/in/foo.c: In function 'main':
/in/foo.c:16:2: warning: implicit declaration of function 'gets_s' [-Wimplicit-function-declaration]
  gets_s(s1), gets_s(s2);
  ^~~~~~
/tmp/ccxnY7ts.o: In function `main':
foo.c:(.text.startup+0x17): undefined reference to `gets_s'
foo.c:(.text.startup+0x21): undefined reference to `gets_s'
collect2: error: ld returned 1 exit status