/in/foo.c: In function 'main':
/in/foo.c:12:9: warning: 'yl' may be used uninitialized in this function [-Wmaybe-uninitialized]
  double diff_y = y1 - y2;
         ^~~~~~
/in/foo.c:18:19: note: 'yl' was declared here
  double x, y, xl, yl;
                   ^~
/in/foo.c:11:9: warning: 'xl' may be used uninitialized in this function [-Wmaybe-uninitialized]
  double diff_x = x1 - x2;
         ^~~~~~
/in/foo.c:18:15: note: 'xl' was declared here
  double x, y, xl, yl;
               ^~