colcon-build for a cmake project doesn't pickup CMAKE_ vars from the environment

[jfinken]

The below describes a reduction exhibiting the symptom:

• ROS2-Humble-Desktop docker image from OSRF (osrf/ros:humble-desktop):

  $ printenv | grep ROS
  ROS_VERSION=2
  ROS_PYTHON_VERSION=3
  ROS_LOCALHOST_ONLY=0
  ROS_DISTRO=humble

• created a ROS2 package via ros2 pkg create --build-type ament_cmake <package_name>
• Export, for example, these variables within the environment. Crucially in my case, CMAKE_C_FLAGS and CMAKE_CXX_FLAGS are exported by a pre-colcon-build step involving conan.

  export CMAKE_C_FLAGS="foo"
  export CMAKE_CXX_FLAGS="bar; baz"

• In the CMakeLists.txt, print out CMAKE_C_FLAGS and CMAKE_CXX_FLAGS:

  message("CMAKE_C_FLAGS are:   ${CMAKE_C_FLAGS}")
  message("CMAKE_CXX_FLAGS are: ${CMAKE_CXX_FLAGS}")

• During a clean colcon build, those variables are empty:

  $ colcon build
  ...
  CMAKE_C_FLAGS are:    
  CMAKE_CXX_FLAGS are: 

The solution is to force the set in CMakeLists.txt. But why?

set(CMAKE_C_FLAGS "$ENV{CMAKE_C_FLAGS} $ENV{CFLAGS}" CACHE STRING "" FORCE)
set(CMAKE_CXX_FLAGS "$ENV{CMAKE_CXX_FLAGS} $ENV{CXXFLAGS}" CACHE STRING "" FORCE)

$ colcon build
...
CMAKE_C_FLAGS are:   foo 
CMAKE_CXX_FLAGS are: foo; bar; baz

[cottsay]

I can't find any evidence that CMAKE_<LANG>_FLAGS is expected to take it's value from an environment variable with the same name. In fact, the documentation specifically says that CMAKE_C_FLAGS and CMAKE_CXX_FLAGS take their initial value from the CFLAGS and CXXFLAGS environment variables respectively.

When I use the proper environment variables, this seems to work as expected. Also, I can reproduce the behavior you described here outside of colcon by simply invoking cmake directly on the project, so I don't see how this could be a problem in colcon itself.

[cottsay]

@jfinken, do you still believe there is a problem with colcon's behavior here, or can this ticket be closed?

[jfinken]

@jfinken, do you still believe there is a problem with colcon's behavior here, or can this ticket be closed?

My apologies. Yes this ticket can be closed. Thank you.