/in/ex4.c: In function 'deg2rad':
/in/ex4.c:11:30: error: 'M_PI' undeclared (first use in this function); did you mean '__P'?
   return deg / (double)180 * M_PI;
                              ^~~~
                              __P
/in/ex4.c:11:30: note: each undeclared identifier is reported only once for each function it appears in
/in/ex4.c:12:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1: all warnings being treated as errors