On 21 Aug 2009, George Spelvin wrote:
> I had some code that was using libusb to usb_get_string, which maps to
>
> struct usb_ctrltransfer ctrl;
> char buf[256];
>
> ctrl.bRequestType = USB_ENDPOINT_IN;
> ctrl.bRequest = request = USB_REQ_GET_DESCRIPTOR;
> ctrl.wValue = (USB_DT_STRING << 8) + index; // 3, 2 or 1
> ctrl.wIndex = 0x0409;
> ctrl.wLength = 256;
>
> ctrl.data = buf;
> ctrl.timeout = 1000;
>
> return ioctl(dev->fd, IOCTL_USB_CONTROL, &ctrl);
>
> Normally, this returns the length of the string descriptor, which
> is 2 + 2 * (number of unicode characters). In fact, buf[0] equals the
> return value.
>
> However, when addressing this request to something with idVendor = 0x1d6b,
> the return value is 1 more. Some code I had a sanity check for that and
> choked.
>
> It appears to be a deliberate decision inside ascii2utf() and rh_string(),
> but it ends up being different from real USB devices.
>
> Isn't that kind of a bug? I mean, the UTF-16 string is full of null
> bytes. What benefit is there in adding another one?
Yes, this is a bug. In ascii2utf, this line:
if (utfmax > 0) {
should be replaced with this:
if (*s && utfmax > 0) {
Alan Stern
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