########################  File Read-Lock Script  ########################
#!/usr/bin/perl -w

use Fcntl ':flock';   #  Get LOCK_SH, LOCK_EX, LOCK_UN, LOCK_NB constants.
open(F, "file") or die "Cannot open file!\n";

print "Cannot get read access!\n" if !flock(F, LOCK_SH | LOCK_NB);
sleep 20;    #  Keep script alive to maintain lock for a while!!

########################  File Write-Lock Script  ########################
#!/usr/bin/perl -w

use Fcntl ':flock';   #  Get LOCK_SH, LOCK_EX, LOCK_UN, LOCK_NB constants.
open(F, "file") or die "Cannot open file!\n";

print "Cannot get write access!\n" if !flock(F, LOCK_EX | LOCK_NB);
sleep 20;
##################  Sample Sessions with Both Scripts  ##################

$ lockr.pl&
[1]     2782792
$ lockw.pl
Cannot get write access!   #  Read lock prevents write lock!

$ lockw.pl&
[1]     2905742
$ lockr.pl
Cannot get read access!    #  Write lock prevents read lock!

$ lockr.pl&
[1]     2944515
$ lockr.pl        
     #  No error message!  Read lock does NOT prevent other read locks!

$ lockw.pl&
[1]     2737801
$ lockw.pl
Cannot get write access!   #  Write locks prevent other write locks!
####################  Guidelines for Using File Locking  ##################

1.  Remember to unlock files (with flock(HANDLE, LOCK_UN | LOCK_NB)) when
    you no longer need a lock i.e., you have finished reading or writing
    a file you have read- or write-locked.

2.  Remember that you can still do any I/O operation on a locked file
    because the locking is *advisory* locking -- the failure of flock
    just reminds you to be nice and not write a file being read or
    read a file being written.

3.  Always use LOCK_NB to cause an immediate failure if a lock cannot
    be obtained.  Otherwise, the program waits INDEFINITELY until the
    lock *can* be obtained.  You may want this -- but not usually!

4.  Always use locking in applications (such as TCP servers with multiple
    child processes running around) where a good chance of simultaneous
    file access exists.  

5.  File locking is irrespective of the file type so don't be confused
    just because it is presented in the DB_File part of Perl Cookbook!!