/in/foo.c: In function 'main':
/in/foo.c:4:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%d %d %d",&year,&mouth,&day);
  ^~~~~
/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:23:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("%d\n",num);
  ^~~~~~
/in/foo.c:23:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:23:2: note: include '<stdio.h>' or provide a declaration of 'printf'