To whom it concerns,
I was reading the RIOT documentation on this page
when I noticed this minor error in the example introduced by “An examplary doxygen documentation in a header file can look like this.”
the last line in the example should be
int set_foobar(int state, int *old_state);
int set_foobar(int state, int old_state);
It is a funny example but it cannot be an out param in C unless old_state is passed in as an address.