up to version 2020a

Off the box, Matlab only supports ROS standard messages and can't handle custom messages. There is an add-on available from Matlab to generate code (mix of Java and Matlab) to handle custom messages. The process is not fully automated and requires a one-time generation and configuration of your Matlab environment.

You can use the Matlab command rosAddons to install rosgenmsg (see https://www.mathworks.com/help/ros/ug/create-custom-messages-from-ros-package.html). For this step, make sure you run Matlab as "super user": sudo matlab. This is the only time you should run Matlab with sudo. Successive users should all install the add-on without sudo.

To note, rosgenmsg has one rather annoying limitation, it expects a directory that contains directories of ROS packages. So if you clone crtk_msgs in catkin_ws/src you will need to use rosgenmsg on all packages in catkin_ws/src. To avoid this, you need to create a sub-directory in catkin_ws/src and then clone crtk_msgs in said sub-directory (for example ~/catkin_ws/src/crtk/crtk_msgs).

If you're cloning crtk_msgs by hand, you can use:

cd ~/catkin_ws/src
# clone in sub-directory crtk and rename to crtk_msgs
git clone https://github.com/collaborative-robotics/crtk_msgs crtk/crtk_msgs

At that point, you can finally generate the code (running Matlab without sudo):

rosgenmsg('~/catkin_ws/src/crtk')

Then follow instructions to edit javaclasspath.txt in ~/.matlab/R2020a (step 1). The tutorial recommends to use savepath (step 2) but this would apply to all users on the workstation so you should probably skip that step and read the following section.

Paths

Instead of setting the path for all users, you can use your startup script. To edit the user's startup.m, use:

edit(fullfile(userpath,'startup.m'))

In your startup.m, you can add the addpath commands that you want executed everytime your start Matlab:

% to locate crtk_msgs
addpath('~/catkin_ws/src/crtk/matlab_gen/msggen')
% to locate crtk client
addpath('~/catkin_ws/src/crtk/crtk_matlab_client')
% to locate dvrk code - only for dVRK users
addpath('~/catkin_ws/src/dvrk-ros/dvrk_matlab')

Then quit Matlab, restart it and test using:

which startup
% show message properties
rosmsg('show', 'crtk_msgs/operating_state')
% create a message
m = rosmessage('crtk_msgs/operating_state')

Very important debugging note

Matlab seems to create a cache of existing messages that is not being refreshed automatically. This is apparently populated when you first call rosmsg or rosmessage. So if you forgot to set your path and then called rosmsg (which will fail to find your new message types), adding the path with addpath is not enough to locate the new message types since the list of available messages will not be refreshed. At that point, it seems the only solution is to quit Matlab, restart Matlab and remember to set your paths before any call to rosmsg or rosmessage. The Matlab command rehash doesn't seem to fix the issue. The script startup.m can ensure that addpath is called before any ROS command.