int usb_get_descriptor(usb_dev_handle *dev, unsigned char type, unsigned char index, void *buf, int size);
usb_get_descriptor retrieves a descriptor from the device identified by the type and index of the descriptor from the default control pipe. Returns number of bytes read for the descriptor or < 0 on error.
See usb_get_descriptor_by_endpoint for a function that allows the control endpoint to be specified.