Hi,
Notice the code the get_string has changed:
https://github.com/walac/pyusb/blob/master/usb/util.py#L295-L340
2015-08-08 1:14 GMT-03:00 Paul Cornelius <paul.cornel...@vuemetrix.com>:
> I found a bug in usb.util.py in the function get_string. This function was
> failing with an "Overflow" error on my Windows 8.1 machine, using Python 3.3
> and libusb-1.0.
>
> If this function is called with the default value for langid (None), it
> first makes a call to get_descriptor to obtain a list of languages supported
> by the device. This second parameter of this function call is the size of
> the reply buffer, which was equal to 1024. But a comment in the libusb-1.0
> source (descriptor.c, function libusb_get_string_descriptor_ascii) reads: /*
> Some devices choke on size > 255 */.
>
> And it seems to be true. When I changed the get_descriptor function call to
> reduce the buffer size parameter to 255 the error disappeared. I also
> thought it would be good to limit the size of any buffer passed to
> get_string. My modified function now reads as follows:
>
> def get_string(dev, length, index, langid = None):
> r"""Retrieve a string descriptor from the device.
>
> dev is the Device object to which the request will be
> sent to.
>
> length is the length of string in number of characters.
>
> index is the string descriptor index and langid is the Language
> ID of the descriptor. If langid is omitted, the string descriptor
> of the first Language ID will be returned.
>
> The return value is the unicode string present in the descriptor.
> """
> from usb.control import get_descriptor
> if langid is None:
> # Asking for the zero'th index is special - it returns a string
> # descriptor that contains all the language IDs supported by the device.
> # Typically there aren't many - often only one. The language IDs are 16
> # bit numbers, and they start at the third byte in the descriptor. See
> # USB 2.0 specification section 9.6.7 for more information.
> #
> # Note from libusb 1.0 sources (descriptor.c)
> buf = get_descriptor(
> dev,
> 255,
> DESC_TYPE_STRING,
> 0
> )
> assert len(buf) >= 4
> langid = buf[2] | (buf[3] << 8)
>
> lenstr = length * 2 + 2
> if lenstr > 255:
> raise ValueError("String buffer length too long")
> buf = get_descriptor(
> dev,
> length * 2 + 2, # string is utf16 + 2 bytes of the
> descriptor
> DESC_TYPE_STRING,
> index,
> langid
> )
> return buf[2:].tostring().decode('utf-16-le')
>
> Paul Cornelius
>
> ------------------------------------------------------------------------------
>
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> pyusb-users mailing list
> pyusb-users@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/pyusb-users
>
--
Best Regards,
Wander Lairson Costa
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