We should keep this on the list since other people besides me can
answer this question, perhaps quicker too, and it might be of general
interest.

On Jul 6, 2006, at 1:55 PM, pix wrote:

hey hans,

i was wondering if you have experienced this behaviour where, on
linux, the hid the hid object hangs if the device disappears (even only briefly) while hid has it open?

i thought it might have just been the problem that the device was getting a new device file name when it returned, but i have now done some udev trickery to make sure the steim junxion box we are using always gets the same event number, and the problem still persists.

once the device disappears hid no longer responds to close or open messages.

it would be cool if there was a work around because we occasionally
have problems with the junxion box we are using 'winking out' for just long enough for linux to kill off the device.

Hmm, I haven't seen that, but its not surprising that it would hang.
Basically, the object is reading that file handle every 5 ms, or
whatever you set the poll time to. So if sometime reads from a non- existant filehandle that's open, then it'll hang, no matter how you
are doing it.

I have used the junxionbox some and I haven't seen that problem. I
think the best way to solve that problem would be to figure out
what's causing the junxionbox to wink out. Its in beta test so I
think they would be interested to hear about that problem. I was
just at STEIM for three weeks working with Rene and Taku, who are
working on the junxionbox. Taku performs with his junxion-related
devices, so he's dealt with a lot of these issues.

I can't think of anything offhand as an easy workaround. You might
be able to test the filehandle before reading from it, but I am
worried about the overhead that might introduce. A key part of hid
is making it as low latency as possible, and not to take a ton of
CPU. Such a check might be pretty minimal but running it 200 times a
second adds up. Can any of the big time Linux heads think of a way
to deal with this?

.hc

The arc of history bends towards justice. - Dr. Martin Luther
King, Jr.