/in/foo.c: In function 'main':
/in/foo.c:20:2: warning: implicit declaration of function 'scanf_s' [-Wimplicit-function-declaration]
  scanf_s("%d%d", &n1, &n2);
  ^~~~~~~
/tmp/ccrzd2cW.o: In function `main':
foo.c:(.text.startup+0x2f): undefined reference to `scanf_s'
foo.c:(.text.startup+0x49): undefined reference to `scanf_s'
foo.c:(.text.startup+0x89): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status