On Friday, March 15, 2002, at 10:31 PM, Barry Twycross wrote:
IOUSBLib has nice friendly calls such as GetDevice<whatever> but what
about more generalised usage of bRequest fields for obtaining other
info. Using the example of USBGetManufacturerStringIndex, how can you,
in the user-space, issue a GET_DESCRIPTOR call to retrieve the
relevant info, specifying the bRequest, wValue, wIndex fields etc. I
guess you'd use something along the lines of kUSBRqGetDescriptor (or
kUSBRqGetStatus or whatever) I'm just not entirely sure how to do
this, or if ot's possible to, from the user-space in
Nano has mentioned which API to use, if you're confused about what the
parameters should be, this is documented in the USB spec, chapter 9.
Hi Barry,
Since my original mail on this subject I've been reading through a lot
more of the USB spec so have a better understanding of the processes,
parameter's etc as defined in those documents, but thanks for the
breakdown.
The wLength you supply is however long you expect the string to be. You
could supply 255, the maximum. However there is a common bug in which
devices can hang on this, so the normal way of doing this is to ask for
2 bytes, which tells you the length of the descriptor (the first byte).
Then you ask for exactly the right number of bytes.
Aha, pre-emptive question answering!
Very handy tip indeed, I was going to go for the rather inefficient
maximum option, or at least a wLength which should reasonably be able to
hold the string. Thanks again for all your help, it's _really_
appreciated.
All the best,
ben
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