Description

flock(resource $stream, int $operation, int &$would_block = null): bool

flock() allows you to perform a simple reader/writer model which can be used on virtually every platform (including most Unix derivatives and even Windows).

The lock is released also by fclose(), or when stream is garbage collected.

PHP supports a portable way of locking complete files in an advisory way (which means all accessing programs have to use the same way of locking or it will not work). By default, this function will block until the requested lock is acquired; this may be controlled with the LOCK_NB option documented below.

Parameters

stream

A file system pointer resource that is typically created using fopen().

operation

operation is one of the following:

LOCK_SH to acquire a shared lock (reader).
LOCK_EX to acquire an exclusive lock (writer).
LOCK_UN to release a lock (shared or exclusive).

It is also possible to add LOCK_NB as a bitmask to one of the above operations, if flock() should not block during the locking attempt.

would_block

The optional third argument is set to 1 if the lock would block (EWOULDBLOCK errno condition).

Return Values

Returns true on success or false on failure.

Examples

Example #1 flock() example

<?php

$fp = fopen("/tmp/lock.txt", "r+");

if (flock($fp, LOCK_EX)) {  // acquire an exclusive lock
    ftruncate($fp, 0);      // truncate file
    fwrite($fp, "Write something here\n");
    fflush($fp);            // flush output before releasing the lock
    flock($fp, LOCK_UN);    // release the lock
} else {
    echo "Couldn't get the lock!";
}

fclose($fp);

?>

Example #2 flock() using the LOCK_NB option

<?php
$fp = fopen('/tmp/lock.txt', 'r+');

/* Activate the LOCK_NB option on an LOCK_EX operation */
if(!flock($fp, LOCK_EX | LOCK_NB)) {
    echo 'Unable to obtain lock';
    exit(-1);
}

/* ... */

fclose($fp);
?>

Notes

Note:
flock() uses mandatory locking instead of advisory locking on Windows. Mandatory locking is also supported on Linux and System V based operating systems via the usual mechanism supported by the fcntl() system call: that is, if the file in question has the setgid permission bit set and the group execution bit cleared. On Linux, the file system will also need to be mounted with the mand option for this to work.

Note:
Because flock() requires a file pointer, you may have to use a special lock file to protect access to a file that you intend to truncate by opening it in write mode (with a "w" or "w+" argument to fopen()).

Note:
May only be used on file pointers returned by fopen() for local files, or file pointers pointing to userspace streams that implement the streamWrapper::stream_lock() method.

Warning

Assigning another value to stream argument in subsequent code will release the lock.

Warning

On some operating systems flock() is implemented at the process level. When using a multithreaded server API you may not be able to rely on flock() to protect files against other PHP scripts running in parallel threads of the same server instance!

flock() is not supported on antiquated filesystems like FAT and its derivates and will therefore always return false under these environments.

Note:
On Windows, if the locking process opens the file a second time, it cannot access the file through this second handle until it unlocks the file.