PHP Cookbook

18.24. Locking a File

Problem

You want to have exclusive access to a file to prevent it from being changed while you read or update it. If, for example, you are saving guestbook information in a file, two users should be able to add guestbook entries at the same time without clobbering each other’s entries.

Solution

Use flock( ) to provide advisory locking:

$fh = fopen('guestbook.txt','a')         or die($php_errormsg);
flock($fh,LOCK_EX)                       or die($php_errormsg);
fwrite($fh,$_REQUEST['guestbook_entry']) or die($php_errormsg);
fflush($fh)                              or die($php_errormsg);
flock($fh,LOCK_UN)                       or die($php_errormsg);
fclose($fh)                              or die($php_errormsg);

Discussion

The file locking flock( ) provides is called advisory file locking because flock( ) doesn’t actually prevent other processes from opening a locked file, it just provides a way for processes to voluntarily cooperate on file access. All programs that need to access files being locked with flock( ) need to set and release locks to make the file locking effective.

There are two kinds of locks you can set with flock( ): exclusive locks and shared locks. An exclusive lock , specified by LOCK_EX as the second argument to flock( ), can be held only by one process at one time for a particular file. A shared lock , specified by LOCK_SH, can be held by more than one process at one time for a particular file. Before writing to a file, you should get an exclusive lock. Before reading from a file, you should get a shared lock.

To unlock a file, call