Locking files for shared access

Multiple users having access to the same data usually requires some sort of mechanism to give exclusive access to all or part of the data for some period of time. Locking is an extensive subject, which would need far more space than we have here, but overall there are two basic concepts: advisory locking (enforced by cooperating processes) and mandatory locking (enforced by the kernel).

Don't panic: this doesn't have to be hard.

Advisory Locking

Advisory locking does involve the kernel: the lockf() and fcntl() system calls are really the same function ;early BSD had a more limited flock(). But the kernel only restricts processes that specifically use the lockf() or fcntl() calls: any other process can read or write at will.

It's trivial to demonstrate advisory locking with Perl:

#!/usr/bin/perl
while (1) {
open(LOCKF, ">yourlockfile");
print "I'm waiting for the other guy\n";
flock LOCKF,2;
print "All set!  Go ahead and type.\n";
$a=<>;
flock LOCKF,8;
close LOCKF;
}
 

If you run that code, you'll see this on your screen:

I'm waiting for the other guy
All set!  Go ahead and type.

Don't type anything (well, don't press ENTER). Switch to another screen and run the script again. This time you'll only see the "I'm waiting for the other guy". It will sit there waiting until you go back to the first instance and press ENTER.

You can fire up even more instances of this - ONE and only one of them will acquire the lock each time you press ENTER at one of the others.

In real use, of course, you'd have two or more entirely different scripts, but they'd all share the locking code:

#!/usr/bin/perl
open(LOCKF, ">yourlockfile");
flock LOCKF,2;
# code in here
flock LOCKF,8;
close LOCKF;
 

Mandatory locking

Mandatory locking is interesting because it is enabled by a hack of setting a file's set-group-id on and turning group execute off:

[root@mail root]# chmod 2760 t
[root@mail root]# ls -l t
-rwxrwS---    1 root     root          550 Dec  5 09:12 
 

That odd "S" is the result of this otherwise senseless combination,

Mandatory locking can be surprising though: Stevens points out that it can be circumvented by unlinking and then opening a new version of the file.

See also:

Who locked that file?
Mac OS X Invisible and Locked Files
Measuring Page Popularity
Race conditions

Comments?

More Articles by Anthony Lawrence - Find me on Google+

Jump to Comments

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.

We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.