How do I lock a file in Perl?

Question

What is the best way to create a lock on a file in Perl?

Is it best to flock on the file or to create a lock file to place a lock on and check for a lock on the lock

Answer 1

If you end up using flock, here's some code to do it:

use Fcntl ':flock'; # Import LOCK_* constants

# We will use this file path in error messages and function calls.
# Don't type it out more than once in your code.  Use a variable.
my $file = '/path/to/some/file';

# Open the file for appending.  Note the file path is quoted
# in the error message.  This helps debug situations where you
# have a stray space at the start or end of the path.
open(my $fh, '>>', $file) or die "Could not open '$file' - $!";

# Get exclusive lock (will block until it does)
flock($fh, LOCK_EX) or die "Could not lock '$file' - $!";

# Do something with the file here...

# Do NOT use flock() to unlock the file if you wrote to the
# file in the "do something" section above.  This could create
# a race condition.  The close() call below will unlock the
# file for you, but only after writing any buffered data.

# In a world of buffered i/o, some or all of your data may not 
# be written until close() completes.  Always, always, ALWAYS 
# check the return value of close() if you wrote to the file!
close($fh) or die "Could not write '$file' - $!";

Some useful links:

• PerlMonks file locking tutorial (somewhat old)
• flock() documentation

In response to your added question, I'd say either place the lock on the file or create a file that you call 'lock' whenever the file is locked and delete it when it is no longer locked (and then make sure your programs obey those semantics).