LanguageExtensions.rst

     __attribute__((__format__ (__printf__, 1, 3)))
     void foo(const char* s, char *buf, ...) {
       va_list ap;
       va_start(ap, buf);

       vprintf(s, ap); // warning
     }

   In this case Clang does not warn because the format string ``s`` and
   the corresponding arguments are annotated.  If the arguments are
   incorrect, the caller of ``foo`` will receive a warning.