/in/foo.c: In function 'main':
/in/foo.c:20:12: warning: zero-length gnu_printf format string [-Wformat-zero-length]
     printf("");
            ^~
/in/foo.c:22:12: warning: zero-length gnu_printf format string [-Wformat-zero-length]
     printf("");
            ^~
/in/foo.c:24:12: warning: zero-length gnu_printf format string [-Wformat-zero-length]
     printf("", a1);
            ^~
/in/foo.c:30:12: warning: zero-length gnu_printf format string [-Wformat-zero-length]
     printf("", a2);
            ^~