interview question how does application connects to device

[Abhijit Pawar]

On 5 July 2011 13:01, Mandeep Sandhu <mandeepsandhu.chd at gmail.com> wrote:

> Oops, accidentally pressed send...
>
> On Tue, Jul 5, 2011 at 12:58 PM, Mandeep Sandhu
> <mandeepsandhu.chd at gmail.com> wrote:
> > On Tue, Jul 5, 2011 at 12:29 PM, Paraneetharan Chandrasekaran
> > <paraneetharanc at gmail.com> wrote:
> >> I think the thread originator is asking about how the application knows
> >> which device file to read or write.
> >> This is done by h/w management system udev. udev creates/manages device
> >> nodes in /dev/ dir and notifes applications based on the udev rules
> written
> >> (via HAL events or DBUS signals).
> >
> > I don't think udev is involved in the read/write file ops. Udev is
> > responsible for handling hotplug events, doing certain actions based
> > on events (as indicated by udev rules),persistent naming of devices
> > etc...but not file i/o.
> >
> > That, I think, is handled by the VFS layer. Each device node is
> > uniquely identified by it's MAJOR-MINOR number combo. I guess the VFS
> > layer uses this to pick the correct file-ops struct to communicate
> > with the device.
>
> >Eg; when we try to open a device, say /dev/ttyS0, it's major-minor
> >numbers (eg: 64-4 on my machine) are used to lookup the file-ops
> >struct and from then on, the VFS passes the read/write calls to this
> >device driver.
>

Yes. whenever we read or write to the device, the file operations structure
would be invoked for the device. And this device is identified by the Major
and Minor number combination.
The device driver / kernel module at registration time mention what major
and minor it would be servicing.

This is what I think it should work. Good to get confirmation though.

Regards,
Abhijit

>
> HTH,
> -mandeep
> >
> > My info is a little dated, so plz CMIIW.
> >
> > HTH,
> > -mandeep
> >
>
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