PX4 Continuous Integration

PX4 builds and testing are spread out over multiple continuous integration services.

Travis-ci also has a MacOS posix sitl build which includes testing.

CircleCI tests the proposed next version of GCC to be used for stable firmware releases using the docker image . It uses the makefile target quick_check which compiles px4fmu-v4_default, posix_sitl_default, runs testing, and verifies code style.