interview question how does application connects to device

[Mandeep Sandhu]

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.

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