Re: How to detect whether media is present in a reader?

[David Zeuthen <david@fubar.dk>]

On Thu, 2004-06-10 at 00:57, Michael Hamilton wrote:
> I've attempted to develop a desktop integration script for usb 
> storage devices. ( http://freshmeat.net/projects/usbmount/ )
> 
> Before anyone reading this thinks this a trivial problem,
> you should read the requirements/issues detailed at the bottom
> of http://users.actrix.co.nz/michael/usbmount.html - many aspects
> of the USB/SCSI system make this task much harder than it should
> be. I would recommend anyone working on this area to take a look
> at my script and see the fun currently involved - and then see
> if what they're working on will make life easier.
> 
> In the mean time, working with what is available on current 
> distributions, I'm looking for a solution to one piece of the
> puzzle...
> 
> I would like to be able to detect whether media is present
> in a reader or not (eg Compact Flash Card reader).  A small 
> piece of C for efficiently obtaining the media state would be 
> good.  I would like a solution for both 2.4 and 2.6, but 
> I'll settle for what I can get.  Does anyone have some sample
> code?
> 
> From looking at what's available I get the impression that
> some variant on polling is the only solution for such problems.  
> Is this really the case - even for USB?
> 
> I've looked at supermount and submount - I'm not sure they
> offer a better solution than my own attempts thus far - 
> I don't think they can tackle all of the requirements listed
> on my webpage. 
> 
> I was also playing with sginfo from sg3_utils ( 
> http://www.torque.net/sg/u_index.html ).  But this can be
> slow, and, in the absence of media, some of the sg3_utils
> probes can get stuck in system calls (which requires a
> reboot).
> 
> If I had a simple/efficient way to poll a device's media 
> insertion state, I think my script could then fully automate 
> almost all aspects of desktop integration of usb storage.
> 
> Thanks.
> 

Hi, have you looked at the HAL project at http://hal.freedesktop.org and
gnome-volume-manager which accomplishes this task on Linux 2.6 kernels?
(It's so much easier on 2.6 thanks to sysfs). The idea is that it's
possible to port this to other kernels which is more or less a
requirement for desktop projects link GNOME or KDE. The IPC used to
communicate with the desktop session is D-BUS.

Regarding media detection I've found that doing the equivalent of
'blockread --rereadpt /dev/sda' every, say, 2 seconds is a good way of
polling for media. In HAL it's straightforward to blacklist or whitelist
devices where this causes trouble.

Hope this helps,
David



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