On Tue, 21 Apr 2015, Dave Mielke wrote:
> [quoted lines by Alan Stern on 2015/04/21 at 10:50 -0400]
>
> >Yes, it's okay. A userspace ABI like that isn't going to go away.
> >It's also true that you can poll for POLLERR or POLLHUP to see when a
> >device has been disconnected, although libusb probably isn't interested
> >in doing that.
>
> Out of curiosity, do you know why one must poll for output (rather than for
> input)?
I have no idea. Perhaps someone decided to do it that way because you
need to have the file open for writing before you can submit or reap
URBs.
> JDoes "writability" precisely mean that there's at least one
> user-submitted URB that can now be reaped?
Yes. The exact test is:
if (file->f_mode & FMODE_WRITE && !list_empty(&ps->async_completed))
mask |= POLLOUT | POLLWRNORM;
where ps->async_completed is the list of completed but not-yet-reaped
URBs.
Alan Stern
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