stdio-common/vfprintf.c %n fortify

    LABEL (form_number):                                                      \
      if (s->_flags2 & _IO_FLAGS2_FORTIFY)                                    \
        {                                                                     \
          if (! readonly_format)                                              \
            {                                                                 \
              extern int __readonly_area (const void *, size_t)               \
                attribute_hidden;                                             \
              readonly_format                                                 \
                = __readonly_area (format, ((STR_LEN (format) + 1)            \
                                            * sizeof (CHAR_T)));              \
            }                                                                 \
          if (readonly_format < 0)                                            \
            __libc_fatal ("*** %n in writable segment detected ***\n");       \
        }