/in/foo.c: In function 'main':
/in/foo.c:6:2: warning: implicit declaration of function 'scanf_s' [-Wimplicit-function-declaration]
  scanf_s("%d%d\n%d%d", &b1, &a1, &b2, &a2);
  ^~~~~~~
/tmp/ccAI9Ocn.o: In function `main':
foo.c:(.text.startup+0x20): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status