Out-of-tree Modules

This describes the possibility to add an external module to the PX4 build.

External modules can use the same includes as internal modules and can interact with internal modules via uORB.

Usage

EXTERNAL_MODULES_LOCATION needs to point to a directory with the same structure as Firmware (and thus contains a directory called src).
There are two options: copy an existing module (eg. examples/px4_simple_app) to the external directory, or directly create a new module.
Rename the module (including MODULE in CMakeLists.txt) or remove it from the existing Firmware cmake build config. This is to avoid conflicts with internal modules.
Add a file $EXTERNAL_MODULES_LOCATION/CMakeLists.txt with content:

set(config_module_list_external
    modules/<new_module>
    PARENT_SCOPE
    )

add a line EXTERNAL to the modules/<new_module>/CMakeLists.txt within px4_add_module, for example like this:

px4_add_module(
	MODULE modules__test_app
	MAIN test_app
	STACK_MAIN 2000
	SRCS
		px4_simple_app.c
	DEPENDS
		platforms__common
	EXTERNAL
	)

Execute make posix EXTERNAL_MODULES_LOCATION=<path>. Any other build target can be used, but the build directory must not yet exist. If it already exists, you can also just set the cmake variable in the build folder. For the following incremental builds EXTERNAL_MODULES_LOCATION does not need to be specified anymore.