/in/foo.c: In function 'main':
/in/foo.c:8:2: warning: implicit declaration of function 'scanf_s'; did you mean 'scanf'? [-Wimplicit-function-declaration]
  scanf_s("%d", &a);
  ^~~~~~~
  scanf
/in/foo.c:25:2: warning: implicit declaration of function 'system' [-Wimplicit-function-declaration]
  system("pause");
  ^~~~~~
/tmp/ccbg0efN.o: In function `main':
foo.c:(.text.startup+0x47): undefined reference to `scanf_s'
foo.c:(.text.startup+0x87): undefined reference to `scanf_s'
collect2: error: ld returned 1 exit status