/in/foo.cc: In function 'int main()':
/in/foo.cc:7:21: warning: format '%lf' expects argument of type 'double', but argument 2 has type 'double*' [-Wformat=]
    7 |         printf("%.9lf",&c);
      |                 ~~~~^  ~~
      |                     |  |
      |                     |  double*
      |                     double