/in/foo.c: In function 'main':
/in/foo.c:4:19: error: expected ';', ',' or ')' before '&' token
  void my_swap(int &x, int &y);
                   ^
/in/foo.c:7:2: warning: implicit declaration of function 'scanf_s' [-Wimplicit-function-declaration]
  scanf_s("%d,%d,%d", &n1, &n2, &n3);
  ^~~~~~~
/in/foo.c:10:3: warning: implicit declaration of function 'my_swap' [-Wimplicit-function-declaration]
   my_swap(n1, n2);
   ^~~~~~~
/in/foo.c: At top level:
/in/foo.c:24:18: error: expected ';', ',' or ')' before '&' token
 void my_swap(int &x, int &y)
                  ^