flock() and PHP, the true scoop?

[Anon]

Having tried to decipher how flock() actually works, there seems to be some confusion amongst the tutorials out there (probably due to different platforms). And the PHP manual's explanation is pure CIPO (clear if previously understood).

Can anybody explain to me in plain English the following:

1. What happens if two users try to write to the same file simultaneously (hence the need for flock() ?). Does the file get trashed, or what?

2. The tutorials show file locking to be used with flock() as :

   flock(fp, 2);

where fp is the pointer to the file (Ok, file handle) and the "magic" number 2 is ?? I cannot find the list of magic numbers for flock() or what they mean.

3. How do I release the lock on the file so others can then right to it?

4. What actually happens when someone tries to write to a file that is locked with flock()? Do I have to warn them that the file is currently locked and they must try their operation again later? That would seem rather dopey.

Again, if anyone can explain these items in plain English, I would greatly appreciate it, and hopefully it would clear up any misunderstandings others may have too.

[brandonschnell]

1) most of the time if two users try to access the file at the same time nothing bad will happen. but with no flocking and heavy access there is a real chance for file corruption. the problem is the order the file system calls are made. for example if a file is truncated by one process so data can be added to the start of the file and if another process just happens to read the file after the truncate and before the fwrite then bad things happen (file has now been erased and only contains some of the new data). but for this to occur the server (running millions of instructions at a time) would have to choose to stop the first process in midstream after the truncate, run another process accessing the same file, and try to read/write the file. if the first process is trying to add data a similiar problem can occur, but most likely the data added by the first process would be lost. either way, if you can't count on the file contents to be consistent, you might as well use paper and pencil.

2) the magic numbers map to constants defined by the OS. according to http://www.php.net/manual/en/function.flock.php:

To acquire a shared lock (reader), set operation to LOCK_SH (set to 1 prior to PHP 4.0.1).

To acquire an exclusive lock (writer), set operation to LOCK_EX (set to 2 prior to PHP 4.0.1).

To release a lock (shared or exclusive), set operation to LOCK_UN (set to 3 prior to PHP 4.0.1).

If you don't want flock() to block while locking, add LOCK_NB (4 prior to PHP 4.0.1) to operation.

3) close the file pointer or call flock with the magic number of 3 (LOCK_UN)

4) here is a decent link that explains how flock is handled by the OS:

http://www.tcp.com/~mink/CLAM/week8.html

scroll down to where it says "How can I have my CGI process lock a file?"

even though the examples are in perl the basic idea is the same.

[kyrlian]

I've performed a quite simple chat, people are reading the log file or adding new lines at the end of it, from time to time the begining of the file is cut.
I'v heavily used flock() to prevent odd file crash, when people read n write, all goes well, but here is the pb :
as soon as file beging cut, a new 'version' of the file is created and you can read one or another, dont know why. dont know what is this new version, but if i try read and read again the same file(using filename) i randomly get the one or the other.
anyone has ever seen such a thing ?