foo.c: In function 'main':
foo.c:18:6: warning: variable 'fu' set but not used [-Wunused-but-set-variable]
  int fu = 1;
      ^~
foo.c:15:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%s",line);
  ^~~~~~~~~~~~~~~~
foo.c: In function 'cal':
foo.c:121:19: warning: 'res' may be used uninitialized in this function [-Wmaybe-uninitialized]
  stack_n[top_n++] = res;
  ~~~~~~~~~~~~~~~~~^~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。