Question

I am trying to insure that only one instance of a perl script can run at one time. The script performs some kind of db_operation depending on the parameters passed in. The script does not necessarily live in one place or on one machine, and possibly multiple OSs. Though the file system is automounted across the various machines.

My first aproach was to just create a .lock file, and do the following:

use warnings;
use strict;
use Fcntl qw(:DEFAULT :flock);
...
open(FILE,">>",$lockFilePath);
flock(FILE,LOCK_EX) or die("Could not lock ");
do_something();
flock(FILE,LOCK_UN) or die("Could not unlock ");
close(FILE);

but I keep getting the following errors:

Bareword "LOCK_EX" not allowed while "strict subs" in use
Bareword "LOCK_UN" not allowed while "strict subs" in use

So I am looking for another way to approach the problem. Locking the DB itself is also not practical since the db could be used by other scripts(which is acceptable), I am just trying to prevent this script from running. And locking a table for write is not practical, since my script is not aware of what table the operation is taking place, it just launches another perl script supplied as a parameter.

I am thinking of adding a table to the db, with just one value, and to use that as a muetex, but I don’t know how practical/reliable that is(a lot of red flags go up in my head). I have a DBI connection to a db that this script useses.

Thanks

Answer 1

The Bareword error you are getting sounds like you’ve done something in that “…” to confuse Perl with regard to the imported Fcntl constants. There’s nothing wrong with using those constants like that. You might try something like LOCK_UN() to see what error that gets you.

If you are using MySQL, you can use the GET_LOCK() and RELEASE_LOCK() mechanism. It works reasonably well for cases like this:

SELECT GET_LOCK("script_lock");

and then when you are finished:

SELECT RELEASE_LOCK("script_lock");

See http://dev.mysql.com/doc/refman/4.1/en/miscellaneous-functions.html for details.