Flock in PHP By Chao Liang

Basic Flock Concept What is flock? Abbreviation for file locking Why flock? Data consistency. Prevent file corruption. Different types of flock Advisory Mandatory

Flock in PHP Availability: PHP 3 >= 3.0.7, PHP 4, PHP 5 Advisory in Unix by default, and Mandatory in Windows Syntax: bool flock ( resource handle, int operation [, int &wouldblock] ) Handle: open file pointer Operation: LOCK_SH - shared lock LOCK_EX - exclusive lock LOCK_UN - release lock LOCK_NB - no block while locking Optional: $wouldblock – true if blocked, usually used with LOCK_NB Return: True for success False for failure

Before Use Flock By default, LOCK_SH, LOCK_EX, and LOCK_UN are blocked. LOCK_SH can be held by more than one process while LOCK_EX can only held by a single one process. A typical use for flock would be try to append to a log file. Request a shared lock when you only need to read from the file. Request an exclusive lock when you need to write to the file. Every file write operation needs to use the same locking mechanism in order for flock to work correctly. Use a database if more stability and security required. Do not forget to release the exclusive lock, or others can not get the lock.

How to Use Flock Example 2: <?php $writeSuccess = false; $fp = fopen(“./test.txt", "w+"); while($fp && !$writeSucess){ $writeSucess = flock($fp, LOCK_EX)); usleep(round(rand(0,10000))); } if (!$fp) {exit;} fwrite($fp, “Lock and write"); flock($fp, LOCK_UN); fclose($fp); ?> Example 1: <?php $fp = fopen(“./test.txt", "w+"); flock($fp, LOCK_EX)) fwrite($fp, “Lock and write"); flock($fp, LOCK_UN); fclose($fp); ?> Request exclusive lock Block if other has the lock Release lock Avoid collision Avoid invalid file pointer No statements in in between to minimize the chance of data lost

How to Use Flock Cont. Blocking can introduce data lost due to fopen does not do file locking. In Linux, flock can return false, even if locking succeeds. Fix: use LOCK_NB and $wouldblock <?php do { if(isset($wouldlock)) { flock($fp,LOCK_UN); fclose($fp); } $fp=fopen(“./test.txt","a") or die(); flock($fp,LOCK_EX | LOCK_NB,$wouldblock); usleep(round(rand(0,10000))); } while ($wouldblock ==1); fwrite($fp, “Begin writing”); flock($lock,LOCK_UN); fclose($fp); ?> Locking without blocking Locking succeed condition Ignore the return since it is not reliable

Workaround for Flock in NFS Use mkdir($file.lock) to simulate flock($file, LOCK_EX), and rmdir($file.lock) to simulate flock($file, LOCK_UN). Example: <?php $fp = fopen(“./test.txt”, ‘a’); Do{ $locked usleep(round(rand(0,10000))); } while ($locked === false); fwrite($fp, “begin writing”); rmdir(“test.txt.lock”); ?> It will fail if directory exists to indicate file is locked Removing the directory acts just like releasing the lock

Limitation of Flock Flock does not work for NFS. Downgrades release the old lock before applying the new lock. Flock does not support for FAT file system. Flock is not reliable for multithread server. Flock is handled differently among Windows, Linux, and Unix: advisory in Unix and Linux, and mandatory in Windows. Other process can still write to the file if it does not request a lock before writing to it.