File Lock with Perl

Question

I can't get Perl flock to work. I lock the file by checking the return result to make sure it is actually locked, and I can still open and write to it as if nothing was happening.

This is how I lock the file:

  #! / usr / bin / perl -w

 use strict;
 use Fcntl ': flock';

 $ |  = 1;

 my $ f = $ ARGV [0];

 open (my $ fh, '>>', $ f) or die "Could not open '$ f' - $!";
 print "locking '$ f' ...";
 flock ($ fh, LOCK_EX) or die "Could not lock '$ f' - $!";
 print "locked \ n";

 sleep 10;
 print "waking up and unlocking \ n";
 close ($ fh);

While this script was sleeping, I can play with the same text file:

  #! / usr / bin / perl -w

 use strict;
 my $ f = $ ARGV [0];
 open (my $ fh, '>>', $ f) or die "Could not open '$ f' - $!";
 print $ fh "This line was appended to a locked file! \ n";
 close ($ fh);

Why am I trying to open a file and write to it without saying that it is locked?

Answer 1

flock() is an advisory lock. You must have all your processes using flock()

Also understand that the way you call flock() will block until it can get the lock. If you want a crash, you need to use the LOCK_NB flag.

 open(my $lf, ">>fileIWantToLockOn"); my $gotLock = flock($lf, LOCK_EX | LOCK_NB); unless ($gotLock) { print "Couldn't get lock. Exiting"; exit 0; } 

EDIT: Also note that flock() will not work on NFS

Answer 2

I don't think flock does what you think it does. Locking a file does not prevent anyone from doing anything in the file, except for trying to get a lock in the same file.

From man 2 flock on my system:

flock (2) places only security locks; if there are appropriate permissions on the file, the process can ignore the use of flock (2) and perform I / O on the file.

Answer 3

flock works with advisory locks. Among other things, this means that only other processes that try to execute flock the same file will understand that it is locked.