Re: How to make sure that the target file to read is not under writing by others?

On Wed, 19 Aug 2009 08:13:56 -0400, kyle.smith@inforonics.com (Kyle Smith) wrote: >Nitebirdz wrote: >> On Wed, Aug 19, 2009 at 11:59:39AM +0100, Ashley Sheridan wrote: >> >>> >>> No, what you're saying is 'use a log file in order to know when to look >>> at another log file'. What would happen if you tried to access the >>> control log file whilst it was in the process of being written to? >>> Admittedly, you reduce your chances of failure because there is less >>> data being written to the control log than the actual log, but the >>> chance of reading incomplete data is still there! >>> >>> >> >> WARNING: total newbie here. >> >> If I understood Arno correctly, he was recommending to implement >> something like the old "/var/run/*pid" files in UNIX. That way, you can >> control whether or not the previous run is already done with the file >> before you move on. >That is definitely the correct approach. Have the script which copies >the log file touch a file called 'log_file.write' or some such. When >it's done, remove the file. Your PHP script should exit if that file >exists. Of course, given the 30 minute cron cycle, it would then have >to wait until the next cycle. Maybe run it more often. I gather from this discussion that PHP allows two users to open a file for R/W? I had assumed it wouldn't. Anyway, how about: $user = 'Fred'; $try_again = true; If (!file_exists('Busy.txt')) { File_put_contents ('Busy.txt', $user); If ($user == File_get_contents ('Busy.txt') { // Do what you have to Unlink ('Busy.txt'); $try_again = false; } } if ($try_again) { // Come back in 5 minutes } This has a theoretical weakness, in that Joe could check for Busy.txt in the interval between your checking and writing it, and then write his copy after you had read your copy. A moments delay between writing and re-reading would fix this.