(PHP 4, PHP 5)
flock — Portable advisory file locking
$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).
On versions of PHP before 5.3.2, 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). By default, this function will block until the
requested lock is acquired; this may be controlled (on non-Windows
platforms) with the LOCK_NB option documented below.
handleA 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 you don't want flock() to
block while locking. (not supported on Windows)
wouldblock
The optional third argument is set to TRUE if the lock would block
(EWOULDBLOCK errno condition). (not supported on Windows)
Returns TRUE on success or FALSE on failure.
| Version | Description |
|---|---|
| 5.3.2 | The automatic unlocking when the file's resource handle is closed was removed. Unlocking now always has to be done manually. |
| 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
<?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);
?>
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.
Assigning another value to handle argument in
subsequent code will release the lock.
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).