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How to get at the indexed strings in a HID device

I have a HID device that works fine, but I now need to access the indexed strings. My device reacts to a specific command that I send to it using a feature report by placing data in the 10th indexed string, and my app now needs to retrieve that.

These should be accessible via the DeviceRequest() method, but this appears to be available only in the IOUSBDeviceInterface. I normally obtain an IOHIDDeviceInterface object, but this doesn't provide the DeviceRequest() method. When I attempt to obtain an IOUSBDeviceInterface object, first step being (right out of the "Working with USB Device Interface" booklet)

kr = IOCreatePlugInterfaceForService(usbDevice, kIOUSBDeviceUserClientTypeID, kIOCFPlugInterfaceID, &plugInterface, &score)

this fails returning 0x2c7, unsupported. The variant on the above code line that I usually use (see below) works:

kr = IOCreatePlugInterfaceForService(io_service_t, kIOHIDDeviceUserClientTypeID, kIOCFPlugInterfaceID, &plugInterface, &score)

I must be missing something basic, can anyone point me in the right direction? Thanks in advance.

Don





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