How to subscribe hark_msgs types.

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Have you declared a dependency on hark_msgs in both your CMakeLists.txt and your package.xml? catkin needs that information to properly setup your include path(s).

See the Resolving dependencies section on the How to do common tasks page of the catkin documentation.

Edit: on second thought, the fact that you get that error message indicates that you are declaring a dependency, but that catkin / CMake cannot resolve it.

You say you "found the folder where hark_msg related (.msg) files are". Can you make sure that the package is on your package path? Does this work: rospack find hark_msgs?

Edit2: as far as I can tell hark_msgs is not a catkin package (ie: it is a 'dry' package), while your own package is a catkin package (ie: 'wet'). Wet packages cannot depend on dry packages, see Overlaying Dry and Wet Workspaces, which is why you are getting the error about catkin not being able to find the hark_msgs configuration file.

There is probably a manifest.xml file in the hark_msgs directory. Hydro should still be able to build dry packages (with rosbuild), so if you create a rosbuild package you should be able to use the hark stacks.

I don't know where you're getting hark_msgs ROS package, but there's at least a catkinzied package jsk_hark_msgs. While it's in an organizational package suite (good'ol ***-ros-pkg manner), it should be able to be depended upon from any ROS package.