PHP simultaneous file access

Question

Ok, ok .. so right now I'm freaking out.

index1.php

<?
function write_file($filepath,$filecontent) {
    $openedfile = fopen($filepath,"w+"); //replace with $openedfile = fopen($filepath,"a"); just in case
    flock($openedfile, LOCK_EX);
//add here fclose($openedfile); to work
//add here $openedfile = fopen($filepath,"w+"); to work
    fwrite($openedfile,$filecontent);
    flock($openedfile, LOCK_UN);
    fclose($openedfile);
}
function read_file($filepath) {
    $openedfile = fopen($filepath,"r+");
    flock($openedfile, LOCK_SH);
    sleep(10);
    $filecontent = file_get_contents($filepath);
    flock($openedfile, LOCK_UN);
    fclose($openedfile);
    return $filecontent;
}
write_file("Readme.txt","test 1");
$f1 = read_file("Readme.txt");
echo $f1;
?>

index2.php

<?
function write_file($filepath,$filecontent) {
    $openedfile = fopen($filepath,"w+"); //replace with $openedfile = fopen($filepath,"a"); to work
    flock($openedfile, LOCK_EX);
//add here fclose($openedfile); to work
//add here $openedfile = fopen($filepath,"w+"); to work
    fwrite($openedfile,$filecontent);
    flock($openedfile, LOCK_UN);
    fclose($openedfile);
}
function read_file($filepath) {
    $openedfile = fopen($filepath,"r+");
    flock($openedfile, LOCK_SH);
    $filecontent = file_get_contents($filepath);
    flock($openedfile, LOCK_UN);
    fclose($openedfile);
    return $filecontent;
}
write_file("Readme.txt","test 2");
$f1 = read_file("Readme.txt");
echo $f1;
?>

I run index1.php, then after 2 sec I run index2.php. Index2.php waits for index1.php as expected but index1.php shows nothing after 10 sec, while index2.php shows "test 2". What is going on?

EDIT: I figured it out :D. I changed

$openedfile = fopen($filepath,"w+");

to

$openedfile = fopen($filepath,"a");

in second php and it doesn't wipe readme.txt upon index2.php execution anymore.

Looks like you release the lock prior writing to the file. See also fflush. — hakre
– hakre, Commented Aug 4, 2012 at 9:46

JarroVGIT · Accepted Answer · 2012-08-04 09:47:51Z

1

Doesn't this has to do something with the fact your readme.txt is actually empty for a very short period of time when you open the file to write in it? I thought PHP took out the entire text, and than replaced it with the entire text + addings. When index1.php wants to read the file, index2.php has just cleared it perhaps? You can check this in the apache logs by the way.

EDIT: also, immediately after unlocking the file, index2.php takes control over it, overwriting TEST 1 with TEST 2.

edited Aug 4, 2012 at 9:47

answered Aug 4, 2012 at 9:39

JarroVGIT

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6 Comments

user148823 Over a year ago

You're right. I checked again and readme.txt is empty after I run index2.php. I was miss-leaded because earlier when I checked readme.txt through FTP (server is not local) after index2.php was executed, readme.txt was containing "test 1". Does FTP have some cache or somthing?

hakre Over a year ago

FTP? With no word in your question you mentioned that this is related to FTP. And yes, FTP does things different than the local file-system because it is just a protocol and a server talking with it. It then depends on the FTP server and it's configuration what exactly happens. Check the configuration and documentation of your FTP server to find out how you can influence its behavior for your good.

JarroVGIT Over a year ago

Not that I'm aware of actually. But perhaps it's just easier/saver to put the content of readme.txt in a database? That's faster to edit and change.

JarroVGIT Over a year ago

@hakre It isn't related to FTP. He just checked the content of readme.txt to verify if it was changed or not.

hakre Over a year ago

@DjerroNeth: OP asked in a comment about FTP, that's why I wondered.

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