/in/foo.c: In function 'main':
/in/foo.c:7:2: warning: implicit declaration of function 'scanf_s' [-Wimplicit-function-declaration]
  scanf_s("%d%d%d",&year,&month,&day);
  ^~~~~~~
/in/foo.c:8:29: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
  if (month%400==0||year%4==0&&year%100!=0){
                    ~~~~~~~~~^~~~~~~~~~~~~
/tmp/cc9bbclw.o: In function `main':
foo.c:(.text.startup+0x85): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status