> >For example, if I was trying to write an
> application that interacted
> >with a Palm or some other handheld device that
> connects via USB,
> >should I be looking into kernel extensions or using
> a driver
> >interface based on the USB family?
>
> That would be a user land driver. One of the magic
> words is
> "application", you're starting with a user land
> application, it can
> do it all without getting into the kernel.
This is true, but user land drivers aren't exactly
Nirvana. In fact, there really isn't an architecture
for "user land drivers"; there is just the IO Kit APIs
which you can use from an application/user-land space.
One gotcha is that you may have to deal with user-mode
USB device arbitration (see
http://developer.apple.com/techpubs/macosx/Darwin/IOKit/DeviceInterfaces/USB/ClassicUSBDeviceArb.html
for more details).
More importantly, there is really no general
architecture for user-land drivers specified by Apple,
so if you want to package your user-land driver code
to make it available to other applications, you're
going to have to "roll your own" mechanism. How do
other applications, especially third-party apps, find
your driver so that they can re-use what you've
developed? All of Apple's examples of calling IOKit
APIs from user-land are in the context of an
application/tool.
Also, suppose you want your user-land driver to get
loaded whenever a USB device gets plugged in, there's
nothing in the OS that will do it automatically for
you, i.e. there's no registry for user-land drivers.
Again, you'll have to write your own code to do this
(I wound up having to write a small app that is a
wrapper around my driver and that runs as a Startup
Item).
Just some things to think about. It's in Apple's best
interest to push for user-land drivers since they're
not liable to destabilize the kernel in the same way
that kernel-mode drivers can, and I'm not really
critisizing the existing architecture, but I think
Apple should consider doing more development to make
life simpler from user-land. It sometimes seems like
developing a user-land driver can be more work from a
developer's point of view.
-Steve
> --
> Barry Twycross
> email@hidden
> ---
> USB, it's not a Dyslexic BUS.
> (Thanks to TC.)
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