[libusb] Getting vendor data from a device

[Richard H. <hug...@gm...>]

I'm trying to export an additional vendor class interface descriptor
on a device like this:

    /* Interface Descriptor */
    0x09+4,                /* Size of this descriptor in bytes */
    USB_DESCRIPTOR_INTERFACE,    /* INTERFACE descriptor type */
    1,                /* Interface Number */
    0,                /* Alternate Setting Number */
    0,                /* Number of endpoints in this intf */
    0xff,            /* Class code */
    0,                /* Subclass code */
    0,                /* Protocol code */
    0,                /* Interface string index */
    'X', 'Y', 'Z', '\0',

i.e. it's a vendor class code, with extra vendor-specific data of
"XYZ\0". I can get find this interface descriptor using
libusb_get_active_config_descriptor() and drilling down the altsetting
and then matching on vendor, subclass and protocol to get the correct
descriptor.

When I read desc->extra it's NULL and desc->extra_length is set to
zero. I kinda assumed they would be "XYZ\0" and 4 as this is the extra
data in the descriptor that can't be parsed. If I'm not supposed to
use ->extra, I don't see any way to get the vendor data out of the
descriptor.

I have libusbx-1.0.19 installed on Fedora 22, and am happy to debug
more or test patches as requried. Ideas welcome, thanks!

Richard