flock() question

[Steve Purcell]

Carsten Gaebler wrote:
> It seems I don't understand file locking. In the following example the
> parent process locks a file and then the child process locks the same file
> but does not wait until the parent unlocks it. What am I doing wrong?

>From the 'flock' man page:

  "A file is locked (i.e., the inode), not the file  descriptor."

So the correct procedure is to open the lock file separately in each process:

pid = os.fork()
if pid == 0:
        f = open("/tmp/locktest", "w")
        time.sleep(1)
        print "Child: locking file"
        fcntl.flock(f.fileno(), FCNTL.LOCK_EX)
        print "Child: file locked"
        time.sleep(5)
        os._exit(0)

else:
        f = open("/tmp/locktest", "w")
        print "Parent: locking file"
        fcntl.flock(f.fileno(), FCNTL.LOCK_EX)
        print "Parent: file locked"
        time.sleep(10)
        fcntl.flock(f.fileno(), FCNTL.LOCK_UN)


-- 
Steve Purcell, Pythangelist
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