io.c:2334
static char*
rb_io_mode_string(fptr)
    OpenFile *fptr;
{
    switch (fptr->mode & FMODE_READWRITE) {
      case FMODE_READABLE:
      default:
	return "r";
      case FMODE_WRITABLE:
	return "w";
      case FMODE_READWRITE:
	return "r+";
    }
}
