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I'm using:

file_put_contents("peopleList.txt", $person, FILE_APPEND | LOCK_EX);

To write to the end of a file and to make sure no one else (or script) is also writing to the same file at the same time.

The PHP manual says it will return a falsy value if unsuccessful.

If it cannot obtain a lock on the file, will it fail, or keep trying until it can? If it does fail when no lock can be obtained, what would be the best way to go about ensuring the data is written?

Perhaps looping the function in a while loop until it does not return false (cringe) or simply providing the user (website visiter) with some kind of GUI requesting they try again?

Daniel Widdis
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hozza
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    Apache spawns a thread in which a PHP process is executed. Multiple users = multiple PHP processes. The process that acquired the lock is the only one allowed to work with the file. All other processes will receive an error from file_put_contents function and file_put_contents issued by those users won't write anything to the file. That means that while 1 user is writing, others are being denied and the function will return boolean `false`. – Mjh Jan 19 '12 at 14:48
  • Reason why LOCK_EX exists is a whole another topic. If you want to write to the file regardless, just remove the LOCK_EX since you don't need it and the last person to access your website/script will be the one who will have the data written to the file. Also, since you're doing append - what's the point of the locking the file at all? Just remove it. – Mjh Jan 19 '12 at 14:59
  • I am using LOCK_EX and FILE_APPEND together because other scripts have the ability to write to the file and I do not what them to be able to while data is being appended to it and vice versa. – hozza Jan 19 '12 at 15:44
  • Do all of the scripts append data to the file? – Mjh Jan 19 '12 at 16:04
  • No, some will overwrite and some will just read. – hozza Jan 19 '12 at 16:35
  • If you really need a lock for a file to write to the end of it it may be better to store this data in mysql and just have each new write be a new row and combind the writes into a single doc. This way you wont have lock errors. But I will also answer the question. – WojonsTech Jan 25 '12 at 16:23

2 Answers2

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Actually, my previous answer was a bit out of date. flock() blocks until the requested lock is acquired:

PHP supports a portable way of locking complete files in an advisory way (which means all accessing programs have to use the same way of locking or it will not work). By default, this function will block until the requested lock is acquired; this may be controlled (on non-Windows platforms) with the LOCK_NB option documented below.

So since file_get_contents() utilizes it, I'd assume it's the same. That said, be warned that it varies per operating system.

A bit more importantly, you don't need to lock the file in the scenario you described, for the reasons N.B. already explained. Unless you are using the CLI SAPI, I can't think of a common scenario you should be worried about file locking.

yannis
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  • @hozza please, no need for guessing what I'm thinking, I'm just explaining things. If you don't understand them, that's a different thing. Using a loop in conjunction with sleep is simply put - stupid. You're doing FILE_APPEND, there's no need to use the LOCK_EX at all. Don't copy examples from php.net blindly, it tells you what's what and what's it used for. – Mjh Jan 19 '12 at 15:01
  • The code I provided is an example. The scenario is simply many scripts being able to append, truncate-write or read the file. I only want one of these things to happen at a time, hence wanting to lock the file with LOCK_EX when using file_put_contents(); – hozza Jan 19 '12 at 15:46
  • Then what @NB said. Try running concurrent scripts and see what happens – yannis Jan 19 '12 at 15:48
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If the function fails to get the lock for the file then it will return an error you can use.

if ($result === false) { print "failed to lock"; }

Now if you want to try to get a the lock a few times just use a loop to keep trying to write to the file.

WojonsTech
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