How to disable message generation from within the CMakeLists.txt?

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Afaik, it only looks at that environment variable. See here.

CMake can set env vars, but only for the current and child processes.

Perhaps setting ROS_LANG_DISABLE like so before the call to find_package(Catkin ..) (I haven't checked the syntax):

set(ENV{ROS_LANG_DISABLE} genlisp)

Note: if you use catkin_make (or anything which coalesces multiple build contexts into a single one) this will -- if it works -- make one package set this value for all others in the active context.

I also have to say I don't find this a very clean approach or a nice thing to do.

And I'm not referring to setting the env var from a build script, but to packages determining for which languages files get generated. That seems like something they should not be doing.

@gvdhoorn what are some examples of ROS-blessed build tools that do _not_ coalesce multiple build contexts into a single one? Could set(ENV{ROS_LANG_DISABLE} genlisp) be made to work with, say, regular catkin and/or colcon?

what are some examples of ROS-blessed build tools that do _not_ coalesce multiple build contexts into a single one?

catkin_tools, catkin_make_isolated and colcon. Although I'd not use the last one for anything but ROS 2 workspaces myself.

And plain CMake itself of course.

Whether it works: I don't know.

What is your rationale for wanting to disable generators from within a package?