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