Callback handle. Callbacks handles are generated by libusb_hotplug_register_callback() and can be used to deregister callbacks. Callback handles are unique per libusb_context and it is safe to call libusb_hotplug_deregister_callback() on an already deregistered callback. Since version 1.0.16, LIBUSB_API_VERSION
>= 0x01000102 For more information, see libusb_hotplug.