io.c:1663
int
rb_io_mode_flags(mode)
    const char *mode;
{
    int flags = 0;
    const char *m = mode;

    switch (*m++) {
      case 'r':
	flags |= FMODE_READABLE;
	break;
      case 'w':
	flags |= FMODE_WRITABLE;
	break;
      case 'a':
	flags |= FMODE_WRITABLE;
	break;
      default:
      error:
	rb_raise(rb_eArgError, "illegal access mode %s", mode);
    }

    while (*m) {
        switch (*m++) {
        case 'b':
            flags |= FMODE_BINMODE;
            break;
        case '+':
            flags |= FMODE_READWRITE;
            break;
        default:
            goto error;
        }
    }

    return flags;
}
