foo.c: In function 'main':
foo.c:4:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%d",&n);
  ^~~~~
foo.c:4:2: warning: incompatible implicit declaration of built-in function 'scanf'
foo.c:4:2: note: include '<stdio.h>' or provide a declaration of 'scanf'
foo.c:10:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("%d\n",r);
  ^~~~~~
foo.c:10:2: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:10:2: note: include '<stdio.h>' or provide a declaration of 'printf'

自豪的采用jd5进行评测(github.com/masnn/jd5)