/in/foo.c: In function 'main':
/in/foo.c:70:2: warning: implicit declaration of function 'scanf_s' [-Wimplicit-function-declaration]
  scanf_s("%d %d %d", &today.year, &today.month, &today.day);
  ^~~~~~~
/tmp/ccLxjFUu.o: In function `main':
foo.c:(.text.startup+0x1b): undefined reference to `scanf_s'
foo.c:(.text.startup+0x2e): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status