Re: [linux-usb-devel] Hotplug Driver Interface to User Level Programs: A Driver Example

[Oliver Neukum <oliver@neukum.org>]

Am Montag, 13. September 2004 21:27 schrieb Linas Vepstas:
> Another alkternative to returning -ENODEV is to have a zero-length read.
> This is a traditional unix way of signifying that the remote end has
> disconnected; e.g. for tcp sockets.  That is, select() will tell you
> that there is something to read, and the read() call will suceed
> (without error), and return zero bytes read.  For tcp, this is
> understood to mean that the far side has closed its socket.
> 
> I'm not sure, but maybe this would also make sense for any USB device
> that can be read with open(), read() calls? 

Unfortunately that would violate POSIX. Quote:

When attempting to read a file (other than a pipe or FIFO) that supports non-blocking reads and has no data currently available:
*If O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].
*If O_NONBLOCK is clear, read() shall block the calling thread until some data becomes available.
*The use of the O_NONBLOCK flag has no effect if there is some data available.

If you need to modify applications, you can just as well return
a proper error.

	Regards
		Oliver



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