CODEWITHSUNDEEP
phpfilefile-io
Amandeep Grewal
Amandeep Grewal

Reputation: 1821

PHP Simultaneous File Writes

I have two different PHP files that both write to the same file. Each PHP script is called by a user action of two different HTML pages. I know it will be possible for the two PHP files to be called, but will both PHP files attempt to write to the file at the same time? If yes, what will happen? Also, it is possible to make one of the PHP fail gracefully (file write will just fail, and the other PHP can write to the file) as one PHP function is less important that the other.

Upvotes: 24

Views: 13228

Answers (5)

chaos
chaos

Reputation: 124297

The usual way of addressing this is to have both scripts use flock() for locking:

$f = fopen('some_file', 'a');
flock($f, LOCK_EX);
fwrite($f, "some_line\n");
flock($f, LOCK_UN);
fclose($f);

This will cause the scripts to wait for each other to get done with the file before writing to it. If you like, the "less important" script can do:

$f = fopen('some_file', 'a');
if(flock($f, LOCK_EX | LOCK_NB)) {
    fwrite($f, "some_line\n");
    flock($f, LOCK_UN);
}
fclose($f);

so that it will just not do anything if it finds that something is busy with the file.

Upvotes: 35

Antti Rytsölä
Antti Rytsölä

Reputation: 1545

Please note posix states atomic access if files are opened as append. This means you can just append to the file with several threads and they lines will not get corrupted.

I did test this with a dozen threads and few hundred thousand lines. None of the lines were corrupted.

This might not work with strings over 1kB as buffersize might exceed.

This might also not work on windows which is not posix compliant.

Upvotes: 13

Marcello Nuccio
Marcello Nuccio

Reputation: 3901

Please note:

As of PHP 5.3.2, the automatic unlocking when the file's resource handle is closed was removed. Unlocking now always has to be done manually.

The updated backward compatible code is:

if (($fp = fopen('locked_file', 'ab')) !== FALSE) {
    if (flock($fp, LOCK_EX) === TRUE) {
        fwrite($fp, "Write something here\n");
        flock($fp, LOCK_UN);
    }

    fclose($fp);
}

i.e. you have to call flock(.., LOCK_UN) explicitly because fclose() does not do it anymore.

Upvotes: 10

xentek
xentek

Reputation: 2645

FYI: flock only works on *nix and is not available on Windows

Upvotes: -4

cdmckay
cdmckay

Reputation: 32250

Take a look at the flock function.

Upvotes: 1

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