Re: Re: Order of flock and open

You are absolutely right -- you must watch out in this case. Most of the other replies don't bring up the fact that you must lock the entire process (read/write/rename). If you do this with a separate file, it is rather straightforard.

UPDATE: merlyn is right that the "code" below is fishy and should not be used. Take it as an example of what can go wrong if you try to get too tricky with locking. There are almost always conditions you will overlook (as I have.)

That said, I believe you can do this by locking both files, but you must pay attention to the order you perform the operations. You should be able to do the following in order...

open and lock the input file
open lock the output file
read the input file 
write the output file
rename the input file
rename the output file
close both files (which unlocks them)
delete old renamed input file
[download]

I'm not sure this works on all operating systems, but should work for Unix/Linux. You must use a non-blocking lock for the input file and you must close/reopen it in a loop if it is already locked. This is because someone can completely replace the input file before you get the lock for it and then your open file descriptor is pointing to the old stale (possibly deleted) input file. You should probably also use unique names (i.e. that don't collide with other processes) for the output file and the old input file.

This all seems rather complicated, which is why locking a separate file is much simpler. But, YMMV...

bluto

Comment on Re: Re: Order of flock and open Download Code

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•Re: Re: Re: Order of flock and open by merlyn (Sage) on Apr 30, 2003 at 20:12 UTC
I think what you want is what I wrote in "Exclusively updating a file that continues to be repeatedly read". Yours looks a bit fishy, like there are still places where two people can have locks on what should be the only copy. And then there's the problem of a dead process after renaming the input file but not yet renaming the output file. Now you have a missing file! -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re: •Re: Re: Re: Order of flock and open by bluto (Curate) on Apr 30, 2003 at 21:54 UTC
I agree that it is fishy and I would never use it in practice (the reason for the lack of real code and the disclaimer in the last line of the post) since over the years I've learned that once you start messing around with operation ordering too much, and start requiring non-blocking locks, chances are good the locking strategy is incorrect. That said, I'm not entirely convinced that it can't be fixed, perhaps by using only a single rename, only that it isn't worth it. Your solution is more elegant in any case. bluto	[reply]