What is the difference between message_generation and genmsg?

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Hello,

I was following the tutorials and I have a question about the message generation. Once you generate a service with messages you need to modify the package.xml file:

<build_depend>message_generation</build_depend>
<run_depend>message_runtime</run_depend>

That part is cover, I have no problems there. Now, after that, you need to modify the CMakeLists file, you need to add:

find_package(catkin REQUIRED_COMPONENTS roscpp rospy std_msgs message_generation)
...
add_dependencies(<node_name> package_name_generate_messages_cpp)

But then, following the tutorials those two lines have different names for the message generation:

find_package(catkin REQUIRED_COMPONENTS roscpp rospy std_msgs genmsg)
...
add_dependencies(<node_name> package_name_gencpp)

What is the difference between message_generation and genmsg?

In the add_dependencies function, the second function declared is for the messages, but why one is package_name_gencpp and the other package_name_generate_messages_cpp??? I am confused here. So, the name for the message generation dependency can be whatever I want. But where is that defined, how does ROS know the second dependency is for messages???

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Thanks.