Re: file lock unnecessary?

On Wednesday April 25 2007 6:14 am, Varjü Tamás wrote:
> Hello everyone!
>
> I have written a perl script which opens a file for reading and writing.
> I tested what happens if the script is run in multiple instances by the
> webserver.  I put a sleep command between seeking writing position in
> the file and actual writing.  I filled the web form in two different
> tabs of my browser and sent it.
>
> My impression was that the following happens:
> - first instance opens the file,
> - first instance seeks the writing position,
> - first instance goes sleeping,
> - second instance opens the same file,
> - second instance seeks the writing position (different from the above
> one),
> - second instance goes sleeping,
> - first instance wakes up,
> - first instance writes the file,
> - first instance closes the file,
> - second instance wakes up,
> - second instance writes the file,
> - second instance closes the file.
>
> After that procedure both changes has appeared in the file.
>
> Does it mean, that on this system the multiple instance writing of a
> file is handled well and I do not have to worry about locking the file
> as long as the different instances write different positions in the
> file?  Even the system administrator had the tip, that only the last
> change will appear.
>
> If I want to be sure that multiple instance writing will work, is append
> a better choice?  I have read that on the archive of this list.
>
> Thanks

you should use flock to  lock the file , so one process can work on it at a 
time , if you don't eventually  a race way condition will occur  and  you 
will see a lock up happen , or   if your luck  one process will  overwrite 
the other  and you will just loose data..  either way  it's  just smart 
programming to lock a file  your working on especially if  it's happening 
under CGI conditions.

Greg