The cross-compilation of ROS2 is a non trivial process. To tackle most of the possible issues, we decided to create a cross-compilation environment based on Docker.
While running sysroots/get_sysroot.sh or while building one of the provided Dockerfile you get the following error:
standard_init_linux.go:207: exec user process caused "exec format error"
Possible reasons: You are trying to build an ARM Docker image on an X86_64 machine. This requires running some ARM binaries which are not supported by default.
Solution: Follow the Allow Docker to run and build ARM containers instructions in our Docker setup.
If when you run the cross-compilation script you get the following output message
Summary: 0 packages finished [0.16s]
[0.384s]
Possible reasons: This means that colcon has not found any package in the provided workspace.
Solution: Check that the workspace contains a src directory with the source code that you want to cross-compile. Moreover, it is important that the packages are actually present in the workspace and that they are not only symlinked.
While cross-compiling the ROS2 SDK you may get the following or a similar error
/usr/bin/arm-linux-gnueabihf-ld: warning: libPocoFoundation.so.50, needed by /root/ws/install/lib/librmw_implementation.so, not found (try using -rpath or -rpath-link)
/usr/bin/arm-linux-gnueabihf-ld: warning: libyaml.so, needed by /root/ws/install/lib/librcl_yaml_param_parser.so, not found (try using -rpath or -rpath-link)
/root/ws/install/lib/librcl_yaml_param_parser.so: undefined reference to `yaml_event_delete'
/root/ws/install/lib/librmw_implementation.so: undefined reference to `typeinfo for Poco::LibraryLoadException'
...
Possible reasons: the linker that you are using is looking for these libraries only in the sysroot, while they should be in the install/lib directory of the current workspace.
Solution: in the .toolchain.cmake file that you are using, add to the CMAKE_C_FLAGS and CMAKE_CXX_FLAGS a linker path to the current workspace library directory, for example -Wl,-rpath-link=/root/ws/install/lib.
If there are still problems, a work-around consists in cross-compiling up to these libraries and then adding them to the sysroot (a symbolic link is enough).
You try to source a cross-compiled workspace, but you get this error:
The build time path "/root/ws/install" doesn't exist.
Either source a script for a different shell or set the environment variable "COLCON_CURRENT_PREFIX" explicitly.
Possible reasons: the setup scripts inside the packages have hardcoded inside the path where they have been built. With cross-compilation, this path will become invalid once you move the workspace to your target platform.
Solution: follow the advice and set the COLCON_CURRENT_PREFIX environment variable to the current location of the install directory of the workspace.
export COLCON_CURRENT_PREFIX=<path_to_cross_compiled_ws>/install
You try to run a ROS2 node using the standard CLI command
ros2 run examples_rclcpp_minimal_publisher publisher_lambda
However, you get a Python error similar to this one:
Failed to load entry point 'launch': No module named 'rclpy._rclpy'
Possible reasons: In order to use the CLI commands you need to have cross-compiled the ROS2 Python packages.
Note that an alternative cause could be that you cross-compiled all the required packages, but the PYTHON_SOABI value specified in the toolchain.cmake file is wrong.
Solution: Besides cross-compiling all the Python packages and checking the PYTHON_SOABI value, there exists a quicker solution, suitable for these platforms where Python3 is not available.
Indeed the install/lib/<package_name> directory of a ROS2 workspace contains all the binaries of a specific package. The ROS2 CLI is just a convenient tool for not having to specify the full path to this binaries, but it is not required.
You cross-compiled the ROS2 SDK, but when you run any executable you get the following error:
error while loading shared libraries: libPocoFoundation.so.50: cannot open shared object file: No such file or directory
Possible reasons: the poco_vendor package checks whether Poco Foundation is installed on your computer/sysrooot. If it finds it, it doesn't do anything, but if the library is not present, it installs it.
The installed library will be placed both in the ros2 workspace install directory as well as in the default library path of your computer/sysroot.
If you cross-compile the ROS2 SDK multiple times without restoring the sysroot, what happens is that after the first run the library is installed and included in the install/lib directory. However a second run will erase the workspace and this time the library will not be added to the workspace as CMake already finds it in the system. Thus if you copy this last workspace to the target board, it will not include the libPocoFoundation library.
Solution: you can generate again the sysroot, create a workspace containing only the poco_vendor package and cross-compile it. Note that you don't need the ROS2 SDK for this operation. Then copy the cross-compiled libraries to the target board.
If your target is a armv7l (i.e. 32-bit Raspberry Pi or similar) there could be a byte alignment error during the cross-compilation.
This problem was present in the first release of ROS2 Crystal and then it has been addressed by this PR, which, after patch 1 has now been merged, so no further actions should be required.
If you have any problems, ensure that the src/ros2/rcl package is checking out version 0.6.4 or above.