Solved

file locker?

Posted on 2000-04-03
4
188 Views
Last Modified: 2013-12-25
how to make a file locker, so only one person can access the file a time?
0
Comment
Question by:syu1
  • 2
4 Comments
 
LVL 16

Accepted Solution

by:
maneshr earned 50 total points
ID: 2680805
if you are using PERL, you can use the flock function.

flock FILEHANDLE, OPERATION

Establishes or removes a lock on a file opened with filehandle. This function calls one of the Unix functions flock, lockf, or
the locking capabilities of fcntl, whichever your system supports. If none of these functions exist on your system, flock will
produce a fatal error.

operation is the type of locking function to perform. The number by each operation name is the argument that Perl's flock
takes by default. You may also use the operation names if you explicitly import them from the Fcntl module with use Fcntl
":flock".

     LOCK_SH (1)

          Establishes a shared lock on the file (read lock).

     LOCK_EX (2)

          Establishes an exclusive lock on the file (write lock).

     LOCK_UN (8)

          Removes a lock from the file.

     LOCK_NB (4)

          Prevents flock from blocking while trying to establish a lock with LOCK_SH or LOCK_EX and instructs it to return
          immediately. LOCK_NB must be ored with the other operation as an expression for the operation argument, i.e.,
          (LOCK_EX | LOCK_NB).



This function calls flock(2) on FILEHANDLE. See the manual page for flock(2) for the definition of OPERATION. Invoking flock will produce a fatal error if used on a machine that doesn't implement flock(2) or emulate it through some other locking mechanism. Here's a mailbox appender for some BSD-based systems:

$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;

sub lock {
    flock MBOX, $LOCK_EX;
    # and, in case someone appended
    # while we were waiting...
    seek MBOX, 0, 2;
}

sub unlock {
    flock MBOX, $LOCK_UN;
}

open MBOX, ">>/usr/spool/mail/$ENV{'USER'}" or die "Can't open mailbox: $!";

lock();
print MBOX $msg, "\n\n";
unlock();

Note that flock is unlikely to work on a file being accessed through a network file system.
=====================================================
0
 

Expert Comment

by:mwhuen
ID: 2684073
what is the use of
seek MBOX,0,2;
???
0
 
LVL 16

Expert Comment

by:maneshr
ID: 2684103
seek MBOX, 0, 2;


seek FILEHANDLE, OFFSET, WHENCE

This function positions the file pointer for FILEHANDLE, just like the fseek(3) call of standard I/O. The first position in a file is at offset 0, not offset 1, and offsets refer to byte positions, not line numbers.
0
 
LVL 84

Expert Comment

by:ozo
ID: 2684834
But since you opened MBOX with >> won't it automatically seek to the end to append?
0

Featured Post

3 Use Cases for Connected Systems

Our Dev teams are like yours. They’re continually cranking out code for new features/bugs fixes, testing, deploying, testing some more, responding to production monitoring events and more. It’s complex. So, we thought you’d like to see what’s working for us.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Making a simple AJAX shopping cart Couple years ago I made my first shopping cart, I used iframe and JavaScript, it was very good at that time, there were no sessions or AJAX, I used cookies on clients machine. Today we have more advanced techno…
Active Directory replication delay is the cause to many problems.  Here is a super easy script to force Active Directory replication to all sites with by using an elevated PowerShell command prompt, and a tool to verify your changes.
The viewer will learn how to count occurrences of each item in an array.
The viewer will learn the basics of jQuery, including how to invoke it on a web page. Reference your jQuery libraries: (CODE) Include your new external js/jQuery file: (CODE) Write your first lines of code to setup your site for jQuery.: (CODE)

910 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

19 Experts available now in Live!

Get 1:1 Help Now