In setup(), it registers the event handlers mainloop.cpp. In loop(), it calls the actual eventloop() codes, i.e. dispatches events. There is a particular catch about this which sort of makes it ...

The GRBL ino file is just a list of includes , all the actual code is elsewhere, including it's own main.c that does not have loop() or setup(). It seems that is it digging main.cpp out of the arduino ...