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(PHP 4, PHP 5)

flock — Portable advisory file locking


bool flock ( resource $handle , int $operation [, int &$wouldblock ] )

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() (which is also called automatically when script finished).

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).



An open file pointer.


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).
  • LOCK_NB if you don't want flock() to block while locking. (not supported on Windows)


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

Return Values

Returns TRUE on success or FALSE on failure.


Version Description
4.0.1 The LOCK_XXX constants were added. Prior to that you must use 1 for LOCK_SH, 2 for LOCK_EX, 3 for LOCK_UN and 4 for LOCK_NB


Example#1 flock() example



if (
flock($fpLOCK_EX)) { // do an exclusive lock
fwrite($fp"Write something here\n");
flock($fpLOCK_UN); // release the lock
} else {
"Couldn't lock the file !";




Note: flock() locks mandatory under Windows.

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()).


flock() will not work on NFS and many other networked file systems. Check your operating system documentation for more details.

On some operating systems flock() is implemented at the process level. When using a multithreaded server API like ISAPI 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 this environments (this is especially true for Windows 98 users).