Question

How to lock/unlock a file in Linux

Asked by: rdashokraj

Hi Folks,

I want to know how to lock a file in Linux?  I did a search on this and found commands like flock, lockf, fcntl etc...but didn't get a clear idea about it.

Along with my above question, I want to know how to check whether a file is been locked or not?


Thanks,
Ashok

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Asked On
2007-02-17 at 11:47:23ID22396432
Tags

linux

,

file

,

unlock

Topic

Linux Administration

Participating Experts
1
Points
400
Comments
7

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Answers

 

by: TintinPosted on 2007-02-17 at 15:32:36ID: 18556904

locking only works if all processes that access the file use the same type of locking.  The most common is flock.

Please give some more detail as to what processes will be locking the file and for what purpose.

 

by: rdashokrajPosted on 2007-02-18 at 15:15:49ID: 18560174

Tintin, I myself not clear with my issue. That's the reason, I tried to ask a generic question about file locking concept.

One of our DBA sent a mail like this:
"Can you make this file available from OS(ltprodgui01.dbf). Files are present in their respective directory but Oracle is not able to acquire a lock on the file(s) mentioned below. This problem originated because DB was aborted after the UPS went down"

oracle@shrek2_cydrs% ls -lrt
total 5253232
drwxr-xr-x    2 oracle   dba          4096 Jan 29 17:27 cydrs
-rw-r-----    1 oracle   dba      1073750016 Feb 16 18:26 ltprodgui03.dbf
-rw-r-----    1 oracle   dba      2147491840 Feb 16 18:26 ltprodgui02.dbf
-rw-r-----    1 oracle   dba      2147491840 Feb 16 18:26 ltprodgui01.dbf
oracle@shrek2_cydrs% pwd
/proj/itg_ora1/u06/oradata

When I asked him, he is telling that while rebooting a system and when oracle starts along with it, it acquires some lock in many files. Only then the oracle will start successfully it seems. Here the problem is, when oracle starts all the files have acquired the lock, except these 3 files. So he is asking something to do fix this. These files are mounted from a NetApp filer. Any idea how to fix this?  Do I need to know something on the NetApp filer like remounting?

Thanks,
Ashok






 

by: rdashokrajPosted on 2007-02-19 at 20:09:43ID: 18568279

Tintin, We found those 3 files got corrupted because of the abrupt system shutdown. We restored those from backup and the database is up now. Thanks.

 

by: TintinPosted on 2007-02-19 at 22:57:54ID: 18568805

I must admit, I was racking my brain as to what type of file locking Oracle used as I wasn't aware of any.

Anyway, the corrupted files makes sense.

 

by: rdashokrajPosted on 2007-02-20 at 00:21:36ID: 18568992

Tintin, Thanks so much for your time. Anyways, I wish to have a simple understanding about this file locking mechanism. Can you give me some good material on it?  I gone through the one given below but couldn't understand much out of it.

http://www.unixguide.net/unix/programming/2.5.shtml

Tried these options:

[root@jester ojk]# chmod -l file2
chmod: invalid option -- l
Try `chmod --help' for more information.
[root@jester ojk]# fcntl file2
bash: fcntl: command not found
[root@jester ojk]# lockf file2
bash: lockf: command not found
[root@jester ojk]# flock file2
bash: flock: command not found
[root@jester ojk]# cat /etc/redhat-release
Red Hat Enterprise Linux AS release 3 (Taroon Update 8)

 

by: TintinPosted on 2007-02-20 at 13:42:41ID: 18574351

In general there are two types of locking:

1.  The existance of a physical file to indicate a file is locked.  A good example of this is the /var/lock/subsys directory which contains lock files for the various daemons that are started.

2. Locking via flock/lockf system calls.  These system calls are typically used in C programs and Perl scripts.

All locking mechanisms are advisory only, meaning that any process that doesn't use/recognise the same locking mechanism can simply write/delete the file (assuming it has permission)

 

by: rdashokrajPosted on 2007-02-22 at 05:34:10ID: 18587228

Now I got the basic idea about file locking mechanism.  Your example (/var/lock/subsys) helped to understand. Thanks a lot !!

20120131-EE-VQP-002

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