More on flocking() and the Mother Ship.

Anon

Apparently flock() can't always be trusted. The PHP manual says this:

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

So if you call to a function something like this, expecting only one process to be able to append to the file at one time:

function WriteFile($FileName, $Data)
{
$FilePtr = fopen($FileName, "a") or die ("Couldn't open file : $FileName");

flock($FilePtr, LOCK_EX);

.
.
.
}

. . . you may think you are safe. But not quite. It seems that flock() only puts other processes on hold that may be writing to your file, until the file is closed. But it does not stop I/O from happening!

Huh? Isn't that a contradiction? Yes, it sure sounds like it to me. IOW, you may think the file is locked, and the other process is TOLD the file is locked, but it goes ahead and writes to the file anyway! That's just swell.

I don't know who designed flock() but I suspect they were either drugged out at the time, or else they had seen one to many flying saucers and were waiting for the Mother Ship :-)

Either way, flock() is basically useless. The only way to fix this is you have to sit there and manually grant a user the right to write to your file. Yes, I know. We all thought that was the purpose of flock(), but surprise!

This reminds of beginning air traffic control school in the Air Force. We had a very simple scheme for this: "Ok number 1, it's your turn to land . . . Ok number 2, you go next. . . Ok number 3, it's your turn!"

mrskhris

Hi, Stinger,

First of all, what operationg system do you use?

Second, here is the excerpt from PHP annotated manual:
On most operation systems flock() is implemented at the process level. When using a multithreaded server API like ISAPI you cannot rely on flock() to protect files against other
PHP scripts running in parallel threads of the same server instance!

Anon

My server (with Verizon) uses UNIX. What version? Don't know. Probably one of the thousands of mutants out there. So I presume that flock() should work as indicated (or not work at all, if you read that little gotcha in the manual correctly).

That gotcha statement about "PHP scripts running in parallel threads" is clear as mud to me. I'm not sure what ISAPI is, or who might even have it. What I think it is saying (the little I know about threads, have pulled a few on a new suit I once bought) is that since UNIX, or NT all support multi-users and multi-processes, (meaning these threads are running around all over the microprocessor's address space and whatever it is doing in there) then flock() is just a crap shoot, like the manual really says.

I guess what I really want to know is, if I call flock() as:

flock($fp, 2); // magic number for PHP < 4.0

will it guarantee (on UNIX) that no one else can write to whatever file $fp points to until the file is closed with fclose($fp) ?

On top of that, will flock() also guarantee that any user who is waiting to write to this file (waiting meaning they received a notice that the file is locked), that when the file closes, they will then be able to carry out their write operation? And just how long can they be in this "wait" state?

Note: I am only allowing users to append to this file, not do anything else. They don't even know it exists, nor do they need to.

Thanks for your help.