fwrite if writable - if not try again later

[barrax]

Hi - i have a flash game that sends xml scores to be written to a file. The problem is that more than one player could be writing to the scoreboard at a particular time. So i end up with a 0kb file and everyone loses their score🙁

To try and overcome this i am trying to modify the php code so that it loops through - attempting to write to the file until it is successful. Here's what i have so far (btw it doesnt work - seems to continually loop)

<?php 
$filename = 'test.txt'; 
$somecontent = $_GET['testtext']; 
do  

{ 
    if (is_writable($filename)) { 
        if (!$handle = fopen($filename, 'w')) { 
             exit; 
        } 
        if (!fwrite($handle, $somecontent)) { 
            exit; 
        } 
        $written=true; 
        fclose($handle);                 

    } 
sleep(100); // to make sure the CPU does not freak out 
} 
while( !$written ); 
?> 

[Garth_Farley]

This is a common enough problem, so have a look at file locking. This ensures that a file is exclusively written to by one program at a time, to prevent 0byte files.

Call the flock() function before you write (it must be passed a file handler, and a type of lock). The manual page is here

Note: The FAT filesystem doesn't support file locking, so if you're testing on your own PC, flock() returns false every time. But you host should be using either a Unix or NTFS, which are ok.

Garth Farley

[barrax]

Sorry you'll have to excuse my newbie-ness in php - but this is how far i got - seems to still be continually looping. I guess a condition isn't being met.

<?php
$newXML = $_GET['newXML'];
$newXML = stripslashes($newXML);
$file = fopen("scores.xml", "w"); 
$written=false;
do {
	if(is_writable($file)){
		flock($file,LOCK_SH);
		fwrite ($file, "$newXML"); 
		flock($file,LOCK_UN);
		fclose($file);
		$written=true;
		print ("Written: $newXML");
		}
	sleep(100);
}
while ( written==false );
?>

[Garth_Farley]

File Locking prevents a file from being written to by 2+ programs simultaneously.

When program A places a lock on a file, program B has to wait until A removes the lock, until it can write.

PHP will patiently wait until the lock on the file is removed before writing to it.

$newXML = $_GET['newXML']; 
$newXML = stripslashes($newXML); 
$file = fopen("scores.xml", "w");  

if(!is_writable($file)) die("File not readable");  //uh oh

if(flock($file,LOCK_EX) ){  //exclusive lock - for writing
     fwrite ($file, "$newXML");  

}//if file not locked successfully, nothing will be written.
flock($file,LOCK_UN); 
fclose($file); 

print ("Written: $newXML"); 

 ?> 

Garth Farley

[barrax]

Thanks so much for your help..

Looking at it then - is this line really needed?

if(!is_writable($file)) die("File not readable"); 

Because the rest of the script seems to take care of opening/locking/writing/unlocking/closing.

[Garth_Farley]

Nope, not really. Only if you're file permissions go haywire, it's a good debug.

GF