PHP flock Function

last modified April 3, 2025

The PHP flock function provides advisory file locking. It helps prevent race conditions when multiple processes access the same file.

Basic Definition

The flock function allows you to acquire or release a lock on an open file. It operates on a file pointer resource and supports shared/exclusive locks.

Syntax: flock(resource $stream, int $operation, int &$would_block = null): bool. The function returns true on success or false on failure.

Basic Exclusive Lock Example

This shows how to acquire an exclusive lock for writing to a file.

basic_exclusive_lock.php

<?php

declare(strict_types=1);

$file = fopen(“data.txt”, “c+”);

if (flock($file, LOCK_EX)) { fwrite($file, “Writing with exclusive lock\n”); flock($file, LOCK_UN); // Release lock } else { echo “Couldn’t get the lock!”; }

fclose($file);

This acquires an exclusive lock (LOCK_EX) before writing. The lock is released with LOCK_UN. Only one process can hold an exclusive lock at a time.

Shared Lock Example

This demonstrates using a shared lock for reading a file safely.

shared_lock.php

<?php

declare(strict_types=1);

$file = fopen(“data.txt”, “r”);

if (flock($file, LOCK_SH)) { $content = fread($file, filesize(“data.txt”)); echo $content; flock($file, LOCK_UN); } else { echo “Couldn’t get the shared lock!”; }

fclose($file);

Multiple processes can hold shared locks (LOCK_SH) simultaneously. This is useful when you want to allow concurrent reads but prevent writes during reading.

Non-blocking Lock Example

This shows how to attempt a non-blocking lock that fails immediately if unavailable.

non_blocking.php

<?php

declare(strict_types=1);

$file = fopen(“data.txt”, “c+”);

if (flock($file, LOCK_EX | LOCK_NB, $would_block)) { if ($would_block) { echo “Lock is held by another process\n”; } fwrite($file, “Non-blocking write\n”); flock($file, LOCK_UN); } else { echo “Couldn’t get non-blocking lock!\n”; }

fclose($file);

The LOCK_NB flag makes the function return immediately if the lock can’t be acquired. The $would_block parameter indicates if the lock failed due to contention.

File Locking with Timeout

This example implements a simple timeout mechanism for acquiring locks.

lock_timeout.php

<?php

declare(strict_types=1);

function acquireLockWithTimeout($file, int $timeout = 5): bool { $start = time();

while (!flock($file, LOCK_EX | LOCK_NB)) {
    if (time() - $start >= $timeout) {
        return false;
    }
    usleep(100000); // Wait 100ms before retry
}

return true;

}

$file = fopen(“data.txt”, “c+”);

if (acquireLockWithTimeout($file)) { fwrite($file, “Write with timeout lock\n”); flock($file, LOCK_UN); } else { echo “Failed to acquire lock within timeout\n”; }

fclose($file);

This implements a custom timeout by repeatedly trying to acquire a non-blocking lock. It waits up to 5 seconds before giving up.

Process Synchronization Example

This demonstrates using file locks for inter-process synchronization.

process_sync.php

<?php

declare(strict_types=1);

$lockFile = fopen(“process.lock”, “c+”);

if (!flock($lockFile, LOCK_EX)) { die(“Could not acquire lock\n”); }

// Critical section file_put_contents(“output.log”, “Process " . getmypid() . " running\n”, FILE_APPEND); sleep(2); // Simulate work

flock($lockFile, LOCK_UN); fclose($lockFile);

echo “Process completed\n”;

This ensures only one instance of the script runs at a time. The lock file acts as a mutex for process synchronization.

Best Practices

• Always release locks: Use try-finally blocks to ensure unlock.

• Keep lock duration short: Minimize time holding locks.

• Handle errors: Check return values from flock calls.

• Use advisory locking: Remember locks are advisory in PHP.

• Clean up: Close files after use to free resources.

Source

PHP flock Documentation

This tutorial covered the PHP flock function with practical examples showing file locking techniques for safe file operations.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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