locking files on Linux

[Grant Edwards]

On 2012-10-18, andrea crotti <andrea.crotti.0 at gmail.com> wrote:

> I'm trying to understand how I can lock a file while writing on it,
> because I might have multiple processes working on it at the same time.
>
> I found the fcntl.lockf function but if I do this:
>
> In [109]: locked = open('locked.txt', 'w')
>
> In [110]: fcntl.lockf(locked, fcntl.LOCK_EX)
>
> I can happily open the file with vim from somewhere and write on it, so
> it doesn't seem to be very useful, or am I missing something?

File locks under Unix have historically been "advisory".  That means
that programs have to _choose_ to pay attention to them.  Most
programs do not.

Linux does support mandatory locking, but it's rarely used and must be
manually enabled at the filesystem level. It's probably worth noting
that in the Linux kernel docs, the document on mandatory file locking
begins with a section titled "Why you should avoid mandatory locking".

http://en.wikipedia.org/wiki/File_locking#In_Unix-like_systems
http://kernel.org/doc/Documentation/filesystems/locks.txt
http://kernel.org/doc/Documentation/filesystems/mandatory-locking.txt
http://www.thegeekstuff.com/2012/04/linux-file-locking-types/
http://www.hackinglinuxexposed.com/articles/20030623.html

-- 
Grant Edwards               grant.b.edwards        Yow! Your CHEEKS sit like
                                  at               twin NECTARINES above
                              gmail.com            a MOUTH that knows no
                                                   BOUNDS --