There is no reason for libusb to ever return a negative number of bytes,
but due to the use of a signed integer, it's technically possible. And
because libdivecomputer returns an unsigned integer, such a negative
number would be reported as a very large size.
The USB communication is now also implemented as an I/O stream
transport. Unlike most I/O devices, USB communication supports multiple
interfaces and endpoints. This requires some some special care:
In the general case, autodetection isn't really possible without
additional knowledge. Hence the need for the filter parameters to pass
this kind of information.
The implementation assumes two bulk endpoints for the standard
read/write interface. Communication with the control endpoint is
supported through the new DC_IOCTL_USB_CONTROL_{READ,WRITE} ioctl's.
