Hello,

I need to put a mandatory lock on /dev/fb0 (presently any file) in Linux OS. But in actual scenario it will be a device file.

Scenario:
1. Process A is super process
2. Process B is normal process.
3. Process A and B are non-cooperating process (So, advisory locking is out of scope).
4. At particular instance Process B is accessing /dev/fb0.
5. Now, Process A wants access to it.
6. Process A locks /dev/fb0
7. It can hold the lock as long it wants & in any case process B cannot write anything to it. (It’s better if B cannot read the contents also).

I also changed permissions of directory and lock file.

directory:
drwxr-Sr-x 2 ists ists 4096 2008-11-21 11:20 test

file to be locked:
-rw-r-Sr-- 1 ists ists 15 2008-11-21 09:32 lock.txt

uname -a:
Linux ists-desktop 2.6.24-21-generic #1 SMP Tue Oct 21 23:43:45 UTC 2008 i686 GNU/Linux

Please help me how to achieve above or what all I need to do it.
I am really stuck.

Regards,
~Nick

Take a look at the kernel documentation. Documentation/filesystems/mandatory-locking.txt. Once you set the proper file permissions you can use flock or lockf to apply the mandatory lock.